ANTIGEN RECOGNITION BY T-LYMPHOCYTES

*Antigens are recognized by cell surface receptors *Antigen receptor referred to as *T-cell receptor *T-cell and B-cell receptors are similar *Structure *Immunoglobulin superfamily *Organization of genes *Non-functional segments *Mechanism which generates diversity and specificity *Somatic recombination

ANTIGEN RECOGNITION BY T-LYMPHOCYTES *T-cell and B-cell receptors recognize different antigens *B-cells recognize *Intact protein, carbohydrate and lipid molecules on bugs and soluble toxins *T-cells recognize *Peptide antigens bound to special antigen-presenting glycoproteins *Antigen-presenting glycoproteins *Major histocompatibility complex (MHC) molecules *Expressed on antigen-presenting cells (APC’s)

T-LYMPHOCYTE (CELL) RECEPTOR *Membrane bound glycoprotein *Composed to 2 polypeptide chains (1 antigen binding site) *Alpha *Beta *Each chain has variable domain, constant domain and transmembrane region *Variable (V) domains of alpha and beta chains each have 3 hypervariable regions (loops) *Complementarity-determining regions (CDR) *Structure resembles single antigen-binding arm of B cell receptor (immunoglobulin) *Fab fragment (membrane-bound)

GENERATION OF DIVERSITY IN T-CELL AND B-CELL RECEPTORS *Mechanisms which generate B-cell receptor diversity *Before antigen stimulation *Somatic recombination *After antigen stimulation *Somatic hypermutation *Mechanisms which generate T-cell receptor diversity *Before antigen stimulation *Somatic recombination *After antigen stimulation *None

ORGANIZATION AND REARRANGEMENT OF T-CELL RECEPTOR GENES *Alpha chain locus * Located on chromosome 14 * Variable domain similar to IG light chain locus * V and J segments *Beta chain locus * Located on chromosome 7 * Variable domain similar to IG heavy chain locus * V, J and D segments *Receptor gene rearrangement takes place during T-cell development in thymus

ORGANIZATION AND REARRANGEMENT OF T-CELL RECEPTOR GENES *Recombination directed by *Recombination signal sequences (RSS) *Alpha chain gene *V segment joined to J segment by somatic recombination *P and N nucleotides inserted at VJ junction *Beta chain gene *D segment joined to J segment *DJ segment joined to V segment *P and N nucleotides inserted at D, J and V junctions

COMPOSITION OF THE T-CELL RECEPTOR COMPLEX *Newly synthesized alpha and beta chains enter endoplasmic reticulum *In ER, chains associate with 4 invariant membrane proteins * Chromosome 11 * Delta, epsilon, gamma * Chromosome 1 *Zeta *Invariant membrane proteins * Transport to cell surface * Signal transduction

COMPOSITION OF THE T-CELL RECEPTOR COMPLEX *Delta, epsilon and gamma proteins collectively termed *CD3 complex *T-cell receptor complex *T-cell receptor, CD3 proteins and Zeta protein *Persons may lack CD3 delta or CD3 epsilon chains *Inefficient transport of receptors to cell surface *Low number of receptors *Impaired signal transduction

ALTERNATIVE FORM OF T-CELL RECEPTOR *Second type of receptor consists of *Gamma and Delta chains *T-cells referred to as *Gamma:Delta T-cells *Comprise approximately 1 to 5% of circulating T-cells *Function is unknown *Not restricted to MHC presentation of peptide antigens *Alpha:Beta and Gamma:Delta receptors never expressed together

T CELL RECOGNITION OF ANTIGENS – PROCESSING AND PRESENTATION *T-cells cannot recognize antigens in native form *T-cell recognition of antigens *Processing *Presentation *Antigen Processing * Pathogen derived proteins broken down into peptides *Antigen Presentation * Peptide combined with MHC molecule and displayed on surface of antigen presenting cells

T-CELLS RESPOND TO INTRACELLULAR AND EXTRACELLULAR PATHOGENS * T-cells classified on basis of cell surface glycoproteins * CD4 * CD8 * Classes have different functions * CD8 * Primary function to kill cells (cytotoxic) infected with virus or other intracellular pathogen * CD4 * Primary function to help other cells of immune system respond to extracellular pathogens

