Paper No. : 10 : Immunology Module : 07 : Structure and function of Major Histocompatibility Complex Development Team Principal Investigator: Prof. Neeta Sehgal Head, Department of Zoology, University of Delhi Co-Principal Investigator: Prof. D.K. Singh Department of Zoology, University of Delhi Paper Coordinator: Prof. Anju Srivastava Department of Zoology, University of Delhi Content Writer: Dr. Soma M. Ghorai Hindu College, University of Delhi Content Reviewer: Prof. Sukhmahendra Singh Banaras Hindu University 1 ZOOLOGY Immunology Structure and function of Major Histocompatibility Complex Description of Module Subject Name ZOOLOGY Paper Name Zool 010: Immunology Module Name/Title Structure and function of Major Histocompatibility Complex Module ID M07 Structure and function of Major Histocompatibility Complex Keywords MHC haplotypes, histocompatible genes, MHC Class-I, MHC-Class- II, MHC-Class-III, Antigen presentation, CD4+ T cells, CD8+ T cells, MHC polymorphism, Natural Selection, Immunoglobulin superfamily, MHCII transactivator (CIITA), HLA typing, Transplantation Contents 1. Learning Objectives 2. Introduction 3. MHC Haplotypes 3.1 MHC Class-I 3.1.1 Structure of MHC Class-I 3.2 MHC Class-II 3.2.1 Structure of MHC Class-I 3.3 MHC Class-III 4. Structure of Peptide Binding Cleft 4.1 Peptide –MHC interaction 5. MHC Class-I and MHC Class-II Antigen Presentation 5.1 Class-I MHC Antigen Presentation 5.2 Class-II MHC Antigen Presentation 6. Role of MHC in Self/Nonself Recognition 7. The Genetic Architecture of MHC 8. Natural Selection and MHC Genes Polymorphisms 9. Epigenetics and MHC 10. MHC in Transplantation 10.1 Allogenic Immune Responses 10.2 HLA- Testing and Clinics 11. Summary 2 ZOOLOGY Immunology Structure and function of Major Histocompatibility Complex 1. Learning Objectives After studying this module, you shall be able to Recognize the various haplotypes of MHC and their structures. Differentiate between the Class-I MHC and Class –II MHC antigen presentation. Learn the role of MHC in self/non-self recognition. Ascertain about the genetic architecture of MHC. Learn about MHC polymorphism and their basis on Natural selection process. How epigenetics has a role in MHC polymorphism. Know the role of MHC in autoimmunity and other disease-conditions. Why MHC (HLA) gene typing is essential for transplantation. 2. Introduction For the body to rein an attack on any pathogen; it must be recognized as self or non-self and this is accomplished by the set of cell surface receptors which not only distinguishes self from non-self but also plays a pivotal role in tissue transplantation and rejection, autoimmunity and susceptibility to infections. These cell surface receptors are encoded by the most diverse gene loci in vertebrates known as the major histocompatibility complex (MHC Class I and II loci). In humans, MHC spans a considerable part of the DNA i.e. about 0.1% of total genome containing 200 coding loci. These are highly polymorphic genes that can be attributed to the natural selection over long periods of evolutionary time. The history behind MHC genes dates back to 1916 when Little and Taylor showed that tissue could be transplanted only in the same strain mice; followed by George D. Snell’s observation in 1935 that several closely linked histocompatible genes are responsible for tissue rejection. He discovered that in tumor transplants on mice from their congenic cousins were immediately rejected but were successful in syngeneics trains. The mystery was solved in 1975 by Zinkernagel and Doherty, who found that T- cell responses were restricted to antigens and the MHC’s presented on the cell surface. The 1980’s and 90’s finally revealed the structure of MHC molecules and their encoded proteins. The Nobel Prize in Physiology or Medicine in 1980 was awarded jointly to Baruj Benacerraf, Jean Dausset and George D. Snell for their contribution in understanding that similar antigens are controlled by dominant autosomal genes termed immune response (Ir) genes located on the MHC loci of all the vertebrates. Other than any region of the genome, the MHC genes are the most associated with autoimmune and infectious diseases. These disease-associated variations in the MHC regions are due to very subtle and minute changes in the gene loci. There is still more to learn about MHC like (i) what mechanism drives MHC polymorphism, (ii) which are naturally and positively selected variations having functional importance, (iii) which signaling pathways does the MHC genes follow at the molecular level. 