Chapter 9 Major Histocompatibility Complex (MHC)

Chapter 9 Major Histocompatibility Complex Contents PartⅠ Several important concepts PartⅡ Composition of HLA complex PartⅢ Genetic characteristics of HLA PartⅣ Molecular structure and distribution of HLA PartⅤ Interaction of MHC molecules and antigen peptide PartⅥ Biological functions of MHC PartⅦ Application of HLA in medicine

PartⅠ Several important concepts Transplantation antigen(Histocompatibility antigen): Antigens which cause immune response to the graft and determine the survival of the graft.They are alloantigen which is specific for each individual.

MHS(Major histocompatibility antigen system )： A group of complex histocompatibility antigens which cause rapid and strong immunoreaction to the graft. mHS(Minor histocompatibility antigen system)： A group of complex histocompatibility antigens which cause slow and weak immunoreaction to the graft.

MHC (Major histocompatibility complex)： A large cluster of linked genes located in some chromosome of human or other mammals encode for MHS and relate to allograft rejection, immune response, immune regulation and cell-cell recognition.

HLA (Human leucocyte antigen)： The MHS of human which is associated with allograft rejection, immune response, immune regulation and cell-cell recognition. HLA complex： The MHC of human,a cluster of the genes which encode for HLA and relate to allograft rejection immune response, immune regulation and cell-cell recognition.

H-2 complex: The MHC of mouse (chromsome 17)

PartⅡ Composition of HLA complex HLA complex is located on chromosome 6 and divided into three regions Class II Class III Class I DP DQ DR C4B C4A Bf C2 HSP TNF B C E A F G RING DP DM LMP2 LMP7 DQ DR B2 A2 B1 A1 A B TAP1 TAP2 B2 A2 B3 B1 A1 B* A 第六对 染色体

Ⅰ. ClassⅠgene region Ⅱ. Class Ⅱ gene region Ⅲ. Class Ⅲ gene region Classical HLA classⅠgenes-------HLA-A,B,C Non-classical HLA classⅠgenes-----HLA-E,F,G MHC classⅠ chain related genes Ⅱ. Class Ⅱ gene region Classical HLA classⅡ genes---HLA-DP,DQ,DR Genes associated with antigen processing Ⅲ. Class Ⅲ gene region Complement genes-----C4,C2,Bf Inflammation-associated genes----TNF,HSP70

Ⅰ. Class Ⅰ gene region Classical HLA class Ⅰ genes ------HLA-A,B,C Participate in endogenous antigen presenting and immune regulation.

2. Non-classical HLA classⅠ genes ----HLA-E,F,G Participate in immune regulation Associated with maternal-fetal immune tolerance. 3. MHC classⅠchain-related genes(MIC) MICA,MICB,MICC,MICD,MICE Associated with cytotoxicity effect of NK cell.

Ⅱ. Class Ⅱ gene region 1. Classical class Ⅱ genes----HLA-DP,DQ,DR Participate in exogenous antigen presenting and immune regulation.

2. Genes associated with antigen processing The genes associated with endogenous antigen-processing and presenting TAP（Transporter associated with antigen processing,抗原加工相关转运体） LMP（Large multifunctional proteasome,巨大多功能蛋白酶体）or （ low molecular weight polypeptide 低分子量多肽）

(2) The genes associated with exogenous antigen-processing and presenting HLA-DM----Processing and presenting of exogenous Ag HLA-DO----Inhibit the function of HLA-DM

Ⅲ. Class Ⅲ gene region 1. Encoded genes of complement ------C4B,C4A,Bf,C2

2. TNF genes family 3. Heat-shock protein(HSP) genes family

PartⅢ Genetic characteristics of HLA Haplotype inheritance Polymorphism Linkage disequilibrium

1. Haplotype inheritance Genotype Phenotype

2. Polymorphism Polymorphism means there are multiple alleles at a gene locus of MHC in a population so that an individual may have variant allele. ------multiple allele ------codominance: The two alleles at a gene locus of MHC in the two homogenous chromosomes are expressed together.

3. Linkage disequilibrium The distribution of haplotypes is not random in a population. The probability in which two or more than two alleles present together in one chromosome is greater than the probability in which they present randomly.

