Direct evidence for the existence of nominal antigen binding sites on T cell surface Ti α-β heterodimers of MHC-restricted T cell clones

The binding of nominal antigen to Ti α-β heterodimers on MHC-restricted human T cell clones specific for fluorescein-5-isothiocyanate (FL) was detected by flow cytometry and affinity chromatography. The FL-Ti interaction is of physiologic significance, since T cell activation is induced by cross-linked arrays of FL in the absence of the specific MHC recognition. High antigen valence is required to achieve stable binding to cells and subsequent activation, which is consistent with estimated Ti-FL association constants of <3 × 10⁵l/mol. In addition to providing direct evidence that the Ti α-β heterodimer is the receptor for antigen, these data suggest that nominal antigen binding sites exist on the Ti molecules of at least some MHC-restricted clones.