Poster Session II - A252 THE EFFECT OF <i>LACTICASEIBACILLUS RHAMNOSUS</i> STRAIN C51 IN THE PREVENTION OF COLITIS IN HIGH PROTEOLYTIC IBD MICROBIOTA COLONIZED MICE

Abstract Background Microbial proteolytic imbalance occurs in about 30% of patients with inflammatory bowel disease (IBD), yet drivers and therapies targeting this dysfunction remain unknown. A high salt diet (HSD) is associated with increased colitis risk and higher predicted microbial proteases. Whether HSD-induced high proteolytic activity (PA) increases colitis risk or responds to anti-proteolytic therapy remains unknown. Aims 1. To determine whether HSD-induced high PA (HPA) microbiota worsens colitis severity. 2. To test whether elafin-producing Lacticaseibacillus rhamnosus (LrC51) prevents spontaneous colitis in mice colonized with HPA-IBD microbiota. Methods Adult germ-free (GF) C57BL/6 mice were colonized with low PA-IBD microbiota that induced a switch to HPA after HSD (HSD “responder”) or IBD microbiota that did not respond to HSD (“non-responder”). Mice were fed either a control diet (CD) or an isocaloric HSD supplemented with 4% NaCl (HSD). After three weeks, all mice were fed a CD and subjected to 1.5% dextran sodium sulfate (DSS) for 5 days followed by 2 days of water. Fecal blood was evaluated daily during colitis. Adult GF Il10-/- mice were colonized with HPA-IBD microbiota and fed a CD. After 3 weeks, mice received daily oral gavages with LrC51, wild-type (WT) control (Lcr35), or PBS+15% glycerol for 2 weeks. Cecal PA was measured (azocasein &amp; FITC-elastin assays), and terminal ileum and colon was collected for histological analysis. Results In mice colonized with responder IBD microbiota, a HSD led to more severe fecal blood on average versus both HSD-responder and non-responder colonized mice fed a CD (p = 0.08 and p = 0.02, respectively). In Il10-/- mice colonized with HPA-IBD microbiota, LrC51 reduced microscopic scores in the colon and ileum compared with both WT (p &amp;lt; 0.001) and vehicle control (p &amp;lt; 0.01). LrC51 treated mice had lower overall proteolytic activity in the cecum (p = 0.04) compared to WT controls. Conclusions Mice colonized with HSD-responder microbiota and fed a HSD prior to low-grade DSS develop severe clinical signs of colitis. In the spontaneous Il10-/- model, the anti-protease producing probiotic LrC51, ameliorated colitis in HPA-IBD microbiota colonized mice. This probiotic is biocontained and its WT form is commercially available, which could rapidly translate to application in humans with IBD. Funding Agencies CIHR