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

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

3. 1. Introduction

4. 1. Introduction MHC 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

5. 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

6. 2. Organization and Structure of MHC Genes and Products

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

8. 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

9. 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

10. Organization of MHC genes
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11. Organization of MHC genes

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

14. 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

15. Products of MHC class I and class II genes
HLA class I molecules
Genes: A, B, C
Distribution: every nucleated cell
Structure:
α chain (45kD)
β2m (12kD)
peptide-binding cleft: α1+α2
CD8+ binding: α3
Function: recognize and present endogenous and cross-present exogenous Ags (from intracellular viruses) to CD8+ T cells

16. MHC HLA class II molecules Genes: DR, DQ, DP
Distribution: APC (B, DC, MΦ), thymic epithelial cells, activated T cells
Structure:
α chain (35kD)
β chain (28kD)
peptide-binding cleft: α1+β1
CD4 binding: β2
Function: recognize and present the exogenous Ags to CD4 T cells

17. 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 CD8 (TC) molecule

19. Cell Membrane MHC molecules MHC class I MHC class II Peptide Peptide
binding groove
Cell Membrane

20. 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 2 ( chain)
1and 1 form antigen binding cleft
 And  heterodimer has been shown to dimerize
CD4 molecule binds 2/2 domains

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

22. Location of MHC Class I and Class II molecules in tissues

23. Differential distribution of MHC molecules in tissues
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.
Tissue MHC class I MHC class II
T cells /-
B cells
Macrophages
Other APC
Thymus epithelium
Neutrophils
Hepatocytes
Kidney
Brain
Erythrocytes

24. Polymorphism of MHC

25. 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 1,000,000,000,000,000 each of which may potentially recognise a different peptide antigen
How can 6 invariant molecules have the capacity to bind to 1,000,000,000,000,000 different peptides?

26. 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

27. 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-2K 55 alleles
HLA-B 470 alleles H-2D 60 alleles
HLA-C 110 alleles
The theoretical class I diversity possible for the human is 240x470x110

28. Polymorphism of MHC MHC 1. Polymorphism
The phenomenon of having multiple stable forms of one gene in the population
HLA (HLA-A 2013; HLA-B 2605 ; HLA-DRB 1260)
HLA-A* HLA-DRB1*1102
Expression of MHC alleles is co-dominant
2. Linkage disequilibrium and haplotype
A2-B46-Cw3-DR9-DQ9-Dw23

29. 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.

30. 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 peptide-binding 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.

31. 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.

32. MHC

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

34. 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

35. Interactions of MHC molecules with antigenic peptides

36. Interactions of MHC molecules with antigenic peptides
Molecular basis
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
Anchor site and anchor residue

37. Cleft geometry b2-M a-chain Peptide a-chain b-chain Peptide
MHC class II accommodate
peptides of >13 amino acids
MHC class I accommodate
peptides of 8-10 amino acids

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39. MHC
Molecular mechanism
anchor site
anchor residue
consensus motif

40. 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.

41. Biological functions of HLA

42. 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)
Participates in the innate immunity serving as regulatory molecules
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

43. HLA and clinical medicine

44. HLA and clinical medicine
MHC
HLA and clinical medicine
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: B27
Adverse drug reaction:
carbamazepine : HLA-B*15:02 (SJS)
allopurinol: HLA-B*58:01
HLA and forensic medicine
Paternity Testing
Vaccine design

45. 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

46. Paternity Testing:
Wife: A2, A11; B27, Bw44 Child 1: A2, A1; Bw44, B15, Cw3 Child 2: A2, Aw24; Bw44, Bw54; Cw3 Husband: A1, A3; B15, -; Cw3 Another man: A2, Aw24; B7, Bw54; Cw3

47. 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

48. MHC