T-Cell Mediated Immunity Regulation and termination of immune ...
T-Cell Mediated Immunity Regulation and termination of immune responses
Lecture Objectives Students should know: What molecules of T cells are involved in T cell activation? Priming and effector phases of T cell response Antigen presenting cells Immature versus mature dendritic cells Co-stimulation signal/CD28/Anergy What happens in T cells after TCR activation? Th1 cells/Cell-mediated immunity Granuloma Th2 cells/ Humoral Immunity Regulatory (suppressor) T cells Functions of Th1/2 cell cytokines Naïve versus memory T cells? Memory and effector T cells?
T cells: Development in thymus Migration out of thymus into blood (Naïve T cells) Priming phase: Antigen-dependent differentiation in lymphoid tissues (to become m emory and effector T cells) Effector phase: Migration to sites of infection and effector function Human: Fetal development Birth School Job
Figure 6-2 Green: MHC class II Red: Lysosomal protein
Naïve T cells enter secondary lymphoid tissues and encounter antigens presented on dendritic cell MHC Infection at skin: Dendritic cells pick up antigen, undergo maturation, and migrate to 2 o LT. Microorganisms in the blood: Trapped in spleen. Infection at mucosal surface: Collected in tonsils, Peyer’s patch, appendix and Bronchial-associated lymphoid tissues . In lymph nodes, naïve T cells migrate from blood to LN through specialized blood vessels called HEV (high endothelial venules).
Figure 6-5 Naive and some memory T cells express L-selectin to adhere on HEV in lymph nodes. Each leukocyte subset expresses unique sets of adhesion molecules and chemokine receptors. Endothelial cells in different tissues express different counter-receptors.
Multi-step homing process into LT <ul><li>Naïve T cells express L-selectin and a chemokine receptor CCR7. </li></ul><ul><li>Endothelial cells express L-selectin receptors (GlyCAM-1 and CD34) and CCR7-activating chemokines SLC and ELC. </li></ul>Rolling (L-selectin) chemokine firm adhesion diapedesis Integrins
Fluorescently labeled T cells migrating into Peyer’s patches Kim lab
Interactions between CD4 and MHC II-expressing antigen presenting cells: (T cell left : APC right) (i) LFA-1 : ICAM-1 (adhesion) (ii) CD2 : LFA-3 (adhesion) (iii) CD4 (Lck) : MHC II (co-activation with TCR) (iv) TCR/CD3 : MHC II/peptide (activation of T cell) (v) CD28 : B7-1/B7-2 (co-activation with TCR) (h) End result : activation of transcription factors. CORE
<ul><li>Interactions between CD8 and MHC I-expressing antigen presenting cells: </li></ul><ul><li>(T cell left : APC right) </li></ul><ul><li>LFA-1 : ICAM-1 (adhesion) </li></ul><ul><li>(ii) CD2 : LFA-3 (adhesion) </li></ul><ul><li>(iii) CD8 (Lck) : MHC I (co-activation with TCR) </li></ul><ul><li>(iv) TCR/CD3 : MHC I / peptide (activation of T cell through PIP2/DAG, IP3 pathway) </li></ul><ul><li>(v) CD28 : B7-1/B7-2 (co-activation with TCR) </li></ul>CORE
Interaction of T cells and APC Interaction between TCR and Ag/MHC alone is not strong enough to sustain the contact between T cells and APC Integrins and their receptors on T and APC strengthen the interaction so that TCR and CD28 receptors on T cells can receive prolonged and stable signals. Signals through the integrins on T cells also enhance T cell activation. TCR-Ag/MHC
TCR-Ag/MHC interaction signals for LFA-1 to bind more firmly to ICAM-1 <ul><li>This is to ensure stable and prolonged interaction between T cells and APC. </li></ul>
