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Notes C-9: Immunology C-9: IMMUNOLOGY UNIT-III Major Histocompatibility complexes class I & class II MHC antigens, antigen processing and presentation. NOTES Major Histocompatibility Complex MHC complex is a large genomic region or group of genes found in most vertebrates on a single chromosome that codes the MHC molecules which plays a vital role in immune system. Major histocompatibility antigens (also called transplantation antigens) mediate rejection of grafts between two genetically different individuals. HLA (human leukocyte antigens) were first detected on leukocytes and so they are called MHC antigens of humans. H-2 antigens are their equivalent MHC antigens of mouse. A set of MHC alleles present on each chromosome is called an MHC haplo-type. Monozygotic human twins have the same histocompatibility molecules on their cells, and they can accept transplants of tissue from each other. Histocompatibility molecules of one individual act as antigens when introduced into a different individual. George Snell, Jean Dausset and Baruj Benacerraf received the Nobel Prize in 1980 for their contributions to the discovery and understanding of the MHC in mice and humans MHC gene products were identified as responsible for graft rejection. MHC gene products that control immune responses are called the immune response genes. Immune response genes influence responses to infections. The essential role of the HLA antigens lies in the induction and regulation of the immune response and defence against microorganisms. The physiologic function of MHC molecules is the presentation of peptide antigen to T lymphocytes. There are two general classes of MHC molecules – Class I and Class II. Class I MHC molecules are found on all nucleated cells and present peptides to cytotoxic T cells. Class II MHC molecules are found on certain immune cells themselves, chiefly macrophages, B cells and dendritic cells, collectively known as professional antigen-presenting cells (APCs). These APCs specialize in the uptake of pathogens and subsequent processing into peptide fragments within phagosomes. The Class II MHC molecules on APCs present these fragments to helper T cells, which stimulate an immune reaction from other cells. 2. Structure of Major Histocompatibility Complex (MHC): The MHC complex resides in the short arm of chromosome 6 and overall size of the MHC is approximately 3.5 million base pairs. The complete three-dimensional structure for both class I and class II MHC molecules has been determined by x-ray crystallography. The class I gene complex contains three loci A, B and C, each of which codes of α chain polypeptides. The class II gene complex also contains at least three loci, DP, DQ and DR; each of these loci codes for one α and a variable number of β chain polypeptides. Class III region is not actually a part of the HLA complex, but is located within the HLA region, because its components are either related to the functions of HLA antigens or are under similar control mechanisms to the HLA genes. Class III antigens are associated with proteins in serum and other body fluids (e.g., C4, C2, factor B, TNF) and have no role in graft rejection. 5. Classification of MHC Molecules: 1. MHC Class I Molecule: MHC Class I is a membrane spanning molecule composed of two proteins. The membrane spanning protein is approximately 350 amino acids in length, with about 75 amino acids at the carboxylic end comprising the trans-membrane and cytoplasmic portions. The remaining 270 amino acids, as shown in the diagram, are divided into three globular domains Alpha-1, Alpha- 2 and Alpha-3 prime, with alpha-1 being closest to the amino terminus and alpha-3 closest to the membrane. The second portion of the molecule is a small globular protein called Beta-2 Micro-globulin. It associates primarily with the alpha-3 prime domain and is necessary for MHC stability. The bound peptide sits within the groove. The MHC molecules ability to present a wide range of antigenic peptides for T cell recognition requires a compromise between broad specificity and high affinity. The peptide main chain is tightly bound whilst peptide side chains show less restrictive interactions. It is primarily the peptide side-chain contacts and conformational variability that ensures that the peptide-MHC complex presents an antigenically unique surface to T cell receptors. 