Donor species complement after liver xenotransplantation. The mechanism of protection from hyperacute rejection

Heat-Inactivation of Fetal and Newborn Sera Did Not Impair the Expansion and Scaffold Engineering Potentials of Fibroblasts

Heat inactivation of bovine sera is routinely performed in cell culture laboratories. Nevertheless, it remains debatable whether it is still necessary due to the improvement of the production process of bovine sera. Do the benefits balance the loss of many proteins, such as hormones and growth factors, that are very useful for cell culture? This is even truer in the case of tissue engineering, the processes of which is often very demanding. This balance is examined here, from nine populations of fibroblasts originating from three different organs, by comparing the capacity of adhesion and proliferation of cells, their metabolism, and the capacity to produce the stroma; their histological appearance, thickness, and mechanical properties were also evaluated. Overall, serum inactivation does not appear to provide a significant benefit.

Pig Liver Xenotransplantation: A Review of Progress Toward the Clinic

Experience with clinical liver xenotransplantation has largely involved the transplantation of livers from nonhuman primates. Experience with pig livers has been scarce. This brief review will be restricted to assessing the potential therapeutic impact of pig liver xenotransplantation in acute liver failure and the remaining barriers that currently do not justify clinical trials. A relatively new surgical technique of heterotopic pig liver xenotransplantation is described that might play a role in bridging a patient with acute liver failure until either the native liver recovers or a suitable liver allograft is obtained. Other topics discussed include the possible mechanisms for the development of the thrombocytopenis that rapidly occurs after pig liver xenotransplantation in a primate, the impact of pig complement on graft injury, the potential infectious risks, and potential physiologic incompatibilities between pig and human. There is cautious optimism that all of these problems can be overcome by judicious genetic manipulation of the pig. If liver graft survival could be achieved in the absence of thrombocytopenia or rejection for a period of even a few days, there may be a role for pig liver transplantation as a bridge to allotransplantation in carefully selected patients.

An in vitro model of pig liver xenotransplantation--pig complement is associated with reduced lysis of wild-type and genetically modified pig cells

BACKGROUND

After pig liver transplantation in humans, the graft will produce pig complement (C). We investigated in vitro the lysis of wild-type (WT), α1,3-galactosyltransferase gene-knockout (GTKO), and CD46 transgenic (CD46) pig peripheral blood mononuclear cells (PBMC) caused by human anti-pig antibodies (Abs) + pig C.

METHODS

Human serum IgM/IgG binding to WT and GTKO PBMC was determined by flow cytometry, and lysis of pig PBMC by a C-dependent cytotoxicity assay using (i) human serum (human Abs + C), (ii) GTKO pig serum (anti-Gal Abs + pig C), (iii) heat-inactivated human serum (human Abs) + rabbit C, or (iv) human Abs + pig C (serum).

RESULTS

Binding of human IgM and IgG to GTKO PBMC was less than to WT PBMC (P < 0.05). In the presence of human Abs, lysis of WT and GTKO PBMC by rabbit C was 87 and 13%, respectively (WT vs. GTKO, P < 0.01), but was only 37 and 0.4% in the presence of pig C (WT vs. GTKO, P < 0.05). Human/rabbit C-induced lysis was greater than pig C-induced lysis for both WT and GTKO PBMC. CD46 pig PBMC reduced rabbit/human C- and pig C-mediated lysis (P < 0.05).

CONCLUSIONS

Pig livers, particularly from GTKO and CD46 pigs, are likely to have an immunologic advantage over other organs after transplantation into humans. In the absence of pig antibodies directed to human tissues, pig complement is unlikely to cause problems after liver xenotransplantation, especially if GTKO/CD46 pigs are used as the source of the livers.

