Lecture 16 - Immunology Study Guide PDF

Functions of Th2 cells Induced in response to helminth infection (and allergens) IL-4 is the defining cytokine Clears surface microbes and enhances barrier Th2 cells are a subset of helper T cells that play a crucial role in orchestrating immune responses against helminth infections and in allergic...

Functions of Th2 cells Induced in response to helminth infection (and allergens) IL-4 is the defining cytokine Clears surface microbes and enhances barrier Th2 cells are a subset of helper T cells that play a crucial role in orchestrating immune responses against helminth infections and in allergic reactions. When the body encounters helminths (parasitic worms) or allergens, such as pollen or certain foods, it triggers the activation and differentiation of Th2 cells. One of the defining features of Th2 cells is their production of interleukin-4 (IL-4), a cytokine that is central to their function. IL-4 stimulates B cells to undergo class switching to produce IgE antibodies, which are particularly effective against helminths. These IgE antibodies then bind to mast cells and basophils, sensitizing them to recognize and respond to subsequent encounters with the allergen or helminth. In the context of helminth infections, Th2 cells coordinate immune responses that involve the recruitment and activation of other immune cells, such as eosinophils, basophils, and mast cells. Eosinophils are specialized white blood cells that are highly effective at killing helminths through the release of toxic granules. Basophils, like mast cells, release histamine and other inflammatory mediators that promote inflammation and help to expel the parasites. Mast cells are particularly important in allergic reactions. When allergens bind to IgE antibodies on the surface of mast cells, they trigger the release of histamine and 1 other inflammatory mediators. Histamine causes blood vessels to dilate and become more permeable, leading to the characteristic symptoms of allergies, such as itching, sneezing, and swelling. Mast cells are also found in tissues throughout the body, including the skin, lungs, and gastrointestinal tract, where they play a role in immune surveillance and defense against pathogens. In the gastrointestinal tract, mast cells contribute to the clearance of surface microbes and enhance barrier function by promoting intestinal mucus secretion and peristalsis, which helps to expel pathogens and allergens from the body. While Th2 cells and the associated immune responses are crucial for defending against helminth infections, they can also contribute to the development of allergic diseases when inappropriately activated. In allergic individuals, exposure to harmless substances, such as pollen or certain foods, can trigger an exaggerated immune response mediated by Th2 cells, resulting in symptoms ranging from mild itching and sneezing to severe anaphylactic reactions. Therefore, while Th2 cells play important roles in host defense and immune regulation, dysregulation of Th2 responses can lead to detrimental outcomes such as allergies and allergic diseases. 1 Functions of Th2 cells Induced in response to helminth infection (and allergens) IL-4 is the defining cytokine IL-4 stimulates B cells to produce IgE IL-4 and IL-13 recruit eosinophils IL-5 activates eosinophils Th2 cells are a subset of helper T cells that play a critical role in immune responses against helminth infections and in allergic reactions. When the body encounters helminths or allergens, it triggers the activation and differentiation of Th2 cells, leading to the secretion of interleukin-4 (IL-4), which is a defining cytokine for this subset. One of the key functions of IL-4 is to stimulate B cells to undergo class switching, leading to the production of immunoglobulin E (IgE) antibodies. IgE antibodies are particularly effective against helminths and are also central to allergic responses. When produced, IgE antibodies bind to Fc receptors on the surface of mast cells and eosinophils, sensitizing them to recognize and respond to subsequent encounters with the antigen. In addition to IL-4, Th2 cells also secrete IL-13, which, along with IL-4, recruits eosinophils to sites of infection or inflammation. Eosinophils are specialized white blood cells that play a crucial role in combating helminth infections. IL-5, another cytokine produced by Th2 cells, activates eosinophils, further enhancing their ability to combat helminths. Both mast cells and eosinophils express Fc receptors for IgE antibodies. When an antigen binds to IgE antibodies bound to the Fc receptors, it triggers degranulation, a process where granules within the mast cells and eosinophils release their contents. 