Augmentation of murine natural killer cell activity by swainsonine, a new antimetastatic immunomodulator

Swainsonine, an indolizidine alkaloid, has been found to inhibit the experimental metastasis of B16-F10 melanoma cells when administered systemically to syngeneic C57BL/6 mice. The inhibition was both potent and dose dependent with greater than or equal to 80% reduction in pulmonary colonization being observed after only 24-h exposure to 3 micrograms/ml of swainsonine in drinking water. In contrast, the inhibitory activity of swainsonine was completely abrogated when assays were performed in mice depleted of their natural killer (NK) cell activity either experimentally (anti-asialo-GM1 antibody- or cyclophosphamide-treated C57BL/6 mice) or as a result of genetic mutation (homozygous C57BL/6bg/bg beige mice). Swainsonine elicited a 32.0% increase in spleen cell number 2 days after administration and induced a concomitant 2- to 3-fold increase in splenic NK cell activity. These results indicate (a) an absolute requirement for a functional NK cell population in order for swainsonine to exert its inhibitory effects on experimental metastasis, and (b) that the antimetastatic activity of swainsonine is mediated primarily through the ability of the drug to augment NK cell reactivity. On the basis of these findings, swainsonine can be classified as a new immunomodulator that has the ability, at least in a prophylactic setting, to block tumor metastasis.