Naïve CD4+ T cells are pleiotropically divided into various T helper (Th) cell subsets, according to their pivotal roles in the regulation of immune responses. The differentiation of Th9 cells, an
interleukin (IL)-9 producing subset, can be impacted by specific environmental cues, co-stimulation with
transforming growth factor β (TGF-β) and
IL-4, and other regulatory factors. Although
IL-9 has been recognized as a classical Th2-related
cytokine, recent studies have indicated that IL-9-producing cells contribute to a group of autoimmune disorders including
systemic lupus erythematosus (SLE),
multiple sclerosis (MS),
inflammatory bowel diseases (IBD),
rheumatoid arthritis (RA) and
psoriasis. Studies of Th9 cells in
autoimmune diseases, although in their infancy, are expected to be of growing interest in the study of potential mechanisms of
cytokine regulatory pathways and autoimmune pathogenesis. Several in vitro and in vivo pre-clinical trials have been conducted to explore potential therapeutic strategies by targeting the
IL-9 pathway. Specifically, anti-IL-9
monoclonal antibodies (mAbs) and
IL-9 inhibitors may potentially be used for the clinical treatment of allergic
diseases, autoimmune diseases or
cancers. Here, we review recent research on Th9 cells and
IL-9 pertaining to cell differentiation, biological characteristics and pivotal cellular inter-relationships implicated in the development of various diseases.