In a finding that could help explain how T cell receptors (TCRs) can identify foreign antigens efficiently despite the relatively small concentration of such antigens, researchers report in Nature Immunology that the CD8 coreceptor can help T cells by interacting with self-proteins displayed on major histocompatibility complexes (MHCs).

A T cell receptor (TCR) "has to be able to find the needle"–the antigen-MHC complex for which it has specificity–"in the haystack," said study coauthor Nicholas Gascoigne of the Scripps Research Institute in La Jolla, Calif. The findings suggest that the interaction of CD8 with nonstimulatory peptide-MHC complexes facilitates this difficult task by making "the T cell much more sensitive to smaller amounts of antigen presented on a cell," Gascoigne said.

In the current model of the immunological synapse, CD8 interacts with a cognate, or T-cell specific, antigenic peptide-MHC class I complex through the nonpolymorphic region of the MHC and recruits the...

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