Comparison of the ability of red cells sensitized with a bispecific anti-D x anti-Fc gamma RIII Fab fragment to activate human K cells and peritoneal macrophages through Fc gamma RIII.

The functional activity of Fc gamma RIII on human K cells from peripheral blood was compared with that of Fc gamma RIII on peritoneal macrophages (PM) separated from the waste material of patients undergoing peritoneal dialysis. Fc gamma R function was assessed in vitro using human monoclonal IgG1 anti-D (AB5) or a bispecific antibody comprising Fab fragments of AB5 chemically linked to Fab fragments of monoclonal anti-Fc gamma RIII, 3G8 (AB5 x 3G8). In antibody-dependent cell-mediated cytotoxicity (ADCC) assays, K cells mediated the lysis of papainized red cells sensitized with the AB5 x 3G8 bispecific antibody but not with AB5. In contrast, red cell lysis by PM was not promoted by AB5 x 3G8 although AB5 was active. However, this lysis, being inhibited by monomeric IgG, was presumably mediated via Fc gamma RI. AB5 x 3G8 also failed to promote the binding and phagocytosis of both papainized and native red cells by PM although 99% of red cells and over 90% of peritoneal cells bound the bispecific antibody. In marked contrast to K cells therefore, Fc gamma RIII on PM was unable to mediate functional interactions with red cells sensitized with anti-D x anti-Fc gamma RIII bispecific antibody.