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Immunology chapter 8
 

Immunology chapter 8

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    Immunology chapter 8 Immunology chapter 8 Presentation Transcript

    • Chapter 8 T Cell-Mediated Immunity
    • T Cell-Mediated Immunity
      • Activation of naïve T cells on encounter with antigen
        • What happens when naïve T cell encounters its specific antigen for the first time and is stimulated to differentiate into an effector cell ?
          • T-cell activation or T-cell priming
      • The properties and functions of effector T cells
        • Describe the interaction of effector T cells with their specific antigen that are presented by APC or target cells.
    • Activation of naïve T cells on encounter with antigen
    • Process of T-cell activation
      • T-cell activation or T-cell priming
      • First stage of a primary adaptive immune response
        • 3 kinds of effector T cells
          • Cytotoxic CD 8 T cells (kill infected cells)
          • CD4 T cells
            • Function = secrete cytokines  activate other immune cells
              • Th1
              • Th2
    • Activation of naïve T cells on encounter with antigen
      • Upon infection, the immune system quickly recruits the small number of naïve T cells that are specific to the pathogen  make contact with antigens derived from the pathogen.
      • Recruitment involves secondary lymphoid tissues.
        • Antigen is brought in from outlying tissues via lymph .
        • Antigen encounters the T cells brought in via blood .
      • Examination of the activation of naïve T cells  effector T cells by professional APC (P-APC) within lymphoid tissues
        • Once activated  Ag-specific CD8 and Th1 cells are sent out to infected areas
        • Th2 cells stay in lymphoid tissues.
      • Interaction of a naïve T cell with antigen presented by cells other than P-APCs
        • leads to inactivation rather than activation of T cells
    • Dendritic cells carry antigens from sites of infection
      • The immune system does not initiate the adaptive immune response wherever a pathogen creates a site of infection .
      • The immune system captures the pathogen and takes it to the secondary lymphoid tissues.
          • Mediated by dendritic cells & macrophages
          • Process is same for infections in peripheral tissues, mucosal surfaces and blood
      • Dendritic cells & macrophages = sentinels in all body tissues
        • Activated with pathogen uptake and antigen processing and presentation by MHC I and II molecules.
    • Dendritic cells carry antigens from sites of infection
      • Dendritic cells – only known function is the triggering T-cell responses  highly specialized and effective.
        • Far superior to macrophages in stimulating naïve T cells.
        • Reason, dendritic cells are migratory cells  carry load of antigen from infection site  nearest secondary lymphoid tissues.
    • Macrophages = function in defense and repair of damaged tissues
      • Macrophages are resident cells (not migratory) and are heavily exposed to pathogens but have no opportunities to interact with naïve T cells .
      • However, some pathogens are carried passively  lymph, blood or other fluid  secondary lymph tissue  ingested by resident macrophages  contributes to T cell activation.
    • Dendritic cells take up antigen, migrate to lymphoid organs, and present foreign antigens to naïve T cells
      • Movement of dendritic cell from site of infection in the periphery to a secondary lymphoid organ causes changes in the dendritic cell’s surface molecules , functions and morphology .
      • In tissues , dendritic are active in the capture , uptake and processing of antigen – called immature dendritic cells .
      • In secondary lymphoid tissues, dendritic cells gain the capacity to interact with T cells – called mature or activated dendritic cells.
      • Mature dendritic cells have finger-like processes called dendrites that contact T cells in the cortex of lymph node .
    • Dendritic cells take up antigen, migrate to lymphoid organs, and present foreign antigens to naïve T cells
      • Wound in the skin
      • Langerhans’ cells (immature dendritic cells) take up the antigen and migrate to a nearby lymph node.
      • Settle in the T-cell areas.
      • Differentiate into mature dendritic cells.
      • Skin and peripheral tissue infection = T cell response in draining lymph nodes .
      • Blood infections = antigen enter the spleen
      • Respiratory mucosa infection = tonsils or other bronchial-associated lymphoid tissue ( BALT ).
      • Gastrointestinal infections = Peyers patches , appendix or gut-associated lymphoid tissues ( GALT )
      • Similar sequence of events in each case
    • Dendritic cells are developmentally related to macrophages
      • Immature dendritic cells phagocytose microbial antigens using DEC 205 receptors (among others)
      • Antigens can also be taken up nonspecifically by macropinocytosis in which a cell engulfs extracellular fluid (ECF).
        • Important for initiation of T-cell viral responses
          • Dendritic cells acquire viral antigens via infection or by taking up virus particles from the ECF or other infected cells .
    • Dendritic Cells Processing and Presentation
    • Dendritic cells change their functions on taking antigen from infected sites to secondary lymphoid tissues
      • MHC class II stained green and a lysosomal protein is stained red .
      • Cell bodies are difficult to discern
      • Dendrites contain endocytic vesicles that stain both for MHC class II & lysosomal protein giving rise to yellow fluorescence (combination of red and green stain).
    • Dendritic cells change their functions on taking antigen from infected sites to secondary lymphoid tissues
      • On activation and migration in the lymph to secondary lymphoid organs the morphology of the dendritic cell changes.
