ImmunologyThese notes were made by Hadley Wickham, hadley@technologist.com and are licensed under the Creative Commons NonCommercial-ShareAlike License. To view a copy of this license, visit http://creativecommons.org/licenses/nc-sa/1.0/ or send a letter to Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA.

Table of ContentsOverview of Immunity and Immune System....................................................................3

How Ag’s get to Meet Immune System...........................................................................5

B Cells and Ab Responses..............................................................................................7

Cells and Cell-Mediated Immunity..................................................................................9

T and B Cell Development and Thymus.........................................................................12

Cytokines, Lymphocyte Surface and Adhesion Molecules...............................................15

How Immune System Deals with Infections...................................................................19

Immunodeficiency (ID).................................................................................................22

Hypersensitivity..........................................................................................................23

Autoimmunity.............................................................................................................25

Immunology in Diagnostic and Clinical Medicine...........................................................28

Factors Influencing Immune Responsiveness......................................................................................32

Overview of Immunity and Immune System

What is the Immune System?

non-specific or innate immune mechanisms 1st line of defence against infectious agents

generally very effective at preventing invasion but if are breached, specific immune system called upon

both immune systems composed of variety of molecules and cells distributed throughout body

interdependent with considerable interaction between them

Non-Specific Immune System Specific Immune System

Repeated infection

resistance not improved resistance improved

Soluble factors Ab’s, complement, acute phase proteins, interferons (IFN and IFN

lysozyme, complement, cytokines

Cells phagocytes, natural killer (NK) cells

lymphocytes, monocytes, APC

Non-Specific Immune System

exterior (especially skin) presents effective barrier to most organisms

importance evident when individual suffers serious burns

most enter via epithelial surfaces (nasopharynx, gut, lungs or GU tract)

variety of physical and biochemical defences protect these areas from most infections

after penetration encounters phagocytic cells of reticulo-endothelial system

macrophages process Ag as well as engulfing them – vital in specific and non-specific immunity

Acute Phase Proteins

acute phase proteins in response to early ‘alarm’ mediators (e.g. IL-1) released by tissue injury

many produced in liver, activate complement, phagocytosis by opsonization and bacterial enzymes

Specific Immune System

8 general properties which characterize immune system:

processing of molecular shape

can recognise diverse range of Ag’s

generates response specific to stimulus

responds to unexpected stimuli

effective discrimination between self and non-self Ag’s

adaptability to provide rapid, appropriate responses

specific immunological memory

regulation by complex 'internal' and 'external' networks

immune system consists of collection of lymphoid organs connected by circulatory and lymphatic system

10 lymphoid organs 20 lymphoid organs

Role mature stem cells into Ag-sensitive T or B lymphocytes

receive mature lymphocytes formed in primary organs

Components

bone marrow (source of stem cells and B lymphocytes)

spleen, lymph nodes, gut-associated lymphoid tissue (GALT – tonsils, adenoids, Peyer’s patches)

10 lymphoid organs 20 lymphoid organs

thymus (source of T lymphocytes)

fetal liver (source of stem cells and B lymphocytes)

blood and lymphatic vessels connect organs to provide comprehensive network throughout body

enables lymphocytes and granulocytes to circulate freely and quickly reach important areas

permits Ag entering body to be brought rapidly into contact with effector mechanisms for disposal

Cells

cells include lymphocytes, macrophages, monocytes and APC (APC)

can be subdivided into number of subpopulations on basis of functions

effector’s responsible for responding to and disposing of Ag’s, regulator’s for control of effector cells

Group Cell Description SurfaceMarkers

Effectors B Ab production CD20

CTL T cell-mediated cytotoxicity CD8

NK/K Natural killer cells (early defence in anti-viral and tumour immunity)

K/Null Ab-dependent cell-mediated cytotoxicity (ADCC) CD16, CD56

Regulators

TH Induction of effector responses

(helper/inducer, recruit non-specific mechanisms. TH1 and TH2 subsets)

CD4

TS Suppression of some effector responses (indistinguishable from those responsible to cytotoxicity)

CD8

How Ag’s get to Meet Immune System

How Foreign Ag’s Meet Immune System

response of immune system to foreign substance depends on nature of substance and route of entry

Route of Entry Response

Skin much foreign matter dealt with at 10 site (local ingestion by macrophages)

Agic fragments are transported in macrophage-like phagocytic cells to local lymph nodes (and spleen), or arrive by themselves at node and are collected by resident scavenger macrophages

in node, Agic fragments passed to dendritic reticular cells (specialized APCs) which process fragments into smaller pieces (epitopes) and present them to Ag-sensitive T and B lymphocytes

Bloodstream filtered out by macrophages in spleen (capable of mounting immune response), liver and lungs

Upper respiratory, GI tract filtered through local lymph nodes and specialized lymphoid organs in gut

Spleen

differentiated into two types of tissue: lymphoid white pulp and erythroid red pulp

white pulp, surrounds small splenic aa. from origins to termini (analogous to cortex of lymph node)

red pulp involved in scavenging old RBCs and serves as reserve site for haematopoiesis

within lymph nodes and spleen, T and B lymphocytes reside in specific areas

Lymph node Spleen

B lymphocytes

follicles of cortical region less well-defined follicular areas of white pulp

T lymphocytes

paracortical areas, medullary cords

peri-arteriolar cuffs

macrophages (especially dendritic reticular macrophages) closely associated with lymphocytes, play vital role in presenting foreign Ag’s to lymphoid cells

Reticulo-Endothelial System

bone marrow-derived myeloid progenitors give rise to cells of mononuclear phagocytic system

two main functions, performed by two different types of cells

‘professional’ phagocytic macrophages remove particulate Ag’s

APCs present Ag’s to specific Ag-sensitive lymphocytes

phagocytic tissue macrophages form network called reticulo-endothelial system (RES)

adherence and ingestion by cells of RES is promoted through opsonisation by complement and Ig

once ingested, degraded by cytoplasmic lysosomal granules (characteristic feature of phagocytes)

both respond to and secrete cytokines

phagocytosis and degradation important method of controlling foreign matter and dead/dying tissues, but process has additional importance as it usually first step in process of lymphocyte activation

Tissue

RES Cell Transport destination

Skin Langerhans cells Regional node

Lungs alveolar macrophages

Spleen or regional nodesBlood blood monocytes, splenic macrophages

Liver Kupffer cells

Gut epithelial M cells Peyer’s patches

Functions of APC

Ag collection, concentration, processing and presentation to lymphocytes

co-stimulation (accessory surface molecules, cytokines)

tolerance induction

Major Histocompatibility Complex (MHC)

studies of grafts between different individuals led to identification of cell surface structures associated with tissue compatibility (Ag’s which caused strong immune responses against grafted tissues)

gene complex responsible called major histocompatibility complex (MHC), located on chr 6

aka human lymphocyte Ag’s (HLA)

Class I and Class II MHC Gene Products

class I and II MHC genes code for surface proteins

class I found on virtually all nucleated cells in body, (A, B, C)

class II restricted to B lymphocytes, macrophages and other specialized APC, (DP, DQ, DR)

class I genes expressed as single glycoprotein (43kD) associated on cell surface with 12kD β2-microglobulin

class II gene products are expressed as heterodimers of α and β chains, encoded by genes of HLA-D locus

both have two other important properties: co-dominant and polymorphic

highly unlikely that two unrelated individuals will have exactly same set of HLA genes

MHC Proteins and Ag Presentation

MHC proteins crucial structures for presenting Ag’s to T lymphocytes

both class I and II MHC have groove in top (peptide-binding groove) into which small pieces of foreign Ag can be fitted

some differences between class I and class II in sort of peptides presented

peptide-binding groove on class I restricted to peptides of 8-10 aa, while class II restricted to ~14 aa

Pathways of Ag Processing

Ag processing within cells differs according to whether Ag’s are taken up by cell or originate from within cell

class I peptides usually derived from infectious process or from breakdown of normal cell products

class II peptides usually result from processing of Ag’s taken up and degraded by APCs

not all fragments resulting from Ag processing will be immunogenic epitopes

only those that can bind to peptide-binding groove can be presented to lymphocytes

different people have different MHC molecules and might present different epitopes to immune system

because class I MHC products expressed on virtually all cells, almost all cells can present Agic fragments derived from metabolic breakdown or from intracellular infectious processes

only B cells and certain specialized Ag-processing cells express class II MHC so only these cells can present Agic fragments from ingested material

B Cells and Ab Responses

Summary

Ab’s

2 heavy and 2 light chain, covalently-bonded structure with globular domains and variable (Ag-binding) and constant (biologically-functional) regions

and light chain and , , , and heavy chain classes

allotypic variation is intraspecies allelic variability

idiotypic variation refers to diversity at binding site (relates to hypervariable regions)

functional properties of Ab’s include:

direct neutralization

agglutination

opsonisation

ADCC

complement activation

Class

Properties Serum Ab

IgM large pentamer, confined to bloodstream, does not cross placenta

produced early in 10 Ab responses - good defence against bacterial spread

efficient agglutinator and complement activator

10%

IgG small monomer, diffuses easily out of blood into extravascular areas, crosses placenta

major class in secondary responses

good complement activator, opsonin and Fc receptor-mediated effector mechanisms

70-75%

IgA predominant Ab class in sero-mucus secretions (dimer)

defence of exterior surfaces

15-20%

IgD highly sensitive to proteolysis

receptor on virgin, Ag-sensitive B lymphocytes

trace

IgE high affinity Fc receptor on mast cells and basophils

involved in symptoms of allergy

elevated levels in some helminthic parasite infections

trace

Complement cascade

classical pathway triggered by Ag-Ab complexes, involves formation of C3 convertase from activation of C1, C2 and C4 components

alternative pathway does not involve Ag-Ab complexes but cell wall components

alternative pathway C3 convertase requires factor B, factor D and C3B

cleavage of C3 into C3a and C3b is central event in complement activation

C3b binds to cell surfaces and serves as focus for activation of late components (C5 to C9) cell lysis

