Xenotransplantation--state of the art

Organ transplantation is limited by the number of cadaveric human donor organs that become available. Xenotransplantation--the transplantation of organs and tissues between animal species--would supply an unlimited number of organs and offer many other advantages. The pig has been identified as the most suitable donor animal. Pig organs, when transplanted into humans or nonhuman primates, are, however, rejected hyperacutely within minutes by antibody-mediated complement activation. Human anti-pig antibodies have been identified as being directed against Gal alpha1-3galactose epitopes on pig vascular endothelium. Major efforts are being made to overcome this hyperacute rejection. Methods being investigated include (i) depletion or inhibition of recipient antibodies or complement, (ii) development of transgenic pigs that do not express the alphaGal epitope and/or express a human complement inhibiting protein (e.g. DAF), and (iii) development of immunological tolerance to pig organs in the recipient. If complement activation is prevented, e.g. by inhibition of complement activation by cobra venom factor, soluble complement receptor 1 or by the use of hDAF transgenic pig organs, then "delayed xenograft rejection" occurs and is again believed to be largely antibody-dependent. Experimental pig-to-primate organ xenotransplantation is now, however, resulting in transplant function for days and weeks rather than minutes, and there is therefore optimism that we are on the threshold of a new era in the field of the transplantation of vital organs.

A histopathologic grading system of hyperacute (humoral, antibody-mediated) cardiac xenograft and allograft rejection

BACKGROUND

If future attempts to introduce xenografting into clinical practice prove successful, it will be essential to have a clinically relevant, reproducible grading system for vascular rejection. No formal attempt has been made to grade hyperacute or delayed vascular rejection on the basis of a review of both experimental and clinical material.

METHODS AND RESULTS

In an attempt to define a microscopic grading system for hyperacute vascular rejection of the heart, we reviewed the clinical and histologic findings in 112 previously personally studied experimental (n = 109) and clinical (n = 3) cardiac xenografts and allografts, most of which showed vascular rejection. The study material comprised 44 discordant xenografts, 41 concordant xenografts, and 27 allografts. We documented, analyzed, and grouped the histopathologic features together with the clinical data. We devised a grading system which allowed us to allocate each sample to one of the following two categories: grade A: unmodified hyperacute rejection; grade B: mixed hyperacute and acute cellular rejection. Both grades A and B can be subcategorized into three stages: (1) mild (initial), (2) moderate (intermediate), or (3) severe (late) stage.

CONCLUSIONS

A common grading system can be applied to both hyperacute rejection and mixed (hyperacute and acute) rejection. The proposed grading system provides a basis for meaningful pathologic evaluation of hyperacute rejection.

How many endomyocardial biopsies are necessary in the first year after heart transplantation?

Since 1989, the immunosuppressive regimen used in all heart transplant (HTx) patients at our center has consisted of a combination of cyclosporin, azathioprine, and prednisone. No prophylactic cytolytic agents have been given. One hundred consecutive patients were followed for periods of 4-56 months (mean 27 months). The incidence of rejection was so low in the initial 18 patients that we felt confident about reducing the number of routine endomyocardial biopsies (EMBs) that were performed. The mean number of EMBs in this subgroup was 10 (median 11). In the next 20 patients, EMB was performed routinely on only three occasions during the 1st post-transplant year (at 2, 4, and 8 weeks). In the subsequent 62 patients, EMB was performed on post-transplant days 10, 20, 30, and 60. Further EMBs were performed after acute rejection episodes had been treated. No noninvasive methods of diagnosing rejection were employed. In 82 consecutive patients, therefore, the mean number of EMBs within the 1st year was five per patient (median four), with 58% undergoing fewer than five EMBs and 25% requiring more than five EMBs. In the entire group of 100 patients, the mean number of EMBs was 5.9. The incidence of acute rejection requiring increased therapy was 24%. Only 7% required i.v. steroids, two of whom (2%) also required ALG and/or OKT3, with 17% requiring increased oral immunosuppression alone. Actuarial survival was 98% at 30 days, 94% at 1 year, and 92% at 2 years. It is possible that we may have missed acute rejection episodes that resolved spontaneously. However, the excellent medium-term results would suggest that any such rejection episode did not progress to become hemodynamically significant. It may be, therefore, that when an effective immunosuppressive regimen is utilized, the number of EMBs performed at many centers is excessive.

