Major Histocompatibility Complex and Antigen Presentation 미생물학교실 권 형 주

Antigen Presentation, MHC-Peptide complex, TCR Medzhitov R, & Janeway C Jr., N. Engl. J. Med., ,

MHC 의 발견 - Polymorphic: many alternative forms of the gene, or allele, exist at each locus among the population - Inbred mice: Homozygous at every genetic locus : Every mouse of an inbred strain is genetically completely identical (syngenic) - Congenic mouse strains: a single genetic region is responsible for most rejection phenomena

Simplified organization of the MHC in the mouse and human

Major Histocompatibility Complex (MHC) molecules - T 세포에 항원을 전달 Recognition by the  TCR requires antigen to be bound to an MHC molecule : Short peptide fragments (endogeneously synthesized)-MHC class I complex : short peptide fragments (extracellular origin)-MHC class II complex : Peptide-MHC complexes serve as ligands for TCRs

MHC molecule provide a sophisticated surveillance system for intracellular antigens : MHC class I molecules - intrinsic antigens 인식 - antigenic peptides from viruses or other pathogens that inhabit the cell - present antigen to cytotoxic T cells (CD8 + T cells) - controlling viral infections by lysing infected cells : MHC class II molecules – extrinsic antigens 인식 - present antigen to helper T cells (CD4 + T cells) - aid B cells in generating antibody responses to extracellular protein antigens : antigenic peptides are produced by proteolytic processing of proteins - Antigen processing and presentation

MHC class I and II molecules

MHC class I - Glycosylated heavy chain (  -chain, 45 kDa) associated with  2 -microglobulin (12 kDa) - Heavy chain : three extracellular domains (  1,  2,  3 ) : transmembrane domain : cytoplasmic tail : intracellular disulfide bond:  2,  3 :  3 domain-homologous Ig C domain interacts with CD8 Tc cells

 2 -microglobulin is essential for expression of MHC class I molecules - non-polymorphic in humans - Ig constant region domain - associate with class I-like molecules (CD1, Fc receptor) - The cell surface-mutant mice lacking  2 m do not express class I  : essential for the expression of all class I molecules  - Daudi cells (tumor) : absence of  2 m, MHC class I  chain –not in membrane  : transfection of  2 m –MHC class I appear on membrane

MHC class II - Heterodimers: heavy  chains: kDa light  chains: kDa - Extracellular domains :  1 and  2,  1 and  2 - Peptidie-binding cleft :  1 -  1 - Transmembrane region : ~30 residues - Cytoplasmic domain : residues -  2 and  2 domains  class I  3 and  2 m -  1,  2,  1 domains: N-glycosylated -  2 domain : binding site for CD4 Blue : HLA-DR1 Red : HLA-A2

The exon/intron arrangement of class I and II genes reflects their domain structure

Class I and II molecules exhibit polymorphism in the region that binds to peptides

- The bound peptides isolated from cell surface MHC molecules  purify and sequence the peptides (HPLC) : foreign peptides – internalized antigens or viral particles : self molecules produced within the cell or endocytosed from extracellular fluids Class I MHC-peptide interaction Example of anchor residues (blue) in nonameric peptides eluted from two class I MHC molecules

Class II MHC-peptide interaction - Incorporates a number of binding pockets, though the locations are somewhat different from that on class I molecules - Class II Is not closed at the ends, so bound peptides extend out of the ends of the groove amino acids Peptides binding MHC class II are less uniform in size than those binding MHC class I molecules - MHC class II : extend beyond the ends of the cleft =  antigen processing pathway (chapter 7)

Class I and Class II molecules exhibit diversity within a species, and multiple forms occur in an individual - Ab, TCR diversity : somatic process : gene rearrangement, somatic mutation -MHC : polymorphism : multiple alleles at a given genetic locus within the species : polygenic (HLA-A, -B, and –C) : HLA-A, -B, and –C : 370X660X190, : 46 million different class I haplotypes possible in the population Linkage disequilibrium : the actual diversity is known to be less, because certain allelic combinations occur more frequently in HLA haplotype than predicted by random combination : Successful organ transplants ????

