Antigen Receptor Structures October 14, 2016 Ram Savan savanram@uw.edu 441 Lecture #8 Slide 1 of 28
Three lectures on antigen receptors Part 1 (Today): Structural features of the BCR and TCR Janeway Chapter 4 Part 2 (Monday): Assembly of antigen receptor genes Janeway Chapter 5 Part 3 (Wednesday): Somatic diversification of BCRs: Hypermutation and class switch recombination Janeway Chapter 5 441 Lecture #8 Slide 2 of 28
Overview B and T lymphocytes are the cells that comprise the adaptive immune system. To initiate an adaptive immune response, B and T cells recognize foreign entities called antigens, using their surface-bound antigen recognition receptors. Today: the structure of these receptors, B cell receptors first, T cell receptors second 441 Lecture #8 Slide 3 of 28
Nomenclature B cell antigen receptor molecules are called Immunoglobulins, or Igs Each B cell expresses Igs of a single antigen specificity Clonal selection theory= One cell one antibody rule Membrane-bound Igs are called B cell receptors, or BCRs Igs secreted by terminally differentiated B cells (plasma cells) are called Antibodies or Abs Ab secretion is the main effector function of B cells in adaptive immunity 441 Lecture #8 Slide 4 of 28
Antibody Structure ribbon diagram of an Ab molecule schematic diagram 441 Lecture #8 Slide 5 of 28
Antibody Structure Antibody molecules are made of four polypeptide chains: Two identical Light or L chains Two identical Heavy or H chains Abs have TWO identical antigen binding sites made of variable (V) regions of the H and L chains. The less diverse regions of the H and L chains are called constant (C) regions. 441 Lecture #8 Slide 6 of 28
Antibody Structure C terminus Disulfide bonds join each L chain to a H chain and the H chains to each other. The arms are joined to the trunk of the Y by a flexible hinge. 441 Lecture #8 Slide 7 of 28
Antibodies have flexible hinges Allows antibodies to reach a variety of antigen structures Constant regions stick out from the bound antigen 441 Lecture #8 Slide 8 of 28
Several flavors of antibodies The constant regions of each of these isotypes mediates the effector function of the antibody molecule Each isotype is specialized to activate a distinct type of immune response Example: IgM and IgG activate classical complement cascade Variable regions that make the antigen binding site can be grafted onto different constant regions More on this next Wednesday 441 Lecture #8 Slide 9 of 28
Protease Cleavage of Ab Molecules Papain cleaves antibodies on the amino terminal side of the S-S bonds that hold the heavy chains together. This results in three pieces: 2 Fab (Fragment Antigen Binding) and 1 Fc (Fragment Crystallizable) This helped immunologists understand what functions are associated with each antibody subunit. 441 Lecture #8 Slide 10 of 28
Protease Cleavage of Ab Molecules Pepsin cleaves antibodies on the carboxy terminal side of the S-S bonds that hold the heavy chains together. This results in two pieces: 1 (Fab ) 2 (2 joined Fab fragments) and 1 pfc (partial Fc) Fab 2 fragments are useful tools because they bind antigen well but have no other effector functions. Fabs and Fab 2s are used as drugs in humans 441 Lecture #8 Slide 11 of 28
Ig light chain constant and variable domains Each domain consists of polypeptide strands running in opposite directions, arranged in sheets (antiparallel b sheets). The sheets are linked by a disulfide bond and folded (rolled) into b barrels. This domain is the Ig fold, very evolutionarily useful: >750 human genes have them (Heavy chain is similar) 441 Lecture #8 Slide 12 of 28
V domains: framework and hypervariable regions 3 especially variable segments within the V H and V L domains: Hypervariable (HV) regions form the loops between the beta sheets. HV3 is the most variable hypervariable region. Framework (F) regions form the beta sheets. Each hypervariable region is encoded by a separate gene segment! (next lecture) 441 Lecture #8 Slide 13 of 28