Co-stimulatory receptor: CD28 (or CTLA4):B7 <ul><li>For full activation, T cells require signals from both TCR and CD28. </li></ul><ul><li>TCR activation alone is not sufficient to activate T cells. </li></ul><ul><li>CD28 and CTLA4 are T cell receptors for B7 molecules on APC. </li></ul><ul><li>CD28 is expressed by all T cells </li></ul><ul><li>CTLA4 is expressed by activated T cells. </li></ul><ul><li>CTLA4:B7 interaction negatively regulate T cell activation, limiting T cell activation and proliferation. </li></ul><ul><li>CD28: Accelerator </li></ul><ul><li>CTLA4: Brake </li></ul>
Figure 6-10 <ul><li>Professional antigen presenting cells </li></ul><ul><li>They are dendritic cells, B cells and macrophages </li></ul><ul><li>They express high levels of MHC I and II molecules </li></ul><ul><li>They express co-stimulatory molecules (e.g. B7) </li></ul><ul><li>They are differently distributed in lymphoid tissues (LT). </li></ul><ul><li>Dendritic cells are particularly important for initiation of T cell activation in lymphoid tissues. </li></ul>Only these cells express B7
Different features of the three types of APC <ul><li>Macrophages are good at presenting extracellular bacteria </li></ul><ul><li>DCs present bacterial and viral antigens </li></ul><ul><li>B cells can efficiently present soluble antigens </li></ul>
Figure 6-12 <ul><li>Bacterial products such as LPS and cell wall products, and cytokines such as TNF-a differentiate immature DCs to mature DCs. Mature DCs express CCR7 and migrate from sites of infection to LT. </li></ul><ul><li>Immature DCs are better in phagocytosis but lack co-stimulatory activity. </li></ul><ul><li>Mature DCs express higher levels of MHCs and B7. They are better in antigen presentation and T cell activation but lack the phagocytic ability. </li></ul>DC maturation
Figure 6-16 T cell activation process CD4 for adhesion to MHC II and signal transduction (lck) Phosphorylation cascade occurs at early stage of signal tranduction
Figure 6-17 part 1 of 2 Intracellular events during T cell activation
Major events involved in T-cell activation Event Example Cell-cell interaction T cell-APC CTL-target cell Receptor-ligand binding TCR-antigen/MHC Transmembrane signal transduction Activation of Lck Generation of second messengers 1,4,5-IP 3 and DG Second-messenger effects Ca 2+ mobilization Protein kinase C activation Biochemical pathways Phosphatidylinositol pathway Ras pathway Cellular events Secretion of cytolytic granules Early gene activation c-Myc, c-Fos Intermediate gene activation Lymphokines, lymphokine receptors, nutrient receptors Late gene activation Genes involved in cell proliferation IL-2, IL-2R , VLA-2 etc
A consequence of T cell activation: Enhanced IL-2 production and induction of IL-2R expression for T cell proliferation <ul><li>Immuno-suppressive drugs such as cyclosporin A and FK506 block NFAT activation induced by TCR activation, thus blocking production of IL-2. </li></ul><ul><li>Therefore these drugs inhibit T cell proliferation and activation. </li></ul>
Figure 6-23 Activated T cell express memory/effector type molecules
TCR activation without CD28/B7 co-stimulation leads to T cell anergy (unresponsiveness) B7 is expressed only by activated professional antigen presenting cells Therefore, recognition of Ag presented by immature APC (which don’t express B7) can induce T cell anergy.
