Mechanisms of interleukin-10 induction in murine spleen and RAW264 cells by Latilactobacillus curvatus K4G4 isolated from fermented Brassica rapa L

Lactic acid bacteria (LAB) are commonly used in fermented foods, and some LAB modulate the immune response. We aimed to investigate the mechanism by which LAB isolates from fermented Brassica rapa L. induce the production of anti-inflammatory interleukin (IL)-10 by the murine spleen and RAW264 cells. Spleen cells from BALB/c mice or the mouse macrophage cell line RAW264 were cultured with heat-killed LAB isolated from fermented B. rapa L., and the IL-10 level in the supernatant was measured. Latilactobacillus curvatus K4G4 provided the most potent IL-10 induction among 13 isolates. Cell wall components of K4G4 failed to induce IL-10, while treatment of the bacteria with RNase A under a high salt concentration altered K4G4 induction of IL-10 by spleen cells. In general, a low salt concentration diminished the IL-10 induction by all strains, including K4G4. In addition, chloroquine pretreatment and knock down of toll-like receptor 7 through small interfering RNA suppressed K4G4 induction of IL-10 production by RAW264 cells. Our results suggest that single-stranded RNA from K4G4 is involved, via endosomal toll-like receptor 7, in the induction of IL-10 production by macrophages. K4G4 is a promising candidate probiotic strain that modulates the immune response by inducing IL-10 from macrophages.