Liver xenografts for the treatment of acute liver failure: clinical and experimental experience and remaining immunologic barriers

A critical element restricting the application of liver transplantation is the shortage of human deceased donor organs. Xenotransplantation using pig organs might be a solution to this shortage. Although the problems that still require resolution include the immunologic barrier, the potential risk of transferring infectious agents with the transplanted organ, and uncertainty about whether the transplanted organ will function satisfactorily in the human environment, recent progress in the genetic manipulation of pigs has led to the prospect that clinical xenografting, at least as a bridge to allotransplantation, may be possible in the foreseeable future. Experience with clinical auxiliary and orthotopic liver xenotransplantation and experimental liver xenotransplantation in nonhuman primate and other large animal models is reviewed, and the remaining immunologic problems are discussed. Evidence suggests that, in patients with hepatic failure, the pig liver may be less susceptible to antibody-mediated injury than other pig organs, such as the heart or kidney. Pig Kupffer cells and other macrophages will recognize and phagocytose primate red blood cells, but this problem should be overcome by pretransplant depletion of macrophages from the organ-source pig. From the evidence currently available, it does not seem unduly optimistic to anticipate that a liver from an alpha1,3-galactosyltransferase gene-knockout pig would survive at least long enough to function as a successful bridge to allotransplantation.

A case of acute humoral rejection in liver transplantation: successful treatment with plasmapheresis and mycophenolate mofetil

We present a case of a 23-year-old female who underwent orthotopic liver transplantation (OLTx) for biliary atresia, 22 years after a failed Kasai operation. Unusually, her postoperative course was complicated by severe acute humoral rejection. In this case report, we discuss her management as well as the role of plasmapheresis in treating allograft dysfunction secondary to acute humoral rejection in liver transplant patients.

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.

Prolongation of liver xenograft survival by adenovirus vector-mediated CTLA-4Ig gene transfer

Cytotoxic T lymphocyte-associated antigen-4/immunoglobulin fusion products (CTLA-4Ig), a structural homologue of CD28, has been shown to inhibit cellular and humoral immune responses. In this study, we investigated the efficacy of an adenovirus vector containing the CTLA-4Ig gene (AdCTLA-4Ig) on recipient survival after hamster-to-rat liver xenografting. AdCTLA-4Ig was administrated intravenously immediately after grafting. Gene expression was achieved a maximum of 7 days after vector injection and continued for more than 4 weeks. The proportion of CD25(+) T-cells in recipient lymph nodes was significantly reduced 7 days after administration of AdCTLA-4Ig, compared to a group given an adenoviral vector containing LacZ gene (AdLacZ) or to an untreated control group. AdCTLA-4Ig markedly reduced CD2(+) T-cell infiltration into the graft and significantly prolonged recipient survival time (9.2+/-4.12 days), compared to the untreated group (5.4+/-0.78 days) (P<0.001) and the AdLacZ-treated group (5.2+/-0.28 days) (P<0.001). These results indicate that a blockade of T-cell co-stimulation by AdCTLA-4Ig inhibited T-cell activation and attenuated CD2(+) T-cell infiltration into the xenograft, resulting in significant prolongation of recipient survival time. Thus, AdCTLA-4Ig therapy may provide a novel approach to immune regulation.

Genetic modification of xenografts

For nearly a century, xenotransplantation has been seen as a potential approach to replacing organs and tissues damaged by disease. Until recently, however, the application of xenotransplantation has seemed only a remote possibility. What has changed this perspective is the advent of genetic engineering of large animals; that is, the ability to add genes to and remove genes from lines of animals that could provide an enduring source of tissues and organs for clinical application. Genetic engineering could address the immunologic, physiologic and infectious barriers to xenotransplantation, and could allow xenotransplantation to provide a source of cells with defined and even controlled expression of exogenous genes. This communication will consider one perspective on the application of genetic engineering in xenotransplantation.

