IMMUNOLOGY Major Histocompatibility Complex MHC and its encoding

IMMUNOLOGY Major Histocompatibility Complex (MHC) and its encoding molecules Dr; Kwanama Phd; GENETIC ENGINEERING

MHC 1. Introduction 2. Structure of MHC genes and products 3. Polymorphism of MHC 4. Inheritance patterns of haplotypes 5. Differential distribution of MHC Class I & Class II in tissues 6. Interactions of MHC molecules with antigenic peptides 7. HLA and clinical medicine 8. Biological function of MHC molecules

1. Introduction

MHC 1. Introduction Histocompatibility The ability to accept grafts between individuals. Histocompatibility antigen The Ags primarily responsible for rejection of genetically different tissues are known as histocompatibility Ags. Major Histocompatibility Complex Cluster of genes found in all mammals Its products play role in discriminating self/non-self Participant in both humoral and cell-mediated immunity

MHC Major histocompatibility complex (MHC) A cluster of genes (complex) on chromosome 6 in humans, encoding cell surface molecules that are polymorphic and that code for antigens causing rapid graft rejection between members of a single species that differ at these loci. Several classes of protein such as MHC class I and II proteins are encoded in this region. Human: Human leukocyte antigens (HLA) Mouse: H-2

2. Organization and Structure of MHC Genes and Products

MHC Organization of MHC Genes and Products Organization of MHC genes Products of MHC class I and MHC class II genes---HLA molecules

Genes encoding MHC (HLA) glycoproteins are the most polymorphic genes in the human genome Located on the short arm of chromosome 6 in humans MHC class I and MHC class II from the particular loci were distinguished using specific antibodies which could recognize polymorphic variants at the loci Using newer antibodies more and more variants of MHC genes will be found

Organization of MHC Genes and Products Genes Of MHC Organized In 3 Classes Class I MHC genes Glycoproteins expressed on all nucleated cells Major function to present processed Ags to TC Class II MHC genes Glycoproteins expressed on M , B-cells, DCs Major function to present processed Ags to TH Class III MHC genes Products that include secreted proteins that have immune functions. Ex. Complement system, inflammatory molecules

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Organization of MHC genes

Gene loci for class I, II & III of MHC molecules

Organization of MHC genes and products Class I MHC genes found in regions A, B and C in humans (K and D in mice) Class II MHC genes found in regions DR, DP and DQ (IA and IE in mice) Class I and class II MHC share structural features Both involved in APC Class III MHC have no structural similarity to class I and II Ex. TNF, heat shock proteins, complement components

MHC Products of MHC class I and class II genes HLA class I molecules Genes: A, B, C Distribution: every nucleated cell Structure: α chain (45 k. D) β 2 m (12 k. D) peptide-binding cleft: α 1+α 2 CD 8+ binding: α 3 Function: recognize and present endogenous and cross-present exogenous Ags (from intracellular viruses) to CD 8+ T cells

MHC HLA class II molecules Genes: DR, DQ, DP Distribution: APC (B, DC, MΦ), thymic epithelial cells, activated T cells Structure: α chain (35 k. D) β chain (28 k. D) peptide-binding cleft: α 1+β 1 CD 4 binding: β 2 Function: recognize and present the exogenous Ags to CD 4 T cells

Structure of MHC class I molecule Comprised of 2 molecules Chain (45 kda), transmembrane 2 -microglobulin (12 kda) Non-covalently associated with each other Association of chain and 2 is required for surface expression Chain made up of 3 domains ( 1, 2 and 3) 2 -microglobulin similar to 3 1 and 2 form peptide binding cleft Fits peptide of about 8 -10 a/a long 3 highly conserved among MHC I molecules Interacts with CD 8 (TC) molecule

MHC molecules MHC class II Peptide binding groove Cell Membrane

MHC class II molecule Comprised of and chains Chain and chain associate non-covalently And chains made up of domains 1 and 2 ( chain) 1 and 1 form antigen binding cleft And heterodimer has been shown to dimerize CD 4 molecule binds 2/ 2 domains

