Major Histocompatibility Complex

Antigen Presentation APC to T-cell

First- and Second-set Allograft Rejection Figure 16.1

MHC Major Histocompatibility Complex Cluster of genes found in all mammals Its products play role in discriminating self/non- self Participant in both humoral and cell-mediated immunity MHC Act As Antigen Presenting Structures In Human MHC Is Found On Chromosome 6 Referred to as HLA complex In Mice MHC Is Found On Chromosome 17 Referred to as H-2 complex

Genes Of MHC Organized In 3 Classes Class I MHC genes Glycoproteins expressed on all nucleated cells Major function to present processed Ags to T C Class II MHC genes Glycoproteins expressed on M , B-cells, DCs Major function to present processed Ags to T H Class III MHC genes Products that include secreted proteins that have immune functions. Ex. Complement system, inflammatory molecules 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 Class I, II and III MHC

Other genes in the MHC MHC Class 1b genes Encoding MHC class I-like proteins that associate with  -2 microglobulin: HLA-G binds to CD94, an NK-cell receptor. Inhibits NK attack of foetus/ tumours HLA-E binds conserved leader peptides from HLA-A, B, C. Interacts with CD94 HLA-F function unknown MHC Class II genes Encoding several antigen processing genes: HLA-DM   and , proteasome components LMP-2 & 7, peptide transporters TAP-1 & 2, HLA-DO  and DO  Many pseudogenes MHC Class III genes Encoding complement proteins C4A and C4B, C2 and FACTOR B TUMOUR NECROSIS FACTORS  AND  Immunologically irrelevant genes Genes encoding 21-hydroxylase, RNA Helicase, Caesin kinase Heat shock protein 70, Sialidase

MHC Products Are Highly Polymorphic Vary considerably from person to person However, Crossover Rate for each mitosis Is Low 4% crossover rate (MHC is 4 centimorgan) Inherited as 2 sets (one from father, one from mother) Haplotype refers to set from mother or father MHC Alleles Are Co-dominantly Expressed Both mother and father alleles are expressed Inbred Mice Haplotypes Are Designated With Italic Superscript Ex. H-2 b Designation refers to entire set of H-2 alleles MHC Genes Are Polymorphic

Major Histocompatibility Complex The major function of the molecules encoded by the MHC is to facilitate the display of unique molecular fragments on the surface of cells in an arrangement that permits their recognition by immune effectors such as T-lymphocytes (Ir gene).

Major Histocompatibility Complex The MHC molecule accomplishes its major role in immune recognition by satisfying two distinct molecular functions: the binding of peptides (or in some cases nonpeptidic molecules) and the interaction with T cells, usually via the ab T-cell receptor (TCR).

Major Histocompatibility Complex The binding of peptides by an MHC-I or MHC-II molecule is the selective event that permits the cell expressing the MHC molecule (the antigen- presenting cell, APC) to sample either its own proteins (in the case of MHC-I) or the proteins ingested from the immediate extracellular environment (in the case of MHC-II).

MHC I Molecules Membrane bound glycoprotein (Ig super family) Alpha chain Three domains Encoded by HLA complex Anchored in cell membrane Cytoplasmic tail Beta chain A single domain Encoded by a different gene on another chromosome Not anchored in cell membrane Bound to alpha chain non covalently Necessary for MHC I expression There are 6 molecule of MHC-I in a human.

MHC I In particular, cell surface MHC class I glycoproteins gather from the cell’s biosynthetic pathway fragments of proteins derived from infecting viruses, intracellular parasites, or self molecules, either normally expressed or dysregulated by tumorigenesis, and then display these molecular fragments at the cell surface.

Fig 4-4

MHC Class I The  chains forms a platform of 8- stranded anti-parallel  pleated sheet supporting two parallel strands of  -helix. The formed cleft can bind peptides of 8-11 amino acids in a flexible extended conformation.

MHC Class I The  3 segment of the MHC I serves as a binding site for CD8. The  2- microglobulin interacts with the  3 non-covalently.

MHC Class II Class II are composed of 2 non- covalent associated peptides,  chain and  chain. The peptide binding cleft can accommodate peptides up to 30 amino (10-30 a.a.) acids in length. B2 bind to CD4

MHC II Molecules Membrane bound glycoprotein (Ig super family) Alpha chain Two domains Encoded by HLA complex Anchored in cell membrane Cytoplasmic tail Beta chain Two domains Encoded by HLA complex Anchored in cell membrane Cytoplasmic tail There are 20 different molecule of MHC-II in a human.

MHC II Molecules Antigen presenting features Alpha 1 and beta 1 domains form peptide-binding pocket amino acid peptides can bind to MHC II molecule beta 2 interact with CD 4 on T helper cells

MHC Class II The  2 is the binding site for the CD4. The  chains are the HLA DR The  chains are the HLA DQ and DP

MHC I Peptides derived from proteins found in the cytosol that are then degraded by the multiproteolytic proteasome complex into peptides.