2 These granules contain various molecules, such as histamine, proteases, and cytokines, which contribute to the immune response against helminths. During degranulation, the Fc receptor on the surface of mast cells or eosinophils (depicted in red) binds to the IgE antibody (depicted in green) that recognizes the antigen (depicted in purple). This binding signals the granule to release its contents, which include molecules that can kill or immobilize helminths. Histamine, for example, can cause vasodilation and increased vascular permeability, leading to the recruitment of other immune cells to the site of infection. Proteases released from the granules can directly damage the helminth, contributing to its elimination. In summary, Th2 cells play a vital role in orchestrating immune responses against helminth infections and allergies by stimulating the production of IgE antibodies and recruiting eosinophils and other immune cells to combat the invading pathogens. Mast cells and eosinophils, armed with Fc receptors for IgE, play a central role in the effector phase of the immune response, where they release potent molecules that contribute to the elimination of helminths and the initiation of allergic reactions. 2 Functions of Th17 cells Promote destruction of extracellular bacteria and fungi Th17 cells are a subset of CD4+ helper T cells that play a crucial role in host defense against extracellular bacteria and fungi. They are characterized by their production of interleukin-17 (IL-17), although they can also produce other cytokines such as IL-22. Th17 cells are generated from naïve CD4 T cells in response to signals from antigenpresenting cells (APCs) exposed to bacteria or fungi. The process begins with the recognition of bacteria or fungi by APCs, such as dendritic cells or macrophages. These APCs engulf the pathogens and present their antigens to naïve CD4 T cells. Upon activation by APCs, naïve CD4 T cells proliferate and differentiate into Th17 cells. Once differentiated, Th17 cells migrate to sites of infection or inflammation, where they play a central role in orchestrating the immune response. Th17 cells produce IL17, which acts on leukocytes and tissue cells to induce the production of antimicrobial peptides and chemokines. These antimicrobial peptides help to directly kill bacteria and fungi, while chemokines recruit neutrophils to the site of infection. In addition to IL-17, Th17 cells can also produce IL-22. IL-22 acts on tissue cells to enhance barrier function, such as increasing the production of mucus and antimicrobial peptides, thereby limiting the spread of pathogens. This reinforces the host's defense against extracellular bacteria and fungi. Th17 cells exhibit functional plasticity, meaning they can adopt different functions 3 depending on the context of the infection. For example, in the presence of certain cytokines and signals, Th17 cells can differentiate into Th1-like or Th2-like cells, each of which may have different roles in combating specific pathogens. While Th17 cells are essential for host defense against pathogens, dysregulated Th17 responses have been implicated in various inflammatory diseases. Conditions such as psoriasis, inflammatory bowel disease (IBD), rheumatoid arthritis (RA), and multiple sclerosis (MS) are characterized by excessive Th17 cell activity and IL-17 production, leading to chronic inflammation and tissue damage. Due to their involvement in inflammatory diseases, agents that target the development or function of Th17 cells have been explored as potential therapeutic strategies. Some of these agents, such as monoclonal antibodies targeting cytokines or receptors involved in Th17 cell signaling, have shown promise in clinical trials and have been approved for use in certain inflammatory conditions. By modulating Th17 cell responses, these agents aim to dampen inflammation and alleviate symptoms associated with Th17-mediated diseases. 