How T cells recognize antigen: The T Cell Receptor (TCR) Identifying the TCR: Why was it so hard to do? Monoclonal antibody approach

Transcription

1 How T cells recognize antigen: The T Cell Receptor (TCR) Identifying the TCR: Why was it so hard to do By the early 1980s, much about T cell function was known, but the receptor genes had not been identified Recall that Ig was purified as soluble myeloma protein Lecture 11, MCB 150 Laurent Coscoy 1 There is no soluble form of T Cell Receptor 2 Monoclonal antibody approach Hedrick & Davis strategy: Subtractive cdna cloning Liver cells B cell lymphoma T cell clones Probed with cdna clone 1 (Allison and colleagues 1982) 3 Probed with cdna clone 2 4

2 A second type of TCR TCR: An αβ heterodimer Hetero dimer of γ and δ chains (like α and β chains) Expressed on a rare subset of T cells with fascinating properties (more later) αβ T cells γδ T cells 5 CD4 T cells CD8 T cells 6 Comparison of TCR and BCR (Ig) T cell receptor loci Analogous to B cell receptor loci 7 8

3 Generation of the T cell receptor TCR structure 9 10 TCR: Peptide/MHC I Interaction Binding groove Peptide Binding Grooves of MHC Molecules Class I Class II Peptide V V CDR3 s Variable domains C C Constant domains 11 12

4 Soluble peptide/mhc complexes: a probe to detect specific T cells HTLV-I and tropical spastic paraparesis Made by genetic engineering; does not exist in nature MHC I Ig HC Fc Region HTLV-I: - human retrovirus, persistent infection - 20 millions people are infected worldwide - encodes the oncogene Tax TSP: - progressive inflamatory disease of the CNS - pathogenesis not fully understood Following Antigen-Specific T Cells CD4 and CD8: Co-receptors for the TCR CD

5 What s with these CD names CD stands for cluster of differentiation Co-receptors bind to non-polymorphic regions of MHC I (CD8) or II (CD4) Many labs generated monoclonal antibodies against cellsurface proteins --> naming of antibodies and their targets was a mess Conference was called to clean up the naming business Target of monoclonal ab directed against the same surface protein or complex was given a number, as in CD3, CD4, etc CD4 and CD8 mark mutually exclusive subsets of mature T cells Flow cytometry of spleen Anti-B220 Anti-CD3 Anti-CD4 Anti-CD8 Anti-CD4 & Anti-CD8 Anti- TCR 19 20

6 Contrast how B and T cells see antigen TCR affinity is quite low B Cell T Cell APC Mysteries in T cell antigen recognition About 10 5 copies of each MHC molecule on cell surface Frequency of any particular Peptide:MHC complex may be as low as 0.01% How does the TCR sort through all those incorrect complexes and find ones with which to trigger 23 Co-receptor and adhesion molecules improve avidity T cell CD2 TCR CD4 or CD8 LFA-1 LFA-3 MHC ICAM-1 target cell 24

7 Immunological Synapse: A model Immunological synapse Red: adhesion Green: Antigen T cell T cell T cell Peptide-MHC complexes APC APC APC 25 Adhesion molecules (ICAM-1) 26 The synapse in real-time The synapse in real-time B-Cell (Peptide-pulsed ) T-Cell 8 minutes in 20 seconds of video 27 28

8 TCR Signal Transduction Why do we care It helps us understand T cell function It gives us molecular targets for immunotherapy It is an amazing example of biological complexity Consequences of T cell activation Proliferation Cell cycle entry and cell division Clonal expansion Differentiation Secretion of cytokines (helper cells) Activation of killer functions (cytotoxic cells) Acquisition of effector function Memory Death Important for down-regulation of immune response How is TCR:peptide:MHC binding linked to these events 29 TCR signaling induces changes in gene expression 30 Example of T cell activation TCR binds peptide:mhc Induces expression of genes Signal Transduction Transmission of external signals across the membrane and conversion of these signals into intracellular biochemical events T cells divides many times creating more T cells with the same TCR Peptide:MHC TCR T cells differentiate and acquire new characteristics 31 Changes in gene expression 32

9 TCR signaling: Big Picture How does the TCR transduce signals with such a little tail RECEPTOR Alterations in Gene expression TCR associates with the CD3 Complex Analogous to the Igα and Igβ proteins that associate with BCR on B cells 35