Differential distribution of MHC Class I and Class II molecules in tissues

Location of MHC Class I and Class II molecules in tissues

Differential distribution of MHC molecules in tissues Tissue T cells B cells Macrophages Other APC Thymus epithelium Neutrophils Hepatocytes Kidney Brain Erythrocytes MHC class II +++ +++ + - +/+++ ++ +++ - Cell activation affects the level of MHC expression. The pattern of expression reflects the function of MHC molecules: • Class I is involved in the regulation of anti-viral immune responses • Class II involved in regulation of the cells of the immune system Anucleate erythrocytes can not support virus replication - hence no MHC class I. Some pathogens exploit this e. g. Plasmodium species.

Polymorphism of MHC

MHC polymorphism Each human usually expresses: 3 types of MHC class I (A, B, C) and 3 types of MHC class II (DR, DP, DQ) The number of different T cell antigen receptors is estimated to be 1, 000, 000 each of which may potentially recognise a different peptide antigen How can 6 invariant molecules have the capacity to bind to 1, 000, 000 different peptides?

Mechanisms of MHC polymorphism Presence of several MHC allelic variants: 6 MHC I and 12 MHC II molecules are expressed in an individual However enormous number of peptides needs to be presented using these MHC molecules than the 6 invariant molecules can handle To achieve this task MHC molecules are not very specific for peptides (unlike TCR and BCR) A peptide can bind a number of MHC An MHC molecule can bind many different peptides; the binding site is flexible at an early, intracellular stage of maturation formed by folding the MHC molecules around the peptide. Export only molecules that have captured a peptide to the cell surface

Polymorphism of MHC class I molecules The diversity of the MHC within a species stems from polymorphism, the presence of multiple alleles at a given genetic locus within the species. The MHC possesses an extraordinarily large number of different alleles at each locus and is one of the most polymorphic genetic complexes known in higher vertebrates. HLA-A 240 alleles H-2 K 55 alleles HLA-B 470 alleles H-2 D 60 alleles HLA-C 110 alleles The theoretical class I diversity possible for the human is 240 x 470 x 110

MHC Polymorphism of MHC 1. Polymorphism The phenomenon of having multiple stable forms of one gene in the population HLA 8016 (HLA-A 2013; HLA-B 2605 ; HLA-DRB 1260) HLA-A*0103 HLA-DRB 1*1102 Expression of MHC alleles is co-dominant 2. Linkage disequilibrium and haplotype A 2 -B 46 -Cw 3 -DR 9 -DQ 9 -Dw 23

MHC 3. Production of MHC Polymorphism The significance of MHC polymorphism: Almost all of the polymorphism among MHC alleles involves amino acid residues located in and around the peptide-binding groove. As a result, each allelic form has its own unique peptide-binding properties. Affects the ability to make immune responses (e. g. resistance or susceptibility to infectious diseases). Ensures survival of the population as a whole when they encounter any new pathogens by presenting new microbial epitopes.

Molecular basis of MHC types and variants Polygenism: several MHC class I and class II genes encoding different types of MHC molecule with a range of peptidebinding specificities. Polymorphism: variation >1% at a single genetic locus in a population of individuals MHC genes are the most polymorphic known The type and variant MHC molecules do not vary in the lifetime of the individual Diversity in MHC molecules exists at the population level This sharply contrasts diversity in T and B cell antigen receptors which are in a constant state of flux within the individual.

Inheritance patterns of MHC haplotypes in a typical human family A haplotype is a collection of specific alleles (particular DNA sequences) in a cluster of tightly-linked genes on a chromosome that are likely to be inherited together. Put in simple words, haplotype is the group of genes that a progeny inherits from one parent.

