Cellular immune responses

T Cell Subsets

• CD8/CTL• CD4/TH cells

1) TH1 - inflammatory response

2) TH2 - anti-inflammatory, B cell response

3) Treg - inhibit immune responses

Th1 express CCR5, CXCR3+ and Th2 express CCR4 and CCR8+

CD4+ Subsets: TH1 vs. TH2• Highly investigated in mouse model of

leishmaniasis: C57BL/6 mouse cures, but Balb/c mouse dies of uncontrolled infection

• Strains differ in cytokine responses• Resistant mice produce interferon- (IFN-),

tumor necrosis factor- (TNF-), and lymphotoxin, which activate inflammatory response and cell-mediated immunity: TH1

• Susceptible mice produce interleukin (IL)- 4, 5, 13, which are anti-inflammatory and promote B cell activation and antibody production: TH2

TH1 vs. TH2 Differentiation Depends on Cytokine Milieu

• TH1 differentiation is driven by the APC-derived cytokine IL-12

• TH2 differentiation is driven by the T cell-derived cytokine IL-4

• IL-12 and IL-4 are mutually antagonisitic: IL-12/IFN- inhibit IL-4 secretion and vice versa, leading to strongly polarized response

Mechanisms of Cytotoxicity

• CTL (but not naïve CD8+ T cells) express lytic granules: perforin, granzymes, granulysin

• Perforin lysis of target cell (inefficient)• Granzyme activation of cytosolic apoptosis

machinery• Fas/FasL induction of apoptosis

CD8 Cytotoxic T cell

MHC Class I

+

CD28

MHC Class II

Dendritic Cell

CD4+ T-cell Help for CD8+ CTL Mediated Through Activation of Dendritic Cell

CD40

CD4+ Helper T cell

CD40L

TCR

TCR

B7 costimulator

IL-15IL-12

IL-15 with vaccine

TFH is non-polarized T cell and Th1, Th2 cell polarize B cells.

Tpath

TpathTpath

Teff

TeffTeff

Teff

Teff

Treg

Treg

Treg

mDC

mDC

Teff

Teff

Teff

iDC

Treg

Tpath

mDC

Treg

Tpath

Teff

ProliferationNo suppression

APC

mDC mDC

Tmem†

Treg Treg

TpathTpath

Treg

Teff

Teff

Teff

TpathTpath

Treg

Treg

APC

A) Steady State: LN

B) Response phase: LN

C) Regulatory phase

LN

InflammatorySite

Tpath

+TLR+TLR

IL-6IL-6

IL-2IL-2

IL-10IL-10

IL-10IL-10

IL-10IL-10

TGF-TGF-

GITR-LGITR-LCD28CD28

Regulatory T Cells

• Long and spotty history of study: “suppressor” cells, “veto” cells, “infectious tolerance”, and now “regulatory” T cells

• Basic premise: one or more populations of T cells act to inhibit the responses of other T cells

Regulatory T Cells cont.

• Sakaguchi and colleagues found that thymectomy in <3 day-old mouse results in multiorgan autoimmunity

• Identified population of CD4+CD25+ T cells that restore self-tolerance

• Differentiation into Treg requires expression of FoxP3 (scurfin)

• Suppression of responses by both contact-dependent (inhibition of APC) and cytokine (IL-10, TGF-) mechanisms

HISTORICAL PERSPECTIVE of T regHISTORICAL PERSPECTIVE of T reg1970’s - existence of specialized suppressor T cells postulated1970’s - existence of specialized suppressor T cells postulated

- usually attributed to Lyt2- usually attributed to Lyt2++ (CD8 (CD8++) T) TSS cells cells

- variety of suppressive phenomena and networks- variety of suppressive phenomena and networks

1980’s - advent of molecular immunology1980’s - advent of molecular immunology

- inability to identify cellular and molecular mechs of suppression- inability to identify cellular and molecular mechs of suppression

- stigma associated with T- stigma associated with TSS cells cells

1990’s - phenotypic identification of CD41990’s - phenotypic identification of CD4++ T cell subsets T cell subsets

- distinct patterns of cytokine secretion Th1, Th2 etc.- distinct patterns of cytokine secretion Th1, Th2 etc.

- immunomodulatory roles for cytokines- immunomodulatory roles for cytokines

1994-present - rebirth of suppressor T cells as regulatory T1994-present - rebirth of suppressor T cells as regulatory TRR cells cells

- phenotypic and functional characterization of T- phenotypic and functional characterization of TRR cells cells

- role in preventing autoimmunity and immune pathology- role in preventing autoimmunity and immune pathology

Th3

Tr1

CD4

TGF- MHC class II-restricted

IL-10 (& TGF-MHC class II-restricted

TCR

Regulatory/Suppressor T cells

TReg

CD25 (IL-2R)

Ts

CD8

? Qa-1-restricted

TCR

IFN-IL-4, IL-13CD1d-restrictedNKTNK1.1

Contact inhibition MHC class II-restricted

CD4

IL-10

Naturally-occurring versus inducible TNaturally-occurring versus inducible TR R cellscells

TGF-?

