Chapter 6a (Major Histocompatibility Complex Molecules) PDF

Cellular & Molecular Immunology 10th Edition Abul Abbas, Andrew Lichtman, Shiv Pillai CHAPTER 6a Major Histocompatibility Complex Molecules Lectures Adapted by Dr. Amin Majdalawieh American University of Sharjah  Ant...

Cellular & Molecular Immunology 10th Edition Abul Abbas, Andrew Lichtman, Shiv Pillai CHAPTER 6a Major Histocompatibility Complex Molecules Lectures Adapted by Dr. Amin Majdalawieh American University of Sharjah  Antigen Presentation to T Lymphocytes u Antigens entering the body are taken up by antigen presenting cells (APCs) through phagocytosis or infection. u Some of the antigens are processed for presentation to T cells as peptides, while some are retained in their native configuration for display to B cells.  The Major Histocompatibility Complex u Unlike B lymphocytes, T lymphocytes cannot interact with free antigens and can only recognize antigens displayed on APCs. u MHC molecules are self markers expressed on all host cells (except RBCs). u MHC molecules enable T lymphocytes to recognize cell-associated antigens. u T cell receptors (TCRs) can specifically interact with MHC-antigen complexes expressed on different cells. u There are two types of MHC molecules: class I & class II § Class I MHC molecules complex with intracellular/cytosolic antigens (e.g. Viral peptides from virus-infected cells, antigens present in tumor cells, self and altered self peptides) § Class II MHC molecules complex with extracellular/vesicular antigens (e.g. All self and non-self proteins taken up by cells via phagocytosis or endocytosis). u Class I MHC molecules present peptides to CD8+ cytotoxic T cells u Class II MHC molecules present peptides to CD4+ helper T cells. T Cell Recognition of a Peptide-MHC Complex Figure 6a-1 Features of MHC Molecules Table 4-1  Discovery of the Mouse MHC u The MHC was discovered as the genetic locus whose products are responsible for rapid rejection of tissue grafts exchanged between inbred strains of mice. u In the 1940s, George Snell and colleagues were using genetic techniques to evaluate graft and tumor rejection. u Inbred strains that are genetically identical as called syngeneic. u Inbred strains that are polymorphic (different alleles) are called allogeneic. Grafts were accepted among members of the same inbred strain. Grafts were rejected among dissimilar inbred strains. u So, the recognition of a graft as self or foreign is an inherited trait. u The genes responsible for graft acceptance or rejection are called “histocompatibility genes”, and differences between self or foreign are attributed to polymorphisms among different histocompatibility gene alleles. u Rejection genes were mapped to small area on chromosome 17, and the genes codes for a blood antigen called antigen II (H-2). MHC Genes Control Graft Rejection/Immune Responses Figure 6a-2  Discovery of the Mouse MHC u MHC genes control immune responsiveness to protein antigens. u For almost 20 years after its discovery, MHC’s only documented role was in graft rejection. u Since transplantation is NOT a normal phenomenon, scientists questioned whether graft rejection was the only role of MHC molecules. u In the 1960s and 1970s, MHC molecules were shown to be of fundamental importance for immune responses to protein antigens. u Some inbred strains of mice could mount good immune responses to structurally simple polypeptide antigens, while others couldn’t (immunization). u The relevant genes were mapped to an area on chromosome 17 (same as rejection locus) and were called immune response genes (Ir genes).  Discovery of the Human MHC u Human MHC molecules are called human leukocyte antigens (HLA) and are equivalent to H-2 molecules in mice. u Was NOT defined in the same way as in mice (breeding and backcrossing). u Was observed that patients who rejected kidney transplants or had transfusion reactions to WBC transfer often times developed antibodies that reacted with blood cells of tissue donor. u Sera that react with the donor’s cells are called alloantisera and it contains alloantibodies that can recognize molecular targets called alloantigens. u Alloantibodies were shown to be products of a gene cluster located on chromosome 6. u HLA-A, HLA-B & HLA-C in humans are equivalent to H-2K, H-2D, & H- 2L in mice (class I MHC molecules). u The Ir genes (I-A & I-E) in mice are equivalent to MLR genes (HLA-DR, HLA-DP, & HLA-DQ) in humans (class II MHC molecules). Schematic Maps of Human & Mouse MHC Loci Figure 6a-3  Genetic Polymorphism u Genes represented by only one nucleic acid sequence in all members of a species (with exception of rare mutations) are said to be nonpolymorphic. u A wild-type (normal) sequence is generally found on both chromosomes (1 donated by mom and the other by dad). u Genes that can exist in many alternative forms or variants and exist at stable frequencies in different members of a population (species), are called polymorphic. u Each common variant of a polymorphic gene is called an allele. u MHC molecules are highly polymorphic.  