<ul><li>Functional subsets of CD4 cells based on cytokine profile. (1) T naive cell = Low levels of IL-2, 4, 5, 6, 10, 13, TNF -alpha , IFN - γ (these may not be detectable; all activated T cells produce IL-2) (2) TH1 = IFN - γ, IL-2(Type 1 cytokines promote cell mediated immunity) (3) TH2 = IL- 2, 3, 4, 5, 9, 13 (Type 2 cytokines promote humoral immunity) (4) TH17 = IL-17, 22 (promote cutaneous inflammation) (5) Subset control: TH1 polarization is promoted by IL-12 and IFN - γ IL-4 &10 inhibit. TH2 polarization is promoted by IL-4; IFN - γ inhibits TH17 polarization is promoted by IL-1 and IL-23; IL-4 and IFN - γ inhibit </li></ul>CORE
T cell differentiation to Th1 or Th2 IgE response for defense against parasites or allergic responses is mediated by Th2 cells Cell-mediated Humoral/Ig response Regulatory T cells: suppress the function of Th1/2 cells <ul><li>T cells can differentiate to Th1, Th2 or other T cell subsets. </li></ul><ul><li>Many people believe that only Th2 cells can induce humoral immune response, while Th1 cells induce only CMI. But this is an overly simplified text book style view. </li></ul><ul><li>Actually Th1 and Th2 cells are required for different aspect s of humoral immune response. </li></ul>TCR activation by APC Thymus
Th1 Th2 IL-4 IFN- Tnp (Nonpolarized T cells) Th1 and Th2 cells produce different cytokines Th0 (IL4 + IFN- + ) Anti-microbial Anti-parasitic
Effector T cell subsets Tregs TGF 1 IL10 etc FoxP3 Target Activation signal No activation
Cytokine secretion and biological activities of T H 1 and T H 2 Subsets Type 1 Type 2 IFN- IL-4 IL-5 IL-13 Cell-mediated Immune response (intracellular Organisms) Some humoral responses that promote opsonization Anti parasitic, and IgE responses General humoral response T cell
12. Cell mediated immune (CMI) responses <ul><li>a. Function: </li></ul><ul><li>(1) Control intracellular pathogens, tumors </li></ul><ul><li>(2) Mediate transplant rejection </li></ul><ul><li>(3) Mediate Type IV hypersensitivity </li></ul><ul><li>(4) Contribute to granuloma formation </li></ul><ul><li>(5) Contribute to chronic inflammation </li></ul>Th1 cells regulate CMI CORE
12. CMI <ul><li>Predominant cells and mediators </li></ul><ul><li>(1) TH1 lymphocytes (activated by Ag, IL-12 and IFN release IL-2 and IFN- induce TC, NK, and macrophage activation </li></ul><ul><li>(2) CD8+ TC1 and TC2 (activated by Ag/MHC I [endogenous pathway] and TH1 cytokines) have cytokine profile similar to TH1 and TH2, respectively; both are cytotoxic via TNFβ, perforin, granzymes, and Fas-induced apoptosis </li></ul><ul><li>(3) NK cells (see above for activation, etc.) release IFN- which activates macrophages and stimulates TH1 activity; also release GM-CSF that stimulates PMNs; lyse targets predominantly via granule components (e.g. perforin) </li></ul><ul><li>(4) Macrophages: next page </li></ul>CORE
<ul><li>Monocytes/macrophages [mononuclear phagocytes] (activated by IFN and TLR ligands [e.g. LPS] or other cytokines [e.g. GM-CSF, TNF ) </li></ul><ul><li>(a) Tissue types: Kupffer, alveolar, etc. </li></ul><ul><li>(b) Surface markers: C3b receptor (CR-1), LFA-1, Fc receptor, MHC II </li></ul><ul><li>(c) Macrophages play a central role in CMI and inflammation: </li></ul><ul><li>(i) Antigen presenting cell, important in killing bacteria and lysing tumor cells </li></ul><ul><li>(ii) effector/mediator of delayed type hypersensitivity (DTH, see Type IV hypersensitivity) </li></ul><ul><li>(iii) release IL-12 & induces TH1 response </li></ul><ul><li>(iv) release IL-10 & suppress immune responses </li></ul><ul><li>(v) release IL-1, 6, TNF and , INF and </li></ul><ul><li>(vi) release enzymes, coagulation factors, complement components, superoxide (etc.), leukotrienes, and prostaglandins </li></ul>12 b (4) Macrophages CORE
Mediators of Cell Mediated Immunity <ul><li>Th1 cells: DCs present Ag to naïve T cells and activate them. IL-12 produced from dendritic cells induce polarization of T cells into Th1. Produce IFN-g which activates CD8 T, NK and macrophages </li></ul><ul><li>CD8 T cells: also called T cytotoxic T cells. Kill virus-infected or tumor cells by TNF-b, perforin, granzymes and FAS ligand. </li></ul><ul><li>NK cells: IL-2, IL-12 and IFN-g activate NK cells to make them more efficient killer cells. </li></ul><ul><li>Macrophages: They are also activated by Th1 cytokine IFN- . Activated macrophages are more efficient in killing of engulfed or intracellular pathogens. They produce IL-12, IL-10, IL-1, IL-6, TNF- and IFN- , and release inflammatory mediators (coagulation factors, complement components, superoxide, leukotrienes and prostaglandins). </li></ul>
CD8 T cells (also called cytotoxic T cells, Tc or CTL) <ul><li>Cytolytic activity of CTL is promoted by IL-2, IL-12 & IFN </li></ul><ul><li>Activated CTLs lyse targets with TNF , degranulation (perforin, granzymes), and/or Fas-induced apoptosis. </li></ul>
Cytotoxic CD8 T cells detect infected cells through TCR/Ag recognition and kill them CD8+ T cells need the CD28 signal from APC at priming stage to become effector CD8+ cells, But effector CD8 + cells don’t need the CD28 signal (at the effector stage) to kill target cells (APOPTOTIC cell death) .
