Mechanisms of hyperacute rejection of discordant liver xenograft

Xenogeneic transplantation

This review summarizes the clinical history and rationale for xenotransplantation; recent progress in understanding the physiologic, immunologic, and infectious obstacles to the procedure's success; and some of the strategies being pursued to overcome these obstacles. The problems of xenotransplantation are complex, and a combination of approaches is required. The earliest and most striking immunologic obstacle, that of hyperacute rejection, appears to be the closest to being solved. This phenomenon depends on the binding of natural antibody to the vascular endothelium, fixation of complement by that antibody, and finally, activation of the endothelium and initiation of coagulation. Therefore, these three pathways have been targeted as sites for intervention in the process. The mechanisms responsible for the next immunologic barrier, that of delayed xenograft/acute vascular rejection, remain to be fully elucidated. They probably also involve multiple pathways, including antibody and/or immune cell binding and endothelial cell activation. The final immunologic barrier, that of the cellular immune response, involves mechanisms that are similar to those involved in allograft rejection. However, the strength of the cellular immune response to xenografts is so great that it is unlikely to be controlled by the types of nonspecific immunosuppression used routinely to prevent allograft rejection. For this reason, it may be essential to induce specific immunologic unresponsiveness to at least some of the most antigenic xenogeneic molecules.

Improved technique for infrahepatic vena cava reconstruction in guinea pig-to-rat liver transplantation

Guinea pig-to-rat orthotopic liver transplantation is associated with serious technical problems contributing to impaired graft perfusion and primary graft failure. In order to shorten the procurement procedure and thereby minimize liver damage before flushing, a simplified technique for infrahepatic caval reconstruction was developed. Dissection of the infrahepatic vena cava (IHVC) from adrenal glands and renal and lumbar veins represents the most difficult and time-consuming part of the donor operation, which is often not well tolerated by the animal; we avoided this step by using an isogeneic vena cava interposition graft (VCIG) following in situ perfusion. This graft is connected with the IHVC transsected just below the liver with a cuff technique. Donor operations lasted 15 to 20 minutes with the new technique (n = 7) compared to 52 to 76 minutes with conventional technique (n = 7). Reduced operating time was associated with markedly improved graft perfusion and significantly better graft survival. This modification of the donor procedure for the guinea pig-to-rat liver xenograft using a VCIG significantly reduces operating time and improves reperfusion and recipient survival.

Delayed hyperacute xenograft rejection in porcine to canine fetal liver transplantation

Fetal tissues are generally considered to express weaker antigenic cell-surface molecules than adult tissues. We have reported that transplantation of porcine fetal liver tissue (fragments) is useful for acute and chronic hepatic failure in rats. We further investigated, in the present study, whether transplantation of a porcine fetal liver has the advantage of delayed hyperacute xenograft rejection (HAR) in comparison with that of an adult liver. Porcine fetal liver heterotopically transplanted into dogs was compared. Haematoxylin-eosin (HE) and immunohistochemical studies using IgM, C3, IgG antibodies were performed in serial biopsies of the liver grafts. Lectin binding to target antigen epitopes on pig and dog tissues was studied by flow cytometry. Carbohydrate expression on the liver was also studied by immunohistochemistry. The macroscopic and HE section findings indicate that HAR started 15 min postgraft in fetal and adult liver grafts. Thereafter, vascular changes and parenchymal damage progressed more rapidly in the adult grafts. The final HAR time in adult liver transplantation was determined to be 60 min, while it was determined to be 180 min in fetal liver transplantation. IgM, C3 and IgG were deposited more strongly in the adult grafts than in the fetal grafts up until 60 min after xenografting. Phaseolus vulgaris erythroagglutinin lectin competitively blocked dog sera binding to porcine PBLs. The fetal liver expressed oligosaccharide at a significantly lower level than the adult liver. We conclude that porcine fetal liver xenografts had a significantly delayed HAR.

Role of UW solution and sodium nitroprusside in reperfusion of liver xenografts from guinea-pig to rat

Guinea-pig livers are poorly reperfused when transplanted into rats. We have observed that, in contrast to that of the rat, the guinea-pig intrahepatic portal vein (PV) has a thick layer of smooth muscle. It is possible that, after perfusion of the liver with ice-cold saline, this could go into spasm, resulting in poor reperfusion. To test this hypothesis, guinea-pig livers were perfused with different solutions stored at varying temperatures and transplanted into LEW rats. To prevent xenograft hyperacute rejection, all xenograft recipients were treated with 80 U/kg cobra venom factor (CVF) i.v. on days -1 and 0. In addition to the percentage reperfusion, PV resistance and recipient survival were also monitored. In group I, liver xenografts perfused with ice-cold saline (4 degrees C) reperfused poorly (20-30%), resulting in the development of portal hypertension (16.5 cmH2O vs. 12 cmH2O in naive LEW rats) and shortened mean survival time (11.7 +/- 4.2 h). In contrast, group II livers perfused with saline at room temperature (23 degrees C) underwent homogeneous reperfusion (98-100%) with no increase in portal vein resistance, indicating that low temperature was the main trigger for the spasm of the PV. Moreover, recipient survival in this group was significantly prolonged to a mean of 22 + 2.6 h (P < 0.01). Although UW solution (group III) and the vasodilator sodium nitroprusside (NP) (group IV) when used alone improved the degree of hepatic reperfusion, it was still not optimal. The supplementation, however, of UW solution with NP in group V animals resulted in homogeneous reperfusion (98%) with no portal hypertension and consistent prolonged graft survival of 21.0 +/- 1.7 h. Therefore, this study has determined that the riddle of the abnormal reperfusion of guinea-pig liver xenografts by rat blood is nonimmune mediated and is due to the spasm of the strong smooth muscle in the PV tree produced by cold perfusates.

Prevention of initial perfusion failure during xenogeneic ex vivo liver perfusion by selectin inhibition

BACKGROUND

Endothelial cell activation triggered by xenoreactive antibodies and complement products is the main feature of discordant xenograft rejection. The contribution of early cell-mediated mechanisms to this rejection process is poorly understood, and the function of adhesion molecules in xenogeneic cell interactions in vivo is unclear. The aim of the study was to investigate the role of selectins in mediating cell-dependent initial perfusion failure and functional restrictions in xenoperfused guinea pig (GP) livers.

METHODS

Isolated GP livers were hemoperfused in a flow-constant, recirculating perfusion system via the portal vein. Microhemodynamic parameters such as sinusoidal perfusion rate and leukocyte flux were analyzed using intravital fluorescence microscopy. Hepatic oxygen consumption and bile production, as well as liver enzymes, potassium level, and numbers of white blood cells and platelets in the perfusate, were determined. The GP livers were perfused either with GP blood (control perfusion), with unmodified rat blood (xenoperfusion), or with rat blood treated with the selectin-blocking polysaccharide Fucoidin.

RESULTS

A significant sinusoidal perfusion failure was observed in the xenoperfusion group, which was accompanied by distinct signs of a functional restriction-like reduced oxygen consumption, bile production, and increased perfusion pressure. However, there were significantly fewer impairments in the Fucoidin group. Furthermore, fewer platelets were trapped and a smaller number of stagnant leukocytes were observed in this group.

CONCLUSION

Fucoidin did not suppress complement activation during xenoperfusion. Considering that Fucoidin inhibits the selectin-dependent interactions among white blood cells, platelets, and sulfate-containing proteoglycans on the surface of vascular endothelium, these findings suggest an important role for early cellular interactions in the development of organ failure during xenogeneic rejection.