L11 CTL Killing, TREG Cells, Memory Cells, NK Cells, and ILCs PDF

Cytotoxic T Cells and Other Lymphocytes POD Immunology 231 Fall 2024 L11 Garwin Kim Sing  Learning Objectives 1. Effector Mechanisms of Cytotoxic T Cell and NK cell-mediated lysis of target cells 2. Characteristics of memory T cells 3. Characteristics of regulato...

Cytotoxic T Cells and Other Lymphocytes POD Immunology 231 Fall 2024 L11 Garwin Kim Sing  Learning Objectives 1. Effector Mechanisms of Cytotoxic T Cell and NK cell-mediated lysis of target cells 2. Characteristics of memory T cells 3. Characteristics of regulatory T cells (TREGS) 4. NK Cells and recognition – “Missing or Altered self” 5. Innate Lymphoid Cells (ILC’s) Cytotoxic T Cells (CTLs)  Function and Properties of Cytotoxic T Cells  CD3+CD8+ recognizing antigen on class I MHC.  CTLs are the major killers of virus-infected cells and cancer cells  In addition to their killing ability, CTLs are also a major source of pro-inflammatory cytokines, mostly IFN  Activation from naïve cells to effector cells relies on APCs to undergo cross-presentation to acquire target cell antigens.  CTL activation may also require TH help. Activation of Cytotoxic CD8+ T Cells Activation of naïve CTL requires Class I and Class II antigen presentation on the same APC to activate TH1 and CTLs at the same time. Cross- presentation. Effector T Naïve T cells cells  T Cell Clonal Expansion Pan T Lymphocytes CD3+ CD3+  T cells CD4+ (2:1) CD8+ TH Cells CTL  CTL Killing by Exocytosis of 1. Effector CTLs Cytotoxic Granules (and NK cells) contain stored lytic granules which contain cytotoxins – granzyme and perforin. 2. Once it interacts with target, a synapse is formed between CTL and target. Perforin punches holes into the target cell membrane allowing granzyme granules to enter the cell. Granzyme then activates caspases to induce the target cell to undergo apoptosis. 3. The CTL can then detach itself from the target cell, make new granules and kill more targets.  Intrinsic and Extrinsic Pathways of  Apoptosis can be cause byApoptosis intrinsic or extrinsic pathways.  The intrinsic pathway involves release of cytochrome C from the mitochondria which then activates caspases to form an apoptosome which activates caspase zymogens to initiate the caspase cascade resulting in programmed cell death. Mitochondria-mediated apoptosis is known as the intrinsic pathway.  The granzymes and perforin released by CTLs initiates this intrinsic pathway. Surface Protein on CTL Ligand on Target Cell  CTLs can also induce apoptosis via the extrinsic pathway: Fas-L Fas Tumor Necrosis Factor α (TNFα) TNFR-I Lymphotoxin β (LTβ) TNFR-II CTLs express membrane proteins like Fas ligand (Fas-L), TNFα and Lymphotoxinβ (LTβ). When these bind to their corresponding receptors (Fas, TNFR-I and TNFR-II) expressed on target cells, apoptosis can also be induced by caspase-dependent mechanisms (but not involving cytochrome C). Memory Cells  Memory Cells mory cells do not require signals provided by antigenic exposure for survi y are extremely long-lived (months or years) compared to naïve or effec s (weeks or months). mory cells respond more rapidly to antigen stimulation than naïve cells sp he same antigen. number of memory cells specific for any antigen is greater than the num e cells specific for the same antigen. mory cells can migrate to peripheral tissues (e.g. skin) and respond to se osure to antigens at these sites. mory cells require less co-stimulation to function compared to naïve cells ntenance of memory cells is dependent on cytokines (IL-7) but not on eated antigen exposure Regulatory T Cells (TREGs)  TREG Cells 1. Characterized by expression or the transcription factor FoxP3 which directs expression of high levels of CD25 (IL-2R), and anti- inflammatory cytokines IL-10 and TGF-. 2. These anti-inflammatory cytokines TGFβ and IL-10 suppress T, B and NK cell activation. They also reduce the ability of APCs to stimulate T cells. 3. Main function is to maintain peripheral tolerance. 