Characterization of the Effects of Endotoxin on Macrophage Tumor Cell Killing

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Abstract
These studies demonstrate the potent effect of LPS on the induction of macrophage-mediated tumor cell killing and show, by using pure populations of cloned macrophages as effector cells, that LPS can act in the complete absence of contaminating lymphocytes. Macrophage differentiation toward the tumoricidal state parallels the responsiveness of macrophages to LPS. Normal macrophages will not kill even in the presence of 10 µg/ml LPS; peptone-induced normal macrophages (stimulated macrophages) are made tumoricidal by ≥500 mg/ml LPS; and nontumoricidal BCG-activated macrophages are made to kill tumor cells by 0.5 to 1.0 ng/ml LPS. The LPS effect is reproduced by lipid A. The effect is abolished by alkaline treatment of LPS or by the presence of polymyxin B. Nontumoricidal BCG-activated macrophages from the lipid A-nonresponder mouse strain C3H/HeJ require 250 times more LPS to become tumoricidal than do nontumoricidal BCG-activated macrophages from the C3H/HeN strain. LPS acts synergistically with MAF-rich peritoneal cell supernatants in causing macrophages to kill tumor cells. Although LPS can elicit MAF production by lymphocytes, it can also act directly on cells of lymphocyte-free macrophage colonies to render them tumoricidal. Reagents are frequently contaminated with LPS, and unless this is noted, valid interpretation of experiments investigating macrophage tumor cell killing is difficult. Means of detecting, detoxifying, and eliminating the LPS of experimental agents are discussed.