CD4 T-CELLS RESPOND TO EXTRACELLULAR PATHOGENS * CD4 cells also known as T-helper cells * Subclasses of CD4 cells * T-helper 1 cells (TH1) * Activate tissue macrophages * T-helper 2 cells (TH2) * Stimulate B-cell proliferation and differentiation * Activation and stimulation mediated by cytokines

STRUCTURE OF THE CD4 AND CD8 GLYCOPROTEINS * CD4 Structure * Four immunoglobulin-like domains (D1- D4) and a membrane- spanning region * CD8 Structure *Alpha, beta chain and extended membrane-spanning region

MAJOR HISTOCOMPATIBILITY MOLECULES (MHC) PRESENT ANTIGENS TO CD4 AND CD8 CELLS * Classes of MHC molecules * MHC class I * MHC class II * Functions of MHC molecules * MHC class I * Present intracellular antigens to CD8 cells * MHC class II * Present extracellular antigens to CD4 cells

MAJOR HISTOCOMPATIBILITY MOLECULES (MHC) PRESENT ANTIGENS TO CD4 AND CD8 CELLS *Mechanisms for recognition between T cells and MHC molecules *T-cell receptor recognition of peptide presented by MHC molecule *Specific interactions between *CD8 and MHC class I molecules *CD4 and MHC class II molecules *CD8 and CD4 molecules *Considered T-cell co-receptors

STRUCTURES OF MHC MOLECULES * MHC molecules are glycoproteins * MHC class I molecule * A single membrane bound alpha chain non-covalently bonded to beta2-microglobulin * Alpha chain has three domains * MHC class II molecule * Two membrane bound chains (alpha and beta) * Each chain has two domains

PEPTIDE BINDING SITES OF MHC MOLECULES *MHC molecule binding sites *Can bind many different amino acid sequences *Length of peptides bound *MHC class I *8 – 10 amino acids *MHC class II *13 – 25 amino acids

PROCESSING OF ANTIGENS FROM INTRACELLULAR AND EXTRACELLULAR PATHOGENS * Intracellular pathogens *Degradation of proteins in cytosol of infected cells *Peptides enter endoplasmic reticulum and bound to MHC class I molecules * Extracellular pathogens * Microorganisms and toxins taken into cells by * Phagocytosis and endocytosis * Degradation of proteins and binding to MHC class II molecules in phagolysosomes and endocytotic vesicles

MECHANISM FOR PROCESSING OF ANTIGENS FROM INTRACELLULAR PATHOGENS * Proteins degraded in cytosol of infected cells by * Proteasome * Barrel shaped protein complex with several proteolytic activities * Peptides transported across ER membrane by protein * Transporter associated with antigen processing (TAP)

MECHANISM FOR PROCESSING OF ANTIGENS FROM INRACELLULAR PATHOGENS *MHC class I heavy chain enters ER and binds to membrane protein * Calnexin *Calnexin released when beta-2-microglobulin binds *MHC class I molecule binds complex of proteins *Peptide-loading complex *Calreticulin, Tapasin, TAP, ERp57 and PDI

MECHANISM FOR PROCESSING OF ANTIGENS FROM INTRACELLULAR PATHOGENS *MHC class I molecule retained in ER until it binds a peptide *Following binding, MHC class I molecule *Released from protein complex *Leaves ER in membrane-bound vesicle *Transported by Golgi complex to cell surface *Process is continuous, not only during infection

FAILURE OF THE INTRACELLULAR PATHOGEN PROCESSING MECHANISM *Bare Lymphocyte Syndrome (MHC class I) * Immunodeficiency disease *Clinical Manifestations * Chronic bacterial respiratory infections * Cutaneous ulceration with vasculitis *Mechanism *Mutations in TAP1 or TAP2 genes *Decreased levels of cell surface MHC class I molecules *Reduce levels of alpha:beta CD8 T cells

MECHANISMS PREVENTING THE PROCESSING OF ANTIGENS FROM INTRACELLULAR PATHOGENS *Herpes Simplex Virus (HSV) * Produce protein which binds to and inhibits TAP * Prevents viral peptide transfer to ER *Adenovirus *Produce protein which binds MHC class I molecule *Prevents MHC class I molecule from leaving ER

MECHANISM FOR PROCESSING ANTIGENS FROM EXTRACELLULAR PATHOGENS *Extracellular microorganisms and toxins engulfed by phagocytosis / endocytosis in *Phagosomes / endosomes *Phagosomes fuse with lysosomes (proteases/hydrolases) forming phagolysosome *Peptides produced bind with MHC class II molecules within vesicular system *Peptide:MHC class II complexes transported to cell surface

