Macrophages act as effectors of tissue damage in acute renal allograft rejection

Background. Macrophages constitute 38% to 60% of infiltrating cells during acute renal allograft rejection. Their contribution to tissue damage during acute rejection was examined by depleting macrophages in a rat model. Methods. Lewis rats underwent bilateral nephrectomy and then received a Dark Agouti renal allograft and liposomal-clodronate, control phosphate-buffered saline liposomes, or saline intravenously (n=7 per group) on days 1 and 3 postsurgery. Grafts were harvested on day 5. Results. Liposomal-clodronate treatment resulted in a 70% reduction in blood ED1⁺ monocytes and 60% reduction in intragraft ED1⁺ macrophages (both P < 0.01). Half of all remaining interstitial ED1⁺ cells were undergoing apoptosis (terminal deoxynucleotide transferase-mediated dUTP nick-end labeling ⁺/ED1⁺), and thus functional depletion of more than 75% of macrophages was achieved. Histologic and functional parameters of acute rejection were attenuated: interstitial infiltrate, tubulitis, and glomerulitis (P < 0.01); tubular cell apoptosis (P < 0.001); tubular cell proliferation (P < 0.001); and serum creatinine (P < 0.01). Production of inducible nitric oxide synthase by infiltrating cells and urinary nitric oxide excretion was reduced by 90% (P < 0.001). In contrast, no reduction in the number of other leukocytes was seen (CD3⁺, CD4⁺, CD8 ⁺, and natural killer cells). Activation of lymphocytes (CD25 ⁺) and production of lymphocyte effector molecules (granzyme B) were unaltered. Conclusion. This study demonstrates that macrophages contribute to tissue damage during acute rejection.