BackgroundThe integration of host genetics, environmental triggers and the microbiota is a recognised factor in the pathogenesis of barrier function diseases such as IBD. In order to determine how these factors interact to regulate the host immune response and ecological succession of the colon tissue-associated microbiota, we investigated the temporal interaction between the microbiota and the host following disruption of the colonic epithelial barrier.Methodology/Principal FindingsOral administration of DSS was applied as a mechanistic model of environmental damage of the colon and the resulting inflammation characterized for various parameters over time in WT and Nod2 KO mice.ResultsIn WT mice, DSS damage exposed the host to the commensal flora and led to a migration of the tissue-associated bacteria from the epithelium to mucosal and submucosal layers correlating with changes in proinflammatory cytokine profiles and a progressive transition from acute to chronic inflammation of the colon. Tissue-associated bacteria levels peaked at day 21 post-DSS and declined thereafter, correlating with recruitment of innate immune cells and development of the adaptive immune response. Histological parameters, immune cell infiltration and cytokine biomarkers of inflammation were indistinguishable between Nod2 and WT littermates following DSS, however, Nod2 KO mice demonstrated significantly higher tissue-associated bacterial levels in the colon. DSS damage and Nod2 genotype independently regulated the community structure of the colon microbiota.Conclusions/SignificanceThe results of these experiments demonstrate the integration of environmental and genetic factors in the ecological succession of the commensal flora in mammalian tissue. The association of Nod2 genotype (and other host polymorphisms) and environmental factors likely combine to influence the ecological succession of the tissue-associated microflora accounting in part for their association with the pathogenesis of inflammatory bowel diseases.

Adenosine possesses potent anti-inflammatory properties which are partly mediated by G i -coupled adenosine A3 receptors (A3Rs). A3R agonists have shown clinical benefit in a number of inflammatory conditions although some studies in A3R-deficient mice suggest a pro-inflammatory role. We hypothesised that, in addition to cell signalling effects, A3R compounds might inhibit neutrophil chemotaxis by disrupting the purinergic feedback loop controlling leukocyte migration. Human neutrophil activation triggered rapid upregulation of surface A3R expression which was disrupted by pre-treatment with either agonist (Cl-IB-MECA) or antagonist (MRS1220). Both compounds reduced migration velocity and neutrophil transmigration capacity without impacting the response to chemokines per se. Similar effects were observed in murine neutrophils, while cells from A3R-deficient mice displayed a constitutively impaired migratory phenotype indicating compound-induced desensitisation and genetic ablation had the same functional outcome. In a dextran sodium sulphate-induced colitis model, A3R-deficient mice exhibited reduced colon pathology and decreased tissue myeloperoxidase levels at day 8 -consistent with reduced neutrophil recruitment. However, A3R-deficient mice were unable to resolve the dextran sodium sulphate-induced inflammation and had elevated numbers of tissue-associated bacteria by day 21. Our data indicate that A3Rs play a role in neutrophil migration and disrupting this function has the potential to adversely affect innate immune responses.