Periphery

ALTERNATIVE FORM OF T-CELL RECEPTOR

• Second type of receptor consists of– Gamma and Delta chains

• T-cells referred to as– Gamma:Delta T-cells

• Gamma:Delta T-cells– Comprise approximately 1 to 5% of circulating T-cells– Function is unknown– Not restricted to MHC presentation of peptide antigens

• Alpha:Beta and Gamma:Delta receptors never expressed together

T-lymphocyte epitope-specific receptor

• Similarities with the Fab portion of a BCR/antibody– Two identical length chains (ά and β)– One antigen-binding site/receptor (one Vά and one Vβ)

– Cά and Cβ

– Beta sheets, intersheet bonds (non-covalent & disulfide)– Domains– HV and FR amino acid sequences

26

-MHC

TCR

CD4 CD8

TCR complex

Co-Receptor

CD28 Co-Stimulator

Integrine Adhesion molecule

MHC II / MHC IRestriktion

“Signal 2”

T cell receptor

Acc

e ss o

ry (

Hil

fs-)

Mo

l ek ü

le

CD3

28

Struktur

Monomer

Heterodimer

3. CD4 und CD8 Co-Rezeptoren

Ig-Superfamilie

CD8 Homodimere kommen ebenfalls auf bestimmten T Zell Populationen vor

29

2. CD3 und Proteine

Expression des TCR Komplexes erfordert alle seine Komponenten

Self MHC limitation

“Immunologische Synapse”

15 nm

Fig 9.6 p249

Fig. 8.12 T-cell activation through the T-cell receptor and CD28 leads to the increased expression of CTLA-4, an inhibitory receptor for B7 molecules.

CTLA-4: Putting on the Brakes

1) CTLA-4 is another CD28-related receptor, and binds both B7-1 and B7-2 - avidity is at least 20x as high as CD28

2) CTLA-4 expression on the surface is undetectable in resting T cells, but is rapidly increased after TCR + CD28 signaling

3) Unlike CD28, CTLA-4 is inhibitory, and blocks T cell proliferation and IL-2 production - combination of competing B7 molecules away from CD28 and bona fide inhibitory signals (possibly phosphatases, but still not well defined)

CD28: The First (and still the champ!) Co-stimulatory Molecule

1) Yet another member of the immunoglobulin superfamily (single Ig-V domain)

2) Expressed on the surface of almost all T cells (100% of mouse T cells, ~80% of human T cells) as a disulfide-linked dimer

3) Binds to B7-1 (CD80) and B7-2 (CD86) expressed on antigen presenting cells

4) Cytoplasmic tail has binding motifs for several signaling molecules (PI3K, Grb2, Itk), but no ITAM. It is still unclear what signals CD28 contributes to T cell activation

5) Signaling by CD28 alone does not stimulate T cells, and only activates PI3K

Polarization of T Cells Part I: The Cytoskeleton

1) T cell responses are directed at the APC/target cell, not in all directions2) This requires reorganization of the cell to have a “front” (toward the APC)

and a “back” - induced by signals from the TCR and costimulatory molecules

3) Result: Reorganization of the cytoskeleton causes reorientation of cytosolic organelles toward APC - Golgi, secretory vessicles, and microtubule organizing center (MTOC). The MTOC connects actin cytoskeletal changes with the tubulin cytoskeleton

Fig. 8.29 The polarization of T cells during specific antigen recognition

The MHC molecules

• Class I– ά chain forms the peptide-binding “groove”– β-2 microglobulin provides stability

• Class II– ά and β chains form peptide-binding “groove”

• TCR binds to peptide• CD binds to MHC

MHC Class I antigen processing and presentation

• Peptides in cytoplasm– viral proteins– Tumor proteins (phenotypic expression of activated oncogenes)– Self-proteins (damaged, no longer needed, excess)

“Cross-presentation” by Dendritic Cells• Usual endocytosis of bacteria, soluble antigens

– Class II presentation as expected (peptides in membrane-bound vesicles)• Infection by viruses

– Class I presentation as expected (peptides in cytoplasm)

• DC involved in “priming” T-cells in lymph node– Need to present peptides using MHC Class II and Class I– Class II using “normal” endocytosis into membrane-bound vesicles– Class I presentation

• Endocytosis of viruses (not an infection)• Peptides moved into cytoplasm (Class I presentation)• Viral peptides also transferred to other DCs (into cytoplasm) to be presented by MHC Class I