One-year monkey heart xenograft survival in cyclosporine-treated baboons. Suppression of the xenoantibody response with total-lymphoid irradiation

Composite tissue xenopreservation: Preliminary results of staged VCA in rat to mouse model

BACKGROUND

The time between procurement and transplantation of composite tissues, especially regarding the limited donor pool, is a challenge effecting the outcomes of the transplantation. Current preservation techniques mainly include either cold preservation with a solution or machine perfusion using blood or certain oxygen-carrying solutions. However, none enables preservation beyond 24 h. Increasing this time to several days will provide better usage of the donor pool, safer transplantation of VCA with significant muscle content, and gives time to stabilize a patient before long surgical procedures. Herein, we described a novel strategy of xenopreservation (preservation via xenotransplantation) to preserve composite tissues for 7 days, followed by staged transplantation.

MATERIALS AND METHODS

We used two concordant species, female Sprague Dawley rats (n = 10) and female CF-1 mice (n = 10) in this study. Four of pair of animals are used for anatomical study. The groin flap of the rat was used as a xenograft and xenotransplanted to the neck area of the carrier mouse. Cyclosporine (CsA) was administered used as immunosuppressant. After 7 days of preservation on the mouse neck, xenotransplanted groin flap (called xenopreserved flap) was re-harvested, skin and vessels samples were collected for histopathological evaluation, and the xenopreserved flap was transplanted to the donor rat's opposite groin area. Anastomoses were performed between the flap's pedicle and the femoral vessels. Clinical observation regarding inflammation and tissue perfusion of the xenopreserved flap was monitored daily. Fifteen days after the second surgical procedure, the rats were euthanized, and skin and vessel samples were collected. Histologic evaluation, including inflammatory cell numbers, was performed. Wilcoxon test was used to compare the changes in inflammation severity and p < .05 was set for statistical significance.

RESULTS

All xenopreserved groin flaps except one survived. Mean lymphocyte count before the second operation (at the end of the xenopreservation procedure) was 20,22 ± 0.44 and reduced to 13,14 ± 0.47 at the end of 15 days, and the difference was statistically significant (p < .05).

CONCLUSION

This proof-of-concept study with preliminary results showed that xenotransplantation might be a novel strategy for preservation of VCA for a certain period of time. However, additional translational studies are needed to modulate the tissue changes following xenopreservation.

The long-term survival of baboon-to-monkey kidney and liver xenografts

The present study was undertaken to develop an optimum immunosuppressive regimen in baboon-to-monkey life-supporting kidney xenografts. Baseline therapy for all groups include cyclosporine (CsA) and steroids. We compared adding (1) cyclophosphamide (CyP) at dose of 20 mg/kg/day given on post-operative day (POD) 0, 2, 5 and 7; (2) mycophenolic mofetil (MMF) at a dose of 40 mg/kg/day by daily gavage; or (3) CyP + rapamycin (Rap). The latter group was divided into high and low dose subgroups. Untreated xenografts were rejected on POD 6, CsA alone treated xenografts survived for 35 days and CsA + CyP treated xenografts survived for 45 days. Adding MMF significantly prolonged mean survival to 111 +/- 53 days, but the xenografts eventually developed rejection. Combination therapy including CsA, CyP and Rap reliably prevented xenogenic rejection and achieved a mean survival of 290 +/- 30 days. However, high dose CyP + Rap led to high incidence of post-transplant lymphoproliferation disorders (PTLD), while the incidence of PTLD was significantly less in the low dose subgroup (P < 0.01). Four animals in this subgroup survived for more than 300 days with normal renal function and histology. In addition, two liver recipients treated with CsA + CyP survived for 91 and 1,076 days. We conclude that long-term survival of kidney or liver xenografts can be achieved in a non-human concordant xenograft model using currently available immunosuppressive agents.

Baboons undergoing orthotopic concordant cardiac xenotransplantation surviving more than 300 days: effect of immunosuppressive regimen

OBJECTIVE

We reviewed long-term survival among hosts in 3 consecutive series of a rhesus monkey-baboon orthotopic cardiac xenotransplantation model with reference to host immune response, including the effectiveness in preventing rejection and limiting toxicity concerning infection, to evaluate specific immunosuppressive regimens for long-term outcomes.

