Functional and molecular analysis of host interactions with commensal Segmented Filamentous Bacterium, a potent physiological driver of gut immune responses – SFBIMPRO

Molecular mechanisms involved in the immunostimulatory properties of SFB are studied in germ-free mice, immunodeficient or not, monocolonized with SFB. As SFB is yet nonculturable, genomic approaches are used to generate clones which activity will be screened on reporter epithelial cell lines in high throughput functional assays designed to monitor the activation of key transcription pathways involved in the host-microbiota cross-talk.

Our results suggest that the immunostimulatory properties of SFB may result from its unique capacity to combine multiple pathways to stimulate host adaptive immune responses. Mechanisms involved are not yet elucidated.

Mecanistic studies in mice of SFB behaviour should not only provide us with a precise insight into the trade-off set up with the immune system to maintain intestinal homeostasis, but also to serve as a basis for the analysis of the impact of these bacteria in human, where they were very recently identified.

Right now, the ongoing project was only presented in oral communications.

Our body surfaces are home to trillions of bacteria, 90% of which inhabit the distal parts of the gastrointestinal tract. The intestinal microbiome contains ~100 times as many genes as the host genome. Intestinal bacteria thereby provide their hosts with an extended repertoire of enzymatic activities, a notion which has led to the concept of host-microbiota “superorganism”. The intestinal microbiota is nevertheless an ominous threat for the hosts, who, during millions years of co-evolution, have deployed complex immune mechanisms that monitor and control this microbial ecosystem. In mammals, mutualistic interactions with the microbiota are maintained by a complex network of local innate and adaptive immune mechanisms. Our recent work suggests that one unculturable Clostridium-related bacterium, the Segmented Filamentous Bacterium (SFB), orchestrates the post-natal maturation of these intestinal immune mechanisms. The overall goal of our project is thus to fully delineate the impact of SFB on the host immune system and to identify the molecular mechanisms of the dialogue between the bacterium and the host immune system.

A first objective will be to determine whether SFB influences the immune status of the host beyond the gut and modulate systemic immune responses. The impact of the microbiota on the maturation of the intestinal immune system is now well established. Yet, mounting evidence indicates that the gut microbiota may also tune host peripheral immune responses and even influence the onset of allergies and autoimmune disorders observed in the developed countries. A first demonstration of this hypothesis has been recently obtained in the non-obese diabetic (NOD) mouse model, which was protected against the development of autoimmune type 1 diabetes by the gut microbiota. One important step is now to determine which bacterial groups or specific members of the normal microbiota can modulate the immune balance and influence the general status of the host. Given its strong impact on host responses in the gut, SFB is an attractive candidate. Using gnotobiotic mice, we will determine the impact of SFB a) on the development of host peripheral immune responses and, b) on the development of autoimmune diabetes in NOD mice.

The second objective of our project is to determine the molecular mechanisms underlying the potent immunostimulatory properties of SFB. One striking feature of SFB is its host-specific attachment to the ileal mucosa and more particularly to Peyer’s patches, which play a key role in the initiation of gut immune responses. The capacity of SFB to stimulate gut immune responses will be compared in mice with and without Peyer’s patches. To identify the receptors which enable attachment of SFB to host epithelium and/or the soluble mediator(s) which may stimulate host immune cells, we will a) attempt to develop in vitro culture methods for SFB and, b) develop genomic approaches using DNA from SFB maintained as single species in gnotobiotic mice. SFB genome will be characterized through sequencing and annotation, and putative candidate genes will be cloned into a culturable bacterial vector. In parallel, a library of large insert clones will be developed in E. coli. Clones will be then screened on mouse reporter epithelial cell lines in high throughput functional assays designed to monitor the activation of key transcription pathways involved in the host-microbiota cross-talk.

Altogether this project should provide new insight on how the microbiota influences host immune status and help the design of strategies and/or tools aiming to preserve/restore the putative beneficial effect of the microbiota in hosts with impaired gut immune defence. Identification of the immunostimulatory constituants of SFB may help to identify new vaccine adjuvants.