The Major Histocompatibility Complex (MHC) January 31, 2018 Pete Burrows 4-6529 peterb@uab.edu What it is What it does Required for antigen recognition by T lymphocytes

Preview of antigen processing/presentation Discovery and function of MHC MHC structure The similarities and differences between MHC class I and class II expression and function The consequences of MHC polymorphisms NKT cells

Antigen-antibody Interaction Heavy Chain Lysozyme Antigenic Determinant Epitope Light Chain

Antigen must be “processed” inside of cells and then displayed on the cell surface to be recognized by T cells Proteases TCR #1 TCR #2 Protein either taken up from the outside or produced intracellularly. In this case a foreign peptide, taken up by phago- or pinocytosis. TCR can recognize peptide but not by itself. Needs to be associated with self MHC. Lysozyme

T cells recognize processed (degraded) protein antigens plus MHC from lysozyme TCR 2

Phagocytosis Processing of external antigens MHC always has a peptide, usually from self This is a foreign peptide that will alert the T cells of the immune system. Dendritic cells even better at this APC function than macrophages. Processing of external antigens Complex is recognized by T helper cells Dendritic cells are even more potent APC

Processing of internal antigens Death Signal! Viral protein Foreign Proteasome Stay tuned for Dr. Collawn’s lecture about how this happens

Major Histocompatibility Complex Discovered using inbred strains of mice and examining tumor immunity

Major Histocompatibility Complex - MHC Discovered using inbred strains of mice and examining tumor immunity Were really studying transplantation immunology – histocompatibility antigens MHC is the major histocompatibility antigen that needs to be matched for organ transplantation A complex of linked genes encodes the MHC proteins Normal function of MHC is to display peptide antigens (self AND non-self) to T cells

MHC MHC I and MHC II MHC and T cell responses Expression and function Structure Genes and polymorphisms MHC and T cell responses

MHC Class I Protein expressed on all nucleated cells Presents peptide to CD8 T cells Cytotoxic “killer” cells Kill virus infected cells Virus infected cell All cells can potentially be infected by viruses Tc killing virus infected cells

MHC Class II Expressed by specialized antigen presenting cells (APC) Dendritic cells B cells Macrophages Presents peptide to CD4 T cells “Helper” T cells Help B cells proliferate, differentiate, isotype switch Help activate macrophages to kill intracellular pathogens DC polarized Th response,Th1 h2, Th17, etc.

CD4 T cell helping a B cell

B Cell #1 Class I Class II Self peptide Foreign protein/peptide BCR The fact that the Th and the B cell are recognizing the same antigen is sometimes a little unclear mechanistically. Self peptide Foreign protein/peptide BCR

B B Cell Cell #1 #2 CD4 T Cell TCR Class I Class II Self peptide Immune surveillance. T cell has to have a TCR specific for this particular peptide-MHC. Lots of other cells doing surveillance that will ignore this because TCR not specific for this. Class I Class II Self peptide Foreign peptide CD4 T Cell

Class I and Class II MHC Molecules Membrane bound glycoproteins Structurally very similar Both have 4 domains 2 membrane proximal domains 2 membrane distal domains that form a peptide binding cleft

MHC Structure Class II Class I

3-D Structure of Human Class I HLA Molecule

Peptide T cell view

The human MHC gene complex HLA – human leukocyte antigen No gene rearrangements in MHC! Classical MHC molecules. No allelic exclusion, so both chromosome genes expressed.

Expanded view of the human MHC Class III includes complement components like C2, C4 and Factor B.

MHC Polymorphisms Allelic variations in MHC genes Concentrated in the peptide binding regions of Class I and Class II

The human MHC gene complex HLA – human leukocyte antigen

MHC Polymorphisms

MHC Polymorphisms Allelic variations in MHC genes Concentrated in the peptide binding regions of Class I and Class II Population level In an individual, all parental HLA-DP are identical, all maternal HLA-B, are identical, etc.

Consequences of MHC Polymorphism Organ and tissue transplants are difficult Polymorphic residues change the peptide binding specificity of the MHC If all MHC were identical, pathogens might avoid immune detection by mutation to prevent MHC binding Can do transplants not because of better MHC matching but potent immunosuppressive drugs. If no MHC-viral peptide, the virus infected cell becomes invisible to adaptive T cell response. No killing.

NKT cells A subset of T cells that express an αβ T-cell receptor, but also express a variety of molecular markers that are typically associated with NK cells, such as NK1.1.  Unlike NK cells, NKT cells express ab TCR and undergo TCR gene rearrangement in the thymus Conventional T cells recognize peptide MHC NKT cells recognize lipid and glycolipid antigens in association with CD1d, an MHC class I-like molecule ⍺-galactosylceramide (αGalCer) is an exogenous NKT cell ligand that can be used to activate NKT cells in vitro and in vivo. Unlike NK cells, NKT cells express ab TCR. Most commonly used mouse strains (apart from C57BL/6) do not express the NK1.1 marker NKT cells arise in the thymus from a common precursor pool of CD4+CD8+ double-positive thymocytes that have undergone random TCR gene rearrangement and expression. Expression of a TCR that binds with appropriate avidity to MHC class II or I (MHCII or MHCI; plus self-peptide) on thymic epithelial cells leads to the positive selection of conventional CD4+ T cells (green) or CD8+ T cells (blue), respectively. Thymocytes that express a TCR that binds to CD1d plus self-lipid or glycolipid antigen, expressed by other double-positive thymocytes, enter the NKT cell lineage (red). Once selected, NKT cell precursors undergo a series of differentiation steps that ultimately results in the NKT cell pool. At least four distinct NKT cell development stages have been defined through differences in expression of CD24, CD44 and NK1.1;

NKT cells Develop normally in MHC KO mice but not in β2-macroglobulin KO mice Have an “invariant” TCR: Vα14-Jα18: Vβ8.2, 7, 2 (mouse) Vα24-Jα18: Vβ11 (human) Jα18 KO mice lack NKT cells TCRβ TCR

NKT cell functions NKT cells are able to produce large quantities of cytokines within minutes to hours of antigenic stimulation Many proposed functions “Initiation of Antiviral B Cell Immunity Relies on Innate Signals from Spatially Positioned NKT Cells. Gaya, et al. Cell 172:517 (2018) NKT cells are the initial source of IL-4 needed for B cell proliferation, isotype switching etc.