Characterization of baboon class I major histocompatibility molecules. Implications for baboon-to-human xenotransplantation

Increasingly strong medical and political pressures are stimulating consideration of the transplantation of baboon organs and cells into humans. Critical to the success of these xenotransplants is management of the immune system such that graft rejection and, in the case of bone marrow transplantation, graft-versus-host disease do not result in transplant failure. The polymorphic products of the major histocompatibility complex (MHC) are the primary barrier to successful allotransplantation, and here we describe class I MHC molecules from baboon (Papio anubis) to gain an understanding of how similarities and differences between baboon and human MHC molecules might affect xenograft survival and function. Comparative analyses of our five novel baboon class I molecules with defined HLA class I molecules demonstrate that the baboon class I molecule are up to 90% identical. Disparity between baboon class I proteins and their human homologues lies predominately at positions in the antigen-binding groove, while C-terminal portions of the class I heavy chain are more conserved between the two species. Such concentration of cross-species differences within the alpha1 and alpha2 domains involves a majority of substitutions at positions demonstrating polymorphism in human alleles; the location of substitutions distinguishing baboon and human molecules thus resembles the positioning of human class I allopolymorphisms. Because this preliminary characterization indicates that both baboon and human T cells with be restricted by xenogeneic class I molecules, immune responses triggered during baboon-to-human transplantation should mimic those arising during MHC mismatched human allotransplantation.

The reducing end of alpha Gal oligosaccharides contributes to their efficiency in blocking natural antibodies of human and baboon sera

Synthetic galactosyl oligosaccharides were tested for their ability to inhibit the cytotoxic reaction of human and baboon natural antibodies on PK15 cells in culture. Methyl-alpha-Gal gave weak inhibition, Gal alpha 1-3Gal substantially inhibited the reaction (400muM), and Gal alpha 1-3Gal beta 1-4GLcNAc was ten times more efficient (30 muM). The modification from alpha to beta anomeric configuration of the nonreducing end resulted in a complete loss of activity, while substitutions at the reducing end induced only a partial loss of activity. These observations suggest that natural anti-alphaGal antibodies recognize the epitope from its nonreducing end, but that substitutions at the reducing terminus can modify the antibody-binding capacity. Modified tri- and tetrasaccharides are better inhibitors than the disaccharide but not as good as Gal alpha 1-3Gal beta 1-4GlcNAc. The reducing terminus therefore contributes some energy to the reaction, indicating that certain oligosaccharides will be of more potential clinical use than others.

Ultrastructure changes associated with brain death in the human donor heart

Electromicroscopic examinations were carried out on 30 myocardial biopsies taken from 22 human donor hearts immediately after excision (prestorage) or immediately before transplantation (poststorage). All electron micrographs were independently examined by two morphologists. Eleven structures were examined in each micrograph, and each structure was scored according to the degree of injury. A good interobserver correlation was obtained in 84% of the structures scored. In the prestorage left ventricular biopsies (n = 11), approximately 20%-25% showed moderate to severe ultrastructural injury. The ultrastructural injury observed in the poststorage left ventricular biopsies (n = 15) was no different from that in the prestorage group, particularly injury to the sarcomere and mitochondria. A similar degree and pattern of injury was seen in the right ventricle (n = 4). There was no evidence that an ischemic storage period of less than 6 h increased the degree of injury seen. However, there was a higher incidence of moderate to severe injury in those hearts excised from donors initially dependent on high inotropic support.