3 CTLs kill cells that contain specific microbes or antigens in their (cytosol) Target cell Cytotoxic T lymphocytes (CTLs), also known as CD8+ T cells, are a crucial component of the adaptive immune response responsible for eliminating cells infected with specific microbes or containing particular antigens. CTLs recognize and kill these target cells by inducing apoptosis, a programmed cell death process. One of the primary functions of CTLs is to target and eliminate cells infected with intracellular pathogens, particularly those that have breached the host cell's defense mechanisms and gained access to the cytosol. This includes viruses and certain bacteria that have escaped from endosomes or phagosomes into the cytosol. Once inside the cytosol, these microbes are less accessible to the immune system and can evade detection by phagocytes. CTLs play a critical role in combating such intracellular pathogens by recognizing specific antigens presented on the surface of infected cells in the context of major histocompatibility complex class I (MHC-I) molecules. These antigens are typically peptides derived from viral or bacterial proteins synthesized within the host cell. CTLs recognize these peptide-MHC-I complexes through their T cell receptors (TCRs), triggering the activation of CTLs. Upon activation, CTLs undergo clonal expansion, leading to the generation of a large population of effector CTLs. Effector CTLs migrate to the site of infection, where they recognize and engage with infected target cells displaying the specific antigen on their 4 surface. CTLs release cytotoxic molecules, such as perforin and granzymes, which induce apoptosis in the target cell. Perforin forms pores in the target cell membrane, allowing the entry of granzymes into the cytosol. Granzymes are proteolytic enzymes that cleave key intracellular substrates, triggering apoptosis through multiple pathways. This leads to the fragmentation of the target cell's DNA, degradation of cellular components, and ultimately cell death. In addition to combating intracellular pathogens, CTLs also play a crucial role in immune surveillance against cancer. Tumor cells often express abnormal or mutated proteins that can be recognized as foreign antigens by the immune system. CTLs are capable of recognizing these tumor-associated antigens presented on MHC-I molecules and initiating the apoptosis of cancerous cells. Overall, CTLs serve as frontline defenders against intracellular pathogens and tumor cells by recognizing specific antigens presented on infected or aberrant cells and inducing their elimination through apoptosis. This process helps to cripple the ability of pathogens to survive and replicate within the host and contributes to the maintenance of immune homeostasis and protection against infectious diseases and cancer. 4 Activation and effector phases of CD8+ T cell responses Activation of naïve CD8+ T cells has two special features: 1. Antigen cross presentation 2. Assistance from CD4+ Helper T cells The activation and effector phases of CD8+ T cell responses are crucial steps in the adaptive immune response, allowing the immune system to recognize and eliminate intracellular pathogens and abnormal cells, such as virus-infected cells and cancer cells. 1.Activation of Naïve CD8+ T Cells: 1. Naïve CD8+ T cells originate from the thymus and circulate in the bloodstream until they encounter their specific antigen. 2. Antigen recognition occurs when dendritic cells (DCs), specialized antigen-presenting cells, capture and process antigens from pathogens or abnormal cells. DCs then present antigenic peptides on their surface in the context of major histocompatibility complex class I (MHC-I) molecules. 3. Naïve CD8+ T cells, bearing T cell receptors (TCRs) that recognize the specific antigen-MHC-I complex, bind to the antigen-presenting DCs in secondary lymphoid organs, such as lymph nodes. 4. Two special features of CD8+ T cell activation are antigen crosspresentation and assistance from CD4+ helper T cells: 1. Antigen cross-presentation occurs when DCs present exogenous antigens on MHC-I molecules to activate CD8+ T cells. This allows 5 CD8+ T cells to recognize and respond to antigens derived from infected or abnormal cells. 2. CD4+ helper T cells provide essential signals, such as CD40-CD40L interactions and cytokines like interleukin-2 (IL-2), that help to optimize CD8+ T cell activation and proliferation. 1.Proliferation and Differentiation of CD8+ T Cells: 1. Upon activation, naïve CD8+ T cells undergo clonal expansion, resulting in the proliferation of antigen-specific T cells. This process involves multiple rounds of cell division, leading to the generation of a large pool of effector and memory T cells. 