      • Dendritic cells stop phagocytosis  indicated by a partial separation of MHC class II (green) from the lysosomal protein (red)
    • Dendritic cells change their functions on taking antigen from infected sites to secondary lymphoid tissues
      • On reaching a lymph node “mature” dendritic cells begins antigen presentation to T-cells , stimulating the T cells instead of uptaking and processing antigen.
      • Lysosomal protein (red) is distinct from the MHC class II molecules (green).
      • MHC class II (green) is displayed at high density on the many dendritic processes .
    • Recirculating, Naïve T cells first encounter antigen on APCs in secondary lymphoid tissues
      • Naïve T cells enter lymphoid tissues via blood capillaries or from the afferent lymph .
      • T cells in blood:
        • Bind to endothelial cells of the thin-walled high endothelial venules (HEV).
        • Squeeze through vessel wall and enter the cortical region of the lymph node .
        • Encounter dendritic cells and use its TCR to examine the peptide:MHC complexes on dendritic surfaces.
      • When the T cell encounters a peptide:MHC complex that it can bind to, the T cell is retained in the lymph node and is activated , then proliferates and differentiates into a clone of effector T cells.
    • Recirculating, Naïve T cells encounter antigen during recirculation through lymphoid organs
      • Naïve T cells ( blue & green ) recirculate through secondary lymphoid organs, such as the lymph node.
      • These leave the blood through HEVs and enter the lymph node cortex , where they mingle with professional APCs (mainly dendritic cell and macrophages).
      • T cells that do not encounter their specific cells and antigen ( green ) leave the lymph node in the efferent lymph and eventually rejoin the bloodstream.
      • T cells that encounter antigen ( blue ) on antigen-presenting cells are activated to proliferate and to differentiate into effector cells.
      • These effector T cells can also leave the lymph node in the efferent lymph and enter the circulation.
    • Naïve T cells encounter antigen during recirculation through lymphoid organs
    • Recirculating Naïve T-cells
    • Once Ag-specific T cells are trapped in the LN by an APC….
      • Naïve T cells specific for an antigen are about 1 in 10 4 or 1 in 10 6 of the total T cell pool .
      • Most T cells that enter a lymph node do not encounter a specific antigen and will recirculate for many years .
      • Once an antigen-specific T cell is trapped in the lymph node by an APC and activated it takes several days for the activated T cell to proliferate and differentiate into effector T cells .
      • = delay between onset of infection and appearance of primary adaptive immune response.
    • Homing of naïve T cells to secondary lymphoid tissues is determined by cell adhesion molecules.
      • Passage of naïve T cell out of the bloodstream, through HEV to the lymph node cortex is controlled by cell-surface molecules on the T cells and endothelial cells .
      • These contacts are initiated by adhesion molecules on the T cell which bind to complementary molecules on other cells .
      • Adhesion molecules of the immune system comprise four structural classes.
      • Drawn into the secondary lymphoid tissue by chemokines that bind to their surface receptors.
        • Receptor on T-cell – CCR7
        • Chemokines produced in lymphoid tissue – CCL21 and CCL19
    • Leukocyte adhesion molecules
      • The four structural classes of adhesion molecule present on white blood cells and the cells with which they interact are:
        • Selectins – are carbohydrate-binding lectins
        • Vascular addressins - contain carbohydrate groups to which selectins bind
        • Integrins
        • Proteins in the immunoglobulin superfamily
    • Leukocyte adhesion molecules
    • Binding of L-selectin to vascular addressins directs naïve lymphocytes homing to lymphoid tissues.
      • Movement of naïve T cells into secondary lymphoid tissues
        • Homing
        • determined by T-cell surface selectin (L-selectin) interacting with two vascular addressins on surface of HEV venules (CD34 & GlyCAM-1)
        • L-selectin on naïve T cells (and naïve B cells) binds to sulfated carbohydrate sialyl-Lewis x moieties of vascular addressins CD34 and GlyCAM-1 on the high endothelial cells of lymph venules
    • Naïve T and B lymphocytes circulate in the blood and enter lymph nodes by crossing high endothelial venules
      • Lymphocytes bind to the endothelium in the lymph node through interaction of L-selectin with vascular addressins .
      • Chemokines, which are also bound to the endothelium, activate the integrin LFA-1 on the lymphocyte surface enabling it to bind tightly to ICAM-1 on the endothelial cell.
      • Establishment of tight binding allows the lymphocyte to squeeze between two endothelial cells , leaving the lumen of the blood vessel and entering the lymph node
      • The T-cells will migrate toward the high concentrations of CCL21 and CCL19 within the lymph node
    • Naïve T and B lymphocytes circulate in the blood and enter lymph nodes by crossing high endothelial venules
    • Cell-surface molecules of the immunoglobulin superfamily initiate lymphocyte adhesion to APC
      • As naïve T cells move through the cortex of the lymph node, these will bind transiently to APCs.
      • The initial encounter of T cells with antigen-presenting dendritic cells involves integrins and members of the immunoglobulin superfamily :
        • The T cell’s LFA-1 binds to ICAM-1 or ICAM-2 on the APC
        • The APC’s LFA-1 binds to ICAM-3 on the T cell
      • Adhesion is strengthened by:
        • CD2 on the T cell binding to LFA-3 on the APC.
        • ICAM-3 on the T cell and the lectin DC-SIGN on activated dendritic cell.
