A wingman for T cell receptors?

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 kinase Lck close to the TCR to facilitate signal transduction, according to the report. However, researchers disagree as to whether CD8 interacts with TCR through its associated intracellular CD3 protein constitutively or only after the T cell has recognized its target antigen. To monitor the intracellular interaction between CD8 and the TCR-CD3 complex in living cells, the group used fluorescence resonance energy transfer (FRET) microscopy to colocalize these fluorescent chimeric proteins to within 10 nm of each other. The researchers found that in fixed cells, the FRET signal–and thus the interaction–was transiently induced 10-12 minutes following antigen recognition. According to Stephen Jameson of the University of Minnesota in Minneapolis, who did not participate in this study, FRET is "much more clear cut than the methods used before" and the results are "pretty definitive." To assess CD8 interaction with nonstimulatory peptide-MHC complexes, the researchers exposed T cells to both antigenic and nonstimulatory peptides. They found that unlike TCR recruitment, CD8 clustering at the immunological synapse was not peptide-specific and was driven primarily by the concentration of MHC, indicating that the nonstimulatory peptides play a role in bringing CD8 into the synapse. Furthermore, they found that the presence of excess nonstimulatory peptide-MHC increased interaction between the CD8 and CD3 molecules and drastically reduced the amounts of cognate peptides needed to form the CD8-CD3-TCR conjugate. Gascoigne has "taken what's been out there, really, as a collection of reagents, and used them in a really precise way to look at the problem," said David Kranz of the University of Illinois at Urbana-Champaign, who did not participate in the study. "He did all the right experiments." According to Kranz, "Once (CD8) are clustered in the synapse, they are positioned and available for the TCR, which is going to engage the actual antigen peptide, and now it can also engage CD8 more easily." Returning to Gascoigne's analogy, Jameson said that immunology researchers, including himself, often "get rid of all the straw… just leaving the needle" in their experimental models. In future work, Jameson told The Scientist, researchers must realize "that the haystack actually contributes in getting to the needle." "You're only going to have a few of these [target antigens] at the synapse," Kranz told The Scientist. "It's been kind of a puzzle—how can only a few [antigenic] peptide MHCs stimulate a T cell?" And though CD8 has long been known to be "essential for [CD8+] T cells to recognize and kill target cells, what has been unclear is the molecular basis for why CD8 is needed," he said. Previous studies have shown evidence that in CD4+ T cells, the CD4 coreceptor enhances TCR response to only some types of nonstimulatory peptide-MHC class II complexes. These authors have shown that CD8 T cells respond equivalently to all nonstimulatory peptides, "which suggests that the TCR is not involved [for the CD8 interaction]," Jameson said. Now, according to Gascoigne, "what we need to address is when the CD8 molecule needs to interact with both endogenous and antigenic [peptides-MHC complexes] or one or the other." Kranz said that the next steps for this research are to study "more details of the interaction" and to "look at whether the TCR itself binds to nonstimulatory pMHCs."

A wingman for T cell receptors?

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