THE MICROBIOME: A CORRELATE OF INFLAMMATION IN CROHN'S DISEASE? The etiopathogenesis of Crohn's Disease-associated SpA remains a puzzle. As with other autoimmune diseases, interplay between genetic factors such as HLA B27 (Human Leukocyte Antigen- B27) and environmental factors likely play a role. The joint symptoms of SpA are not consistently correlated with bowel disease flares.19 Intestinal microbiota plays a critical role in evolution of our entire immune system, since axenic laboratory animals (germfree animals raised in sterile environment) were noted to have partial restoration of T cell population when these animals are colonized with filamentous bacteria. A symbiotic relationship between the main bacterial phyla is necessary for proper functioning of immune system, since notable alterations in the intestinal microbiome (i.e. dysbiosis) have been suggested in various autoimmune diseases. Reduction in taxa-diversity (such as, enterobacteriaceae, Bacteroidales and Clostridiales) and expansion of certain phyla in the intestine have been recently reported in a large cohort of new onset treatment-naïve Crohn's disease (CD) patients.20 In addition, Dr. Longman's lab has shown that the expansion of immunologically relevant Enterobacteriaceae correlates with Crohn's related SpA among a mixed group of patients with Crohn's and ulcerative colitis, (in press). However, while these are exciting data, SpA cases were identified using a non-validated clinical diagnosis, without systematic rheumatology evaluation and no imaging studies. This will be first study evaluating the microbiome in a carefully phenotyped cohort of Crohn's associated SpA, who will also have detailed MRI imaging.