4. Express high levels of the negative co-stimulator CTLA-4 which competes with CD86(B7) on APC for binding to CD28 on T cells. 5. Generated mainly by self-antigen recognition in the thymus and recognition of self- and foreign antigens in peripheral lymphoid organs 6. Express high levels of the receptor for the T cell growth factor IL-2  Interleukin 2 (IL-2) T cell Growth Factor Stimulates the proliferation and differentiation of antigen-activated T cells (clonal expansion) Promotes survival of T cells by upregulating expression of Bcl-2, an anti-apoptotic protein Acts in an autocrine and paracrine manner Stimulates production of effector cytokines like IFN and IL-4 by T cells Important for the survival and function of TREG cells which express Natural Killer (NK) Cells  Natural Killer Cells (NK Cells)  NK cells recognize infected and stressed cells, and respond by killing the cells directly and by secreting IFN which activates macrophages to become more efficient at killing phagocytosed microbes.  While they have lymphoid morphology, they lack T cell receptors and are negative for CD3, CD4 and CD8.  Identified by expression of the surface molecule CD16 which is the Fc receptor for IgG and CD56  They also known as Large Granulocytic Lymphocytes (LGL).  Recognition and Killing By NK Cells  NK cells are controlled by a combination of negative and positive signals which determine whether to attack a target cell or to leave it alone.  They recognize self-MHC class I on healthy cells which give negative signals to the NK cell not to attack them.  They kill stressed cells, tumour cells or virus-infected cells where MHC is not expressed or altered. Missing or “altered self”.  They can kill targets by either of three mechanisms:  Perforin and granzyme-mediated lysis  They express a surface molecule TRAIL (tumor necrosis factor- related apoptosis-inducing ligand) which binds to TNFR superfamily ‘death’ receptors DR4 and DR5 on target cells resulting in apoptosis.  Antibody-dependent cellular cytotoxicity (ADCC).  The Activation of NK cells is Determined by a Balance Between Activating  NK and cells are regulated by both Inhibitory Receptors activating and inhibitory receptors. They distinguish infected and stressed cells from healthy cells.  The receptors recognize activating signals from cells needing to be killed and downregulatory signals from healthy cells.  The inhibitory receptors of NK cells  When a NK cell interacts with block signalling by activating another cell, the outcome is receptors and are specific for class I determined by the sum total of MHC molecules, which are up- and down-regulatory signals expressed on healthy cells. generated by the target cell surface.  There are two major families of inhibitory receptors: 1. Killer cell  One of the activating receptors is immunoglobulin-like receptors Antibody-Dependent Cellular Cytotoxicity  In addition to NKG2D, another activating receptor for killing is CD16, the Fc receptor for IgG.  When cell targets are coated by specific IgG, NK cells can bind to the Fc region via CD16. This brings the NK cell in close proximity to the target allowing it to discharge its perforin and granzyme granules.  This process is called antibody-dependent cellular cytotoxicity (ADCC).  Innate Lymphoid Cells (ILC’s)  Consists of 3 subgroups based on the types of cytokines they produce: 1. ILC1 – IFNγ. Protects against virus infections and intracellular pathogens 2. ILC2 – IL-4, IL-5, IL-13. Promotes mucosal and barrier immunity and against parasites. 3. ILC3 – IL-17, IL-22.Protection against extracellular bacteria and fungi.  Although they function as TH1, TH2 and TH17 responses, they do not circulate in blood or lymphatics but are tissue fixed.  They also don’t express a T cell receptor and most don’t express CD3.  They are activated by cytokines.  Reading Material Abbas, AK, Lichtman, AH and Pillai S. Basic Immunology: Functions and Disorders of the Immune System. 6th ed, 2020