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 r eceptor for aggregated immunoglobulins, is expressed by polymorphonucl ear leukocytes (PMN), natural killer (NK) cells and differentiated mon onuclear phagocytes. 2B1 potentiates the in vitro lysis of c-erbB-2 ov er-expressing tumors by NK cells and macrophages. In this report, the interactions between 2B1 and PMN were investigated to assess the impac t 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 conc entration of 10 mu g/ml 2B1. However, 2B1 rapidly dissociated from PMN in vitro at 37 degrees C in non-equilibrium conditions. This dissocia tion was not caused by CD16 shedding. When PMN were labeled with I-125 -2B1 and incubated at 37 degrees C and the supernatants examined by HP LC analysis, the Fab regions of dissociated 2B1 were not complexed wit h shed CD16 extracellular domain. While most of the binding of 2B1 to PMN was solely attributable to Fab-directed binding to Fc gamma RIII, PMN-associated 2B1 also bound through Fc gamma-domain/Fc gamma RIII in teractions. 2B1 did not promote in vitro PMN cytotoxicity against c-er bB-2-expressing SK-OV-3 tumor cells. When PMN were coincubated with pe ripheral blood lymphocytes, SK-OV-3 tumor and 2B1, the concentration o f 2B1 required for maximal tumor lysis was lowered. Although PMN may s erve as a significant competitive binding pool of systemically adminis tered 2B1 in vivo, the therapeutic potential of the targeted cytotoxic ity properties of this bsmAb should not be compromised.