Experimental discordant hepatic xenotransplantation in the recipient with liver failure: implications for clinical bridging trials

BACKGROUND

Clinical xenotransplantation might start with bridge-to-bridge trials. Situations where hyperacute rejection is avoided would provide opportunities for the initiation of bridging trials. Patients with liver failure have a diminished capacity to initiate antibody and complement-induced injury of xenogeneic endothelium. Hyperacute rejection of a liver xenograft manifests as a coagulopathy. We examined the ability of a recipient with liver failure to hyperacutely reject a liver xenograft in the dog-to-pig model in the immediate postoperative period.

STUDY DESIGN

Liver failure in pigs was induced with galactosamine. Canine livers were transplanted into pigs with liver failure and into healthy pigs. The postoperative course was monitored for 1 hour for histologic changes in the xenograft, changes in platelet counts, and whole blood clotting with Sonoclot analysis. In vitro assays with pig serum and canine hepatic sinusoidal endothelial cells were used to assess the effect of liver failure on serum cytotoxicity and xenoreactive antibody levels.

RESULTS

All untreated pig recipients of liver xenografts died from a coagulopathy. Recipients with liver failure manifested no signs of coagulopathy, and had minimal change in platelet counts or Sonoclot (Sienco Inc., Morrison, CO) tracings. Liver xenograft biopsies from recipients with liver failure showed no evidence of the tissue injury that characterized the biopsies of control recipients. Serum from pigs was less cytotoxic to the canine hepatic sinusoidal endothelium after induction of liver failure. The xenoreactive antibody levels and repertoire were similar in the pig serum before and after liver failure was induced. CH50 (total complement) levels were diminished in pigs after the induction of liver failure.

CONCLUSIONS

Liver xenotransplantation used in bridging trials in recipients with liver failure might not face the barrier of hyperacute rejection.

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

We reported previously that no classical features of hyperacute rejection (HAR) could be found in liver grafts in the guinea-pig (GP)-to-rat model and that recipients died shortly after transplantation of non-immunologic causes. Thus, the GP-to-rat model is not suitable for studying the mechanisms of discordant liver xenograft rejection. In the hamster to rat model, long-term survival of a liver graft is possible, but extremely low levels of xenoreactive natural antibodies are present. To mimic a discordant situation with pre-formed IgM and IgG antibodies, we sensitized rats 1 or 5 weeks before grafting. Specific anti-hamster IgM antibodies were found in recipients sensitized at week -1 but not week -5. Anti-hamster IgG was present in all recipients, albeit considerably higher in animals sensitized 5 weeks before grafting. In these two models, we examined the mechanism of HAR of liver grafts and compared this with heart xenografts. Control heart and liver grafts were rejected 4 and 7 days after transplantation respectively. Liver grafts in recipients sensitized at week -5 showed venous congestion and bleeding after reperfusion, indicating HAR, however this was not observed after sensitization at week -1. This surprising finding was confirmed by histology. Massive extravasation, edema, and acute liver cell degradation were noticed in grafts subjected to HAR. Liver grafts of recipients sensitized at week -1 showed only minimal changes. Heart grafts were rejected hyperacutely in both sensitization models. IgG antibodies could be detected on liver grafts in the group sensitized at week -5 but not in the group sensitized at week -1. Minimal IgM depositions were found on liver grafts of animals sensitized 1 week before grafting. Rejected heart grafts from similar sensitization groups showed identical antibody depositions; only IgM depositions were massive. Complement depositions were found in all groups. These results indicate that IgG, but not IgM, mediates HAR in hepatic xenografting. Such a predominance of IgG over IgM does not exist for heart grafts.

