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• W2517085020 abstract "During the period, January–February, 2007, four reviews on xenotransplantation were published [1–4]. Robert Zhong reviewed the recent experience acquired using α1,3-galactosyltransferase gene-knockout (GT-KO) pig organs in non-human primate transplantation [1]. Although graft survival has been prolonged, as hyperacute humoral rejection was overcome, the overall results have not been markedly improved when compared with those using a Galα1,3Gal (Gal)-positive pig organ transgenic for a human complement-regulatory protein with the administration of a synthetic Gal-conjugate to inhibit anti-Gal antibodies. Therefore, future work in xenotransplantation should focus on further modification of donors, such as combining human complement-regulatory genes with GT-KO, deleting non-Gal antigens and adding protective genes or a gene that inhibits coagulation. Tai et al. discussed the progress achieved in xenotransplantation following the introduction of GT-KO pigs [2]. They reviewed current knowledge on porcine organ and islet transplantation, and activation of the coagulation and complement cascades. Like Zhong, they also concluded that further genetic manipulations are needed to overcome delayed rejection. Cowan reviewed the role of coagulation and porcine endothelium in the rejection process of xenotransplanted organs [3]. Transgenic overexpression of anti-thrombotic molecules on xenograft endothelium might be a future approach to prevent acute humoral rejection. Vajta et al. discussed recent achievements of somatic nuclear transfer in pigs with the aim of generating animals as sources for organ xenotransplantation [4]. Meschia et al. used porcine skin collagen implants to prevent recurrent vaginal prolapse in women undergoing primary surgery for pelvic organ prolapse in a randomized prospective study [5]. They randomized 206 patients into two groups, with and without Pelvicol (porcine skin collagen) implants. They showed that, at 1 yr, women having been treated with Pelvicol implants had a better clinical outcome and a significantly lower rate of prolapse recurrence when compared with women operated without implant insertion. The authors concluded that porcine skin collagen can be used to augment and reinforce anterior vaginal wall repair with good clinical outcome. Stone et al. studied the long-term anti-non-Gal response in human patients suffering from a ruptured anterior cruciate ligament who were treated with a porcine patellar tendon transplant lacking Gal epitopes [6]. They followed five patients who received porcine patellar tendon grafts that had been previously treated by recombinant alpha-galactosidase to eliminate Gal epitopes. They showed that the xenografted tendon continued to function for over 2 yr, but they also found that the patients developed anti-non-Gal antibodies against multiple pig xenoepitopes. The authors concluded that these antibodies might induce a low level of inflammation that promotes a gradual replacement of the xenograft by infiltrating host fibroblasts. In two letters to the Editors published in Xenotransplantation, the Council of the International Xenotransplantation Association (IXA) and researchers from the Diabetes Research Institute in Miami replied to a letter published earlier by Dr. Rafael Valdes-Gonzalez [7] about the clinical trial of pig-to-human islet transplantation carried out in Mexico [8,9]. The opinion of the IXA Council was that this clinical trial should not continue, mainly because of a lack of preclinical data and a lack of rigor in safety monitoring. Concerning the data analysis by the Miami Group, both letters stated that the collected samples and data were difficult to interpret. The IXA Council and the Miami group both concluded that no convincing evidence of graft function had been shown in these patients. Nagata et al. transplanted freshly isolated porcine hepatocytes into the spleen of cynomolgus monkeys using conventional immunosuppression to control rejection [10]. They demonstrated the function of transplanted hepatocytes for over 80 days after a single infusion and up to 253 days after re-transplantation. The authors concluded that xenotransplantation of hepatocytes is feasible, that xenogeneic hepatocytes function up to 3 and 8 months after one or two infusions, respectively, and that hepatocyte xenotransplantation should be explored as an alternative therapy for hepatic failure. Wu et al. investigated the role of coagulation in the pathogenesis of hyperacute rejection and early graft failure in pig-to-baboon heart xenotransplantation, using pigs transgenic for human complement-regulatory proteins [11]. They observed that despite strong immunosuppression and complement inhibitors, over 50% of animals demonstrated either hyperacute rejection or early graft failure. In the rejected grafts, important fibrinogen and platelet deposition was observed. The authors concluded that dysregulated coagulation correlated closely with primary failure of pig hearts, and efforts should be undertaken to develop strategies to prevent dysregulated coagulation in pig organ xenografts. Majado et al. analyzed the evolution of xenoantibodies after depletion in a naïve, non-immunosuppressed baboon over a period of 4 yr [12]. They performed immunoadsorption using an apheresis machine and depleted the plasma of xenoantibodies by circulation of the plasma through a pig liver. They observed that approximately 1 week