Xenotransplantation: The Challenge to Current Psychosocial Attitudes

Because of the limited availability of transplantable human organs, xenotransplantation, the use of animal organs as an alternative source, has received considerable attention in recent years. Xenotransplantation would provide an unlimited supply of organs, and these organs would be available whenever required. Although the pig is considered the best source for organs, significant immunologic barriers currently prohibit the implementation of a clinical trial of organ transplantation. However, as medical research gains more insight into the mechanisms underlying rejection of pig organs in primates, therapeutic xenotransplantation is becoming more feasible. Clinical trials of porcine cell transplants are currently underway. Although xenotransplantation will minimize the waiting period for an organ and obviate the feelings of guilt or indebtedness commonly experienced by recipients of human organs, several psychosocial issues may hinder the reintegration of patients into society. For example, concerns that infectious pathogens could be transferred to recipients of pig organs will necessitate life-long monitoring and perhaps even temporary isolation of patients. The possible risk of the spread of a xenozoonosis from the patient to other members of the community may inspire public controversy and even fear, which may have an adverse impact on the patient's emotional state. Additionally, some patients may be psychologically disturbed by the need to incorporate pig organs into their body. This article addresses these and other psychosocial issues that may be associated with clinical xenotransplantation.

Adherence to controller asthma medications: 6-month prevalence across a US community pharmacy chain

WHAT IS KNOWN AND OBJECTIVE

Non-adherence to controller asthma medications is an important public health problem. It is estimated to occur in 30-70% of individuals and is a significant risk factor for asthma morbidity and mortality. The aim of this study was to determine the level of adherence, as indicated by refill rates, to controller asthma medications in a community pharmacy setting.

METHODS

Secondary analyses of a community pharmacy dispensing database in 15 locations throughout Utah.

RESULTS AND DISCUSSION

The dispensing records of 2193 patients who received controller medications for asthma in a 12-month period, and had a minimum of 6-month potential coverage (180 days) from the date of their first receipt of a controller medication in that period, were examined. Using standard metrics to gauge adherence, the proportion of days covered (PDC) and the medication possession ratio (MPR), the average coverage for controller asthma medications across a 6-month period (180 days) was poor, averaging less than 50% of days' availability. Standard cut-offs (≥80% medication availability) indicated that only 14-16% of patients had 'satisfactory' adherence over their 6-month follow-on period. Females and older patients had significantly greater satisfactory adherence. Medication adherence was significantly greater with inhaled corticosteroid (ICS)-long-acting β₂ -agonist (LABA) combinations than with ICS alone.

WHAT IS NEW AND CONCLUSION

This study confirms the considerable scope of the asthma therapy non-adherence problem. Therefore, it is imperative to conduct survey-based research linked directly to pharmacy-based dispensing data to derive patient behavioural, attitudinal and environmental factors that may contribute to the issue, and then pilot and evaluate interventions for change.

Pig hematopoietic cell chimerism in baboons conditioned with a nonmyeloablative regimen and CD154 blockade

BACKGROUND

In an attempt to induce mixed hematopoietic chimerism and transplantation tolerance in the pig-to-primate model, we have infused high-dose porcine peripheral blood progenitor cells (PBPC) into baboons pretreated with a nonmyeloablative regimen and anti-CD154 monoclonal antibody (mAb).

METHODS

Group 1 baboons (n=2) received a nonmyeloablative regimen including whole body irradiation, pharmacological immunosuppression, porcine hematopoietic growth factors, and immunoadsorption of anti-Galalpha1,3Gal (Gal) antibody before infusion of high doses of PBPC (2.7-4.6x10(10) cells/kg). In group 2 (n=5), cyclosporine was replaced by anti-CD154 mAb. Group 3 (n=3) received the group 1 regimen plus anti-CD154 mAb.

RESULTS

In group 1, pig chimerism was detected in the blood by flow cytometry (FACS) for 5 days (with a maximum of 14%), and continuously up to 13 days by polymerase chain reaction (PCR). In group 2, pig chimerism was detectable for 5 days by FACS (maximum 33%) and continuously up to 28 days by PCR. In group 3, initial pig chimerism was detectable for 5 days by FACS (maximum 73%). Two of three baboons showed reappearance of pig cells on days 11 and 16, respectively. In one, in which no anti-Gal IgG could be detected for 30 days, pig cells were documented in the blood by FACS on days 16-22 (maximum 6% on day 19) and pig colony-forming cells were present in the blood on days 19-33, which we interpreted as evidence of engraftment. Microchimerism was continuous by PCR up to 33 days.

CONCLUSIONS

These results suggest that there is no absolute barrier to pig hematopoietic cell engraftment in primates, and that this may be facilitated if the return of anti-Gal IgG can be prevented.

Transplantation of islets of Langerhans: new developments

The clinical results recently reported by the Edmonton group in recipients of allogeneic islet grafts, all of whom achieved at least temporary insulin independence, has rekindled interest in transplantation of islets of Langerhans as a means to cure diabetes. Long-term islet graft survival has been achieved in a non-human primate pre-clinical model with a protocol of T-cell signaling blockade using a new monoclonal antibody. Islet xenotransplantation (namely the use of animal islets, with the aim of transplanting them into humans), or stem cell technology (the controlled differentiation of stem cells to obtain specialised cells for the treatment of diabetes) are other procedures currently being evaluated in animal models. The recent clinical success suggests that, in the near future, diabetes might be treated by islet transplantation early in the clinical course of the disease before the development of complications, and without the risks associated with conventional immunosuppression.

Transplantation of islets of Langerhans: new developments

The clinical results recently reported by the Edmonton group in recipients of allogeneic islet grafts, all of whom achieved at least temporary insulin independence, has rekindled interest in transplantation of islets of Langerhans as a means to cure diabetes. Long-term islet graft survival has been achieved in a non-human primate pre-clinical model with a protocol of T-cell signaling blockade using a new monoclonal antibody. Islet xenotransplantation (namely the use of animal islets, with the aim of transplanting them into humans), or stem cell technology (the controlled differentiation of stem cells to obtain specialised cells for the treatment of diabetes) are other procedures currently being evaluated in animal models. The recent clinical success suggests that, in the near future, diabetes might be treated by islet transplantation early in the clinical course of the disease before the development of complications, and without the risks associated with conventional immunosuppression.

