Interleukin 9 (IL-9) is an extensively glycosylated protein of 14 kDa containing ten cysteine residues involved in disulfide bonding.The protein encoded by this gene is a cytokine produced by T-cells and specifically by CD4 helper cells that acts as a regulator of a variety of hematopoietic cells. This cytokine stimulates cell proliferation and prevents apoptosis. It functions through the interleukin-9 receptor (IL-9R), which activates different signal transducer and activator (STAT) proteins and thus connects this cytokine to various biological processes. The gene encoding this cytokine has been identified as a candidate gene for asthma. Genetic studies on a mouse model of asthma demonstrated that this cytokine is a determining factor in the pathogenesis of bronchial hyperresponsiveness.
The existence of an IL-9-mediated autocrine loop has been suggested for some malignancies such as Hodgkin’s disease and large cell anaplastic lymphoma for Hodgkin’s cell lines. IL-9 is expressed by Reed-Sternberg cells and Hodgkin lymphoma cells and some large aplastic lymphoma cells, while non-Hodgkin lymphomas and peripheral T-cell lymphomas do not express it.