1 How T cells recognize antigen: The T Cell Receptor (TCR) Lecture 11, MCB 150 Laurent Coscoy

2 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 There is no soluble form of T Cell Receptor

3 Monoclonal antibody approach (Allison and colleagues 1982)

4 Probed with cDNA clone 1 Probed with cDNA clone 2 T cell clones Liver cells B cell lymphoma Hedrick & Davis strategy: Subtractive cDNA cloning

5 TCR: An  heterodimer

6 A second type of TCR Hetero dimer of  and  chains (like  and  chains) Expressed on a rare subset of T cells with fascinating properties (more later)  T cells  T cells CD4 T cells CD8 T cells

7 Comparison of TCR and BCR (Ig)

8 T cell receptor loci Analogous to B cell receptor loci

9 Generation of the T cell receptor

10 TCR structure

11 TCR: Peptide/MHC I Interaction Constant domains Variable domains CDR3’s Peptide Binding groove CC CC VV VV

12 Class I Class II Peptide Binding Grooves of MHC Molecules

13 Soluble peptide/MHC complexes: a probe to detect specific T cells Ig HC Fc Region MHC I Made by genetic engineering; does not exist in nature

14 HTLV-I and tropical spastic paraparesis 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

15 Following Antigen-Specific T Cells CD8

16 CD4 and CD8: Co-receptors for the TCR

17 What’s with these “CD” names? “CD” stands for cluster of differentiation Many labs generated monoclonal antibodies against cell- surface 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.

18 Co-receptors bind to non-polymorphic regions of MHC I (CD8) or II (CD4)

19

20 CD4 and CD8 mark mutually exclusive subsets of mature T cells Anti-CD8 Anti-CD4 Anti-CD3 Anti-B220 Flow cytometry of spleen Anti-  TCR Anti-CD4 & Anti-CD8

21 Contrast how B and T cells see antigen B Cell T Cell APC

22 TCR affinity is quite low

23 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?

24 Co-receptor and adhesion molecules improve avidity CD2LFA-3 LFA-1ICAM-1 TCR CD4 or CD8 MHC T cell target cell

25 Immunological Synapse: A model T cell APC Red: adhesion Green: Antigen

26 Immunological “synapse” Peptide-MHC complexes Adhesion molecules (ICAM-1)

27 The synapse in real-time T-Cell B-Cell (Peptide-pulsed ) 8 minutes in 20 seconds of video

28 The synapse in real-time

29 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

30 Consequences of T cell activation How is TCR:peptide:MHC binding linked to these events? 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 TCR signaling induces changes in gene expression

31 Example of T cell activation TCR binds peptide:MHC Induces expression of genes T cells divides many times creating more T cells with the same TCR T cells differentiate and acquire new characteristics

32 Signal Transduction Transmission of external signals across the membrane and conversion of these signals into intracellular biochemical events ? TCR Peptide:MHC Changes in gene expression

33 RECEPTOR ? ? ? ? ? ? Alterations in Gene expression TCR signaling: Big Picture ? ? ?

34 How does the TCR transduce signals with such a little tail?

35 TCR associates with the CD3 Complex Analogous to the Ig  and Ig  proteins that associate with BCR on B cells