2. Differentiation occurs as CD8+ T cells transition into effector cytotoxic T lymphocytes (CTLs), characterized by changes in gene expression and acquisition of effector functions. 3. Effector CTLs acquire cytotoxic capabilities and migrate to the site of infection or inflammation. 2.Migration of Effector T Cells and Other Leukocytes: 1. Effector CTLs, along with other leukocytes such as macrophages and neutrophils, migrate to the site of antigenic challenge in tissues or organs. 2. Chemokines and adhesion molecules guide the migration of immune cells to the site of infection, where they coordinate their effector functions to combat the pathogen. 3.Effector Functions of CD8+ T Cells: 1. Effector CTLs recognize and eliminate infected or abnormal cells by inducing apoptosis through the release of cytotoxic molecules, such as perforin and granzymes. 2. Perforin forms pores in the target cell membrane, allowing the entry of granzymes into the cytosol. Granzymes induce apoptosis by cleaving key intracellular substrates, leading to DNA fragmentation and cell death. 3. Additionally, effector CTLs produce cytokines, such as interferon-gamma (IFN-γ), which promote inflammation, enhance immune responses, and activate other immune cells, further contributing to pathogen clearance. Overall, the activation and effector phases of CD8+ T cell responses are highly coordinated processes that enable the immune system to mount a targeted and effective response against intracellular pathogens and abnormal cells, thereby maintaining immune homeostasis and protecting the host from infectious diseases and cancer. 5 Antigen cross-presentation Naïve CD8+ T cells are most effectively activated by antigens presented by DCs => Dcs (dentritic cells) provide costimulation via B7 and other signals But: viruses can infect many cell types and cancer affects many cell types CD8+ TCRs only recognizes class 1 MHC-presented antigens Class I MHC molecules only present cytosolic antigens DCs are not readily infected by most viruses How then to best present viral or tumor antigens to naive CD8+ T cells? Cross-presentation Antigen cross-presentation is a critical mechanism by which dendritic cells (DCs) effectively activate naïve CD8+ T cells by presenting antigens derived from extracellular sources, such as viruses or tumor cells, on major histocompatibility complex class I (MHC-I) molecules. This process is essential for initiating CD8+ T cell responses against pathogens that infect a wide range of cell types or tumor cells that arise from various tissues. 1.DCs Provide Costimulation for CD8+ T Cell Activation: 1. Naïve CD8+ T cells require two signals for full activation: antigen recognition and costimulation. Antigen recognition occurs when the T cell receptor (TCR) on CD8+ T cells binds to specific peptides presented by MHC-I molecules on the surface of antigen-presenting cells, such as DCs. 2. Costimulation is provided by additional signals, such as those mediated by the interaction between B7 molecules (CD80/CD86) on DCs and CD28 receptors on T cells. Costimulation is necessary for optimal T cell activation, proliferation, and differentiation into effector CTLs. 2.Challenges in Antigen Presentation to CD8+ T Cells: 1. Viruses and cancer cells can infect or arise from diverse cell types, making it challenging to ensure that all infected or cancerous cells present antigens to CD8+ T cells. 6 2. CD8+ T cell responses are primarily directed against antigens presented by MHC-I molecules. However, MHC-I molecules typically present peptides derived from cytosolic antigens, limiting the ability of CD8+ T cells to recognize extracellular antigens. 1.Role of Antigen Cross-Presentation: 1. DCs are specialized antigen-presenting cells equipped with the unique ability to capture, process, and present exogenous antigens on MHC-I molecules, a process known as antigen cross-presentation. 2. DCs efficiently internalize extracellular antigens through various mechanisms, such as receptor-mediated endocytosis, phagocytosis, or macropinocytosis. 3. Once internalized, antigens are processed within the endosomal or phagosomal compartments of DCs. Specialized pathways and molecular machinery within DCs allow them to transport these exogenous antigens into the cytosol for proteasomal degradation. 4. Processed antigenic peptides are then loaded onto MHC-I molecules in the endoplasmic reticulum (ER) and transported to the cell surface for presentation to CD8+ T cells. 