Properties of MHC Genes u The two types of polymorphic MHC genes (class I & II) encode two groups of structurally distinct but homologous proteins. u MHC genes are the most polymorphic genes present in the genome of all species analyzed. u MHC genes are always co-dominantly expressed in each individual (i.e. both classes I & II should be expressed in an individual). u The set of MHC alleles present on each chromosome is called an MHC haplotype.  Structure of MHC Molecules u Key advance came with solution of crystal structure of the extracellular portions of class I & class II molecules by Don Wiley, Jack Strominger and colleagues. u Each MHC molecule consists of an extracellular peptide-binding cleft (groove) followed by Ig-like domains, transmembrane, and cytoplasmic domains. u The polymorphic amino acid residues of MHC molecules are located in and adjacent to the peptide-binding cleft. u The nonpolymorphic Ig-like domains of MHC molecules contain binding sites for the T cell molecules CD4 and CD8. CD4 and CD8 are considered co-receptors; they do not bind to the antigen. CD8 interacts selectively with MHC I molecules (cytotoxic T lymphocytes). CD4 interacts selectively with MHC II molecules (helper T lymphocytes).  Class I MHC Molecules u Class I MHC molecules consist of 2 non-covalently linked polypeptide chains: 1. MHC-encoded a chain (44-47 kD) 2. Non-MHC-encoded b chain (12 kD) u The a chain possesses three main moieties: 1. Extracellular region (more than 75%) (N-terminus at the end) 2. Transmembrane region (25 aa residues) 3. Cytoplasmic region (C-terminus at the end) (30 aa residues) u The extracellular region of the a chain has two segments (a1 & a2) made of anti-parallel b-strands supporting parallel a-helices. u This arrangement forms the antigen-binding cleft of MHC I molecules. u The antigen-binding cleft is somewhat “closed” and cannot support native globular proteins (large size) unless such proteins are “processed”.  Class I MHC Molecules u There is great variability in the a1 & a2 sequences, which provides specificity to MHC-antigen interaction. u The a3 segment (nonpolymorphic) forms Ig domain, and it is conserved among all MHC I molecules. u a3 binds to CD8. u The light chain of MHC I molecule (b chain) is called b2-microglobulin (structurally homologous to Ig domain). u The b chain interacts with the a chain non-covalently. u MHC I is a heterodimer (when there is no bound peptide). u The “fully assembled” MHC I molecule is a heterotrimer consisting of a chain, b2-microglobulin, and a bound antigenic peptide (required to stabilize the molecule and retain the expression of only helpful peptide-loaded MHC I molecules on the cell surface). Structure of Class I MHC Molecule Figure 6a-4  Class II MHC Molecules u Class II MHC molecules consist of 2 non-covalently linked polypeptide chains: 1. a chain (32-34 kD) 2. b chain (29-32 kD) u a & b chains of MHC II molecules possess three main moieties: 1. Extracellular region (more than 75%) (N-terminus at the end) 2. Transmembrane region (25 aa residues) 3. Cytoplasmic region (C-terminus at the end) u Unlike MHC I molecules, the genes encoding both chains of MHC II molecules are polymorphic. u The N-termini of both a1 & b1 chains form the peptide-binding cleft. u Unlike MHC I molecules, the peptide-binding cleft in MHC II molecules is open, and peptides of 30 aa residues or more can fit.  Class II MHC Molecules u a2 & b2 chains (nonpolymorphic) of MHC II molecules form Ig domains. u b2 chain bind to CD4. u MHC II is a heterodimer (when there is no bound peptide). u The “fully assembled” MHC II molecule is a heterotrimer consisting of a chain, b chain, and a bound antigenic peptide (required to stabilize the molecule and retain it’s the expression of only helpful peptide-loaded MHC II molecules on the cell surface). Structure of Class II MHC Molecule Figure 6a-6 Polymorphic Residues of MHC Molecules Figure 6a-5 Features of Class I & II MHC Molecules Table 6a-1  MHC Molecules Helper T cell Cytotoxic T cell TCR CD4 CD8 peptide antigen Class II MHC Class I MHC Antigen Presenting Cell (APC)  Binding of Peptides to MHC Molecules u For a protein to be immunogenic in an individual, processing by an APC must produce peptides that can bind to the MHC molecules of that individual. u For a given allele and a given peptide, some can and some cannot elicit an immune response. u Information about peptide-MHC interaction could be very helpful in developing effective vaccines by inserting the MHC-binding amino acid sequences into antigens used for immunization.  