after depletion, xenoantibody titers increased rapidly and stayed high for about 2 months, when they decreased again. Four years later, xenoantibody titers were still higher than before depletion. The authors suggested that this immune response was initiated by soluble antigens that entered the plasma during the pig liver perfusion, and concluded that exposure to only a small amount of antigen resulted in significant sensitization. Kim et al. compared the outcome for pancreatic islet isolation of specific pathogen-free Chicago Medical School miniature swine, Prestige World Genetics miniature swine, and adult market pigs from a Korean slaughterhouse [13]. They observed that islet yield was higher and islet size bigger for islets isolated from the Chicago Medical School miniature pigs when compared with other breeds. The pancreases of the Chicago Medical School miniature pigs contained a higher mean islet volume density and larger islets than those of the two other breeds. The in vivo and in vitro function of isolated islets showed no significant difference. Therefore, specific pathogen-free Chicago Medical School miniature pigs seem to be candidates as donors for future clinical islet xenotransplantation [13]. Bottino et al. isolated islets from Yorkshire and White Landrace wild-type pigs and GT-KO pigs of three different age groups: younger than 6 months, 6–12 months old, and >2 yr of age [14]. They observed that islet yield, in vitro and in vivo islet function from donors >2 yr was significantly higher when compared with younger donors. The authors concluded that, in spite of higher costs involved, organ-source pigs for islet isolation need to reach adult age before being considered optimal donors. Layton et al. produced a monoclonal antibody (mAb) identifying porcine CD34, as common hematopoietic stem cell marker [15]. Using this anti-CD34 mAb, the authors showed that, similar to human and mouse, pig CD34 identifies a subset of bone marrow progenitor cells. Further studies are needed to demonstrate that these porcine CD34+ cells have hematopoietic stem cell activities. Aronovich et al. harvested fetal pig spleens from 42-day-old fetuses, before the appearance of T cells, and transplanted this tissue into hemophiliac severe combined immunodeficient (SCID) mice [16]. Transplanted mice showed a complete alleviation of hemophilia 2–3 months after transplantation, and normal coagulation and factor VIII blood levels. This study provided a proof of principle to the concept that transplantation of fetal spleen, obtained at a developmental stage prior to the appearance of T cells, could become a novel strategy in the treatment of genetic deficiencies for enzymes or coagulation factors. Meyerrose et al. studied the distribution of human adipose-derived mesenchymal stem cells (AMSC) after transplantation into immunodeficient mice that had been previously irradiated [17]. AMSC were transduced with a lentiviral vector coding for enhanced green fluorescent protein to identify transplanted cells in the recipient mice. For up to 75 days after transplantation, donor-derived AMSC could be identified in multiple tissues, but AMSC did not proliferate extensively at the sites of engraftment. As AMSC are easily isolated and highly amenable to current protocols for retroviral transduction, the authors stated that AMSC represent an important tool for cell-based therapies that involve a wide range of tissue targets. Séveno et al. analyzed the effect of CTLA4-Ig in a model of hamster-to-rat heart transplantation [18]. After CTLA4-Ig treatment, xenografted heart survival was prolonged to 19 days (controls survived 6 days), and this was accompanied by the appearance of regulatory cells exhibiting nonspecific suppressive activity, dependent on interleukin-2, nitric oxide, and indoleamine 2,3-dioxygenase. These regulatory cells were different from those observed in allogeneic transplantation. Rejection occurred through an immunoglobulin-M humoral response and complement activation. The authors concluded that CD28/B7 blockade by CTLA4-Ig delayed T cell-mediated rejection and induced cellular regulatory mechanisms. Chandra et al. analyzed the expression of various chemokines and inflammatory molecules in a model of adoptive transfer of activated macrophages into non-obese-diabetic severe combined immunodeficient (NOD-SCID) mice, transplanted with fetal porcine pancreatic fragments [19]. Chemokine receptor 2 and chemokine receptor 5 gene expression was 10-fold greater in graft-infiltrating macrophages when compared with other sites, suggesting that graft-mediated pro-inflammatory signals were important for macrophage recruitment. Therefore, xenografts provide additional signals that activate macrophages, and thereby induce macrophage-mediated recognition and rejection of porcine fetal pancreatic fragments. Schrepfer et al. studied the effect of FK778 (a leflunomide analog) in concordant hamster-to-rat aortic xenotransplantation [20]. Recipients were treated for 14 days with FK778, tacrolimus, sirolimus alone, or a combination regimen. The authors observed that FK778 suppressed humoral and cellular aortic xenograft rejection. When compared with other immunosuppressive drugs, the efficacy of FK778 was similar to sirolimus, but not as strong as tacrolimus. Liu et al. analyzed the relationship between the expression of human decay-accelerating factor (hDAF) on porcine fibroblasts and its inhibitory effect on human serum cytotoxicity [21]. They transfected