Cells of the Immune System

Cells of the Immune System• During embryonic development blood cell precursors

mainly originate in fetal liver and yolk sac• In postnatal life, all derive from the bone marrow• Two main lineages derive from the bone marrow

hematopoietic stem cells:1. Lymphoid lineage T cells, B cells, Natural Killer (NK) cells2. Myeloid lineage• Monocytes, Macrophages, Dendritic cells,

Megakaryocytes, Granulocytes (eosinophils, neutrophils, basophils)

• In mammals, T cells develop in the thymus while B cells develop in the fetal liver and bone marrow

Hematopoiesis

Myeloidprogenitor

Lymphoidprogenitor

Tccell

Thcell

HematopoieticStem cell

NK

B cell

Granulocyte

Platelets Megakaryocyte

Macrophage Monocyte

Dendritic cell AFC

Plasma cell

Mast cell

Thymic Processing of T cells HematopoieticStem cell

CD4-

CD8-TCR-

CD4+CD8+TCR+

CD4+CD8-TCR+

CD4-

CD8+TCR+

• T-cell precursors differentiate into immunocompetent T-cells within thymus

• Prior to thymus all lack antigen receptors & CD3, CD4 & CD8-Double negative• In thymus first differentiate to express both

CD4 & CD8 - Double positive

Double negative

Double positive

• A double-positive will become CD4+ if it contacts a cell bearing MHC II proteins

• A double-positive will become CD8+ if it contacts a cell bearing MHC I proteins• These are now Single positive

MHC IMHC II

No MHC engagement

Single positives

Apoptosis

Thymic Processing of T cellsHematopoietic

Stem cell

CD4-

CD8-TCR-

CD4+CD8+TCR+

CD4+CD8-TCR+

CD4-

CD8+TCR+

Double negative

Double positive

• Double-positive & double negative cells are located in thymus

• Single-positive are located in medulla and migrate to blood and extrathymic tissues

MHC IMHC II

• Each double-positive T cell synthesizes a highly specific Ag receptor (TCR)• The rearrangement of variables, diversity accounts for the remarkable ability of T cells to recognize millions of different Ags

No MHC engagement

Single positives

Apoptosis

• CD4+ & CD8+ cells bearing Ag receptors for “self” proteins are killed (clonal selection) by a process of programmed cell death called apoptosis

• The removal of these self reactive cells called negative selection results in self tolerance and prevents autoimmune reactions

• CD4+ & CD8+ cells bearing Ag receptors that do not react with self MHC proteins are also killed: Positive selection for T cells

• Thymic Processing produces T cells that are selected for their ability to react with foreign Ags and with self MHC

Thymic Processing of T cells: Thymic Education

CD3+ Cells• All T cells have CD3 proteins on their surface in

association with TCR

• Is complex of 5 transmembrane proteins that transmit information inside the cell that TCR is occupied

CD4+ & CD8+ cells• T cells are divided into two major categories

whether they have either CD4 or CD8 proteins on their surface but not both

T cells Types

• 65-80% of circulating pool of small lmphocytes

• Within LN, are located in the inner subcortical region

• Life span is months to years

Features of T cells

TCR: Specificity of Immune Responses Resides in Receptors

• T cell receptor (TCR) recognizes pathogen (antigen)-associated with MHC and is univalent.

• TCR consists of two polypeptides α & β associated with CD3: are similar to immunoglobulin heavy chain

• Each T cell has a unique TCR on its surface; millions of T cells exist in each person

• Clones of T cells are produced specific for an Ag

Tcell

What Does the TCR Recognize?

