Human neutrophil interactions of a bispecific monoclonal antibody targeting tumor and human Fc gamma RIII.

2B1 is a bispecific murine monoclonal antibody (bsmAb) targeting the c-erbB-2 and CD16 (Fc gamma RIII) antigens. c-erbB-2 is over-expressed by a variety of adenocarcinomas, and CD16, the low-affinity Fc gamma receptor for aggregated immunoglobulins, is expressed by polymorphonuclear leukocytes (PMN), natural killer (NK) cells and differentiated mononuclear phagocytes. 2B1 potentiates the in vitro lysis of c-erb-2 over-expressing tumors by NK cells and macrophages. In this report, the interactions between 2B1 and PMN were investigated to assess the impact of these associations on in vitro 2B1-promoted tumor cytotoxicity by human NK cells. The peak binding of 2B1 to PMN was observed at a concentration of 10 microgram/ml 2B1. However, 2B1 rapidly dissociated from PMN in vitro at 37 degrees C in non-equilibrium conditions. This dissociation was not caused by CD16 shedding. When PMN were labeled witn 125I-2B1 and incubated at 37 degrees C and the supernatants examined by HPLC analysis, the Fab regions of dissociated 2B1 were not complexed with shed CD16 extracellular domain. While most of the binding of 2B1 PMN was solely attributable to Fab-directed binding to Fc gamma RIII, PMN-associated 2B1 also bound through Fc gamma-domain/Fc gamma RII interactions. 2B1 did not promote in vitro PMN cytotoxicity against c-erbB-2-expressing SK-OV-3 tumor cells. When PMN were coincubated with peripheral blood lymphocytes, SK-OV-3 tumor and 2B1, the concentration of 2B1 required for maximal tumor lysis was lowered. Although PMN may serve as a significant competitive binding pool of systemically administered 2B1 in vivo, the therapeutic potential of the targeted cytotoxicity properties of this bsmAb should not be compromised.