T Cell Mediated Immunity
Immunology Ch 5 93-115
T Cell Mediated Immunity
-Major role for lymphocytes is cell-mediated immunity; defense against intracellular microbes
-microbes are ingested by phagocytes as innate immunity, but it is not enough sometimes, as is the case with viruses or microbes that damage insides of phagocytes
-T cells also defend against non-intracellular microbes, like bacteria, fungi, and parasites
Phases of T cell response – naïve T cells recognize antigens in peripheral lymphoid organs, which stimulates their proliferation and differentiation into effector and memory cells, and the effector cells are activated by the same antigens in peripheral tissues or lymphoid organs
-naïve T cells express antigen receptors but not capable of performing effector function
-responses of naïve T cells to cell-associated microbial agents consists of series of steps that result in an increase in the number of antigen specific T cells and conversion of naïve T cells to effector and memory cells
-earliest responses is secretion of cytokines to stimulate proliferation and clonal expansion
-some lymphocytes undergo differentiation; naïve T cell à effector T cell
-many effector T cells leave lymphoid organ à circulation and migrate to infection
-some of the T cells develop into memory T cells which are long-lived and inactive, but are ready to respond quickly to repeated antigen exposure
-When T cell recognizes antigen, it is transiently arrested on dendritic cell and initiates an activation program
3 types of stimuli are critical for T cell activation: (1) antigen recognition, (2) costimulation, (3) cytokines
Antigen Recognition and Costimulation – initiation of responses requires multiple receptors on T cells and ligands on APCs; TCR recognizes antigen on MHC, CD4/CD8 recognizes MHC itself
Recognition of MHC-Associated Peptides – TCR and CD4/CD8 coreceptor together recognize complex of peptide antigens and MHC molecules on APCs, providing first signal
-TCR contains alpha and beta chains, both capable of antigen recognition
-every mature T cell has either CD4 or CD8 and recognizes MHC-II or MHC-I respectively at a site on MHC that isn’t the peptide binding site
-proteins ingested through vesicles into APCs are presented on MHC-class II
-proteins present in cytosol are processed into peptides presented on MHC-class I
-CD4 recognizes extracellular microbe antigens, CD8 recognizes cytosolic peptides
-TCR and coreceptor need to be engaged simultaneously to initiate T cell response, and multiple TCRs need to be engaged to activate the program
biochemical signals that lead to T cell activation are triggered by set of proteins linked to TCR that are part of TCR complex and by CD4/8 coreceptor
-in T cells, antigen recognition and signaling are performed by different molecules
-TCR αβ chain can recognize antigens but cannot transmit signal inside cell
-CD3 and ζ chain: signaling molecules that TCR uses to transmit signals (TCR complex)
-other molecules that bind TCR can activate T cells: phytohemagglutinin (PHA) and superantigens can activate T cells
Role of Adhesion Molecules in T Cell Responses – adhesion molecules on T cells recognize their ligands on APCs and stabilize the binding of the T cells to the APCs
-TCR binds peptide/MHC complex with low affinity, and not long enough to stabilize binding
-Adhesion molecules called integrins stabilize binding of T cell to APC. Major T cell integrin involved in binding to APCs is leukocyte function-associated antigen 1 (LFA-1) whose ligand on APCs is intercellular adhesion molecule 1 (ICAM-1)
-LFA-1 is in LOW affinity state in naïve T cells, and antigen recognition increases affinity of LFA-1, therefore upon antigen recognition, cell strengthens stabilizer molecule
Role of Costimulation in T cell Activation – full activation of T cells depends on recognition of costimulators on APCs in addition to antigen, acting as second signals for T cell activation
-best defined costimulatory molecules are B7-1 (CD80) and B7-2 (CD86) which are expressed on APCs, and whose expression is increased in response to microbe
-B7 proteins bind CD28 receptor expressed on all T cells to generates signal that works synergistically with TCR/MHC signal to activate naïve T cells (not sufficient without 2nd signal)
-another set of molecules that participate in T cell responses are CD40 ligand (CD40L or CD154) on activated T cells and CD40 on APCs
-CD40/CD40L does not activate T cells directly, but instead upregulates B7 costimulators and cytokines from APCs to enhance T cell differentiation
-protein antigens fail to elicit T-cell immune responses unless antigens are administered with substances that activate APCs. Such substances are called adjuvants, and they function to activate APCs to express costimulators and cytokines for T cell activation
