Analysis of the interaction of monoclonal antibodies with surface IgM on neoplastic B-cells

In vitro studies identified three Burkitts lymphoma cell lines, Ramos, MUTU -I and Daudi, that were growth inhibited by anti-IgM antibody. However, onl y Ramos and MUTU-I were sensitive to monoclonal antibodies (mAb) recognizin g the Fe region of surface IgM (anti-Fc mu). Experiments using anti-fc mu m Ab (single or non-crossblocking pairs), polyclonal anti-mu Ab, and hyper-cr osslinking with a secondary layer of Ab, showed that growth inhibition of B -cell lines was highly dependent on the extent of IgM crosslinking. This wa s confirmed by using Fab', F(ab')(2) and F(ab)(3) derivatives from anti-fc mu mAb, where increasing valency caused corresponding increases in growth a rrest and apoptosis, presumably as a result of more efficient BCR-crosslink ing on the cell surface. The ability of a single mAb to induce growth arres t was highly dependent on epitope specificity, with mAb specific for the Fc region (C mu 2-C mu 4 domains) being much more effective than those recogn izing the Fab region (anti-L chain, anti-id and anti-fd mu, or C mu 1). Onl y when hyper-crosslinked with polyclonal anti-mouse IgG did the latter resu lt in appreciable growth inhibition. Binding studies showed that these diff erences in function were not related to differences in the affinity, but pr obably related to intrinsic crosslinking capacity of mAb.