Activation of hemolytic complement by mouse monoclonal IgG1 antibody: comparison with rabbit IgG.

We investigated the ability of a mouse anti-hapten monoclonal IgG1 antibody (Ab) to bind to cell-bound specific hapten and to fix and activate C1 and thus the lytic sequence of complement (C). In a comparative study with polyclonal rabbit anti-hapten IgG Ab, we found that about 6 times more monoclonal Ab molecules than polyclonal were necessary for the generation of 1 hemolytic site/cell: the data were interpreted to mean that a cluster of four cell-bound monoclonal Ab molecules was necessary to bind C1 and activate C-mediated hemolysis. Experiments performed under conditions of low density of cell-bound hapten and excess of antibody showed that both monoclonal and polyclonal IgG Abs were able to react only with 20-30% of the cell-bound hapten and that both Abs recognized the same hapten specificity. We also found that even though monoclonal IgG1 Ab was able to bind strongly to a protein A-Sepharose column and could be eluted only by a low-pH buffer, the purified Ab, when bound to cell surface hapten, showed a weak ability to react with free protein A.