porcine fibroblasts with the hDAF gene, and sorted these cells according to the level of hDAF expression. When compared with hDAF expression of human endothelial cells, a 15- to 30-fold higher hDAF expression was required in pig cells for effective inhibition of human sera with the highest cytotoxicity capacity. Therefore, transgenic pigs with higher expression levels of hDAF might be indicated for xenotransplantation. In cancer immunology, a potential therapeutic strategy is to deliver α1,3-galactosyltransferase to tumor vasculature and thereby sensitize transduced cells to natural antibody/complement-mediated lysis, mimicking the pathophysiology of xenograft rejection. Larkin and Porter used nude GT-KO mice which allowed the growth of human tumor xenografts, and the use of ecotropic retrovirus producer cells to generate expression of Gal on tumor vasculature [22]. After administration of serum containing anti-Gal antibody with cytolytic activity, the destruction of Gal-positive tumor endothelium was minimal. The authors concluded that this experimental model was limited by the use of a xenograft in which additional xenoreactivities led to complement insufficiency. Human embryonic stem cells have recently been a major focus in regenerative medicine. One problem encountered with embryonic stem cells is their tumorigenicity when transplanted at an undifferentiated stage. To engineer protection against the unintentional transplantation of undifferentiated cells, Hewitt et al. generated human embryonic stem cells carrying a construct in which α1,3-galactosyltransferase (GT) open reading frame was transcribed from the hTERT promoter [23]. Because the endogenous GT gene is inactive, GT expression was limited to undifferentiated cells. These cells were then assessed for their response to human serum containing anti-Gal antibodies. Undifferentiated cells were largely sensitive to human serum, but became resistant after differentiation. The authors concluded that this method protects the recipient from tumorigenicity of undifferentiated embryonic stem cells and may even provide ongoing immune surveillance after engraftment against dedifferentiation or against de novo tumorigenesis involving hTERT reactivation. To study the contribution of the host immune system to the potential for porcine endogenous retrovirus (PERV) transmission from pig islet tissue xenografts to host tissues, Popp et al. examined two immunoincompetent animal models, i.e. thymectomized fetal lambs and NOD/SCID mice, as recipients of porcine islets [24]. In fetal lambs, PERV A, B and C were detected in one-third of fetal lamb liver samples in the first 3 weeks post-transplantation. Porcine cells were also detected, demonstrating microchimerism in lamb livers. Histology, however, showed complete xenograft rejection by 23 days after transplantation to fetal lambs. In contrast, pig islets survived long-term in NOD/SCID mice, but no PERV transmission was observed. Transcription of PERV A, B and C, and possibly replication, continued in the donor pig islet tissue after transplantation. The authors concluded that the absence of stable PERV transmission and microchimerism in fetal lambs and the rejection of pig islet xenografts correlated in time with the establishment of host immunocompetence. They also suggested that the frequent failure to detect PERV transmission long term after porcine tissue xenotransplantation could be due to the immunologic destruction of PERV-infected host cells. Chiang et al. developed two mAbs, named 8E10 and 7C4, able to neutralize PERV infection in HEK293 cells [25]. mAb 8E10 bound directly to PERV, indicating that the epitope was exposed at the surface of the virion. mAb 7C4 bound native PERV inefficiently, suggesting that its epitope was accessible only after the virus interacted with its receptor. The authors stated that both mAbs effectively neutralized PERV infection, and might be used as a means to prevent PERV infection in patients receiving xenotransplantation. Bisset et al. developed in vitro co-cultures, consisting of phagocytic human fibroblasts and apoptotic or necrotic porcine B cells [26]. They demonstrated the presence of porcine DNA, including PERV, in the nucleus of human fibroblasts exposed to apoptotic porcine cells. This PERV transfer was characterized by a very low and transient efficiency, being present in only 0.22% of co-cultured human cells and disappearing to undetectable levels after 4 weeks of exposure. The transferred PERV was unable to replicate. This study suggested that apoptosis mediated horizontal PERV transfer and did not represent an overt infectious hazard. However, these results remain only in vitro, and further in vivo studies are needed to exclude PERV transmission. Lee et al. developed a multiplex polymerase chain reaction to simultaneously detect pseudorabies virus, porcine cytomegalovirus, and porcine circovirus, as these viruses are considered to be of significance for the microbiological safety of xenotransplantation [27]. No non-specific reaction was observed, and this technique was also showed to be effective in detecting various combinations of one or more of these viruses in pig specimens." @default.
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• W2517085020 date "2007-05-01" @default.
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• W2517085020 title "Xenotransplantation literature update January?February, 2007" @default.
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