Induction of tumor cytotoxicity in murine bone marrow-derived macrophages by two synthetic lipopeptide analogues.

Lipoprotein from the outer membrane of Escherichia coli and the synthetically prepared lipopeptides Pam3Cys-Ala-Gly and Pam3Cys-Ser-[Lys]4 derived from the N-terminus of lipoprotein constitute potent macrophage and polyclonal B-lymphocyte activators. The compounds have also been shown to induce tumor cytotoxicity in murine bone marrow-derived macrophages (BMDM). Bone marrow stem cells were cultured in the presence of colony-stimulating factor 1 to yield BMDM of 98 to 99% purity at day 8. After stimulation with the lipopeptides on days 4, 6, 8 and 10 of bone marrow culture, the cytotoxic effect of BMDM on the tumor cell line L929 was determined in a [3H]thymidine release assay. Maximum tumor cytotoxicity was found on day 8 with an optimal effector/target-cell ratio of 10:1, and a duration of lipopeptide stimulation of 4 h. The supernatants of lipopeptide stimulated BMDM also showed cytotoxic activity that could be inhibited by antiserum against tumor necrosis factor alpha. The effects of the lipopeptides Pam3Cys-Ala-Gly and Pam3Cys-Ser-[Lys]4 were comparable or superior to those exerted by lipopolysaccharide. Our results demonstrate that synthetic lipopeptides are potent activators for murine BMDM and may therefore prove to be an important tool for the elucidation of the role of macrophages in the host defence mechanisms against tumor cells.