Systemic dissemination of commensal bacteria is CX3CR1-dependent and is mediated by CD11c + CD103 + dendritic cells

Bifidobacteria are one of the earliest colonisers of the human gut. Previously, our group has shown that certain surface molecules of bifidobacteria, such as exopolysaccharide (EPS) and Tad pili, are able to modulate colonisation and influence host immunity 1,2. Specifically, surface EPS seems to be important for the immunomodulatory and host protective effects of bifidobacteria 3,4. However, the precise mechanisms through which EPS contributes to these effects remain poorly understood. To investigate the effects of EPS on host-microbe interaction at the interface of the intestinal mucosal immune system we colonised the murine gastro intestinal tract with Bifidobacterium breve UCC2003 wild type strain (B.breve EPS +) and its isogenic mutant lacking EPS (B.breve EPS-). To facilitate tracking of the bacteria, both strains stably expressed the red fluorescent protein mCherry. We observed that B.breve EPS + colonised the gut to a higher level compared to B.breve EPS-, indicating that EPS is important for initial colonisation of the host possibly due to the fact that it provides stress tolerance to low pH and bile acids. Interestingly, we found that, after colonisation and in the absence of overt inflammation, live colonies of B.breve EPS + but not B.breve EPS-could be recovered from several systemic tissues (liver, kidney, spleen, thymus, adipose tissue) in addition to intestinal tissues and faeces. FACS analysis of these tissues from colonised mice, indicates that a specific subset of colonic dendritic cells (DC), the migratory CD11c + CD103 + DC, can carry B.breve EPS + but not B.breve EPS-to the draining mesenteric lymph node (MLN) and also to different systemic tissues. Furthermore, sorting of these specific DC subsets from dissociated systemic tissues of colonised mice, allowed for recovery of live colonies of B.breve EPS +. Considering the role that CX 3 CR1 mononuclear phagocytes play in sampling and delivery of luminal antigen to migratory DC 5,6 , we investigated if CX 3 CR1 + cells might be involved in the systemic spread of B.breve EPS +. Our data demonstrate that CX 3 CR1 mononuclear phagocytes facilitated the colonisation of B. breve in the murine gut and were required for systemic dissemination of the bacteria. In addition, in vitro experiments showed that B.breve EPS + but not B.breve EPS-could persist inside a pure population of sorted bone marrow derived dendritic cells (BMDDC) for 72 hours in the absence of inflammatory cytokine production. These data indicate that bifidobacteria may specifically target CD11c + CD103 + DC to facilitate their systemic dissemination and to mediate their diverse effects on host immunity and physiology. References 1. Bifidobacterial surface-exopolysaccharide facilitates commensal-host interaction through immune modulation and pathogen protection. Functional genome analysis of Bifidobacterium breve UCC2003 reveals type IVb tight adherence (Tad) pili as an essential and conserved host-colonization factor.