      • These transitory interactions allow the T cell to screen the peptide:MHC complexes on the APCs.
    • Cell-surface molecules of the immunoglobulin superfamily initiate lymphocyte adhesion to APC
    • Transient adhesive interactions between T cells & dendritic cells are stabilized by specific Ag recognition
      • When a naïve T cell binds to its specific ligand on an antigen-presenting dendritic cell, intracellular signaling through the TCR induces a conformational change in LFA-1 that causes it to bind with higher affinity to ICAMs on the antigen-presenting cell. These T-cells will suppress expression of S1P receptors .
      • This interaction is stable and can last several days during which the T cell proliferates and its progeny differentiates into effector cells.
      • If antigen isn’t met, then the T-cells will be drawn out of the lymph node by a different chemotactic molecule called sphingosine 1-phosphate (S1P).
        • Now the T-cell can continue to recircluate until it meets it’s antigen.
    • Transient adhesive interactions between T cells & dendritic cells are stabilized by specific Ag recognition
    • Activation of naïve T cells requires a co-stimulatory signal delivered by a professional antigen-presenting cell.
      • Initial intracellular signal generated by TCR:peptide:MHC complex = necessary for naïve T cell activation but…
        • Not sufficient …..a second co-stimulatory signal is required.
        • Co-stimulatory signals delivered by APC .
        • Both the antigen-specific stimulation and the co-stimulation must be delivered by ligands on the same APC.
    • The principal co-stimulatory molecules on professional APC are B7 molecules, which bind CD28 proteins on the T-cell surface
      • Binding of the T-cell receptor and its co-receptor CD4 to the peptide:MHC class II complex on the dendritic cell delivers a signal 1 .
      • This signal induces clonal expansion of T cells only when the co-stimulatory signal 2 is also given by the binding of CD28 (on the T cell ) to B7 (on the APC).
      • Both CD28 and B7 are members of the immunoglobulin superfamily.
      • There are two forms of B7 , called B7.1 (CD80) and B7.2 (CD86), but their functional differences have yet to be understood.
        • Also known to as co-stimulatory molecules or co-stimulator molecules
    • In absence of infection professional APC do not express co-stimulatory molecules
      • Capacity to activate naïve T cells is only acquired during an infection.
        • B7 expression is a direct consequence of infection, induced by interaction of a potential APC with microbial products via cell-surface receptors that contribute to innate immune response.
      • Activated T cells express another B7 receptor called CTLA-4.
      • CTLA-4 binds B7 twentyfold more strongly than does CD28 and functions as an antagonist .
      • So B7 binding to CD28 activates a T cell, while B7 binding to CTLA-4 slows down activation and limits cell proliferation .
    • Professional antigen-presenting cells versus other antigen-presenting cells
      • The characteristic that distinguishes professional antigen-presenting cells (APCs) from other antigen-presenting cells is the presence of B7 co-stimulatory molecules on their surface.
      • The three kind of professional APCs are: the dendritic cell, the macrophage and the B cell .
      • All three types are present in secondary lymphoid tissues but at different locations .
    • Three types of professional APCs populate different parts of the lymph node Dendritic cells (or interdigitating cells) are situated in the cortical T-cell areas of the lymph node. Dendritic cells are the most effective antigen-presenting cell.
    • Three types of professional APCs populate different parts of the lymph node
      • Although macrophages are distributed throughout the cortex and medulla of the lymph node, they concentrate in the marginal sinus , where afferent lymph collects before percolating through the lymphoid tissue, and also in the medullary cords , where efferent lymph collects before leaving the node.
      Macrophages are less effective than dendritic cells but more effective than B cells in presenting antigen.
    • Three types of professional APCs populate different parts of the lymph node These distributions reflect differences in the functions and importance of the three types of professional antigen-presenting cells. B cells mainly populate the lymphoid follicles in the cortex.
    •  
    • Dendritic cells are developmentally related to macrophages
      • Immature dendritic cells phagocytose microbial antigens using DEC 205 receptors (among others)
      • Antigens can also be taken up nonspecifically by macropinocytosis in which a cell engulfs extracellular fluid (ECF).
        • Important for initiation of T-cell viral responses
          • Dendritic cells acquire viral antigens via infection or by taking up virus particles from the ECF or other infected cells .
    • Dendritic cells are developmentally related to macrophages
      • Langerhans’ cell of the skin is a typical immature dendritic cell with large granules (Birbeck granules = phagosome like form).
      • On skin infection , local Langerhans’ cells will take up and process microbial antigen before traveling to the T-cell cortex areas of the draining lymph node and maturing to become a professional APC.
      • In lymph nodes , mature dendritic cells have a distinctive morphology which led to them being called interdigitating reticular cells .
      • Activated dendritic cells not only express B7 and class II MHC molecules but also express high levels of adhesion molecules (i.e. DC-SIGN) and secrete the chemokine CCL18 to attract naïve T-cells.
        • DC-SIGN binds to ICAM-3 on the T-cell
    • Immature DC: DEC205 facilitates phagocytosis and pinocytosis of antigens. Mature DC: Increase expression of B7 co-stimulators, MHC molecules and adhesions molecules like DC-SIGN
    • Macrophages in lymph nodes have different functions
      • Macrophages are found throughout the lymph node tissue and have several different functions:
        • Are phagocytic cells that take up microbes and particles from the extracellular environment.
