Th9 lymphocytes: A recent history from IL-9 to its potential role in rheumatic diseases

• Th9 cells are a new Th subset characterized by high expression of IL-9.
• They are involved in immunity against parasites and allergic diseases.
• Increased levels of IL-9 has been demonstrated in patients with rheumatic diseases.
• Their pathogenic role has not yet elucidated.

Various subtypes of effector T cells have been described up to date, and each one has its specific immunological function and a defined cytokine secretion profile. Th9 lymphocytes, recently described, are characterized by a high IL-9 expression. Their differentiation requires the integration of IL-4 and TGF-β signaling pathways and the coordinated participation of multiple transcription factors. Their role has been mainly found in immunity against parasites and in allergic inflammatory processes. Nevertheless, they have been implicated in processes as autoimmunity, cancer and recently in rheumatic diseases. The objective of this review is to describe the discovery of this cellular subtype, its differentiation, expression regulation and its potential role in rheumatic diseases.