IgG, but not IgM, mediates hyperacute rejection in hepatic xenografting

Application of modified two-cuff technique and multiglycosides tripterygium wilfordii in hamster-to-rat liver xenotransplant model

AIM: To modify the hamster-to-rat liver xenotransplant technique to prevent postoperative complications, and to study the inhibiting effect of multiglycosides tripterygium wilfordii (T_II) on immune rejection.

METHODS: Female golden hamsters and inbred male Wistar rats were used as donors and recipients, respectively. One hundred and twelve orthotopic liver xenotransplants were performed by Kamada’s cuff technique with modifications. Over 72 hour survival of the animal after operation was considered as a successful operation. When the established surgical model became stable, 30 of the latest 42 cases were divided into untreated control group (n = 15) and T_II group (n = 15) at random. Survival of recipients was observed. Liver specimens were collected at 2 and 72 h from the operated animals and postmortem, respectively, for histological study.

RESULTS: The successfully operative rate of the 30 operations was 80%, and the survival of the control and T_II group was 7.1 ± 0.35 was days and 7.2 ± 0.52 d, respectively (t = 0.087, P = 0.931). The rate of conjunctival hyperemia in control group (100%) differed significantly from that (31%) in T_II group (P = 0.001). Rejection did not occur in both groups within 2 h postoperatively, but became obvious in control group at 72 h after surgery and mild in T_II group. Although rejections were obvious in both groups at death of recipients, it was less severe in T_II group than in control group.

CONCLUSION: This modified Kamada’s technique can be used to establish a stable hamster-to-rat liver xenotransplant model. Monotherapy with multiglycosides tripteryguiumwilfordii (30 mg•kg^-1•l^-1) suppresses the rejection mildly, but fails to prolong survival of recipients.

Prospects for the temporary treatment of acute liver failure

At present, the most successful treatment of acute liver failure is orthotopic liver transplantation, with survival rates ranging from 70% to 85%. However, mortality rates for liver failure remain high because of the shortage of available donor organs. Therefore, there has been renewed interest in temporary treatment methods for patients with acute liver failure to either allow liver regeneration or await liver transplantation. It is thought that the function of the liver can only be replaced with the biological substrate, e.g. liver cells or a whole liver specimen, which requires the availability of liver tissue from xenogeneic or human sources. In this review, existing temporary liver support techniques are summarized and the potential hazards are described. These include the immunological implications of these techniques, e.g. the host versus graft reaction, which may influence the effectivity of the support system, and in the long run may sensitize the patient to subsequent allogeneic transplantation. The graft versus host reaction is also considered. At present, one of the major concerns is the threat of pig-to-human transmission of activated endogenous retrovirus present in the pig genome. An overview is given of literature concerning the transmission of retrovirus particles in vitro and in vivo. Finally, new solutions for the development of ex vivo systems for temporary treatment of patients with acute liver failure are discussed. These include the use of new immortalized human cell lines and human fetal hepatocytes, and the possibility of isolating, expanding and genetically manipulating stem cells in order to have stable differentiated and committed cells.

Detection of xenoantibodies using a simple flow cytometric assay

Higher primates, including humans, have high levels of pre-existing naturally circulating antibodies that predominantly recognize the epitope Gal (1,3-Gal), which is highly expressed on the surface of xenogenic cells. Deposition of these antibodies on the endothelial cell surface of vascularized xenografts leads to an activation of the classical pathway of the complement system, resulting in tissue ischemia and necrosis with rapid demise of the xenograft. This hyperacute rejection (HAR) is always a major barrier in xenograft transplantation and should be minimized by accurately monitoring the naturally occurring antibodies. In the present study, we utilized a simple and rapid flow cytometric (FCM) assay to monitor the presence of these naturally occurring antibodies. We found that the FCM assay is very effective in measuring human antibodies bound to the xenogenic cells, which cause cytotoxicity. This assay could be useful in the pre- and post-xenotransplantation monitoring of xenoantibodies, thus, helping in the development of strategies to block the binding of preformed human antibodies to the xenograft in order to overcome the problem of HAR.

Both IgG and IgM anti-pig antibodies induce complement activation and cytotoxicity