CD40-CD154 pathway blockade requires host macrophages to induce humoral unresponsiveness to pig hematopoietic cells in baboons

The effect of CD154 blockade and macrophage depletion or inhibition on baboon humoral and cellular immune responses to pig antigens was studied in a pig-to-baboon peripheral blood mobilized progenitor cell (PBPC) transplantation model aimed at inducing tolerance. We infused pig PBPCs in baboons pretreated with a nonmyeloablative regimen along with murine anti-human CD154 monoclonal antibody (mAb) and macrophage-depleting or -inhibiting agents. Group 1 baboons (n=2) underwent a nonmyeloablative regimen and immunoadsorption of anti-Gal(alpha)1,3Gal (Gal) antibody (Ab) before intravenous infusion of high doses (1.3-4.6 x 10(10)cells/kg) of PBPCs. In group 2 (n=5), cyclosporine was replaced by 8 doses of anti-CD154 mAb over 14 days. Group 3 (n=3) received the group 2 regimen plus medronate liposomes (n=2) or commercially available human intravenous immunoglobulin G depleted of anti-Gal Ab (n=1) to deplete/inhibit recipient macrophages. Group 1 developed sensitization to Gal and also developed new Ab to non-Gal porcine antigens within 10 to 20 days. In group 2, no sensitization to Gal or non-Gal determinants was seen, but Gal-reactive antibodies did return to their preleukocyte transplantation levels. CD154 blockade, therefore, induced humoral unresponsiveness to pig cells. In group 3, sensitization to Gal was seen in all three baboons at 20 days, and Abs against new porcine determinants developed in one baboon. The depletion or inhibition of host macrophages, therefore, prevented the induction of humoral unresponsiveness by CD154 blockade. These results suggest that CD154 blockade induces humoral unresponsiveness by a mechanism that involves the indirect pathway of antigen presentation. In vitro investigation of baboon anti-pig mixed lymphocyte reaction confirmed that only the indirect pathway is efficiently blocked by anti-CD154 mAb. The mechanism in which blockade of the CD40-CD154 pathway induces its effect remains to be determined, but it could involve the generation of regulatory cells capable of suppressing the direct pathway.

Pig kidney transplantation in baboons: anti-Gal(alpha)1-3Gal IgM alone is associated with acute humoral xenograft rejection and disseminated intravascular coagulation

BACKGROUND

Kidneys harvested from miniature swine or pigs transgenic for human decay-accelerating factor (hDAF) were transplanted into baboons receiving an anti-CD154 monoclonal antibody (mAb) and either a whole body irradiation (WBI)- or cyclophosphamide (CPP)-based immunosuppressive regimen.

METHODS

Group 1 baboons (n=3) underwent induction therapy with WBI and thymic irradiation, pretransplantation antithymocyte globulin, and immunoadsorption of anti-Gal(alpha)1-3Gal (Gal) antibody (Ab). After transplantation of a miniature swine kidney, maintenance therapy comprised cobra venom factor, mycophenolate mofetil, and an anti-CD154 mAb (for 14-28 days). In group 2 (n=2), WBI was replaced by CPP in the induction protocol. Group 3 (n=3) animals received the group 2 regimen, but underwent transplantation with hDAF pig kidneys.

RESULTS

Group 1 and 2 animals developed features of disseminated intravascular coagulation (DIC), with reductions of fibrinogen and platelets and increases of prothrombin time, partial thromboplastin time, and fibrin split products. Graft survival was for 6-13 days. Histology showed mild acute humoral xenograft rejection (AHXR) of the kidneys, but severe rejection of the ureters. Group 3 animals developed features of DIC in two of three cases during the fourth week, with AHXR in the third case. Graft survival was for 28 (n=1) or 29 (n=2) days. Histology of day 15 biopsy specimens showed minimal focal mononuclear cellular infiltrates, with predominantly CD3+ cells. By days 28 and 29, kidneys showed mild-to-moderate features of AHXR. In all groups, the humoral response was manifest by reappearance of anti-Gal IgM below baseline level, with no or low return of anti-Gal IgG. All excised kidneys showed IgM deposition, but no complement and no or minimal IgG deposition. No baboon showed a rebound of anti-Gal Ab immediately after excision of the graft, and anti-Gal Ab increased over pretransplantation levels only when anti-CD154 mAb was discontinued.

CONCLUSIONS

DIC was observed with WBI- or CPP-based therapy, and after miniature swine or hDAF kidney transplantation. AHXR+/-DIC was observed in all recipients even in the absence of complement and no or low levels of anti-Gal IgG, but was significantly delayed in the hDAF recipients. These results confirm our earlier observation that CD154 blockade prevents T cell-dependent sensitization in baboons to pig antigens, but that baseline natural anti-Gal Ab production is not inhibited. We suggest that IgM deposition, even in the absence of IgG and complement, leads to endothelial cell activation with the development of DIC, even when there are only minimal histologic changes of AHXR.

Assessment of methotrexate as a potential immunosuppressive agent in baboons

Methotrexate is an anti-proliferative agent that affects both T-cell and B-cell immunity, and therefore might be expected to suppress antibody (Ab) production. Although it has been used in xenotransplantation studies to suppress anti-pig Ab production, it has always been used in combination with other immunosuppressants. The purpose of this study was to measure its effect as a single immunosuppressant on anti-Gal Ab production in baboons (n=4). Pharmacokinetic studies showed that methotrexate was not detected in the blood when administered per os. Prolonged daily IV or IM administration (i) reduced T-cell and B-cell numbers by 50% to 70% and modestly reduced responsiveness on mixed lymphocyte reaction (but only at toxic doses) and (ii) did not result in lowered anti-Gal Ab levels, only marginally reducing the rate of return of Ab after extracorporeal immunoadsorption. Our observations would suggest that methotrexate will not contribute significantly to immunosuppressive regimens in the baboon at non-toxic doses.

Clearance of mobilized porcine peripheral blood progenitor cells is delayed by depletion of the phagocytic reticuloendothelial system in baboons

INTRODUCTION

Attempts to achieve immunological tolerance to porcine tissues in nonhuman primates through establishment of mixed hematopoietic chimerism are hindered by the rapid clearance of mobilized porcine leukocytes, containing progenitor cells (pPBPCs), from the circulation. Eighteen hours after infusing 1-2 x 10(10) pPBPC/kg into baboons that had been depleted of circulating anti-alphaGal and complement, these cells are almost undetectable by flow cytometry. The aim of the present study was to identify mechanisms that contribute to rapid clearance of pPBPCs in the baboon. This was achieved by depleting, or blocking the Fc-receptors of, cells of the phagocytic reticuloendothelial system (RES) using medronate liposomes (MLs) or intravenous immunoglobulin (IVIg), respectively.

