Tumor inhibition in mice by lipopolysaccharide-induced peritoneal cells and an induced soluble factor

Lipopolysaccharide (LPS) was shown to prevent tumor growth in BALB/c mice when administered either prior to or after the inoculation of lethal doses of tumor cells. An attempt to elucidate the mechanism of this phenomenon utilizing in vivo protocols was made by the adoptive transfer of tumor protection with peritoneal cells as well as with cell-free peritoneal fluids obtained from non-tumor-bearing, LPS-stimulated syngeneic mice. The in vivo-activated peritoneal cells from LPS-treated mice were capable of adoptively transferring tumor protection at peritoneal cell to tumor cell ratios ranging from 1,000:1 to 100:1. Experiments were also performed that indicate that: (i) LPS exerts no direct toxic or inhibitory effect on the tumor cells, and (ii) that residual LPS present in cell and fluid preparations was not responsible for such protection.