Antigen Processing and Presentation PDF

Antigen processing and presentation George Nadăș,Professor of Immunology, [email protected] Antigen processing Introduction Einii.it To initiate immune responses, antigens are captured from their site of entry and concentrated in secondary (peripheral) lymphoid organs through which naive T cells c...

Antigen processing and presentation George Nadăș,Professor of Immunology, [email protected] Antigen processing Introduction Einii.it To initiate immune responses, antigens are captured from their site of entry and concentrated in secondary (peripheral) lymphoid organs through which naive T cells circulate constantly Microbes and other antigens most often enter the body through epithelium-lined surfaces, which interface with the external environment Microbes may also colonize any tissue, and antigens may be produced in these tissues 2 Antigen processing Introduction Because the total number of lymphocytes in the body is finite and the immune system generates a large number of lymphocyte clones each with a different specificity, there are very few naive T and B cells specific for any one antigen, in the range of 1 in 105 or 106 lymphocytes This small number of naive T cells has to be able to locate and respond to the foreign antigen – impossible unless APC’s trap antigen and carry it to the secondary lymphoid organs T lymphocytes recognize and respond to cell-associated antigens and not to soluble, cell-free antigens – MHC class I and II 3 Antigen processing Properties of antigens recognized by T lymphocytes Most T lymphocytes recognize only short peptides, whereas B cells can recognize peptides, intact folded proteins, nucleic acids, carbohydrates, lipids, and small chemicals As a result, T cell– mediated immune responses are usually induced by foreign protein antigens (the natural source of foreign peptides), whereas humoral immune responses are induced by protein and nonprotein antigens The antigen receptors of CD4+ and CD8+ T cells are specific for peptide antigens that are displayed by MHC molecules 4 Antigen processing Properties of antigens recognized by T lymphocytes The function of MHC molecules is to bind and display peptides for recognition by CD4 + and CD8 + T cells MHC molecules can bind and display peptides and no other types of molecules; this is why CD4+ and CD8+ T cells recognize peptides MHC molecule binding and displaying a peptide and a T cell receptor recognizing the complex of peptide and MHC molecule 5 Antigen processing Antigen capture and the functions of APC’s ProfessionalAPC's semiprofessional The three major types of antigenpresenting cells for CD4 + T cells function to display antigens at different stages and in different types of immune responses 6 Antigen processing Antigen capture and the functions of APC’s Different cell types function as APCs to activate naive T cells or previously differentiated effector T cells FYI 7 Antigen processing Antigen capture and the functions of APC’s DCs are the most effective APCs for activating naive T cells and therefore for initiating T cell responses Macrophages and B lymphocytes also function as APCs, but mostly for previously activated CD4 + helper T cells rather than for naive T cells APCs display peptide-MHC complexes for recognition by T cells and also provide additional stimuli that are required for the full responses of the T cells Antigen is the first signal, and these additional stimuli are sometimes called second signals or costimulators. They are more important for activation of naive T cells than for restimulation of previously activated effector and memory cells 8 Antigen processing Antigen capture and the functions of APC’s The antigen-presenting function of APCs is enhanced by exposure to microbial products. This is one reason that the immune system responds better to microbes than to harmless, nonmicrobial substances Microbes and protein antigens that enter through epithelia are concentrated in lymph nodes, and blood- borne antigens are captured mostly in the spleen The common routes through which foreign antigens, such as microbes, enter a host are the skin and the epithelia of the gastrointestinal and respiratory systems 9 Antigen processing DCs DCs are the cells that are best able to capture and transport antigens for presentation to naive T cells DCs are divided into several subsets based on phenotypes and functions: 1. Conventional (or classical) DCs (cDCs) are present in most epithelia that interface with the external environment, such as the skin and the intestinal and respiratory tracts, and in tissues, and are enriched in lymphoid organs They are the DC subset that captures antigens and transports them to secondary lymphoid organs and are thus involved in antigen presentation to naive CD4+ and CD8+ T cells 10 Antigen processing DCs types Conventional DCs are divided into two groups: ○ Type 1 cDCs (cDC1) are especially efficient at transferring ingested antigens from vesicles into the cytosol - essential step in the process of cross-presentation, in which ingested antigens are presented on class I MHC molecules to CD8+ T cells ○ Type 2 cDCs (cDC2) are the major DC subset that presents captured antigens to CD4+ T cells, and thus the subset that is most important for initiating responses of these T cells 11 Antigen processing DCs types 2. Plasmacytoid DCs (pDC) are the body’s major source of type I IFN and are thus essential for innate immune responses to viruses. pDCs also may capture antigens in the blood and transport them to the spleen 3. Monocyte-derived DCs (moDC) can be induced to develop from monocytes under inflammatory