Attenuated acute cardiac rejection in NOS2 -/- recipients correlates with reduced apoptosis

Background-The mechanisms through which NOS2-mediated pathways regulate gra ft failure in acute cardiac rejection are ill defined. To determine whether apoptosis promoted by NOS2 may contribute, we used a heterotopic transplan t model to study mouse cardiac allografts placed in recipients with targete d gene deletion of NOS2. Methods and Results-Using 5 different indexes of apoptosis, we showed that mouse cardiac allografts placed in NOS2 -/- recipients (n=7) had reduced ap optotic activity compared with those in NOS2 +/+ controls (n=8). There were significantly fewer TUNEL-positive nuclei per high-powered field (P<0.01), less DNA fragmentation (antinucleosome ELISA; P<0.05), lower corrected tra nscript levels for caspase-1 and -3 (P-32 reverse transcriptase-polymerase chain reaction; P<0.01), and reduced caspase-3 activity (cleavage of DEVD-p NA [P<0.001] and poly [ADP-ribose] polymerase) in grafts from NOS2 -/- reci pients. This concordant reduction in apoptotic indexes paralleled the impro ved histological outcome of grafts transplanted into NOS2 -/- recipients (a ssessed as rejection scores; P=0.012), To identify pathways controlled by N OS2, we compared intragraft transcript levels of potential triggers and reg ulators. Whereas Fas ligand/Fas and tumor necrosis factor (TNF)-alpha/TNF r eceptor-1 levels were not altered by NOS2 deficiency, transcript levels for p53 were significantly lower in grafts from NOS2 -/- recipients, coincidin g with a significant increase in the antiapoptotic Bcl-2/Bax balance and de crease in Bcl-X-1 levels. Conclusions-Using NOS2 knockout mice, we demonstrated that NOS2-mediated pa thways can promote acute rejection, at least in part, by inducing apoptotic cell death. When NOS2 is present, p53 might control NOS2-mediated apoptosi s by stimulating Bar and repressing Bcl-2 and Bcl-X-1 expression, which may activate the cell death program in the rejecting heart.