Th9 and IL9 in Chronic Superior Airway Inflammation: A Narrative Review

Inflammation at the superior airway level has multiple manifestations, and allergic rhinitis and chronic rhinosinusitis with or without polyps are two of the most frequent and troublesome of them, with innate and adaptive immunity being implicated. Dendritic cells, epithelial cells, neutrophils, macrophages, mucosal mast cells, eosinophils, basophils, innate lymphoid cells (ILCs), and NK cells are the players in innate immunity, while regulatory T (Treg), TH1, TH2, TH17, T follicular helper, and B cells are components of the adaptative immune system. Th9 cells, a subset of T helper cells discovered in 2008 that produce interleukin-9 (IL-9), play a vital role in the adaptive immune response and have advantageous and harmful effects in different diseases due to the induction pattern. We queried international databases for current, up-to-date information regarding the interplay between interleukin 9 (IL-9) and helper T cells (especially Th9 cells), and by other immune cells. Interleukin-9 has multiple immunological functions, acting on various target cells through its specific receptor (IL-9R), such as the following: the regulation of allergic (Th2-type) immune responses; effects on epithelial and mucosal cells, mast cells, and eosinophils; chronic inflammation; and autoimmunity. Thus, there is a further need to translate laboratory findings into clinical practice regarding IL-9.