          • Degrade microorganism in phagolysosomes that are loaded with hydrolytic enzymes.
          • Trap and degrade pathogens  this enables macrophages to process and present antigen to naïve T cells and prevents infection from reaching blood .
          • Prevents noninfectious particulates from lymph nodes from entering blood and blocking small blood vessels.
          • Remove and degrade lymphocytes that die in secondary lymphoid tissues.
    • Macrophages in lymph nodes have different functions
      • In the absence of infection , macrophages do not express B7 co-stimulators but have receptors that recognize bacterial components that are part of the innate immune recognition response: mannose receptor, scavenger receptors, complement receptors and Toll-like receptors .
      • When these receptors are engaged signals transmitted to the macrophage induce expression of B7 and increase expression MHC molecules .
      • Macrophages in the secondary lymphoid tissues can present antigen with MHC to activate naïve T-cells but those macrophages in the peripheral tissues (ex. skin) don’t have the opportunity to present because they don’t migrate to the secondary lymphoid tissue where the T-cells are.
      • In this manner the presence of an infection results in macrophage becoming an professional APC.
    • Microbial substances induce co-stimulatory activity in macrophages
      • Phagocytosis of bacteria by macrophages and their breakdown in phagolysosomes leads to the release of substances such as bacterial lipopolysaccharide which induce the expression of Co-stimulatory B7 molecules.
      • Peptides derived from the degradation of bacterial protein in the macrophage vesicular system are bound by MHC class II molecules  presented on the MAC’s surface.
      • Activation of naïve T cells  CD28 binding to B7 along with peptide:MHC complexes binding to TCR and co-receptor binding.
    • Third Type of P-APC
      • B cell Ig binds specific antigen from extracellular environment.
      • Ag:Ig complex  internalized by receptor-mediated endocytosis  transported to endocytic vesicles  degraded into peptides  peptides bind MHC class II molecules  peptide:MHC class II complex  transported to cell surface
      • In the primary immune response, the naïve B-cells are activated by effector T-cells that were activated in the same secondary lymphoid tissue.
        • They don’t typically activate naïve T-cells in the primary immune response.
    • Signals from TCR and co-receptors alter the pattern of gene transcription in activated T cells.
      • The signal that antigen has bound the TCR is transmitted by the cytoplasmic tails of CD3 chains , associated with the TCR  - &  -chains .
      • CD3 cytoplasmic tails contain sequences called immunoreceptor tyrosine-based activation motifs (ITAMs), which associate with cytoplasmic protein tyrosine kinases .
        • Kinases are activated by receptor clustering and phosphorylate ITAM tyrosine residues.
        • Enzymes and other signaling molecules bind to the phosphorylated tyrosine residues and become activated.
        • Pathways of intracellular signaling are initiated to effect alterations in gene expression .
    • Clustering of the TCR and a co-receptor initiates signaling within the T cell
    • Signaling pathways initiated by the T-cell receptor complex, its CD4 co-receptor and CD28
    • Proliferation and differentiation of activated T cells are driven by the cytokine interleukin-2 (IL-2)
      • In the previous slide, you saw that gene expression would be changed. One of the most important genes to be transcribed codes for the cytokine IL-2
      • Activation of T cell by professional APC initiates a program of differentiation controlled by the cytokine IL-2.
        • A burst cell division
        • Acquisition of effector function
      • IL-2 is synthesized and secreted by activated T cells .
      • IL-2 binds IL-2 receptors of T cell to drive clonal expansion of the activated cell
      • IL-2 is one of a number of cytokines produced by activated and effector T cells to control the development differentiation of cells in the immune response.
    • Proliferation and differentiation of activated T cells are driven by the cytokine interleukin-2 (IL-2)
      • IL-2 production requires both the signal delivered through the TCR:co-receptor complex and the co-stimulatory signal delivered through CD28.
        • Signals through the TCR:CD3 complex activate NFAT  activate transcription of the IL-2 gene (IL-2 mRNA is inherently unstable).
        • Co-stimulation (CD28:B7) stabilizes the IL-2 mRNA which causes an increase in the synthesis of IL-2 by T cells (X 20-30).
        • Co-stimulation also activates other transcription factors that stimulate the transcription of the IL-2 gene (3X).
        • Principal effect of co-stimulation is to increase synthesis of IL-2 by 100 fold.
    • Activated T cells secrete and respond to IL-2
      • Naïve T cells express the low-affinity receptor for IL-2 which consists of  and  chains
      • Activation of a naïve T cell by the recognition of a peptide:MHC complex accompanied by co-stimulation induces the synthesis and secretion of IL-2 and the synthesis of the IL-2 receptor  chain
      • Cell enters the first phase ( G1 ) of cell-division cycle
      •  chain combines with the  and  chains to make a high-affinity receptor for IL-2
      • IL-2 binds to the IL-2 receptor producing an intracellular signal that promotes T-cell proliferation
    •  
    • Antigen recognition without co-stimulation leads to a nonresponsive T cell
      • When a TCR on a mature naïve T cell binds to a peptide:MHC complex on an APC that does not express the co-stimulatory molecule B7 the T cell becomes nonresponsive or anergic and cannot be activated with subsequent antigen encounter.
