ACTIVATION OF HUMAN-COMPLEMENT BY MOUSE AND MOUSE-HUMAN CHIMERIC MONOCLONAL-ANTIBODIES

The complement (C)-activating capabilities in human serum of 32 mouse and 10 mouse/human chimeric MoAbs of different isotypes, and their fra gments, were tested in vitro. Activation of C via the classical pathwa y (CP) was performed in 1% factor D-deficient serum in gelatin contain ing Veronal buffer in the presence of calcium and magnesium (GVB++), w hile activation of the alternative pathway of C (AP) was assessed in 1 0% C1q-depleted serum in the presence of 5 mm MgCl2 in GVB++. The C-ac tivating ability of MoAbs was expressed relative to the degree of acti vation of complement by aggregated IgG for the CP and relative to mous e IgG1 for the AP. All of seven mouse IgG2a MoAbs were potent activato rs of the CP. The results of CP activation by IgG1, IgG2b and IgG3 iso types were different for individual MoAbs. Only three (two IgG1 and on e IgG3) of 32 mouse MoAbs were potent activators of the AP. IgG2a and IgG2b were relatively poor AP activators. There were a few MoAbs which activated both the AP and CP. Of 10 chimeric MoAbs, two IgG1, one IgG 2 and one IgG4 were poor or non-activators of the CP. On the other han d, IgG2 and IgG4 were good AP activators. IgG3 was the most potent AP activator. Most of the F(ab')2 fragments were activators of the AP and displayed no activation of the CP. Fc fragments only activated the CP , whereas Fab' did not activate the CP or the AP. These studies sugges t that the route Of complement activation by class and subclass MoAbs can not always be predicted in advance and based only on their subclas s identity.