Fine specificity of a proliferating T-cell clone activated by a conformational determinant of the I-Ek molecule

We have examined the fine specificity of a stable Thy-1.2⁺, Lyt-1.2⁺, Lyt-2^–, and I-A^s– anti-I-E^k proliferating T-cell clone isolated from an A.TH anti-A.TL secondary mixed lymphocyte culture. Spleen cells from various I-A^k, E^k strains induced either a strong (A.TL, OH, and CBA) or a weak (AKR and B10.BR) proliferative response, although such cells expressed at their surface similar amounts of I-E^k antigens. Analysis of H-2 recombinant strains indicated that this clone recognized a conformational determinant carried by the E^kE^kdimer, but not on the Ea chain per se. Among the Fl hybrid strains in which the combinatorial E^kE^kproduct was detected by cellular binding with monoclonal E^k-specific antibodies (mAb), some [(BIO.S(8R) × BlO.HTT) but not others (for example, B10.A(4R) × B10.A(5R)] were stimulatory. Seventeen anti-E^k mAb, regardless of the three spatially separated domains that they defined by antibody binding competition, completely inhibited the restimulation of this clone, whereas 15 other anti-A^k mAb failed to do so. This clone was not reactivated by stimulating cells from strains with the H-2 haplotypes p, j, v, b, r, and s but it proliferated strongly against cells from several H-2^d or H-2^q strains. Genetic evidence or blocking studies with selected mAb assigned these cross-reactive mixed lymphocyte reaction determinants to the A^d or A^q molecules, respectively. The data support the conclusion that alloreactive T cells may define a polymorphism of I-region coded products not detected by serological analyses and extend at the T-cell level the observations of serological cross-reactions between A and E molecules.