METHODS

Baboons (preliminary studies, n=4) were used in a dose-finding and toxicity study to assess the effect of MLs on macrophage depletion in vivo. In another study, baboons (n=9) received a nonmyeloablative conditioning regimen (NMCR) aimed at inducing immunological tolerance, including splenectomy, whole body irradiation (300 cGy) or cyclophosphamide (80 mg/kg), thymic irradiation (700 cGy), T-cell depletion, complement depletion with cobra venom factor, mycophenolate mofetil, anti-CD154 monoclonal antibody, and multiple extracorporeal immunoadsorptions of anti-alphaGal antibodies. The baboons were divided into three groups: Group 1 (n=5) NMCR+pPBPC transplantation; Group 2 (n=2) NMCR+ML+pPBPC transplantation; and Group 3 (n=2) NMCR+IVIg+pPBPC transplantation. Detection of pig cells in the blood was assessed by fluorescence-activated cell sorter and polymerase chain reaction (PCR).

RESULTS

PRELIMINARY STUDIES

ML effectively depleted macrophages from the circulation in a dose-dependent manner. Group 1: On average, 14% pig cells were detected 2 hr postinfusion of 1 x 10(10) pPBPC/kg. After 18 hr, there were generally less than 1.5% pig cells detectable. Group 2: Substantially higher levels of pig cell chimerism (55-78%) were detected 2 hr postinfusion, even when a smaller number (0.5-1 x 10(10)/kg) of pPBPCs had been infused, and these levels were better sustained 18 hr later (10-52%). Group 3: In one baboon, 4.4% pig cells were detected 2 hr after infusion of 1 x 10(10) pPBPC/kg. After 18 hr, however, 7.4% pig cells were detected. A second baboon died 2 hr after infusion of 4 x 10(10) pPBPC/kg, with a total white blood cell count of 90,000, of which 70% were pig cells. No differences in microchimerism could be detected between the groups as determined by PCR.

CONCLUSIONS

This is the first study to report an efficient decrease of phagocytic function by depletion of macrophages with MLs in a large-animal model. Depletion of macrophages with MLs led to initial higher chimerism and prolonged the survival of circulating pig cells in baboons. Blockade of macrophage function with IVIg had a more modest effect. Cells of the RES, therefore, play a major role in clearing pPBPCs from the circulation in baboons. Depletion or blockade of the RES may contribute to achieving mixed hematopoietic chimerism and induction of tolerance to a discordant xenograft.

Modulation of platelet aggregation in baboons: implications for mixed chimerism in xenotransplantation. II. The effects of cyclophosphamide on pig peripheral blood progenitor cell-induced aggregation

BACKGROUND

The induction of tolerance to pig antigens in primates may facilitate the development of successful clinical xenotransplantation protocols. The infusion of mobilized porcine peripheral blood leukocytes (PBPCs, comprised of approximately 2% peripheral blood progenitor cells) into splenectomized preconditioned (whole body irradiation (WBI)-based) baboons, intended to induce mixed hematopoietic cell chimerism, however, results in a severe thrombotic microangiopathy (TM) that includes pronounced thrombocytopenia. Previous studies have indicated that the infused PBPCs initiate platelet aggregation, but that the various individual components of the conditioning regimen are not associated with the development of aggregation. We have now investigated the effects of cyclophosphamide (CPP) as an alternative to WBI on platelet aggregation.

METHODS

Splenectomized baboons (n=3) were treated with CPP. Blood samples were collected and platelet-rich plasma (PRP) was prepared. Using light transmission aggregometry, the extent of aggregation induced by platelet agonists (thrombin, adenosine diphosphate (ADP), collagen, ristocetin, and arachidonic acid) was determined in vitro. PRP was also prepared from untreated baboons and from baboons receiving CPP, PBPCs were added, and platelet aggregation was measured in the absence of exogenous platelet agonists.

RESULTS

CPP markedly inhibited platelet aggregation induced by all standard agonists. In vitro addition of PBPCs to PRP stimulated platelet aggregation in the absence of any agonists. Prior treatment of baboons with CPP, however, inhibited this effect by 55% to 65%. TM was not evident in baboons receiving a conditioning regimen that included CPP instead of WBI.

CONCLUSIONS

Aggregation of baboon platelets and TM is directly induced by PBPCs. CPP has direct anti-aggregatory properties and may provide an alternative strategy to WBI in this pig-to-primate model intended to induce mixed hematopoietic cell chimerism.

Modulation of platelet aggregation in baboons: implications for mixed chimerism in xenotransplantation. I. The roles of individual components of a transplantation conditioning regimen and of pig peripheral blood progenitor cells

BACKGROUND

The induction of tolerance to pig antigens in primates may facilitate the development of successful clinical xenotransplantation protocols. The infusion of mobilized porcine peripheral blood leukocytes (PBPCs, comprised of approximately 2% peripheral blood progenitor cells) into splenectomized preconditioned baboons, intended to induce mixed hematopoietic cell chimerism, however, results in a severe thrombotic microangiopathy (TM) that includes pronounced thrombocytopenia. Because the mechanisms responsible for this phenomenon are unclear, we have explored the effects of individual components of the conditioning regimen, of therapeutic adjuncts, and of PBPCs on platelet aggregation.

METHODS

Groups of splenectomized baboons (n = at least 2 in each group) were treated with single components of the conditioning regimen--whole body irradiation (WBI), antithymocyte globulin (ATG), extracorporeal immunoadsorption (EI), mycophenolate mofetil (MMF), anti-CD40L monoclonal antibody (mAb), cobra venom factor (CVF), pig hematopoietic growth factors (interleukin-3 (pIL3) and stem cell factor (pSCF))--or with potential adjuncts, prostacyclin (PGI2), heparin, methylprednisolone, and eptifibatide (a GPIIb/IIIa antagonist). Blood samples were collected and platelet-rich plasma (PRP) was prepared. Using light transmission aggregometry, the extent of aggregation induced by platelet agonists (thrombin, adenosine diphosphate (ADP), collagen, ristocetin, and arachidonic acid) was determined in vitro. PRP was also prepared from untreated baboons, PBPCs were added, and platelet aggregation was measured in the absence of exogenous platelet agonists.

