Some strains of lactobacilli can stimulate macrophages and dendritic cells to secrete IL-12, which plays a key role in activating innate immunity. We examined the IL-12-inducing ability of 47 Lactobacillus strains belonging to 10 species in mouse peritoneal macrophages, and characterized the properties important for the induction of IL-12. Although considerable differences in IL-12-inducing ability were observed among the strains tested, almost all strains belonging to the Lactobacillus casei group (L. casei, Lactobacillus rhamnosus, and Lactobacillus zeae) or to Lactobacillus fermentum induced high levels of IL-12. Phagocytosis of lactobacilli was necessary for IL-12 induction, and the strains with strong IL-12 induction were relatively resistant to lysis in the macrophages. The sensitivity of Lactobacillus strains to in vitro treatment with M-1 enzyme, a member of the N-acetylmuramidases, was negatively correlated with IL-12-inducing ability. Using a probiotic strain, L. casei strain Shirota (LcS), we showed that the cell wall of LcS could be digested by long-term treatment with a high dose of M-1 enzyme and that the IL-12-inducing ability was diminished according to the duration of the enzyme treatment. The soluble polysaccharide-peptidoglycan complex released from the cell wall of LcS did not induce IL-12, whereas the insoluble intact cell wall of LcS induced IL-12. These results suggest that the intact cell wall structure of lactobacilli is an important element in the ability to induce IL-12 and that Lactobacillus strains having a rigid cell wall resistant to intracellular digestion effectively stimulate macrophages to induce IL-12.

Material

We examined the effect of a probiotic strain, Lactobacillus casei strain Shirota, on cytokine production and natural killer (NK) cell activity in human peripheral blood mononuclear cells (

Summary In order to clarify the probiotic features of immunomodulation, cytokine production by murine spleen and Peyer’s patch (PP) cells was examined in response to probiotic and pathogenic bacteria. In spleen cells, probiotic Lactobacillus casei induced interleukin (IL)‐12 production by CD11b+ cells more strongly than pathogenic Gram‐positive and Gram‐negative bacteria and effectively promoted the development of T helper (Th) type 1 cells followed by high levels of secretion of interferon (IFN)‐γ. Although the levels of IL‐12 secreted by PP cells in response to L. casei were lower in comparison with spleen cells, Th1 cells developed as a result of this low‐level induction of IL‐12. However, IFN‐γ secretion by the L. casei‐induced Th1 cells stimulated with a specific antigen was down‐regulated in PP cells. Development of IL‐17‐producing Th17 cells was efficiently induced in PP cells by antigen stimulation. Lactobacillus casei slightly, but significantly, inhibited the antigen‐induced secretion of IL‐17 without a decrease in the proportion of Th17 cells. No bacteria tested induced the development of IL‐10‐producing, transforming growth factor‐β‐producing or Foxp3‐expressing regulatory T cells, thus suggesting that certain probiotics might regulate proinflammatory responses through as yet unidentified mechanisms in PP cells. These data show probiotic L. casei to have considerable potential to induce IL‐12 production and promote Th1 cell development, but the secretion of proinflammatory cytokines such as IL‐12 and IL‐17 may be well controlled in PP cells.

Summary We previously showed that Lactobacillus strains having a rigid cell wall resistant to intracellular digestion can stimulate macrophages to induce large a quantity of interleukin‐12 (IL‐12). In this study, we examined the influence of lactobacilli and bacterial cell wall components on IL‐12 production by macrophages that was induced by Lactobacillus casei, which has a rigid cell wall. Easily digestible lactobacilli such as Lactobacillus johnsonii and Lactobacillus plantarum or their intact cell walls (ICWs) weakly or very weakly induced IL‐12 production by macrophages, and inhibitedL. casei‐induced IL‐12 production. While the ICW of L. casei was resistant to intracellular digestion and did not inhibit L. casei‐induced IL‐12 production, its polysaccharide‐depleted ICW, i.e. intact peptidoglycan, was sensitive to intracellular digestion and inhibited L. casei‐induced IL‐12 production. Furthermore, the peptidoglycans of L. johnsonii, L. plantarum and Staphylococcus aureus also inhibited L. casei‐induced IL‐12 production. Peptidoglycans from lactobacilli suppressed L. casei‐induced expression of IL‐12p40 but not IL‐12p35 mRNA. Inhibition of IL‐12 production by peptidoglycan was mitigated in Toll‐like receptor 2 (TLR2)‐deficient macrophages compared with the inhibition in wild‐type macrophages. A derivative of the minimal structural unit of peptidoglycan (6‐O‐stearoyl‐muramyl dipeptide) recognized by nucleotide‐binding oligomerization domain 2 (NOD2) could also suppress L. casei‐induced IL‐12 production. These findings demonstrate that easily digestible bacteria and peptidoglycan suppress IL‐12 production through pattern recognition receptors such as TLR2 and NOD2. IL‐12 production in the gut may be negatively regulated by the simultaneous inhibitory actions of various resident bacteria that are susceptible to intracellular digestion.