The binding of monoclonal antibodies to cell surface molecules. Quantitative analysis of the reactions and cross-reactions of an antibody (MB40.3) with four HLA-B molecules.

The MB40.3 monoclonal antibody binds to four distinct HLA-B molecules; B7, B40, B40*, and B27. With Fab' fragments only the interaction with B7 and B40 was detected and the affinity for both was the same (1-2 X 10(8) M-1) suggesting the epitope is shared by the two molecules. Unlike many antibodies for which low affinity is due to a high-dissociation constant, that of MB40.3 results from a very low-association rate constant, coupled with a low-dissociation constant. In consequence, the affinity and avidity of Fab', F(ab')2, and IgG for B7 and B40 were found to be of a similar magnitude, soluble B7 was a more efficient competitor for antibody than cell surface B7 and in practice antibody bivalency was of little importance. The forward rate constant could be increased by removing Fc from the antibody or by removing sialic acid from the cells by treatment with neuraminidase. The neuraminidase treatment also produced an increase in the number of detectable cell surface HLA-A,B molecules. The affinity of MB40.3 for B40* and B27 was estimated to be less than 4 X 10(6) as no binding with Fab' was detected due to a high-dissociation rate. For these two HLA-B molecules bivalent attachment was critical, and it increased the strength of interaction with cell surface B40* and B27 to a point where the avidities were comparable to those obtained with B7 and B40, with B40* interacting more strongly than B27. The epitopes recognized by MB40.3 on B40* and B27 were thus shown to be structurally different from each other and from those on B7 and B40. The properties of this antibody contrast with those of other anti-HLA-A,B we have studied (Ways, J.P., and Parham, P. (1983) Biochem. J. 216, 423-432).