    • Antigen recognition without co-stimulation leads to a nonresponsive T cell
      • Some mature naïve T cells may be specific for self-proteins expressed by cells not found in the thymus.
      • These T cells will not be activated because the cells expressing these self antigens will not express the co-stimulatory molecule, B7 .
      • A mature, naïve T cell that binds a self-peptide:MHC complex on a cell without B7 does not receive a co-stimulatory signal and undergoes anergy.
        • Anergic T-cells don’t make IL-2  no proliferation or differentiation
      • Anergic T cells can never be stimulated to proliferate or differentiate.
      • This process induces tolerance in the mature T cell repertoire .
    • Studying Ab and T-cell responses in lab animals
      • Immunization of protein antigen alone rarely induces an immune response.
      • Protein antigens + certain bacterial breakdown products (adjuvants) are required for a strong immune response.
      • Microbial products, known as adjuvants , induce co-stimulatory activity in DC, macrophages and B cells.
          • Whole microorganism = more effective vaccines than highly purified Ag molecules
          • Mechanism which allows the immune system to distinguish between antigens borne by infections agent and antigens associated with innocuous proteins.
    •  
    • On activation, CD4 T cells can acquire different helper functions
      • Towards the end of the proliferative phase , T cells acquire the capacity to synthesize the proteins they need to perform their specialized functions .
      • CD4 T (helper) cells produce cell-surface molecules and soluble cytokines to activate and help other cell types.
          • Macrophages and B cells
      • CD4 T cells differentiate into CD4 TH1 or TH2 cells .
      • The main cytokines secreted by TH1 cells (INF-  and IL-2) lead to macrophages activation , inflammation and production of opsonizing antibodies to enhance phagocytosis of pathogens.
      • The main cytokines secreted by TH2 cells (IL-4 and 5) lead to B-cell differentiation and production of neutralizing antibody .
    •  
    • Cytokine Environment Plays Large Role in Type of T-cell Produced
      • Whether a T-cell will become a T helper 1 or T helper 2 or a T cytotoxic cell is determined by the local cytokine environment where it is activated
      • One of the main cells responsible for these cytokines are dendritic cells
      • Once a pathway of differentiation is chosen the cytokines expressed by the T-cell itself can continue/reinforce the pathway of differentiation.
    • Stages of activation of CD4 T cells
      • A response biased towards TH1 cells is called cell-mediated immunity because the response is mediated by effector cells.
      • A response biased toward TH2 cells is called humoral immunity because the response is dominated by the production of antibodies.
    • Cytokines produced by TH1 cells can suppress the differentiation of TH2 cells and Cytokines produced by TH2 cells can suppress the differentiation of TH1 cells.
    • Naïve CD8 T cells can be activated in different ways to become cytotoxic cells
      • The activation of naïve CD8 T cells requires stronger co-stimulation than CD4 T cells.
      • Only dendritic cells – the most potent APC – can provide sufficient co-stimulation to produce cytotoxic T cells .
      • CD8 T cells stimulated by antigen and co-stimulation will synthesize IL-2 and the high affinity IL-2 receptor , which induces their proliferation and differentiation.
      • Under conditions of suboptimal co-stimulation , CD4 T cells can help activated naïve CD8 T cells.
      • CD4 and CD8 T cells must recognize the their specific antigen on the same APC , then the CD4 T cells will secrete cytokines that upregulate co-stimulation on the APC , which then activates the CD8 T cell.
    •  
    • T Cell-Mediated Immunity
    • T Cell-Mediated Immunity
      • Activation of naïve T cells on encounter with antigen
        • What happens when naïve T cell encounters its specific antigen for the first time and is stimulated to differentiate into an effector cell?
          • T-cell activation or T-cell priming
      • The properties and functions of effector T cells
        • Describe the interaction of effector T cells with their specific antigen that are presented by APC or target cells.
    • The properties and functions of effector T- cells
    • After differentiating (secondary lymphoid organs) effector T cells detach from the APC that nursed their differentiation Effector T cells CD4 TH2 CD4 TH1 Cytotoxic CD8 T cells Sites of infection -CD8 kills infected cells -CD4 secrete cytokines Stays in the secondary lymphoid organs -CD4 secrete cytokines
    • T-cell effector functions are turned on when TCRs bind to peptide:MHC complexes
      • This binding stimulates the T cell to release effector molecules that act on the target cell .
      T cell Target Cell TCR------ peptide:MHC
    • Effector T cells can be stimulated by antigen in the absence of co-stimulation
      • Activated effector T cell VERSUS Resting naïve T cells
        • How are they different?
          • Types of cell surface molecules expressed
          • Abundance of these cell surface molecules
    • Example of CD4 T cell Surface Molecules
    • Effector T cells can be stimulated by antigen in the absence of co-stimulation
      • A major change is that…
        • Effector T cells can respond to their specific antigen without the need for co-stimulation via B7-CD28 interaction.
        • Effector T cells can respond to antigen presented on cells other than professional antigen-presenting cells .
      • What’s the benefit of these relaxed activation requirements for effector T cells ?
        • Cytotoxic CD8 T cells must recognize many types of cells.
