Mechanistic studies of the effects of anti-factor H antibodies on complement-mediated lysis

We have recently reported that complement factor H, a negative regulator of complement-mediated cytotoxicity, is produced and secreted by most bladder cancers. This observation was exploited in the development of the BTA stat (TM) and BTA TRAK(TM) diagnostic assays, both of which make use of two fact or H-specific monoclonal antibodies in sandwich format. Here we show that b oth antibodies exert interesting effects on the biochemistry of complement activation in in vitro systems. Antibody X13.2 competes with C3b for associ ation with factor H and strongly inhibits factor H/factor I-mediated cleava ge of C3b, thereby evidently inactivating a negative regulator of complemen t; yet, the antibody strongly inhibits complement-mediated lysis as well. C onversely, antibody X52.1, which does not compete with C3b and has no effec t on solution-phase cleavage of C3b, is capable of enhancing complement-med iated lysis of various cell types, including cancer cells, by over 10-fold. Our observations indicate that it is possible to deconvolute the biochemic al roles of factor H in complement by means of appropriate inhibitors, a fi nding with potentially valuable implications for both basic research and ca ncer therapy.