5. By cross-presenting extracellular antigens on MHC-I molecules, DCs effectively activate naïve CD8+ T cells, initiating specific immune responses against viruses or tumor cells that infect or arise from various cell types. 2.Importance of DCs in CD8+ T Cell Activation: 1. DCs play a central role in priming CD8+ T cell responses by capturing and cross-presenting antigens to naïve CD8+ T cells in secondary lymphoid organs, such as lymph nodes. 2. Optimal activation of CD8+ T cells requires the interaction of naïve T cells with mature, activated DCs presenting both antigen and costimulatory signals. 3. Through antigen cross-presentation, DCs bridge the gap between innate and adaptive immunity, linking the detection of pathogens by innate immune cells to the activation of adaptive immune responses mediated by CD8+ T cells. In summary, antigen cross-presentation by dendritic cells is a crucial mechanism for efficiently activating naïve CD8+ T cells and initiating specific immune responses against viruses and tumors that infect or arise from diverse cell types. This process ensures that CD8+ T cells are primed to recognize and eliminate infected or cancerous cells, contributing to host defense against infectious diseases and cancer. 6 Steps in Antigen cross-presentation 1. DCs INGEST INFECTED CELLS OR TUMOR CELLS 2. DCs transfer antigens to their cytosol (have to display it on Class I antigen presnation) 3. DCs process the antigen via class I MHC antigen presentation pathway 4. cd8+ T cells recognize MHC/peptide AND receive costimulatory signals The process of antigen cross-presentation by dendritic cells (DCs) involves several distinct steps, ensuring efficient presentation of exogenous antigens on major histocompatibility complex class I (MHC-I) molecules to CD8+ T cells. Here are the detailed steps involved in antigen cross-presentation: 1.Ingestion of Infected Cells or Tumor Cells: 1. Dendritic cells encounter infected cells or tumor cells through various mechanisms, such as migration to sites of infection or inflammation. 2. Upon encountering infected or tumor cells, DCs can phagocytose these target cells, internalizing them into intracellular vesicles known as phagosomes. 2.Transfer of Antigens to the Cytosol: 1. Following internalization, antigens derived from the ingested cells are processed within the phagosomes of DCs. 2. To initiate antigen cross-presentation, DCs must transfer these exogenous antigens from the phagosomes to their cytosol, where they can access the MHC-I antigen presentation pathway. 3. Several mechanisms facilitate the transfer of antigens to the cytosol, including the action of specialized proteins and molecular chaperones that promote the translocation of antigens across phagosomal 7 membranes. 3. Processing of Antigens via MHC-I Antigen Presentation Pathway: 1. Once in the cytosol, exogenous antigens undergo proteasomal degradation, mediated by the proteasome complex. 2. The proteolytic products, including antigenic peptides, are further trimmed and processed by cytosolic proteases to generate peptides suitable for binding to MHC-I molecules. 3. Processed antigenic peptides are transported into the endoplasmic reticulum (ER) by the transporter associated with antigen processing (TAP) complex, where they bind to nascent MHC-I molecules. 4.Presentation of Antigen-MHC-I Complexes: 1. Mature MHC-I/peptide complexes are then transported from the ER to the Golgi apparatus and subsequently to the cell surface in specialized vesicles. 2. Once on the cell surface, MHC-I/peptide complexes are displayed to CD8+ T cells, enabling specific recognition by T cell receptors (TCRs) expressed on the surface of CD8+ T cells. 3. Concurrently, DCs upregulate costimulatory molecules, such as CD80 (B71) and CD86 (B7-2), which interact with CD28 receptors on CD8+ T cells, providing essential secondary signals required for T cell activation. 5.Recognition by CD8+ T Cells and Costimulation: 1. CD8+ T cells scan the surface of DCs for the presence of antigen-MHC-I complexes that match their TCR specificity. 2. Binding of the TCR to the MHC-I/peptide complex, in conjunction with costimulatory signals provided by CD80/CD86-CD28 interactions, triggers the activation of CD8+ T cells. 