1. T Cell Activation/Function
Ned Braunstein, MD

2. The Major T Cell Subsets
CD4+ T cells
CD8+ T cells
lck
p56
lck
p56 g z z z z d e g d e
CD3
CD3 C a C b C a C b
CD8
CD4
TCR V a V
TCR b V a V b
peptide
peptide
MHC II
MHC I
(1) Interacts with MHC class II expressing cells (B cells, macrophages)
(2) Induce(help) B cells to synthesize antibody
(3) Induce and activate macrophages
(4) Secretes lymphokines
(1) Interacts with MHC class I expressing cells (all nucleated cells)
(2) Kill MHC class I expressing target cells
(3) Suppress immune responses
(4) Secretes lymphokines

3. Naïve T cells are activated by antigen in LNs where they mature into effector cells
(1) Naive T cells home continuously from the blood to lymph nodes and other secondary lymphoid tissues. Homing to lymph nodes occurs in high endothelial venules (HEV), which express molecules for the constitutive recruitment of lymphocytes.
(2) Lymph fluid percolates through the lymph nodes; the fluid is channeled to them from peripheral tissues, where dendritic cells collect antigenic material. In inflamed tissues, dendritic cells are mobilized to carry antigen to lymph nodes, where they stimulate antigen-specific T cells. On stimulation, T cells proliferate by clonal expansion and differentiate into effector cells, which express receptors that enable them to migrate to sites of inflammation.
(3) Although most effector cells are short-lived, a few antigen-experienced cells survive for a long time. These memory cells are subdivided into two populations on the basis of their migratory ability: the effector memory cells migrate to peripheral tissues, whereas central memory cells express a repertoire of homing molecules similar to that of naive T cells and migrate preferentially to lymphoid organs.

4. The Biology of Dendritic Cells: Antigen capture and presentation to T cells
Antigens are captured by DCs in peripheral tissues and processed to form MHC-peptide complexes. As a consequence of antigen deposition and inflammation, DCs begin to mature, expressing molecules that will lead to binding and stimulation of T cells in the T-cell areas of lymphoid tissues. If the antigen has also been bound by B cells, then both B and T cells can cluster with DCs. After activation, B blasts move to the lining of the intestine, the bone marrow, and other parts of the lymphoid tissue with some becoming antibody-secreting plasma cells. T blasts leave the blood at the original site of antigen deposition, recognizing changes in the inflamed blood vessels and responding vigorously to cells that are presenting antigen. This limits the T-cell response to the site of microbial infection. Banchereau, J. and Steinman, R. Nature 392, (1998)

5. The T cell activation cycle
Minutes
Hours
Days
IL-2R P P
Ca++
IP3
DAG c-
fos
IL-2 etc. c-
jun
Antigen
recognition c-
myc
NF-KB
NF-AT
Cytokine production and
autocrine stimulation
Immediate
events
Effector functions:
Help
DTH
Killing (CTL)
regulation
Proliferation

6. MHC + Ag CD4 CD3 Cytoplasm Nucleus ICAM-1 (CD54) CD80
LFA-1 (CD11a/CD18)
CD3
CD28
IL2R
lck
CD45
fyn
ZAP-70
PTK
RAS
Cytoplasm
PLC
PKC
PIP2
DAG
IP3
MAP-kinase Ca
Nucleus
NF-AT
NF-KB
OTF1
IL-2

7. MHC + Ag CD4 CD3 Cytoplasm Nucleus ICAM-1 (CD54) CD80
LFA-1 (CD11a/CD18)
CD3
CD28
IL2R
lck
CD45
fyn
ZAP-70
PTK
RAS
Cytoplasm
PLC
PKC
PIP2
DAG
IP3
MAP kinase
Ca2+ P
I-kB
NF-kB
Nucleus
NF-AT
NF-kB
OTF1
IL-2

8. MHC + Ag CD4 CD3 Cytoplasm Nucleus CD80 ICAM-1 (CD54)
LFA-1 (CD11a/CD18)
CD3
CD28
IL2R
lck
CD45
fyn
ZAP-70
PTK
Cytoplasm
PLC
CNB
Cal-
modulin
CNA
PKC
PIP2
DAG
IP3
Ca2+ P
NF-AT
Nucleus
Jun
Fos P
NF-AT
NF-kB
OTF1
IL-2

9. The T cell activation cycle
Minutes
Hours
Days
IL-2R P P
Ca2+
IP3
DAG c-
fos
IL-2 etc. c-
jun
Antigen
recognition c-
myc
NF-kB
NF-AT
Cytokine production and
autocrine stimulation
Immediate
events
Effector functions:
Help
DTH
Killing (CTL)
regulation
Proliferation

