P159 PHENOTYPIC CHARACTERIZATION OF INFLAMMATORY BOWEL DISEASE BIOPSIES REVEAL THAT MAST CELLS ARE SIGNIFICANTLY ELEVATED AND ACTIVATED IN PATIENTS WITH ULCERATIVE COLITIS

Abstract Rationale Accumulation and activation of mast cells and eosinophils have been implicated in the pathogenesis of several chronic inflammatory gastrointestinal (GI) diseases, including eosinophilic gastrointestinal diseases (EGIDs) and inflammatory bowel disease (IBD). Despite the strong association of mast cell and eosinophil numbers and activation with the pathogenesis of IBD, no further characterization of these cells has been performed. Current treatment options for IBD include aminosalicylates, antibiotics, immunomodulators, biologic agents and small molecules. These therapies are only moderately effective. A significant proportion of patients fail to respond, do not fully respond, or lose response over time. Therefore, there is significant need for more selective and effective therapy options. Siglec-8 is an inhibitory receptor selectively expressed on human eosinophils and mast cells and represents a novel target for the treatment of IBD with the anti-Siglec-8 mAb, antolimab (AK002). We aimed to quantify and evaluate the activation state of mast cells and eosinophils in colonic tissue from IBD or non-diseased patients. In addition, we quantified the production of TNFa from human colon tissue mast cells and evaluated the inhibitory activity of antolimab (AK002) on these cells. Methods Single-cell suspensions were prepared by enzymatic digestion of fresh colonic biopsies from patients clinically diagnosed with IBD (n=29) or non-diseased control tissues (n=16). Multi-color flow cytometry was performed to identify major immune cell populations and evaluate the activation state of mast cells and eosinophils. Mast cells were FACS-sorted from human colon tissue to evaluate cytokine production and inhibitory activity of antolimab. Results The percentage of mast cells and the expression of the mast cell degranulation marker CD107a were significantly increased in ulcerative colitis (UC) patient biopsy tissue compared to Crohn’s disease (CD) and non-diseased colonic tissue (Figure 1A and B). Furthermore, FACS-sorted mast cells from human colon tissue produced significant quantities of TNFa that was reduced after ex vivo antolimab treatment. Colonic tissue eosinophils were also elevated in a subset of UC and CD patient biopsies, and all UC and CD tissue eosinophils displayed increased expression of the activation marker CD11b compared to control colonic tissue. Figure 1. (A) Percentage of colonic tissue mast cells and (B) mast cell degranulation in patie