Chimerism and xenotransplantation. New concepts

In both transplant and infectious circumstances, the immune response is governed by migration and localization of the antigen. If the antigenic epitopes of transgenic xenografts are sufficiently altered to avoid evoking the destructive force of innate immunity, the mechanisms of engraftment should be the same as those that permit the chimerism-dependent immunologic confrontation and resolution that is the basis of allograft acceptance. In addition to "humanizing" the epitopes, one of the unanswered questions is whether the species restriction of complement described in 1994 by Valdivia and colleagues also necessitates the introduction of human complement regulatory genes in animal donors. Because the liver is the principal or sole source of most complement components, the complement quickly is transformed to that of the donor after hepatic transplantation. Thus, the need for complementary regulatory transgenes may vary according to the kind of xenograft used. Much evidence shows that physiologically important peptides produced by xenografts (e.g., insulin, clotting factors, and enzymes) are incorporated into the metabolic machinery of the recipient body. To the extent that this is not true, xenotransplantation could result in the production of diseases that are analogous to inborn errors of metabolism. In the climate of pessimism that followed the failures of baboon to human liver xenotransplantation in 1992-1993, it seemed inconceivable that the use of even more discordant donors, such as the pig, could ever be seriously entertained; however, this preceded insight into the xenogeneic and allogeneic barriers that has brought transplantation infectious immunity to common ground. With this new insight and the increasing ease of producing transgenic donors, the goal of clinical xenotransplantation may not be so distant.

Comparative histopathology of hepatic allografts and xenografts in the nonhuman primate

Liver transplantation was performed in the following groups: Group 1, baboon-to-baboon allografting (n=8) (control group); Group 2, ABO-compatible vervet monkey-to-baboon xenografting (n=8); Group 3, ABO-incompatible vervet monkey-to-baboon xenografting (n=6); Group 4, pig-to-baboon xenografting (n=2); and Group 5, pig-to-rhesus monkey xenografting (n=6). Immunosuppressive therapy (cyclosporine, cyclophosphamide, and methylprednisolone) was begun 2-7 days before liver transplantation (LTx) and continued indefinitely after LTx. The liver grafts were biopsied pre-LTx and subsequently post-LTx at approximately 1 hr, 2-3 hr, 7-10 days, 20-30 days, 60 days, 120 days, and at euthanasia or spontaneous death. There were 19 successful LTxs with grafts functioning from one hour to 123 days. No pig liver (Groups 4 and 5) survived more than 5.5 hr, as there was an immediate severe vascular response after reperfusion, typical of hyperacute rejection (congestion and hemorrhage). Vascular rejection was not seen in allografts (Group 1), but early mild-to-moderate congestion and neutrophil infiltration were present in concordant xenografts (Groups 2 and 3), which were associated with moderate deposition of immunoglobulin, C3, and fibrinogen. Lymphoid cell infiltration, bile duct damage, and portal vein endothelialitis in the portal zones occurred later in both allografts (Group 1) and concordant xenografts (Groups 2 and 3), developing earlier in the presence of ABO-incompatibility (Group 3). In concordant xenografts it was usually followed by fibrosis.

Will xenotransplantation ever be feasible?

In several recent conferences, the principal questions have been whether xenotransplantation technology should be encouraged and, if so, how it should be regulated. Because the prospect of successful transplantation of animal organs into humans is still remote, the rush to achieve consensus about clinical application would be inexplicable were it not for two ostensibly unrelated issues. The first is the small but undeniable theoretical hazard of causing new human infections with the intermingling of tissues from different species. The second, advanced by animal-rights advocates, concerns the spiritual and ethical relationship of humans to animals.

The future of transplantation: with particular reference to chimerism and xenotransplantation

The assumption for the last third of a century that stem cell-driven hematolymphopoietic chimerism was irrelevant to successful conventional whole organ transplantation has prompted alternative inadequate explanations of organ allograft acceptance. This assumption clouded the biologic meaning of successful organ as well as bone marrow transplantation, and precluded the development of a cardinal principle that accommodated all facets of transplantation. Recognition of this error and the incorporation of the chimerism factor into a two-way paradigm have allowed previous enigmas of organ as well as bone marrow engraftment to be explained. No credible evidence has emerged to interdict this interactive concept. If the two-way paradigm is correct, it will allow the remarkable advances that have been made in basic immunology to be more meaningfully exploited for transplantation, including that of xenografts.

