Background. Interleukin (IL)-2, IL-4, IL-7, IL-9, and IL-15, all T-cell growth factors (TCGFs), utilize the common IL-2 receptor γc chain as a critical signaling component in their receptor complexes. We have bred IL- 2(-/-) and IL-4(-/-) double knockout (DKO) mice and showed vigorous islet allograft rejection by DKO hosts. The identity of TCGFs that support the IL- 2- and IL-4-independent allograft rejection is unclear. Methods. We analyzed IL-9 gene expression in rejecting islet allografts in wild-type and in DKO mice, as well as in human renal transplant biopsy specimens, by reverse transcriptase polymerase chain reaction and compared the expression of IL-9 with that of other TCGFs. Results. IL-9 gene expression was not detected in rejecting murine islet allografts in either wild-type or DKO recipient mice despite robust expression of other TCGFs, including IL-7 and IL-15. IL-9 transcripts were also not expressed in any of the human renal transplant biopsies obtained 4 to 251 days after transplantation, regardless of the presence or absence of histological evidence of rejection. Despite expression of IL-9 by DKO splenic cells upon in vitro mitogenic stimulation, IL-9 alone was unable to stimulate the proliferation of concanavalin A-activated splenic leukocytes harvested from DKO mice. Conclusion. IL-9 is conspicuously absent despite vigorous expression of IL-2, IL-4, IL-7, and IL-15 genes during acute allograft rejection.
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