Generation Of Ab Responses

10 Ab responses result from activation of naive, Ag-sensitive B cells, 20 (or anamnestic) responses result from activation of (long-lived) memory B cells

generation of Ab responses occurs in 20 lymphoid organs after Ag-sensitive B cells have bound sufficient Ag through their surface Ig (sIg) receptors and received activation signals from TH

cells

activation of Ag-sensitive B cells results in proliferation and maturation to form population of plasma cells which secrete Ab’s of same Ag-binding specificity as was present on sIg of their precursor B cell, and population of memory B cells which can be restimulated subsequently to produce secondary responses

IgM 1st class of Ab’s produced in 10 response and then class switches to IgG

IgG predominates in 20 responses although small IgM component generally still evident

term affinity refers generally to strength of binding between receptor and its ligand

In Ab response, there will be range of Ab affinities generated and average affinity of Ab population produced will depend on concentration of Ag and immunological history of individual

Immunological Memory

during course of Ab response, new population of small B lymphocytes (memory cells) distinct from Ab-forming plasma cells are generated

histologically, memory B cells appear very similar to original small, Ag-sensitive B cells

role is 'super' Ag-sensitive cells

serve as replacements for their original Ag-sensitive precursors but have lower activation threshold so can be stimulated more easily

can mount more vigorous response than naive B cells so give higher concentrations of Ab’s (usually of different class) that persist for longer periods in serum

Ab Class Switching

IgM 1st class to be synthesized in 10 Ab response but after time replaced by production of IgG

individual activated B cell clones begin by producing IgM but subsequently switch to IgG production

B cell remains true to Ag that originally stimulated it, although some mutation of Ag-binding region fine tuning specificity

IgG synthesis dramatically increased in 20 responses because memory B cells preferentially make IgG

IgM component of 20 response mostly due to activation of new naive Ag-sensitive B cells

Cells and Cell-Mediated Immunity

Natural Killer (NK) Cells

NK cells generally large granular leukocytes comprising 1-5% mononuclear cells in blood

present in spleen and peritoneal exudate, absent from thymus and few in lymph nodes and bone marrow

can be identified by presence of Fcγ receptor (CD16) and CD56 surface marker

do not have to recognize target Ag’s in association with MHC determinant

do not require activation by Ag, although activity by certain cytokines (e.g. IL-2, IFN-γ)

limited repertoire of receptors directed at CHO-containing target structures

recognize targets by variety of mechanisms:

e.g. IgG Fc-γ receptors (CD16) to bind Ab’s bound to target cells and activate NK cell

even target cells resistant to NK cytotoxicity in absence of Ab can be killed by ADCC mechanism

another way NK cells might recognize target cells is through combinations of adhesion molecules such as various of integrin and cell adhesin molecules

very little known about origins or their lineage relationships, but thought to develop in bone marrow

mice with SCID genetic mutation don’t develop T and B cells but have normal NK cells, indicating that NK cell development diverges from B and T lymphocyte development relatively early

Adoptive Transfer

possible to temporarily transfer immunity passively by injecting Ab’s from donor

provides short-term immunity, dependent on half-life of transferred Ab’s in recipient

situations where adoptive transfer of Ab’s does not protect against infection (e.g. intracellular parasites)

transfer of immunity can often be effected by injection of lymphocytes from immune donor

cytotoxic T-cells, rather than B-cell, important in this effect

Cell-Mediated Immunity and Graft Rejection

cell-mediated immunity (CMI) = any response in which Ab’s play subordinate role

most responses involve both Ab and cell-mediated events, each influencing degree and magnitude of other

tissue transplanted from one animal to a genetically-dissimilar animal does not survive long

initially graft tissue begins to grow and be accepted but after days-weeks (depending on type of tissue and amount of genetic dissimilarity) graft begins to die

histologically, lymphocytes infiltrate graft and some appear to attack and kill cells of graft

by using adoptive transfer techniques, found that T-cells most important

if 2nd graft from same donor placed on same recipient at later time, rejected much more rapidly, showing that immune responses capable of developing immunological memory

grafts between genetically-identical pairs not rejected because Ag’s not recognised as foreign

tissue from genetically-identical donor syngeneic, from genetically-unrelated donor allogeneic

immune responses that occur in graft rejection called alloreactive responses

Mixed Lymphocyte Reaction

similar immunological rejection processes occur when blood is transplanted from one individual to another

immune responses occurring during graft rejection can be mimicked by mixing lymphocytes from different individuals in culture and observing reaction over few days by measuring uptake of radioactive nucleotides

test called mixed lymphocyte reaction (MLR) and is still important way to test immunological compatibility

Cells and MHC-Restricted Recognition of Ag’s

T cells and B cells do not respond to exactly same foreign determinants

T cells respond best to Ag’s associated with cell surface

B cells bind soluble Ag’s

proof:

infect mouse from one inbred strain (strain 1) with lymphocytic choriomeningitis virus (LCM virus)

short time later, when cytotoxic T cells specific for virus-infected target cells developed, remove spleen from mouse and study specificity

found that virus-specific TCT could effectively kill infected target cells derived from syngeneic mouse (i.e. strain 1) but would not kill infected target cells derived from allogeneic mouse (i.e. strain 2)

indicates that cytotoxic T cells not only recognizing Ag’s specified by LCM virus but also some normal component of infected target cells

so T cell receptor for Ag more complex than surface immunoglobulin receptor in that it recognizes foreign Ag’s in association with some normal 'self' Ag’s

shown to be MHC

MHC-restricted recognition of Ag general property of T lymphocytes, although different T cells recognize Ag’s associated with different MHC structures

Ag-Recognition Surface Molecules on T-Cells

all T cells have complex of transmembrane proteins (CD3) on surface, known to be required for activation

TH cells also have CD4 molecules and TCT cells CD8

both CD4 and CD8 are intimately associated with CD3

T cells have two other transmembrane polypeptides involved in Ag recognition

called α and β chains, together known as T cell receptor (TCR)

striking similarities between structure of Ig and TCR: both chains have constant and variable domain, hydrophobic membrane-spanning region and short intra-cytoplasmic domain

genes that coding for TCR possess similar arrangement to that found in Ig genes

variable regions undergo rearrangement during T lymphocyte maturation in thymus

Ag Recognition and Roles Of CD4, CD8 & CD3

both CD4 and CD8 T lymphocytes use same genes to synthesize TCR

CD4 helper T lymphocytes: class II MHC-restricted recognition of Ag’s

CD8 cytotoxic T lymphocytes: class I MHC-restriction Ag

CD4 and CD8 bind to non-polymorphic regions of class II and class I respectively

TCT lymphocytes potentially able to recognize Ag’s expressed on any nucleated cell in body

TH lymphocytes have much more limited Agic universe

CD3 (intimately associated with TCR) consists of complex of 5 polypeptide chains

must be involved in activation, as monoclonal Ab’s against CD3 block induction and effector phases

other surface molecules involved in complex process of interaction, but TCR, CD4/CD8, and MHC contribute most to specificity

Clonal Selection and T Cell Activation

20 lymphoid organs and circulating lymphocyte pool contain repertoire of Ag-sensitive T cells, each cell bearing set of TCRαβ receptors through which it can recognize outside world

foreign Ag’s come into contact with these T cells

those that can bind strongly to foreign Agic determinants presented in MHC are selected and activated

as well as requirement for binding to Ag presented by MHC activation requires and interaction between other surface molecules called co-stimulators

without co-stimulator activation does not occur and T cell can become anergized

include B7 expressed on activating APC and CD28 expressed on T cell

for cytotoxic T lymphocyte precursors (CTLP), Ag binding and other surface interactions induces cell to express cytokine-R, in particular IL-2-R (secreted by activated TH cells)

in response to Ag, IL-2 and other cytokines, CTLP can then proliferate and differentiate to form cytotoxic effector cells

cells in cytotoxic clone have same Ag-R and can recognize same foreign Ag presented in MHC

seek out and destroy any cells which bear foreign determinants that they can recognize

during processes of T-cell activation by Ag, memory cells also generated

similar to and have same specificity as precursors but more sensitive and can respond more vigorously

responsible for maintenance of immune state, causes rapid 2nd set rejection in transplant immunology

Cytotoxic Mechanisms Used by CD8 T Cells

do not release effector molecules but require direct physical contact with targets to exert cytotoxic effect

kill targets by three mechanisms

insert complex of molecules called perforin into membrane which assembles into doughnut-shaped holes and compromise integrity of target cell’s membrane

release enzymes which digest target cell membrane

release cytokines that can bind to receptors on target cells and induce apoptosis

T and B Cell Development and Thymus

T Cell Receptor Genes

very similar to Ig genes and are considered part of Ig gene superfamily

divided into arrays of interchangeable V, D and J exons scattered over large tracts of DNA

undergo rearrangement during T cell development in same way as Ig genes during B cell development, so that individual mature T cells end up with ability to produce only one type of receptor

separate gene complexes for α, β and γ chains and each contains V, J and D exons followed by C exons

δ chain unusual in that D, J and C exons are located between V and J exons of α chain gene complex

As result, α and δ chains use same V segments

C gene segments made up of 4 exons which include constant domain segment, short hinge-like region, transmembrane and intracytoplasmic parts

during T cell development in thymus, V, D and J segments are selected and recombined to form complete V-coding domain adjacent to particular C region