conditions. Their roles in immune responses are not clear 4. Langerhans cells of the epidermis were one of the earliest DCs identified. These cells are related to tissue-resident macrophages and develop early in life from progenitors in the yolk sac or fetal liver and seed the skin, with probably similar functions to that of cDC2 12 Antigen processing DCs activation DCs that are resident in epithelia and tissues capture protein antigens Tissue-resident cDCs express numerous membrane receptors, such as C-type lectins, that bind microbes DCs use these receptors to capture and endocytose microbes or microbial proteins and then process the ingested proteins into peptides capable of binding to MHC molecules In addition to receptor-mediated endocytosis and phagocytosis, DCs can ingest antigens by pinocytosis, a process that does not involve specific recognition receptors but serves to internalize whatever molecules might be in the fluid phase in the vicinity of the DCs 13 Antigen processing DCs activation Simultaneously with antigen capture, DCs are activated by microbial products to mature into APCs that transport the captured antigens to draining lymph nodes At the time that microbial antigens are being captured, microbial products (PAMPs) recognized by Toll-like receptors and other innate recognition receptors – innate responses 14 Antigen processing DCs activation The DCs are activated by these signals and by cytokines, such as tumor necrosis factor (TNF), produced in response to the microbes The activated DCs (also called mature DCs) lose their adhesiveness for epithelia or tissues and begin to express a chemokine receptor called CCR7 that is specific for two chemokines, CCL19 and CCL21, that are produced in lymphatic vessels and in the T cell zones of lymph nodes These chemokines attract the DCs bearing microbial antigens into draining lymphatics and ultimately into the T cell zones of the regional lymph nodes Naive T cells also express CCR7, and this is why they localize to the same regions of lymph nodes where antigen-bearing DCs are concentrated 15 Antigen processing DCs activation Activation also converts the DCs from cells whose primary function is to capture antigen into cells that are able to present antigens to naive T cells and to activate the lymphocytes Activated DCs express high levels of MHC molecules with bound peptides and costimulators required for T cell activation In the absence of infection or inflammation, conventional DCs capture antigens in the tissues but are not activated to produce the high levels of cytokines and costimulators that are required to induce effective immune responses 16 Antigen processing DCs activation Several properties of conventional DCs make them the most efficient APCs for initiating primary T cell responses: 1. DCs are strategically located at the common sites of entry of microbes and foreign antigens (in epithelia) and in tissues that may be colonized by microbes 2. DCs express receptors that enable them to capture and respond to microbes 3. In response to chemokines, activated DCs migrate from epithelia and tissues via lymphatics, preferentially into the T cell zones of lymph nodes, and naive T lymphocytes also circulate through the same regions of the lymph nodes 17 Antigen processing DCs activation Several properties of conventional DCs make them the most efficient APCs for initiating primary T cell responses: 4. Mature DCs express high levels of peptide-MHC complexes, costimulators, and cytokines, all of which are needed to activate naive T lymphocytes 5. Specialized DCs (cDC1) can transfer internalized proteins from phagosomes into the cytosol and are thus efficient at cross-presenting antigens to CD8+ T cells. This process is essential for initiating CD8+ T cell responses to many viruses and tumors 18 Antigen processing Functions of other APCs In cell-mediated immune responses, macrophages present the antigens of phagocytosed microbes to effector T cells, which respond by activating the macrophages to kill the ingested microbes In humoral immune responses, B lymphocytes internalize protein antigens and present peptides derived from these proteins to helper T cells All nucleated cells can present peptides, derived from cytosolic protein antigens, to CD8 + CTLs Other cell types that express class II MHC molecules and may present antigens to T cells include endothelial and some epithelial cells 19 Antigen processing Processing of protein antigens Proteins that are present in the cytosol are degraded by proteasomes to yield peptides that are displayed on class I MHC molecules, whereas proteins that are ingested from the extracellular environment and sequestered in vesicles are degraded in lysosomes (or late endosomes) to generate peptides that are presented on class II MHC molecules 20 The class I MHC pathway for processing and presentation Antigen presentation of cytosolic proteins Microbial proteins present in the cytosol that undergo proteasomal degradation microbes are that derived either from produce antigens in the cytosol of cells or whose antigens are transferred to the cytosol The same principles apply to tumor antigens 21 The class I MHC pathway for processing and presentation Antigen presentation of cytosolic proteins Cytosolic antigens come from a number of sources: All viruses replicate and survive in infected cells and thus synthesize proteins in the infected cell cytoplasm Some bacteria are internalized into phagosomes but are able to damage phagosome membranes and create pores through which the microbes and their antigens enter the cytosol – Listeria monocytogenes Some bacteria have type