METHODS

Six juvenile baboons surviving more than 300 days after transplantation were reviewed. Regimen A consisted of splenectomy, FK506, methotrexate, and antilymphocyte globulin. Regimen B consisted of pretransplantation and chronic maintenance with cyclosporine A (INN: ciclosporin), methotrexate, and antithymocyte globulin. Regimen C was the same as regimen B plus pretransplantation total lymphoid irradiation and intraoperative donor bone marrow cell infusion. Rejections were detected by means of echocardiography.

RESULTS

Long-term survivors in 3 groups were followed for a range of 332 to 515 days (mean, 436 days). Rejection frequency in regimens A, B, and C was 0.35, 0.58, and 0.18 per month, and rescue therapy days were 23 (4.8%), 123 (9.5%), and 20 (2.4%), respectively (P <.0001). Infection frequency was 0.58, 0.56, and 0.19 per month, and therapy days were 192 (38.2%), 164 (12.6%), and 7 (0.9%), respectively (P <.0001). Concerning the host immune response, interleukin 2-activated T cells of all groups during rejection-free periods showed lower numbers compared with those of control animals (P <.0005), and regimen C was the lowest among 3 groups (P <.01). The production of xenoantibody was sufficiently attenuated in all groups.

CONCLUSION

Regimen C leads to long-term survival with fewer rejection and infection episodes by means of suppression of the interleukin 2 pathway and xenoantibody production.

Total lymphoid irradiation for refractory rejection in pediatric heart transplantation

BACKGROUND

We evaluated the role of total lymphoid irradiation (TLI) in the management of refractory rejection among pediatric heart transplant patients.

METHODS

Eleven of 298 patients underwent TLI at 6 to 195 months of age and were divided into subgroups: those who survived (group A, n = 7) and those who did not survive beyond 1 year after TLI (group D, n = 4). Non-TLI recipient data were considered as the controls.

RESULTS

Six out of 11 patients died eventually (54%). TLI was initiated 3 to 107 months after transplantation with a dosage of 600 to 840 cGy. The pre-TLI rejection rate (0.62 +/- 0.40 per month) was higher (p < 0.0001); however, the post-TLI rejection rate (0.24 +/- 0.65 per month) showed no significant difference from the control rejection rate. The Cox proportional hazard model found significance for TLI as a risk factor for development of posttransplant coronary artery disease (relative risk, 4.8; 95% CI, 1.1 to 21.3) and posttransplant lymphoproliferative disease (relative risk, 47.9; 95% CI, 1.6 to 1,475.3), respectively. Although the rejection rate decreased after TLI in both groups (group A pre/post, 0.51 +/- 0.31/0.06 +/- 0.08 per month; group D pre/post, 0.82 +/- 0.49/0.57 +/- 1.09 per month), significance was obtained only in group A (p = 0.018).

CONCLUSIONS

TLI was an effective adjunct for reversal of refractory rejection in pediatric heart transplantation by reducing the rejection rate. Great care must be taken for the risk of development of coronary artery disease or lymphoproliferative disease.

Lack of cross-species transmission of porcine endogenous retrovirus infection to nonhuman primate recipients of porcine cells, tissues, or organs

BACKGROUND

Nonhuman primates (NHPs) have been widely used in different porcine xenograft procedures inevitably resulting in exposure to porcine endogenous retrovirus (PERV). Surveillance for PERV infection in these NHPs may provide information on the risks of cross-species transmission of PERV, particularly for recipients of vascularized organ xenografts for whom data from human clinical trials is unavailable.

METHODS

We tested 21 Old World and 2 New World primates exposed to a variety of porcine xenografts for evidence of PERV infection. These NHPs included six baboon recipients of pig hearts, six bonnet macaque recipients of transgenic pig skin grafts, and nine rhesus macaque and two capuchin recipients of encapsulated pig islet cells. Serologic screening for PERV antibody was done by a validated Western blot assay, and molecular detection of PERV sequences in peripheral blood mononuclear cells (PBMCs) and plasma was performed using sensitive polymerase chain reaction and reverse transcriptase-polymerase chain reaction assays, respectively. Spleen and lymph node tissues available from six bonnet macaques and three rhesus macaques were also tested for PERV sequences.

RESULTS

All plasma samples were negative for PERV RNA suggesting the absence of viremia in these xenografted animals. Similarly, PERV sequences were not detectable in any PBMC and tissue samples, arguing for the lack of latent infection of these compartments. In addition, all plasma samples were negative for PERV antibodies.

CONCLUSION

These data suggest the absence of PERV infection in all 23 NHPs despite exposure to vascularized porcine organs or tissue xenografts and the use of immunosuppressive therapies in some animals. These findings suggest that PERV is not easily transmitted to these NHP species through these types of xenografts.