2. MHC Class II Molecule: Although similar to Class I, the MHC Class II molecule is composed of two membrane spanning proteins. Each chain is approximately 30 kilodaltons in size, and made of two globular domains as shown in the diagram. The domains are named Alpha-1, Alpha-2, Beta-1 and Beta-2. The two regions farthest from the membrane are alpha-1 and beta-1. The two chains associate without covalent bonds. The bound peptide is within the groove. The MHC molecules ability to present a wide range of antigenic peptides for T cell recognition requires a compromise between broad specificity and high affinity. The peptide main chain is tightly bound whilst peptide side chains show less restrictive interactions. It is primarily the peptide side-chain contacts and conformational variability that ensures that the peptide-MHC complex presents an antigenically unique surface to T cell receptors. Class II molecules are dimers consisting of an alpha and beta polypeptide chain. Each chain contains an immunoglobulin like region, next to the cell membrane. The antigen binding cleft, composed of two alpha-helices above a beta-pleated sheet, specifically binds short peptides, about 15 to 24 residues long. The amino acid sequence around the binding site, which specifies the antigen binding properties, is the most variable site in the MHC molecule. Differences between Class I and Class II structures can explain the different length requirements for the bound peptide. The ends of the antigen binding cleft of Class I molecules taper and are blocked by bulky tyrosine that bind the N terminus of the peptide. These conserved residues are not found in Class II molecules where smaller residues (glycine or valine) replace the larger tyrosine. 3. MHC Class III Molecule: This class includes genes coding several secreted proteins with immune functions-components of the complement system (such as C2, C4 and B factor) and molecules related with inflammation (cytokines such as TNF-α, LTA, LTB) or heat shock proteins (hsp). Class-III 000002 molecules do not share the same function as class-I and II molecules, but they are located between them in the short arm of human chromosome 6. For this reason they are frequently described together. Fig. MHC I are found on all nucleated body cells, and MHC II are found on macrophages, dendritic cells, and B cells (along with MHC I). The antigen-binding cleft of MHC I is formed by domains α1 and α2. The antigen-binding cleft of MHC II is formed by domains α1 and β1. Antigen-Presenting Cells (APCs) All nucleated cells in the body have mechanisms for processing and presenting antigens in association with MHC molecules. This signals the immune system, indicating whether the cell is normal and healthy or infected with an intracellular pathogen. However, only macrophages, dendritic cells, and B cells have the ability to present antigens specifically for the purpose of activating T cells; for this reason, these types of cells are sometimes referred to as antigen- presenting cells (APCs). While all APCs play a similar role in adaptive immunity, there are some important differences to consider. Macrophages and dendritic cells are phagocytes that ingest and kill pathogens that penetrate the first-line barriers (i.e., skin and mucous membranes). B cells, on the other hand, do not function as phagocytes but play a primary role in the production and secretion of antibodies. In addition, whereas macrophages and dendritic cells recognize pathogens through nonspecific receptor interactions (e.g., PAMPs, toll-like receptors, and receptors for opsonizing complement or antibody), B cells interact with foreign pathogens or their free antigens using antigen-specific immunoglobulin as receptors (monomeric IgD and IgM). When the immunoglobulin receptors bind to an antigen, the B cell internalizes the antigen by endocytosis before processing and presentting the antigen to T cells. Antigen Presentation with MHC II Molecules MHC II molecules are only found on the surface of APCs. Macrophages and dendritic cells use similar mechanisms for processing and presentation of antigens and their epitopes in association with MHC II; B cells use somewhat different mechanisms that will be described 000003 further in B Lymphocytes and Humoral Immunity. For now, we will focus on the steps of the process as they pertain to dendritic cells. After a dendritic cell recognizes and attaches to a pathogen cell, the pathogen is internalized by phagocytosis and is initially contained within a phagosome. Lysosomes containing antimicrobial enzymes and chemicals fuse with the phagosome to create a phagolysosome, where degradation of the pathogen for antigen processing begins. Proteases (protein- degrading) are especially important in antigen processing because only protein antigen epitopes are presented to T cells by MHC II. APCs do not present all possible epitopes to T cells; only a selection of the most antigenic or immune dominant epitopes are presented. The mechanism by which epitopes are selected for processing and presentation by an APC is complicated and not well understood; however, once the most antigenic, immunodominant epitopes have been processed, they associate within the antigen-binding cleft of MHC II molecules and are translocated to the cell surface of the dendritic cell for presentation to T cells. Figure .A dendritic cell phagocytoses a bacterial cell and brings it into a phagosome. Lysosomes fuse with the phagosome to create a phagolysosome, where antimicrobial chemicals and enzymes degrade the bacterial cell. Proteases process bacterial antigens, and the most antigenic epitopes are selected and presented on the cell’s surface in conjunction with MHC II molecules.
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