Augmentation of antimetastatic effect on Lewis lung carcinoma (3LL) in C57BL/6 mice by priming with Lactobacillus casei

The augmentation of the antimetastatic effect of heat-killed cells of Lactobacillus casei YIT9018 (LC 9018) on Lewis lung carcinoma (3LL) in C57BL/6 mice by presensitization (priming) with LC 9018 was examined. Intralesional injection of LC 9018 into 3LL-bearing mice inhibited both the growth of the primary tumors and the formation of lung metastases, and this effect was significantly augmented by subcutaneous injection of LC 9018 before the tumor inoculation. In the LC 9018-primed mice, intraperitoneal administration of LC 9018 into syngeneic hosts after priming induced a high level of interleukin-2 (IL-2) and interferon- (IFN-) in the peritoneal cavity. At this time, T cells of the spleen cells from the LC 9018-primed mice proliferated and produced IL-2 when co-cultured with LC 9018 as antigen in vitro. Also, the phenotype of these T cells was found to be L3T4⁺ and Ly-2.2^– T cells by analysis by flow cytometry. These results suggest that LC 9018-reactive helper T (T_h) cells were induced by the priming and subsequent challenge with LC 9018, and that IL-2 or IFN-, which was produced by the activated LC 9018-reactive T_h cells, augmented a host immune response resulting the antitumor activity.