HLA and antigen presentation Institute of Immunology University Hospital Motol

Structure and function of HLA history structure of HLA gens and molecules function of HLA moleculs nomenclature of HLA system HLA association with disease antigen presentation

HLA - MHC HLA molecules are responsible for the compatibility of the tissues of genetically different individuals combination of HLA molecules is unique in each individual monozygotic twins have the same histocompatibility molecules on their cells HLA molecules are responsible for the rejection of transplant HLA molecules present antigens and provoke immune reaction leading to the transplant rejection and destruction histocompatibility molecules are called histocompatibility antigens or transplantation antigens.

HLA - immune system development Histocompatibility molecules are glycoproteins expressed at the surface of all vertebrate cells. HLA molecules represent the latest stage of the development of the immune system.

structure of HLA molecules Two classes of HLA molecules are recognized and designed HLA class I. and HLA class II. HLA molecules of both classes are glycoproteins, heterodimers, composed from two chains. At least one of the chains is polymorphic. Structure of HLA molecules of both classes enables antigen binding and contact with T cell receptors.

HLA class I. molecules Class I molecules consist of three parts a transmembrane protein, called the heavy chain. The outermost domains contain two segments of polymorphic alpha helix that form two ridges with a groove between them called a1and a2, a3 is nonpolymorphic domain close to the membrane.. a molecule of beta-2 microglobulin (ß2m), which is attached noncovalently to the heavy chain peptide bound to the groove formed by a1and a2 domaines

structure of HLA class I. molecules HLA class I. molecules are composed from heavy a chain b-2 microglobulin peptide

HLA A,B,C Humans synthesize three different types of class I. molecules designated HLA-A, HLA-B, and HLA-C. These differ only in their heavy chain, all sharing the same type of beta-2 microglobulin. HLA class I. molecules are expressed on all nucleated cells of the body. HLA class I. molecules are not expressed on red blood cells and only minor level of expression is detected on the cells of central nervous system. The genes encoding the different heavy chains are clustered on chromosome 6 in the major histocompatibility complex.

HLA class II. molecules Class II molecules consist of two transmembrane polypeptides: an alpha chain and a beta chain. Both chains are polymorphic. The alpha and beta chains are encoded by clusters of loci in the region of chromosome 6 designated HLA-D. HLA class II. molecules form 3 types, designed HLA DR, DP and DQ. Class II molecules are not as widely expressed in the body as the class I molecules are. However, cells where inflammation is occurring express class II strongly and provide a powerful stimulus to the immune system.

Structure of HLA class II. molecules Class II molecules consist of two transmembrane polypeptides and bound peptide.

Structure of HLA class I. and II. molecules HLA class I. HLA class II.

HLA and peptides antigenic peptids in the binding sites of HLA molecules class I. class II.

HLA genes chromozom 6 HLA region D HLA class III. B C E A G F DP DQ DR HLA class III. are soluble molecules as complement, TNF, HSP HLA DR a chain is not polymorphic

HLA haplotypes Haplotype is combination of allelic forms of HLA molecules on one chromosome. We inherit 3 types of heavy chains for HLA class I. molecules from each parent . Everybody expresses 6 different types of HLA class I. molecules unless honmozygous status for some of the types was inherited. A B C A B C A B C A B C A B C A B C

HLA polymorfism Genes for HLA are the most polymorhic structures from all known systems. The diversity of alleles in the population makes possible thousands of different combinations. In a study of 1000 blood and organ donors in San Francisco that were typed for HLA-A and HLA-B, Over half the group had a combination that was unique. Another 111 donors had a set of these molecules that they shared with only one other person in the group. The most frequent phenotype (HLA-A1, HLA-A3, HLA-B7, and HLA-B8) was found in 11 donors.

HLA typing The MHC of humans is now completely sequenced! A consortium of laboratories reports in the 28 October 1999 issue of Nature that they have determined the sequence of 3,673,800 nucleotides on chromosome 6 that encode the genes of the MHC as well as many other genes involved in immune function.

HLA nomenclature HLA Alleles There are currently (2015) 14,015 alleles described by the HLA nomenclature and included in the IMGT/HLA Database.

HLA nomenclature

HLA and antigen presentation Although histocompatibility molecules were discovered because of the crucial role they play in graft rejection, clearly evolution did not give vertebrates these molecules exclusively for that function. The main function of HLA molecules is the process called antigen presentation to T lymphocytes.

Immune system General defense Innate immunity Adaptive immunity TCD4 B neutrophil B TCD8 Plasma cell Monocyte/macrophage Immune synapse Dendritic cell

HLA and antigens Antigens presented to the immune system come from different sources. The main division of the antigens recognizes extracellular antigens and intracellular antigens. The nature of extracellular and intrcellular antigens differs markedly and both induce different type of immune reaction. Intracellular antigens are presented in connection with HLA class I. Extracellular antigens are presented in connection with HLA class II.

Antigen presentation Antigens are molecules able to induce immune response. Antigens are mostly proteins, glycoproteins or polysaccharides. Extracellular antigens enter the body from the environment. They are inhaled or ingested macromolecules or molecules that are introduced beneath the skin . Intracellular antigens are antigens that are generated within the cells of the body; these would include proteins encoded by the genes of viruses that have infected a cell or aberrant proteins that are encoded by mutant genes such as mutated genes in cancer cells.

