Antigen Processing and Presentation PDF

Antigen processing and presentation Dr Felix N. Toka, DVM, PhD, DSc., DACVM Professor, Veterinary Immunology & Virology Department of Biomedical Sciences Objectives for the topics Define what antigen processing and presentation is Know the different types of antigen presenting...

Antigen processing and presentation Dr Felix N. Toka, DVM, PhD, DSc., DACVM Professor, Veterinary Immunology & Virology Department of Biomedical Sciences Objectives for the topics Define what antigen processing and presentation is Know the different types of antigen presenting cells (APCs) and their distribution Understand how APCs process and present antigens and the roles of MHC class I and class II molecules in this process Understand where antigen presentation takes place What is antigen processing and presentation? It is the process by which antigen-presenting cells digest antigens acquired from inside or outside the cell and display the resulting antigenic fragments on cell surface MHC molecules for recognition by T lymphocytes content/uploads/2014/04/dendritic-cell.gif http://www.hemacare.com/blog/wp- What cells are involved in the process? Antigen-presenting cells (APC) - they digest (processing) and display (presentation) Later, T lymphocytes recognize the presented antigens and get activated to initiate a cellular immune response  Professional antigen presenting cells Both are mice Dendritic cells - present antigen to naïve CD4+ and CD8+ T lymphocytes, so they initiate T-cell responses  Ralph Steinman Prize motivation: "for his discovery of the dendritic cell and its role in adaptive immunity" Dr. Steinman died on 30th September 2011, three days before his Nobel Prize was announced  but Panipatfire phagocytosis Macrophages - present antigens to differentiated CD4+ T lymphocytes in the effector phase of cell-mediated immunity  antibodies Principal fund do but B lymphocytes - present antigens to helper T-cells (CD4+) during humoral immune response Antigen presenting cells *Non-professional antigen presenting cells Neutrophils Eosinophils Basophils T cells Fibroblasts NK cells Smooth muscle cells Astrocytes Microglial cells Thymic epithelial cells Corneal cells, etc. * - all other nucleated cells when they become infected, particularly with virus Origin of antigen presenting cells  Distribution and properties of APCs so Macrophage DC B cell MHC-II expression Low levels; induced Always expressed Always expressed; by PAMPs, inducible DAMPs, and on activation cytokines Antigen type and Extracellular Intracellular & Extracellular antigen presentation antigens: extracellular binds to by MHC presentation antigens: specific Ig receptors: via MHC II presentation presentation via MHC I & MHC II via MHC II Location Lymphoid tissue Lymphoid tissue Lymphoid tissue Connective tissue Connective tissue Blood Body cavities Epithelium  Antigen presenting cells Experimental evidence The responses of antigen-specific T-cells to protein antigens require the participation of antigen presenting cells (APCs), which capture, digest and display the antigens to T-cells  Two important functions of antigen presenting cells 1. They convert protein antigens to peptides, and they display peptide- MHC complexes for recognition by T-cells Conversion of proteins to peptides within APCs is called antigen processing 2. APCs provide additional stimulus to T-cells, the stimulus is called co- stimulation  Antigen presentation begins with antigen processing The pathways of antigen processing convert protein antigens derived from the extracellular space or the cytosol into peptides and load these peptides into MHC molecules for display to lymphocytes Protein antigens present in acidic vesicular compartments of APCs generate MHC class II-associated peptides, whereas antigens present in the cytosol generate MHC class I-associated peptides Antigen processing and presentation pathways confirms antigen pipa  Scientific evidence of the different pathways of antigen presentation was derived from the following experiment  Processing of endocytosed antigens for MHC class II- associated presentation: sequence of events 1. Uptake of extracellular proteins into vesicular compartments of APCs 2. Processing of internalized proteins in endosomal or lysosomal vesicles 3. Biosynthesis and transport of MHC class II molecules to endosomes 4. Association of processed peptides with MHC class II molecules in vesicles 5. Expression of peptide-class II complexes on the APC surface  Uptake of extracellular proteins into vesicular compartments of APCs Internalization/uptake is receptor mediated, macrophages and dendritic cells express a variety of receptors on their cell surface that are responsible for binding microbes B lymphocytes use the BCR to internalize antigen Once internalized, uptaken material is localized in vesicles called endosomes  Endosomes are acidic compartments that contain proteolytic enzymes Endosomes communicate with lysosomes to form phagolysosomes