محاظرات مادة المناعة / المرحلة الثانية
تاريخ النشر : 2015-04-09 09:56:18
عدد المشاهدات : 1696
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Response to Antigen: Processing and Presentation MHC Restriction and Role of the Thymus
MAJOR HISTOCOMPATABILITY COMPLEX
MHC - The Major Histocompatibility Complex
1) Originally identified in mice as blood cell antigens by R.A. Gorer and G.D. Snell in 1930s, and defined on the basis of tissue graft rejection - 1980 Nobel prize awarded to Snell.
2) Work by Rolf Zinkernagel and Peter Doherty in the 1970s revealed that it is the complex of MHC molecule plus antigen that is recognized by T cells (MHC restriction of T cell responses) - 1996 Nobel Prize
3) Two classes of MHC molecule: Class I (single MHC a chain + b2-microglobulin) and Class II (a chain plus b chain) - more Ig-superfamily members (Ig-C domains)
Major Histocompatibility complex (MHC)
The Major Histocompatibility complex (MHC) is a large genomic region or gene family found in most vertebrates. It is the most gene-dense region of the mammalian genome and plays an important role in the immune system, autoimmunity, and reproductive success
Major Histocompatibility Complex
Definition
A group of genes that code for cell-surface Histocompatibility antigens and are the principal determinants of tissue type and transplant compatibility. They are the most diverse genes in humans and are used to determine if a sample of DNA comes from a specific person
MHC
• 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 MF, 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
Basis of MHC system
· In man and mouse, as in most species, each class of MHC is represented by more than one locus( polygeny), in man these are called HLA for Human Leucocyte Antigen. The class I loci are HLA-A,-B and -C and the class II loci HLA-DR, -DQ and -DP. All the MHC genes map within a single region of the chromosome (hence the term Complex); the physical map of the MHC of man and mouse has been determined by cloning, mapping and sequencing
Class I, II and III MHC
• 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
Composition of Class I Molecules
Class I molecules are composed of two polypeptide chains; one encoded by the BCA region and another (ß2-microglobulin) that is encoded elsewhere. The MHC-encoded polypeptide is about 350 amino acids long and glycosylated, giving a total molecular weight of about 45 kDa. This polypeptide folds into three separate domains called alpha-1, alpha-2 and alpha-3. ß2-microglobulin is a 12 kDa polypeptide that is non-covalently associated with the alpha-3 domain
Class II molecules
• MHC (major Histocompatability complex) Class II molecules are found only on a few specialized cell types, including macrophages, dendritic cells and B cells, all of which are professional antigen-presenting cells (APCs).
Humans are very
Diversified MHC Plays a Great Role
The loci which specify MHC’s are polymorphic.
Many alleles may exist at a locus:
HLA A locus has ~60 alleles
HLA B locus ~110 alleles
HLA C locus ~40 alleles
The high level of allelism creates diversity within a species (thus restricting allografting) but does not produce diversity within an individual
What is polygenecity?
• Humans have DP, DQ, and DR “regions” specifying a and b chains of MHC-II.
Why are these called “regions”?
MHC Molecules Control the Immune response
• The Major Histocompatibility Complex (MHC) is a set of molecules displayed on cell surfaces that are responsible for lymphocyte recognition and "antigen presentation". The MHC molecules control the immune response through recognition of "self" and "non-self" and, consequently, serve as targets in transplantation rejection
• Peptide Binding to MHC Molecules: Class I vs. II
• Class I Class II
• • Generated by proteasome (cytosol) • Generated in endosomes/lysosomes
• • Transported to ER by TAP1/2 • Bind in specialized vesicles (MIIC)
• • Restricted length, 8-9 residues • Any length, extend from MHC
• • Binding energy from N, C termini • Binding energy from backbone
• • “Anchor” residues fill specific pockets • “Anchor” residues fill specific pockets
MHC- I MHC-II
MHC-I
Heavy chain (alpha) and
“microglobulin” (beta two)
Heavy is 45 kilodaltons, has three domains +
a transmembrane component (40 aa) +
a cyto- plasmic tail (30 aa)
The three alpha domains are called: a1, a2, & a3
a1 and a2 interact to present processed Ag
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Process Ag is optimally a nonomer
Microglobulin (12 kDa) associates non-covalently with a3
Microglobulin and a3 are part of immunoglobulin superfamily
Microglobulin is the only member of the superfamily that does
not have a component linking it to a membrane
The Major Histocompatibility Complex
1) MHC Class I and Class II genes are encoded in a large (>4 Mb, >200 genes) tightly linked gene cluster: the MHC
2) The MHC is similarly organized in mice (Ch17) and humans (Ch6) - syntenic
3) Known in humans as HLA (human leukocyte antigen) genes; in mouse as H-2 (histocompatibility group 2) genes
4) Genes are highly polymorphic (many alleles)
5) Class I: HLA-A, -B, -C (human); H-2K, -2D, -2L (mouse)
6) Class II: HLA-DP, -DQ, -DR (human), I-A, I-E (mouse)
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b2-microglobulin is located on a different chromosome.
Class I and II MHC
MHC- I & MHC-II
Class I And II Specificity
• Several Hundred Allelic Variants Have Been Identified In Humans
• However, up to 6 MHC I And 12 MHC II Molecules Are Expressed In An Individual
• Enormous Number Of Peptides Needs To Be Presented Using These MHC Molecules
• To Achieve This Task MHC Molecules Are Not Very Specific For Peptides (Unlike TCR and BCR)
• Promiscuous Binding Occurs
– A peptide can bind a number of MHC
– An MHC molecule can bind numerous peptides
Class I And II Diversity And Polymorphism
• MHC Is One Of The Most Polymorphic Complexes Known
• Alleles Can Differ Up To 20 a/a
• Class I Alleles In Humans: 240 A, 470 B, 110 C
• Class II Alleles In Humans: HLA-DR 350 b, 2 a!
