T-Cell-Mediated Immunity PDF - ETH Zurich Lecture

T-Cell-Mediated Immunity Literature: Chapter 9, Janeway’s Immunobiology ETH Zurich Lecture on “Pharmaceutical Immunology I” Prof. Dr. Cornelia Halin Winter 535-0830-00L HS 2024  Revision from Chapter 1 Circulating ly...

T-Cell-Mediated Immunity Literature: Chapter 9, Janeway’s Immunobiology ETH Zurich Lecture on “Pharmaceutical Immunology I” Prof. Dr. Cornelia Halin Winter 535-0830-00L HS 2024  Revision from Chapter 1 Circulating lymphocytes encounter antigen in peripheral lymphoid organs, e.g. in lymph nodes Most T lymphocytes constantly recirculate between blood and lymph nodes: blood => lymph node => efferent lymphatic vessels => thoracic duct => blood Make this round several times / day to increase their chances of finding a cognate (specific) antigen: increases immunesurveillance Once they find a cognate antigen on an antigen-presenting DC: T cells proliferate (generate millions of copies) and differentiate into effector or memory cells 1  4. Effector mechanisms T cells that have encountered pathogen-derived antigen differentiate into different types of effector cells, which are specialized in fighting the pathogen Þ In this Chapter we will learn how T cells get primed (= activated) in secondary lymphoid organs, the different types of effector cells generated and the signals that induce them. 2 Content 1) Function of secondary lymphoid organs – sites of initiation of adaptive immunity 2) Antigen-presenting cells (APCs) 3) Priming of naïve T cells by pathogen-activated dendritic ells 4) General properties of effector T cells and their cytokines 5) T-cell-mediated cytotoxicity 3  1. SLO function 1. Function of secondary lymphoid organs (SLO) – sites of initiation of adaptive immunity Secondary lymphoid organs serve as anatomical crossroads for interactions of antigens and lymphocytes => If you do not remember: Revise the anatomy of SLOs (Chapter 1) 4  1. SLO function Similarities and differences between SLOs Similar cellular composition: T& B cells (90%), dendritic cells, macrophages, stromal cells (scaffold), endothelial cells Similarly defined B and T cell zones Positioning coordinated by chemokines CCL21/CCL19 (T cells) CXCL13 (B cells) Distinct routes of antigen arrival lymph nodes: afferent lymphatics, spleen: blood, Peyer’s patches: gut lumen Routes for cell entry / exit Involving blood and lymphatic vessels Naïve T cells recirculate several times per day through secondary lymphoid organs (SLOs) in search of antigen Example: blood => lymph node => efferent lymphatic vessels => thoracic duct => blood Aim: Increase chances of encountering antigen (helps immune surveillance) Description: Entry into lymph nodes occurs through High Endothelial Venules (HEVs; specialized endothelium) After entry, the naïve T cells migrates into the T ell area to meet with antigen- presenting dendritic cells (DCs) If they do not encounter a cognate antigen: The naïve T cells leaves the lymph node via efferent lymphatics If they encounter a cognate antigen: The activated T cell proliferates and looses the ability to exit from the node. A few days later the differentiated effector cells exit the node via efferent lymphatics  1. SLO function In presence of antigen, antigen-specific T cells are transiently retained in the lymph nodes Lasts 3-5 days During this time, naïve T cells become activated, proliferate and differentiate into effector cells Þ Increase cell numbers by several thousand-fold Þ Generate an “army” of cells to fight the infection Lymphocyte entry into lymphoid tissues depends on chemokines and adhesion molecules Blood flow velocity in capillaries (1 mm/sec) generates high sheer forces Extravasation occurs in distinct steps: multistep adhesion cascade Overall process of T cells extravasation through high endothelial venules (HEVs) in lymph nodes 1 2 3 Þ The same intravascular adhesion steps also occur during extravasation at sites of inflammation / infection. However, depending on the tissue and inflammatory stimulus, other chemokines and adhesion molecules are involved. Þ The set of chemokines/adhesion molecules expressed in a vascular bed dictates which type of leukocytes will be