MHC I T he resulting peptides, transported from the cytosol to the endoplasmic reticulum with the aid of the intrinsic membrane transporter, the transporter associated with antigen processing (TAP), are then cooperatively folded into the newly synthesized MHC-I molecule.

MHC I Differential response Potentially any nucleated cell

MHC II MHC class II molecules bind peptides derived from the degradation of proteins ingested by MHC-II–expressing APC, and display them at the cell surface for recognition by specific T- lymphocytes.

MHC II The MHC-II antigen presentation pathway is based on the initial assembly of the MHC-II  heterodimer with a dual function molecule.

MHC II The invariant chain (Ii) that serves as a chaperone to direct the  heterodimer to an endosomal, acidic protein– processing location.

MHC II Where Ii encounters antigenic peptides, it serves to protect the antigen-binding site of the MHC-II molecule so that it preferentially will be loaded with antigenic peptides in this endosomal– lysosomal location.

MHC II The loading of the MHC-II molecule with antigenic peptide, a process dependent on the release of the Ii- derived CLIP peptide, in part dependent on the MHC-II-like molecule leads to the cell surface expression of MHC-II peptide complexes.

Molecular Basis of Direct Recognition

MHC II TH 1 TH 2

MHC I versus MHC II The MHC-I antigen presentation pathway is most easily thought of as an inside-out pathway by which protein fragments of molecules synthesized by the cell are delivered to and bound by the MHC- I molecule during its biosynthesis. In contrast, the MHC-II antigen presentation pathway is best more clearly visualized as an outside-in one in which ingested proteins are degraded by enzymes in the endosomal– lysosomal system and are delivered to the MHC-II molecules in that degradative compartment.

MHC I versus MHC II The MHC-I and MHC-II molecules also show preferential restriction to T cells of the CD8- or CD4-bearing subsets. This is related to the observation that CD8 binds to the nonpolymorphic  3 domain of MHC-I molecules, while CD4 interacts with membrane proximal domains of MHC-II.

Viruses and MHC There is little question that the immune system plays a decisive role in host defense against microbial pathogens, and it should not have come as a surprise to find that many pathogens have the ability to produce molecules that modulate the host response.

Viruses and MHC The most striking results have been obtained for viruses and MHC class I molecules. (a) peptide binding to TAP (ICP47 of herpes simplex) or peptide translocation by TAP (US6 of human cytomegalovirus); (b) stable insertion of the class I heavy chain into the ER membrane (US2 and US11 of human cytomegalovirus and HIV vpu;

Viruses and MHC The most striking results have been obtained for viruses and MHC class I molecules. (c) release of assembled class I–peptide complexes from the ER (E19 of adenovirus); (d) inhibition of MHC class I heavy-chain gene transcription (adenovirus, HIV); and (e) accelerated clearance of surface class I (HIV Nef).

Viruses and MHC The most striking results have been obtained for viruses and MHC class II molecules. Epstein-Barr virus produces a molecule that binds the human DR  chain. The T-cell response to herpes simplex in humans has been found to have the unusual feature of eliciting primarily CD4+ cytotoxic cells, and not stimulating much in the way of a CD8+ CTL response.

Viruses and MHC The most striking results have been obtained for viruses and MHC class II molecules. Cells infected with herpes simplex were found to have a defect in the stable assembly of MHC class I heavy chain–-  2m dimers and the export of class I molecules to the cell surface, which later was recognized to resemble the situation in cells lacking effective peptide import into the ER as a result of mutations in TAP-1 or -2.

MHC I versus NK Cells The expression by the target cell of MHC class I molecules can, in certain cases, protect the target from killing by the NK effector events, and target cells defective in the expression of normal MHC class I cells are susceptible to such NK-cell lysis.

Expression Is Regulated By Many Cytokines IFN , IFN , IFN  and TNF Increase MHC expression Transcription Factors That Increase MHC gene Expression CIITA (Transactivator), RFX (Transactivator) Some Viruses Decrease MHC Expression CMV, HBV, Ad12 Reduction Of MHC May Allow For Immune System Evasion MHC Expression

MHC Diversity Genetically determined Polygenic characteristic Different alleles Genes with overlapping functions ~12 million haplotypes Diversity in HLA type

MHC I Expression Varies by cell type Lymphocytes high Hepatocytes low Regulated by cytokines Interferons TNF Suppressed by viruses Herpes family Hepatitis B

Class I MHC and Class II MHC MHC Class I MHC Class II Nomenclature HLA-A, HLA-B, HLA-C HLA-DP, HLA-DQ, HLA-DR Found on All nucleated somatic cells Macrophages, B-cells, Dentritic cells, langerhans cells of skin and activated T cells Recognized by CD8 TC cells CD4 TH cells Functions Presentation of Ag to TC cells leading to elimination of tumor or infected host cell Presentation of Ag to TH cells which secrete cytokines

Types of grafts 1) Autografts : The transfer of an individual’s own tissues from place to place e.g. Skin grafts (regularly accepted) 2) Isografts : Transfer of tissues between genetically identical persons e.g. Identical twins ( accepted permanently)