TCR activation polarization of CD8+ T cell cytoskeleton release lytic granules to kill target cells Cytotoxins of CD8+ cells: Granzyme Granulolysin Perforins FAS-ligand: activates FAS to induce apoptosis
12 c. Role of endothelium <ul><li>Inflamed enothelial cells express P- and E-selectin and recruit effector cells that mediate CMI (i.e. phagocytosis and cell killing) </li></ul><ul><li>Detection of cell killing activity by CD8 T cells: </li></ul><ul><li>Cytolytic assays (51Cr release assay) </li></ul>51Cr CD8 51Cr CD8 released Target cells 51Cr 1. Labeling 2. Co-culture 3. Lysis & release of 51Cr CORE
Figure 6-33 Th1 cells activate macrophages, making them highly microbicidal
Granuloma Aggregated infected macrophages surrounded by T cells A sign of chronic microbial infection Pathogens persist in macrophages for long time Insufficient macrophage activation by Th1 cells Typical in M. tuberculosis infection It is a defense mechanism that limits the spread of pathogens A compromised situation between pathogens and the immune system
IFN- promotes antigen presentation, Th1 cell development, production of opsonizing Ab and macrophage activation
Figure 6-36 Th2 cells activate B cells through cytokines and CD40L for IgG and IgE response IgE, IgG4
Th1/2 choice made by the immune system has profound effect on the outcome of some diseases (e.g. Leprosy) Tuberculoid Leprosy Infection by Mycobacterium leprae. Th1 cells produce IFN- Activation of Macrophages Destruction of intracellular bacteria. Milder damage. Patients survive. Lepromatous Leprosy Infection by Mycobacterium leprae. Th2 cells cannot activate Macrophages. Unchecked proliferation of intracellular bacteria. Gross destruction of tissues. Fatal.