• HLA-DR
– b genes vary from 2-9 in different individuals!!!,
– 1 a gene (a can combine with all b products increasing number of APC molecules)
• DP (2 a, 2 b) and DQ (2 a, 3 b)
MHC I and II belong to Immunoglobulin Supergene Family
MHC location on Chromosome 6
MHC Expression
• Expression Is Regulated By Many Cytokines
– IFNa, IFNb, IFNg and TNF Increase MHC expression
• Transcription Factors That Increase MHC gene Expression
– CIITA (Trans activator), RFX (Trans activator)
• Some Viruses Decrease MHC Expression
– CMV, HBV, Ad12
• Reduction Of MHC May Allow For Immune System Evasion
MHC system important in Rejection of Transplants
• Gene products encoded in the Major Histocompatibility Complex (MHC) were first identified as being important in rejection of transplanted tissues. Furthermore, genes in the MHC were found to be highly polymorphic (i.e. in the population there were many different allelic forms of the genes)
Class I MHC proteins
• Class I MHC proteins (found on all nucleated cell surfaces) present antigens to cytotoxic T lymphocytes (CTLs) . Most CTLs possess both T-cell receptors (TCR) and CD8 molecules On their surfaces. These TCRs are able to recognize peptides when they are expressed in complexes with MHC Class I molecules. For the TCR to bind a peptide-MHC complex two conditions must be met. Firstly, the TCR must have a structure which allows it to bind the peptide-MHC complex. Secondly, the accessory molecule CD8, must bind to the alpha-3 domain of the MHC Class I molecule. Due to genetic recombination events each CTL expresses a unique TCR which only binds a specific MHC-peptide complex
Class I MHC molecules
• Class I MHC molecules are composed of two polypeptide chains, a long α chain and a short β chain called β2-microglobulin (figure 2). The α chain has four regions. First, a cytoplasmic region, containing sites for phosphorylation and binding to cytoskeletal elements
Class II MHC molecules
Class II MHC molecules are composed of two polypeptide chains an α and a β chain of approximately equal length (Figure 6). Both chains have four regions: first, a cytoplasmic region containing sites for phosphorylation and binding to cytoskeletal elements;
MHC II are Complex in Organization
Within the MHC there are 5 loci that encode class II molecules, each of which contains a gene for an α chain and at least one gene for a β chain. The loci are designated as HLA-DP, HLA –DQ, HLA-DR, HLA-DM, and HLA-DO. Among these, HLA-DP, HLA –DQ, and HLA-DR are the most important and are most polymorphic..
MHC is highly polymorphic
• The MHC is highly polymorphic. There are a large number of genetic variants (alleles) at each genetic locus. Crucially many of these alleles are represented at significant frequency (> 1%) in the population, and in addition the alleles generally differ from one another by many (up to 30) amino acid substitutions. For example in humans there are more than 200 alleles described at some MHC loci. Such a remarkable degree of polymorphism implies a selective pressure to establish and maintain it.
• MHC Polymorphism
• MHC Class I and Class II molecules have many alleles (point mutations, gene conversion, gene recombination)
• Diversity ensures that a wide range of peptides can be presented within the population (even if a much more limited set is presented by any individual)
• For Class II, both a and b chains are polymorphic (except DRa in humans and Ea in mice), adding more diversity
• Collection of Class I + Class II alleles on one chromosome is called the haplotype
• MHC molecules are expressed co-dominantly: alleles from both chromosomes are expressed in each cell
• MHC Polymorphism
• MHC Class I and Class II molecules have many alleles
• Diversity ensures that a wide range of peptides can be presented within the population (even if a much more limited set is presented by any individual)
• For Class II, both a and b chains are polymorphic (except DRa in humans and Ea in mice), adding more diversity
Inheritance Of HLA Haplotypes
MHC Polymorphism
4)Collection of Class I + Class II alleles on one chromosome is called the haplotype
5)MHC molecules are expressed co-dominantly: alleles from both chromosomes are expressed in each cell
Biological role of the MHC
• The products of the MHC play a fundamental role in regulating immune responses. T cells must recognise antigen as a complex with MHC molecules. This requires antigen to be processed by unfolding and proteolytic digestion before it complexes with the MHC molecule
Biological role of the MHC
• Once formed the complex of antigenic peptide and MHC are generally very stable (half life ~ 24hrs). Thus the biological role of MHC proteins is to bind small peptides and to "present" these at the cell surface for the inspection of T cell antigen receptors.
MHC-Linked Diseases
• Defects in MHC gene expression lead to immunodeficiencies (MHC molecules are required for both T cell development and activation)
• Some MHC alleles are associated with susceptibility or resistance to autoimmune diseases
MHC-Linked Immunodeficiencies
Bare Lymphocyte Syndromes lead to loss of MHC molecule expression:
• Defects in TAP genes prevent MHC Class I protein surface expression (even though MHC proteins are normal), so no CD8+ T cells - surprisingly mild immunodeficiency (respiratory and skin infections)
• Defects in TF’s controlling Class II gene expression (CIITA, RFXANK, RFX5, RFXAP) block CD4+ T cell development - result in SCID (severe combined immunodeficiency)