RESULTS

WBI, ATG, MMF, anti-CD40L mAb, CVF, pIL3, pSCF, and PGI2 had no effect on purified baboon platelet aggregation profiles in vitro. Eptifibatide markedly inhibited platelet aggregation induced by all standard agonists. EI or heparin inhibited thrombin-induced platelet aggregation, and methylprednisolone inhibited ADP-induced aggregation to some extent. In vitro addition of PBPCs to PRP stimulated platelet aggregation in the absence of any agonists. Prior treatment of baboons with eptifibatide, however, inhibited this effect by 70% to 80%.

CONCLUSIONS

Aggregation of baboon platelets and TM is directly induced by PBPCs, but not by individual components of the conditioning regimen. GPIIb/IIIa antagonists, such as eptifibatide, interfere directly with xenogeneic PBPC-platelet interactions and may further ameliorate TM in the pig-to-primate model.

The pig as a source of cardiac xenografts

The inadequate availability of human donor hearts and other organs has inspired interest in the field of xenotransplantation. Historically, ten attempts to transplant animal hearts into human recipients have been reported. Those who received hearts from nonhuman primates (i.e., baboons and chimpanzees) survived rather longer than did those who received hearts from nonprimates (i.e., sheep and pigs). Nevertheless, current opinion is that the pig is the best candidate as a source of hearts for humans despite the considerable immunologic disparity between the two species. Pigs are available in large numbers and can be bred easily and rapidly. They grow to appropriate sizes and their cardiovascular system is similar to that of humans. Substantial knowledge has been accumulated regarding both genetic engineering and tolerance induction in pigs, two strategies that may help to overcome the existing immunologic barriers. Concern has been raised, however, with regard to the potential for the transfer of a porcine infection with the pig organ to the human recipient. This brief review addresses these and other aspects of the use of the pig as a source of hearts for patients with end-stage cardiac disease.

Sex trading and psychological distress among women on methadone

This study examined the relationship between sex trading and psychological distress among a nonrandom sample of women recruited from 3 methadone maintenance clinics in New York City. Face-to-face interviews with 280 women (M age = 40.7) revealed that 32% had traded sex for money or drugs in the previous year. Compared to other participants, these women reported less education and higher rates of incarceration in the past year, sexually transmitted diseases, childhood sexual abuse, partner abuse, and current regular crack/cocaine and alcohol use. Hierarchical multiple regression analysis indicated that sex traders scored 0.41 units higher than non-sex traders on the General Severity Index of the Brief Symptom Inventory after controlling for all relevant covariates. The findings emphasize the need to consider the interrelation of psychological distress, abuse, and addiction in designing public health interventions addressing methadone maintained women.

Effects of specific anti-B and/or anti-plasma cell immunotherapy on antibody production in baboons: depletion of CD20- and CD22-positive B cells does not result in significantly decreased production of anti-alphaGal antibody

Anti-Galalpha1-3Gal antibodies (antialphaGal Ab) are a major barrier to clinical xenotransplantation as they are believed to initiate both hyperacute and acute humoral rejection. Extracorporeal immunoadsorption (EIA) with alphaGal oligosaccharide columns temporarily depletes antialphaGal Ab, but their return is ultimately associated with graft destruction. We therefore assessed the ability of two immunotoxins (IT) and two monoclonal antibodies (mAb) to deplete B and/or plasma cells both in vitro and in vivo in baboons, and to observe the rate of return of antialphaGal Ab following EIA. The effects of the mouse anti-human IT anti-CD22-ricin A (proportional to CD22-IT, directed against a B cell determinant) and anti-CD38-ricin A (proportional to CD38-IT, B and plasma cell determinant) and the mouse anti-human anti-CD38 mAb (proportional to CD38 mAb) and mouse/human chimeric anti-human anti-CD20 mAb (proportional to CD20 mAb, Rituximab, B cell determinant) on B and plasma cell depletion and antialphaGal Ab production were assessed both in vitro and in vivo in baboons (n = 9) that had previously undergone splenectomy. For comparison, two baboons received nonmyeloablative whole body irradiation (WBI) (300 cGy), and one received myeloablative WBI (900 cGy). Depletion of B cells was monitored by flow cytometry of blood, bone marrow (BM) and lymph nodes (LN), staining with anti-CD20 and/or anti-CD22 mAbs, and by histology of LN. EIA was carried out after the therapy and antialphaGal Ab levels were measured daily. In vitro proportional to CD22-IT inhibited protein synthesis in the human Daudi B cell line more effectively than proportional to CD38-IT. Upon differentiation of B cells into plasma cells, however, less inhibition of protein synthesis after proportional to CD22-IT treatment was observed. Depleting CD20-positive cells in vitro from a baboon spleen cell population already depleted of granulocytes, monocytes, and T cells led to a relative enrichment of CD20-negative cells, that is plasma cells, and consequently resulted in a significant increase in antialphaGal Ab production by the remaining cells, whereas depleting CD38-positive cells resulted in a significant decrease in antialphaGal Ab production. In vivo, WBI (300 or 900 cGy) resulted in 100% B cell depletion in blood and BM, > 80% depletion in LN, with substantial recovery of B cells after 21 days and only transient reduction in antialphaGal Ab after EIA. Proportional to CD22-IT depleted B cells by > 97% in blood and BM, and by 60% in LN, but a rebound of B cells was observed after 14 and 62 days in LN and blood, respectively. At 7 days, serum antialphaGal IgG and IgM Ab levels were reduced by a maximum of 40-45% followed by a rebound to levels up to 12-fold that of baseline antialphaGal Ab by day 83 in one baboon. The results obtained with proportional to CD38-IT were inconclusive. This may have been, in part, due to inadequate conjugation of the toxin. Cell coating was 100% with proportional to CD38 mAb, but no changes in antialphaGal Ab production were observed. Proportional to CD20 mAb resulted in 100% depletion of B cells in blood and BM, and 80% in LN, with recovery of B cells starting at day 42. Adding 150cGy WBI at this time led to 100% depletion of B cells in the BM and LN. Although B cell depletion in blood and BM persisted for > 3 months, the reduction of serum antialphaGal IgG or IgM Ab levels was not sustained beyond 2 days. Proportional to CD20 mAb + WBI totally and efficiently depleted CD20- and CD22-positive B cells in blood, BM, and LN for > 3 months in vivo, but there was no sustained clinically significant reduction in serum antialphaGal Ab. The majority of antibody secretors are CD38-positive cells, but targeting these cells in vitro or in vivo with proportional to CD38-IT was not very effective. These observations suggest that CD20-and CD22-positive B cells are not the major source of antialphaGal Ab production. Future efforts will be directed towards suppression of plasma cell function.