Functional relevance of MHC polymorphism

Detailed genomic Map of MHC genes

Cellular expression of MHC Molecules - Class I MHC molecules : Expressed on most nucleated cells : Lymphocytes – 5 x 10 5 molecules/cell : Cell 에 따라 발현정도 다름 - Class II MHC molecules : APC, macrophages, mature dendritic cells, thymic epithelial cells…… : Cytokine stimulation, differentiation stage 에 따라 다름 ……

Human class II genes are located in the HLA-D region - HLA-D region, Three loci : DR, DQ, DP - DR family: single a gene (DRA) up to nine b genes (DRB1-9)including pseudogenes - The class II region also contains genes that encode proteins involved in antigen presentation that are not expressed at the cell surface

- The organization and length of the DRB region varies in different haplotypes, with different numbers of  chain expressed MHC polymorphism is concentrated in and around the peptide-binding cleft - Extreme degree of polymorphism (structural variability) - Class I :  1 and  2 domains - Class II : DR , DQ , DQ  chains, DP   less polymorphic --  peptide binding site DR  chains are invariant - Individuals have two MHC haplotypes

Regulation of MHC Expression TRAF6 p65 c-Rel IBIB HLA-DRA XX2Y RFX CIITA BB MyD88 CpG-DNA LPS NF-  B IRAK TLRs NF-Y IRF-1 CIITA Stat1 JAK1, 2 IFN-  USF-1 P Stat1 p38 MEK-1 ? - Regulation MHC class II expression - CIITA : class II transcriptional activator

- Regulation MHC class I expression

MHC and Diseases Susceptibility An individual’s MHC haplotype affects their susceptibility to disease Different MHC molecules affect: - The ability to make immune responses, including the level of antibody production - Resistance or susceptibility to infectious disease - Resistance or susceptibility to autoimmune diseases and allergies Why the MHC is so polymorphic? - many different pathogens  selective advantage in having different MHC molecules - select for different MHC molecules in each area

Self-MHC Restriction of T cells CD4 + and CD8 + T cells can recognize antigen only when presented by a self-MHC molecule - CD4 + T H cell is class II MHC restricted - CD8 + Tc cells is class I MHC restricted

Role of Antigen-Presenting cells Processing of antigen is required for recognition by T cells

Most cells can present antigen with class I MHC; presentation with class II MHC is restricyed to PACs - Display peptides associated with class II MHC to CD4 + T H cell : Antigen-presenting cells - Display peptides associated with class I MHC to CD8 + Tc cells : Target cells - Professional antigen-presenting cells : constitutively ecpress class II MHC, costimulatory molecules - Nonprofessional antigen-presenting cells

Evidence for different antigen-processing and presentation pathways Overview of cytosolic and endocytic pathways for processing antigen

- Inhibitor of protein synthesis : emetine : Class I presentation inhibition - Endocytic processing blocker : chloroquine : Class II presentation inhibition

Endogenous antigens : The Cytosolic Pathway Cytosolic proteolytic system for degradation of intracellular proteins - Immunoproteasome : induced by IFN- , TNF- , virus-infected cells

TAP (transporter associated with antigen processing)

Assembly and stabilization of class I MHC molecules Molecular chaperones : calnexin, calreticulin, tapasin

Exogenous antigens : The Endocytic Pathway

- Ii (CD74) : invariant chain - CLIP : class II-associated invariant chain peptide -HLA-DM : catalyze the exchange of CLIP with antigenic peptide - HLA-DO : negative regulator of class II antigen processing by binding to HLA-DM and inhibiting its role in the dissociation of CLIP from class II MHC molecule (b) HLA class II-peptide or CLIP

Cross-Presentation of Exogenous Antigens

- CD1 molecules : structurally related to MHC class I non-polymorphic - Group I : CD1a, CD1b, and CD1c - Group II : CD1d - Identified on cortical thymocytes: T-cell differentiation marker - Found on B cells and dendritic cells Presentation of Nonpeptide Antigens

Antigen presentation by CD1 - CD1 molecules : structurally related to MHC class I non-polymorphic - Group I : CD1a, CD1b, and CD1c - Group II : CD1d - Identified on cortical thymocytes: T-cell differentiation marker - Found on B cells and dendritic cells

CD1 is an MHC class I-like molecule which presents lipid antigens - CD1d 의 구조분석 : X-ray crystallography : deep electrostatically neutral antigen-binding groove : hydrophobic acyl groups of the lipids into the large hydrophobic pockets : polar groups (phosphate, carbohydrate) interact with the TCR : lipid antigen-specific T cells : NK-T cells : acidic endosomal compartments  lipid antigens: partially unfolded at low pH -Group CD1 : present lipids from mycobacteria and Haemophilus influenzae  stimulate CD4+, CD8+ T cells  role in antimicrobial defense - CD1d : present lipids from parasites such as Plasmodium falciparum and Trypanosoma brucei