Report of the Xenotransplantation Advisory Committee of the International Society for Heart and Lung Transplantation: the present status of xenotransplantation and its potential role in the treatment of end-stage cardiac and pulmonary diseases

An urgent and steadily increasing need exists world-wide for a greater supply of donor thoracic organs. Xenotransplantation offers the possibility of an unlimited supply of hearts and lungs that could be available electively when required. However, anti-body- mediated mechanisms cause the rejection of pig organs transplanted into non-human primates, and these mechanisms provide major immunologic barriers that have not yet been overcome. Having reviewed the literature on xenotransplantation, we present a number of conclusions on its present status with regard to thoracic organs, and we make a number of recommendations relating to eventual clinical trials. Although pig hearts have functioned in heterotopic sites in non-human primates for periods of several weeks, median survival of orthotopically transplanted hearts is currently ,1 month. No transplanted pig lung has functioned for even 24 hours. Current experimental results indicate that a clinical trial would be premature. A potential risk exists, hitherto undetermined, of transferring infectious organisms along with the donor pig organ to the recipient, and possibly to other members of the community. A clinical trial of xeno-transplantation should not be undertaken until experts in microbiology and the relevant regulatory authorities consider this risk to be minimal. A clinical trial should be considered when approximately 60% survival of life-supporting pig organs in non-human primates has been achieved for a minimum of 3 months, with at least 10 animals surviving for this minimum period. Furthermore, evidence should suggest that longer survival (.6 months) can be achieved. These results should be achieved in the absence of life-threatening complications caused by the immunosuppressive regimen used. The relationship between the presence of anti-HLA antibody and anti-pig antibody and their cross-reactivity, and the outcome of pig-organ xenotransplantation in recipients previously sensitized to HLA antigens require further investigation. We recommend that the patients who initially enter into a clinical trial of cardiac xenotransplantation be unacceptable for allotransplantation, or acceptable for allotransplantation but unlikely to survive until a human cadaveric organ becomes available, and in whom mechanical assist-device bridging is not possible. National bodies that have wide-reaching government-backed control over all aspects of the trials should regulate the initial clinical trial and all subsequent clinical xenotransplantation procedures for the foreseeable future. We recommend coordination and monitoring of these trials through an international body, such as the International Society for Heart and Lung Transplantation, and setting up a registry to record and widely disperse the results of these trials. Xenotransplantation has the potential to solve the problem of donor-organ supply, and therefore research in this field should be actively encouraged and supported.

Hyperacute rejection in the guinea pig-to-rat model without formation of the membrane attack complex

The guinea pig (GP)-to-rat transplantation model has been widely used to study hyperacute rejection (HAR) of xenografts. In this model heart graft survival beyond 8 days has never been reported. In contrast, survival times of kidney and heart grafts up to 62 days have been reported in the discordant pig-to-primate model. It is not clear why it is so much more difficult to obtain long-term graft survival in the GP-to-rat model as compared to the pig-to-primate model. We hypothesized that mechanisms other than activation of complement may be involved in the rejection of guinea pig grafts by rat recipients. Therefore, we have studied in detail the rejection of GP aortic grafts by rat recipients, either PVG/c+ (complement competent, group 1), or PVG/c- (complement C6 deficient, group 2). PVG/c- rats are not able to form the membrane attack complex (MAC) of complement. Forty-four GP-to-rat aortic transplantations were performed successfully. Recipient rats were sacrificed at various intervals after transplantation (4, 24 and 48 h, and 7 and 28 days, three to six animals per time point per group). Twenty-four hours after transplantation the number of cells in the media was significantly decreased from 11.1 +/- 0.9 cells/mm2 to 3.1 +/- 2.8 cells/mm2 in group 1, whereas the number of medial cells in group 2 remained the same. The number of medial cells was significantly decreased in both groups at 48 h post-transplantation (group 1: 1.8 +/- 2.2 cells/mm2; group 2: 5.5 +/- 3.0 cells/mm2). At that time no infiltrating cells were apparent in the grafts of either two groups. Seven days after transplantation, the number of medial cells remained low in group 1 (1.8 +/- 2.9 cells/mm2) but was increased in group 2 (10.7 +/- 2.6 cells/mm2) as a consequence of infiltrating immune cells. These infiltrating cells consisted mainly of macrophages, but also T cells and NK cells. At 28 days after transplantation the grafts in both groups were completely reorganized and no distinction could be made between media and adventitia. These results show that rejection of GP grafts by rat recipients can occur in the absence of both MAC of complement and immune competent cells. This MAC and immune cells independent type of rejection has not been described before and may explain the difficulty in obtaining long-term graft survival in the GP-to-rat xenotransplantation model.