Two Types of TCR

most T cells use α and β chains but small subset use γ and δ chains

represent lineage distinct from αβ T cell lineage

found in small numbers in peripheral blood and lymphoid organs (1-10%)

present in larger numbers in intestinal epithelium and represent bulk of dendritic T cells in skin

home to lungs, reproductive tract and spleen

in lymphoid tissue, γδT cells reside in regions distinct from those occupied by αβ T cells; splenic γδ T cells predominate in sinusoids, αβ in follicular areas

majority have CD3 associated with their TCR but lack CD4 and CD8 (although small population of intestinal γδ also have CD8 on their surface)

cannot recognize Ag’s in same way as αβ T cells because do not have CD4 or CD8 molecules

physiological role uncertain, but may have role in control of infectious processes and autoimmunity

relative abundance epithelial surfaces suggests importance as early line of defence

proliferate in response HSPs

T Cell Receptor Diversity

at least 3 highly diverse regions in both α and β chains (correspond to CDRs of Ig)

2 encoded by V gene segments

1 encoded by V and J gene segments (CDR3, in α chain) or V, D and J segments (in β chain)

many fewer V genes employed by TCR than by Ig, other ways are used by TCR to generate diversity - particularly within CDR3 region

include:

N-region diversity documented in all 4 TCR chains but only occurs in 1 Ig chain (heavy chain locus)

large number of TCR J region gene segments (cf. small number in Ig loci)

several Vαs, Vγs and Vδs flexible with respect to position of their 3' joining points (not seen in Ig)

can use both D regions in δ chain and N-region addition occurs at 3 different positions

additionally, translation of D region sequences in all possible reading frames rare in Ig heavy chains but common in TCR β and δ chains, compensating for smaller number of TCR segments

Ig TCR αβ TCR γδ

H κ α β γ δ

Variable (V) segments

~100 ~70 50 57 8 3

Diversity (D) segments

~4 - - 2 0 3

Joining (J) segments 6 5 70 7 3 3

Combinatorial diversity

~106 ~106 ~103

N-region addition V-D

D-J

None V-J V-D

D-J

V-J V-D1

D1-D2

D2-J

Ds read in all frames Rarely - - Often - Often

Somatic mutation yes yes no no no no

Total diversity

by all mechanisms

~1010 ~1010 ~106

concentration of diversity in V-J junction of TCRs may be reflection of known function of TCRs (recognize very diverse small molecules embedded in much less diverse and larger MHC molecules)

γδT cells, in spite of great potential for diversity, show very limited repertoire for V, D and J gene usage

e.g. within given epithelium, γδT cells show little or no diversity

however, certain receptor genes used appear to be specific for location where γδ cell found

B And T Cell Ontogeny and Self-Tolerance

both B and T cell repertoires are designed for maximum recognition versatility

only cells that can recognize Ag through surface receptors activated to generate responses

because there does not have to be exact complementarity between sIg receptors and Ag, cells with range of affinities triggered in response to any particular Ag

high probability that there will always be some cells capable of responding to even most obscure Ag’s

but immune system must be able to discriminate between self and non-self and respond differently

B Cell Ontogeny

lymphoid progenitor cells and early pro-B cells bind to VCAM-1 on bone marrow stromal cells through integrin VLA-4 and other adhesion molecules

promotes binding of surface c-Kit tyrosine kinase to stem-cell factor, inducing proliferation

late pro-B cells require both stem cells factor of stromal cell surface and IL-7 for growth and maturation

first type of cell that can be identified as being committed to B cell lineage is pre-B cell

can synthesize (but not secrete) μ heavy chains (can be observed in cytoplasm)

have undergone DNA rearrangements in heavy but not light chain genes

differentiate further and become fully committed by rearranging either their κ or λ light chain genes so that they acquire ability to make complete immunoglobulins

next, negative selection of self-reactive cells occurs

if immature B cell expresses s-IgM-R that recognizes self Ag in bone marrow it will bind to it

unlike mature B cells, any immature B cells that bind Ag become inactivated

never reach full maturity, and do not appear in Ag-sensitive B cell repertoire of 20

lymphoid organs

immature B cells that don’t recognize self Ag’s permitted to differentiate into fully mature B-cells and express s-IgD and IgM

s-IgD functions as B cell receptor delivering activation signal following Ag recognition, often referred to as triggering receptor

T Cell Ontogeny

T cell development occurs in thymus (bilobed organ overlying heart, each lobe organized into lobules)

in each lobule, bone marrow-derived lymphoid cells arranged in outer cortex of immature proliferating thymocytes and inner medulla of more mature cells

interdigitating bone marrow-derived APCs expressing MHC molecules

both thymic ep cells and interdigitating cells are involved in intrathymic ‘education’ of T lymphocytes

requirements to selection T cell lineage:

effective T cell receptor gene rearrangement

+ve selection for recognition of self MHC

-ve selection to eliminate strongly auto-reactive cells

differentiation of CD4 and CD8 subpopulations

development of αβ and δγ T cell subpopulations

during development, stem cells move from cortex to medulla, becoming progressively more mature

each thymocyte will rearrange either its TCRαβ or TCRδγ genes during sojourn in thymus

cortical thymocytes express CD4 and CD8 accessory molecules (double positive), subsequent maturation leads to these cells becoming ‘single positive’ medullary thymocytes

immature CD4/CD8 cells express only 1/5-1/10 TCR molecules of mature peripheral blood T cells

mature thymocytes develop TCR density ~20-40,000 per cell, similar to peripheral T cells

Positive And Negative Selection In Thymus

of cells expressing TCRαβ are two types of selection that must occur before develop into fully mature T cells

must express TCR which recognizes MHC molecules with low affinity so that they will be able to recognize MHC in association with foreign epitopes with higher affinity (+ve)

must not recognize MHC + self peptide so strongly to be self-reactive in absence of foreign Ag (-ve)

Positive Selection

recognize Ag exclusively in association with self-MHC

not property of all possible TCRαβ combinations

T cells unable to interact with self MHC die in thymus

only T cells with TCRαβ capable of interacting with self-MHC are positively selected for survival

self-MHC molecules responsible reside on thymic epithelial cells of cortex

probably occurs when thymocytes express both CD4 and CD8 accessory molecules

Negative Selection

T cell repertoire arising from random rearrangement of TCRαβ genes would include combinations that would recognize self Ag’s plus self-MHC

strongly self-MHC autoreactive T-cells clonally deleted to maintain self tolerance

for clonal deletion to explain all tolerance every self-Ag in body must manifest itself in thymus

some researchers think idea credible because thymus crossroads for macrophages, other APC and soluble molecules circulating through body

other mechanisms, such as clonal anergy or suppression probably operate as well

Mature Repertoire

as well as selection constraints, repertoire of mature T cells is dynamic and in constant process of flux, influenced by such factors as exposure during life to various microbial Ag’s

analysis of frequency of TCR gene usage shows variation from person to person at any one time

Cytokines, Lymphocyte Surface and Adhesion Molecules

T Cell Subsets And Cytokine Production

TH major source of many cytokines and very influential in modulating immune and inflammatory responses

TH can be divided into different populations on basis of different cytokines produced

TH0 thought to be common precursors

Cytokines produced TH0 TH1 TH2

Cytoxic/Inflammatory (IL-2, IFN-γ, lymphotoxin)

IL-12

TNF, GM-CSF, IL-3

Ab response

(IL-4, 5, 6, 10)

TH1 important in elimination of intracellular infections (e.g. Leishmania)

TH2 responses amplify humoral immune response against extracellular micro-organisms

TH1 and TH2 responses regulated themselves by cytokines produced

IL-4 produced by TH2 cells promotes further TH2 expansion and down-regulates TH1

response while IL-12 promotes TH1 responses and down-regulates TH2 cells

Activated T cells and their ProductsCD8 T cells

Cytotoxins perforin 1, granzymes

Others Fas ligand, IFN-, lymphotoxin

(TNF- and TNF-)

TH1 cells Macrophage-activating cytokines

IFN-, GM-CSF, TNF

Others IL-3, lymphotoxin, IL-2

TH2 cells B cell-activating cytokines CD40 ligand, IL-4, IL-5, IL-6

Others IL-3, GM-CSF, IL-10, TGF-

Cytokines

glycoproteins that act as soluble messenger molecules between cells

initiate many different cellular events including production of further cytokines and other mediators of inflammation, expression of adhesion molecules, cell growth and differentiation

actions of some are inhibitory, playing important role in regulating immune response

number of cytokines termed interleukins (IL) meaning “between white cells”

most cytokines display pleiotropism (diverse actions), redundancy (many cytokines have similar functions), diverse origins, networking and high affinity binding

Class Cytokines Function

Multifunctional

IL-1, TNFIL-6 T and B cell activation, hepatocyte acute phase response, fever induction, IL-1 and TNF mediate fibroblast proliferation, PGE2 and collagenase synthesis, bone resorption

T cell IL-2

(IL-4, IL-6, IL-7)

activation and growth of T helper and T cytotoxic cells

B cell IL-4, IL-5,

IL-6

IL-7

B cell stimulation and growth

B cell differentiation to plasma cells

stimulation of pre-B cells

Haematopoietic

IL-3

G-CSF, M-CSF

GM-CSF

proliferation of haematopoietic cells

granulocyte and macrophage growth /differentiation

eosinophil growth factor

Class Cytokines Function

IL-5,

IL-9

erythrocyte and megakaryocyte growth

Chemotactic IL-8 neutrophil migration and degranulation

Interferons IFNα, IFNβ, IFNγ

anti-viral

enhance HLA Class I & II expression

activate NK cells, macrophages

Growth factors TGFβ, PDGF fibroblast proliferation and matrix synthesis

TGFα, EGF anchorage independent growth

IGF, FGF wound healing

Inhibitory IL-10

TGFβ

CSIF (cytokine synthesis inhibitory factor)

inhibits IL-1 and TNF synthesis

Properties

can be classified on basis of their functions, emphasizing role in regulating immune response

Innate Immune Response important in viral and bacterial infections

e.g. IFN-α & β play vital roles in viral infections, TNF-α, IL-1 & 6 important in bacterial infections

TNF-, IL-1 and IL-6 all inflammatory responses (“pro-inflammatory” cytokines)

