4. Nature of T cell B cell receptors.pptx

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1. Nature of T cell & B cell receptors Dr. K. Padma Dorothy

2. The molecule responsible for T-cell specificity - a heterodimer composed of either α and β or γ and δ chains. Cells that express TCRs have approximately 10 5 TCR molecules on their surface. The T-cell receptor is associated on the membrane with a signal-transducing complex, CD3 I n the early 1980s , T-cell receptors, first isolated by means of clonotypic monoclonal antibodies, are heterodimers consisting of an α and β or γ and δ chain. When antisera were prepared using heterodimers isolated from membranes of various T-cell clones, some antisera bound to heterodimers from all the clones, whereas other antisera were clone specific - suggested that the amino acid sequences of the TCR and chains have constant and variable regions In human and mouse, the majority of T cells express the α β heterodimer; the remaining T cells express the γδ heterodimer. T cell receptor

3. The domain structures of α β and γδ TCR heterodimers are similar to immunoglobulins - classified as members of the immunoglobulin superfamily Each chain in a TCR: 2 domains with an intrachain disulfide bond between the domains that spans 60–75 amino acids. Amino-terminal domain in both chains, exhibits marked sequence variation T he sequences of the remainder of each chain are conserved. TCR domains–one variable (V) and one constant (C)–TCR molecule resembles a Fab fragment TCR variable domains - 3 hypervariable regions, equivalent to CDRs in Ab light and heavy chains. - another area of hypervariability (HV4) in the β chain - does not contact antigen Each TCR chain contains - short connecting sequence -cysteine residue form a S-S link with the other chain of the heterodimer. A transmembrane region of 21 or 22 amino acids - anchors each chain in the plasma membrane. - unusual in that they contain positively charged amino acid residues - enable the chains of the TCR heterodimer to interact with chains of the signal-transducing CD3 complex. E ach TCR chain contains a short cytoplasmic tail of 5–12 amino acids at the carboxyl-terminal end. The TCRs differ in the orientation of the V & C regions - elbow angle between the long axes of V & C regions of γδ TCR is 111°; in the αβ TCR, the elbow angle is 149°, giving the molecules distinct shapes - contributes to differences in signaling mechanisms and in how the molecules interact with coreceptor molecules. In humans the predominant receptor expressed - recognizes a microbial phospholipid antigen, 3-formyl-1-butyl pyrophosphate - on M. tuberculosis and other bacteria and parasites. This specificity for frequently encountered pathogens - cells may function as an arm of the innate immune response, allowing rapid reactivity to certain antigens without the need for a processing step. α β and γδ T-Cell Receptors - Structure

4. T-Cell Receptors

5. The genes that encode the α β and γδ T-cell receptors are expressed only in cells of the T-cell lineage. The four TCR loci ( α , β , γ and δ ) are organized in the germ line similar to the multigene organization of the immunoglobulin (Ig) genes Functional TCR genes are produced by rearrangements of V and J segments in the α -chain and γ - chain families and V, D, and J segments in the β -chain and δ -chain families. Allelic Exclusion of TCR Genes - The δ -gene segments are located on chromosome 14 between the V & J segments – a productive rearrangement of the α -chain gene segments deletes C δ , so that, in a given T cell, the α β TCR receptor cannot be co- Sexpressed with the γδ receptor. Organization of TCR Genes TCR Multigene families in humans

6. The α chain - encoded by V, J, and C gene segments. The β chain - encoded by V,D, J, and C gene segments. Rearrangement of the TCR α - and β -chain gene segments results in: VJ joining for the α chain VDJ joining for the β chain After transcription of the rearranged TCR genes, RNA processing, and translation, the α and β chains are expressed as a disulfide-linked heterodimer on the membrane of the T cell. The heterodimer is expressed only in a membrane-bound form Each TCR constant region includes a connecting sequence, a transmembrane sequence, and a cytoplasmic sequence. The germ-line DNA encoding the TCR α and β chain constant regions - TCR α chain DNA has only a single C gene segment; the β - chain DNA has two C gene segments The protein products differ by only a few amino acids and have no known functional differences. TCR V & C - Region Gene Rearrangement Mechanisms that contribute to diversity of TCRs