Only fragments of proteins (peptides) associated with MHC molecules on surface of cells

Helper T cells (Th) recognize peptide associated with MHC class II molecules

Cytotoxic T cells (Tc) recognize peptide associated with MHC class I molecules

Generally, Class I MHC proteins present endogenously synthesized Ags e.g. viral proteins and class II MHC presents Ags of extracellular microbes which are phagocytised like bacterial proteins

T Cell FunctionsRegulatory: mainly mediated by CD4+ cells (Th

cells)-Th 1 & Th 2. Play a central role in humoral and cell mediated immune responses

Th1 cells: • Help CD8 T cells by IL-2 to become activated Tc cells• Activate macrophages by Gamma-INF in delayed

hypersensitivity Th2 cells: • Help B cells by IL-4 & IL-5 to develop into Ab-

producing plasma cells * IL-12 produced by macrophages is important regulator

of balance between Th1 & Th2

Helper (Th1) T Cells

Macrophage MacrophageTh1cell

Lysosome

Mycobacteria Antigen

Infected macrophage Activated infectedmacrophage

Mycobacteria resist intraphagocytic killing by inhibiting fusion with lysosomes. Th1 cell recognizes an infected macrophage and releases macrophage-activating

molecules that induce lysosomal fusion and the activation of macrophage bactericidal activities

• Four types of T cells : Th-1, Th-2, Th-17 (CD4+ cells) & CD8+ cells

• Th-1 cells: mediate delayed hypersensitivity reactions against intracellular organisms including certain fungi and bacteria. Th-1 cells secrete gamma INF and activate macrophages to kill the organism

• Th-2 cells along with eosinophils mediate protection against worms by producing IL-4 which activates eosinophils to secrete enzymes to kill worms

• Th-17 cells : protect against the spread of bacterial infections by recruiting neutrophils by IL-17 at mucosal surfaces

T Cell: Effector Functions

• CD8+ cells (T cytotoxic cells) : Protect by killing: Virus infected cells Tumor cells Allografts

T Cell: Effector Functions

Cytotoxic (Tc) T Cells Effector Function

Virus infects cell

AB

C

Cell expresses viral antigens

CytotoxicT cell

Infected cell is killed by cytotoxic T cellby activation of perforins and granzymes

that cleave host and viral DNA

Th 2 cells recognize viral Ags bound to MHC II on APC and secrete IL-2

IL-2 activates Tc cell to kill virus infected cell

Fas-Fas ligand (FasL) Interaction• Fas is a protein on surface of many cells• FasL is in Tc cell • When Tc TCR recognizes an epitope on surface

of target cell, FasL is induced in Tc cell• When Fas & FasL interact apoptosis of target

cell occursADCC• Ab bound to the surface of infected cell is

recognized by IgG receptors on surface of phagocytic cell and the infected cell is killed

Cytotoxic (Tc) T Cells Effector Function

B Cells Processing

• B Cell precursors differentiate into immunocompetent B cells in BM

• B cells also undergo clonal deletion of cells bearing Ag receptors for self proteins and reduce occurrence of autoimmune diseases.

• The site of clonal deletion is uncertain

B Cells Processing• Maturation of B cells has two phases:

• Ag-independent phase consists of stem cells, pre-B cells, B cells

• Ag-dependent phase consists of activated B cells and plasma cells

• Pre-B cells lack surface receptors (Ig) and light chains. Are found in bone marrow

• 30% of circulating pool of small lmphocytes

• Within LN, are located in the germinal center, also in spleen and Peyer’s patches

• Life span is days to weeks

• 109 B cells are produced daily

Features of B cells

B cell receptor (surface immunoglobulin-IgM) recognizes antigen and is bivalent. This IgM is monomer

Bcell

B Cell Activation

• An Ag interacts with IgM receptor on B cell (APC)

• The Ag is processed and epitopes appear on the surface in conjunction with class II MHC

• This complex is recognized by Th2 cell with a receptor for the Ag on its surface

• Th cell gets activated and produces IL-2, IL-4, IL-5 that activate B cells

Cellular Events in 1o Response to T-dependent Ags

• Lag Phase• Clonal selection

• Log Phase• Initially IgM• Class switching:

some B cells switch from making IgM to IgG, IgA or IgE.

• Stationary Phase• Decline• Memory Cell Pool

IgM

Memory Cells

IgG

1o Ag

IL2, BCGF (IL4), BCDF (IL5)

Activated Th2 cell

Activated B cells transform to plasma cells to produce thousands of specific Ig molecules per sec for few days and then die.