or secondary lysosomes, where further degradation of engulfed material takes place  Processing of internalized proteins in endosomal and lysosomal vesicles Internalized proteins are degraded enzymatically in late endosomes and lysosomes About 14-20aa long peptides are generated that bind to the peptide- binding clefts of class II MHC molecules Cathepsins are the most important proteolytic enzymes involved in protein degradation  Biosynthesis and transport of MHC class II molecules to endosomes The α and β chains of the MHC class II molecule are synthesized and associated with each other in the endoplasmic reticulum (ER)  Upon synthesis in the ER, MHC class II molecules are transported to the Golgi apparatus and then to endosomes with an associated invariant chain (Ii) O  Association of processed peptides with MHC class II molecules in endocytic vesicles In endosomes, the Ii dissociates and the antigenic peptides are associated with MHC class II molecules  Expression of peptide-MHC class II complexes on the APC surface Stable peptide-MHC class II complexes are delivered to the surface of the APC, where they are displayed for recognition by CD4+ T lymphocytes Experimental evidence of antigen processing Not a 0  Processing of cytosolic antigens for MHC class I- associated presentation: sequence of events 1. Cytosol location of foreign proteins say tatisdff.de 2. Proteolytic degradation of cytosolic proteins in the proteosome endosa 3. Transport of peptides from the cytosol to the ER 4. Assembly of peptide-MHC class I complexes in the ER 5. Surface expression of peptide-MHC class I complexes  Sources of cytosolic antigens make in he cells Proteins that are found in the cytosol are endogenously synthesized in nucleated cells Cytosolic proteins may originate from viruses or other intracellular microbes that infect cells Sometimes even endocytosed antigens may be presented through MHC class I molecules  Processing of cytosolic antigens Proteins synthesized in the cytosol are ubiquitinated thus destined for fgn proteosomal degradation b proteins Processing of cytosolic proteins Peptides are derived from cytosolic protein antigens by proteolysis in the proteasome The proteasome is a large multi- enzyme complex found in the cytoplasm of most cells and has a broad range of proteolytic activity Transport of peptides from the cytosol to the ER Peptides generated in the proteasome are transported to the ER The main transporting system is the “transporter associated with antigen presentation” (TAP) Bindinto re peptides andtinspotte the ER to  TAP is located in the membrane of ER where it mediates the transport of peptides into the ER  Assembly of peptide-MHC class I complexes in the ER Peptides translocated into the ER bind MHC class I molecules that are attached to the TAP dimer Surface expression of MHC class I-peptide complexes MHC class I-peptide complexes move through the Golgi complex and are delivered to the cell surface MHC class I molecules with bound peptides are structurally stable and are expressed on the cell surface  Once expressed on the cell surface, complexes may be recognized by peptide antigen-specific CD8+ T lymphocytes  Alternatives to antigen presentation through MHC Apart from the MHC antigen presentation system, there is another pathway to present lipid antigens Lipid antigens are presented by class I-like non-polymorphic molecules called CD1 CD1 – cont’d CD1 is found on APCs and on some epithelial cells CD1 presents lipid antigens to T-cells that are not MHC restricted such as NKT-cells  Nature of T cell responses The presentation of vesicular or cytosolic proteins by the MHC class II or class I pathways, respectively, determines which subsets of T-cells will respond to antigens Exogenously acquired proteins will generate peptides that activate CD4+ T-cells ended in Mite I up Endogenously acquired proteins will generate peptides that activate CD8+ T-cells ended in Miter up Nature of T cell responses E.fiiaes  The T cell receptor or commonly called the TCR The TCR is a surface molecule found on T- cells that recognizes antigen presented in the correct MHC context family The TCR is similar to immunoglobulin in structure and is part of the immunoglobulin superfamily  There are two types of TCRs, the predominant αβ TCR which is commonly found on T lymphocytes in lymphoid tissues, and the γδ TCR Teinterumnosed which is found on T lymphocytes at mucosal surfaces swines  The TCR Each T-cell bears a TCR of only one specificity (i.e., there is allelic exclusion) V Recognition of antigen by the αβ TCR is MHC- C dependent The γδ TCR recognizes antigen in an MHC- independent manner V = Variable, C = Constant  The TCR and CD3 complex The TCR is closely associated with a group of 4 proteins collectively called the CD3 complex The CD3 complex is composed of one γ, one δ, two ε and 2 ξ chains  The TCR and CD3 complex All of the proteins of the CD3 complex are invariant and they do not contribute to the specificity of the TCR