Distinct epitopes on Ik gene products identified by monoclonal antibodies

Analysis of reactivity of monoclonal anti-Ia^k alloantibodies, obtained by fusion between NS 1 myeloma and spleen cells from alloimmune A.TH mice, permitted the identification of 9 distinct determinants of the I^k gene products. Competitive binding experiments indicated that 2 private epitopes (defined by H 8–109.13 and H 8–138.4 antibodies) of the I-A^k product could be separated, thereby apparently splitting the Ia.2 specificity. A public determinant of the I-A^k molecule (identified by H 8–15.9 antibody) was found expressed not only on the 1-A products of the H-2^b, H-2^d, H-2^ja, H-2^P and H-2^q murine haplotypes, but also on human HLA-DR antigens. Four determinants of the I-E/C^k antigen (defined by H 7–8.26, H 10–81.10, H 10–93.2 and H 8–86.2 antibodies) had a strain distribution analogous to the Ia.7 specificity. However, competitive binding experiments, and the cross-reactivity pattern with la-like antigens from other species (e.g. human HLA-DR antigens) indicated that these antibodies detected distinct determinants on the I-E/C^k molecule, thereby subdividing the broad Ia.7 specificity. Two other determinants (defined by H9–14.8 and H9–15.4 antibodies) had a strain distribution that did not permit a precise assignment to a given Ia antigen, even though preliminary data suggested that they could detect separate determinants on the I-E/C^k product. All these monoclonal antibodies identified membrane antigens with the expected Ia tissue distribution pattern, and most of them could precipitate a molecule containing two chains of 28 kD and 35 kD, from mouse spleen cell lysates.