Adaptive Immune Response IL-1 (and to lesser extent TNF-) secreted by APC following Ag presentation, and has been termed second messenger for T cell activation

stimulates secretion of another cytokine, IL-2, by TH, which stimulates T cell proliferation

link initial “innate” response with specific “adaptive” immune response driven by Ag

IL-2 originally called T cell GF as its main action is T cell growth and clonal proliferation

produced mainly by TH, has autocrine and paracrine effects

IL-2 also important stimulator of NK cell activity and may help to stimulate B cell growth

IFN-γ

IL-4 performs important function in switching Ab production from IgM to other isotypes, particularly important in allergy where it promotes production of IgE

IL-1,6 important in promoting B cell differentiation are IL-1 and IL-6

Chemotaxis IL-8 representative of large family of chemokines, small (8-10kD) chemotactic glycoproteins

all contain 2 internal disulphide loops, structurally divided into 2 groups depending on whether cysteines are adjacent (C-C) or separated (C-X-C)

IL-8 is C-X-C chemokine, is produced at sites of inflammation promoting chemotaxis of neutrophils

binds to receptors on neutrophils coupled to GTP-binding proteins and have characteristic structure featuring 7 transmembrane domains

important participants in leukocyte trafficking as they activate integrins, promoting firm adhesion of leucocytes to endothelium as well as directing movement of neutrophils

may be thought of as 20 mediators of inflammation as their secretion is stimulated by pro-inflammatory cytokines

C-C chemokines include monocyte chemotactic protein and macrophage inflammatory protein, play important role in allergic inflammation by stimulating basophils to release histamine

Haematopoietic Growth Factors

number of cytokines appear to act primarily to promote growth and differentiation of various lineages of haematopoietic cells

Anti-Inflammatory Cytokines - Promotion Of Repair

TGFβ produced by certain tumours, allows normal cells to grow in soft agar

important immunoregulatory properties, secreted by Ag-activated T cells and macrophages

highly pleiotropic – can or growth of many cell types depending on experimental conditions

important actions on T cell proliferation and maturation of cytotoxic T cells

can synthesis of pro-inflammatory cytokines such and counteract effects of others

promotes wound healing by generation of connective tissue and new blood vessels

IL-10 (aka cytokine synthesis inhibitory factor, CSIF) important in regulating inflammatory response

FGF, PDGF, IGF all important growth factors participating in wound healing

Cell Adhesion Molecules

fundamentally important to cell, determining whether it circulates or moves from blood vessels

granulocytes and monocytes migrate through vessels in response to changes in adhesion molecules expressed by endothelial cells and home to target areas within tissue - do not recirculate

lymphocytes continually patrol body for foreign Ag by recirculating from blood stream, into tissue, through lymph nodes, into lymph and ultimately back to blood stream

also involved in growth and development of tissues, wound healing and tumour invasion

4 major families of cell adhesion molecules which work in co-ordinated way to facilitate movement of leucocytes and their interaction with endothelial cells and ECMFamily Molecule Notes

Sele

cti

ns

mediate processes of tethering and rolling

interactions have rapid association and dissociation constants and so adhesion is short-term and labile

selectins bind to mucin-like ligands, which contain sialylated CHO residues which specifically bind to Ca2+-dependent lectin domain

L-selectin expressed on all circulating leucocytes except for memory lymphocytes

ligands: GlyCAM and CD34 produced by HEV

P-selectin in α-granules of platelets and in pre-formed Weibel-Palade bodies of endothelial cells

ligands: P-selectin glycoprotein ligand (PSLG-1) present on neutrophils

E-selectin induced on vascular endothelial cells by cytokines such as IL-1 and TNF-ligands: sialyl LewisX epitopes on leucocytes

Inte

gri

ns

one of most versatile and diverse families of adhesion molecules, important in cell-cell and cell-ECM adhesion

adhesiveness can be rapidly regulated by chemokines such as IL-8

composed of α and β subunit, subfamilies are classified according to subunit

β1 responsible for cell-ECM adhesion

some recognize tripeptide sequence ‘RGD’ found on ligands (e.g. fibronectin)

interaction between 1 integrin VLA-4 on monocytes, lymphocytes and eosinophils and VCAM-1 on endothelial cells provides important means of leucocyte-endothelium adhesion

2 found on surface of lymphoid and myeloid cells, mediate adhesion of these leucocytes to endothelium

particularly important in neutrophil adhesion as these cells lack 1 integrins

present constitutively on leucocytes and undergo conformational change upon activation to adhesiveness

3 found on platelets

platelet adhesion to fibrinogen, fibronectin, von-Willebrand factor and vitronectin

7 found on lymphocytes and mediates homing of lymphocytes to Peyer’s patches in gut

recognizes ligand on HEV of Peyer's patch called mucosal addressin cell adhesion molecule (MAdCAM-1)

Imm

un

og

lob

ulin

S

up

erf

am

ily M

ole

cu

les

(Ig

SF)

ICAM-1 expressed on inflamed endothelium

binds to LFA-1

ICAM-2 constitutively expressed on endothelial cells

binds to LFA-1 on leucocytes

ICAM-3 expressed on leucocytes

VCAM-1 expressed on activated endothelium

binds VLA-4

MAdCAM expressed by HEV of Peyer’s patches

MAdCAM-1 also contains mucin-like domain which can bind L-selectin and mediate lymphocyte rolling

Cad

heri

ns

Ca2+-dependent adhesion molecules, mediate “homophilic” cell-cell binding

N-cadherin (neural tube)

expressed in embryonic ectoderm and influences separation of neural tube tissue from ectoderm

Leucocytes in Inflammation

leucocytes circulating in bloodstream must be able to slow down and stop within blood vessels near site of inflammation, adhere strongly to endothelium, penetrate into underlying tissue and migrate through ECM

Rolling and Tethering

selectin molecules bind mucin-like CHO ligands

initially (within seconds) P-selectin expressed on surface of endothelial cells, hooks on to PSLG-1 on neutrophils causing them to roll along endothelial surface

L-selectin on leucocytes then interacts with CD34 on endothelial cells

E-selectin and sialyl-Lewis X interaction occurs later as E-selectin only presented 2-8 hours after stimulation with pro-inflammatory cytokines

Triggering

chemokines act as leukocyte chemoattractants and are produced at sites of inflammation

diffuse outward producing concentration gradient down which leucocytes migrate

also produced by endothelial cells under influence of other cytokines (e.g. IL-1 and TNF-α)

platelet activating factor, CD31 and C5a other important chemoattractants

chemoattractants from endothelial cells attach to chemoattractant-R on leucocytes

ligand bound to receptor activates G protein cascade

G proteins transduce signals which activate integrin adhesiveness bringing

Strong Adhesion

leukocyte integrins (e.g. LFA-1) express “activation epitopes” which affinity for ligands >200x

2 integrin/ICAM interactions used by all leucocytes, while T lymphocytes also use VLA-4/VCAM

strong adhesion followed by decay of integrin function allowing cells to be released and extravasate

release phase also associated with shedding of L-selectin from leukocyte surface

migration of leucocytes within tissues associated with β1 integrin interactions as ligands for these adhesion molecules are found within ECM

leucocytes then migrate down chemoattractant concentration gradients to sites of inflammation

Lymphocyte Trafficking

~10% lymphocytes circulating at any time

re-circulating lymphocytes pass from blood through lymphoid system and back to blood

tend to recirculate preferentially to site where they have previously encountered Ag

naive T cells specifically migrate to lymph nodes and memory T cells to non-lymphoid tissue (homing)

specialized cuboidal endothelial cells in HEV express various adhesion molecules (e.g. GlyCAM-1 and CD34) to direct flow of lymphocytes into tissues

molecules which function to direct traffic of T cells through lymph node called addressins

suggested that lymphocyte homing process may also involve sequential steps including selectin-mucin interactions, followed by integrin activation and then coupling with IgSF molecules

interestingly, memory T cells lack L-selectin and therefore do not move through peripheral lymph nodes

long-lived lymphocytes (mostly T and memory cells), most mobile and participate in continual pattern of movement through lymph nodes, spleen, blood and lymphatic system

T-Cell Activation

T cells also use adhesion molecules during adaptive immune response

activation occurs after encounter with Ag-presenting cell when Ag interacts with T cell receptor

adhesion molecules on both cell surfaces are required to stabilize this process

important molecules include 2 integrins, LFA-1, MAC-1 and CD2

corresponding ligands on APC are ICAM-1 and LFA-3

Clinical Applications

current research on cell adhesion molecules extended into clinical field

monoclonal Ab’s to ICAM-1 used in transplantation research to block renal graft rejection

Ab’s to 2 integrins have been used experimentally to prevent cardiac re-perfusion injury by blocking migration of leucocytes to ischaemic tissue

Ab to 1 integrin has been shown to block experimental allergic encephalitis in rats (animal model for MS)

How Immune System Deals with Infections

Process of Infection

often initial focus of infection does not stimulate immune enough to dispose of infectious agent completely

not until agent spread to 20 sites and replicated further that immune systems responds adequately

summary of events: (e.g. bacterial infection occurring at cut in skin)

initial lesion damages tissues and capillaries, provoking local inflammation

chemotactic factors released, attracting phagocytic cells – resulting neutrophil influx cleans up site by ingesting and digesting foreign matter and dead tissue

local mononuclear phagocytes as well as some monocytes also take up foreign matter, break it down into fragments and transport fragments to nearest lymph node through lymphatic vessels

bacteria that have entered lesion may begin multiplying at site

many bacteria will be ingested by local phagocytes, but some may escape and migrate through lymphatic and/or blood systems

other phagocytic cells reside in lymph nodes and spleen which serve to filter such bacteria

