Do Close Encounters Exclude Large Proteins?

Science's STKE  02 Aug 2005:
Vol. 2005, Issue 295, pp. tw283
DOI: 10.1126/stke.2952005tw283

Signaling from the T cell antigen receptor (TCR) is initiated (TCR triggering) when the TCR is bound by peptide presented by the major histocompatibility antigens (pMHC) on antigen-presenting cells (APCs). Whereas the TCR-pMHC interaction spans an intermembrane distance of about 14 nm (as do various receptor-ligand pairs that contribute to TCR triggering), receptor tyrosine phosphatases such as CD45 and CD148, which inhibit TCR triggering, have much larger ectodomains. Choudhuri et al. created murine pMHC class I complexes as single-chain trimer (SCT) fusion proteins containing the ovalbumin-derived peptide SIINFEKL (pOVA), β2-microglobulin, and the H-2Kb MHC class I heavy chain. SCTs were elongated by including two, three, or four immunoglobulin superfamily domains to test the hypothesis that TCR triggering involves size-dependent exclusion of proteins with large ectodomains and thus depends on the intermembrane distance spanned by the TCR-pMHC. When expressed in Chinese hamster ovary (CHO) cells, elongated SCTs were less effective at activating a T cell hybridoma (B3Z cells) or CD8+ T cells that specifically recognized pOVA-H2Kb. Transmission electron microscopy (TEM) showed that the mean intermembrane distance between B3Z cells and APCs expressing the elongated SCT containing four immunoglobulin superfamily domains (SCT-CD4) was substantially greater (17.8 nm) than that with cells expressing unelongated SCT (13.1 nm). Moreover, an early depletion of CD45 from the T cell-APC interface seen when T cells were incubated with APCs expressing SCT was not apparent when T cells were incubated with APCs expressing SCT-CD4. Similar results were obtained with an artificial TCR-ligand system. Thus, the authors conclude that TCR triggering depends on close contact between the T cell and the APC and the size-dependent segregation of proteins with large ectodomains from the T cell-APC interface.

K. Choudhuri, D. Wiseman, M. H. Brown, K. Gould, P. A. van der Merwe, T-cell receptor triggering is critically dependent on the dimensions of its peptide-MHC ligand. Nature 436, 578-582 (2005). [PubMed]