MHC

Inheritance of MHC haplotypes Parents DP-1, 2 DQ-3, 4 DR-5, 6 B-7, 8 C-9, 10 A-11, 12 DP DP DQ DR B C DP-9, 8 DQ-7, 6 DR-5, 4 B-3, 2 C-1, 8 A-9, 10 A Children DP DQ DR B C A DP DQ DR B C A DP-2, 8 DQ-4, 6 DR-6, 4 B-8, 2 C-10, 8 A-12, 10 DP DQ DR B C A DP-2, 9 DQ-4, 7 DR-6, 5 B-8, 3 C-10, 10 A-12, 9 DP DQ DR B C A DP-1, 8 DQ-3, 6 DR-5, 4 B-7, 2 C-9, 8 A-11, 10 A X DP-1, 9 DQ-3, 7 DR-5, 5 B-7, 3 C-9, 1 A-11, 9

Inheritance of MHC haplotypes • MHC molecules are encoded by huge numbers of alleles • It is unlikely that any two people will inherit exactly the same set • The repertoire of MHC proteins vary from person to person • Only identical twins will inherit Exactly the same set of MHC molecules

Interactions of MHC molecules with antigenic peptides

MHC Interactions of MHC molecules with antigenic peptides Molecular basis o Antigen-binding cleft class I Molecules: 1/ 2 closed at both ends 8~10 amino acids Class II Molecules: 1/ 1 open at both ends 11~15 amino acids o Anchor site and anchor residue

Cleft geometry -chain Peptide 2 -M MHC class I accommodate peptides of 8 -10 amino acids Peptide -chain MHC class II accommodate peptides of >13 amino acids

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Molecular mechanism anchor site anchor residue consensus motif MHC

MHC Characteristics of peptide-MHC binding Specificity: special anchor residues The important point is that peptide binding by a given MHC protein is selective but less specific than antigen binding by a TCR or a BCR. Flexibility: a series of different antigenic peptides with the same consensus binding motif can be presented by a given MHC molecule.

Biological functions of HLA

Biological functions of HLA Participates in the adaptive immune response serving as antigen presenting molecules T cells recognize both the antigenic peptide and MHC molecules by TCR (double recognition) 2. Participates in the innate immunity serving as regulatory molecules 1. Classical MHC class III genes encode complement, hereby participating in complement reactions and in the pathogenesis of some immune diseases. Non-classical HLA class I molecules (HLA-E, G) regulate the activity of NK cells. Inflammation related genes initiate and control inflammation. They also play a role in stress responses (such as HSP). MHC polymorphism is a crucial determinant of disease susceptibility in different individuals MHC contributes to the genetic heterogeneity of the population 3. 4.

HLA and clinical medicine

HLA and clinical medicine MHC HLA and transplantation : HLA typing; cross-match Abnormal expression of HLA Tumor: class I molecule↓ Autoimmune diseases: class II molecule↑ HLA and disease susceptibility Ankylosing spondylitis: B 27 Adverse drug reaction: carbamazepine : HLA-B*15: 02 (SJS) allopurinol: HLA-B*58: 01 HLA and forensic medicine Paternity Testing Vaccine design

Clinical application of MHC genetics Matching of transplant donors and recipients The biology, diversity and complexity of the MHC locus and its pattern of inheritance explains: The need to match the MHC of the recipient of a graft with the donor The difficulties faced in matching unrelated donors with recipients There is only ~20% chance of finding a match in siblings

Paternity Testing: Wife: A 2, A 11; B 27, Bw 44 Child 1: A 2, A 1; Bw 44, B 15, Cw 3 Child 2: A 2, Aw 24; Bw 44, Bw 54; Cw 3 Husband: A 1, A 3; B 15, -; Cw 3 Another man: A 2, Aw 24; B 7, Bw 54; Cw 3

HLA polymorphism and vaccine design Few human beings will share the same set of HLA alleles Different persons will react to a pathogen infection in a non-similar manner A CTL based vaccine must include epitopes specific for each HLA allele in a population A CTL based vaccine must consist of ~800 HLA class I epitopes and ~400 class II epitopes

MHC