Immune Responses to Infectious Agents

complex interactions among microbe, host factors, immunity, and pathogenesis occur during infections and degree and importance of each interaction varies depending on infectious agent

specific immune system can respond to infectious agents in variety of ways, in particular elaboration or inflammatory or inhibitory cytokines, cytotoxic T cell generation, and Ab formation

importance of each depends on nature of infectious agent

Ab’s can be generated against structural Ag’s of infectious agent, or against metabolic products

TCT generation is important in dealing with virus infected cells while of less value with extracellular pathogenic infections such as bacteria

Factor

Micro-organism

Type of micro-organism (e.g.virus, bacterium, parasite)

Dose (i.e. degree of exposure)

Virulence of organism

Route of entry

Host Integrity of non-specific defences

Competence of specific immune system

Genetic capacity to respond normally to specific organism

Evidence of previous exposure (natural or acquired)

Existence of co-infection

Immune Direct neutralization by Ab’s

Opsonization and phagocytosis

Complement-mediated effects

MHC-restricted T cell-mediated cytotoxicity

Inflammatory and immunoregulatory cytokines

Anti-viral cytokines (interferons)

Ab’s In Infection

specific Ab’s in high concentrations at time of infection may act to prevent viruses infecting target tissues

however, if Ab’s not initially present, virus infection will often become well established in target tissues before production has been induced

other immune mechanisms, particularly TCT and TH-mediated effects will be more important

of value in bacterial and parasitic infections if they lead to inactivation or destruction of micro-organisms, but many bacteria have capsules or cell walls that are inherently resistant to complement-mediated damage

in situations where pathogen’s preferred site of infection is phagocytic cell, Ab’s may actually help

may occur in mycobacterial infections where mycobacteria grow inside macrophages and are fairly resistant to killing by Ab-mediated mechanisms

Ab’s at Mucosal Surfaces

adherence to epithelial cells of mucous membranes often essential for viral and bacterial infection

IgA affords protection in external body fluids (i.e. tears, saliva, nose, intestines and lungs)

if infectious agent penetrates IgA barrier, comes up against IgE facet of secretory system

most serum IgE arises from plasma cells in mucosal tissues and in lymph nodes that drain them

although present in low concentration, IgE bound very firmly to Fc receptors of mast cells, and contact with Ag leads to mediator release

Cytotoxic T Cells

because TCT cell activity restricted to recognition of peptides presented by class I MHC, not all Ag’s of pathogens will be accessible as targets of these effector cells

peptides presented by class I MHC arise from cytoplasmic processing of proteins and so only in pathogens that have intracellular cytoplasmic phase will T cell-mediated cytotoxicity play important role

TCT important in most viral infections but will be relatively unimportant in handling extracellular bacteria

recovery from viral infections strongly influenced by TCT activity and interferon rather than Ab’s

Cytokines in Infection

Secondary messengers

important second messengers in cytotoxic T cell and B cell activation

Pro-inflammatory TNFα TNFα mediates defence against bacteria (especially G-), release is triggered by LPS

anti-TNFα results in widespread sepsis and sometimes death

TNFα may enter bloodstream and act as hormone resulting in systemic host response to infection (e.g. fever, acute phase response, cachexia)

IL-1 very similar actions to TNFα, both termed “endogenous pyrogens”

primary source is activated macrophage but also produced by endothelium, ep cells and chondrocytes

second messenger for T cell activation

IL-6 shares many actions with IL-1 and TNF-promotes “acute phase response”, acts upon hepatocytes to stimulate secretion

elevation of fibrinogen is basis of clinical test which gives non-specific indication of degree of inflammation in body; erythrocyte sedimentation rate or ESR

Chemotaxis e.g. IL-8 function to limit infectious processes by aiding chemotaxis and mobilization of effector cells as described

Cytotoxicity IL-2, IFN- NK cell activity and Class I MHC expression NK and cytotoxic T cells lysis

IFN- activates intracellular killing of bacteria by macrophages

Interferon important antiviral proteins

IFN- product of CD4 T cells, IFN– and IFN- transiently induced in most cells of body during viral infection

act on uninfected cells to induce transient antiviral state by inducing of specific enzymes that cleave viral RNA

Helper Cell Subsets in Infection

infectious disease immunology focused for many years on circulating Ab’s or capacity to generate rapid 20 Ab response as most effective means of protective immunity

Ab’s play important role in protection from many viral infections but it is are not only effector mechanism nor are they protective in all infections

not only do TCT responses against virus-infected target cells constitute means of limiting viral infection but many bacterial and parasitic infections appears to depend more on TH than Ab’s

in some cases one class of T cells (e.g. TH1) associated with protection while another class (e.g. TH2) associated with exacerbation

protective immunity must be seen in context of each particular infection as relationship among B cells, TCT, TH and innate (e.g. NK cells) responses which may vary greatly depending on organism associated with infection

unstimulated macrophages do not deliver co-stimulatory signal to T cells recognizing non-bacterial Ag’s and consequently result in T cell anergy

bacteria stimulate macrophages to deliver co-stimulatory signal to T cells recognizing bacterial Ag resulting in proliferation and differentiation to specific T cell effectors

if non-bacterial Ag’s also presented by macrophages induced to express co-stimulatory signal then they too will lead to activation of specific T cells which recognize them

Long-Lasting Immunity

generally speaking, vaccines designed to prevent viral infections do not give same degree of long-lived immunity as natural infection

one of factors contributing to this may be length of time Ag persists in infected or vaccinated individual

Ab-secreting B cells and effector T cells generally have short half-lives (days-weeks)

when Ag persists after infection, memory cells recruited over time and become Ab-secreting cells, thus providing for continual presence of Ab’s or at least allowing rapid production following re-exposure

injected Ag’s trapped in germinal centres of lymphoid tissues and undegraded Ag’s in form of Ag-Ab complexes can persist for long periods on follicular dendritic cells

in this form, Ag is highly immunogenic and can be endocytosed by B cells and presented to T cells, resulting in rapid B cell proliferation with formation of Ab-secreting cells in germinal centres

Host Pathology in Infection

when large numbers of Ab’s produced with high concentrations of Ag’s, Ab’s will bind readily with Ag’s and immune complexes formed will tend to lodge in capillaries of kidney

activation of complement system can result in localized inflammatory reaction causing damage to surrounding tissues and compromising function of organs

when some Ag’s of infectious organism sufficiently similar to some host Ag’s, Ab’s generated against infectious Ag’s may cross-react and damage host tissues

when large numbers of infected cells in vital organ killed rapidly by TCT, organ may be unable to fulfil its functions properly

mechanism important in hepatitis B infections and in AIDS

Immunodeficiency (ID)

Classification

immune system has many components: Ab’s, complement, B and T lymphocytes, cytokines and receptors, adhesion and other accessory molecules, HLA Ag’s, phagocytes etc

malfunction of any may result in state of immunodeficiency

Classification

congenital (due to genetic defect or in utero disease)

acquired (in addition to HIV, acquired ID may be secondary to drug therapy or systemic disease)

primary (inherited / genetic)

secondary (due to some other factor, e.g. HIV)

according to part of immune system affected (humeral, cellular, combined, complement, and phagocytic)

selective IgA deficiency is common (~1/500 people), but other primary ID are rare

Primary ID Syndromes

very heterogenous range of syndromes

molecular basis of many ID is now known, and in future they will be classified by their cause

most due to autosomal or X-linked recessive gene defects

Name Notes

Adenosine Deaminase (ADA) deficiency

build-up toxic byproducts of purine metabolism inside cells, T-cells especially affected

first use of gene therapy, with limited success

Bruton agammaglobulinaemia

mutation in tyrosine kinase enzyme in B lymphocytes

first ID described, X-linked, presents in childhood with recurrent pyogenic infections

Severe Combined Immunodeficiency (“SCID”)

many potential causes, e.g. mutation in γ chain of IL-2-R critical cytokine, also part of receptor for cytokines IL-4, -7, -9, and -15, so has widespread effects

defects in both Ab and cellular immunity

compatible bone marrow transplantation offers only chance of permanent cure

Wiskott Aldrich syndrome defective gene codes for protein that regulates actin cytoskeleton

platelet abnormalities as well as addition to B and T cell problems

Leukocyte Adhesion Deficiency

defect in CD18 adhesion molecule, leukocytes can’t adhere properly, so can’t get to infected and inflamed sites

Hyper IgM Syndrome mutation in CD40 molecule so can’t make class switch

high levels of IgM Ab’s but low IgG and IgA

bare lymphocyte syndrome

transcription of class II MHC genes abnormal, so can’t be expressed on surface

MHC deficiency severely limits ability of T cells to respond to Ag’s

Chronic Granulomatous Disease (CGD)

defective enzymes in NADPH oxidase system needed to produce respiratory burst

defective killing of phagocytosed bacteria

Complement Deficiency Syndromes

may be asymptomatic or unusually prone to infection with certain bacteria

Hereditary angioneurotic oedema

deficiency of C1 inhibitor

Clinical Presentation

usually presents clinically with infection

type of infection is clue to underlying immune defect, but is usually not in itself diagnostic

Organism Immunological Responses

Ab T-cells Other

Extracellular bacteria

IgM, IgG complement, phagocytes

Intracellular bacteria

macrophages

Viruses IgG, IgA complement, IFN

Parasites IgE eosinophils/mast cells

Fungi IgA neutrophils

should suspect ID if patient has recurrent bacterial infections, or infections with unusual organisms

may be family history, many genetically are X-linked so will appear only in boys, most others autosomal recessive

20 ID may be suspected:

in patients taking steroid or cytotoxic regimens for other disorders

with known major disease in other organ systems

with lifestyle risk factors for HIV

should always seek specialist advice before going beyond basic screening investigations

infections should be treated aggressively, and may require prolonged antibiotic therapy

except for Ig replacement therapy for Ab deficiency syndromes, no specific curative therapies for 10 ID

If ID is 20 to medication or disease, if possible underlying problem should be remedied

