Activation of macrophages (Mφ) for host defense against tumor cells follows a sequence of priming events followed by an initiating stimulus that results in production and release of cytotoxic molecules that mediate target cell killing. We have developed a model to study specific macrophage cytotoxicity in vitro utilizing a cultured murine Mφ cell line, J774. Specific cytotoxicity of cultured human gastrointestinal tumor cells is achieved in the presence of murine IgG2a monoclonal antibody (mAb) 17-1-A. The ability of these cells to mediate antibody-dependent cell-mediated cytotoxicity (ADCC) is greatly enhanced following γ-irradiation. ADCC can be demonstrated at mAb 17-1-A concentrations ≥ 1 μg/ml and effector/target cell ratios ≥2. Exposure to doses ≥ 10 Gy of γ-irradiation increases ADCC threefold. Varying the duration form J774 Mφ exposure to γ-irradiation until addition of antibody-coated target cells showed that the primed state for ADCC is stable for at least 8 days but approximately 24 hr is required for complete development of the primed state. mAb-dependent target cell death begins 8 hr after addition of mAb and labeled target cells to primed effector cells and is complete by 24 hr. Incubation of unirradiated J774 Mφ effector cells with recombinant murine interferon-γ (rmIFN-γ) also results in enhanced ADCC, but the extent of target cell killing achieved is less than that following priming by γ-irradiation. Concomitant priming of γ-irradiated J774 Mφ with rmIFN-γ increases the extent of ADCC. Further study of irradiated J774 cells may elucidate the molecular pathways utilized by Mφ for achieving and maintaining the primed state for ADCC. Irradiated J774 cells will also provide a homogenous, stably primed cell type in which to examine the mechanism(s) of cytotoxicity employed by tumoricidal Mφ.
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