-most adjuvants are products of microbes or microbe mimics
-adjuvants trick immune system into responding to purified protein antigens
-Proteins homologous to CD28 also are critical for limiting and terminating immune responses – CTLA-4 recognizes CD80/86 on APCs, and PD-1 which recognizes different but structurally similar ligands on many types
-CTLA-4 and PD-1 are induced in activated T cells and inhibit responses to some tumors (PD-1 inhibits responses against viruses too)
Stimuli for CD8 T cell Activation – activation of CD8+ T cells stimulated by MHC-I associated peptides and requires Costimulation and/or helper T cells
-CD8 T cell activation requires cytoplasmic antigen from one cell to be cross-presented by dendritic cells
-differentiation of CD8+ T cells to cytotoxic T cells and memory cells requires concomitant activation of CD4 T cells
-when virus-infected cells are ingested by phagocytes, both MHC-I and MHC-II are expressed, and so both CD8 and CD4+ T cells are activated near one another
-CD4 cells may produce cytokines or membrane molecules to help CD8 cells
Biochemical Pathways of T Cell Activation – on recognition of antigens and costimulators, T cells express proteins that are involved in proliferation, differentiation, and effector functions
-biochemical pathways that link antigen recognition with T cell responses consist of enzyme activation, recruitment of adaptor proteins, and production of transcription factors
-TCR complex, CD4/8 and CD28 coalesce and form a peripheral ring to optimally induce cells
-region between T cell and APC that are together is called immunological synapse
-some effector molecules/cytokines may be secreted here as well as enzymes that mediate/inhibit signaling molecules
-CD4/8 facilitate signaling through protein tyrosine kinase called Lck that is noncovalently attached to these recepotrs
-Signaling proteins associated with TCR, CD3 and ζ-chain contain motifs called immunoreceptor tyrosine-based activation motifs (ITAMs) which is critical for signaling
-Lck is brought to TCR complex by CD4/8 and phosphorylates ITAMs of CD3 and ζ chains
-phosphorylated ITAMS of the ζ chain become docking sites for tyrosine kinase ZAP-70 which is also phosphorylated by Lck and made enzymatically active
-ZAP-70 phosphorylates adaptor proteins assembling near TCR complex to initiate calcium NFAT pathway, RAS/RAC-Map Kinase pathway, and PKC-NF-kB pathway, and PI-3 kinase pathway
-Nuclear Factor of Activated T cells (NFAT) is a transcription factor in an inactive phosphorylated form in resting T cells
-NFAT depends on intracellular Ca and is initiated by ZAP-70 phosphorylation and activation of enzyme called phospholipase Cγ (PLCγ), which catalyzes plasma PIP2 à IP3, which binds to IP3 receptors on ER to stimulate Ca release
-cytoplasmic Ca binds calmodulin to activate calcineurin, which removes phosphates from NFAT, enabling it to enter nucleus to bind promoters of T cell growth factor IL-2
-cyclosporine is a drug that inhibits calcineurin to inhibit NFAT-dependent production of cytokines by T cells (it is an immunosuppressive drug)
-Ras/Rac-Map Kinase Pathways include GTP binding Ras/Rac proteins to initiated a cascade that activates MAP kinases, and are initiated by ZAP-70 dependent phosphorylation
-ZAP-70 phosphorylation and accumulation of adaptor proteins at plasma membrane recruits Ras/Rac, and activated by exchanging GDP for GTP to initiate cascades leading to activation of distinct MAP kinases
-terminal MAP kinases, known as ERK and JNK induce expression of c-Fos and phosphorylation of c-Jun
-c-Fos combines with c-Jun to form activating protein 1 (AP-1) to activate T cell genes
-another big pathway involves activation of Θ isoform of serine-threonine kinase called protein kinase C (PKCΘ) and activation of transcription factor NF-kB
-PKC is activated by DAG (generated by PLC-mediated hydrolysis of inositols)
-PKCΘ acts through adaptor proteins recruited to TCR complex to activate NF-kB, which exists in resting T cell cytoplasm bound to inhibitor IkB, and after TCR-induced signals, IkB is destroyed and NF-kB is released and moves to nucleus where it induces many genes
-T cell signaling involves lipid kinase called PI-3 Kinase –which phosphorylates PIP2 à PIP3, and PIP3 is required for activation of serine-threonine kinase called protein kinase B, or Akt, which has roles in expressing anti-apoptotic proteins and promoting survival of antigenic T cells
-PI-3 kinase/Akt pathway triggered by TCR and CD28 and IL-2 receptors
-NFAT, AP-1, NF-kB stimulate transcription of cytokines, cytokine receptors, cell cycle inducers, and effector molecules such as CD40L, and all of these are induced by antigen recognition
Functional Responses of T cells to Antigen and Costimulation – many responses of T cells are mediated by cytokines that are secreted by T cells and act on themselves and other cells
