Inhibition of responder cell activity in mixed leukocyte culture reactions. II. Influence of antibody binding and shedding properties within a B7-cross-reactivity group.

An HLA-B7 antiserum showed cross-reactivity with HLA-B8, Bw41, a split of B40 (Bw60) and possibly Bw22 and B27, thus detecting one or more determinants on these antigens similar to an antigenic site on HLA-B7 usually not detected by other B7-antisera. The cross-reactions were demonstrable by adsorption of antibodies on lymphocytes followed by release at 37 degrees C, which enabled the detection of weak and otherwise hardly detectable reactivity. Released antibody molecules were detected in two different assays: (1) Antibody-dependent cellular cytotoxicity (ADCC) with HLA-B7 positive target cells (fluorochromasia micro ADCC). (2) Inhibition of MLC reactions with B7 positive stimulator cells. The B7-antibody, as detected in both assays, was released in decreasing activity from Bw41 greater than B8 greater than Bw60 much greater than B7 greater than (B27 = Bw22) positive cells. The order of sensitivity in which the various antigens were detected in ADCC assays in which the antiserum activity was measured directly on various target cells was different, viz. HLA-B7 greater than Bw60 = B27 greater than Bw41 greater than B8. Bw22 was not detected. Absorption studies demonstrated that HLA-B7 positive cells bound more B7 antibody activity than B8 positive cells. However, antibody molecules bound to B7 positive cells were mainly released as immune complexes, which could be dissociated by treatment with acid. In contrast, B7 antibody molecules bound to B8 positive cells were released as free antibody molecules. This marked difference in shedding properties further explained the previously described B7 specific unresponsiveness in MLC of HLA-B8 (and also Bw41) positive responder cells after sensitization with the B7 antiserum (de Rooij et al. 1980).