HOMOLOGOUS RESTRICTION OF COMPLEMENT-MEDIATED CELL-LYSIS CAN BE MARKEDLY ENHANCED BY BLOCKING DECAY-ACCELERATING FACTOR

Regulation of complement (C') dependent lysis of cells is attributed t o certain membrane proteins. One of these is decay-accelerating factor (DAF), CD55, a 70kD glycosylated protein which functions to protect h ost cells from damage by autologous C'. We hypothesized that blockade of DAF function by a monoclonal antibody (mAb) could augment C'-depend ent lysis mediated by another mAb to a cell surface antigen expressed on leukaemia cells. Thus, we tested the effects of the anti-DAP mAb 1C 6 on the ability of both rabbit and human C' to lyse human leukaemia c ells through activation by complement-fixing murine mAb. DAF antigen w as highly expressed on most leukaemia cell lines and primary acute leu kaemia blast cells tested. Murine mAb to CD15 (PM-81) and to gp 120 (A ML-99), both IgM, also bound to the majority of myeloid and lymphoid l eukaemia cells. Using human serum as a source of C', the addition of m Ab 1C6 reduced by an additional 85-94% the numbers of clonogenic HL-60 (myeloid leukaemia) cells lysed by mAb PM-81 alone. Similarly, the ad dition of mAb 1C6 reduced by an additional 87% the numbers of HL-60 co lonies eliminated by mAb AMG1-99 alone. Similar results were observed in an experimental purging model using the myeloid leukaemia cell line s HL-60, U937 or NB4 cells as targets. These results show that mAb 1C6 can effectively block the actions of DAF. In the presence of mAb 1C6, the cytotoxic activity mediated by human C' was similar to that of ra bbit C'. These results show that increased tumour cell killing can be achieved through DAF blockade. This finding has relevance to clinical trials using C'-fixing mAb for purging bone marrow of occult tumour ce lls prior to autologous transplantation.