Interleukin-15, a leukocyte activator and growth factor, is produced by cortical tubular epithelial cells

Many renal diseases, including transplant rejection, are mediated by mononuclear cells. Interleukin-15 (IL-15) has been recently described as a cytokine with IL-2-like activity. IL-15 is an effective leukocyte growth factor, activator, and chemoattractant. In rejected human kidney allografts, elevated IL-15, but not IL-2, mRNA is expressed, suggesting a role for IL-15 in the rejection process. The aim of this study was to investigate whether human cortical tubular epithelial cells (HTC) are able to produce IL- 15 and whether IL-15 expression is regulated by inflammatory mediators. HTC were isolated and characterized, and IL-15 expression was analyzed by reverse transcription-PCR, enzyme-linked immunosorbent assay, and bioactivity. It was found that HTC constitutively express IL-15. Upon stimulation of HTC with interferon-γ (IFNγ), the levels of both mRNA and protein increased up to twofold. In contrast, lipopolysaccharide, IL-1, IL-2, and tumor necrosis factor-α had no detectable effect. IFNγ action on HTC was dose-dependent from concentrations of 5 U/ml, reaching a plateau at 50 U/ml. HTC supernatants induced proliferation of the T cell line CTLD, which could be partially blocked (50%) by specific IL-15 antibodies. This study shows that IL-15 is secreted by HTC and that the Th₁-cytokine IFNγ upregulates IL-15 expression. This suggests that HTC play a role in cell-mediated renal diseases by releasing IL-15.