Hypervariable regions form the antigen binding sites Antigen binding site: 3 HV regions from heavy chain 3 HV regions from light chain (Two of these sites per Ab molecule) These regions are complementary to the antigen: complementarity determining regions (CDRs). 441 Lecture #8 Slide 14 of 28
Antibodies can bind antigens in very diverse ways Epitopes: conformational shapes on the antigen surface bound by the Ab Epitopes can be proteins, carbohydrates, chemicals, almost anything Interaction is non-covalent: antibodies can let go, based on affinity and avidity 441 Lecture #8 Slide 15 of 28
T cell receptors and Igs are built of similar domains Can be membrane-bound OR secreted Binds to soluble antigens (epitopes are in native conformation) Membrane-bound ONLY Binds to peptides presented by MHC molecules 441 Lecture #8 Slide 16 of 28
Division between T and B cells is ancient Jawless vertebrates first appeared around 500 million years ago IgZ Jawless fish have lymphocytes that can secrete their receptors (Blike) and cells that have only membrane-bound receptors (T-like) Their receptors look very different from BCRs/TCRs, but the principles of antigen detection are very similar (convergent evolution) Adapted from Nature Reviews Immunology 2, 688-698 (September 2002) 441 Lecture #8 Slide 17 of 28
TCRs resemble one Fab: one arm of the antibody Y Each T cell expresses many copies of a single type of TCR on its surface Most TCRs consist of two disulfide-bonded a and b glycoproteins. Each a and b chain consists of a V and a C domain, and together they form a single antigen binding site per TCR. 441 Lecture #8 Slide 18 of 28
T cell receptor structure TCR V regions are homologous to Ig V domains TCR C regions are homologous to Ig C domains The TCR a and b chains have short cytoplasmic tails and positively charged residues in the transmembrane regions. (More on this in another lecture) 441 Lecture #8 Slide 19 of 28
T cell receptor crystal structure CDR loops a chain in pink, b chain in blue Hypervariable regions (CDRs): similar organization as in BCR: 3 from the a chain 3 from the b chain Side view of TCR The single antigen binding surface of TCRs is flatter than that of BCRs 441 Lecture #8 Slide 20 of 28
Refresher: TCRs bind to peptide-mhc complexes 441 Lecture #8 Slide 21 of 28
Refresher: TCRs bind to peptide-mhc complexes Peptide-MHC Class I Peptide-MHC Class II The peptide-loaded MHC forms a continuous 3-dimensional surface 441 Lecture #8 Slide 22 of 28
TCR binding to the peptide-mhc complex The TCR contacts both MHC molecule and peptide The six CDRs are focused mostly on the peptide Just as with BCRs, these CDRs are all encoded by separate gene segments 441 Lecture #8 Slide 23 of 28
CD4 and CD8 structure CD4: single chain, 4 Ig-like domains. CD8: disulfide bonded heterodimer, 2 Ig-like domains CD4 T cells: Helper T cells, MHC class IIrestricted CD8 T cells: Cytotoxic T cells, MHC Class I- restricted 441 Lecture #8 Slide 24 of 28
Coreceptors bind to specific MHC molecules CD4: binds to side Of MHC Class II molecules CD8: binds to side Of MHC Class I molecules 441 Lecture #8 Slide 25 of 28
Coreceptors stabilize MHC-peptide interactions Affinity of a TCR for MHC-peptide is fairly low (much lower than Ab-antigen) Coreceptors add binding energy Coreceptors also bring signaling components to the activated TCR (more on this later) 441 Lecture #8 Slide 26 of 28
Refresher: helper vs cytotoxic T cells MHC Class I expressed on all nucleated cells MHC-I/peptide : CD8 T cell: T cell usually kills target MHC Class II expressed on specialized antigen-presenting cells: DCs and macrophages, B cells Also expressed on epithelial cells in thymus (more on this later) MHC-II/peptide : CD4 T cell: T cells secrete cytokines to help that cell perform its function 441 Lecture #8 Slide 27 of 28
Summary: antigen receptor structure BCRs: soluble, native antigens TCRs: MHC-peptide complexes Antigen receptors use a structural framework as a platform to generate diversity of antigen binding How is this diversity encoded? More on this on Monday 441 Lecture #8 Slide 28 of 28