T Cell-Mediated Immunity

Pathway to T Cell Activation

LCK Immunoreceptor tyrosine-based activation motifs ZAP-70 Phosphatidylinositol-3 (PI-3) kinase Phosphatidyl Inositol specific phospholipase C (PLC) NFAT NFkB AP-1 Akt mTOR IL-2/IL-2R

Naïve T cells undergo multiple steps before becoming effector cells

- After T cells recognize peptides presented by antigen presenting cells, they become activated & clonally expand to provide sufficient number of antigen-specific T cells to eliminate pathogen. - They receive signals that also determine the subset of T cell to which they will differentiate. - After controlling infection, most will die from apoptosis but memory T cells will remain & rapidly expand upon re-exposure to antigen.

CD4+ cytokines mediate many cell-mediated immune responses

- Although many cells can secrete cytokines, T cells are major producers during an adaptive immune response. - T cell-secreted cytokines are very important in almost every type of adaptive immune response. - Note that IL-2 is an autocrine hormone that stimulates proliferation of the cells that produced it. - It will be important for you to recognize the function of these molecules and the key subset of T cells that secrete them.

T Cell APC Interactions

- Antigen Recognition - Specificity - Signal Transduction (or costimulation) - Adhesion - These processes are not necessarily exclusive

CD4+ T cells secrete and are responsive to numerous cytokines

- Cytokines are hormones of immune system and just like many cells of innate immune system, T cells can also secrete autocrine, paracrine, & endocrine hormones. - Some cytokines, such as interferon-gamma, are produced by innate cells (NK cells) and by T cells (TH1 subset).

Summary II

- Cytokines drive CD4+ T cell differentiation by activation of transcription factors - Cytokines mediate many of functions of CD4+ T cells and populations of CD4 + T cells may be defined by cytokines they secrete - CD4 + T cells eliminate pathogens by activating macrophages and inducing antibody production by B cells - TH1 cells coordinate elimination of intracellular pathogens - TH2 cells coordinate eliminate of extracellular pathogens - TH17 cells promote inflammation

Synthesis of IL-2 stimulates T cell proliferation: Clonal Expansion

- IL-2 is produced by antigen-stimulated T cells and is secreted early in the activation process. - It binds to the IL-2R which is composed of 3 peptides to form a high affinity complex. - Naïve T cells only express 2 chains (b and the common g chain {gc) which bind to IL-2 with low affinity. - The gc chain is utilized by several cytokines receptors and a deficiency or defect of this protein results in profound immunodeficiencies.

Integrins promote a stable interaction between the naïve T cell and dendritic cell

- In order for a T cell to be activated following recognition of a foreign peptide, it must increase its adherence to APC long enough for signal threshold to be reached. - This is mediated primarily by a clustering of integrin molecules (LFA-1) on the T cells by increasing affinity of LFA-1 on T cell so that it binds more tightly to ICAM-1 on APC. - These changes are induced by the engaged TCR/CD3 complex and by chemokines that are synthesized by APCs in response to the infection.

CD4+ T cells may help CD8+ T cells

- In some cases, CD8+ T cells can be activated directly by peptide/MHC class I complexes on APCs plus costimulation. - They still may require cytokines produced by CD4+ T cells to become fully activated and to proliferate. - In other cases, an infected cell may be phagocytized by an APC and peptides presented on both class I and class II MHC molecules. - CD4+ T cells will recognize peptides presented by class II molecules & CD8+ T cells will recognize the peptide presented by class I molecules 7 be stimulated by cytokines secreted by activated CD4+ T cells.

T cell recognition & activation involve several points of contact with APC

- Interactions must occur between TCR & its co-receptor (CD4 or CD8) & peptide: a. MHC complex on antigen presenting cell. b. There must be more than one TCR engaged for activation to occur. - Signal transduction molecules: a. CD28 must be engaged with co-stimulatory molecules, B7-1 (CD80) & B7-2 (CD86). b. Members of TCR complex, CD3 & zeta, will transduce activation signals to interior of the cell. - Adhesion molecules engage the two cells in a stable manner long enough for signals to be received by the T cell.

T-cell mediated immunity

- Naïve T cell are constantly re-circulating through secondary lymphoid tissues because this is the site where they will encounter foreign protein antigens presented by dendritic cells. - After activation and differentiation into effector cells, they leave the lymphoid tissue to migrate to the site of the foreign antigen where they will carry out their effector functions. - Naïve T cells enter lymph nodes through HEV via L-selectin and LFA-1 mediated interactions with L-selectin ligand (carbohydrates on the surface of the HEVs) and ICAM-1. - Chemokines , CCL19 and CCL21, are also involved in inducing T cells, via CCR7, to migrate to the T cell zone to sample peptides presented by dendritic cells. - Most T cells migrate through some lymph nodes at least once per day.

