T_H9 cells and interleukin-9 in parasitic infections: Updates, opportunities, and challenges

Over the past decades, Interleukin-9 (IL-9) and T_H9 cells, have gained attention as important regulators of diverse responses throughout the promotion of activation and expansion of diverse cell subtypes. T_H9 cells have been studied in several immunological contexts, including allergic and autoimmune responses, cancer and infectious diseases; being the latest one, specifically those induced by parasites, the main focus of this review. T_H9 cells have risen as critical mediators of the immune response against parasites, including roundworms, flatworms, tapeworms, and hookworms; however, the number of parasitic diseases influenced by this cell subset was still limited in studies published up to 2016. In this review, we aimed to summarize the latest findings regarding the roles of T_H9 cells and IL-9 in parasitic infections, seeking to provide a comprehensive overview of latest knowledge in the field including the models of parasites never studied. We compile recent evidence highlighting both protective and pathogenic roles for IL-9, discuss the challenges to unravel the complexity of T_H9-IL-9 mediated responses during parasitic infections, as well as the opportunities for therapeutic intervention that will help to translate new findings into future clinical applications.