Immunology Of HIV Infection

well established that HIV infects and depletes CD4 T-cells, molecular pathogenesis still poorly understood

co-factors (e.g. other infectious agents) may be involved

also infects macrophages, Ag-presenting dendrtitic cells, and some haemopoietic stem cells

virus’ gp120 molecule binds to CD4 on lymphocyte and low Mr cytokines receptors (chemokines-R)

some strains have preference for macrophages rather than T cells, and may reflect chemokine-R that predominate on different cell types

certain receptor polymorphisms seem to provide degree of protection, as does strong host chemokine response (competitive inhibition of binding to receptor?)

treatments based on chemokines being developed

infected cells may undergo cell death because of cell fusion/syncytia formation and can act as source of infection in cell to cell transmission

may also have more subtle functional effects on cells it infects, poorly understood

immune destruction results in loss of tumour surveillance, tumour cytotoxicity, and IFN-γ production

autoimmune problems can occur in AIDS, but not usually of clinical significance

result of losing T lymphocytes which normally suppress self-reactive immune responses?

due to excess of Th2 (B cell help) helper T lymphocytes?

vaccination against HIV possibility, and clinical trials with number of vaccines using gp120 are underway

produce successful Ab responses in host, crucial thing is probably to induce cytotoxic T lymphocytes

Reasons for Poor Immune Response

progression associated with preferential loss of TH1 cells

virus mutates very rapidly, and immune system has trouble keeping up

good at “hiding” in dormant integrated state in lymphocytes resting in lymph nodes

current anti-retroviral therapies viral load and prolong survival but cannot eliminate latent virus

Hypersensitivity excessive immune response, so that host is damaged in some way

traditionally been divided into four types of reaction

1: IgE mediated (anaphylactic, or “true” allergy)

2: Ab mediated

3: Immune complex mediated

4: Cell mediated (delayed type hypersensitivity, DTH)

Type I Hypersensitivity : IgE mediated

occurs when divalent allergen cross-links 2 IgE molecules, previously passively bound to high affinity IgE Fc-R

mast cell mediators are released

granule-associated preformed mediators:

histamine, heparin

enzymes: tryptase, b-glucosaminidase

chemotactic and activating factors: eosinophil chemotactic factor (ECF), neutrophil chemotactic factor (NCF) and platelet activating factor (PAF)

newly formed mediators:

lipoxygenase products: SRS and leucotrienes

cyclooxygenase products: prostaglandins, thromboxanes

main effects are vasodilatation, vascular leakiness, pruritis, and smooth muscle contraction

may be localised, effects depend on which part of body is involved:

Organ Symptoms

Respiratory tract

allergic rhinitis (hay fever, perennial rhinitis)

sinusitis (NB ? secondary bacterial infection)

asthma (allergic component VERY common)

Eyes allergic conjunctivitis

Skin urticaria (wheals)

angioedema (deeper skin involvement)

Gut food allergy (diarhoea, abdominal cramps, vomiting)

Multiple organ anaphylaxis

generalised anaphylactic reactions can be fatal, as can severe asthma

patient’s with tendency to make IgE Ab’s to multiple allergens called atopic

runs in families, but basis poorly understood: one gene may be for variant of IgE Fc receptor

bronchial reactions to allergens show immediate and late phase reaction

Type 2 Hypersensitivity: Ab mediated

complement fixation: (eg haemolytic disease of newborn, autoimmune haemolytic anaemia)

receptor-binding: blocking (anti-AChR in myasthenia gravis) or stimulating (anti-TSH receptor, Grave’s)

K/ADCC cells: receptors for Fc portion of Ig molecules, and lyse target cells when cross-linked

can be mediated by neutrophils, macrophages, eosinophils, and possibly other cell types

in vivo importance uncertain

Type 3 Hypersensitivity: Immune complex mediated

binding of Ag and complementary Ag

conformational change in Ig Fc allows binding and activation of complement

direct action of complement split products

recruitment of other inflammatory cells by soluble mediators

systemic: classic example is serum sickness, caused by injection of foreign protein

complexes deposit in skin (rash), joints (arthritis) and kidney (nephritis)

formerly seen with horse Ig for tetanus, problem with non-human monoclonal Ab’s used in therapy

postulated mechanism for vasculitis and renal disease seen in SLE

localised: occurs in tissues

farmer’s lung - extrinsic allergic alveolitis caused by inhaled actinomycete fungi which grow in hay, IgG Ab’s from circulation meet this Ag in alveoli

complement activation and recruitment of inflammatory cells

deposition of immune complexes in blood vessel walls

Type 4 Hypersensitivity: Cell mediated (delayed type, DTH)

T cell mediated, takes 24- 48 hours

classic example: reaction to intradermal injection of purified protein derivative of Tuberculin, to test for cell mediated immunity against Mycobacteria

used to assess immunity (e.g. mumps, candida) as part of investigation of suspected immunodeficiency

also mechanism underlying contact sensitivity (e.g. to metals in jewellery)

initial phase involves uptake, processing, and presentation by dendritic cell in skin

presents Ag to T cells locally and in paracortical zones of nearby lymph nodes

T lymphocyte (mainly TH1) secretes cytokines, macrophages are recruited and activated

up-regulation of adhesion molecules and MHC expression on keratinocytes

Autoimmunity tolerance of “self” is central property of normal immune system

if discrimination broken, immune system may react to self structures and cause autoimmune diseases

Mechanisms of Tolerance

Self-reactive lymphocytes are removed from immune repertoire when they are at immature stage of development

There are several possible mechanisms:

clonal deletion - complete removal of self-reacting cells

dominant mechanism for Ag’s expressed in primary lymphoid organs (thymus and bone marrow)

clonal anergy - self-reacting lymphocytes still exist, but are usually resistant to stimulation

important for Ag’s found only in periphery

immunological ignorance

self-reactive cells present, but do not mount pathological response, because Ag’s sequestered or lacking adequate T cell help

immune system needs to see Ag’s in context of certain “danger” signals before it responds

suppression

self-reacting lymphocytes present and potentially active, but are kept in check by “suppresser cells”

discrete suppressor population not found, now thought to be either TH2 or competing anergic cells

AutoAbs

mechanisms causing autoimmune disease are not fully understood

Natural AutoAbs

loss of self-tolerance does not always lead to autoimmune disease

healthy immune repertoire has some B cells which have potential to produce autoAbs

usually IgM, low titre, and/or low affinity

directed against Ag’s that are not normally accessible in significant amounts (“sequestered” Ag’s)

referred to as “natural” autoAbs

postulated that they have regulatory role, or help dispose of breakdown products

Why don’t they cause disease? amounts or affinity are low

need “help” from corresponding self-reactive T helper lymphocyte to get stronger response going

Ab not pathogenic (many of Ab’s associated with connective tissue diseases don’t cause disease)

patient DOES have autoimmune disease, but this is still sub-clinical

AUTOIMMUNITY IS COMMON, AUTOIMMUNE DISEASE IS RARE

Sequestered Ag’s

tolerance not always established to Ag’s normally hidden from immune system (e.g. lens crystallin)

autoAbs can often be detected following trauma (allows contact of Ag with immune system)

may cause frank autoimmune disease (e.g. sympathetic ophthalmia, damaging good eye after trauma or surgery to other)

Cross Reactions

tolerance can sometimes be by-passed by immunization with closely cross-reacting Ag

e.g. Ab’s against viral RNA or DNA may cross-react with self RNA or DNA, may help explain autoAbs to RNA or DNA in connective tissue disease

infection with certain bacteria (e.g. salmonella, shigella, chlamydia) can trigger reactive arthritis

rheumatic fever

similar cross-reactive theories put forward for infection triggering range of autoimmune diseases

alternative term for this theory of autoimmune disease is “molecular mimicry”

Adjuvant Effects

adjuvants non-specifically enhance immune response

mixed with vaccines to improve efficacy and with Ag’s when experimental animals used to produce antisera

can help produce autoimmune disease when mixed with self or cross-reactive Ag’s

experimental examples:

collagen arthritis produced in rats and mice by immunising with bovine collagen in adjuvant, used as model (albeit not very convincing one) for human rheumatoid arthritis

thyroiditis can be induced in rabbits with thyroglobulin in adjuvant

experimental autoimmune encephalomyelitis produced in mice and rats using myelin basic protein, model for human multiple sclerosis

postulated that infections (both bacterial and viral) may have adjuvant-like effect, in particular:

lipopolysaccharide in G- bacteria is polyclonal B lymphocyte activator

superAgs secreted by certain bacteria are polyclonal T lymphocyte activators

Abnormal MHC Expression

viruses may have analogous adjuvant-like effect:

viral infection leads to release of interferons

IFN-γ causes up-regulation of Class II HLA Ag synthesis

most cells (except for immune and haemopoietic) don’t normally express Class II HLA

may present peptides derived from intracellular autoAgs

immune system isn’t normally exposed to these - intracellular version of sequestered Ag’s

T lymphocytes then respond to HLA+ sequestered-self

CD4+ T cells can now provide “help” for B lymphocytes that previously ignored intracellular Ag

examples of MHC Class II up-regulation include:

islet cells in type I diabetes mellitus

thyroid glandular cells in autoimmune thyroiditis

synovial endothelia and lining cells in rheumatoid arthritis

keratinocytes in psoriasis

unresolved issue is whether class II increase is cause or effect

Regulatory Abnormalities

older theory for autoimmune disease was that regulatory suppressor T lymphocytes became defective, allowing emergence of autoimmune processes previously held in check

not widely believed, may regulate size of some immune responses rather than all-or-nothing tolerance effect

