Antibody-dependent antitumor cytotoxicity by human monocytes cultured with recombinant macrophage colony-stimulating factor. Induction of efficient antibody-mediated antitumor cytotoxicity not detected by isotope release assays

Macrophage colony-stimulating factor (M-CSF) is known to stimulate proliferation of monocyte/macrophage progenitors and enhance in vitro antitumor cytotoxicity by murine macrophages. In this paper we have shown that recombinant human M-CSF causes human peripheral blood monocytes to differentiate in culture into metabolically active macrophage-like cells. These cells mediate very efficient antibody-dependent cellular cytotoxicity (ADCC) against human melanoma and neuroblastoma cell lines in the presence of two murine IgG₃ mAbs (3F8 and R24). They also mediate antibody-independent cytotoxicity (or cytostasis) to a lesser extent. Human serum had an inconsistent effect on ADCC, but often induced similar high levels of ADCC. Cytotoxicity was measured using a novel ELISA to detect surviving tumor cells after ADCC. Two conventional isotope-release assays (⁵¹Cr and [³H]TdR) underestimated or entirely failed to detect ADCC by M-CSF-activated monocytes. Optimal activation occurred with 100-300 U/ml of M-CSF, and required 9-11 d for completion. Most of the M-CSF cultured monocytes expressed the low-affinity Fc receptor (CD16). ADCC by cells of the monocyte/macrophage lineage using murine IgG₃ mAbs may have significance for the immunotherapy of human malignancies.