Catestatin regulates gut immune homeostasis and microbial composition Introduction The pro-hormone chromogranin A (CgA) and its bioactive cleavage product catestatin (CST) are both associated with inflammatory bowel disease (IBD) and low diversity of the gut microbiome, but their exact role is unknown. Here, we demonstrate that CST regulates mucosal immunity, barrier function and microbial composition. Method We generated mice with selective deletion of the CST-coding region of the ChgA gene (CST-KO). The microbial composition of cecum was determined and colon tissue was analyzed by immunohistochemistry or electron microscopy from CST-KO mice. Leukocyte recruitment was measured by intravital imaging. CgA and CST levels were measured in blood and stool of IBD patients. Results Blood plasma and feces CgA levels were increased in patients with IBD in flare-ups while CST was higher in remission, suggesting that the contrasting activities of CgA and CST link to IBD activity. Microbial composition was less diverse and showed an increase in firmicutes in CST-KO. Especially the mucus layer was thicker in colon of CST-KO, which was in line with the amount of goblet cells. Both KO mice, showed increased macrophage infiltration, while both CgA and CST promoted leukocyte recruitment. Conclusions CST is a key modulators of intestinal health and disease. Thereby, CgA and CST levels not only seem relevant markers for disease severity of IBD, but might also be targets for therapeutic interventions. Authors & affilitaions: Elke M. Muntjewerff1*, Kechun Tang2*, Lisanne Lutter3,4, Gustaf Christoffersson5,6, Mara J.T. Nicolasen1, Hong Gao10, Gajanan D. Katkar7, Soumita Das8, Martin ter Beest1, Wei Ying10, Pradipta Ghosh7,9,10, Sahar El Aidy11, Bas Oldenburg4, Geert van den Bogaart1,11#, Sushil K. Mahata2,10# 1Department of Tumor Immunology, Radboud Institute for Molecular Life Sciences, Radboud University Medical Center, Nijmegen, the Netherlands; 2VA San Diego Healthcare System, 3350 La Jolla Village Drive, San Diego, CA, USA; 3Center for Translational Immunology, Utrecht University Medical Center, Utrecht, the Netherlands; 4Department of Gastroenterology and Hepatology, Utrecht University Medical Center, Utrecht, the Netherlands; 5Science for Life Laboratory and 6Department of Medical Cell biology, Uppsala University, Uppsala, Sweden; 7Departments of Cellular and Molecular Medicine, 9Pathology, 10Medicine, University of California San Diego, La Jolla, CA, USA; 11Department of Molecular Immunology and Microbiology, Groningen Biomolecular Sciences and Biotechnology Institute, University of Groningen, Groningen, the Netherlands.