Secretion of Cytokines and Expression of Cytokine Receptors – in response to antigen and costimulators, CD4 cells especially rapidly secrete cytokines
-cytokines are diverse group of proteins that function as mediators of immunity and inflammation
-first cytokine produced by CD4 cells after 1-2 hours of activation is IL-2, and activation also rapidly enhances T cells ability to bind to IL-2 by increasing expression of IL-2 receptor
-IL-2 is a 3-chain molecule, and Naïve T cells express 2 chains but not the third until the cell has been activated by antigen and costimulators; IL-2 produced by antigen stimulated T cells preferentially binds on same T cell
-function of IL-2 is to stimulate survival and proliferation of T cells to result in increased numbers
Clonal Expansion – T cells activated by antigen and Costimulation begin to proliferate within 1-2 days to result in expansion of antigen-specific clones to provide large pool of T cells from which effector cells can be generated
-expansion of CD4+ cells is much less than CD8+ cells, up to 100-1000-fold, which may reflect differences in function (CD8 directly kills and CD4 releases cytokines where small # is sufficient)
Differentiation of Naïve T cells into Effector Cells – some progeny of antigen-stimulated, proliferating T cells differentiate into effector cells whose function is to eradicate infections
-genes encoding cytokines (CD4) or cytotoxic proteins (CD8) and begins in concert with clonal expansion, and differentiated cells appear 3-4 days after exposure to microbes
-effector cells of CD4+ helper lineage function to activate phagocytes and B lymphocytes by expressing surface molecules and secreting cytokines
-most important cell surface protein in effector CD4 cells is CD40L, which resembles TNF
-CD40L binds to CD40 on macrophages, B cells, and dendritic cells to activate them
Subsets of CD4 helper T cells Distinguished by Cytokine Profiles – CD4 helper T cells may differentiate into at least 3 subsets of effector cells that produce dif cytokines: Th1, Th2 and Th7
Th1 Subset – Th1 cells stimulate phagocyte-mediated ingestion and killing of microbes, a key component of cell-mediated immunity
-most important cytokine produced is interferon-γ (IFN-γ), which is a potent activator of macrophages to kill ingested microbes
-IFNγ also stimulates production of antibody isotypes to promote phagocytosis
-these antibodies bind to phagocyte Fc receptors and activate complement
Th2 Subset – stimulate phagocyte-independent, eosinophil-mediated immunity, especially effective against helminthic parasites
-Th2 cells produce IL-4 which stimulates production of IgE antibodies, and IL-5, which activates eosinophils
-IgE activates mast cells and binds to eosinophils to kill helminthic parasites
-IgE coats heminths, eosinophils bind IgE and are activated to release granules to kill
-IL-4 and IL-13 promote expulsion of parasites from mucosal organs
-cytokines from Th2 also activate macrophages to synthesize ECM proteins for tissue repair
-some cytokines like IL-4, IL-10, and IL-13 inhibit microbicidal activity of macrophages
Th17 Subset – induce inflammation which functions to destroy bacteria and fungi and can contribute to inflammatory diseases
-secretes IL-17 and IL-22
Development of Th1, Th2 and Th17 – generation of these subsets is regulated by stimuli that naïve CD4 cells receive when they encounter microbial antigens
-Most important transcription factors differentiation into the subsets are:
-Th1 = T-bet
-Th2 = GATA-3
-Th17 = RORγT
-all of these work in concert with transcription factor STATs
Th1 Differentiation – driven by cytokines IL-12 and IFN-γ. In response to bacteria and viruses, dendritic cells and macrophages produce IL-12 and NK cells produce IFN-g, and when T cells recognize antigens from these microbes and are exposed to these cytokines, they differentiate into Th1 cells
-Th1 produce IFN-g which activates macrophages to KILL but also promotes more Th1 and inhibits Th2 and Th17
Th2 Differentiation – stimulated by IL-4 produced by active mast cells or eosinophils stimulated by helminth parasites, or by the T cells themselves
Th17 Differentiation – requires IL-6 and IL-1 produced by macrophages and dendritic cells; IL-12 produced by the same cells, and TGF-β
Differentiation of CD8 T cells into CTLs – CD8 cells activated by antigen and costimulators differentiate into cytotoxic T lymphocytes that are able to kill infected cells
Development of Memory T cells – fraction of antigen-activted T cells differentiates into long-lived memory cells
-memory cells found in lymphoid organs, in peripheral tissues (mucosa and skin) and in circulation
-memory T cells require signals from cytokines such as IL-7 to stay alive