Transcript 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 membranespanning 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