Commensal-induced regulatory T cells mediate protection against pathogen-stimulated NF-κB activation

Host defence against infection requires a range of innate and adaptive immune responses that may lead to tissue damage. Such immune-mediated pathologies can be controlled with appropriate T regulatory (Treg) activity. The aim of the present study was to determine the influence of gut microbiota composition on Treg cellular activity and NF-kappa B activation associated with infection. Mice consumed the commensal microbe Bifidobacterium infantis 35624 followed by infection with Salmonella typhimurium or injection with LPS. In vivo NF-kappa B activation was quantified using biophotonic imaging. CD4(+)CD25(+)Foxp3(+) T cell phenotypes and cytokine levels were assessed using flow cytometry while CD4(+) T cells were isolated using magnetic beads for adoptive transfer to naive animals. In vivo imaging revealed profound inhibition of infection and LPS induced NF-kappa B activity that preceded a reduction in S. typhimurium numbers and murine sickness behaviour scores in B. infantis-fed mice. In addition, pro-inflammatory cytokine secretion, T cell proliferation, and dendritic cell co-stimulatory molecule expression were significantly reduced. In contrast, CD4(+)CD25(+)Foxp3(+) T cell numbers were significantly increased in the mucosa and spleen of mice fed B. infantis. Adoptive transfer of CD4(+)CD25(+) T cells transferred the NF-kappa B inhibitory activity. Consumption of a single commensal micro-organism drives the generation and function of Treg cells which control excessive NF-kappa B activation in vivo. These cellular interactions provide the basis for a more complete understanding of the commensal-host-pathogen trilogue that contribute to host homeostatic mechanisms underpinning protection against aberrant activation of the innate immune system in response to a translocating pathogen or systemic LPS.