Renal IL-18 Production Is Macrophage Independent During Obstructive Injury

Background: Interleukin 18 (IL-18) is a pro-inflammatory cytokine that mediates fibrotic renal injury during obstruction. Macrophages are a well-known source of IL-18; however, renal tubular epithelial cells are also a potential source of this cytokine. We hypothesized that IL-18 is predominantly a renal tubular cell product and is produced during renal obstruction independent of macrophage infiltration. Methods: To study this, male C57BL6 mice were subjected to unilateral ureteral obstruction (UUO) vs. sham operation in the presence or absence of macrophage depletion (liposomal clodronate (1 ml/100 g body weight i.v.)). The animals were sacrificed 1 week after surgery and renal cortical tissue harvested. Tissue levels of active IL-18 (ELISA), IL-18 receptor mRNA expression (real time PCR), and active caspase-1 expression (western blot) were measured. The cellular localization of IL-18 and IL-18R was assessed using dual labeling immunofluorescent staining (IFS). Results: Immunohistochemical staining of renal tissue sections confirmed macrophage depletion by liposomal clodronate. IL-18 production, IL-18R expression, and active caspase 1 expression were elevated in response to renal obstruction and did not decline to a significant degree in the presence of macrophage depletion. Obstruction-induced IL-18 and IL-18R production localized predominantly to tubular epithelial cells (TEC) during obstruction despite macrophage depletion. Conclusion: These results demonstrate that renal tubular epithelial cells are the primary source of IL-18 production during obstructive injury, and that tubular cell production of IL-18 occurs independent of macrophage infiltration.