Hyperacute rejection of pig xenografts transplanted in humans is caused by endothelial cell binding of pre-formed xenoreactive antibodies (XAb) and activation of the classical pathway of complement. Human XAb mainly consist of anti-Galalpha1 3Gal antibodies, which occur in IgM, IgG and IgA classes. Whereas IgM anti-Galalpha1 3Gal antibodies have an established role in hyperacute rejection, the potential role of IgG XAb in this process is still controversial. The aim of the present study was to assess the specificity and functional properties of IgG and IgM XAb. Both classes were present in all human plasma samples tested, with a high inter-individual variability. Levels of IgG XAb did not correlate with levels of IgM XAb. Binding to Galalpha1 3Gal is strongly correlated with binding to the pig cell line PK15, both for IgG and for IgM, pointing to Galalpha1 3Gal as the major antigen recognized. Both purified IgM and IgG induced C3 deposition on PK15 cells and complement-dependent cytotoxicity in a dose-dependent way. The combination of IgG and IgM XAb resulted in an additive effect on cytotoxicity. Affinity-purified IgG anti-Galalpha1 3Gal antibodies were 22 times less potent than IgM in induction of cytotoxicity. These results indicate a quantitative, but not a qualitative, difference between IgM and IgG anti-pig antibodies concerning their complement-activating properties. Therefore, both classes of XAb are of importance in the pathogenesis of hyperacute rejection, and the relative importance of each class may differ considerably between individual patients, depending on the ratio of IgG and IgM XAb present in serum.

Preconditioning with the prostacyclin analog epoprostenol and cobra venom factor prevents reperfusion injury and hyperacute rejection in discordant liver xenotransplantation

Liver xenografts transplanted from guinea pig to rat suffer from inadequate organ reperfusion and initial dysfunction, despite sufficient complement depletion using cobra venom factor (CVF). Reperfusion injury is prevented when complement depleted donors are treated with the prostacyclin analog epoprostenol. Histological analysis suggests that epoprostenol preconditioning prevents post-reperfusion spasms of the intrahepatic branches of the portal vein and strongly reduces appearance of hepatocyte apoptosis shortly after transplantation. Cobra-venom-treated rats show breakdown of glucose metabolism and die in acute hypoglycaemia, whereas the additional application of epoprostenol restores gluconeogenesis. Consequently, recipient survival after epoprostenol and CVF treatment is significantly improved compared with animals receiving CVF only (5.1 +/- 2.6 h vs. 17.9 +/- 5.1 h). These data demonstrate that initial dysfunction of discordant liver grafts in the guinea-pig-to-rat species combination, can be overcome by the application of epoprostenol combined with CVF. Using this pharmacologic regimen, the discordant guinea-pig-to-rat model appears useful to study further questions concerning functional and immunological compatibility of a discordant liver xenograft.

Extracorporeal perfusion for the treatment of acute liver failure

OBJECTIVE AND SUMMARY BACKGROUND DATA

Because of the shortage of available donor organs, death rates from liver failure remain high. Therefore, several temporary liver-assisting therapies have been developed. This article reviews various approaches to temporary liver support as well as immunologic and metabolic developments toward a solution for this problem.

METHODS

A literature review was performed using Medline and additional library searches to obtain further references. Only articles with a well-defined aim of study and methodology and a clear description of the outcome of the experiments were included.

CONCLUSIONS

Renewed interest has developed in old and new methods for an extracorporeal approach to the treatment of acute liver failure. Although temporary clinical improvement has been established, further research is needed to achieve a successful long-term clinical outcome. New developments in the field of genetic modification and tissue engineering await clinical application in the near future.

Immunohistologic evaluation of mechanisms mediating hyperacute lung rejection, and the effect of treatment with K76-COOH, FUT-175, and anti-Gal column immunoadsorption

Although most investigators agree that lung dysfunction occurs rapidly in various pig-to-primate hyperacute lung rejection (HALR) models, the basic mechanisms mediating this phenomenon remain in question. Here we describe an immunohistochemical method for assessment of mechanisms driving HALR. Using an established model wherein piglet lungs are perfused ex vivo with human blood, six experimental groups (K76 COOH; FUT-175; K76 with FUT; anti-alpha-Gal column adsorption; column with FUT; and column with K76) and two control groups (unmodified human blood; autologous pig blood) were studied. Each lung was biopsied serially during perfusion, and assessed using an immunohistochemical technique, with vWF staining as an internal control to quantitate binding of human IgM, IgG, C3, C5b-9, properdin, and C1q. The effect of each treatment and subsequent lung perfusion on IgG and IgM anti-alpha-Gal titers(by ELISA) and on pig endothelial cell cytotoxicity were correlated with histologic findings. We found that [1] the classical complement activation pathway was activated, as has been shown for other pig organs in primate or human blood environments [2]; alternative complement pathway activation is also seen, which has not been described for other organs in pig-to-primate models, but only in the context of classical pathway activation; and [3] anti-Gal column absorption, pharmacologic inhibition of complement, or combination therapy each was associated with histologic evidence of partial protection, consistent with what would be predicted for each intervention. Further, immunohistologic differences correlated with physiologic outcomes [8] and with antibody assay results, and revealed that treatments used were incompletely effective. Our data suggest that more complete inhibition of antibody- and complement-driven pathways than was achieved in these experiments will be necessary to prevent the antibody and complement-mediated facets of hyperacute lung rejection. This immunohistologic technique may also help us identify additional pathogenic mechanisms important to eventual clinical application of pig-to-human lung xenografts.