Rejection of Cardiac Xenografts by CD4+ or CD8+ T Cells

We recently showed that brief complement inhibition induces accommodation of hamster cardiac transplants in nude rats. We have reconstituted nude rats carrying an accommodated xenograft with syngeneic CD4+ or CD8+ T cells to investigate the cellular mechanism of xenograft rejection. We show that CD4+ T cells can initiate xenograft rejection (10 ± 1.7 days) by promoting production of IgG xenoreactive Abs (XAb). These XAb are able to activate complement as well as to mediate Ab-dependent cell-mediated cytotoxicity. Adoptive transfer of these XAb into naive nude rats provoked hyperacute xenograft rejection (38 ± 13 min). The rejection was significantly (p &lt; 0.001) delayed by cobra venom factor (CVF; 11 ± 8 h in four of five cases) but was still more rapid than in control nude rats (3.3 ± 0.5 days). CVF plus NK cell depletion further prolonged survival (&gt;7 days in four of five cases; p &lt; 0.01 vs CVF only). CD8+ T cell-reconstituted nude rats rejected their grafts later (19.4 ± 5.8 days) and required a larger number of cells for transfer as compared with CD4+ T cell-reconstituted nude rats. However, second xenografts were rejected more rapidly than first xenografts in CD8+ T cell-reconstituted nude rats (9 ± 2 days), indicating that the CD8+ T cells had been activated. This study demonstrates that CD4+ and CD8+ T cells can both reject xenografts. The CD4+ cells do so at least in part by generation of helper-dependent XAb that act by both complement-dependent and Ab-dependent cell-mediated cytotoxicity mechanisms; the CD8+ cells do so as helper-independent cytotoxic T cells.

Prolonged discordant xenograft survival and delayed xenograft rejection in a pig-to-baboon orthotopic cardiac xenograft model

OBJECTIVE

Our objectives were to study delayed xenograft rejection and the effectiveness of pretransplantation total lymphoid irradiation combined with immunosuppression on rejection in a pig-to-baboon cardiac xenograft model.

METHODS

Baboons were treated with pretransplantation total lymphoid irradiation, cyclosporine A (INN: ciclosporin), and methotrexate. Orthotopic pig-to-baboon cardiac transplantations were performed after depletion of circulating xenoreactive natural antibody by pretransplantation donor organ hemoperfusion. Tissue samples were collected for immunologic and immunopathologic evaluation.

RESULTS

Pig cardiac xenografts survived more than 18 and 19 days without evidence of hyperacute rejection. Immunologic analysis of serum samples demonstrated that circulating xenoreactive natural antibody levels did not return to pretransplantation levels. The production of xenoreactive natural antibodies from the recipient's splenocytes was inhibited completely. Histologic examination of xenografts showed the feature of acute vascular rejection. Immunohistochemical studies demonstrated infiltration of cardiac xenografts by large numbers of macrophages, small numbers of natural killer cells, and a few T cells. The infiltrating macrophages also showed expression of interleukin-1 and tumor necrosis factor. Diffuse deposition of immunoglobulin G, C1Q, C3, and fibrin on xenograft vasculature was observed. Interleukin-2 expression was not found in rejected cardiac xenografts. Xenograft endothelial cells also showed evidence of activation (expression of cytokines interleukin-1 and tumor necrosis factor).

CONCLUSIONS

This study demonstrates prolonged discordant cardiac xenograft survival and delayed xenograft rejection in a pig-to-baboon model. The delayed xenograft rejection is mediated by both humoral and cellular mechanisms. Pretransplantation total lymphoid irradiation combined with cyclosporine A and methotrexate can inhibit xenoreactive natural antibody production but not elicited antipig antibody production and the xenoreactivity of macrophages.

Immunology of xenotransplantation

The transplantation of tissues and organs between individuals of different species, that is, xenotransplantation, engenders a variety of immune responses. Xenogeneic immune responses mediated by naturally-occurring antibodies and complement lead to hyperacute and acute vascular rejection of vascularized organ grafts and may also cause vascular rejection of cell and tissue grafts. Under some circumstances, however, a vascularized organ graft may evade humoral rejection despite the presence of anti-donor antibodies in the circulation of the recipient; this condition is called accommodation. Xenogeneic immune responses mediated by T lymphocytes and natural killer cells may cause acute cellular rejection. The extent to which cellular rejection of xenografts resembles cellular rejection of allografts remains to be determined. New insights into the molecular mechanisms underlying the immune responses to xenotransplantation has shed light on the pathogenesis of immunological disease and has allowed the development of specific immunomodulatory strategies that may facilitate clinical application of xenotransplantation.

