Relationship between the in vitro response of dendritic cells to Lactobacillus and prevention of tumorigenesis in the mouse

Background. Some strains of lactobacilli stimulate immune cells, yet little is known about their potency in cancer prevention. We have previously reported that Lactobacillus casei Shirota (LcS) suppresses murine tumorigenesis through immune modulation. In this study, differences were compared among six representative strains of lactobacilli in regard to their ability to stimulate bone marrow cell-derived dendritic cells (BMDCs) in vitro and tumor suppression in vivo. Methods. BM-DCs were cocultured with a Lactobacillus strain in vitro, and the interleukin (IL)-12 released into the culture supernatant was measured by enzyme-linked immunosorbent assay. Tumors were chemically induced by a single subcutaneous injection of 3-methylcholanthrene (MC) in BALB/c mice. The test diets containing Lactobacillus were given from the day of the MC injection, and the tumor incidences were monitored. Peyer's patches were dissected from Lactobacillus-fed mice, and the status of c-Src, a regulator of DCs, in Peyer's patch cells was examined by Western blotting. Results. In the coculture system, L. fermentum FERM P-13857 and LcS potently elicited IL-12 production. LcS but not the other strains of lactobacilli showed tumor suppression. The inactive form of c-Src, phosphorylated at Tyr527, was dominantly detected in Peyer's patches resected from L. fermentum FERM P-13857-fed mice compared with LcS-fed mice. Conclusions. The responses of DCs may be associated with tumor suppression by an ingested Lactobacillus strain.