Inducible nitric oxide synthase inhibitors prolonged the survival of skin xenografts through selective down-regulation of pro-inflammatory cytokine and CC-chemokine expressions

To elucidate the possible immunoregulatory role of nitric oxide (NO) in cellular xenograft rejection we performed rat-to-mouse skin xenotransplantation. The rat skin engrafted mice were treated with the inducible NO synthase (iNOS) inhibitors, aminoguanidine (AMG, 200 mg/kg) and N^G-nitro-L-arginine methyl ester (L-NAME, 60 mg/kg) every other day until rejection. Skin xenograft survival was monitored and immune cell infiltration and intragraft cytokine and chemokine mRNA expressions were analyzed 7 days after grafting. Compared with the control mice, the AMG- and L-NAME treated mice showed delayed xenograft rejection by approximately 3 days (8.9±0.7 days vs. 11.7±1.2 and 12.0±0.9 days, respectively). Infiltrations of CD11b⁺, MOMA-2⁺ cells and neutrophils were significantly reduced in both AMG- and L-NAME treated graft but CD4⁺ and CD8⁺ cells were not. The expression of cytokines such as IL-1β, IL-2, IL-6, IL-12 and IFNγ in AMG- and L-NAME treated grafts were significantly decreased (P<0.01), whereas IL-10, TNF-α and TGF-β1 were unchanged or enhanced. Additionally, the expressions of CC-chemokines, such as RANTES and MIP-1α, were significantly reduced (P<0.01) whereas the expressions of CXC-chemokines, such as IP-10 and MIG, were unchanged. These results imply that prolonged rat-to-mouse skin xenograft survival by iNOS inhibitors may be due to the selective inhibition of pro-inflammatory cytokines and chemokines and suggest the possible regulatory role of NO in cytokine and chemokine expressions during xenotransplant rejection.