Accommodation and T-Independent B Cell Tolerance in Rats With Long Term Surviving Hamster Heart Xenografts

It was previously reported that treatment with leflunomide (LF; 10 mg/kg/day) together with cyclosporine (CsA; 10 mg/kg/day) resulted in long term survival of hamster heart xenografts (Xg) in rats and that LF could be withdrawn 2 to 4 wk after transplantation. To study the mechanisms allowing withdrawal of LF, second hamster heart Xgs were transplanted 6 wk after the first xenograft. Only the rats that received LF for 4 wk accepted second Xgs (&gt;30 days; n = 5). Hence, after 4 wk of LF, the rats developed partial B cell tolerance, as they were unable to produce T-independent (CsA-resistant) XAbs. Rejection of second Xgs (2–4 days; n = 5) in the 2-wk LF group resulted in the formation of IgM xenoantibodies (XAbs) localizing together with complement within rejected grafts. However, these XAbs did not affect first Xgs, suggesting that the latter Xgs became resistant to this IgM XAb-mediated rejection, a phenomenon referred to as accommodation. Accommodation was further confirmed as adoptive transfer of IgM XAbs, which resulted in hyperacute Xg rejection in naive rats (&lt;1 h; n = 5), did not cause rejection in long term survivors (&gt;30 days; n = 4). This was associated with a down-regulation of the expression on the graft endothelial cells of adhesion molecules (believed to be important expressers of xenogeneic epitopes), such as P- and E-selectins. Interestingly, these adhesion molecules reappeared after retransplanting the accommodated Xgs to naive recipients. In conclusion, depending on the duration of the LF treatment, long term survival of hamster hearts in CsA-treated rats is based in part on accommodation and in part on T-independent B cell tolerance.

Cardiac xenotransplantation

Heart failure is an important medical and public health problem. Although medical therapy is effective for many people, the only definitive therapy is heart transplantation, which is limited severely by the number of donors. Mechanical devices presently are used as "bridges" to transplantation. Their widespread use may solve the donor shortage problem, but at present, mechanical devices are limited by problems related to blood clotting, power supply, and foreign body infection. Cardiac xenotransplantation using animal donors is a potential biologic solution to the donor organ shortage. The immune response, consisting of hyperacute rejection, acute vascular rejection, and cellular rejection, currently prevents clinical xenotransplantation. Advances in the solution of these problems have been made using conventional immunosuppressive drugs and newer agents whose use is based on an understanding of important steps in xenoimmunity. The most exciting approaches use tools of molecular biology to create genetically engineered donors and to induce states of donor and recipient bone marrow chimerism and tolerance in xenogeneic organ recipients. The successful future strategy may use a combination of a genetically engineered donor and a chimeric recipient with or without nonspecific immunosuppressive drugs.

Recent advances in the immunology of xenotransplantation

The transplantation of tissue and organs between individuals of different species, that is xenotransplantation, engenders a variety of severe immune responses. Xenogeneic immune responses mediated by naturally occurring antibodies and complement lead to hyperacute and acute vascular rejection of vascularized organ grafts and may also cause vascular rejection of cell and tissue grafts. Under some circumstances, however, a vascularized organ graft may evade humoral rejection despite the presence of antidonor antibodies in the circulation of the recipient; this condition is called accommodation. Xenogeneic immune responses mediated by T-lymphocytes and natural killer cells may cause acute cellular rejection. The extent to which cellular rejection of xenografts resembles cellular rejection of allografts remains to be determined. New insights into the molecular mechanisms underlying the immune responses to xenotransplantation have shed new light on the pathogenesis of immunological disease and have allowed the development of specific immunomodulatory strategies that may facilitate clinical application of xenotransplantation.