    •  
    • Effector T cells Respond Without the need for Co-stimulation
      • Cytotoxic CD8 T cells must recognize many types of cells that can be infected with viruses or intracellular pathogens which may not express co-stimulators .
      • Effector CD4 T cells that interact with B cells in lymphoid tissues or macrophages (at sites of infection) also benefit from the relaxed activation requirements.
          • B cells or macrophages express varying levels of co-stimulators, relaxing the requirements for activation increases the number of APCs that can stimulate CD4 T cells.
    • Effector T cells express more adhesion molecules than naïve T cells
      • Effector T cells express two to four times more adhesion molecules – CD2 and LFA-1 – than naïve cells.
      • Effector T cells can interact with target cells expressing lower levels of ICAM-1 and LFA-3.
      • Interactions of effector T cell with target cells is short-lived UNLESS the TCR is engaged by specific antigen .
      • The interaction leads to a conformational change in LFA-1 that strengthens the adhesion between the two cells.
    • Example of Resting CD4 T cell
      • Resting naïve T cells express L- selectin (for homing to LN) and relatively low levels of CD2 & LFA-1 (lymphocyte function –associated antigen-1 on surface of lymphocytes) adhesion molecules
      • When activated , L-selectin expression stops & increased amounts of integrin LFA-1 are made
    • Example of Activated CD4 T cell
      • Integrin VLA-4 is expressed ( homing receptor for vascular endothelium at sites of inflammation ) and guides activated T cells to the infection site .
      • Activated T cells have more surface CD2 which leads to increased adhesion to target cells.
    • Example of CD4 T cell increased sensitivity
      • Alternative splicing of RNA made from the CD45 genes causes activated T cells to express the CD45RO isoforms that associates with the TCR and CD4 …this change makes the T cell more sensitive to stimulation by lower concentrations of peptide:MHC complexes.
    • Changes in adhesion molecule expression = changes in migration patterns
      • Changes in the adhesion molecule expression of activated effector T cells versus resting naïve T cells also results in different patterns of migration .
      • Without the expression of L-selectin effector T cells no longer recirculate through lymph nodes by leaving the blood through HEVs.
      • Effector T cells instead express VLA-4 which enables binding to endothelial cells of blood vessels in infected/ inflamed tissues – where their effector function is needed.
    • Integrin VLA-4 enables effector T cells to home to inflamed tissue
      • Integrins are heterodimeric proteins comprising a  chain , which defines the class of integrin, and an  chain , which defines the different Integrins within a class.
      • LFA-1 is a  2 integrin present on all leukocytes including T cells.
      • LFA-1 binds ICAMs and is important in the adhesive interaction that mediate cell migration and in the interactions of T cells with APC or target cells.
      • LFA-1 expression is increased in effector T cells.
      • VLA-4 is a  1 integrin that increases its abundance upon T-cell activation.
      • VLA-4 binds to the cell adhesion molecule VCAM-1 which is selectively expressed on the endothelium of blood vessels in inflamed tissue.
    • Integrin VLA-4 enables effector T cells to home to inflamed tissue Recruitment of effectors T cells into inflamed or infected tissues T cell migration or antigen presentation
    • Effector T-cell functions are performed by cytokines and cytotoxins
      • Molecules that carry out T cell effector functions:
        • Cytokines , which alter the behavior of target cells.
          • All effector T cells produce cytokines (just different types and in different combinations ).
        • Cytotoxins , secreted cytotoxic proteins used to kill target cells.
          • Cytotoxins are specialized products of cytotoxic CD8 T cells .
    • Secreted cytokines
      • Secreted cytokines and related membrane-bound proteins that act through cell-surface receptors generally to induce changes in gene expression within target cells .
        • Work locally and work over short period of time
          • Autocrines : act on the cell that produced them
          • Paracrines : act locally on another cell
        • Colony stimulating factors : work at a distance : stimulate bone marrow
      • Many cytokines made by T cells are called interleukins (numbered in order of discovery, IL-2 or IL-7).
      • Cytokines (general term) made by lymphocytes are called lymphokines (more specific term).
    • Many cytokine receptors signal through a pathway in which receptor-associated kinases activate transcript factors directly
      • Cytokines bind to cytokine receptors composed of two or three chains.
      • The cytoplasmic tail of most cytokine receptors are associated with protein kinases called Janus kinases (JAKs).
      • Cytokine binding causes dimerization of the cytokine receptors which stimulate the JAKs to phosphorylate members of a protein family called STATs (Signal Transducers and Activators of Transcription).
      • On phosphorylation, two STAT molecules dimerize and move from the cytoplasm to the nucleus where they activate specific genes .
      • The genes that are activated is determined by the cytokine/s that were bound.
    • Many cytokine receptors signal through a pathway in which receptor-associated kinases activate transcript factors directly
      • Three types of effector T cell produce distinct sets of effector molecules .
    • TH1 &TH2 cells make characteristic sets of cytokines that have distinct effects on the immune response
    • Cytotoxic CD8 T cells are selective and serial killers of target cells at sites of infection
      • Once inside of a cell, the pathogen becomes inaccessible to antibody and other immune system proteins (i.e. complement ).
      • Elimination will be either through the efforts of the infected cell itself or by direct attack on the infected cell by the immune system.
      • Function of cytotoxic CD8 T cells is to kill cells that have become overwhelmed by intracellular infection.