Primary Concept of T-cell mediated immunity

- Naïve T cells circulate through lymphoid tissues where they can be activated. - Once activated, these antigen specific T cells egress to the periphery to carry out their effector function.

CD4, CD3, and zeta (z) signal intracellularly that an antigen has been recognized by the TCR

- Once naïve T cells arrive in lymph node, they will scan dendritic cells for peptide recognition. - Remember that peptides processed from cytoplasmic or endogenous proteins are loaded onto MHC Class I molecules and activate CD8+ T cells. - Peptides that are processed from phagocytized microbes will be loaded onto MHC Class II molecules & activate CD4+ T cells. - Antigen recognition by more than one TCR & co-receptor (CD4 or CD8) complex is SIGNAL #1 in T cell activation. - However, T cell receptor by itself cannot initiate activation because its cytoplasmic tail is too short. - Other noncovalently associated proteins are involved and include CD3 and zeta (homodimer). - The TCR complex includes TCR, CD3 and zeta.

Activation of naïve T cells is dependent on expression of co-stimulators

- Resting APCs constantly present self-peptides to naïve T cells but because these APCs have not been activated by microbes or by cytokines generated in response to microbes, they do not express sufficient levels of B7 costimulators to activate T cells. - When T cells recognize self-peptides on APCs in the absence of CD28, they will not respond (anergy; opposite of energy). - This is one way of preventing an immune respond against self and the development of autoimmunity. - However, if the APC has been activated by a microbe, cytokines, or adjuvants, expression of B7 will increase and stimulate T cells. - There are other costimulatory molecules expressed on T cells in addition to CD28 such as inducible co-stimulator (ICOS).

Summary I

- T cell receptor plus CD4 or CD8 recognition of peptide:MHC complexes on APCs is Signal #1 for T cell activation - CD3 and zeta, as well as CD4 or CD8, transduce intracellular signals that promote T cell activation - Stable interaction between T cells and APCs is further mediated by adhesion molecules (integrins, etc.) - Costimulation by B7 binding to CD28 is another example of a second signal (Signal #2) for T cell activation - Receptors on the T cell cluster to form an immune synapse that is conducive to T cell proliferation

Summary III

- The immunologic synapse is the point of contact between an APC and a T cell - Activation of signal transduction pathways results in the synthesis and expression of new proteins in T cells undergoing activation - IL-2 is the major growth factor that drives proliferation of activated T cells - The high affinity IL-2R consists of 3 chains; a (CD25), b, and gc - Proliferation of activated T cells is antigen-specific and contraction occurs after antigen clearance

T Cell Expansion and Contraction

- The process of a naïve CD4+ T cell differentiating to one of these highly specialized subsets is called polarization. - This differentiation process is highly influenced by the cytokine milieu generated during the innate immune response macrophages, dendritic cells, NK cells, and mast cells. - The cytokines produced by these cells is probably driven by the pathogen recognition receptors that are engaged. - Once a CD4+ T cell commits to a certain subset, it produces cytokines that promote its own development and may inhibit the development of other subsets. - committed TH1 cells produce interferon-gamma which dampens the development of TH2 cells. - Polarization occurs because the microbial signals drive the differentiation of the subset best able to respond to the infectious agent.

Signal Transduction & Gene Expression

- There are multiple signal transduction pathways that are involved in T cell activation. - These pathways are initiated when immunoreceptor tyrosine-based activation motifs (ITAMs) in the cytoplasmic tails of CD3 and zeta are phosphorylated by Lck. - The tyrosine kinase ZAP-70 is also phosphorylated by Lck. - The downstream events result in the activation of the transcription factors nuclear factor of activated T cells (NFAT), nuclear factor kappa B (NFkB), and AP-1. - A congenital absence of CD3 or ZAP-70 results in profound T cell deficiencies. Calcineurin is the target for the drug, cyclosporine. - This potent immunosuppressive drug inhibits the production of cytokines.

Adjuvants

- products designed to enhance the immune response to a protein antigen. - are used in vaccines (alum) and in research settings to prolong presence of antigen, induce cytokine secretion, & increase expression of costimulators.