10. Key molecular interactions between T cells and APCs
MHC class II/
autopeptide
CD3
TCR
CD40
CD80
Activated T cell
CD40
CD40L
CD40
CD28
CD80
MHC
class II
(1) induction of cytokines/chemokines (IL-8, IL-12, TNF-a, MIP-1a)
(2) stimulation of CD80 and CD86 expression and co-stimulatory function with activation of T cell growth
(3) augmentation of antigen-presenting function

11. Naïve CD4+ T cells differentiate into Th1 and Th2 subsets
Th1 Cells
IL-2 IFN-g TNF
IL-2
IFN-g, IL-12
Antigen + APC
IFN-g
(–)
IL-4 IL-10
(–)
Resting CD4+ cell
“pTh”
Activated CD4+ cell
IL-4 IL-5 IL-6 IL-10
IL-4, IL-13
Th2 Cells

12. Functions of Th subsets
Th1 Cells
• Activate macrophages/dendritic cells augment antigen presentation
• induce delayed type hypersensitivity (DTH) responses important in eradicating intracellular pathogens (TB, leprosy, listeria
• mediate Th1 diseases (ie; rheumatoid arthritis, multiple sclerosis and type I diabetes
IL-2 IFN-g TNF
IL-4 IL-10
(–)
IFN-g
(–)
Functions of Th2 subsets
IL-4 IL-5 IL-6 IL-10
• Help B cells and induce humoral immunity
• mediate allergic and immediate hypersensitivity responses
• involved in antibody mediated immune diseases like SLE and ITP
Th2 Cells

13. Major Functions of T Lymphocytes
(1) Induction and Activation of B cells (Help)-required for most antibody responses
(2) Delayed Type Hypersensitivity (DTH) - important in elimination of intracellular pathogens (virus, fungi and mycobacteria)
(3) Cell mediated Cytotoxicity (Killer function)-important in the immune response to virus infected cells and cancer cells
(4) Suppressor Cell Function regulates the cell mediated and antibody responses

14. The Induction and Activation of B cells
(Helper Function)
Antigen binds specifically to SmIg, is internalized into vesicles and cleaved into peptides which displace CLIP and bind to MHC class II molecules in the endocytic compartment. The peptide/MHC complex is then transported to the surface membrane.
The CD4+ T cell a,b TCR/CD4 complex binds to the MHC class II/peptide complex on the surface of B cells. The CD4+ Th2 cells are triggered to secrete IL-2, IL-4, IL-5, IL-6 and IL-10 and begin to express CD40L. These lymphokine and contact dependent signals (CD40L) induce B cells to proliferate, class switch and differentiate into antibody (IgM, IgG, IgA and IgE) secreting B cells and plasma cells.

15. Antigen Processing and Presentation by B cells
MHC Class ll
BCR
(SmIg)
B cell
Peptide
Internalization of antigen/Ig
Antigenic peptides
Bind to MHC class II
molecules
Antigen binds specifically to BCR (surface membrane Ig), is internalized into vesicles and cleaved into peptides which displace and bind to MHC class II molecules. The peptide/MHC complex is then transported to the surface membrane.

16. Antigen Presentation by B cells
BCR
(SmIg)
hapten
Antigen
MHC class II
carrier protein
MHC class II/
carrier peptide complex
B cell
carrier peptides
Antigen binds specifically to SmIg, is internalized into vesicles and cleaved into peptides which displace and bind to MHC class II molecules. The peptide/MHC complex is then transported to the surface membrane.

17. Resting EffectorT cell
Expression of Membrane Proteins Following Antigen Specific Activation of T and B Cells
BCR
(SmIg)
TCR
Resting EffectorT cell
Resting B cell
MHC class II
CD4
BCR
(SmIg)
CD23
IL-2R
TCR
Activated
Effector T cell
CD40
CD40L
MHC class II
CD4
CD80
CD86
Activated B cell
MHC class II

18. CONSEQUENCES OF CD40L/CD40 INTERACTIONS DURING T-B CELL INTERACTIONS
TCR
Sm Ig
CD4
CD40
Activated Effector T cell
Activated B Cell
Triggering of B cell proliferation
Rescue from apoptosis
Induction of Ig isotype class switching
Up-regulation of CD80 and CD86
Germinal center formation
Up-regulation of CD23
Downregulation of CD40L expression