Cardiac transplantation in patients aged 60 years or older

One hundred eighty-four consecutive patients who underwent heart transplantation from January 1987 to December 1994 have been reviewed. Patients who were 60 years of age or older at the time of transplant (Group A, n = 50) showed improved overall survival when compared with younger patients (Group B, n = 134), though this was not statistically significant. Survivals in groups A and B were 94% and 90%, respectively, at 1 year; 86% and 80% at 5 years; and 86% and 78% at 8 years. Overall survival of older patients in the U.S. and worldwide is inferior when compared with younger patients. More thorough pretransplant evaluation of the older patient and improved compliance post-transplantation may be factors in the good results obtained in this group at our own center.

A comparison of transplant patient and social worker attitudes in regard to transplant patient psychosocial selection criteria, role expectations and communication style

Though much is known about organ transplant (TX) patient and family psychosocial needs, there is virtually no information about the nature of the interaction that exists between TX social workers and patients. A comparison of TX patient-social worker attitudes and beliefs found (i) patients supported a more egalitarian "consumer-centered" approach to health care, but were reluctant to assume the accompanying responsibility, and (ii) social workers were more "traditional" in their approach, which focused primarily on assessment and social support. Results argue for a more comprehensive clinical approach by social workers to deal with problems of patient compliance, substance abuse, and psychological needs in a TX service where emphasis is placed on results rather than subjective process measures.

Induction therapy with cyclosporine without cytolytic agents results in a low incidence of acute rejection without significant renal impairment in heart transplant patients

Since 1989, the immunosuppressive regimen used in all heart transplant patients at our center has consisted of (i) cyclosporine induction therapy (pretransplant p.o. 2-6 mg/kg depending on serum creatinine level, with immediate post-transplant i.v. therapy at 1-3 mg/h until p.o. therapy alone maintains a whole blood trough level of 300 ng/ml by RIA); (ii) azathioprine (2.5 mg/kg/d i.v./p.o.); (iii) methylprednisolone i.v. for 24 h and then prednisone p.o. at 1 mg/kg/d, tapering to 0.1 mg/kg/d at 1 yr. No prophylactic cytolytic agents (ALG, OKT3) were given. One hundred consecutive patients have been followed for periods of 4-56 months (mean 27 months). The incidence of acute rejection requiring increased therapy was 24%, with only 7% requiring i.v. steroids, 2 of whom (2%) also required ALG and/or OKT3, and with 17% requiring increased oral immunosuppression alone. Mean creatinine levels (mg/dl) were 1.3 pretransplant, 1.4 on d 7, 1.5 at 30 d, and 1.8 after 2 yr. Only 1 patient required temporary hemodialysis. Survival was 98% at 30 d, 94% at 1 yr, and 92% at 2 yr. We conclude that cyclosporine induction therapy with steroids and azathioprine without any cytolytic agent results in a low incidence of acute rejection without jeopardizing renal function.

Auxiliary liver allografting and xenografting in the nonhuman primate

Auxiliary liver transplantation has been performed in the baboon using allografts (n = 8) and concordant xenografts from donor African green monkeys (n = 8). The native portal vein was ligated in all cases and the native common bile duct was ligated in 5 cases. The immunosuppressive therapy used was identical in both the allografts and xenografts and consisted of triple drug therapy (cyclosporine, cyclophosphamide, and methylprednisolone), all at dosages consistent with clinical use. During the determination of the surgical technique to be applied, there were 5 early failures (3 allografts, 2 xenografts), and 2 deaths at 10 and 20 days from multiorgan failure and sepsis, respectively (xenografts). The remaining 9 baboons (5 allografts, 4 xenografts) were electively euthanized at 16-62 days (allografts) and 35-120 days (xenografts). Hyperacute rejection or antibody-mediated rejection was not seen in the grafted livers. Episodes of acute cellular rejection occurred in the majority of animals within the first 30 days and recurred in the longer-term survivors, but could be controlled by bolus therapy with intravenous methylprednisolone. Satisfactory donor liver function was confirmed using a number of tests, including scintigraphy in 3 cases. We conclude that auxiliary liver transplantation using a closely related donor species is feasible in baboons and might be extended to humans with terminal liver failure. A baboon-to-man auxiliary liver graft may serve as a "bridge" until either a human cadaver donor liver became available or native liver function recovers in patients with fulminant hepatic failure.