3. Activated CD8+ T cells undergo clonal expansion, generating a population of effector cytotoxic T lymphocytes (CTLs) capable of recognizing and eliminating infected or tumor cells displaying the same antigenic peptides. In summary, antigen cross-presentation by dendritic cells is a highly regulated process that involves the internalization, processing, and presentation of exogenous antigens on MHC-I molecules to CD8+ T cells. This mechanism enables the priming of CD8+ T cell responses against pathogens and tumor cells that infect or arise from diverse cell types, contributing to the host's immune defense against infections and cancer. 7 Helper T cells can promote differentiation of CTLs Vai cytokine production Helper T cells play a crucial role in promoting the differentiation of cytotoxic T lymphocytes (CTLs), especially when the innate immune response is weak or when encountering challenges such as latent viral infections or tumors. There are two main mechanisms through which helper T cells contribute to CTL differentiation: via cytokine production and through the "licensing" of antigen-presenting cells (APCs). 1.Cytokine Production by Helper T Cells: 1. CD4+ helper T cells, also known as T helper (Th) cells, produce various cytokines that influence the differentiation and activation of CTLs. 2. One of the key cytokines produced by Th cells that promotes CTL differentiation is interleukin-2 (IL-2). IL-2 acts as a growth factor for T cells and supports their proliferation and differentiation into effector CTLs. 3. Additionally, other cytokines produced by Th cells, such as interleukin-12 (IL-12), interleukin-15 (IL-15), and interferon-gamma (IFN-γ), also play important roles in promoting CTL differentiation and effector functions. 4. These cytokines act on naïve CD8+ T cells, enhancing their activation, proliferation, and differentiation into functional CTLs capable of recognizing and eliminating infected or tumor cells. 2."Licensing" of APCs by Helper T Cells: 1. Helper T cells provide critical signals to antigen-presenting cells (APCs), 8 such as dendritic cells (DCs), that enhance their ability to stimulate CTL differentiation. 2. The interaction between CD40 ligand (CD40L) on activated Th cells and CD40 receptors on APCs is particularly important in this process. CD40CD40L signaling leads to the upregulation of costimulatory molecules, such as CD80 and CD86, on the surface of APCs. 3. Increased expression of costimulatory molecules on APCs enhances their ability to provide essential secondary signals required for optimal CTL activation and differentiation. 4. Furthermore, Th cell-derived cytokines, such as IL-2 and IFN-γ, can further enhance the licensing of APCs by promoting their maturation, increasing their expression of MHC molecules, and amplifying their secretion of cytokines necessary for CTL differentiation. In summary, helper T cells play a central role in promoting the differentiation of CTLs, particularly in situations where the innate immune response is insufficient to control infections or tumors. Through cytokine production and the licensing of APCs, Th cells provide the necessary signals and support for the optimal activation and differentiation of CTLs, thereby contributing to the host's immune defense against pathogens and cancer. 8 T cell exhaustion => Can occur during chronic viral infection Evolved to limit damage associated with chronic infections? But may contribute to some chronic infections (HIV, HepC) and cancers… Role of T cell exhaustion in COVID-19 is active area of research T cell exhaustion is a state of functional impairment that occurs in CD8+ cytotoxic T lymphocytes (CTLs) during chronic viral infections, cancer, and other persistent antigenic stimulation. This phenomenon is characterized by a progressive loss of effector functions and the expression of inhibitory receptors on T cells, ultimately leading to a weakened immune response. Here's a detailed description of T cell exhaustion and its implications: 1.Occurrence During Chronic Infections and Cancer: 1. T cell exhaustion commonly occurs during chronic viral infections, such as HIV and hepatitis C virus (HCV), as well as in certain cancers where tumors persistently stimulate the immune system. 2. The exhaustion of CD8+ T cells is believed to be an evolutionary mechanism aimed at limiting immunopathology and tissue damage associated with prolonged immune activation during chronic infections. 2.Role in Limiting Damage During Chronic Infections: 1. T cell exhaustion is thought to serve as a regulatory mechanism to prevent excessive tissue damage caused by prolonged and uncontrolled immune responses. 