Mechanisms of thrombotic microangiopathy following xenogeneic hematopoietic progenitor cell transplantation

INTRODUCTION

Attempts to induce tolerance though mixed hematopoietic chimerism in the discordant pig-to-baboon xenotransplantation model are sometimes complicated by a potentially fatal thrombotic microangiopathy in the recipient baboons. This state develops immediately after the infusion of porcine mobilized peripheral blood leukocytes, containing progenitor cells (PBPC). In our study, we examined the interaction of infused porcine PBPC with recipient platelets in vivo in baboons and investigated the underlying mechanisms using an in vitro model.

METHODS

Two naïve baboons and six baboons preconditioned with irradiation and immunosuppression that received porcine PBPC were evaluated in vivo. The interaction of porcine and baboon PBPC with baboon platelets was investigated by an in vitro platelet aggregation assay. Fresh and cryopreserved PBPC were evaluated as well as PBPC obtained from growth-factor mobilized and unmobilized pigs. Furthermore, cellular subsets of PBPC were assessed for potential to induce platelet aggregation. Immunohistochemical staining was performed on platelet-leukocyte aggregates and potential inhibition of aggregation with anti-P-selectin and anti-CD154 mAbs, or eptifibatide (a GPIIb/IIIa receptor antagonist), was tested.

RESULTS

All baboons that received porcine PBPC rapidly developed marked thrombocytopenia (<20,000/microl), elevated serum lactate dehydrogenase (>1,500U/liter), schistocytosis, and platelet aggregates on blood smear. Three baboons died (two untreated and one preconditioned), and substantive platelet aggregates containing porcine leukocytes were observed in the microvasculature of lungs and kidneys. In vitro, porcine, but not baboon, PBPC induced aggregation of baboon platelets in a dose-dependent manner. Immunohistological examination of these aggregates confirmed the incorporation of porcine leukocytes. Cryopreserved PBPC caused less aggregation than fresh PBPC, and growth-factor-mobilized PBPC induced less aggregation than unmobilized PBPC. Aggregation was fully abrogated by the addition of eptifibatide, and modulated by anti-P-selectin and anti-CD154 monoclonal antibodies that recognize adhesion receptors on activated platelets. Purified fractions (granulocytes, CD2+, and CD- cells) of porcine PBPC did not initiate aggregation, whereas addition of exogenous porcine PBPC membranes (erythrocytes, dead cells, and/or platelets) to the purified fractions exacerbated the aggregation response.

CONCLUSIONS

These data indicate that porcine PBPC mediate aggregation of baboon platelets. This process likely contributes to the thrombotic microangiopathy observed after PBPC transplantation in the pig-to-baboon model. Eptifibatide can fully abrogate platelet aggregation induced by porcine PBPC in vitro. Purification of the progenitor cells from porcine PBPC and/or treatment of baboons with eptifibatide may be beneficial.

Christiaan Barnard and his contributions to heart transplantation

Christiaan (Chris) Barnard was born in South Africa in 1922 and qualified in medicine at the University of Cape Town in 1946. Following surgical training in Cape Town and Minneapolis, Barnard established a successful open heart surgery program at Groote Schuur Hospital. In 1967, he led the team that performed the world's first human-to-human heart transplant. Although his first patient survived only 18 days, 4 of his first 10 patients survived for more than 1 year, 2 living for 13 and 23 years, respectively. With his junior colleague, Jaques Losman, Barnard then developed the operation of heterotopic heart transplantation. In 1981, his group was the first to successfully transport donor hearts using a hypothermic perfusion storage device. Several studies on the hemodynamic and metabolic sequelae of brain death were carried out in his department. Barnard retired from the University of Cape Town in 1983 at the age of 61. Now in his 79th year, he continues to pursue his interest in writing for the public. In the words of one of his former colleagues, he is a "surgical visionary and simply the most unforgettable character of the second generation of cardiac surgeons."

Porcine hematopoietic cell xenotransplantation in nonhuman primates is complicated by thrombotic microangiopathy

Thrombotic microangiopathy (TM) is a serious complication of bone marrow transplantation (BMT) that resembles thrombotic thrombocytopenic purpura (TTP). In attempting to achieve hematopoietic cell chimerism in the pig-to-baboon model, we have observed TM following infusion of high doses (>10(10) cells/kg) of porcine peripheral blood mobilized progenitor cells (PBPC) into baboons. We performed investigations to analyze the pathobiology of this TM and to test therapeutic interventions to ameliorate it. PBPC were obtained by leukapheresis of cytokine-stimulated swine. The initial observations were made in two baboons that underwent a non-myeloablative regimen (NMR) prior to PBPC transplantation (TX) (group 1). We then studied three experimental groups. Group 2 (n = 2) received NMR without PBPC TX. Group 3 (n = 2) received PBPC TX alone. Group 4 (n = 6) received NMR + PBPC TX combined with prostacyclin, low-dose heparin, methylprednisolone, and cyclosporine was replaced by anti-CD40L mAb in five cases. Baboons in groups 1 and 3 developed severe thrombocytopenia (<10,000/mm3), intravascular hemolysis with schistocytosis (>10/high powered field (hpf)), increase in plasma lactate dehydrogenase (LDH) (2500-9000 U/l), transient neurologic changes, renal insufficiency, and purpura. Autopsy on two baboons confirmed extensive platelet thrombi in the microcirculation, and, similar to clinical BMT-associated TM/TTP, no unusually large vWF multimers or changes in vWF protease activity were observed in the plasma of baboons with TM. In group 2, self-limited thrombocytopenia occurred for 10-15 days following NMR. Group 4 baboons developed thrombocytopenia (<20,000/mm3) rarely requiring platelet transfusion, minimal schistocytosis (<3/hpf), minor increase in LDH (<1000 U/l), with no clinical sequelae. We conclude that high-dose porcine PBPC infusion into baboons induces a microangiopathic state with vWF biochemical parameters resembling clinical BMT-associated TM/TTP and that administration of antithrombotic and anti-inflammatory agents can ameliorate this complication.

