Dr. Jianming Xie, Stanford University School of Medicine

Chemistry meets Immunology: photocrosslinkable peptide-MHCs detect rare antigen-specific T cells and elucidate TCR signaling

T cells, which play a central role in many aspects of the immune system, express T cell receptors (TCRs) to probe a diverse milieu of antigenic peptides presented by molecules of the major histocompatibility complex (MHC) on the surface of antigen presenting cells (APCs)[1]. Upon encountering cognate antigens (such as those derived from microbes or cancer cells), T cells mount a variety of immune responses to clear the pathogen and protect the host.  My talk will focus on using protein photochemistry and live-cell imaging to elucidate the macromolecular basis of T cell activation[2].  TCR–pMHC binding governs T cell activation, but this interaction is incredibly weak, having a dissociation constant (Kd) of 1-100 µM and a half-life (t1/2) of 0.5-10 s.  I will first introduce the development of a photocrosslinkable derivative of pMHC monomers, which can be used to stably stain and purify antigen-specific T cells in a mixture.  I will then discuss using this reagent to probe TCR dynamics and T cell activation.  This study has led to two novel findings.  One is that ligand-engaged TCRs are preferentially transported into the center of the immunological synapse, which suggests that ligand engagement enables linkage of the TCR to the cytoskeleton and subsequent transport of ligated TCRs to a central platform for signaling regulation.  The other is that covalently bound pMHC ligands in solution cannot stimulate T cells, but are stimulatory after being multimerized or being presented on planar lipid bilayers.  This result shows unambiguously that TCR conformational change induced by ligand binding is not sufficient to activate T cells, and that aggregating TCRs and/or mechanical forces are required.  This photochemical technique has the potential for broad application to critical questions in immunology, for example, to identify tumor antigens for the purpose of T-cell-targeted cancer vaccine development.

References:

[1]   M. M. Davis, M. Krogsgaard, M. Huse, J. Huppa, B. F. Lillemeier, Q. J. Li, Annu Rev Immunol 2007, 25, 681.

[2]   J. Xie, J. B. Huppa, E. W. Newell, J. Huang, P. J. Ebert, Q. J. Li, M. M. Davis, Nat Immunol 2012, 13, 674.

 For more information contact Prof. Donald Doyle (404-385-0631).