MECHANISM FOR PROCESSING ANTIGENS FROM EXTRACELLULAR PATHOGENS *MHC class II alpha and beta chains transported into ER *In ER, associated with “invariant chain” which functions *Prevent peptide binding *Chaperones MHC II molecules to endosomes *In endosomes, invariant chain degraded by *Cathepsin L *Degradation results in small fragment which covers MHC II peptide binding site *Class II associated invariant chain peptide (CLIP)

MECHANISM FOR PROCESSING ANTIGENS FROM EXTRACELLULAR PATHOGENS *CLIP removal associated with *Interaction of MHC II and endosome membrane glycoprotein * HLA-DM * Similar structure to MHC II * Does not bind peptides or appear on cell surface *MHC II quickly binds peptide or is degraded *Peptide:MHC II transported to cell surface for recognition by specific T-cell receptor

EXPRESSION OF MHC I AND MHC II ON HUMAN CELLS *MHC class I * Guard the intracellular territory * Constitutive expression on virtually all cells * Comprehensive surveillance by CD8 T-cells *MHC class II * Guard the extracellular territory * Constitutive expression only on APC’s *Macrophages *B lymphocytes *Dendritic cells (immature)

EXPRESSION OF MHC I AND MHC II ON HUMAN CELLS *Antigen uptake by APC’s * Macrophages * Phagocytosis and pinocytosis in all tissues * B lymphocytes * Internalize antigens bound to surface IG * Receptor-mediated endocytosis * Dendritic cells (immature) * Phagocytosis and macropinocytosis in all tissues *Cytokine upregulation of MHC I and II in immune response *Interferons

MAJOR HISTOCOMPATIBILITY COMPLEX (MHC) *Named MHC following identification of region responsible for rejection of tissue or organ transplant *MHC molecules encoded by a number of closely linked genes on chromosome 6 * Conventional gene configuration *Large number of variants in human population *Variants responsible for * Host versus graft * Graft versus host

MAJOR HISTOCOMPATIBILITY COMPLEX (MHC) *Complex also called *Human leukocyte antigen (HLA) complex *Antibodies originally used to identify MHC molecules react with leukocytes *HLA I genes and HLA II genes *Located on short arm of chromosome 6 *Beta-2-microglobulin (C-15) and invariant chain (C-5) not located in HLA region

MECHANISMS OF DIVERSITY IN MHC MOLECULES *Polygeny (polygenic) * Multiple genes encode alpha chain of MHC I molecules * Multiple genes encode alpha and beta chains of MHC II molecules *Polymorphism (polymorphic) * Multiple alternative forms of MHC I and MHC II genes in human population *Alternative gene forms called “alleles”

POLYGENY AND POLYMORPHISM IN HUMAN MHC CLASS I MOLECULES *Polygeny (multiple genes) *3 genes for alpha chain *HLA-A, HLA-B and HLA-C *Polymorphism (multiple alleles) *Alleles *HLA-A (506) * HLA-B (872) * HLA-C (274)

POLYGENY AND POLYMORPHISM IN HUMAN MHC CLASS II MOLECULES *Polygeny (multiple genes) *HLA-DP *1 gene for each alpha and beta chian *HLA-DQ *1 gene for each alpha and beta chain *HLA-DR *1 gene for alpha chain *DRA *4 genes for beta chain *DRB1, DRB3, DRB4, DRB5 *Polymorphism (alleles) *Multiple alleles for all genes except DRA

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MHC POLYMORPHISM AND REJECTION OF TRANSPLANTED TISSUES AND ORGANS *MHC molecules primary reason for transplant rejection *Allogeneic *Genetic differences between two members of same species *Alloantigens *Antigens which differ between members of same species *Alloreaction *Immune response to alloantigens *MHC allotype variation is clustered in peptide binding site

HUMAN LEUKOCYTE ANTIGEN (HLA) COMPLEX *HLA type *Combination of HLA class I and HLA class II allotypes *HLA typing in medicine *Selection of donors and recipients for transplantation *Transplantation of organs *Problem of graft rejection by recipient *HLA mismatches overcome using immunosuppressive agents *Transplantation of bone marrow *Problem of alloreaction of graft against recipients tissues