Mouse IL-9 was originally identified as a T cell-derived T cell growth factor III/P40 which could support the long term growth of certain mouse T helper clones in the absence of antigen or antigen-presenting cells. IL-9 can also prolong the in vitro survival of other T cell clones as well as potentiate the IL-2 dependent proliferation of mouse fetal thymocytes. However, this cytokine has no growth-stimulating activity on mouse cytolytic T cell clones or fresh T cells. In addition to its activities on T cells, mouse IL-9 also has mast cell enhancing activity (MEA) and can enhance the mIL-3- or mIL-4-dependent proliferation of mouse bone marrow-derived mast cells. Furthermore, IL-9 will synergize with erythropoietin to support erythroid colony formation in vitro.
The gene for mIL-9 has been mapped to mouse chromosome 13. The mouse IL-9 cDNA encodes a 144 amino acid residue precursor protein with an 18 amino acid signal peptide that is cleaved to form the mature cysteine-rich protein with a predicted molecular mass of 14 kDa. Mouse IL-9 contains four potential N-linked glycosylation sites and the native mIL-9 is a highly glycosylated protein.
Human IL-9 was independently cloned as a novel growth factor which is mitogenic for the human megakaryoblastic leukemic cell line, M07e. Human and mouse IL-9 share 56% and 67% homology at the amino acid and nucleotide levels, respectively. Although mouse IL-9 is active on human cells, human IL-9 is not active on mouse cells.
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