Bispecific antibody-targeted phagocytosis of HER-2/neu expressing tumor cells by myeloid cells activated in vivo

Studies from our laboratory and others have established that both mononucle ar phagocytes and neutrophils mediate very efficient cytotoxicity when targ eted through Fc receptors using a suitable monoclonal or bispecific antibod y (BsAb). Cross-linking an Fc receptor for IgG (Fc gammaR) triggers multipl e anti-tumor activities including superoxide generation, cytokine and enzym e release, phagocytosis and antibody-dependent cellular cytotoxicity (ADCC) . In this report, using unfractionated leukocytes and two color flow cytome tric analysis, we describe the phagocytic capacity of peripheral blood poly morphonuclear cells (PMN) and monocytes isolated from patients enrolled in a phase I clinical trial of MDX-H210 given in combination with IFN gamma. M DX-H210 is a BsAb targeting the myeloid trigger molecule Fc gamma RI and th e HER-2/neu proto-oncogene product overexpressed on a variety of adenocarci nomas. In this trial, cohorts of patients received escalating doses of MDX- H210 3 times per week for 3 weeks. Interferon-gamma (IFN gamma) was given 2 4 h prior to each BsAb infusion. Our results demonstrate that monocytes fro m these patients were inherently capable of phagocytosing the HER-2/neu pos itive SK-BR-3 cell Line and that addition of MDX-H210 into the assay signif icantly enhanced the number of targets phagocytosed. Two days after adminis tration of an immunologically active dose of MDX-H210 (10 mg/m(2)), monocyt es from these patients were able to phagocytose greater amounts of target c ell material, indicating that these cells remained armed with functionally sufficient BsAb for at least 48 h. PMN from these patients very efficiently mediated phagocytosis through Fc gamma RI after being treated with IFN gam ma, but not before. We conclude that phagocytosis is not only an efficient mechanism of myeloid cell-mediated cytotoxicity, but may also be a mechanis m by which antigens from phagocytosed cells can enter a professional antige n presenting cell for processing and presentation. (C) 2001 Elsevier Scienc e B.V. All rights reserved.