Studies on the mechanism of macrophage-mediated tumor cell lysis induced by Mycoplasma orale

Following our previous demonstration that both viable and heat-killed Mycoplasma orale induce selective tumor cell killing by murine peritoneal macrophages, further investigations reported here showed that also macrophages from a continuously proliferating cell line established from long-term cultures of murine bone marrow explants can effectively be induced by the heat-killed mycoplasmas to express cytolysis. The use of single-cell suspensions of M. orale from a 0.45-micron filtrate or following either sonication or treatment with DNase did not significantly affect the level of cytolysis. Minute quantities of M. orale acted synergistically with ineffectively low levels of either lymphokines (LK) or lipopolysaccharide (LPS) to produce killing. The exceptional resistance of M109 lung adenocarcinoma cells to macrophage-mediated killing induced by LK and LPS, as previously reported by us, could not be overcome by the addition of M. orale. These data appear to indicate a mechanism of macrophage activation by M. orale similar to that caused by LPS.