Hyperacute rejection in the xenogenic transplanted rat liver is triggered by the complement system only in the presence of leukocytes and free radical species.

BACKGROUND

Reactive oxygen species (ROS) and nitric oxide species (NOS) are pivotal after ischemia-reperfusion. However, the role of different cells on the formation of free radical species after xenotransplantation remains elusive. We hypothesized that ROS and NOS formed during hyperacute rejection are dependent on leukocytes, erythrocytes, activated thrombocytes, and Kupffer cells (KCs). To address this issue, we developed a model of xenoperfused rat liver and assessed the relationship between free radical production and graft dysfunction.

METHODS

Livers from Sprague-Dawley rats were isolated, flushed with cold Ringer solution, and perfused at physically flow rates for 120 min after 1 h of ischemia. The control group was perfused with rat whole blood (n = 9). In the study groups, the livers were perfused with human whole blood, human plasma with erythrocytes, and plasma with erythrocytes and isolated thrombocytes (n = 9/group). In an additional group, gadolinium chloride (GdCl3), a selective Kupffer cell (KC) toxic agent, was applied. Liver damage, hyperacute rejection, and the depletion of KCs were monitored histologically. Liver damage and function were determined by means of liver enzymes, portal pressure, and bile production. Malondialdehyde (MDA), nitric oxide formation, and peroxynitrite concentration, as well as total glutathione (tGSH) level, were measured as indicators for free radical formation and anti-oxidative status.

RESULTS

Significant differences in the MDA, NO, peroxynitrite levels, and GSH levels after reperfusion with various cell populations were observed. Markedly high ROS/RNS production was evident in the KCs and the xenogeneic whole-blood group. The oxidative stress was mainly caused by leukocytes and to lower extent by KCs, but only in combination with leukocytes. Neither erythrocytes, thrombocytes, nor hepatocytes had an effect on the release of ROS and RNS, as we could not observe significant differences in the MDA, peroxynitrite, and NO levels in these groups compared with control. Tissue injury and hyperacute rejection were more evident in the KC and whole-blood livers. No sign of damage was observed for the control, erythrocyte, and thrombocyte group. Removal of leukocytes from the perfusate by filtration had a major protective effect on the liver function and the grade of hyperacute rejection, whereas KC depletion reduced the ROS production, but did not have an impact on the hyperacute rejection and liver damage. In all xenogeneic perfused groups, the activation of the complement was histologically observed by positive C3c and C9b. Neither KC depletion nor the removal of leukocytes or thrombocytes from the perfusate had an effect on the activation of the complement system. Damage of the rat liver by the complement system was only observed in association with leukocytes.

CONCLUSION

Our data revealed that various cell populations contribute to the formation of free radicals in our model. The production of free radicals was mainly linked to leukocytes and to a minor extent to KCs, but only in combination with leukocytes. Free radicals critically contribute to injury, rejection, and dysfunction of the xenotransplanted liver. Furthermore, hyperacute rejection in the xenogeneic perfused liver is triggered by the complement system only in the presence of leukocytes and free radical formation.