PartⅣ Molecular structure and distribution of HLA molecules

Ⅰ. Molecular Structure of HLA molecules 1. Class Ⅰ HLA molecules (1)Class Ⅰ HLA molecules contain two separate polypeptide chains HLA-encoded alpha (or heavy) chain Non-HLA-encoded beta chain (beta2 microglobulin,β2m)---15 chromosome (2) Only alpha chain is trans-membrane HLA

(3) Class Ⅰ HLA molecules have four regions Peptide binding region----α1, α2 IgSF region----α3, β2m Trans-membrane region Cytoplasmic region

(4) Function of each domain 1 and 2 domains: Interact to form a peptide-binding region which is a groove(cleft) having a "floor"of 8 beta-pleated strands and two "walls" with alpha-helices. ( Containing a peptide of 8-11 aa in the groove)

3 domain: Binding to CD8 on Tc cells 2 microglobulin domain: To maintain proper conformation of class Ⅰ HLA molecules. Trans-membrane region: Anchoring class Ⅰ HLA molecules Intra-membrane region: Transmitting the signal

2. Class Ⅱ HLA molecules (1) Class Ⅱ HLA molecules consist of a non-covalent complex of two chains:  and . (2) Both  and  chain span the membrane. (3) Both  and  chain are HLA-encoded HLA

(4) Class Ⅱ HLA molecules have four regions Peptide binding region----α1, β1 IgSF region----α2, β2 Trans-membrane region Cytoplasmic region

(5) Function of each domain α1 and 1: Interact to form the peptide-binding region which is a groove having a "floor" of 8 beta-pleated strands and two "walls" with alpha-helices. (Containing a peptide of 10-30aa in the groove)

 2 and  2 domain : Form the immunoglobulin-like region.  2 domain can bind to CD4 on Th cells Trans-membrane region: Anchoring class Ⅱ HLA molecules Intra-membrane region: Transmitting the signal

β α

Ⅱ. Distribution of HLA molecules 1.Class Ⅰ HLA molecule: Expressed on nucleate cells 2.Class Ⅱ HLA molecule Expressed on professional antigen-presenting cell(macrophage, dentritic cell and B cell), activated T cell, thymus epithelial cell et al. 3.sHLA

PartⅤ Interaction between MHC molecule and antigenic peptide Class I MHC molecule -antigenic peptide complex Class II MHC molecule-antigenic peptide complex

Ⅰ. Molecular basis of interaction between MHC molecule and antigenic peptide

Interaction between MHC and antigen Anchor sites: Two or more sites on antigenic peptide bind to “groove” or “pocket” of MHC molecules. Anchor residues: The amino acid residues on anchor sites of antigenic peptide.

Antigenic peptides which can combine with Interaction between MHC and antigen Certain MHC molecule binds the corresponding anchor residue of antigenic peptides. Antigenic peptides which can combine with the same kind of MHC molecule have same or similar anchor sites and anchor residues (consensus motif).

Ⅱ. Characteristics of interaction Relative specificity (1) Specificity: Certain MHC molecule recognizes corresponding consensus motif (2) Flexibility: Amino acids “x” and anchor residues are variable Different MHC molecules can recognize similar consensus motif

MHC restriction: T cell only recognize antigenic peptide combining with MHC molecules. That is to say, interaction of T cell and antigen-presenting cell(APC) or target cell need restriction by MHC molecules.

Interaction of Th(CD4+T) cell and antigen-presenting cell (APC) is restricted by class Ⅱ molecules. Interaction of Tc(CD8+T) cell and antigen-presenting cell (APC) or target cell is restricted by class Ⅰ molecules.

CD8+T cell(Tc) CD4+T cell(Th) T cell Receptor Peptide MHC Class II Antigen Presenting Cell CD4 CD8

PartⅥ Biological functions of MHC 1.Induce the differentiation and maturation of T cell to form functional T cell repertoire 2.Present antigen to initiate immune response

1.Induce the differentiation of T cell to form T cell repertoire

2. Present antigen to initiate immune response Endogenous antigen: antigens synthesized within cells Exogenous antigen: antigens comes outside the cell Endogenous Ag is presented to CD8+ T cell by MHC class Ⅰ molecule Exogenous Ag is presented to CD4+ T cell by MHC class Ⅱ molecule

CD8+T cell

CD4+T cell

PartⅦ Application of HLA in medicine HLA and transplantation

MLR Mixed lymphocyte reaction to determine identity of class II HLA antigens between a potential donor and recipient. Lymphocytes from the donor are irradiated or treated with mitomycin C. If the class II antigens on the two cell populations are different, the recipient cells will divide rapidly and take up large quantities of radioactive nucleotides into the newly synthesized nuclear DNA. The amount of radioactive nucleotide uptake is roughly proportionate to the MHC class II differences between the donor and recipient lymphocytes.

Some diseases are associated with HLA genotypes

HLA and forensic medicine: Paternity testing Criminal investigation