The influence of concordant xenografts on the humoral and cell-mediated immune responses to subsequent allografts in primates

The humoral and cell-mediated immune responses to subsequent allografts were determined in primate recipients after concordant xenotransplantation as a bridge to allotransplantation. Heterotopic heart transplants (n = 4) were performed from cynomolgus monkeys into ABH type-matched olive baboons followed 2 weeks later by allotransplantation from ABH type-matched baboon donors. Allografts were explanted at 8 weeks. All recipients underwent splenectomy at the time of xenotransplantation and received immunosuppression with cyclosporine, azathioprine, and methylprednisolone. Concordant xenotransplantation in these primates did not induce humoral or cell-mediated immune responses that jeopardized subsequent allografts. The degree of xenospecific immune reactivity, as determined by specific cytotoxicity of recipient T-cell lines derived from the xenograft and extent of histologic xenograft rejection, did not predict the severity of subsequent allograft rejection. In two of the four recipients, xenotransplantation induced an alloreactive humoral response against antigens expressed by the B cells of more than 50% of members from a panel of 12 unrelated baboons. In all recipients, priming with xenogeneic splenocytes in vitro induced an accelerated proliferative T-cell response to allogeneic lymphocytes from 16% of this panel. This study affirms the role of concordant xenografts as appropriate biologic bridges to human allotransplantation. However, our results suggest that xenoreactive baboon memory CD4 T cells may recognize major histocompatibility complex class II--like structures shared between the xenogeneic and allogeneic targets. The potential allorecognition induced by a xenograft may affect the process of subsequent allograft donor selection.

Chronic rejection of concordant aortic xenografts in the hamster-to-rat model

Several groups have demonstrated that it is possible to obtain long-term graft survival of concordant xenografts. One of the important questions that remains is whether xenografts are susceptible to chronic rejection. To answer this question we used the aorta transplantation model. One centimetre of hamster aorta was interposed in the abdominal aorta of Lewis rat recipients. The recipients were either untreated (group 1), or treated with 10 mg/kg cyclosporine (CsA), given intramuscularly three times a week (group 2). Rats were sacrificed at day 7, 14, 21, 28, 56 and 84 and the thickness of the intima, the media and the adventitia was measured. Furthermore, the cellularity of the media and the adventitia was assessed by counting the number of nuclei per 0.05 mm2 and immunohistochemistry of the aortic grafts was performed. Graft arteriosclerosis developed in aortic xenografts of both group 1 and group 2. In group 1, intimal lesions were already present from day 21 onwards in all rats, whereas in group 2 they were present only in 33% (2/6) of the rats. At day 84 all the grafts in group 1 were totally occluded, while those in group 2 were still open. The thickness of the media was slightly increased in both groups during the whole observation period, mainly due to edema. Although a few infiltrating macrophages could be seen, the number of nuclei per 0.05 mm2 of the media remained constant during the first 21 days, but declined sharply from day 21 onwards, as a consequence of disappearing myocytes. Thickness of the adventitia in both groups increased after transplantation due to infiltrating macrophages and T cells, reaching a peak at day 14. After day 14 the adventitial thickness in group 1 decreased rapidly to reach values comparable to group 2 from day 28 onwards. In conclusion, graft arteriosclerosis, as a sign of chronic rejection, occurs in concordant aortic xenografts. The lesions in the xenografts develop extremely rapidly, and compared to data from the literature, faster than in aortic allografts. The process of chronic rejection in aortic xenografts can be reduced by CsA.

Inhibition of chronic vascular rejection in primate cardiac xenografts using mycophenolate mofetil

Similar to human allografts, cardiac xenografts also appear susceptible to chronic vascular rejection. The study described here evaluated the influence of mycophenolate mofetil on the incidence and severity of vascular rejection in a primate model of heart xenotransplantation. Nine baboons received heterotopic cardiac xenografts from donor cynomolgus monkeys. All baboons were placed on a cyclosporine and methylprednisolone-based immunosuppressive regimen. In addition, group 1 baboons received azathioprine (4 mg.kg-1.day-1) and group 2 baboons received mycophenolate mofetil (70 mg.kg-1.day-1). Biopsy specimens were obtained at regular intervals and reviewed blindly by a pathologist. A total of 50 biopsy specimens, 29 from group 1 and 21 from group 2, were reviewed. Histologic evidence of vascular rejection was present in 16 of the 29 biopsy specimens from the group 1 animals and in only 2 of the 21 specimens from the group 2 animals (p < 0.005). The mean graft survival was 3 months in group 1 versus 10 months in group 2. At 1-year follow-up, profound intimal proliferation in the coronary vasculature was noted in the biopsy specimens from group 1, whereas the coronary vessels were found to be normal in the specimens from group 2. The use of mycophenolate mofetil, in combination with cyclosporine and steroids, resulted in reduced vascular rejection and prolonged xenograft survival.