Antigen contacts by Ni-reactive TCR: Typical αβ chain cooperation versus α chain-dominated specificity

Abstract

VB17+ TCR dominate in Ni-driven T cell cultures from highly Ni-sensitized patients. Using transfection of TCR from three CD4+, VB17+, Ni-specific human T cell clones, we studied their Ni-MHC contacts by site-directed TCR mutation and combination of α and β chains between different TCR. All three TCR exhibited N-nucleotide-determined Arg-Asp motifs in their CDR3-β sequences. Two of them were specifically restricted to HLA-DR13, while the third one accepted a variety of HLA-DR alleles. The highly similar α or β chains of the DR13-restricted TCR were interchangable without loss of specificity, but α or β chains of other TCR were not tolerated. Mutations of their Arg-Asp motif revealed loss of reactivity upon exchanging Asp for Glu or Ala and of Arg for Ala but not of Arg for Lys or the Ni binding His. Reactivity was also destroyed by mutation of α chain position 51, proposed as a general contact site for MHC. Hence, in these two TCR the Arg-Asp motif is clearly involved in contacting Ni-MHC complexes, and close cooperation between α and β chain is required. In contrast, the third TCR retained Ni reactivity upon mutation of α chain position 51 or of its β chain Arg-Asp motif, which rather affected the pattern of DR cross-restriction. Moreover, its α chain paired with various β chains from other, even mouse TCR, irrespective of their specificity, retaining Ni reactivity as well as promiscuous HLA-DR restriction. This preponderance of an α chain in defining specificity indicates fundamental differences in Ni interactions of individual TCR and implies that β chain similarities may not necessarily result from antigen selection.