Th17 cells Another effector T cell subset Induced from naïve CD4+ or CD8+ T cells Induced by IL-1, IL-6, or IL-23 Produce IL-17 and IL-22; IL-17 induces expression of G-CSF and chemokines for inflammation with neutrophils; IL-22 induces anti-bacterial proteins. Important for anti-bacterial and anti-fungal infcetion; and chronic autoimmune inflammation in skin, joints and CNS IL-2, IL-4 and IFN- inhibit the generation of Th17 cells
e. Regulatory T cells (also called Tregs or FoxP3+ cells) <ul><li>Specialized T cells that suppress immune cells such as T cells (naïve, Th1 and Th2), B cells, dendritic cells, macrophages etc. </li></ul><ul><li>FOXP3+ CD4+ CD25+T cells constitute a major subset of regulatory cells. </li></ul><ul><li>Express CD25 and CTLA4 </li></ul><ul><li>Produce TGF-β, and/or IL-10 to suppress target cells . </li></ul><ul><li>Prevent autoimmune diseases </li></ul><ul><li>Regulatory T cell deficiency due to mutations of FOXP3 causes immune dysregulation, polyendocrinopathy, enteropathy, X linked syndrome (called IPEX or XLAAD ) autoimmune diseases in multiple organs </li></ul><ul><li>FOXP3 is the master transcription factor for regulatory T cells </li></ul><ul><li>Commonly called Tregs. </li></ul>
Regulatory T cells Effector T cells I nfection Cancer Hyper-responses or Autoimmune diseases Balanced for health Too much Too much TGF- 1 IL-10 IL-4/5/13 IFN-
13. Regulation and termination of immune responses <ul><li>a. Regulation by the nature of the antigen (e.g. polysaccharides IgM) </li></ul><ul><li>b. Regulation by individual history </li></ul><ul><li>(and genetic background), antigen dose and route of administration </li></ul><ul><li>(e.g. oral immunization tolerance) </li></ul><ul><li>c. Regulation by antigen presenting cell </li></ul><ul><li>(1) Co-stimulatory signals (e.g. B7 ligands for CD28 or CTLA4; </li></ul><ul><li>Activated T cells express CTLA4 for downregulation of T cell activation) </li></ul><ul><li>(2) Suppressive cytokines (e.g. IL-10 and TGF-beta) </li></ul><ul><li>d. Modulation by antibody </li></ul><ul><li>Immune complexes, receptor cross-linking, and activation and suppressive Fc receptors (e.g. Fc receptors of NK cells and B cells) </li></ul><ul><li>e. T cells (regulatory T cells and other cells) </li></ul><ul><li>(1) Th1/2 and Tc1/2 subsets and cytokine control (e.g. IL4 versus IFN-g) </li></ul><ul><li>(2) soluble forms of TCRs (?) and cytokine receptors </li></ul><ul><li>(3) CD4+CD25+ regulatory T cells suppress immune responses </li></ul><ul><li>(4) FAS/FASL-mediated lymphocyte apoptosis </li></ul><ul><li> (activated lymphocytes express FAS, susceptible to cell death) </li></ul>CORE
13. Regulation and termination of immune responses <ul><li>f. Neuroendocrine controls </li></ul><ul><li>(1) hormonal controls (e.g. stress-induced immunosuppression by glucocorticoids and catecholamines. </li></ul><ul><li>g. Tolerance </li></ul><ul><li>(1) Central tolerance (positive and negative selection) </li></ul><ul><li>(2) Peripheral tolerance (clonal deletion, apoptosis and anergy) </li></ul>CORE
Brain Hypothalamus Pituitary Adrenal gland Immune cells (through specific receptors) (results: suppression of immune cells) Testes/ovary Cortisol Corticosterone testosterone , estradiol , progesterone Corticotropin LH/FSH Neuroendocrine controls Negative regulation of the immune system by stress hormones and sex hormones Stress pathway Reproductive pathway
CNS and Liver Activation of the immune system TNF- /IL-1/IL6 fever, sleepiness, fatigue, loss of appetite decreased libido Regulation of the CNS by inflammatory cytokines Stress Cortisol Infection
CNS regulates the immune system (IS): The central nervous system affects the immune system through the neuroendocrine humoral outflow via the pituitary, and through direct neuronal influences via the sympathetic, parasympathetic (cholinergic) and peptidergic/sensory innervation of peripheral tissues. Thus, circulating hormones or locally released neurotransmitters and neuropeptides regulate major immune functions such as antigen presentation, secretion of cytokines, chemokines and antibodies, selection of T helper (Th)1 or Th2 responses, lymphocyte activity, proliferation and traffic. IS regulates CNS: Certain cytokines such as interleukin (IL)-1, IL-6 and tumor necrosis factor (TNF)-a , released during an immune response activate the central components of the stress system, alter neurotransmitter networks activity and, thus induce fever, sleepiness, fatigue, loss of appetite and decreased libido. IS affects the liver: Cytokines activate the hepatic synthesis of acute phase proteins - changes referred to as 'sickness behavior' and 'acute-phase response', respectively. Adapted from http://www.endotext.org/adrenal/adrenal28/adrenalframe28.htm By Ilia J. Elenkov