Mouse monoclonal antibodies direct phagocytosis of tumor cells by human monocytes

Antibody-independent phagocytosis of tumor cells by human monocyte-derived macrophages cultured in recombinant macrophage colony-stimulating factor

Human monocytes exposed in vitro to recombinant macrophage-colony-stimulating factor (rhMCSF) differentiate into monocyte-derived macrophages (MDM), which mediate efficient antibody-dependent cytotoxicity (ADCC) against tumor cells. We and others have shown that this form of ADCC is unusual in that phagocytosis, rather than extracellular lysis, appears to play the major role in target cell killing. In this study, we asked whether the phagocytic form of cytotoxicity seen with ADCC could occur in the absence of an opsonizing antibody. We now report that, whereas cell lines derived from solid tumors are often resistant to antibody-independent cytotoxicity, malignant cells of lymphoid origin appear particularly susceptible to such antibody-independent killing. We found that all of nine lymphocytic leukemia and lymphoma cell lines tested in a total of 35 experiments, plus all four samples of fresh leukemic blasts, were consistently susceptible to antibody-independent MDM cytotoxicity. Antibody-independent cytotoxicity against these cells was efficient (40%-63% killing) at effector: target (E:T) ratios as low as 2:1. Like ADCC, antibody-independent cytotoxicity involved phagocytosis of target cells, as demonstrated by ingestion of fluorescently labeled targets and analysis by flow cytometry. At the time of phagocytosis, the majority of target cells retained membrane integrity, as indicated by the direct transfer of intracellular [51Cr]chromate from radiolabeled targets to phagocytosing MDM, without release of the label into the medium. However, in contrast to ADCC, we found that the degree of antibody-independent cytotoxicity was not a function of the E:T ratio. Instead, a constant proportion of the available target cells were killed regardless of the E:T ratio, suggesting that target cell recognition, rather than effector cell potency, might be the limiting factor in determining cytotoxicity. In additional experiments, we have also identified a second tumor cell type, nueroblastoma, as being susceptible to antibody-independent phagocytosis (all of five cell lines tested, cytotoxicity 40%-93%, E:T = 3:1). Our data thus indicate that the cytotoxicity induced by rhMCSF is not confined to antibody-mediated killing, and that phagocytosis can play a significant role in target cell destruction even in the absence of opsonizing antibody.

Liver macrophage-mediated cytotoxicity toward mastocytoma cells involves phagocytosis of tumor targets

Macrophage-mediated cytotoxicity toward tumor cells usually involves extracellular lysis of the targets. In this study, we report that liver macrophages from rats treated with lipopolysaccharide (5 mg/kg, intravenous) also kill certain tumor cell targets by phagocytosis. Liver macrophages were coincubated with P815 mouse mastocytoma cells for 24 to 72 hr at an effector/target ratio of 10:1. Macrophage phagocytosis was characterized by flow cytometry and by light and electron microscopy. For flow-cytometric studies, P815 cells were prelabeled with the fluorescent dye 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate. We found that coincubation of macrophages with labeled targets resulted in a time-dependent increase in macrophage-associated fluorescence, reaching a maximum at 72 hr. This correlated with light-microscopic observations of increased numbers of tumor cells in the macrophages and enhanced macrophage surface area and density. Electron microscopic studies revealed that the initial event in the phagocytic process involved the capture of P815 cells by the pseudopodia of the macrophages. Target cells were then surrounded by lamellipodia, internalized in phagosomes and destroyed. These data, together with previous studies, provide evidence for multiple mechanisms of cytotoxicity mediated by activated liver macrophages.

Phagocytosis of tumor cells by human monocytes cultured in recombinant macrophage colony-stimulating factor

Macrophages and cultured human monocytes can mediate efficient antibody-dependent cytotoxicity (ADCC) against human tumor cells using monoclonal antibodies (mAbs). The mechanism of this killing is usually assumed to involve secreted factors (reactive oxygen intermediates, tumor necrosis factor, or other cytotoxic factors) leading to target cell lysis. In this study, we present evidence that phagocytosis of intact target cells is the principal mechanism of antitumor cytotoxicity in our in vitro model of ADCC by cultured monocytes. Human monocytes cultured in recombinant human macrophage colony-stimulating factor ingested up to 100% of fluorochrome-labeled melanoma and neuroblastoma target cells, in the presence of an appropriate antitumor mAb. Electron microscopy demonstrated phagocytosis of intact tumor cells by cultured monocytes during ADCC. All of the radionuclide in radiolabeled target cells was taken up by monocytes during phagocytosis. By preventing the release of radioisotope tracers, phagocytosis thus prevents the detection of this very efficient form of cytotoxicity by most conventional assays.

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.