2. By dampening the activity of effector T cells, particularly CTLs, during chronic infections, the host's immune system aims to balance pathogen 9 control with tissue preservation and immune tolerance. 3. Implications in Chronic Infections and Cancer: 1. While T cell exhaustion may initially evolve as a protective mechanism, it can also contribute to the persistence and progression of chronic infections and cancer. 2. In chronic viral infections like HIV and HCV, T cell exhaustion hampers the ability of CTLs to eliminate infected cells, leading to viral persistence and disease progression. 3. Similarly, in cancer, exhausted T cells fail to mount effective antitumor responses, allowing tumor cells to evade immune surveillance and proliferate. 4.Active Research in COVID-19: 1. The role of T cell exhaustion in COVID-19, caused by the SARS-CoV-2 virus, is currently an active area of research. 2. Studies have suggested that severe cases of COVID-19 may be associated with T cell exhaustion, characterized by the presence of exhausted CD8+ T cells and elevated levels of inhibitory receptors such as PD-1. 3. Understanding the mechanisms underlying T cell exhaustion in COVID-19 could provide insights into disease progression and inform the development of immunotherapeutic strategies. 5.Two Paths of T Cell Response: 1. During acute infection, naïve CD8+ T cells differentiate into effector CTLs upon encountering the antigen. Effector CTLs exert antiviral effects by secreting cytokines, proliferating, and killing virus-infected cells. 2. In chronic infections where the antigen persists, such as in HIV or HCV, a subset of CD8+ T cells becomes exhausted over time. These exhausted CTLs exhibit functional impairments, reduced cytokine production, and upregulation of inhibitory receptors like PD-1. In summary, T cell exhaustion represents a complex immunological phenomenon observed during chronic infections and cancer, characterized by functional impairment of CTLs and the expression of inhibitory receptors. While initially serving as a regulatory mechanism to limit tissue damage, T cell exhaustion can contribute to the persistence and progression of chronic infections and cancer. Research into T cell exhaustion in various diseases, including COVID-19, is ongoing and may inform the development of novel therapeutic interventions. 9 How do CTLs kill target cells? 1. Perforin / granzyme-mediated cell killing Immunne synapse Lethal hit Within hours CTL granules contain: Perforin => perturbs membrane Granzymes => proteases Cytotoxic T lymphocytes (CTLs) eliminate target cells primarily through a process known as perforin/granzyme-mediated cell killing, which involves the release of cytotoxic granules containing perforin and granzymes. Here's a detailed description of how CTLs kill target cells: 1.Formation of the Immune Synapse: 1. Before CTLs initiate the killing of target cells, they form a specialized junction called the immune synapse at the interface between the CTL and the target cell. 2. The immune synapse allows for the precise delivery of cytotoxic molecules from the CTL to the target cell, ensuring targeted and efficient killing. 2.Release of Cytotoxic Granules: 1. CTLs contain cytotoxic granules within their cytoplasm, which store molecules essential for inducing apoptosis (programmed cell death) in target cells. 2. Upon recognition of specific antigens presented by target cells, CTLs polarize their cytotoxic granules towards the immune synapse. 3. At the immune synapse, CTLs release the contents of their cytotoxic granules, which include perforin and granzymes, into the synaptic cleft 10 between the CTL and the target cell. 3. Role of Perforin: 1. Perforin is a pore-forming protein that forms transmembrane channels in the plasma membrane of target cells. 2. Upon release into the immune synapse, perforin molecules assemble to form oligomeric pores in the lipid bilayer of the target cell membrane. 3. These pores allow for the influx of water and ions, disrupting the integrity of the target cell membrane and inducing osmotic lysis. 4.Role of Granzymes: 1. Granzymes are serine proteases stored within cytotoxic granules, typically in an inactive form. 2. Perforin-mediated membrane disruption facilitates the entry of granzymes into the cytoplasm of the target cell. 3. Once inside the target cell, granzymes activate and exert their proteolytic activity by cleaving specific intracellular substrates, particularly proteins involved in apoptosis regulation. 