Co-capping studies reveal CD8/TCR interactions after capping CD8 beta polypeptides and intracellular associations of CD8 with p56(lck).

CD8 is a T cell surface glycoprotein that participates in recognition of peptide/MHC class I molecules by binding to their alpha 3 domains. In addition, the cytoplasmic domain of CD8 associates with the intracellular tyrosine kinase p56(lck) (lck) promoting recruitment of lck to the TCR signaling complex. Recent data have suggested also that CD8 may interact with the TCR to promote energetically favorable conformations which increase its ligand binding. We have used the techniques of co-capping and confocal microscopy to ask whether we can detect an association between CD8 and the TCR independently of their binding to MHC class I molecules. We show that capping CD8 heterodimers with antibodies to the CD8 beta polypeptide is significantly more efficient than antibodies to the CD8 alpha polypeptide at inducing co-localization of TCR molecules with CD8, suggesting that there may be preferred conformations of CD8 which stabilize interactions with the TCR. In addition, we show by microscopy that intracellular lck redistributes very efficiently to the area of a CD8 cap, suggesting that there is a stronger association between lck and CD8 than has been proposed from immunoprecipitation analyses.