recruited into the tissue  Blocking leukocyte extravasation: A therapeutic target in autoimmunity a4b1 VCAM-1 a4b7 MadCAM Adapted from Sathish et al., Nat. Rev. Drug Discovery 2013 Natalizumab (Tysabri) blocks the a4 integrin subunit on effector T cells (forms part of integrins a4b1 and a4b7) inhibits interactions with VCAM-1 and MadCAM-1 (expressed in blood vessels in the CNS and mucosal tissues, respectively) What could be reduced leukocyte recruitment to inflamed CNS and intestine side effects of Natalizumab? approved for treatment of multiple sclerosis and Crohn’s disease  T cells constantly recirculate through secondary lymphoid organs HEV in the lymph node T cell RECIRCULATION blood circu- Lymph- lation lymph knoten node thoracic duct efferent DC T cell lymphatic Naive T cells recirculate vessel approximately 3-4 times / day through different lymph nodes This helps immunesurveillance HEV: high endothelial venule DC: dendritic cells Blockade of lymphocyte recirculation by modulation of sphingosine 1-phosphate receptors (S1PRs) Blocks emigration of T cells out of lymph node parenchyma (tissue) into efferent lymphatic vessels T cells are „stuck“ in the lymph node: even if a T cells gets activated by antigen in a lymph node, it can no longer leave the node to travel to the site of infection/ disease. First-in-class drug approved in 2010 for the treatment of the autoimmune disease multiple sclerosis: Fingolimod (FTY720, Novartis) 12 The egress of lymphocytes from lymph nodes is me- diated by a sphingosine 1-phosphate (S1P) gradient S1P: sphingolipid with chemotactic activity on T cells (and other leukocytes) S1P levels are low in tissue but high in lymph (e.g. in lymph nodes efferent lymphatics) To leave the lymph node, S1P receptor 1 (S1PR1)-expressing T cells sense the S1P gradient and transmigrate across lymphatic endothelial cells into the efferent lymphatics. FTY720 induces internalization and degradation of S1PR1. As a result, T cells no longer “find” the way out of the lymph node. Physiologic function of S1PR1: If a T cell gets activated in the lymph node by antigen, it transiently downregulates S1PR1, in order to remain in the lymph node, proliferate and differentiate into and effector cell. 2. Professional antigen-presenting cells (APCs) 3 main types of professional antigen- presenting cells exist Macrophages, dendritic cells (DCs), B cells These constitutively express MHCII have the ability to take up antigen Þ Different distribution in lymph nodes  2. APCs Different routes by which DCs can take up, process and present proteins antigens Cross-presentation: Ability to present phagocytosed / macropinocytosed antigen on MCH-I (rather than MHC-II) Important for induction of cytotoxic T cells Dendritic cells, but not macrophages or B-cells are able to cross-present  2. APCs Langerhans cells and dermal dendritic cells take up antigen in the skin and transport it via afferent lymphatic vessels to draining lymph node Langerhans cells in the mouse epidermis Langerhans cells: specialized type of dendritic cells (DCs) in the epidermis Microbe-induced TLR-signaling in tissue-resident dendritic cells (DCs) induces their migration to lymphoid organs and enhances antigen processing PAMP: pathogen associated molecular pattern Upon encountering PAMPs, DCs undergo TWO maturation steps: 1) Upregulation of CCR7 for migration towards CCL21- expressing lymphatics and into lymph nodes, and enhanced antigen processing 2) Upregulation of MHC and costimulatory molecules (e.g B7 = CD80, CD86) to boost T cell activation in dLNs  2. APCs B cells can use their surface immunoglobulin to present specific antigen very efficiently to T cells Þ B cell receptor-mediated uptake allows to enrich for presentation of antigen-derived peptides on the B cell’s MHCII molecules Þ BUT: B cells are much less efficient in priming naïve T cells, since they express less co-stimulatory molecules than dendritic cells  2. APCs Dendritic cells are called “professional antigen-presenting cells” => activation of naïve T cells typically requires antigen presentation by dendritic cells  3. T cell priming 3. Priming of naive T cells by pathogen- activated dendritic cells dendritic