7. The T-cell receptor associates with CD3, forming the TCR-CD3 membrane complex. T he accessory molecule participates in signal transduction after interaction of the T cell with antigen – however it does not influence interaction with antigen. E xperiments by J. P. Allison and L. Lanier using cross-linking reagents demonstrated that the two chains must be within 12 Å. It was demonstrated that CD3 is closely associated with the heterodimer & its expression is required for membrane expression of αβ and γδ T-cell receptors—each heterodimer forms a complex with CD3 on the T-cell membrane. Loss of the genes encoding either CD3 or the TCR chains results in loss of the entire molecular complex from the membrane. CD3 - a complex of five invariant polypeptide chains that associate to form three dimers: a heterodimer of gamma and epsilon chains – γε a heterodimer of delta and epsilon chains - δε a homodimer of two zeta chains or a heterodimer of zeta and eta chains – ζζ or ζ ή The cytoplasmic tails of the CD3 chains contain a motif called the immunoreceptor tyrosine-based activation motif (ITAM) - found in a number of other receptors – ITAM sites interact with tyrosine kinases - play an important role in signal transduction. The T-cell receptor complex can thus be envisioned as four dimers: the αβ or γδ TCR heterodimer determines the ligand-binding specificity the CD3 dimers ( γε , δε , and ζζ or ζ ή ) - required for membrane expression of TCR & for signal transduction. T-Cell Receptor Complex: TCR-CD3

8. Schematic diagram of the TCR-CD3 complex

9. Formation of immunological synapse at the T-cell–APC interface

10. All isotypes of mIg have very short cytoplasmic tails: the mIgM and mIgD cytoplasmic tails contain only 3 amino acids; the mIgA tail, 14 amino acids; and the mIgG and mIgE tails, 28 amino acids. In each case, the cytoplasmic tail is too short to mediate an activating signal after contact with an antigen and to associate with intracellular signaling molecules - tyrosine kinases and G proteins The mIg does not constitute the entire antigen-binding receptor on B cells. T he B cell receptor (BCR) - a transmembrane protein complex composed of mIg and disulfide-linked heterodimers called Ig- α /Ig- β Michael Reth and his colleagues in the early 1990s has discovered the Ig- α /Ig- β heterodimer Ig- α chain has a long cytoplasmic tail containing 61 amino acids; the tail of Ig- β chain contains 48 amino acids. The tails in both Ig- α and Ig- β are long enough to interact with intracellular signaling molecules. Molecules of this heterodimer associate with an mIg molecule to form a BCR - an extracellular receptor with an intracellular signal-transducing unit Many cells feature membrane glycoproteins - Fc receptors ( FcR ) - have affinity for Fc portion of Ab molecule. These receptors are essential for many of the biological functions of antibodies: P oly Ig receptor - essential for transport of polymeric immunoglobulins (IgA & IgM) across epithelial surfaces Neonatal Fc receptor ( FcR N ) - transfers IgGs from mother to fetus during gestation across the placenta & also plays a role in the regulation of IgG serum levels The B-Cell Receptor

11. Fc receptors have been discovered for all Ig classes: Fc α R receptor that binds IgA Fc ε R that binds IgE Fc δ R that binds IgD Fc μ R that binds IgM several Fc γ R receptors that bind IgG & its subclasses Fc receptor is part of a signal-transducing complex that involves the participation of other accessory polypeptide chains - a pair of γ chains; for IgE receptor, a more complex assemblage of two γ chains and a β chain. These receptors also allow passive acquisition of antibody by many cell types, including B and T lymphocytes, neutrophils, mast cells, eosinophils, macrophages, and natural killer cells. Fc receptors provide a means by which antibodies—the products of the adaptive immune system—can recruit such key cellular elements of innate immunity as macrophages and natural killer cells. Engagement of antibody-bound antigens by the Fc receptors of macrophages or neutrophils provides an effective signal for the efficient phagocytosis, opsonization & ADCC C rosslinking of Fc receptors by Ag-mediated crosslinking of FcR -bound Abs generates immunoregulatory signals that affect cell activation, induce differentiation and, in some cases, downregulate cellular responses.

12. Structure of B-cell receptor (BCR) Structure of human Fc-receptors, Fc-binding polypeptides & accessory signal-transducing polypeptides