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