Lecture outline

• Signals for T cell activation

• Costimulation and the B7:CD28 family

• Responses of T cells

• Cytokines

The life history of T lymphocytes

Abbas, Lichtman and Pillai. Cellular and Molecular Immunology, 7th edition, 2011 c Elsevier

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

Steps in the activation of T lymphocytes

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.

Recognition of antigen by the TCR

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: clustering of receptors --> activation of kinases (often via “adaptor proteins”) --> activation of transcription factors

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.

Receptor-ligand pairs involved in T cell activation

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

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

Formation of the immunological synapse

Signaling molecules orient to one region of the cell within seconds of antigen recognition

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

The two-signal requirement for lymphocyte activation

Abbas, Lichtman and Pillai. Cellular and Molecular Immunology, 7th edition, 2011 c Elsevier

Costimulation: signal(s) in addition to antigen that are needed to initiate adaptive immune responses

Best defined for CD4+ Tcells

Multiple pairs of ligands on APCs and receptors on T cells may serve this function; best defined are the B7-CD28 families of proteins

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 dangerous insults, such as tumors) and not to harmless antigens

– Adjuvants stimulate expression of costimulators

Role of costimulation in T cell activation

The B7: CD28 family

Different membersof the B7-CD28families servedifferent roles instimulating and suppressing immune responses.

Major functions of selected B7-CD28 family members

• B7-CD28: initiation of immune responses

• ICOS-ICOS-L: role in B cell activation in germinal centers

• B7-CTLA-4: inhibits early T cell responses in lymphoid organs

• PD-1:PD-L1,2: inhibits effector T cell responses in peripheral tissues

APCTCR

CD28

NaïveT cell

B7

B7-CD28interaction

B7-CTLA-4interaction

CTLA-4

Proliferation,differentiation

Functional inactivation

The opposing functions of CD28 and CTLA-4

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

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.

Inhibitory role of PD-1 in a chronic infection

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.

Virus-specific T cell response

Residual virus

Inhibitory receptors

• Prevent reactions against self antigens

• Mediate immunosuppression in chronic infections (HCV, HIV)

• Limit responses to tumors • Similar roles are established for

both CTLA-4 and PD-1

The balance between activation and inhibition

• How does a T cell choose to use CD28 to be activated or CTLA-4 to shut down?

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

Therapeutics based on the B7:CD28/CTLA-4 family1. Costimulatory blockade

CTLA-4.Ig is used for diseases caused by ….?

CTLA-4.Ig is used for diseases caused by excessive T cell activation -- rheumatoid arthritis, graft rejection; not yet approved for IBD, psoriasis

Therapeutics based on the B7:CD28/CTLA-4 family1. Costimulatory blockade

Therapeutics based on the B7:CD28/CTLA-4 family2. Inhibiting the inhibitor

Anti-CTLA-4 antibody is used for ….?

Anti-CTLA-4 antibody is approved for tumor immunotherapy (enhancing immune responses against tumors)Even more impressive early clinical trial results with anti-PD-1 in cancer patients

Therapeutics based on the B7:CD28/CTLA-4 family2. Inhibiting the inhibitor

• 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

Costimulation

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

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Infection

Clonal expansion

Contraction (homeostasis)

Memory

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)

Production of IL-2 and expression of high-affinity IL-2 receptors are both dependent on antigen recognition + costimulation

Role of IL-2 and IL-2 receptors in T cell proliferation

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

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

Naïve T cell:

Can recognize antigen but incapable of

any functions

APC

+ antigen Differentiation

+ a

nti

gen

CD4+ helper T cells

CD8+ CTLs

Cytokine secretion

Cell killing

Effector T cells

Naïve T cells differentiate into functional effector cells

Show naïve CD8 also?

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

• Develop from antigen-stimulated T cells• 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

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