Bispecific-Armed, Interferon γ-Primed Macrophage-Mediated Phagocytosis of Malignant Non-Hodgkin's Lymphoma

To show that macrophages can be effectively targeted against malignant B cells, bispecific antibodies (BsAb) were constructed from two antibodies having specificity for the high-affinity Fc receptor for IgG (FcγRI/CD64) and the B-cell differentiation antigens CD19 and CD37. Using a flow cytometry-based assay and confocal imaging, we show that these constructs mediated significant phagocytosis of B lymphocytes by macrophages that could be enhanced with interferon γ (IFNγ) and IFNγ in combination with macrophage colony-stimulating factor. BsAb-dependent phagocytosis was triggered through FcγRI and could be blocked only by using F(ab')2 fragments from the parent molecule or by cross-linking FcγRI. BsAb-dependent phagocytosis was not blocked by antibodies to the other Fc receptors, FcγRII and FcγRIII. Because these antibody constructs bind to an epitope outside the FcγRI ligand binding site, we show that autologous serum, polyclonal IgG, and monomeric IgG1 did not block BsAb-dependent phagocytosis, whereas autologous serum and the IgG fractions blocked parent molecule monoclonal antibody-dependent phagocytosis due to the avid binding of monomeric IgG to FcγRI. Finally, BsAb-mediated phagocytosis was effective against the malignant B cells of patients with mantle cell lymphoma, prolymphocytic leukemia, and chronic lymphocytic leukemia. Based on these studies, we propose that BsAbs may provide an effective means of immunomodulation for patients with B-cell malignancies.