Background: Food allergy is caused by production of IgE against dietary antigen induced by T(H2) response. IL-12 inhibits T(H2) responses and strongly suppresses IgE production. We have recently established a murine model for IgE production with a predominant T(H2) response induced by feeding antigen. Objective: We here show a suppressive effect of heat-killed Lactobacillus plantarum L-137, a potent inducer of IL-12, on IgE production against naturally fed antigen in a murine model. Methods: The ability of L. plantarum L-137 to induce IL-12 production was examined in vitro and in vivo. DBA/2 mice were fed a casein diet and injected intraperitoneally with L. plantarum L-137 from the beginning of feeding or 2 weeks later. Recombinant mouse IL-12 was also injected 2 weeks after the start of feeding. Casein- specific IgE and IgG1 in plasma were determined by ELISA. Results: L. plantarum L-137 directly induced IL-12 production by the peritoneal macrophages and also stimulated spleen cells to produce both IL-12 and interferon-γ in vitro. In vivo treatment of L. plantarum L-137 also increased the plasma level of IL-12 in mice. Plasma anti-casein IgG1 and IgE levels were gradually elevated in DBA/2 mice fed a casein diet. Administration of L. plantarum L-137 from the beginning of feeding suppressed the elevation of anti-casein IgE levels, whereas the levels of anti-casein IgG1 were rather augmented by L. plantarum L-137. IL-12 production of the peritoneal macrophages was enhanced, but IL-4 production of concanavalin A (Con A)-stimulated spleen cells was suppressed in the L. plantarum L-137- treated mice compared with control mice fed a casein diet. When L. plantarum L-137 was given from 2 weeks after the start of feeding, anti-casein IgE levels were also significantly suppressed, which was similar to the result found in mice treated with IL-12. Conclusion: Our results suggest that L. plantarum L-137, a potent IL-12 inducer, is useful for prevention and treatment of food allergy.
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