Immune response of non-pathogenic Gram(+) and Gram(−) bacteria in inductive sites of the intestinal mucosa

The gut associated lymphoid tissue (GALT) is anatomical and functionally divided in inductive and effectors sites. In previous works we demonstrated that non-pathogenic bacteria with probiotic characteristics can improve the gut mucosal immune system, with an increase in the number of IgA and cytokines producing cells in the effector site of the intestine. In the present work we studied the effect of non-pathogenic Gram(+), Gram(−) bacteria and a Gram(+) probiotic strain on the inductor site (PP) after the oral administration to BALB/c mice. We also studied some signals induced by the assayed strain in the effectors site, such as the enzyme calcineurin and TLR-9 as a way to understand the mechanisms induced in such bacterial stimulation. The implicance of the lipoteichoic acid (LTA) in the immunostimulation was analyzed. All strains increased the number of IFN-γ and TNF-α(+) cells, but not of IL-10(+) cells in the total population of PP. The release of IFN-γ and TNF-α was only induced by LPS stimulation. All assayed strains increased the number of calcineurin(+) cells, while only Gram(+) strains increased the number of TLR-9(+) cells. The immunostimulatory properties of the purified LTA from Gram(+) strains was evaluated on a monocyte-macrophage U937 cell line. These cells showed capacity to release TNF-α and IL-10 in response to all LTA assayed in a dose-dependent way. Gram(+) strains induced signals through the calcineurin enzyme able to activate the transcriptional factor NFAT and through TLR-9. The LTA molecule from Gram(+) strains would not be the only structure involved in the immunostimulatory properties observed, specially for the probiotic strain.