antigen recognition by t-lymphocytes. *antigens are recognized by cell surface receptors *antigen...
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ANTIGEN RECOGNITION BY T-LYMPHOCYTES
ANTIGEN RECOGNITION BY T-LYMPHOCYTES
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
* 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
* 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
* 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
Figure 3-23
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