The Role of Eosinophils in Non-Parasitic Infections

Eosinophils are a subset of leukocytes the traditionally associated with Th2-related diseases and helminth infections. However, accumulating evidence suggests that eosinophils play a more prominent role in the immune response to bacterial and viral pathogens than previously realized. Specifically, eosinophils possess antimicrobial properties against a broad range of pathogens, and release specific and secondary granules as a result of pathogen recognition. Pathogen recognition is accomplished through expression of Toll-like receptors, as well as other surface and intracellular receptors expressed by the eosinophil. Interestingly, specific killing mechanisms employed by each granule protein differ based on pathogen recognition, but ultimately release of eosinophil granules leads to direct killing of many different pathogens. The precise mechanisms of killing by granule proteins and the circumstances in which specific proteins are secreted are only now being determined. Future efforts to understand these mechanisms may lead toward clinical use of granule proteins as antimicrobial agents in humans, in addition to revealing implications regarding the use of eosinophil-depleting therapies for allergic disorders. This review will summarize the literature to date regarding the role of eosinophils in non-parasitic infections.