Background. Interleukin 18 (IL-18) is primarily a macrophage-derived, pro-inflammatory cytokine. As macrophages can act as effector cells in acute rejection, we examined the role of IL-18 in a rat model of acute renal allograft rejection.

Methods. Life-sustaining orthotopic DA to Lewis allograft and Lewis-Lewis isograft kidney transplants were performed. In the same model, macrophage-depleted animals, achieved with liposomal-clodronate therapy, were also studied. Macrophage (ED1⁺) accumulation and IL-18 expression was assessed by immunohistochemistry. CD11b⁺ cells (macrophages) were isolated from kidney and spleen by micro beads. Real-time PCR was used to assess IL-18 and INF-γ mRNA expression in tissue and cell isolates.

Results. Allografts, but not isografts, developed severe tubulo-interstitial damage and increased serum creatinine by day 5 (P<0.001). Immunohistochemistry revealed a greater ED1⁺ cell accumulation in day 5 allografts compared with isografts (P<0.001). IL-18 mRNA expression was increased 3-fold in allografts compared to isografts (P<0.001). Accordingly, IL-18 protein was increased in allografts (P<0.001), and was predominantly expressed by ED1+ macrophages. CD11b⁺ macrophages isolated from allografts had a 6-fold upregulation of IL-18 mRNA expression compared to isograft macrophages (P<0.001). Macrophage depletion resulted in a marked attenuation of allograft rejection, ED1⁺ and IL-18⁺ cells were significantly reduced (P<0.05) as was IL-18 mRNA expression (29.28±2.85 vs 62.48±3.05, P<0.001). INF-γ mRNA expression (P<0.01) and iNOS (P<0.001) production were also significantly reduced in the macrophage-depleted animals.

Conclusion. This study demonstrates that IL-18 is significantly increased during acute rejection and is principally produced by intra-graft macrophages. We hypothesize that IL-18 upregulation may be an important macrophage effector mechanism during the acute rejection process.