III secretion systems that inject bacterial proteins into the cytosol - Yersinia pestis, Salmonella typhi, Shigella dysenteriae, Vibrio cholerae, and Chlamydia species 22 The class I MHC pathway for processing and presentation Antigen presentation of cytosolic proteins Cytosolic antigens come from a number of sources: The products of mutated genes in tumors produce antigens in the cytosol of the tumor cells most viruses infect cells other than DCs, and tumor antigens are produced in the tumor cells, not in DCs. The process by which antigens of other cells (virus-infected or tumor cells) are presented by DCs is called cross-presentation (or cross-priming), to indicate that one cell type (the DC) can present antigens from another cell (the virusinfected or tumor cell) and prime, or activate, T cells specific for these antigens The vesicles fuse with the endoplasmic reticulum (ER), and by mechanisms that remain poorly defined, proteins from the vesicles are transported into the cytosol 23 The class I MHC pathway for processing and presentation Antigen presentation of cytosolic proteins Digestion of Proteins in Proteasomes Proteasomes are large multiprotein enzyme complexes with a broad range of proteolytic activity that are found in the cytoplasm and nuclei of most cells appears as a cylinder composed of a stacked array of two inner β rings and two outer α rings, each ring being composed of seven subunits, with a caplike structure at each end of the cylinder Two types of proteasomes: Immunoproteasomes are present in immune cells, such as DCs and other APCs Thymoproteasome because it is present in thymic epithelial cells 24 The class I MHC pathway for processing and presentation Antigen presentation of cytosolic proteins Transport of Peptides From the Cytosol to the Endoplasmic Reticulum Peptides generated by proteasomes in the cytosol are translocated by a specialized transporter into the ER, where newly synthesized class I MHC molecules are available to bind the peptides This delivery is mediated by a dimeric protein located in the ER membrane called transporter associated with antigen processing (TAP) it optimally transports peptides ranging from 8 to 16 amino acids in length and containing carboxyl termini that are basic or hydrophobic these are the characteristics of the peptides that are generated in the proteasome and are able to bind to class I MHC molecules 25 The class I MHC pathway for processing and presentation Antigen presentation of cytosolic proteins Assembly of Peptide–Class I MHC Complexes in the Endoplasmic Reticulum Peptides translocated into the ER bind to newly synthesized class I MHC molecules that are associated with the TAP dimer through tapasin On the luminal side of the ER membrane, the TAP protein associates with a protein called tapasin, which also has an affinity for newly synthesized empty class I MHC molecules Once class I MHC molecules are loaded with peptide, they no longer have an affinity for tapasin, so the peptide-loaded class I MHC molecules are released and are able to exit the ER and be transported to the cell surface 26 The class I MHC pathway for processing and presentation Antigen presentation of cytosolic proteins Peptides transported into the ER preferentially bind to class I but not class II MHC molecules for two reasons: 1. newly synthesized class I MHC molecules are attached to the luminal aspect of the peptide-loading complex, and they capture peptides rapidly as the peptides are transported into the ER by TAP 2. the peptide-binding clefts of newly synthesized class II molecules in the ER are blocked by a protein called the invariant chain 27 The class I MHC pathway for processing and presentation Antigen presentation of cytosolic proteins Surface Expression of Peptide–Class I MHC Complexes Class I MHC molecules with bound peptides are structurally stable and are expressed on the cell surface Stable peptide–class I MHC complexes that were produced in the ER are guided by chaperones to move through the Golgi complex and are transported to the cell surface in exocytic vesicles the peptide–class I complexes may be recognized by peptide antigen– specific CD8+ T cells 28 The class II MHC pathway for presentation of proteins degraded in Antigen presentation acidic vesicles Most class II MHC–associated peptides are derived from protein antigens that are ingested into and digested in endosomes and lysosomes in APCs commonly extracellular proteins captured by endocytosis, phagocytosis, surface pinocytosis, but also proteins that endocytosed and intracellular proteins or include are cell being degraded that may and be membrane-bound, vesicular, or cytosolic 29 The class II MHC pathway for presentation of proteins degraded in Antigen presentation acidic vesicles Different APCs can bind native protein antigens in several ways and with varying efficiencies and specificities: 1. DCs and macrophages express a variety of surface receptors, such as lectins, that recognize structures shared by many microbes 2. Macrophages also express receptors for the Fc portions of antibodies and receptors for the complement protein C3b, which bind antigens that are opsonized by antibodies or complement proteins and enhance antigen internalization 3. the surface Ig on B cells, which, because of its high affinity for antigens, can effectively mediate the internalization of proteins present at very low concentrations in the extracellular fluid 30 The class II MHC pathway for presentation of proteins degraded in Antigen presentation acidic vesicles Proteolytic Digestion of Antigens in Acidic Vesicles Internalized proteins are degraded enzymatically