Antigen Recognition by B-Cell & T-cell Receptors PDF

Antigen Recognition by B-Cell Receptors and T-cell Receptors Literature: Chapter 4, Janeway’s Immunobiology ETH Zurich Lecture on “Pharmaceutical Immunology I” Prof. Dr. Cornelia Halin Winter 535-0830-00L HS 2024 Cont...

Antigen Recognition by B-Cell Receptors and T-cell Receptors Literature: Chapter 4, Janeway’s Immunobiology ETH Zurich Lecture on “Pharmaceutical Immunology I” Prof. Dr. Cornelia Halin Winter 535-0830-00L HS 2024 Content 1. The structure of a typical antibody molecule 2. The interaction of the antibody molecule with specific antigen 3. Antigen recognition by T cells 1  Revision from Chapter 1 During an infection (or vaccination) lymphocytes are activated and contribute to the specific and efficient elimination of the pathogen T-cells: - recognize and destroy infected cells - activate other leukocytes transmembrane antibody secreted antibody B-cells: - activated by pathogen-specific T cells to secrete antibodies Antibodies: - bind specifically to foreign structures (antigens) and make them inactive 2 2  Revision from Chapter 1 Antibodies and T cell receptors (TCRs) recognize antigens by fundamentally different mechanisms: Antibody: Binds directly to the native antigen (as is occurs in blood or tissue) T cell receptor (TCR): Only recognizes peptide fragments of the antigen when presented on major histocompatibility (MHC) molecules => In this Chapter, we will study in detail how T cells 3 and B cells (i.e. antibodies) recognize antigen  1. Antibody structure Antibodies Effector molecules of the humoral immune response mainly used to fight pathogens (bacteria, viruses, fungi) in the extracellular space Present on B cells, as surface receptor for antigen (B cell receptor; BCR) Composed of constant and variable immunoglobulin domains Variable domains bind the antigen (Ag) 1 Ab has two Ag binding sites => avidity effect! 4  1. Antibody structure Antibody structure - molecular weight of IgG: 150 kDa 2 heavy chains (50 kDa), 2 light chains (25 kDa) - linked by disulfide bonds - Ag binding site composed of variable heavy and variable light chain domain - 5 major immunoglobulin classes, i.e. isotypes: IgM, IgD, IgG, IgA, IgE - Isotypes differ in their distribution in the body and in the effect they trigger (“effector functions”) 5  1. Antibody structure Folding of the immunoglobulin domain each domain consists of two anti-parallel b sheets b sheets are stabilized by an internal disulfide bond forms a b sandwich characteristic structure that has given the name of an entire superfamily: Immunoglobulin fold 6  1. Antibody structure Antibody cleavage by pepsin or papain gives rise to different fragments Cleavage occurs in the hinge region Fc fragment: ” fragment crystallizable” Fab fragment: ”fragment antigen-binding” 7  1. Antibody structure The hinge region allows flexibility in antibody (Ab) binding of multiple antigens affinity vs. avidity 8  1. Antibody structure Antigen binding occurs via the hypervariable regions present in the variable domains of the heavy and light chain FR: Framework HV: hypervariable regions 9 The hypervariable regions lie in discrete loops of the folded structure Hypervariable regions are also called complementarity- determining regions (CDRs) => named CDRs because the surface they form is complementary to that of the antigen they bind 3 CDRs in the heavy chain and 3 CDRs in the light chain variable domain may contact the antigen => total of 6 CDRs  2. Ab-antigen interaction The antigen-binding site is shaped by the CDRs CDR: complementarity-determining regions 11  2. Ab-antigen interaction The antigen-binding site is shaped by the CDRs CDR: complementarity- determining regions 12  3. T cell-antigen interaction 3. Antigen recognition by T cells The T cell receptor resembles a membrane-bound Fab fragment It is composed of an T-cell receptor (TCR) a and TCR b chain The variable domains (Va and Vb) differ between different T cell clones Each T cell bears approx. 