cell T cell Priming: first-time activation of a naïve T cells typically only possible upon encountering antigen on a dendritic cell (higher costimulatory molecules than e.g. macrophages of B cells) typically takes place in a secondary lymphoid organ (not in peripheral tissue) 20  3. T cell priming T cells and DCs interact via an immune synapse cSMAC: inner ring, comprising peptide-MHC/ TCR complexes and costimulatory molecules pSMAC: outer ring, comprising adhesion molecules for stabilisation (integrin LFA-1 on T cells and ICAM-1 on DC) enable effective signaling Purpose of targeted release of cytokines immune synapse (important of T cell differentiation)  3. T cell priming Activation of naïve T cells requires 3 types of signals Peptide-MHC Specificity and activation B7 molecules (CD80, CD86): Co-stimulation (required for survival) Cytokines: Differentiation into a specific type of effector T cell (e.g. TH1, TH2, TH17)  3. T cell priming Autocrine T-cell survival signal: IL-2/IL-2 receptor signaling Upon activation, T cells express interleukin-2 (IL-2) and the IL-2 receptor alpha chain (CD25), which forms part of the high-affinity IL-2 receptor trimer (bg & a chain) Upregulation of IL-2 and CD25 is induced by T cell receptor (TCR) and co- stimulatory signaling (CD28) => via signals 1 & 2 (see last page)  3. T cell priming Activated T cells secrete and respond to IL-2 IL-2 signaling is targeted by powerful immuno- suppressive drugs: Cyclosporin Basiliximab (antibody directed against IL-2Ra (CD25))  3. T cell priming CTLA-4 is an inhibitory receptor for B7 molecules It competes wit CD28 for binding to B7 molecules Since CTLA-4 has a higher avidity than CD28 for B7, it typically “wins” => B7-CTLA- interaction blocks T cell activation, by preventing co- stimulation by CD28 4. General properties of effector T cells and their cytokines CD8+: cytotoxic T lymphocytes (CTLs) CD4+: TH1 – TH2 - TH17 – TFH – Tregs 26  4. Effector T cells Activation of T cells changes the expression of several cell surface molecules Þ Altered trafficking behavior and tissue tropism Þ e.g. less homing to lymph nodes (L-selectin) and more homing to peripheral tissues (PSGL-1, CD44, VLA-4)  Revision from Chapter 1 The categories of disease-causing microorganisms (pathogens) Size comparison: Different strategies neutrophils: 10-12 µm needed to fight different types of pathogens! T cell: approx. 7 µm macrophages: approx. 20 µm 28  Revision from Chapter 1 The major types of pathogens confronting the immune system and some of the diseases they cause => Specialized T cells help to fight different types of pathogens 29  4. Effector T cells Different types CD4 effector cells: specialized to fight different types of infections Variation in signal 3 induces CD4 T cell differentiation into different types of effector cells  4. Effector T cells Induction of lineage-specific transcription factors regulates T cell differentiation transcription factor Differentiation-inducing cytokines activate different STAT family transcription factors. This results in induction of lineage-specific transcription factors. => If the latter are defective, differentiation into the particular lineage is compromised  4. Effector T cells CD4 T-cell subsets can cross-regulate each other’s differentiation through the cytokines they produce Relationship TH1-TH2-TH17: IL-4 produced by TH2 cells inhibits development of TH1 or TH17 IFNg produced by TH1 cells inhibits development of TH2 and TH17 Tregs produce TGFb, which inhibits the development of TH1 and TH2 (but not of TH17 – see Fig.9.31!) Matching exercise: Match each CD4 T helper cell subset with its subset-specific cytokine and its respective effector function: T helper subset: cytokine produced specialized T cell function TH1 IL-17 Eradication of intracellular pathogens A TH2 IL-4 Response to extracellular bacteria TH17 IFNg Suppression of adaptive immunity Tregulatory IL-10 Control of extracellular parasites T helper subset: cytokine produced specialized T cell function TH1 IFNg Eradication of intracellular pathogens B TH2 IL-4 Control of extracellular parasites TH17 IL-17 Response to extracellular bacteria Tregulatory IL-10 Suppression of adaptive