Cellular Events in 2o Response to T-dependent Ags

• Lag phase• Virgin cells• Memory cells

• Log phase• Pool size• IgG, IgA or IgE

• Stationary• Decline

• Sustained production

IgM

Memory Cells

IgG

IgG

Memory Cells

Memory Pool

Virgin B cell

Comparison of T cells and B cellsFeatures T cells B cells

• Ag receptor on surface Yes Yes• Ag receptor recognizes only processed Yes No

peptides in association with MHC• Ag receptor recognizes unprocessed No Yes

proteins without requirement of MHC• IgM on surface No Yes• CD3 proteins on surface Yes No• Ig synthesis No Yes• Regulator of Ab synthesis Yes No• Effector of CMI Yes No• Maturation In thymus Bursa or

its equivalent

Lymphocyte Recirculation

Naïve lymphocytesenter lymph nodes

from the blood circulation

Lymphocytes return to blood

via thoracic duct

Antigens from infected area go to lymph nodes

via the lymphatic system

Lymphocytes recirculate and encounter antigen in

peripheral lymphoid tissues

Leukocyte Migration and Localization

Tcell

Tcell

Tcell

Bcell

Bcell

Bcell

APC

Tcell

Bcell

Naivelymphocytes

Bone marrow

Thymus

Spleen and lymph nodes Tissues

Primed lymphocytes

Dendritic cell

Macrophage

Classes of Major Histocompatibility Complex (MHC) Molecules

MHC class I and MHC class II

Class I MHC Molecules

• Are about 20 glycoproteins expressed on surface of all nucleated cells

• Polymorphism of MHC is important in recognition of self & non-self

• Are recognized by TCR of Tc (CD8+) cells

• CD8 binds to class I MHC-peptide complex

Class II MHC Molecules

• Are glycoproteins expressed on surface of some nucleated cells, mainly on APC, B cells, dendritic cells and Langerhans cells

• Are recognized by TCR of Th (CD4+) cells

• CD4 binds to class II MHC-peptide complex

Cells Expressing Class I and Class II MHC

Class I MHC

Class IIMHC

All nucleated cells express class I MHC

Cells expressing class II MHC also express class I MHC

Biological importance of MHC

The ability of T cells to recognize Ag is dependent on association of the Ag with either class I or II MHC

Self MHC Restriction in the Thymus4 -

8 low4 low8 low

4 + 8 +TCR

4 + 8 +TCR

macrophage

Productive TCRrearrangement

Non-productive TCRrearrangement

Recognise self MHC

Not recognise self MHC

TCR does not recognise self Antigens Negative selection

TCR recognisesself antigens

APOPTOSIS

4 + 8 +TCR 4 - 8 -

Sub-capsular region

Cortex

Cortico-medullary region

4 + 8 -TCR 4 - 8 +

TCRvessel

Medulla

Class I MHC Pathway

Viral protein is madeon cytoplasmic

ribosomesPlasma membrane

Proteasome degrades protein topeptides

Peptide transporterprotein moves peptide into ER

MHC class I alpha and beta proteinsare made on the rER

Peptide associateswith MHC-I complex

Peptide with MHCgoes to Golgi body

Peptide passeswith MHC from Golgi

body to surface

Peptide is presentedby MHC-I to CD8cytotoxic T cell

Golgi body

rER

Globular viralprotein - intact

Class II MHC Pathway

Globular protein

Endocytosis

Protein is processed topeptides in endosome

or lysosome

Endosome

Lysosome

Fusion of endosomeand exocytic vesicle

Endoplasmic reticulum

Class II MHCSynthesis

3 chains: α,β and Ii

Golgibody

Exocytic vesicle fuseswith endosome

releasing Ii from αβ dimer

αβ Ii

Immunodominantpeptide binds

to class II MHC

Endosome fuses withplasma membrane

Peptide MHC-II complex is presentedto CD4 helper T cell CD4 helper T cell

Leukocyte Migration and Localization

• Bone marrow and thymus (primary lymphoid tissues) produce B cells and T cells, respectively

• B cells and T cells recirculate through spleen and lymph nodes (secondary lymphoid tissues)

• Recirculation of lymphocytes through lymphoid tissues optimizes productive encounters with Ag to initiate response

• Antigen presenting cells (APC) pick up antigen and migrate to secondary lymphoid tissues (main sites where lymphocytes encounter Ag) and interact with T cells and B cells