Antigen presenting cells and T lymphocytes The recognition of antigen by T cells is necessary for induction of the immune response. The nature of the outcome of the immune response is directed according to the nature of presented antigen. exogenic antigen presentation HLA class II. antigen TCR CD4 T lymfocyte APC lysozom ER, Golgi

Antigen presenting cells and T lymphocytes HLA class I. antigen TCR CD8 T lymfocyte APC ER, Golgi The recognition of antigen by T cells is necessary for induction of the immune response. The nature of the outcome of the immune response is directed according to the nature of presented antigen. endogenous antigen presentation

Exogenous antigens Exogenous antigens (inhaled, ingested, or injected) are taken up by "professional" antigen-presenting cells These include: phagocytic cells like macrophages and dendritic cells B lymphocytes which are responsible for producing antibodies against the antigen. professional antigen-presenting cells All these cells expressed HLA class II. molecules

Exogenous antigens and cells Antigen presenting cells engulf the antigen by endocytosis degrade it into fragments (e.g. break an antigenic protein into short peptides) display these peptides at the surface of the cell nestled within a class II. histocompatibility molecule here they may be recognized by CD4+ T cells

Exogenous pathway Steps of the exogenous antigen presentation: CD4+ Exogenous pathway Steps of the exogenous antigen presentation: The two chains of the class II molecule are inserted into the membrane of the RER. They bind (in their groove) one molecule of invariant chain. This trimolecular complex is transported through the Golgi apparatus and into vesicles called lysosomes. Invariant chain is actively deleted by HLA DM. Invariant chain is replaced by antigen. Complex of HLA and antigens moves to the surface of the cell. Complex HLA-antigen is recognized by CD4 positive lymfocyte. APC endosome Golgi ER

Endogenous antigens Antigens that are generated within a cell are degraded into fragments (e.g., peptides) within the cell in proteasome selected antigens are transported to ER with active action of TAP (transport associated protein) stable complex of HLA class I. and antigen is formed in ER, moves to Golgi displayed at the surface Here they may be recognized by CD8+ T cells. Most CD8+ T cells are cytotoxic. They have the machinery to destroy the infected cell (often before it is able to release a fresh crop of viruses to spread the infection).

Endogenous pathway HLA antigens class I. are expressed on all nucleated cells There are three subunits in each class I histocompatibility molecule: the transmembrane polypeptide - heavy chain the antigenic peptide beta-2 microglobulin complex on the cell surface meets CD8+ T lymphocyte

TAP Transport associated protein - TAP is responsible for the peptide transport from cytoplasm to ER. Proteins are degraded to peptide in proteasome. The peptides are picked up by TAP proteins and transported from the cytosol into the RER where they assemble with the transmembrane polypeptide and beta-2 microglobulin. this trimolecular complex then moves through the Golgi apparatus and is inserted in the plasma membrane proteazome TAP

Antigen presentation pathways endogenous antigens, HLA class I. exogenous antigens, HLA class II.

CD4 T lymfocyte B lymfocyte B lymfocytes B lymphocytes process antigen by the class II pathway. However, antigen processing by B cells differs from that of phagocytic cells like macrophages in crucial ways.

BCR B cells engulf antigen by receptor-mediated endocytosis B lymfocyte B cells engulf antigen by receptor-mediated endocytosis The affinity of B cells for an epitope on an antigen may be so high that the B cell can process the antigen when it is present in body fluids in concentrations thousands of times smaller than a macrophage would need The remaining steps of antigen processing occur by the same class II pathway The B cell grows into a clone of plasma cells under the influence of cytokines produces by CD4+ lymohocyte The antibodies are secreted into the surroundings

Antibodies antibody production B lymfocyte CD4 cytokines T lymfocyte plasma cell Antibodies CD4 T lymfocyte B lymfocyte antibody production cytokines

Signal pathways, cell activation CD3 ion channels signal pathways protein phosphorylation protein kinases APC T lymfocyte

Summary - antigen presentation pathways CD4 T lymfocyte B lymfocyte HLA class I. antigen TCR CD8 APC ER, Golgi HLA class II. lysozome endogenous exogenous B lymfocytes cell destruction immune response antibody production

Antigen presentation and T lymphocyts

Polarisation of T lymphocytes

Presentation of antigen and infection

Presentation of antigen and autoimmunity

Presentation of antigen and transplantation

Presentation of antigen and tumours

Presentation of antigen and vaccines

Presentation of antigen - summary

Presentation of antigen - summary defective ribosomal products (DRiPs) mature proteins (retirees) ER aminopeptidases (ERAP) Peptides that fail to bind to MHC class I molecules are removed by the translocon SEC61 and enter the cytoplasm Making sense of mass destruction: quantitating MHC class I antigen presentation Jonathan W. Yewdell, Eric Reits & Jacques Neefjes Nature Reviews Immunology 3, 952-961 (December 2003)

Antigen presentation and immune defense

Antigen presenting cell and lymphocyte

Antigen presenting cell and lymphocyte