This report illustrates, at a cellular and molecular level, the impact of the commensal microbiota on host immune defense and immune homeostasis. The deliberate consumption of one commensal organism, B. animalis AHC7, resulted in the attenuation of NF-κB activation within mice infected with a pro-inflammatory translocating microbe, S. typhimurium. In addition, B. animalis AHC7 consumption was associated with modulation of cytokine signaling within the mucosa of healthy animals. In vitro, dendritic cells bound B. animalis AHC7, secreted IL-10 and IL-12p70, and dendritic cells stimulated with B. animalis AHC7 enhanced Foxp3 expression in naïve T cells.
Infection by microbes such as Salmonella species results in severe immunopathology characterized by loss of intestinal barrier function, tissue cell death and loss of function, fever and ultimately can lead to death of the host . The pathology is not only a direct effect of the microbe itself but is a consequence of the inflammatory response induced by the presence of the organism. Therefore Salmonella infection of mice is a good model system to examine the regulatory mechanisms which protect against excessive pro-inflammatory responses to a range of pro-inflammatory stimuli and not only Salmonella itself. The resident microbiota can aid in the protection against aberrant inflammatory activity as suggested by studies in germ-free mice where infection with S. typhimurium results in colitis which was not observed in conventionally colonized animals . In addition, specific commensal organisms such as B. animalis AHC7 protect against S. typhimurium infection and disease severity . While B. animalis AHC7 may directly antagonize Salmonella within the gastrointestinal tract, this is unlikely to be the primary mechanism as other commensal organisms which were significantly more effective than B. animalis AHC7 at killing S. typhimurium in vitro, did not protect against infection in vivo. These studies suggest that the biological activity of certain commensal microbes is exerted well beyond a direct influence on the microbiota within the gastrointestinal tract. Therefore, we investigated whether B. animalis AHC7 could exert an effect on the host via regulation of the pro-inflammatory response as a potential mechanism underpinning its protective effect.
S. typhimurium infects the host via intestinal epithelial cells and dendritic cells. In addition, Salmonella infects via M cells which transport the bacterium to underlying Peyer's patches for immunological processing . Infection of the Peyer's patches leads to recruitment of a large number of pro-inflammatory infiltrating leukocytes which further aggravate intestinal inflammation and promote systemic dissemination of the pathogen. B. animalis AHC7 consumption leads to an altered cytokine profile within Peyer's patches which may protect against excessive inflammation. Release of TNF-α and IFN-γ by stimulated Peyer's patch lymphocytes in vitro is significantly reduced. In contrast, release of the Th1 cytokine IL-12 is enhanced with no alteration in IL-6, IL-10 or MCP-1 levels. Interpretation of these results is complex as IFN-γ and IL-12 are both considered to be Th1 cytokines. IL-12p70 is released by cells of the innate immune system, such as dendritic cells, while IFN-γ is secreted by T cells suggesting that B. animalis AHC7 consumption may differentially regulate dendritic cell and T cell cytokine production. In vitro, B. animalis stimulated dendritic cells secrete IL-12 and therefore the increased IL-12 release by Peyer's patch cells may be dendritic cell derived. Regardless of the mechanism, it is clear that in vitro stimulated cytokine responses from Peyer's patch cells are modulated by B. animalis AHC7 consumption and it is likely that these altered responses contribute to the anti-inflammatory effect observed in the Salmonella model. In particular, reduced secretion of TNF-α, which is a key pro-inflammatory cytokine, would reduce the inflammatory burden following S. typhimurium infection.
Innate immune activation to Salmonella is mediated via pattern recognition receptors, such as TLR-5, which rapidly up-regulate NF-κB activity . Both mucosal and systemic NF-κB activation in response to Salmonella infection was noted suggesting that widespread activation of the innate immune system occurs rapidly and this response is modulated by B. animalis AHC7 consumption. The molecular basis for this inhibitory activity is not known and may involve induction of suppressor molecules, induction of regulatory cells (such as Tregs), down-regulation of TLR expression and/or activity and enhancement of the mucosal immunological barrier. Previously we have shown that increased numbers of CD25+Foxp3+ T cells can reduce NF-κB activation in vivo. We did not assess CD25+Foxp3+ T cell polarization in B. animalis AHC7- fed animals but the in vitro co-culture model using B. animalis conditioned dendritic cells clearly demonstrated that this bacterium can induce a dendritic cell response which induces CD25+Foxp3+ T cells. However, it remains to be determined if this mechanism is responsible for the B. animalis AHC7 anti-inflammatory effect in vivo.