Background—Inducible NO synthase (NOS2, or iNOS) is upregulated in grafts with transplant arteriosclerosis. However, the functional role of NOS2 in the pathogenesis of transplant arteriosclerosis remains unclear. NOS2 may regulate lesion development by modulating the early alloimmune response and/or late myointimal thickening.

Methods and Results—To determine whether NOS2-mediated pathways protect against or promote transplant arteriosclerosis, we used NOS2-deficient mice as recipients in our vascularized chronic cardiac rejection model. The severity of vascular thickening in 55-day grafts placed into NOS2 −/− recipients (n=13) was compared with that in wild-type recipients (n=15). Computer-assisted analysis of all elastin-stained vessels (n=283) showed significantly increased luminal occlusion (77.1±9.4% versus 40.8±13.6%, P<.0001) and intima/media ratios in allografts from NOS2 −/− recipients (1.9±1.3 versus 0.4±0.3, P=.0002). To elucidate potential mechanisms, we studied NOS2 effects on T-cell differentiation (Th₁/Th₂) and neointimal smooth muscle cell accumulation. Normalized mRNA levels for Th₁- (signal transducer and activator of transcription [STAT] 4, interleukin [IL]-2, interferon-γ) and Th₂- (STAT 6, IL-4, and IL-5) associated factors were comparable in both groups. In contrast, quantitative analysis of the α-actin–positive area showed a significant increase in the contribution of smooth muscle cells within the neointima in allografts from NOS2 −/− recipients (28.2±2.0%) compared with wild-type controls (13.2±2.3%; P<.0001).

Conclusions—NOS2 plays a protective role in the development of transplant arteriosclerosis, suppressing neointimal smooth muscle cell accumulation.