1. Signals for T cell activation Costimulation and the B7:CD28 family
Lecture outline
Signals for T cell activation
Costimulation and the B7:CD28 family
Inhibitory receptors of T cells
Responses of T cells
Cytokines

2. The life history of T lymphocytes
Abbas, Lichtman and Pillai. Cellular and Molecular Immunology, 7th edition, 2011 c
Elsevier

3. Principles of lymphocyte activation
Lymphocytes are normally in a resting state in lymphoid organs and circulation
Rapid response to antigen (activation) --> proliferation, change to functionally active effector cells (differentiation)
Migration to tissues, where they perform their function of eliminating infections
Multiple possible steps for therapeutic targeting

4. Steps in the activation of T lymphocytes

5. Recognition of antigen by the TCR
The TCR of CD4+ and CD8+
T cells recognizes MHC-bound peptide + portions of the MHC.
Other T cells (gd T cells, NKT cells) recognize non-peptide antigens; these are small cell populations whose function is unclear.

6. Antigen recognition by T cells
Each T cell sees a (self) MHC molecule and a bound peptide
Dual recognition determines specificity and MHC restriction
Multiple ligands on APCs and receptors on T cells, in addition to the TCR, participate in orchestrating responses to antigens
Signaling: engagement of receptors --> activation of kinases (often via “adaptor proteins”) --> activation of transcription factors

7. Receptor-ligand pairs involved in
T cell activation
Different molecules involved in T cell responses to
antigen serve distinct functions, seen even in this partial listing.
Drugs that block these ligand-receptor pairs have been developed
to treat immune-mediated inflammatory diseases, graft rejection.

8. Molecules involved in T cell activation
Signal transduction
CD4 and CD8 “co-receptors” recognize MHC molecules (class II or class I) at the same time as the TCR sees the peptide-MHC; CD4 and CD8 provide necessary activating signals for T cells
CD28 is a receptor for “costimulators” expressed on APCs

9. Molecules involved in T cell activation
Signal transduction
CD4 and CD8 co-receptors recognize MHC molecules (class II or class I) at the same time as the TCR sees the peptide-MHC; CD4 and CD8 provide necessary activating signals for T cells
CD28 is a receptor for “costimulators” expressed on APCs
Strengthen adhesion with antigen-presenting cells
Integrins function as adhesion molecules
Affinity of integrins is increased by chemokines produced during inflammation, and by antigen recognition by TCRs
Control routes of T cell migration
Selectins and integrins control migration of naïve T cells through lymph nodes and of effector and memory T cells to sites of infection
Therapeutic targets

10. Therapeutic targeting of molecules involved in T cell responses
CD3: signaling molecule attached to the TCR on all T cells; anti-CD3 MAb to deplete T cells (transplants)
Integrins (LFA-1, VLA-4, others): adhesion to APCs, endothelium; anti-integrin MAb’s to block leukocyte migration into tissues
“Costimulators”: CD28, others; costimulatory blockade

11. The two-signal requirement for lymphocyte activation
Costimulation: signal(s)
in addition to antigen that are needed to initiate adaptive immune
responses
Best defined for CD4+ T
cells
Multiple pairs of ligands
on APCs and receptors
on T cells may serve this
function; best defined
are the B7-CD28 families of proteins
Abbas, Lichtman and Pillai. Cellular and Molecular Immunology, 7th edition, 2011 c
Elsevier

12. Role of costimulation in T cell activation

13. Two signal requirement for T cell activation
Naïve lymphocytes need two signals to initiate their responses
Signal 1: antigen recognition
Ensures that the response is antigen-specific
Signal 2: costimulators induced on APCs during infection (or upon recognition of necrotic cells)
Ensures that the immune system responds best to microbes (or to necrotic cells, as in tumors) and not to harmless antigens
Adjuvants stimulate expression of costimulators

14. The B7: CD28 family Different members of the B7-CD28 families serve
different roles in
stimulating and suppressing
immune responses.

15. Major functions of selected B7-CD28 family members
B7-CD28: initiation of immune responses
ICOS-ICOS-L: T cell help in germinal center reactions (antibody responses)
B7-CTLA-4: inhibits early T cell responses in lymphoid organs
PD-1:PD-L1,2: inhibits effector T cell responses in peripheral tissues
Activation
Inhibition

16. Inhibitory receptors of the immune system
One mechanism by which the system maintains a balance between activation and inhibition is to use different receptors for different outcomes
Inhibitory receptors are present in NK cells, T cells and B cells; other immune cells?
In many instances, activating receptors work by recruiting kinases (often via ITAMs) and inhibitory receptors activate phosphatases (via ITIMs)

17. The opposing functions of CD28 and CTLA-4B7
CD28
APC
B7-CD28
interaction
Naïve
T cell
TCR
Proliferation,
differentiation
CTLA-4
B7-CTLA-4
interaction
Functional
inactivation
Inhibitory pathways function normally to prevent
responses to self antigens: demonstrated by the
finding that blocking or eliminating these inhibitors
(CTLA-4, PD-1) causes autoimmune disease

18. Blocking CTLA-4 promotes tumor rejection
Tumor recognition by T cells leads to engagement of CTLA-4 on the T cells and inhibition of immune responses. Blocking CTLA-4 increases anti-tumor response and leads to tumor rejection.

