Eosinophils in infection and intestinal immunity

Purpose of review Inflammatory bowel diseases (IBDs, e.g., Crohn's disease and ulcerative colitis) are thought to be a consequence of an uncontrolled inflammatory response against luminal antigens, including commensal bacteria. The observed link between eosinophil levels and severity and remission rates in IBD has led to speculation that eosinophils may contribute to the antimicrobial inflammatory response in IBD. Recent findings Eosinophils express the necessary cellular machinery (innate immune receptors, proinflammatory cytokines, antibacterial proteins, and DNA traps) to mount an efficient antibacterial response; however, the rapid decline in eosinophil numbers following acute systemic bacterial infection suggests a very limited role for eosinophils in bacterial responses. Summary We describe the clinical evidence of eosinophil involvement in IBD, summarize the in-vitro and in-vivo evidence of eosinophil antibacterial activity and the biology of eosinophils focusing on eosinophil-mediated bactericidal mechanisms and the involvement of eosinophil-derived granule proteins in this response, and conceptualize the contribution of eosinophils to a response against commensal bacteria in IBD.