Characteristics of Peptide-MHC Interactions u MHC molecules show a broad specificity for peptide binding, in contrast to the fine specificity of antigen recognition by TCRs. u Each MHC I and MHC II molecule has a single peptide-binding cleft (but it can bind many, different peptides). Evidence? If a T cell specific for one peptide is stimulated by an APC presenting that peptide, the response is inhibited by addition of similar, but non-identical, peptides. Using purified MHC molecules, different peptides were shown to interact with MHC molecules in a competitive manner. u The ability of one MHC molecule to interact with multiple (4 to 6) peptides is NOT surprising given that each individual has only a small number of different MHC molecules (6 MHC I molecules; 10-20 MHC II molecules). MHC II molecules are more specific since they are more diverse. Antigen Competition for T Cells Figure 6a-7  Characteristics of Peptide-MHC Interactions u The peptides that bind to MHC molecules share structural features that promote such interaction (to fit physically and chemically). Size of peptide (8-10 aa for class I and 10-30 aa for class II) (optimal 12-16) Amino acid sequence of peptide u The association of antigenic peptides and MHC molecules is a saturable interaction with a very slow off-rate (stable interaction). By the help of chaperones/enzymes, peptide-MHC complexes are somewhat stable for a relatively long time (a few hours to days). Thisprovides enough time for T cells to encounter an APC carrying the peptide-MHC complex. u MHC molecules of an individual do NOT discriminate between foreign peptides and self-proteins. T cells can survey the peptide-MHC complexes displayed by APCs and recognize only foreign peptides. Important to recognize cancerous cells; applies more to MHC I molecules Structural Basis of Peptide Binding to MHC Molecules u The binding of peptides to MHC molecules is a non-covalent interaction mediated by residues both in the peptides and in the peptide-binding clefts of MHC molecules. u Initially, the peptides interact with the clefts in an extended conformation. u Conformational changes allow a better “fit” between the peptide & the peptide-binding cleft forming b-sheets of the floor of the cleft and a-helices of the walls of the cleft. u Often, the b-sheets of the cleft contain “pockets” where aa residues of the peptide (called anchor aa residues) fit into through hydrophobic interactions (complementarity). u The aa residues on the a-helices of the cleft also contribute to peptide-MHC interaction via hydrogen bonding & charge interactions (salt bridges). u The antigen receptors of T cells (TCRs) recognize both: the antigenic peptide (provides fine specificity for antigen recognition) the MHC molecule (accounts for MHC restriction of T cells) Peptide Binding to MHC Molecules Figure 6a-8  Genomic Organization of MHC Molecules u Many of the proteins involved in the processing of antigenic peptides and presentation of such peptides to T cells are encoded by genes located within the MHC genes. u Within class II and I loci are genes that encode several proteins that play critical roles in antigen processing. u Between class I and class II gene clusters are genes (class III MHC genes) that encode other proteins not involved in antigen processing: components of the complement system three structurally-related cytokines tumor necrosis factor (TNF) lymphotoxins a & b (LT-a & LT-b) heat shock proteins u Class III MHC molecules do not present like MHC I & II molecules (i.e. they are functionally unrelated to MHC I & II molecules) Map of the Human MHC Figure 6a-9  Expression of MHC Molecules u Class I molecules are constitutively expressed on virtually all nucleated cells (this excludes RBCs since they do not have a nucleus and doesn’t get cancerous). u Class II molecules are normally expressed on APCs (dendritic cells, B cells, macrophages) and a few other cell types following induction. u The expression of MHC molecules is modulated by cytokines produced during both innate and adaptive immune responses. IFN-a, IFN-b, IFN-g, TNF, and LT (lymphotoxin) increase MHC I expression. IFN-g increases MHC II expression. u The rate of transcription is the major determinant of the level of MHC molecule synthesis and expression on the cell surface. Cytokines can stimulate target cells to turn on certain transcription factors (e.g. CIITA), which in turn transactivate MHC gene expression. “bare lymphocyte syndrome” is a condition of immunodeficiency in humans where mutations in several transcription factors lead to defective expression of MHC I and/or MHC II molecules. Enhancement of Class II MHC Expression by IFN-γ Figure 6a-10