4. Granzyme-mediated cleavage of substrates leads to the activation of caspases, a family of proteases that orchestrate the apoptotic process. 5.Induction of Apoptosis: 1. Activation of caspases within the target cell triggers a cascade of events culminating in apoptosis, characterized by DNA fragmentation, nuclear condensation, membrane blebbing, and cell shrinkage. 2. Apoptosis ensures the orderly and controlled elimination of the target cell without eliciting inflammation or tissue damage. 3. Importantly, the process of CTL-mediated killing is rapid, typically leading to the death of the target cell within hours of CTL-target cell interaction. In summary, CTLs kill target cells through the perforin/granzyme-mediated pathway, involving the release of cytotoxic granules containing perforin and granzymes at the immune synapse. Perforin disrupts the target cell membrane, allowing granzymes to enter the cytoplasm and induce apoptosis by activating caspases. This mechanism ensures the precise and efficient elimination of infected or aberrant cells by CTLs as part of the immune response. 10 How do CTLs kill target cells? 2. Fas / FasL-mediated cell killing Fas = death receptor (CD95) Expressed on many cell types FasL = Fas Ligand Expressed on activated CD8+ cells Cytotoxic T lymphocytes (CTLs) have another mechanism for killing target cells, known as Fas/FasL-mediated cell killing. This pathway involves the interaction between Fas (CD95), a death receptor expressed on the surface of target cells, and Fas ligand (FasL), expressed on the surface of activated CD8+ CTLs. Here's a detailed description of how CTLs kill target cells through the Fas/FasL-mediated pathway: 1.Fas and Fas Ligand (FasL): 1. Fas (also known as CD95 or APO-1) is a cell surface receptor belonging to the tumor necrosis factor (TNF) receptor superfamily. It is expressed on the surface of many cell types, including both normal and abnormal cells. 2. FasL is the ligand for Fas and is expressed on the surface of activated CD8+ CTLs. Upon activation, CTLs upregulate FasL expression, enabling them to induce apoptosis in target cells. 2.Initiation of Cell Death Signaling: 1. When CTLs encounter target cells expressing cognate antigen-MHC complexes, they form immunological synapses and deliver cytotoxic signals. 2. The interaction between FasL on CTLs and Fas on the surface of target cells triggers a signaling cascade within the target cell, leading to apoptosis. 11 3. Binding of FasL to Fas induces trimerization of Fas receptors, resulting in the recruitment of intracellular adaptor proteins, such as FADD (Fasassociated death domain), to the cytoplasmic domain of Fas. 1.Activation of Caspases and Apoptosis: 1. Recruitment of FADD to the Fas receptor promotes the assembly of a death-inducing signaling complex (DISC) at the cytoplasmic side of the cell membrane. 2. The DISC serves as a platform for the activation of initiator caspases, particularly caspase-8, through proteolytic cleavage. 3. Activated caspase-8 then initiates a caspase cascade, leading to the cleavage and activation of downstream effector caspases, such as caspase-3 and caspase-7. 4. Activation of effector caspases results in the execution of apoptotic programs, including DNA fragmentation, chromatin condensation, cytoskeletal disruption, and membrane blebbing. 5. Ultimately, the target cell undergoes programmed cell death (apoptosis) in response to the Fas/FasL signaling axis initiated by CTLs. 2.Role in Tissue Damage: 1. While the Fas/FasL pathway is crucial for eliminating infected or aberrant cells, excessive or dysregulated activation of CTLs can lead to unintended tissue damage. 2. CTLs may inadvertently target and kill normal cells expressing Fas, contributing to tissue injury and inflammation, which can be particularly problematic in autoimmune diseases or in conditions of chronic immune activation. In summary, CTLs kill target cells through the Fas/FasL-mediated pathway by engaging Fas receptors on target cells with FasL expressed on activated CTLs. This interaction triggers a signaling cascade within the target cell, leading to apoptosis through the activation of caspases and execution of apoptotic programs. While essential for immune surveillance and defense, the Fas/FasL pathway can also contribute to tissue damage under certain conditions. 11