in late endosomes and lysosomes to generate peptides that are able to bind to the peptide-binding clefts of class II MHC molecules mediated by proteases - the most abundant proteases of late endosomes are cathepsins, which are thiol and aspartyl proteases with broad substrate specificities Class II MHC molecules are synthesized in the ER and transported to endosomes with an associated protein, the invariant chain (I i), which occupies the peptide-binding clefts of the newly synthesized class II MHC molecules 31 The class II MHC pathway for presentation of proteins degraded in Antigen presentation acidic vesicles Class II–associated invariant chain peptide (CLIP), are transported into late endosomes and lysosomes, where the I i is degraded and the remaining CLIP is removed by the action of DM. Antigenic peptides generated in the vesicles are then able to bind to the class II MHC molecules 32 The class II MHC pathway for presentation of proteins degraded in Antigen presentation acidic vesicles Expression of Peptide–Class II MHC Complexes on the Cell Surface Class II MHC molecules are stabilized by the bound peptides, and the stable peptide– class II complexes are delivered to the surface of the APC, where they are displayed for recognition by CD4+ T cells Once expressed on the APC surface, the peptide–class II complexes are recognized by peptide antigen–specific CD4+ T cells, with the CD4 coreceptor playing an essential role by binding to nonpolymorphic regions of the class II MHC molecule 33 Nature of Effector T Cell Responses Antigen presentation Presentation of extracellular and cytosolic antigens to different subsets of effector T cells A, Cytosolic antigens are presented by nucleated cells to CD8 + cytotoxic T lymphocytes (CTLs), which kill (lyse) the antigen-expressing cells B, Extracellular antigens are presented by macrophages or B lymphocytes to CD4 + helper T lymphocytes, which activate the macrophages or B cells and eliminate the extracellular antigens 34 Summary Antigen processing and presentation The antigen receptors of most T cells recognize only peptides displayed by major histocompatibility complex (MHC) molecules on the surface of antigen-presenting cells (APCs). CD4+ helper T lymphocytes recognize antigens in association with class II MHC molecules, and CD8 + CTLs recognize antigens in association with class I MHC molecules APCs capture protein antigens, process them, and display MHC-associated peptides to T cells. DCs are the most efficient APCs for initiating primary responses by activating naive T cells, and macrophages and B lymphocytes present antigens to helper T cells in the effector phase of cell-mediated immunity and in humoral immune responses, respectively. All nucleated cells can present class I–associated peptides, derived from cytosolic proteins, such as viral and tumor antigens, to CD8+ T cells 35 Summary Antigen processing and presentation DCs capture antigens from their sites of entry (usually through epithelia) or production (in tissues) and transport these antigens to secondary (peripheral) lymphoid organs. Naive T cells that recirculate through these organs recognize the antigens, and primary immune responses are induced in these organs Antigen processing is the conversion of native proteins into MHC-associated peptides. This process consists of the introduction of exogenous protein antigens into vesicles of APCs or the synthesis of antigens in the cytosol, the proteolytic degradation of these proteins into peptides, the binding of peptides to MHC molecules, and the display of the peptide-MHC complexes on the APC surface for recognition by T cells 36 Summary Antigen processing and presentation For the class I MHC pathway, protein antigens are degraded in the proteasome, generating peptides that bind to class I MHC molecules. Most of these antigens are synthesized in the cytosol or introduced into the cytosol from microbes or vesicles. These peptides are delivered from the cytosol to the endoplasmic reticulum (ER) by an ATP-dependent transporter called transporter associated with antigen processing (TAP) Stable complexes of class I MHC molecules with bound peptides move out of the ER, through the Golgi complex, to the cell surface 37 Summary Antigen processing and presentation Specialized APCs, mainly DCs, can ingest virus-infected or tumor cells and transport their antigens into the cytosol for presentation by class I MHC molecules. This process, called cross-presentation, enables DCs to initiate CD8+ T cell responses to the antigens of ingested cells For the class II MHC pathway, protein antigens are internalized into endosomes, and these proteins are proteolytically cleaved by enzymes in lysosomes and late endosomes Newly synthesized class II MHC molecules associated with the invariant chain (Ii ) are transported from the ER to the endosomal vesicles 38 Summary Antigen processing and presentation These pathways of MHC-restricted antigen presentation ensure that most of the body’s cells are screened for the possible presence of foreign antigens The pathways also ensure that proteins from extracellular microbes preferentially generate peptides bound to class II MHC molecules for recognition by CD4 + helper T cells, which activate effector mechanisms that eliminate extracellular antigens Conversely, proteins synthesized by intracellular (cytosolic) microbes generate peptides bound to class I MHC molecules for recognition by CD8 + CTLs, which function to eliminate cells harboring intracellular infections 39 Antigen processing and presentation ©2024 Ross University School of Veterinary Medicine. All rights reserved.