INSTITUTE FOR IMMUNOBIOLOGY Major Histocompatibility Complex MHC Department

INSTITUTE FOR IMMUNOBIOLOGY Major Histocompatibility Complex MHC Department of Immunology Fudan University Bo GAO, Ph. D 021 -54237154 gaobo@fudan. edu. cn

Major Histocompatibilty Complex, MHC 1. Discovery of MHC 2. MHC Genes 3. Binding of Peptides to MHC Molecules 4. MHC polymorphism 5. Function and significance

Types of graft

MHC of mice 1940 s Inbred mouse strains H-2 d H-2 b BALB/c C 57 BL/6 （George D. Snell) Inbred mouse strains - all genes are identical

Genetic basis of transplant rejection Transplantation of skin between strains showed that rejection or acceptance was dependent upon the genetics of each strain ACCEPTED Skin from an inbred mouse grafted onto the same strain of mouse REJECTED Skin from an inbred mouse grafted onto a different strain of mouse

MHC of mice H-2 ( Histocompatibility-2) A single genetic region is identified by Snell's group, which is primarily responsible for rapid rejection of tissue grafts, and this region was called the major histocompatibility locus. The particular locus encodes a blood group antigen called antigen II, and therefore this region was named histocompatibility-2, or simply H-2.

MHC of human HLA ( Human leukocyte antigen) Discovered by searching for cell surface molecules in one individual that would be recognized as foreign by another individual Jean Dausset leukocyte because the antibodies were tested by binding to the leukocytes of other individuals, and antigens because the molecules were recognized by antibodies

HLA proteins and the mouse H-2 proteins had essentially identical structure. Genes encoding HLA are homologous to the H-2 genes. They are all called MHC genes.

Immune Response Genes Inbred strains of guinea pigs and mice differed in their ability to make antibodies against some simple synthetic polypeptides Responsiveness was inherited as a dominant mendelian trait (Baruj Benacerraf ) The relevant genes were called immune response (Ir) genes, and they were all found to map to the MHC

Immune Response Genes These immune response (Ir) genes, are, in fact, MHC genes that encode MHC molecules that differ in their ability to bind and display peptides derived from various protein antigens.

1980 Noble prize (Baruj Benacerraf ) （Jean Dausset）（George D. Snell)

Major Histocompatibilty Complex, MHC 1. Discovery of MHC 2. MHC Genes 3. Binding of Peptides to MHC Molecules 4. MHC polymorphism 5. Function and significance

MHC of human MHC III MHC I

Expression Because MHC molecules are required to present antigens to T lymphocytes, the expression of these proteins in a cell determines whether foreign (e. g. , microbial) antigens in that cell will be recognized by T cells. There are several important features of the expression of MHC molecules that contribute to their role in protecting individuals from diverse microbial infections.

Expression Class I molecules are constitutively expressed on virtually all nucleated cells. Class II molecules are expressed only on dendritic cells, B lymphocytes, macrophages, and a few other cell types.

Expression Why two types of polymorphic MHC genes are needed? DC, B, MΦ Nucleated cells

Expression The expression of MHC molecules is increased by cytokines produced during both innate and adaptive immune responses. IFN-α, IFN-β , IFN-γ MHC II

Expression The rate of transcription is the major determinant of the synthesis of MHC molecule and its expression on the cell surface. Class II transcription activator (CIITA): highly inducible by IFN-γ MHC I, MHC II IFN-γ TAP, LMP 2, LMP 7

Structure Crystal structures Extracellular portions of MHC molecules with bound peptides Important for us to understand how MHC molecules display peptides

MHC-I Structure General Properties MHC- II a 2 a 1 b 1 Peptide-binding cleft a 3 b 2 m a 2 b 2 Ig-like domain transmembrane domain Each MHC molecule consists of an extracellular peptidebinding cleft, or groove, followed by immunoglobulin (Ig)-like domains and transmembrane and cytoplasmic domains. The polymorphic amino acid residues of MHC molecules are located in and adjacent to the peptide-binding cleft. The nonpolymorphic Ig-like domains of MHC molecules contain binding sites for the T cell molecules CD 4 and CD 8.

Major Histocompatibilty Complex, MHC 1. Discovery of MHC 2. MHC Genes 3. Binding of Peptides to MHC Molecules 4. MHC polymorphism 5. Function and significance

Characteristics of Peptide-MHC Interactions 1. Each class I or class II MHC molecule has a single peptidebinding cleft that binds one peptide at a time, but each MHC molecule can bind many different peptides. Why? Each individual contains only a few different MHC molecules (6 class I and more than 10 to 20 class II molecules in a heterozygous individual)

Characteristics of Peptide-MHC Interactions 2. The peptides that bind to MHC molecules share structural features that promote this interaction. MHC I: 8 to 11 residues MHC II: 10 to 30 residues (optimal length 13 to 18) Complementary interactions between the peptide and that allelic MHC molecule The residues of a peptide that bind to MHC molecules are distinct from those that are recognized by T cells

Characteristics of Peptide-MHC Interactions 3. MHC molecules acquire their peptide cargo during their biosynthesis and assembly inside cells. MHC molecules display peptides derived from microbes that are inside host cells MHC-restricted T cells recognize cell-associated microbes. They are the mediators of immunity to intracellular microbes.

Characteristics of Peptide-MHC Interactions 4. The association of antigenic peptides and MHC molecules is a saturable interaction with a very slow offrate. chaperones enzymes MHC-peptide interaction Stable peptide-MHC complexes Maximize the chance that a particular T cell will find the peptide Long half-lives

Characteristics of Peptide-MHC Interactions 5. Very small numbers of peptide-MHC complexes are capable of activating specific T lymphocytes. As few as 100 complexes of a particular peptide with a class II MHC molecule on the surface of an APC can initiate a specific T cell response. This represents less than 0. 1% of the total number of class II molecules likely to be present on the surface of the APC. .