30’000 identical TCRs on its surface (the estimated number of different TCR clones in a human is 2 x 107) 13 The T cell receptor (TCR) chain structure resembles the one of a Fab fragment Ig-like domain folding with two anti-parallel b sheets (Fig. 4.15a) The framework variable domains (Va and Vb) can be superimposed with antibody variable domains (Fig. 4.15c) Similarly to the variable domains of antibodies , each TCR variable domain contains 3 CDR loops. Some differences in the constant domain folding and domain interactions (see textbook) 14  3. T cell-antigen interaction A T cell receptor (TCR) recognizes antigen in the form of a complex of a foreign peptide bound to an MHC molecule Examples: MHC class I : MHC class II: expressed by virtually all cells in the body expressed by antigen-presenting cells (APC – i.e. DCs, macrophages, B cells) present fragments of proteins expressed by the cell itself (i.e. derived from cytosol) present fragment of proteins that were taken up into the APC from the outside recognized by cytotoxic CD8+ T cells interact with CD4+ T cells  3. T cell-antigen interaction Structure of MHC class I two polypeptide chains a chain and b2-miroglobulin Only the a chain spans the membrane Folded a1 and a2 form the peptide binding cleft / groove: b-sheet floor and two a-helices as “walls” 16  3. T cell-antigen interaction Structure of MHC class II two polypeptide chains a chain and b chain both span the membrane Folded a1 and b1 form the peptide binding cleft / groove: b-sheet floor and two a-helices as “walls” a2 and b2 have an immuno- globulin fold 17  3. T cell-antigen interaction MHC molecules bind peptides within the cleft MHC I MHC II Peptides are stably (non-covalently!) bound to MHC Bound peptide stabilizes the MHC molecule 18  3. T cell-antigen interaction MHC class I molecules bind short peptides by both ends Peptide length: 8– 10 aa 19  3. T cell-antigen interaction Example of Peptides bound to MHC class I Example of peptides binding to two different MHC I versions (alleles) Peptide length: 8–10 aa Presence of characteristic binding motifs  3. T cell-antigen interaction Peptides are bound to MHC class II by several residues within the cleft Peptide length: 13 – 17 aa 21  3. T cell-antigen interaction Example of Peptides bound to MHC class II variable peptide length (13-17 aa) => overhangs anchor residues at various distances from the end Relevance of knowing peptide binding preferences of MHC molecules (e.g. algorithms exist to predict peptide binding to MHC molecules): Tumor immunology (tumor vaccines) Immunogenicity of therapeutic proteins (de-immunization) 22  3. T cell-antigen interaction The T cell receptor (TCR) binds to the peptide:MHC complex Binding involves TCR interactions with both the peptide and the MHC molecule Interactions mainly involve the CDR loops of the TCR (Fig. 4.15c) TCRs display baseline affinity for MHC molecules (peptide-independent). => This concept will be important for under- standing MHC restriction (Chapter 6) and T cell development in the thymus (Chapter 8). 23  3. T cell-antigen interaction T cell contacts with MHC molecules involve CD4 and CD8 CD8 is a disulfide-linked heterodimer and binds to MHCI CD4 is a monomer (comprising 4 Ig- like domains) and binds to MHCII CD4 and CD8 bind to invariant sites of the MHC molecules binding contributes to the overall effectiveness of the T cell response: Increases T cell sensitivity for Ag by approx. 100-fold! CD4 and CD8 are therefore called co- receptors 24 Role of CD4 and CD8 is two-fold: Enhancement of TCR signaling => recruitment of Lck to TCR complex Stabilization of TCR-MHC complex => Formation of the immune synapse  MHCI and MHCII are differentially expressed on cells MHCII highly expressed on professional antigen- presenting cells (APCs) in the body: => dendritic cells, B-cells and macrophages highly expressed on thymic epithelial cells, where it is important for T cell development (Chapter 8) MHCI expressed on all nucleated cells in the body => important to detect viral infection of these cells highly expressed on APCs and some other leukocytes (T cells, neutrophils) 26 Take-home messages B and T cells use different but structurally related molecules – i.e. TCRs and immunoglobulins - to recognize antigen TCRs and immunoglobulins are highly variable molecules; variability is concentrated in the part of the molecule binding to the antigen Immunoglobulins bind a variety of chemically different antigens abTCRs only recognize processed antigen; i.e. peptide derived from intracellularly degraded proteins and bound to MHC for cell surface presentation Two classes of MHC molecules exist with different expression patterns amongst cells in the body T cell binding to MHCI or MHCII is supported by the co-receptors CD8 and CD4, respectively 27