19. Inhibitory role of PD-1 in a chronic infection
Virus-specific
T cell response
Residual virus
In a chronic viral infection in mice, recognition of virus by specific T cells leads to PD-1 engagement, inhibition of T cell responses, and persistence of the virus. Blocking the PD-1 pathway releases the inhibition, enhances the T cell response, and leads to viral clearance.

20. Inhibitory receptors of T cells
Prevent reactions against self antigens (their physiologic function)
Suppress immune responses to some tumors, chronic infections (HCV, HIV)
Similar roles are established for both CTLA-4 and PD-1

21. The balance between activation and inhibition
How does a T cell choose to use CD28 to be activated or CTLA-4 to shut down?
Both recognize B7 on APCs
CTLA-4 (the inhibitory receptor) binds to B7 with much higher affinity than CD28 (the activating receptor)

22. The balance between activation and inhibition
How does a T cell choose to use CD28 to be activated or CTLA-4 to shut down?
Low B7 (e.g. when DC is displaying self antigen) --> engagement of high-affinity CTLA-4
High B7 (e.g. after microbe encounter) --> engagement of lower affinity CD28
Not well understood for the PD-1 pathway

23. Therapeutics based on the B7:CD28/CTLA-4 family
1. Costimulatory blockade
CTLA-4.Ig is used for diseases caused by …?

24. Therapeutics based on the B7:CD28/CTLA-4 family
1. Costimulatory blockade
CTLA-4.Ig is being tested in diseases caused by excessive T cell activation -- rheumatoid arthritis (FDA approved), psoriasis, graft rejection (approved)

25. Therapeutics based on the B7:CD28/CTLA-4 family
2. Removing the brakes on the immune response
Anti-CTLA-4 antibody is used for ….?

26. Therapeutics based on the B7:CD28/CTLA-4 family
2. Removing the brakes on the immune response
Anti-CTLA-4 antibody is approved for tumor
immunotherapy (enhancing immune responses against tumors)
Even more impressive results with anti-PD-1 in cancer patients

27. The emerging paradigm for cancer therapy
Signaling (e.g. Kinase) inhibitor to block oncogenic pathways in tumor cells +
Immune modulator
Inhibiting endogenous regulators allows the host to mount an effective immune response (let the immune system do the work itself)
Appears to be more effective than vaccination and other approaches for stimulating immunity
Anti-PD1 may be more effective than anti-CTLA-4 (less toxic, greater effect on CTLs)

28. Costimulation Required for initiating T cell responses
Many proteins on APCs and their receptors on T cells shown to “costimulate” (function with antigen to activate T cells); most important costimulators are B7:CD28
Ensures that T cells respond to microbes (the inducers of costimulators) and not to harmless antigens
Source of costimulation in responses to tumors and transplants: products of dead cells?
Therapeutic targets

29. T cell expansion and contraction (decline)
Many aspects of T cell responses and functions are mediated
by cytokines: initial activation -- IL-2; maintenance of memory cells -- IL-7; effector functions -- various

30. Cytokines
Secreted proteins that mediate immune and inflammatory reactions, and communications among leukocytes and other cells
Produced transiently in response to extrinsic stimuli
Bind to high-affinity receptors on target cells
Actions are most often autocrine and paracrine, rarely endocrine
Cytokines are pleiotropic (one cytokine has multiple actions) and redundant (different cytokines have similar actions)

31. Role of IL-2 and IL-2 receptors in T cell proliferation
Production of IL-2 and expression of high-affinity IL-2 receptors
are both dependent on antigen recognition + costimulation

32. Clonal expansion (proliferation) of T cells
Stimulated mainly by autocrine IL-2
T cell stimulation by antigen + costimulators induces secretion of IL-2 and expression of high-affinity IL-2 receptors
Therefore, antigen-stimulated T cells are the ones that expand preferentially in any immune response, keeping pace with replicating microbes

33. Clonal expansion (proliferation) of T cells
Stimulated mainly by autocrine IL-2
T cell stimulation by antigen + costimulators induces secretion of IL-2 and expression of high-affinity IL-2 receptors
Therefore, antigen-stimulated T cells are the ones that expand preferentially in any immune response, keeping pace with replicating microbes
CD8+ T cells may expand >50,000-fold within a week after an acute viral infection with minimal expansion of cells not specific for the virus (up to 10% of all CD8+ T cells in the blood may be specific for the pathogen)
Some of the progeny of the expanded clone differentiate into effector and memory cells; the majority die by apoptosis

34. Naïve T cells differentiate into functional effector cells
Effector T cells
CD4+ helper
T cells
Cytokine
secretion
+ antigen
Differentiation
+ antigen
APC
Naïve T cell:
Can recognize antigen but incapable of any functions
Cell
killing
CD8+ CTLs

35. Memory T cells Long-lived, functionally silent
More numerous than naïve cells specific for the antigen; respond more rapidly than do naïve cells -- explains why secondary response is “better” than primary response
May consist of multiple subsets
Some migrate to lymphoid organs (like naïve T cells), and proliferate and differentiate rapidly in response to antigen challenge (repeat infection)
Others migrate to peripheral sites of infection, and rapidly perform effector functions upon encountering the antigen

36. The life history of T lymphocytes
Precursors mature in the thymus
Naïve CD4+ and CD8+ T cells enter the circulation
Naïve T cells circulate through lymph nodes
and find antigens
Clonal expansion;
differentiation into effector and memory cells
Effector T cells migrate to sites of infection
Eradication of infection