Characteristics of Peptide-MHC Interactions 6. The MHC molecules of an individual do not discriminate between foreign peptides and peptides derived from self antigens. MHC molecules display both self peptides and foreign peptides. Most of peptides displayed by APCs derive from self proteins.

Characteristics of Peptide-MHC Interactions Question 1: How can a T cell recognize and be activated by any foreign antigen if normally all APCs are displaying mainly self peptide-MHC complexes? Answer: T cells are remarkably sensitive and need to specifically recognize very few peptide-MHC complexes to be activated. Thus, a newly introduced antigen may be processed into peptides that load enough MHC molecules of APCs to activate T cells specific for that antigen, even though most of the MHC molecules are occupied with self peptides.

Question 2: If individuals process their own proteins and present them in association with their own MHC molecules, why do we normally not develop immune responses against self proteins? Answer: T cells specific for such complexes are killed or inactivated. Therefore, T cells cannot normally respond to self antigens

Structural Basis of Peptide-MHC Interactions The binding of peptides to MHC molecules is a noncovalent interaction mediated by residues both in the peptides and in the clefts of the MHC molecules. Anchor residue Anchor pocket

Structural Basis of Peptide-MHC Interactions These peptides bind to the clefts of MHC molecules in an extended conformation. Once bound, the peptides and their associated water molecules fill the clefts, making extensive contacts with the amino acid residues that form the β strands of the floor and the α helices of the walls of the cleft.

Structural Basis of Peptide-MHC Interactions In the case of MHC I, association of a peptide with the MHC groove depends on the binding of the positively charged N terminus and the negatively charged C terminus of the peptide to the MHC molecule. In most MHC molecules, the β strands in the floor of the cleft contain "pockets. " Many class I molecules have a hydrophobic pocket that recognizes one of the following hydrophobic amino acidsvaline, isoleucine, or methionine-at the C-terminal end of the peptide. Anchor pocket

Structural Basis of Peptide-MHC Interactions Such residues of the peptide are called anchor residues because they anchor the peptide in the cleft of the MHC molecule. Each MHC-binding peptide usually contains only one or two anchor residues, and this presumably allows greater variability in the other residues of the peptide, which are the residues that are recognized by specific T cells. The 2 and 9 anchor residue play critical roles

Structural Basis of Peptide-MHC Interactions Many of the residues in and around the peptide-binding cleft of MHC molecules are polymorphic and different alleles favor the binding of different peptides. This is the structural basis for the function of MHC genes as "immune response genes"; Only animals that express MHC alleles that can bind a particular peptide and display it to T cells can respond to that peptide.

Structural Basis of Peptide-MHC Interactions The antigen receptors of T cells recognize both the antigenic peptide and the MHC molecules, with the peptide being responsible for the fine specificity of antigen recognition and the MHC residues accounting for the MHC restriction of the T cells. Variations in either the peptide antigen or the peptidebinding cleft of the MHC molecule will alter presentation of that peptide or its recognition by T cells. In fact, one can enhance the immunogenicity of a peptide by incorporating into it a residue that strengthens its binding to commonly inherited MHC molecules in a population.

Structural Basis of Peptide-MHC Interactions MHC II

Major Histocompatibilty Complex, MHC 1. Discovery of MHC 2. MHC Genes 3. Binding of Peptides to MHC Molecules 4. MHC polymorphism 5. Function and significance

MHC polymorphism In the human population gene locus Polymorphic alleles multiple alleles co-dominant expression

MHC polymorphism Co-dominance polygeny Diversity of MHC molecules Co-dominance and polygeny both contribute to the diversity of MHC molecules expressed by an individual

The MHC possesses an extraordinarily large number of different alleles at each locus MHC I MHC II A B C DRA DRB DQA 1 DQB 1 DPA 1 DPB 1 total Multiple allele 506 851 276 MHC protein * Up to 2007. 03 28 62 10 3 559 24 34 81 9 23 126 6 2581 1645

Major Histocompatibilty Complex, MHC 1. Discovery of MHC 2. MHC Genes 3. Binding of Peptides to MHC Molecules 4. MHC polymorphism 5. Function and significance

Significance of MHC polymorphism Bind to various Ag peptide Genetically determine the immune responsiveness Self-MHC restriction of T cell response

Rolf Zinkernagle Peter Doherty Nobel Prize 1996

T cell response is self-MHC restricted

Other function of MHC molecules Individual marker Mediate transplantation rejection Association of MHC alleles with risk of disease

Other function of MHC molecules Regulate the T cell development

SUMMARY The major histocompatibility complex (MHC) comprises a stretch of tightly linked genes that encode class I/II proteins associated with intercellular recognition and antigen presentation to T lymphocytes. MHC genes are polymorphic in that there are large numbers of alleles for each gene, and they are polygenic in that there a number of different MHC genes. Class I MHC molecules consist of an a chain, in complex with b 2 microglobulin. Class II MHC molecules are composed of two noncovalently associated glycoproteins, the a and b chain, encoded by separate MHC genes. Both class I and class II MHC molecules present antigen to T cells. Class I molecules present processed endogenous antigen to CD 8+ T cells. Class II molecules present processed exogenous antigen to CD 4+ T cells. Class I molecules are expressed on most nucleated cells; class II antigens are restricted to B cells, macrophages, and dendritic cells.