Clinical Features Of Autoimmune Disease

vary widely, depending on organ and exactly what immune response is directed against

examples include:

thyrotoxicosis, caused by Ab’s against TSH receptor, mimic action of TSH and stimulate thyroid gland

pernicious anaemia, autoimmune destruction of gastric parietal cells or Ab’s directed against IF itself

idiopathic thrombocytopenic purpura, due to Ab’s against platelets

Addison’s disease, due to destruction of adrenal cortex

myasthenia gravis, due to autoAbs blocking acetylcholine receptor on NMJ

pemphigus, bullous inflammatory skin disease caused by autoAbs against cadherins

Genetic Predisposition

Factor Notes

Immunoglobulin genes genes for some autoAbs encoded in germline

somatic hypermutation may also be responsible for producing autoAbs

T cell receptor (TCR) some TCR Vß genes are used preferentially by autoreactive T cells

elimination of T cells which use specific Vß genes can prevent or control disease in animal models

with Vß-specific anti-TCR Ab, T cells mediating experimental allergic encephalomyelitis can be eliminated, curing disease

Complement human SLE is associated with deficiencies of complement components

thought to be due to impaired clearance of immune complexes

MHC many autoimmune disease patients have HLA-B8, -DR3 haplotype

rheumatoid arthritis HLA-DR4, ankylosing spondylitis HLA-B27, IDDM aa substitution in β chain of HLA-DQ

area of intensive research

several mechanisms may account for this: peptide binding, molecular mimicry, receptor model

Other genes genome screens have been performed for several autoimmune diseases, and several candidates identified

tend to code for proteins involved in regulating immune response

New Treatment Approaches

cytokine inhibitors: antagonists of TNF-alpha and IL-1 are now in clinical use for rheumatoid arthritis

oral tolerance: works well in some animal models, e.g. multiple sclerosis and arthritis, but so far has had little success in human disease

Immunology in Diagnostic and Clinical Medicine

Summary

make use of exquisite specificity of Ag-binding part of Ab molecule, and detection system attached to constant portion of molecule

can be used to detect either Ag’s, or Ab’s directed against them

ELISA

enzyme-linked immunosorbent assay (ELISA) widely used technique

Step Instructions

1 Ag attached to solid phase (often –ve plastic plate)

best under alkaline conditions (pH 9-10), so Ag’s have net positive charge

2 plate then washed with saline solution (to remove any unattached Ag’s)

wash solution contains weak detergent to help reduce non-specific binding

3 patient’s serum added, diluted in saline solution, incubated with Ag on plate

specific Ab’s attach to Ag, while other Ab’s stay in solution

plate is washed again, removing any unattached Ab

4 addition of “secondary” Ab” directed against Fc portion of patient’s Ab, chemically coupled to enzyme

plate washed again

5 enzyme substrate then added (often horseradish peroxidase (HRP), in presence of H2O2 produces brown colour)

measured by automated spectrophotometer

Results amount of colour proportional to amount of HRP, in turn proportional to how much secondary Ab has bound, in turn proportional to amount of Ab in patient’s serum

Sandwich ELISA Technique

simple variation on basic ELISA principle, and used to detect Ag’s

Ag “sandwiched” between two Ab’s

used to measure wide range of proteins (e.g. polypeptide hormones)

instructions:

coat 1st Ab onto solid phase

incubate patient’s serum, containing Ag, on plate

add Ab directed against same Ag (can coupled directly to enzyme detected by 3rd Ab)

wash plastic plates between successive incubations

colour developing when substrate added proportional to amount of Ag in patient’s serum

Similar TechniquesName Differences Notes

Radio-Immunoassay(RIA)

instead of enzyme, 20 Ab labelled with radio-isotope

most assays are moving towards ELISA instead of RIA because of safety concerns

Micro Particle Enzyme Immunoassay (MPEIA)

solid phase made of numerous microscopic beads coated with Ag

much greater surface area, so much more Ag can be used, making it more sensitive assay

many of serological assays performed in Auckland now performed by MPEIA (eg Hep B, Hep C, HIV 1 & 2)

Radio Allergo-Sorbent Assay(RAST)

20 Ab directed against Fc portion of IgE only

performed to detect IgE Ab’s against specific Ag’s

use Ag that might be causing patient’s allergy (e.g. house dust mite)

most RAST tests are now performed by ELISA or MPEIA, but old terminology still used

allergy usually diagnosed on clinical grounds or with skin testing, and RAST tests are $$$ so only used in special circumstances

Name Differences Notes

Western Blot Ag’s 1st dissociated with detergent, then electrophoresed (SDS-PAGE), separates components according to Mr

transferred to nitrocellulose membrane, incubated with patient’s serum, Ab bind to their respective Ag on membrane

after washing, Ab’s detected using enzyme-coupled 20 Ab

can tell size of Ag

important in diagnosing HIV infection: if patient’s serum tests +ve by MPEIA, Western blot is performed

ideally, Ab’s should bind to p24, p31, gp40, and either gp120 or gp160 of envelope for definite +ve

Immunofluorescence (IF)

fluorochromes (chemicals that emit light when stimulated) can be chemically coupled to Ab

fluorescein (FITC) emits green (520nm) colour, phycoerythrin (PE) emits orange (570nm) colour

Direct IF

can be used to detect Ag’s found in tissue sections, or expressed on outsides of cells

tissue or cells incubated with fluorochrome-coupled Ab

after washing, specimen is examined under fluorescence microscope

areas with Ag show up as bright, coloured regions on dark background (band-pass filtering)

more than one Ag can be detected if Ab’s coupled to difference fluorochromes

Confocal Microscopy

fluorescent images hard to examine at high magnification because of glare from slightly out-of-focus planes

confocal microscope focuses sharp laser beam on fine plane within tissue

photomultiplier tubes scans area, collects emitted light only from same plane, analysed digitally

result is high definition image that looks like very thin cross section of tissue

can image organelles within cells, and examine proteins secreted immediately surrounding cell

Indirect IF

analogous to ELISA technique, often used to detect autoAbs

ANA

anti-nuclear Ab’s (ANA) present in many patients with SLE, detection useful for diagnosis

assay starts with tissue section (or cultured cells) with prominent nuclei

attached to glass microscope slide, treated with chemical fixatives to stop Ag’s degrading and make membrane permeable (allows ANA to access nucleus)

patient’s serum added, and incubated with fixed tissue

after washing, FITC-coupled anti-Ig 20 Ab is added

after washing, anti-Ig will only remain if serum contained anti-nuclear Ab’s

quantitated by performing serial dilutions (1:10, 1:20, 1:40, 1:160, etc) of patient’s serum

result expressed as highest dilution that gave positive result

because some non-specific binding can still occur, ANA below 1:40 regarded as -ve

different nuclear Ag’s (e.g. nucleoli, centromere, DNA) distributed differently within nucleus

ANCA

anti-neutrophil cytoplasmic Ab’s (ANCA) found in patients with vasculitis

assay performed as above, but using neutrophils

different patterns (perinuclear (pANCA) and cytoplasmic (cANCA)) associated with different types

Ag’s involved are enzymes myeloperoxidase and proteinase 3, respectively

Flow Cytometry

cells in suspension incubated with fluorochrome-tagged Ab’s, then washed

labelled cells pass through flow cytometer one by one

laser beam focused on each cell and fluorescence analysed, size and granularity of cell also measured

by using Ab’s against surface Ag’s, composition of cell population can be precisely determined

measurements are quantitative, so that estimate can be made of amount of Ag on particular cell type

possible to simultaneously examine up to 4 different Ag’s on each cell, but usually only 1 or 2 are

e.g. determination of ratio of CD4+ to CD8+ lymphocytes in AIDS patients: lymphocytes incubated with mixture of anti-CD4 (coupled to fluorescein) and anti-CD8 (coupled to phycoerythrin)

also used to examine white blood cells from patients suspected to have leukaemia: different types of leukemic cells have particular combinations of CD Ag’s on their surfaces

able to sort each cell into different receptacles

e.g. peripheral blood lymphocytes could be separated into CD4 and CD8 subpopulations

currently only done in research settings

Immunocytochemistry

used to examine tissue under microscope for presence and distribution of particular Ag

enzyme-coupled Ab used, yielding insoluble coloured product

stays in tissue where Ab bound to Ag, and can be seen down microscope

disadvantages: lower definition but adequate for lower power microscopy

advantages: can be examined with ordinary microscope, colours also last long time

similar technique can be used with transmission electron microscopy (EM), but Ab’s coupled with gold This is

Complement Fixation Tests (CFT)

often used to diagnose recent infections with organisms that are hard to culture

e.g. in diagnosis of recent infection with adenovirus, respiratory syncytial virus, influenza and B, parainfluenza 1- 3, Mycoplasma pneumoniae and Chlamydia psitticae

steps:

patient’s serum incubated with preparation of viral Ag’s

if patient’s serum contains Ab’s against virus these will bind virus to form immune complex

complement then added and immune complexes bind, then fix complement

sheep red blood cells (SRBC) that incubated with anti-SRBC added to mixture

if there complement left (i.e. no anti-viral Ab’s present), SRBC will be lysed

if no complement left (i.e. anti-viral Ab’s present), SRBC will not be lysed

made quantitative by using serial dilutions of serum

many patients will have Ab’s against common viruses because of prior infection, so to diagnose recent infection, need to show 4-fold rise in titre between acute and convalescent phases

Complex Formation

Immunodiffusion

when Ab’s combine with complementary Ag’s, insoluble complex forms and precipitates

can be visualised in several ways

Ab and Ag diffusing towards each other from two small wells cut in agar gel will form visible line

Ag embedded in agar and serum put in well, visible ring will form as Ab slowly diffuses away from well

to assay for Ag, Ab embedded in gel, and serum added to wells

diameter of ring proportional to concentration of Ab

technique known as radial immunodiffusion

used to measure auto-Ab’s against certain of nuclear Ag’s (e.g. ANA SSA and SSB)

