Defining the murine norovirus as a murine commensal

The body`s surfaces are in constant contact with microorganisms, especially the gastrointestinal tract which is heavily colonised by bacteria, fungi and viruses. The microbiota provides essential and important signals to the host for a number of processes like the development of the architecture of the gastrointestinal tract and the associated immune system, it prevents colonization by pathogens and helps utilization of nutrients from consumed food. Increasing knowledge is available on individual components of the microbiota, especially as germfree mice have been established as an invaluable tool to gain deeper insight into the role of the microbiota. These mice are born and housed under sterile conditions and display a plethora of defects regarding their gastrointestinal development and immune system. Re-colonization of germfree mice with a normal microflora may revert many of the observed defects. The influence of individual bacterial species can be evaluated in this reductive system very elegantly. Viruses are part of our metagenome and are present in the gastrointestinal tract, their role however in the gastrointestinal homeostasis or as commensals has not been analysed yet. We propose murine Norovirus (MNV) to be a potential candidate virus which could act as a murine commensal. MNV is abundant in mouse colonies but does not cause inflammation in mice with a competent immune system. Preliminary data suggests that MNV has the potential to restore some defects observed in germfree mice. In this proposal we want to evaluate the degree to which MNV provides beneficial signals to restore a normal homeostasis and which parts of the gastrointestinal tract can be affected. Therefore we are going to characterize the effects MNV induces on the level of tissue development as well as on the T- and B- cell repertoire found in the lamina propria. We are aiming to describe the signaling pathways evoked by MNV by transcriptionally profiling changes in the epithelium and lamina propria. Furthermore, we will try to elucidate the mechanism behind the MNV induced changes with the help of different knock-out mice for signaling pathways or the usage of blocking antibodies. The functionality of our novel described contribution of MNV will be addressed in aim 3 where we will measure the protection against bacterial superinfections. If MNV is able to stimulate a balanced immune response in germfree mice we expect to see a beneficial effect when we infect with a local pathogen, Salmonella enterica typhimurium, or a systemic pathogen, Listeria monocytogenes. Using these different approaches we are going to define MNV as a murine commensal and we will be able to attribute distinct functions to this enteric virus. This will be the first report to characterize a murine virus as murine commensal.

Our body is constantly colonized with an enormous amount of commensals, which is the collection of all microorganisms in and on our body's surfaces. The gastrointestinal tract shows the greatest amount of these microorganisms. Commensals do not pose any harm or threat to the body, in the contrary they are important and help us digest food, resist infections and train our defense system, the immune system. Many different bacteria have been characterized and studied for their role as commensals, however, no one has ever studied if also viruses can act as commensals in the gastrointestinal tract. We studied a small virus which needs host cells in the gastrointestinal tract to replicate and establish an infection, the so called murine norovirus, MNV. Characteristics of MNV which have been reported previously pointed us to the direction that this virus might fulfill many of the characteristics of bacterial commensals and this we set out to test in different animal models. First, we studied its effect on the gastrointestinal defense system and could determine that this small virus indeed stimulates various branches of the immune system without causing harmful inflammation. Subsequently, we systematically analyzed the responses to the virus within the gastrointestinal tissue and compared these responses to those after bacterial colonization. Importantly, we found overlapping but also distinct responses between these two groups of commensals. Finally, we analyzed if these changes initiated by colonization with the virus have indeed a functional relevance and tested animals with the virus alongside uninfected animals in different models of gastrointestinal injury. We could determine that MNV colonization makes animals more resistant to injuries to the intestine and that it enhances the survival rate dramatically. In summary, we could characterize a virus, MNV, as a novel commensal which fulfills beneficial functions in the gastrointestinal tract and enhances the host's fitness. The effects of MNV on the immune system are broad and the responses are partly comparable to what can be seen with bacterial commensals. We hope that this exciting new avenue of commensal research will help foster a better understanding of the complexity of the gastrointestinal tract and its inhabitants.