        • Infected cell is sacrificed to prevent spread of infection to healthy cells .
    • Effector cytotoxic T cells contain lytic granules
      • Lytic granules are modified lysosomes with a mixture of cytotoxins.
      • CD8 T cells synthesize cytotoxins in inactive forms and package them into lytic granules when T cells are activated by specific antigen in the secondary lymphoid organs.
      • CD8 T cells then migrate to sites of infection and will recognize specific peptides (made from proteins from infected cell) in the context of MHC class I molecules presented by the infected cell.
      • TCR binding signals the cytotoxic T cell to secrete the contents of lytic granules directly onto a small localized area on the surface of the infected target cell.
    • Delivery of cytotoxins by CD8 T cells is aimed directly at the target cell. LG= lytic granules (red) MTOC= microtubule organizing center (microtubules – green) GA = golgi apparatus Initial contact has no effect Cytoskeleton reorganizes TCR:MHC Die
    • Cytotoxic CD8 T cells kill infected cells selectively
      • Specific recognition of peptide:MHC complex on an infected cell by a cytotoxic CD8 T cell (CTL) programs the infected cell to die and leaves healthy cells alone.
      • The CTL detaches from the first target cell (which dies), synthesizes a new set of lytic granules and then seeks out and kills another infected cell ( new target ).
    • In addition to cytotoxic functions, CD8 cells also contribute to the immune response by secreting cytokines
      • Secretion of interferon-gamma ( IFN-  ) inhibits the replication of viruses in the infected cells
      • IFN-  also increases the processing and presentation of viral antigen by MHC class I molecules.
      • IFN-  activates macrophages in the vicinity of the cytotoxic T cells.
      • Activated macrophages then get rid of dying infected cells helping damaged tissue to repair.
    • Apoptosis Versus Necrosis
      • Necrosis = death due physical or chemical injury in which cells lyse and disintegrate.
      • Apoptosis or programmed cell death = cell suicide in which cells shrivel or shrink but remain intact .
      • Apoptosis of the target cell is induced by the cytotoxic CD8 T cells releasing cytotoxins .
      • Target cells die by apoptosis which prevents pathogen replication as well as the release of the infectious pathogen .
    • Apoptotic cell in early stages showing chromatin condensation (red) but well-defined plasma membrane that is shedding vesicles . Apoptotic cell in late stages has very condensed nucleus , no mitochondria , and the cytoplasm and cell membranes are largely lost through vesicle shedding . Necrotic cell with plasma membrane poorly defined Healthy cell with normal nucleus Shriveled and shrunken
    • Time course of programmed cell death
      • Contact is made with target cell and granules are distant from the point of contact.
      • After one minute, granules have begun to move toward the target cell.
      • Movement of the granules is complete after four minutes.
      • After 40 min granules have been secreted and are between the T cell and target. Target cell is undergoing apoptosis, shown by fragmented nuclei.
      1. 2. 3. 4.
    • Cytotoxic T cells kill their targets by inducing apoptosis – two pathways
      • T cells induce apoptosis by two pathways (1) initiated by cytotoxins and (2) interactions with cell surface molecules :
        • Perforin (protein that polymerizes to form transmembrane pores in cell membranes, Granulysin also perturbs the membrane and granzymes , a family of three serine proteases .
        • Perforin and granulysin make pores in the target cell membrane for the granzymes to enter and cleave certain cell proteins leading to the activation of nucleases and other enzymes to initiate apoptosis .
    • Cytotoxic T cells kill their targets by inducing apoptosis – two pathways
      • Second death pathway involves inducing apoptosis between cell surface molecules on the cytotoxic T cell and the target cells.
        • Activated T cells surface cytokine – Fas ligand (FasL) , which binds to the Fas receptor molecule on the surface of the target cell.
        • Fas:FasL interaction sends a signal to the target cell to undergo apoptosis .
        • Main pathway to kill unwanted or excess lymphocytes .
    • Lymphadenopathy in autoimmune lymphoproliferation syndrome (ALPS)
      • Individuals who lack functional Fas molecules cannot control the size of their lymphocyte population nor remove autoimmune cells
      • Secondary lymphoid organs become swollen in the absence of infection.
      • Young girl with ALPS with very enlarged lymph nodes in her neck.
    • TH1 CD4 cells induce macrophages to become activated
      • Macrophage have receptors that bind to microorganisms and facilitate their phagocytosis, destruction and intracellular degradation.
      • As a result, macrophages present pathogen-derived peptide on MHC class II molecules to activate naïve CD4 T cells to become TH1 effector cells .
      • Some microorganisms can adapt to the macrophage and interfere with macrophage function by living and replicating inside the phagosome .
      • Thus, a principal function of TH1 cells is to act back on macrophages to:
        • Increase their phagocytic ability.
        • Increase their capacity to kill ingested microorganisms.
    • TH1 CD4 cells induce macrophages to become activated
        • The enhancement of macrophage function is called macrophage activation and requires interaction of peptide:MHC class II complexes on the macrophage with the TCR on a TH1 cell.
        • Macrophage activation causes phagosomes that contain captured microorganism to be more efficiently fused with lysosomes .
        • Increase synthesis by activated macrophages of highly reactive and microbicidal molecules (oxygen radicals, nitric oxide (NO) and proteases) will kill engulfed pathogens.