Types of grafts 3) Allografts (homograft): - Transfer of a graft between genetically different members of same species e.g from one human to another - Rejection occur if donor and recipient are not matched 4) Xenograft (heterograft): - Transfer of tissues between different species - Always rejected

Mechanism Of Graft Rejection 1) Both TH and TC are activated - TC cells destroy graft cells by direct contact TH cells secrete cytokines that attract and activate macrophages, NK cells and polymorphs leading to cellular infiltration and destruction of graft (Type IV) - B cells recognize foreign antigens on the graft and produce antibodies which bind to graft cells and. Activate complement causing cell lysis. Enhance phagocytosis, i.e. opsonization (Type II). Lead to ADCC by macrophages, NK,PML - Immune complex deposition on the vessel walls induce platelets aggregation and microthrombi leading to ischemia and necrosis of graft (Type II)

Effector Mechanisms of Allograft Rejection Hyperacute Rejection Acute Rejection Chronic Rejection

Types Of Graft Rejection !) Hyperacute rejection: - It occurs hours after transplantation - In individual with preformed antibodies either due to - blood groups incompatibility or previous sensitization by blood transfusion, previous transplantation 2) Acute Rejection: - It occurs 10 to 30 days after transplantation - It is mainly T-cell mediated 3) Chronic or late rejection: - It occurs over a period of months or years - It may be cell mediated, antibody mediated or both

Hyperacute Rejection Characterized by thrombotic occlusion of the graft Begins within minutes or hours after anastamosis Pre-existing antibodies in the host circulation bind to donor endothelial antigens Activates Complement Cascade Xenograft Response

Hyperacute Rejection: the early days Mediated by pre-existing IgM alloantibodies Antibodies come from carbohydrate antigens expressed by bacteria in intestinal flora ABO blood group antigens Not really a problem anymore

Hyperacute Rejection: Today Mediated by IgG antibodies directed against protein alloantigens Antibodies generally arise from Past blood transfusion Multiple pregnancies Previous transplantation

Hyperacute Rejection 1. Preformed Ab, 2. complement activation, 3. neutrophil margination, 4. inflammation, 5. Thrombosis formation

Acute Rejection Vascular and parenchymal injury mediated by T cells and antibodies that usually begin after the first week of transplantation if there is no immunosuppressant therapy Incidence is high (30%) for the first 90 days

Acute Rejection 1.T-cell, macrophage and Ab mediated, 2.myocyte and endothelial damage, 3.Inflammation

Chronic Rejection Occurs in most solid organ transplants Heart Kidney Lung Liver Characterized by fibrosis and vascular abnormalities with loss of graft function over a prolonged period.

Chronic Rejection 1.Macrophage – T cell mediated 2.Concentric medial hyperplasia 3.Chronic DTH reaction

Types of Rejection Acute Rejection: CD4 controlled CD8 mediated (8-11 days) Acute Rejection: CD4 controlled CD8 mediated (8-11 days) Hyperacute Rejection: pre-existing antibodies to donor tissue (7 min) Hyperacute Rejection: pre-existing antibodies to donor tissue (7 min) Chronic Rejection: Mixed CD4 and antibody – ”DTH like” (3 m to 10 years) Chronic Rejection: Mixed CD4 and antibody – ”DTH like” (3 m to 10 years) Xenograft Rejection: pre-existing antibodies to donor tissue (7 min) Xenograft Rejection: pre-existing antibodies to donor tissue (7 min)

Graft Versus Host (GVH) Reaction * An immunologically competent graft is transplanted into an immunologically suppressed recipient (host) * The grafted cells survive and react against the host cells i.e instead of reaction of host against the graft, the reverse occurs * GVH reaction is characterized by fever, pancytopenia, weight loss, rash, diarrhea, hepatsplenomegaly and death

CD4 + and CD8 + CD4 + differentiate into cytokine producing effector cells Damage graft by reactions similar to DTH CD8 + cells activated by direct pathway kill nucleated cells in the graft CD8 + cells activated by the indirect pathway are self MHC-restricted

Autoimmunity The mechanisms of tissue damage in autoimmune diseases are essentially the same as those that operate in protective immunity. Autoimmune responses are a natural consequence of the open repertoires of both B-cell and T-cell receptor that allows them to recognize any pathogen.

Autoimmunity HLA genotype Diabetes MellitusDR3, DR4 Juvenile Rhematoid Arth.B27 Multiple SclerosisDR2

Differential distribution of MHC molecules Cell activation affects the level of MHC expression. The pattern of expression reflects the function of MHC molecules: Class I is involved in the regulation of anti-viral immune responses Class II involved in regulation of the cells of the immune system Anucleate erythrocytes can not support virus replication - hence no MHC class I. Some pathogens exploit this - e.g. Plasmodium species. Tissue MHC class I MHC class II T cells+++ +/- B cells Macrophages Other APC Thymus epithelium Neutrophils+++ - Hepatocytes + - Kidney + - Brain + - Erythrocytes - -

We love learning about MHC. Monday we will Have an exam