The potential role of xenotransplantation in treating endstage cardiac disease: a summary of the report of the Xenotransplantation Advisory Committee of the International Society for Heart and Lung Transplantation

There is an increasing shortage of human donor hearts for transplantation. One potential solution is using hearts from a suitable animal source, such as the pig. A committee of the International Society for Heart and Lung Transplantation has reviewed the current status of research into xenotransplantation. Furthermore, the committee considered what results in the pig-to-nonhuman primate experimental model would justify a clinical trial of xenotransplantation, and the criteria for selecting patients to be entered in the trial. This review emphasizes initial patient selection issues, although the committee's overall conclusions and recommendations are summarized. Although the current experimental results do not presently justify initiating a clinical trial, the committee concluded that xenotransplantation theoretically has immense potential, and that research in this field should be encouraged and supported. Human cadaveric organ donation will still be of the highest priority for the foreseeable future.

Xenotransplantation: the challenge to current psychosocial attitudes

Because of the limited availability of transplantable human organs, xenotransplantation, the use of animal organs as an alternative source, has received considerable attention in recent years. Xenotransplantation would provide an unlimited supply of organs, and these organs would be available whenever required. Although the pig is considered the best source for organs, significant immunologic barriers currently prohibit the implementation of a clinical trial of organ transplantation. However, as medical research gains more insight into the mechanisms underlying rejection of pig organs in primates, therapeutic xenotransplantation is becoming more feasible. Clinical trials of porcine cell transplants are currently underway. Although xenotransplantation will minimize the waiting period for an organ and obviate the feelings of guilt or indebtedness commonly experienced by recipients of human organs, several psychosocial issues may hinder the reintegration of patients into society. For example, concerns that infectious pathogens could be transferred to recipients of pig organs will necessitate life-long monitoring and perhaps even temporary isolation of patients. The possible risk of the spread of a xenozoonosis from the patient to other members of the community may inspire public controversy and even fear, which may have an adverse impact on the patient's emotional state. Additionally, some patients may be psychologically disturbed by the need to incorporate pig organs into their body. This article addresses these and other psychosocial issues that may be associated with clinical xenotransplantation.

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.

Coagulation and thrombotic disorders associated with pig organ and hematopoietic cell transplantation in nonhuman primates

BACKGROUND

Efforts to achieve tolerance to transplanted pig organs in nonhuman primates by the induction of a state of mixed hematopoietic chimerism have been associated with disorders of coagulation and thrombosis. Activation of recipient vascular endothelium and platelets by porcine hematopoietic cells and/or activation of donor organ vascular endothelium and/or molecular differences between the species may play roles. Irradiation or drug therapy could possibly potentiate endothelial cell activation and/or injury.

METHODS

We have investigated parameters of coagulation and platelet activation in nonhuman primates after (1) a regimen aimed at inducing mixed hematopoietic chimerism and tolerance (TIR that included total body irradiation, T cell depletion, and splenectomy; (2) pig bone marrow or pig peripheral blood mobilized progenitor cell transplantation (PCTx); and/or (3) pig organ transplantation (POTx). Five experimental groups were studied. Baboons were the recipient subjects in all groups except Group 1. Gp 1 Cynomolgus monkeys (n=6) underwent TIR + allotransplantation of hematopoietic cells and a kidney or heart or TIR + concordant xenotransplantation (using baboons as donors) of cells and a kidney; Gp 2 Baboons (n=4) underwent TIR with or without (+/-) autologous hematopoietic cell infusion; Gp 3 (n=12) PCTx+/-TIR; Gp 4 (n=5) POTx+/-TIR; Gp 5 (n=4) TIR + PCTx + POTx. Platelet counts, with plasma prothrombin time, partial thromboplastin time, fibrinogen levels, fibrin split products and/or D-dimer were measured.

RESULTS

In the absence of a discordant (porcine) cellular or organ transplant (Groups 1 and 2), TIR resulted in transient thrombocytopenia only, in keeping with bone marrow depression from irradiation. PCTx alone (Group 3) was associated with the rapid development of a thrombotic thrombocytopenic (TTP)-like microangiopathic state, that persisted longer when PCTx was combined with TIR. POTx (+/-TIR) (Group 4) was associated with a gradual fall (over several days) in platelet counts and fibrinogen with disseminated intravascular coagulation (DIC); after graft excision, the DIC generally resolved. When TIR, PCTx and POTx were combined (Group 5), an initial TTP-like state was superseded by a consumptive picture of DIC within the first week, necessitating graft removal.

CONCLUSIONS

Both PCTx and POTx lead to profound alterations in hemostasis and coagulation parameters that must be overcome if discordant xenotransplantation of hematopoietic cells and organs is to be fully successful. Disordered thromboregulation could exacerbate vascular damage and potentiate activation of coagulation pathways after exposure to xenogeneic cells or a vascularized xenograft.

Inhibition of platelet aggregation in baboons: therapeutic implications for xenotransplantation

Activation of endothelial cells and platelet sequestration play major roles in rejection of xenografts. The histopathology of both hyperacute and acute vascular or delayed rejection of vascularized discordant xenografts is characterized by interstitial hemorrhage and intravascular thrombosis. Agents that prevent platelet activation and consequent microthrombus formation have proven beneficial in xenograft rejection but do not fully preclude vascular thrombosis. Recently, several new anti-platelet therapies have undergone extensive clinical testing for atherosclerotic thrombotic vascular disorders; other putative therapies are undergoing pre-clinical evaluation. We have investigated the effect of several of these novel agents on platelet aggregation in baboons in order to screen for future potential in xenograft rejection models.

METHODS

Drugs tested in these experiments were aurintricarboxylic acid (ATA, von Willebrand Factor-GPIb inhibitor), fucoidin (a selectin-inhibitor), 1-benzylimidazole (1-BI, thromboxane synthase antagonist), prostacyclin (PGI2, endothelial stabilizer), heparin (thrombin antagonist), nitroprusside sodium or nicotinamide (NPN or NA, both NO-donors), and eptifibatide (EFT, GPIIb/IIIa receptor antagonist). These were infused intravenously to nine baboons. Coagulation parameters and platelet counts were monitored and baboons were observed for adverse side-effects. The efficacy of these agents in inhibiting platelet aggregation was assayed in a platelet aggregometer.

