IL-9 and Th9 Cells in Tumor Immunity

T cells can be categorized into functionally diverse subpopulations, which include Th1, Th2, Th9, Th17, Th22, and Tfh cells and Foxp3(+) Tregs, based on their role in maintaining normal immune homeostasis and affecting pathological immune-associated diseases. Among these subpopulations, Th9 cells are relatively new, and less is known about their signaling and effects on tumor immunity. Recently, some studies have focused on regulation of the IL-9/IL-9R signaling pathway and Th9 cell differentiation and their roles in tumor environments. Herein, we summarize recent progress in understanding the regulatory signaling of IL-9 and Th9 cells and their critical roles and mechanisms in antitumor immunity.