19. Final Phases of B cell Differentiation are Mediated by
Contact T cell signals (CD40L/CD40) and Lymphokines
SmIg
Activated B cell
CD40
CD23
CD40L
TCR
CD4
Activated Effector T cell
Lymphokines
IL-2, IL-4, IL-5, IL-6, IFN-g, TGFb
IgG
IgA
IgE
Plasma Cell

20. The Hyper IgM Syndrome (HIM)
The Hyper IgM Syndrome (HIM) is an X chromosome-linked Ig deficiency characterized by low serum levels of IgG, IgA and IgE with normal numbers of circulating IgM expressing mature B cells. Germinal centers and splenic follicles due not develop.
Affected patients (usually males) are susceptible to pyogenic infections, autoimmune disease and lymphoproliferative disease. In addition, patients are also susceptible to Pneumocystis carini infections.
The genetic defect in the majority of HIM patients is associated with mutations in the gene encoding CD40L and can be corrected functionally by soluble CD40 ligand, in vitro. A few HIM patients have normal CD40L but defects in CD40 signaling.

21. Major Functions of T Lymphocytes
(1) Induction and Activation of B cells (Help)-required for most antibody responses
(2) Delayed Type Hypersensitivity (DTH) - important in elimination of intracellular pathogens (virus, fungi and mycobacteria)
(3) Cell mediated Cytotoxicity (Killer function)-important in the immune response to virus infected cells and cancer cells
(4) Suppressor Cell Function regulates the cell mediated and antibody responses

22. Delayed Type Hypersensitivity (DTH)
a. DTH is initiated principally by CD4+ Th1 cells and is the primary defense mechanism against intracellular parasites including the mycobacteria (TB), fungi and intracellular bacteria (listeriae monocytogenes).
b. The cognitive phase of DTH involves CD4+ T cell -macrophage/dendritic cell (MHC class II/peptide) interaction resulting in the local secretion of lymphokines.
c. The effector phase of DTH is effected by lymphokines which activate macrophages to secrete lysozyme, TNF, IL-1 and IL-12 as well as chemotactic and migration inhibitory factors restricting granulocytes, macrophages and eosinophils to the site of inflammation.
Delayed Type Hypersensitivity (DTH)

23. DTH Pathway of Contact Hypersensitivity

24. Th1 CD4+ T Cells Induce Inflammation and DTH
Activated CD4+ Th1 cell
APC
CD4
TCR
CD40L
CD40
CD4+ Th1 cell
CD40L
dendritic cell
CD40
Macrophage/
mesenchymal cell
Inflammation inducing
CD40L/CD40 interactions
activated
dendritic cell
Activated macrophages,
mesenchymal cells and
endothelial cells
Proinflammatory molecules
Chemokines (IL-8, SDF-1)
Lymphokines (IL-1, GM-CSF,TGFb
IFNb, IL-12, IL-6
NO, proteolytic enzymes, PGE2

25. T Cell- Macrophage Interactions
Fc receptor
TCR a,b
CD3
MHC class II
CD4 Th1 Cell
CD4
Macrophage
CD28
B7 (CD80)
IL-2
IL-1
IL-6
IL-12
TNF
TGF-b
IL-12
CD40L
CD80
TCR a,b
CD4
IL-2
Receptor
IFN-g
cytotoxic granules
CD80
MHC II
CD28
Activated Th1 Cell
Activated Macrophage

26. Physiology of the DTH Response
CD2
MHC II/peptide
Antigen/IgG
TCR a,b
Phagocytosis
killing
CD4+ TH1
T Cell
CD4
Fc Receptor
Macrophage/
Dendritic cell
IL-12
IL-2
TCR
a,b
IL-1, TNF, IL-6
IFN-g
IL-2R
CD4
endothelial cell
fibroblasts
hypothalamus
granulocytes
Mast Cell
Eosinophil
fever
IL-3, IL-8
IL-3, IL-4, IL-5

28. Functions of Th subsets
Th1 Cells
• Activate macrophages/dendritic cells augment antigen presentation
• induce delayed type hypersensitivity (DTH) responses important in eradicating intracellular pathogens (TB, leprosy, listeria
• mediate Th1 diseases (ie; rheumatoid arthritis, multiple sclerosis and type I diabetes
IL-2 IFN-g TNF
IL-4 IL-10
(–)
IFN-g
(–)
Functions of Th2 subsets
IL-4 IL-5 IL-6 IL-10
• Help B cells and induce humoral immunity
• mediate allergic and immediate hypersensitivity responses
• involved in antibody mediated immune diseases like SLE and ITP
Th2 Cells