RESULTS

Treatment with ATA and fucoidin resulted in complete inhibition of platelet aggregation but also in major perturbation of coagulation parameters. 1-BI and PGI2 had no effect when administered alone, but in combination resulted in moderate inhibition of aggregation without disturbance in PT or PTT. NPN and NA had no substantive effects on platelet aggregation. Heparin resulted in specific inhibition of thrombin-induced platelet aggregation and, as anticipated, was associated with moderate prolongation of PTT. Importantly, EFT caused complete inhibition of platelet aggregation without changes in coagulation. Platelet counts, fibrinogen levels, and fibrinogen degradation products remained within the normal ranges in all experiments.

CONCLUSIONS

Although excellent inhibition of platelet activation was obtained with ATA and fucoidin, clinical use may be precluded by concomitant disturbances of coagulation. Combinations of heparin and EFT may prove beneficial in preventing the thrombotic disorders associated with xenograft rejection while maintaining adequate hemostatic responses. These agents are to be evaluated in our pig-to-primate xenotransplantation models.

Plasma perfusion by apheresis through a Gal immunoaffinity column successfully depletes anti-Gal antibody: experience with 320 aphereses in baboons

BACKGROUND

Anti-Galalpha1-3Gal (Gal) antibodies (Gal Ab) contribute to the rejection of porcine organs transplanted into primates. Extracorporeal immunoadsorption (EIA) has been developed to eliminate Gal Ab from the circulation.

METHODS

Between 1995 and 1999 we performed 320 EIAs in baboons using a COBE-Spectra apheresis unit incorporating a synthetic Gal immunoaffinity column. Three plasma volumes were immunoadsorbed on each occasion. The 221 consecutive EIAs performed in 41 immunosuppressed baboons between January 1997 and April 1999 form the basis of this review. Of these 41 baboons, 29 underwent a series of three or four EIAs at daily intervals, seven had multiple series of three EIAs, and the remainder underwent single or double EIAs. Serum Gal Ab levels were monitored by ELISA before and at intervals after the course of EIA.

RESULTS

There were two fatal complications, one from a respiratory mishap (unrelated to the EIA) and one from persistent hypotension unresponsive to therapeutic interventions. Seven procedures (3%) were terminated early owing to technical difficulties and/or persistent hypotension. Mean pre-EIA Gal Ab levels in naive baboons were 33.1 microg/ml (IgM) and 14.5 microg/ml (IgG). Immediately after three consecutive EIAs, IgM was depleted by a mean of 97.3% and IgG by 99.4%. By 18 to 24 h later, Gal Ab was returning but depletion remained at 80.1% (IgM) and 84.7% (IgG). The subsequent rate of return of Gal Ab depended on the immunomodulatory protocol used.

CONCLUSIONS

(1) With appropriate monitoring, EIA is an acceptably safe procedure, even in small (<10 kg) baboons. (2) Three consecutive EIAs are effective in removing >97% of Gal Ab. (3) In the majority of cases, return of Gal Ab begins within 24 h, irrespective of the immunomodulatory protocol.

Disordered regulation of coagulation and platelet activation in xenotransplantation

Rejection of xenografts is associated with vascular-based inflammation, thrombocytopenia and the consumption of coagulation factors that may evolve into disseminated intravascular coagulation (DIC). Similarly, bone marrow-derived cellular xenotransplantation procedures are associated with endothelial cell activation and thrombotic microangiopathic injury. These complications generally develop despite the best available measures for depletion of xenoreactive natural antibody, inhibition of complement activation and suppression of T- and B-cell mediated immune responses. The mechanisms underlying the DIC and thrombotic microangiopathy associated with xenotransplantation are unclear. A proposed primary biological dysfunction of xenografts with respect to regulation of clotting could amplify vascular injury, promote immunological responses and independently contribute to graft failure. Disordered thromboregulation could have deleterious effects, comparable to unregulated complement activation, in the pathogenesis of xenograft rejection and may therefore represent a substantive barrier to xenotransplantation.

Oesophageal syncope

Oesophageal syncope is loss of consciousness on swallowing, which is thought to be associated with an abnormal vagovagal reflex that leads to transient bradycardia. Patients with this potentially lethal condition may present to neurologists, cardiologists or gastroenterologists. It may be associated with cardio-active drug therapy, previous myocardial infarction, or with an organic lesion of the lower oesophagus. Barium and manometric studies, in association with ECG monitoring, should, therefore, be carried out in all cases. In many patients, however, it appears to be a functional abnormality for which no cause can be determined. In the absence of a condition necessitating surgical correction, medical therapy in the form of anticholinergic or sympathomimetic agents is occasionally helpful. Total denervation of the affected portion of the oesophagus has successfully prevented further symptoms, but insertion of a programmed cardiac pacemaker is currently the definitive treatment of choice.

High-dose porcine hematopoietic cell transplantation combined with CD40 ligand blockade in baboons prevents an induced anti-pig humoral response

BACKGROUND

In pig-to-primate organ transplantation, hyperacute rejection can be prevented, but the organ is rejected within days by acute vascular rejection, in which induced high-affinity anti-Gal alpha1-3Gal (alphaGal) IgG and possibly antibodies directed against new porcine (non-alphaGal) antigenic determinants are considered to play a major role. We have explored the role of an anti-CD40L monoclonal antibody in modifying the humoral response to porcine hematopoietic cells in baboons pretreated with a nonmyeloablative regimen.

METHODS

Porcine peripheral blood mobilized progenitor cells obtained by leukapheresis from both major histocompatibility complex-inbred miniature swine (n=7) and human decay-accelerating factor pigs (n=3) were transplanted into baboons. Group 1 baboons (n=3) underwent whole body (300 cGy) and thymic (700 cGy) irradiation, T cell depletion with ATG, complement depletion with cobra venom factor, short courses of cyclosporine, mycophenolate mofetil, porcine hematopoietic growth factors, and anti-alphaGal antibody depletion by immunoadsorption before transplantation of high doses (2-4 x 10(10)/cells/kg) of peripheral blood mobilized progenitor cells. In group 2 (n=5), cyclosporine was replaced by eight doses of anti-CD40L monoclonal antibodies over 14 days. The group 3 baboons (n=2) received the group 1 regimen plus 2 doses of anti-CD40L monoclonal antibodies (on days 0 and 2).