3. The MHC Haplotypes The MHC genes encode three classes of molecules: 3.1 MHC Class I The MHC class-I heavy chains are encoded by three genes (HLA-A, HLA-B, HLA-C) and presents small peptides to CTLs (cytotoxic T-lymphocytes) enabling each cell of the body to the immune 3 ZOOLOGY Immunology Structure and function of Major Histocompatibility Complex system with possible chances of infection, therefore every nucleated cells of the body present MHC-I molecules. Mostly these peptides are self-peptides, but in case of virus-infected cell or cancer cell or parasite infected cell, the foreign peptides are presented resulting in their elimination by CTLs. The CTLs are very unique cells with highly specific TCR (T-cell receptor) that scan for MHC-antigen complexes and CD8+ molecules to stabilize and help bind the cells to MHC – antigens. The cells with foreign antigens help activate the CTLs, resulting in the clonal selection of these specific CTLs, thus eliminating other infected cells. But it is still very difficult to envisage that how MHC molecules achieve the specificity in immune recognition with such small pool of TCR. In fact, MHC molecules can bind to a varied range of peptides rather than specific peptides. 3.1.1 Structure of MHC Class-I It is composed of α and β polypeptide chains held together with non-covalent bonds with a short cytoplasmic tail (Figure 1). The α-chain is the MHC-encoded integral membrane glycoprotein of ~45000 Da; while β2-microglobulin is non-MHC encoded extracellular ~12kDa peptide. The α-chain is of 350 aminoacids with three globular domains (α1, α2, and α3). α2 and α3, both have intra-chain disulphide linkages. It also has a 26 hydrophobic amino acids transmembrane segment spanning the plasma membrane. The α1 and α2 forms the platform as the peptide-binding unit mainly comprising of β-sheets. The groove formed between them is blocked at both the ends and is enough to hold 8- 10amino acids, so it is imperative for the foreign residues to be processed and presented before they bind to MHC-I. The β2-m chain is non-polymorphic ~90a.a. peptide is encoded by chromosome 15 and interacts non-covalently with α3. This part of the MHC molecule binds to CTL. Class-I MHC molecule is a glycoprotein with one or two N-linked oligosaccharide. Peptide-binding groove (β-sheets) Peptide (8-10 a.a.) Hydrophobic stretch (25 a.a.) Cytoplasmic tail (10-15 a.a.) Figure 1. Structure of MHC-1 molecule 3.2 MHC Class-II Class II peptides are expressed on specialized cells called antigen presenting cells (APCs); mainly the macrophages, dendritic cells and B-lymphocytes. MHC –II molecules are encoded by three polymorphic genes (HLA-DR, HLA-DQ and HLA-DP) that bind to different peptides. The immunodominant epitopes are largely confined to the pathways dependent on lysosomal degradation and peptide binding of newly synthesized MHC-II molecules in the acidic endosomal compartment. 4 ZOOLOGY Immunology Structure and function of Major Histocompatibility Complex The MHC-II molecules that are involved in the CD 4+ T-cell response against intracellular pathogens may trigger immunity towards infection, thymic selection, autoimmunity and tumor immunity 3.2.1 Structure of MHC Class -II Like MHC-I, the class-II MHC molecules have an extracellular domain, a transmembrane domain and an intracellular carboxy terminal tail. The extracellular domain is composed of two non-covalently associated domains (α1, α2 or β1, β2) of ~90 a.a anchored to the cell membrane. The heavy chain (α 1 and α 2) has a molecular weight of ~30-34 KDa and the light chain (β 1 and β 2) has a molecular weight of ~26-29 KDa. α2, β1 and β2 domain contains disulphide linkages, while α1, α2 and β1 domains are glycosylated but β2 domain is not. The transmembrane domain is comprised of ~25 hydrophobic amino acids and the cytoplasmic tail of ~10-15 hydrophilic amino acids. Peptide antigen binds in a groove formed from a pair of α-helices (on a floor of anti-parallel β-strands) involving α1 or β1 segments, respectively. The antigenic peptide – binding domain can hold 13-18 a.a. amino acid residues as it is open at both the ends of the peptide binding groove (Figure 2). Peptide-binding groove Peptide binding cleft (13-17 a.a.) Transmembrane stretch Cytoplasmic tail Figure 2. Structure of MHC-II molecule. 3.3 MHC Class-III These molecules are mainly comprised of serum proteases and complement enzymes and do not have any role in antigen presentation. The complement component includes C2 (serine protease), C4A and C4B (pro-proteins of the multichain forms) and factor B (component of alternate complement pathway). They also include the TNF-α and TNF-β, as well two heat-shock proteins. 4. Structure of Peptide Binding Cleft MHC I and class II molecules fold into a highly similar conformations featuring a peptide‐ binding groove to present T-cell epitopes. Peptide binding grooves of MHC I molecules are composed of two α‐ helices and eight β‐ strands formed by one heavy chain, while MHC II uses two domains from 5 ZOOLOGY Immunology Structure and function of Major Histocompatibility Complex different chains to construct the peptide‐ binding groove.