in any way The CD3 complex is necessary for cell surface expression of the TCR during T-cell development The CD3 complex transduces activation signals to the cell nucleus following antigen interaction with the TCR  Properties of antigens recognized by T lymphocytes Most T lymphocytes recognize only peptides T-cells are specific for amino acid sequences of peptides They recognize only a few residues (anchor residue) in the peptide sequence http://edoc.hu-berlin.de/dissertationen/voigt-antje-2004-04-19/HTML/chapter1.html  T-cells recognize and respond to foreign peptide antigens only when they are attached to the surface of APCs within the appropriate MHC context Exception Some T-cells may be specific for small chemicals called haptens conjugates, such as dinitrophenol, urushiol of poison ivy, beta- lactams of penicillin derived antibiotics  T-cells from one individual recognize foreign peptides only when these peptides are bound to and displayed by the MHC molecules of that individual – this is called self MHC restriction the  CD4+ T-cells recognize peptides bound to MHC class II molecules - i.e., CD4+ T-cells are MHC class II restricted CD8+ T-cells recognize peptides bound to MHC class I molecules - i.e., CD8+ T-cells are MHC class I restricted  CD4+ MHC class II restricted T-cells recognize peptide antigens derived from extracellular (exogenous) proteins that are internalized into vesicles of APCs CD8+ MHC class I restricted T-cells recognize peptides derived from cytosolic (endogenously) synthesized proteins  The B cell receptor (BCR) The BCR does not interact with the MHC molecules B-cells can recognize soluble antigens as well as antigens exposed on the cell surface B-cells recognize peptides, proteins, nucleic acids, polysaccharides and lipids  B Lymphocyte – antigen interaction, in a T-dependent manner An antigen-processing B cell interacts with a helper T cell. Antigen is processed by a dendritic cell and presented to the helper T cell. The B cell itself can act as an antigen-presenting cell. Co-stimulators, such as CD154 and CD28, engage serially to trigger IL-4 secretion by the T cell and IL-4R production by the B cell. Antibody production occurs. Dendritic Cell: Maturation date Prepare.tn call to prep the Tall 0 caring begington intent  Two important functions of antigen presenting cells 1. They convert protein antigens to peptides and they display peptide- MHC complexes for recognition by T-cells Conversion of proteins to peptides by APCs and loading of peptides into MHC molecules is called antigen processing 2. APCs provide stimulus to T-cells, the stimulus is called co-stimulation  Role of co-stimulatory molecules Co-stimulatory signals are required for full T-cell activation Bothsignals rad2 The signal 1 is generated by interaction of an antigenic peptide bound in the MHC cleft with the TCR-CD3 complex (specific signal)  Co-stimulation - cont’d B7 Signal 2 is provided by interaction between CD28 on the T-cells and members of the B7 (CD80/CD86) family on the antigen CD28 presenting cells (co-stimulatory signal)  When T lymphocytes are stimulated without a co-stimulatory signal clonal anergy occurs Clonal anergy - a state of no responsiveness, which occurs when a co- stimulatory signal is absent not respond all and under at go apoptosis Regulation of co-stimulation  Antigen presenting function of APCs is enhanced by exposure to microbial products – APCs express Toll-like receptors that respond to microbes Detection of microbes through Toll-like receptors: induces expression of MHC molecules and co-stimulators  improves efficiency of antigen presentation activates APCs to produce cytokines that stimulate T-cells stimulates production of chemokines that promote their migration to sites of infection Antigen presentation to Naïve T cells: Role of dendritic cells in initiating T-cell responses T-cell responses are initiated in the peripheral lymphoid organs, to which protein antigens are transported after being collected from their portal of entry  Dendritic cells that are resident in epithelia and tissues, capture protein antigens and transport them to draining lymph nodes Several properties of dendritic cells (DCs) make them the most effective APCs for initiating primary T-cell responses DCs are strategically located at common sites of entry of microbes and in tissues that may be colonized by microbes Dendritic cells express receptors that enable them to capture microbes and respond to microbes  They preferentially migrate to T-cell-rich zones of lymph nodes through which naïve T-cells circulate Mature dendritic cells express high levels of co-stimulatory molecules, which are needed to activate naïve T-cells  Dendritic cells can ingest infected cells and tumor cells and present antigens from these cells to CD8+ T cells This contradicts the knowledge that ingested antigen is presented through the MHC class II pathway Presentation of such antigens through class I pathway is called cross- presentation or cross-priming