        • Activated macrophages have increased MHC class II molecules and B7 which enhances antigen presentation , recruiting more naïve CD4 T cells into the immune response to further the activation of macrophages.
    • TH1 CD4 cells activate macrophages to become highly microbicidal
      • Macrophages require two signals for activation , both of which are delivered by TH1 cells.
        • The primary signal is provided by IFN-  , the characteristic cytokine produced by TH1 cells.
        • The second signal is delivered by the CD40 ligand (CD40L) on T cells interacting with the CD40 receptor on macrophages.
      • When a TH1 cell specific for a bacterial peptide contacts a macrophage that presents that peptide , the TH1 cell is induced to secrete IFN-  and express CD40 ligand .
      • Together these newly synthesized proteins activate the macrophage to kill the bacterial living inside of its vesicles.
    • TH1 CD4 cells activate macrophages to become highly microbicidal
      • CD8 T cells can also produce IFN-  and activate macrophages .
      • In place of CD40 ligand (which CD8 T cells do not express), small amounts of bacterial polysaccharides have the same effect activating macrophages .
      • However, microbicidal substances produced by macrophages can be harmful to tissues.
      • CD4 TH2 cells produce cytokines that inhibit macrophage activation , showing the TH2 cells can control the TH1 response.
    • TH1 cells coordinate the host response to intravesicular pathogens
      • Intracellular pathogens can enjoy a protected life in the vesicular system of macrophages (tuberculosis and leprosy).
      • Intravesicular pathogens:
        • Can not be reached by antibody , nor can their peptides be presented by MHC class I molecules to CD8 T cells .
        • Avoid digestion by lysosomal enzymes by preventing the acidification of the phagolysosome that is required to activate the lysosomal hydrolases
      • Infections of this type are fought by TH1 CD4 T cells (that help the macrophages become activated so intracellular pathogens are killed ).
    • Immune response to intravesicular bacteria is coordinated by TH1 cells
    • Granulomas form when an intracellular pathogen resists elimination
      • Mycobacteria (red) can resist the killing effects of macrophage activation.
      • A characteristic localized inflammatory response called a granuloma develops.
      • The granuloma consists of a central core of infected macrophages which can include multinucleated giant cells formed by the macrophage fusions , surrounded by large single macrophages often called epitheloid cells.
      • Mycobacteria can persist in the cells of the granuloma .
      • The central core is surrounded by T cells many of which are CD4 T cells
    • TH2 cells stimulate the proliferation and differentiation of naïve B cells
      • The specific interaction of an antigen binding B cell with a helper TH2 cell leads to the expression of CD40 ligand (CD40L) and the secretion of IL-4, IL-5 and IL-6.
      • In concert these TH2 products drive the proliferation of B cells and their differentiation to form plasma cells dedicated to the secretion of antibody .
      For B cell to receive T cell help, both cells must recognize the same antigen – cognate interactions .
    • Molecular complexes recognized by both B and T cells make effective vaccines
      • Naïve B cell’s surface immunoglobulin binds a carbohydrate epitope on a vaccine composed of a Haemophilus polysaccharide ( blue ) conjugated to tetanus toxoid ( red ) a protein
      • This results in receptor-mediated endocytosis of the conjugate and its degradation in the endosomes and lysosome of a B cell.
      • Peptides derived from degradation of the tetanus toxoid part of the conjugate are bound by MHC class II molecules and presented on the B cell’s surface .
      • The TCR of a TH2 cell recognizes the peptide:MHC complex on the B cell.
      • This induces the T cell to secrete cytokines that activate the B cell to differentiate into plasma cells which produce protective antibody against Haemophilus polysaccharide .
    • Molecular complexes recognized by both B and T cells make effective vaccines
    • Regulatory CD4 T cells limit the activities of effector CD4 and CD8 T cells
      • Cytokines made by TH1 cells can suppress the generation of TH2 cells, while cytokines made by TH2 cells can suppress the generation of TH1 cells.
      • Populations of antigen-specific CD4 T cells whose actions can suppress the rejection response of CD4 and CD8 T cells to allogeneic MHC molecules
      • Regulatory or suppressor CD4 T cells make inhibitory cytokines.
        • IL-4, IL-10, TGF-  with high expression of CD25 , and the  chain of IL-2 receptor
        • Inhibition depends on physical contact between the regulatory CD4 T cell and its target
    • Summary: The properties of the P-APC
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    • Summary: The T-cell response has two distinct stages
      • First stage naïve T cells encounter their cognate antigen on P-APC and are induced to proliferate and differentiate into effector T cells
      • Second stage these effector cells recognize target cells bearing specific antigen and interact with them.
      • Naïve CD8 T cells become CD8 cytotoxic (CTL) effector cells which kill target cells that present peptides derived from viruses and other cytosolic pathogens bound to MHC class I molecules.
      • CD4 T cells differentiate into TH1 or TH2 cells which recognize antigen presented by MHC class II moleucles
      • TH1 cells activate macrophages , thus enhancing their general capacity to eliminate extracellular infection and more specifically to eliminate organism colonizing the macrophage’s vesicular system .
      • TH2 CD4 cells activate naïve B cells and control many aspects of the antibody response.