Lactobacillus casei Inhibits Antigen-Induced IgE Secretion through Regulation of Cytokine Production in Murine Splenocyte Cultures

<b>Background:</b><i>Lactobacillus casei</i> is a nonpathogenic gram-positive bacterium widely used in dairy products and has been shown to enhance the cellular immunity of the host. <b>Methods:</b> To examine the inhibitory effect of <i>L. casei</i> on IgE production, splenocytes obtained from ovalbumin (OVA)-primed BALB/c mice were restimulated in vitro with the same antigen in the presence of heat-killed <i>L. casei</i>. The effect of this bacterium on T helper (Th) phenotype development was also examined with naive T cells from OVA-specific T cell receptor-transgenic mice. <b>Results:</b><i>L. casei</i> induced IFN-γ, but inhibited IL-4 and IL-5 secretion, and markedly suppressed total and antigen-specific IgE secretion by OVA-stimulated splenocytes. The inhibitory effect of <i>L. casei</i> on IgE, IL-4, and IL-5 production was partially abrogated by addition of neutralizing antibody to IFN-γ. Augmented IL-12 production was also observed in the cell cultures containing <i>L. casei</i>, and anti-IL-12 monoclonal antibody completely restored the IgE, IL-4, and IL-5 production to the control levels. The IL-12 augmentation by <i>L. casei</i> was macrophage-dependent. The Th cell development assay showed the ability of <i>L. casei</i> to induce Th1 development preferentially. This effect was also completely blocked by anti-IL-12 antibody. <b>Conclusions:</b> This is the first demonstration that a nonpathogenic microorganism, <i>L. casei</i>, can inhibit antigen-induced IgE production through induction of IL-12 secretion by macrophages. The findings suggest a potential use of this organism in preventing IgE-mediated allergy.