RESULTS

In group 1, sensitization to alphaGal (with increases in IgM and IgG of 3- to 6-fold and 100-fold, respectively) and the development of antibodies to new non-alphaGal porcine antigens occurred within 20 days. In group 2, no sensitization to alphaGal or non-alphaGal determinants was seen, but alphaGal-reactive antibodies did return to their pre- peripheral blood mobilized progenitor cells transplant levels. In group 3, attenuated sensitization to alphaGal antigens was seen after cessation of cyclosporine and mycophenolate mofetil therapy at 30 days (IgM 4-fold, IgG 8-30-fold), but no antibodies developed against new porcine determinants. In no baboon did anti-CD40L monoclonal antibodies prevent sensitization to its own murine antigens.

CONCLUSIONS

We believe these studies are the first to consistently demonstrate prevention of a secondary humoral response after cell or organ transplantation in a pig-to-primate model. The development of sensitization to the murine elements of the anti-CD40L monoclonal antibodies suggests that nonresponsiveness to cell membrane-bound antigen (e.g., alphaGal) is a specific phenomenon and not a general manifestation of immunological unresponsiveness. T cell costimulatory blockade may facilitate induction of mixed hematopoietic chimerism and, consequently, of tolerance to pig organs and tissues.

Acute humoral xenograft rejection: destruction of the microvascular capillary endothelium in pig-to-nonhuman primate renal grafts

The major cause of xenograft loss beyond hyperacute rejection is a form of injury, traditionally termed delayed xenograft rejection (DXR), whose pathogenesis is unknown. Here we analyze the immunologic and morphologic features of DXR that develops in pig kidney xenografts transplanted into nonhuman primates. Kidneys from miniature swine were transplanted into cynomolgus monkeys (n = 14) or baboons (n = 11) that received regimens aimed to induce mixed chimerism and tolerance. No kidney was rejected hyperacutely. Morphologic and immunohistochemical studies were performed on serial biopsies, and an effort was made to quantify the pathologic features seen. The early phase of DXR (Days 0-12) was characterized by focal deposition of IgM, IgG, C3, and scanty neutrophil and macrophage infiltrates. The first abnormality recognized was glomerular and peritubular capillary endothelial cell death as defined by in situ DNA nick-end labeling (TUNEL). Damaged endothelial cells underwent apoptosis and, later, frank necrosis. The progressive phase developed around Day 6 and was characterized by progressive deposition of IgM, IgG, C3, and prominent infiltration of cytotoxic T cells and macrophages, with a small number of NK cells. Thrombotic microangiopathy developed in the glomeruli and peritubular capillaries with TUNEL+ endothelial cells, platelet aggregation, and destruction of the capillary network. Only rare damaged arterial endothelial cells and tubular epithelial cells were observed, with rare endothelialitis and tubulitis. In the advanced phase of DXR, interstitial hemorrhage and infarction occurred. During the development of DXR, the number of TUNEL+ cells increased, and this correlated with progressive deposition of antibody. The degree of platelet aggregation correlated with the number of TUNEL+ damaged endothelial cells. We conclude that peritubular and glomerular capillary endothelia are the primary targets of renal DXR rather than tubular epithelial cells or arterial endothelium and that the earliest detectable change is endothelial cell death. DXR was characterized by progressive destruction of the microvasculature (glomeruli and peritubular capillaries) and formation of fibrin-platelet thrombi. Both cytotoxic cells and antibodies potentially mediate the endothelial damage in DXR; however, in this model, DXR is largely humorally mediated and is better termed "acute humoral xenograft rejection."

Gene therapy and transplantation

Advances in molecular biology and in techniques of gene transfer have resulted in the development of practical approaches to human gene therapy. Many applications are of relevance to manipulation of the immune system and have potential in organ and cell transplantation. For example, gene therapy approaches may facilitate the induction of immunological tolerance to a donor organ or protect it locally against the host's immune response. Based on a comprehensive review of the world literature, examples of current research efforts in both allogeneic and xenogeneic transplantation are presented and discussed.

An approach to the control of disease transmission in pig-to-human xenotransplantation

Although several major immunologic hurdles need to be overcome, the pig is currently considered the most likely source animal of cells, tissues and organs for transplantation into humans. Concerns have been raised with regard to the potential for the transfer of infectious agents with the transplanted organ to the human recipient. This risk is perceived to be increased as it is likely that the patient will be iatrogenically immunocompromised and the organ-source pig may be genetically engineered in such a way to render its organs particularly susceptible to infection with human viruses. Furthermore, the risk may not be restricted to the recipient, but may have consequences for the health of others in the community. The identification of porcine endogenous retroviruses and of hitherto unknown viruses have given rise to the most concern. We document here the agents we believe should be excluded from the organ-source pigs. We discuss the likelihood of achieving this aim and outline the potential means by which it may best be achieved.

Pharmacotherapeutic agents in xenotransplantation

The ability to transplant pig organs into humans would resolve the current crisis in the supply of cadaveric human organs for the treatment of end stage disease. Several immunologic barriers need to be overcome if pig-to-primate transplantation is to be successful. The presence of preformed antibodies in humans, apes and Old World monkeys directed against galactose epitopes on pig vascular endothelium provides the major barrier, as binding of antibody to antigen leads to graft destruction by complement activation and other mechanisms. Hyperacute rejection can result from the action of complement. If this is prevented, delayed antibody-mediated rejection develops, which can be associated with a state of consumptive coagulopathy (disseminated intravascular coagulation, DIC). Efforts being made to overcome antibody-mediated rejection include depletion of antibody by extracorporeal immunoadsorption, prevention of an induced antibody response by co-stimulatory blockade, B-cell and/or plasma cell depletion, depletion or inhibition of complement, or the use of organs from pigs transgenic for a human complement regulatory protein, such as hDAF. The ultimate solution would be the induction of both B- and T-cell tolerance to the transplanted pig organ, which is being explored by attempting to induce haematopoietic cell chimerism. One complication of this is a thrombotic microangiopathy, similar to thrombotic thrombocytopenic purpura. The many and diverse roles in which pharmacotherapy is involved in attempts to overcome the barriers of xenotransplantation are reviewed and current progress, particularly in our own laboratory, is discussed.