Interleukin-9 (IL-9) is a proinflammatory cytokine historically believed to be involved in type 2 immune responses. However, recent evidence suggests IL-9 may be secreted by other T Helper lineages such as Treg and Th17 in addition to a new category called Th9. This Th9 lineage can either be derived from Th4 cells with TGF beta or differentiated directly from naïve CD4+ Tcells with TGF beta and IL-4. The IL-9 expression in subsets such as Treg and Th17 illustrates the plasticity of cells to reprogram to alternative fates.
IL-9 is a member of the common cytokine receptor gamma chain-dependent family of cytokines which also includes IL-2, IL-4, IL-7, IL-15 and IL-21. IL-9 is an extensively glycosylated protein of 14 kDa containing ten cysteine residues involved in disulfide bonding. The human gene maps to chromosome 5q31-32 which is a chromosomal region sometimes deleted in patients with myelodysplastic syndrome. Its pleiotropic effects on Th4 lymphocytes, B lymphocytes, mast cells, eosinophils, IgE production and gut and airway epithelial cells have implicated IL-9 in asthma and other allergy-related disorders. 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 lymphomasdo not express it.