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.