immunity T helper subset: cytokine produced specialized T cell function TH1 IL-17 Eradication of intracellular pathogens C TH2 IL-10 Suppression of adaptive immunity TH17 IFNg Response to extracellular bacteria Tregulatory IL-10 Control of extracellular parasites 34  5. Cytotoxic T cells 5. T-cell-mediated cytotoxicity CD8+ cytotoxic T cells: Recognize cells infected with intracellular pathogens  5. Cytotoxic T cells Cytotoxic T cells can respond to their target without co-stimulation 36 APC: antigen-presenting cell  5. Cytotoxic T cells T cell cytotoxicity requires the formation of an immunological synapse with the target cells An immunological synapse is an area of contact between T cell and antigen-presenting cell or between effector T cell and target cell Purpose: - stabilization of interaction - targeted release of effector molecules (cytokines, death-inducing molecules)  5. Cytotoxic T cells Cellular mechanism of T-cell-mediated cytotoxicity Specific recognition of antigen on a target cell induces T cell polarization Effector molecules are released towards the target cells 38  5. Cytotoxic T cells Cytotoxic CD8 T cells can induce apoptosis in target cells Apoptosis: programmed cell death; cell destroys itself from within chromatin condensation & DNA fragmentation, cell shrinking, blebbing Necrosis: induced by hypoxia, physical damage, antibody or complement activation bursting of the cell wall, release of lysosomal enzymes => auto-digestion  5. Cytotoxic T cells Lytic granules of cytotoxic T cells: Contain proteins that can trigger apoptosis => Same mechanism / same molecules in NK cells 40  5. Cytotoxic T cells Delivery of granzyme into the cytosol induces apoptosis Caspase-3 activation induces DNAse activity : => DNA fragmentation => apoptosis 41  5. Cytotoxic T cells Effector molecules are released from T-cell granules in a highly polar fashion Cytotoxic T cell with lytic granules Target cell  5. Cytotoxic T cells Cytotoxic T cells kill target cells bearing specific antigen receptors but spare neighboring cells Ligation of the TCR triggers new perforin and granzyme synthesis: => cytotoxic T cells are “serial killers” 43  4. Effector T cells Activation of CD8+ T cells mostly requires help from CD4+ T cells Requires simultaneous interaction of three cell types: CD4+ T cell & CD8+ T cell & APC Antigen-specific CD4+ T cells helps by: upregulating co-stimulatory molecules in APC synthesizing IL-2 for the activation of the CD8+ T cell 1) Why could it be beneficial for the immune system make the activation of CD8+ T cells dependent on simultaneous activation of a CD4+ T cells? 2) During an HIV infection CD4+ T cells are gradually lost (die). As a result, the patient develops AIDS (acquired immunodeficiency syndrome) and - if left untreated - dies of opportunistic infections. Knowing about the functions of CD4+ T cells, what type of infections (with which class of pathogens) would you expect? APC: antigen-presenting cell  Take-home messages Chapter 9 Induction of T cell immunity (i.e. T cell priming) is initiated in secondary lymphoid organs (SLOs) when naïve T cells encounter their cognate antigen on the surface of an antigen-presenting cell (APC) Typically, the APC is a dendritic cell expressing high levels of the costimulatory molecules B7.1 and B7.2 (also known as CD80 and CD86) Activated T cell produce IL-2, which is important for T cell activation, proliferation and survival (signal 1 & 2) The differentiation of activated T cells into different T helper subsets (TH1, TH2, TH17 or Treg) is dependent on the cytokines present during T cell activation (signal 3) While naïve T cells continuously recirculate between blood and SLOs, differentiated T cells, also called effector cells, change their trafficking behavior and gain ability to migrate into peripheral tissues, i.e. sites of infection Effector T cells are activated when they recognize cognate antigen on MHC and no longer require costimulatory signals Cytotoxic CD8+ T cells kill peptide-MHCI expressing target cells by induction of apoptosis: this requires the formation of an immunologic synapse and targeted release of cytotoxic granules containing granzyme and perforin 45