Nephelometry

serum mixed with Ag in clear tube

light passing through tube is impeded by complexes, measured by spectrophotometer

light absorption proportional to amount of Ab present

used to quantitate Ab’s against DNA in SLE patients

Agglutination

IgM Ab’s are much more effective at agglutination than IgG

classical application is blood grouping

Ag’s can also be artificially coated onto red blood cells

other particles (e.g. microscopic latex beads coated with Ag’s) can also be used

HLA Tissue Typing

performed to achieve close match between recipient and donor before organ transplant

Lymphocytotoxicity

traditional method uses cytotoxicity assay

peripheral lymphocytes incubated with wide range of Ab’s directed against all main HLA Ag’s

complement added, if lymphocytes have Ag, will be lysed by respective antisera

>100 different antisera are used to determine person’s HLA-A, B, and C Ag’s

anti-HLA antisera are scarce and expensive, so assays scaled down to microlitre proportions

Polymerase Chain Reaction (PCR)

now use PCR to determine HLA types

match PCR primers (“sequence specific primer”, SSP-PCR)

probe PCR products with labelled nucleotides

directly determine DNA sequence of patient’s HLA genes

currently PCR mainly used for class II typing but may eventually replace all cytotoxicity assays

Mixed Lymphocyte Reaction (MLR)

if lymphocytes from 2 patients mixed together, recognise other HLA Ag’s (mainly class II) as “foreign”, and begin to proliferate

in vitro equivalent to early stages of graft rejection

Factors Influencing Immune Responsiveness

Immune System Control Mechanisms

numerous factors that affect nature, magnitude and effectiveness of immune response to Agic challenge

include:

nature of Ag, route of Ag entry and concentration of Ag

presence, concentration and class of specific Ab’s

current immunological state and intercurrent Agic stimuli, Agic history of individual

idiotype/anti-idiotype networks

nature and diversity of Ig and TCR gene families

Ag processing and association with MHC gene products

status of helper T cell involvement and cytokine production

interactions within neuro-immune network

Adaptive and Innate Immunity

diversity imposed by need to recognize all possible self and non-self molecules produces two limitations

cells which recognize self molecules must be rendered ineffective to avoid autoimmunity

lymphocytes of any one specificity will be rare in total repertoire

significant lag phase during which rare lymphocytes undergo expansion before sufficient generated

initial phase characterized by absence of effective adaptive immunity and innate system plays crucial part

main difference between innate and adaptive immunity is system of recognition

innate immunity utilizes limited repertoire that discriminates some common pathogens from self

adaptive immunity distinguishes among infectious agents using receptors that diversify in somatic cells

Ag And Ab Effects

nature of Ag affects sort of immune responses which can be generated against it

foreign Ag’s similar to self Ag’s may not stimulate good response

how Ag processed and nature of MHC presentation will affect epitopes available for recognition

Ab’s already present in individual can either enhance (by opsonisation) or diminish (by leading to rapid Ag removal) immune response

IgG and IgM Ab’s exert inhibitory and enhancing effects, respectively, on B cells

Idiotype/Anti-Idiotype Networks

hypervariable loops on Ig and TCR can be recognized by appropriate Ab’s

many present significant concentrations only during immune responses

can appear to immune system as novel ‘Ag’s’ and so can stimulate and anti-idiotype response

these anti-idiotypes, in turn, behave as Ag’s to stimulate anti-anti-idiotype response

immune system can contain large network of idiotype/anti-idiotype reactions and response to external Ag will be affected by state of idiotypic interactions

evidence that idiotypic networks formed during development and constitute regulatory mechanism that relates self and non-self Ag’s and controls responses against certain important conserved Ag’s

Cytokines as Modulators

focus on immunological autonomy and self-regulation led to discovery of cytokines, which control information exchange within immune and haematopoietic systems

because of powerful influence on immune function, cytokines often called “hormones” of immune system and has been tendency to consider immunological effects to be 10 function

consideration of immune system as essentially autonomous system tended to reinforce belief “external” factors are of less relevance than “internal” factors

recently, become apparent that to appreciate structure and function of immune system within body, necessary to adopt more expanded view of its boundaries and interactions

has come about as result of four general molecular biological observations:

many cytokines have multiple activities within and without of immune system

some cytokines are also produced by cells not of lymphoid or myeloid lineages

increasing numbers of molecules and hormones from outside immune system (particularly from within nervous system) are being found to have direct influences on immune responsiveness

variety of neuropeptides produced by cells of immune system under appropriate conditions

Nervous Effects on Immune System

nervous system can be considered as having two arms with extensive interconnections: ANS (PNS + SNS) and neuroendocrine system centred on hypothalamic-pituitary-adrenal (HPA) axis

HPA Axis and Immune System

limbic system directly regulates neurohormonal and autonomic outflow of CNS

hypothalamus central to this process, so perturbations of hypothalamus affect immune responses

e.g. lesions in ant hypothalamus Ab response, while electrical stimulation Ab response

lesions in hypothalamus and hippocampus also alter NK activity and T cell function

known for many years that corticosteroids have immunosuppressive effects on variety of immune parameters

phagocytosis , macrophages , B and T cells , lymphocyte migratory patterns, cells in thymus

used for immunoproliferative disorders, some autoimmune diseases, and transplant acceptance

number of non-steroidal hormones associated with HPA axis recently shown to influence immune responses

e.g. sex hormones ; insulin, GH, T4

Neuropeptides, Neurological Mediators and Immune Function

over last 5 years large number of neuropeptides discovered and found to affect immune responses

neurological mediators might act directly on immune cells or indirectly by inducing mediator production

receptors for number of these mediators present immune cells, so likely at least some effects direct

Autonomic Nervous System and Immune System

extensive innervation of all lymphoid tissue by NA nerve fibres

terminate predominantly in T cell-rich regions in lymph nodes

NA predominant NT, but A and SER also used

ablation of SNS augments Ab responses

ablation of PNS depresses immune responsiveness

Classical conditioning of immune responses

1975, discovered that immune responsiveness could be influenced by behavioural conditioning

paired sweetened drinking water with injection of cyclophosphamide (IS drug)

found subsequent administration of sweetened water alone, immune responses

since then, classical conditioning of immune responses has been studied extensively

although changes are not large, has been observed for Ab responses, T cell proliferation, T cell mediated cytotoxicity, TH1/TH2 balance, NK activity and allergic responses

display extinction properties characteristic of classical conditioning

Ag-specific conditioned enhancement also demonstrated

conditioned changes have been shown to affect health and/or survival

recent work implicates vagus nerve as integrally involved in psychoimmune conditioning phenomena

conditioning effects probably play potentially important role in placebo phenomena and in most forms of medical and surgical treatment

Immune Effects on Nervous System

changes in electrical activity and NT levels in hypothalamus correlate with time course of response – information must be passing from immune system to brain

potentially 2 ways this could occur and both probably do

responding lymphocytes might secrete neurological mediators

variety of neuroendocrine mediators produced by immune cells under various conditions

cytokines might directly affect neural tissues

cytokines appear to exert effects on CNS and some may act as intrinsic neuromodulators

activation of macrophages produces IL-1 and leads to altered electrical activity and metabolism of NA, SER and DA, also produced by injected IL-1, can be blocked by IL-1 receptor antagonist

does IL-1 acts directly on brain? preoptic nucleus has IL-1-R but as IL-1 is lipophobic it may have difficulty crossing BBB

active transport might carry it across barrier or might cross vascular endothelium

alternatively, IL-1 might stimulate peripheral nerves (demonstrated by recent findings that if vagus nerve cut below diaphragm, stress-induced IL-1-mediated effects )

Effects of cytokines outside immune system

Stress and immune system

difficult to find many measurable aspects of immune system that cannot be altered by some stressor

impact and direction of effects depends on: quality, quantity, duration (e.g. acute: , chronic: sometimes ), temporal relationship, socioenvironmental conditions, various host factors (e.g. species, strain, sex)

stressors can growth of transplanted tumours

effect might be mediated through non-immune mechanisms (e.g. changes in blood flow)

immune effects probably important because NK activity also compromised, and if effect on NK cells pharmacologically blocked, no in tumour growth

many stressors have involved physical ‘insults’, but impact cannot be explained simply in physical terms

e.g. being placed into established territory of another rat, even for brief period of time, severely inhibits production of Ab to Ag administered before intruder introduced

in such situations, adoption of submissive behaviours correlates most closely with Ab production – Ab responses in non-submissive animals relatively unaffected

if animal perceives stressor to be controllable then effects on immune responses are much less

Stress Immunity in Human Studies

some studies conducted into effect of short-term, laboratory stressors but most centred on more ‘naturalistic’ stressful situations

where perception of stress assessed, -ve correlation between perceived stress and immune responses

most significant correlation is with NK cell activity

notion that critical parameter is perception of stress has important implications, and illustrates how we respond psychologically to our environment can affect how we respond physiologically

Immune System Models

classical immunology, through reductionist scientific research, has shown firstly that our immune systems can respond to foreign Ag’s (such as infectious agents) as well as to Ag’s that are normal components of our bodies, and secondly, that these responses are subject to intricate and complex networks of control

psychoneuroimmunology added another dimension to our understanding by revealing important controlling influence on immune responsiveness of what we believe, think and feel

immune system can be considered as extension of nervous system in many ways

called 'molecular sense organ' which gathers information about our molecular environment

immune responses can also be regarded as forms of behaviour essentially no different from other forms of behaviour and potentially amenable to manipulation by psychological behaviour modification techniques

e.g. deliberate intervention at psychological level or at social level affects both immunological measures and disease activity

extensive associations between 'traditional' immunological parameters, elements of nervous system activity, and psychological variables indicate need to consider immune system not just as autonomous defence mechanism but more as integral aspect of self-definition and self-determination of individual

rather than solely protective defence against invasion from potentially hostile environment, immune system could be better viewed as ‘cognitive’ system involved in continual self generation of individual and, in doing so, supporting integrity of body in optimal relationship to its environment