Assessing risk in liver transplantation. Special reference to the significance of a positive cytotoxic crossmatch

OBJECTIVE

The authors determined the impact of a positive cytotoxic crossmatch on the outcome of liver transplantation.

SUMMARY BACKGROUND DATA

Liver allografts rarely undergo hyperacute rejection, but transplants performed across a positive cytotoxic crossmatch tend to follow a different clinical course, with higher intraoperative blood use, postoperative graft dysfunction, and, in some cases, graft loss. How this affects overall graft survival has not been determined.

METHODS

The authors provide a retrospective analysis of 1520 liver transplants performed between November 1989 and December 1993, with a minimum follow-up of 1 year. All cases had a cytotoxic crossmatch using serum pretreated with dithiothreitol.

RESULTS

There were 1390 negative crossmatch and 130 positive crossmatch cases. There was no difference in overall graft survival, although early survival rates were lower in the positive crossmatch group, with the maximum difference at 6 months: 0.76 (95% confidence interval, 0.74-0.78) for a negative crossmatch versus 0.68 (95% confidence interval, 0.61-0.77) for a positive crossmatch. These differences become negligible by the 2-year mark. Using stepwise logistic regression, the authors identified seven variables independently associated with outcome: 1) donor age, 2) donor gender, 3) prior liver transplant, 4) medical urgency status, 5) ischemia time, 6) indication for transplantation, and 7) primary immunosuppressant.

CONCLUSIONS

The cytotoxic crossmatch is not statistically associated with overall graft survival after liver transplantation. However, early failure rates are higher in the positive crossmatch cases, a difference that disappears by the second year.

The biological basis of and strategies for clinical xenotransplantation

Recent discoveries have suggested that the exchange of multiple leukocyte lineages between grafts and host and subsequent long-term chimerism in both is the seminal mechanism of the acceptance of organs transplanted from the same (allografts) or different species (xenografts). This insight suggests new strategies which may allow xenotransplantation, the principal obstacle to which has been humoral rejection. We have defined humoral rejection as a family of complement activation syndromes afflicting allografts and xenografts in which there is a strong (but not invariable) association with performed antigraft antibodies, invariable evidence of complement activation, histopathologic stigmas of vascular endothelial damage, and a concomitant local or systemic coagulopathy. The generic descriptive term hyperacute rejection is a misnomer because a slow-motion version of the same "humoral" process can occur with some allografts and is the rule with the so-called concordant species xenotransplantations. The pathway of experience and discovery leading to this conclusion shows clearly that the distinction frequently made between allograft versus xenograft humoral rejection does not actually exist in principle, but only in details and intensity. Breaking down this barrier to xenotransplantation, whether or not it is associated with antibodies, is unrealistic. However, the possibility of avoiding the barrier has been exposed by showing that animal organs can be humanized, with a mixed donor and recipient cell population similar to the chimerism seen in long surviving allografts or even with complete leukocyte replacement. Pilot experiments in rodents suggest that organs from fully xenogeneic chimeras can be made into xenogeneic targets that are no more provocative of complement activation than allografts when they are transplanted into the donor bone marrow species. Although the validity of this concept of organ xenograft preparation is only at the pilot stage of verification, there is reason to suspect that the complement trigger of humoral rejection can be thereby disarmed. If this can be accomplished, independent evidence suggests that cellular rejection can be controlled with conventional T-cell directed immunosuppression, perhaps even with surprising ease. The potential subtle liability of synthetic products of xenogeneic parenchymal cells is not yet known.

Clinical xenotransplantation

Two baboon liver xenografts transplanted to patients with B virus hepatitis supported life for 70 and 26 days but did not function optimally despite minimum or no histopathologic findings of overt humoral or cellular rejection in serial biopsies. However, there was evidence of complement activation in both cases, which in retrospect was thought to explain the unsatisfactory outcome. Strategies to deal with this problem are discussed.