INSTITUTE FOR IMMUNOBIOLOGY Major Histocompatibility Complex MHC Department of Immunology Fudan University Bo GAO, Ph.D

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

（ George D. Snell) 1940s H-2 d H-2 b BALB/cC57BL/6 Inbred mouse strains MHC of mice Inbred mouse strains - all genes are identical

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

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 mice

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

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.

(Baruj Benacerraf ) 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 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. Immune Response Genes

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

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 IIMHC IIIMHC I MHC of human

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 I 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 IFN-γ MHC I, MHC II TAP, LMP2, LMP7 Expression

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

General Properties Each MHC molecule consists of an extracellular peptide- binding 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 CD4 and CD8. Structure MHC-IMHC- II 22  11 mm 11  Peptide-binding cleft Ig-like domain transmembrane domain

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 peptide- binding 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)

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 off- rate. chaperones enzymes MHC-peptide interaction Stable peptide-MHC complexes Long half-lives Maximize the chance that a particular T cell will find the peptide 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? Characteristics of Peptide-MHC Interactions 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. T cells are remarkably sensitive and need to specifically recognize very few peptide-MHC complexes to be activated. Answer:

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? Question 2: Answer: T cells specific for such complexes are killed or inactivated. Therefore, T cells cannot normally respond to self antigens

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 acids-valine, isoleucine, leucine, 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 peptide- binding 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 IMHC 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

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

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

DwDw * Up to MHC I MHC II A B C DRA DRB DQA1 DQB1 DPA1 DPB1 total Multiple allele MHC protein The MHC possesses an extraordinarily large number of different alleles at each locus

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

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

Rolf Zinkernagle Peter Doherty Nobel Prize 1996

T cell response is self-MHC restricted

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

Regulate the T cell development Other function of MHC molecules

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 are a number of different MHC genes. Class I MHC molecules consist of an a chain, in complex with b2- 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 CD8+ T cells. Class II molecules present processed exogenous antigen to CD4+ T cells. Class I molecules are expressed on most nucleated cells; class II antigens are restricted to B cells, macrophages, and dendritic cells. SUMMARY