Transgenic pigs expressing human CD59, in combination with human membrane cofactor protein and human decay-accelerating factor

Complement networks in gene-edited pig xenotransplantation: enhancing transplant success and addressing organ shortage

The shortage of organs for transplantation emphasizes the urgent need for alternative solutions. Xenotransplantation has emerged as a promising option due to the greater availability of donor organs. However, significant hurdles such as hyperacute rejection and organ ischemia-reperfusion injury pose major challenges, largely orchestrated by the complement system, and activated immune responses. The complement system, a pivotal component of innate immunity, acts as a natural barrier for xenotransplantation. To address the challenges of immune rejection, gene-edited pigs have become a focal point, aiming to shield donor organs from human immune responses and enhance the overall success of xenotransplantation. This comprehensive review aims to illuminate strategies for regulating complement networks to optimize the efficacy of gene-edited pig xenotransplantation. We begin by exploring the impact of the complement system on the effectiveness of xenotransplantation. Subsequently, we delve into the evaluation of key complement regulators specific to gene-edited pigs. To further understand the status of xenotransplantation, we discuss preclinical studies that utilize gene-edited pigs as a viable source of organs. These investigations provide valuable insights into the feasibility and potential success of xenotransplantation, offering a bridge between scientific advancements and clinical application.

Inhibition of complement activation by CD55 overexpression in human induced pluripotent stem cell derived kidney organoids

End stage renal disease is an increasing problem worldwide driven by aging of the population and increased prevalence of metabolic disorders and cardiovascular disease. Currently, kidney transplantation is the only curative option, but donor organ shortages greatly limit its application. Regenerative medicine has the potential to solve the shortage by using stem cells to grow the desired tissues, like kidney tissue. Immune rejection poses a great threat towards the implementation of stem cell derived tissues and various strategies have been explored to limit the immune response towards these tissues. However, these studies are limited by targeting mainly T cell mediated immune rejection while the rejection process also involves innate and humoral immunity. In this study we investigate whether inhibition of the complement system in human induced pluripotent stem cells (iPSC) could provide protection from such immune injury. To this end we created knock-in iPSC lines of the membrane bound complement inhibitor CD55 to create a transplant-specific protection towards complement activation. CD55 inhibits the central driver of the complement cascade, C3 convertase, and we show that overexpression is able to decrease complement activation on both iPSCs as well as differentiated kidney organoids upon stimulation with anti-HLA antibodies to mimic the mechanism of humoral rejection.

Suppression of macrophage-mediated xenogeneic rejection by the ectopic expression of human CD177

Cellular xenogeneic rejection by the innate immune system is a major immunological obstruction that needs to be overcome for the successful clinical use of xenografts. Our focus has been on macrophage-mediated xenogeneic rejection, since suppressing macrophage function has considerable potential for practical applications in the area of xenotransplantation. We report herein on an investigation of the suppressive effect of human CD177 (hCD177) against macrophage-mediated xenogeneic rejection. Wild type swine aortic endothelial cell (SEC) and an SEC transfectant with hCD177 (SEC/hCD177) were co-cultured with macrophages, and the degree of cytotoxicity was evaluated by WST-8 assays, and phagocytosis was examined using Calcein-AM labeling methods. The expression of anti/pro-inflammatory cytokines was evaluated by RT-qPCR and the phosphorylation of SHP-1 on macrophages in co-culture was evaluated by Western blotting. The result of cytotoxicity assays indicated that hCD177 suppressed M1 macrophage-mediated xenogeneic rejection (vs. SEC, p < 0.0001). Similarly, the result of phagocytosis assays indicated that hCD177 suppressed it (vs. SEC, p < 0.05). In addition, hCD177 significantly suppressed the expression of IL-1β, a pro-inflammatory cytokine, in M1 macrophages (vs. SEC, p < 0.01). Luciferase assays using THP1-Lucia NF-kB also showed a significant difference in NF-kB activation (vs. SEC, p < 0.001). In addition, hCD177 was found to induce the phosphorylation of SHP-1 in M1 macrophages (vs. SEC, p < 0.05). These findings indicate that hCD177 suppresses M1 macrophage-mediated xenogeneic rejection, at least in part via in the phosphorylation of SHP-1.

Bioinformatic analysis as a first step to predict the compatibility of hematopoiesis and immune system genes between humans and pigs

The shortage of allogeneic donor organs leaves its supply far short of clinical need. There are great expectations on xenotransplantation, especially with pigs' organs. With the genetic modification of donor pigs, the rejection and cross-species transmission issues have now been widely addressed. However, research on the compatibility of genes between humans and pigs was limited. We performed a systematic screening analysis of predicted incompatible genes between humans and pigs, judged by low protein sequence similarities or different predicted protein domain compositions. By combining with gene set enrichment analysis, we screened out several key genes of hematopoiesis and the immune system with possible incompatibilities, which might be important for establishing chimera and xenotransplantation between humans and pigs. There were seven chemokine genes, including CCL1, CCL5, CCL24, CCL25, CCL28, CXCL12, and CXCL16, that exhibited limited similarity between humans and pigs (similarity < 0.8). Among hematopoiesis process-related genes, 15 genes of adhesion molecules, Notch ligands, and cytokine receptors exhibited differences between humans and pigs. In complement and coagulation cascades, 19 genes showed low similarity and 77 genes had different domain compositions between humans and pigs. Our study provides a good reference for further genetic modification of pigs, which might be beneficial for xenotransplantation.

A high-resolution cell atlas of the domestic pig lung and an online platform for exploring lung single-cell data

The genetically engineered pig is regarded as an optimal source of organ transplantation for humans and an excellent model for human disease research, given its comparable physiology to human beings. A myriad of single-cell RNA sequencing (scRNA-seq) data on humans has been reported, but such data on pigs are scarce. Here, we apply scRNA-seq technology to study the cellular heterogeneity of 3-month-old pig lungs, generating the single-cell atlas of 13,580 cells covering 16 major cell types. Based on these data, we systematically characterize the similarities and differences in the cellular cross-talk and expression patterns of respiratory virus receptors in each cell type of pig lungs compared with human lungs. Furthermore, we analyze pig lung xenotransplantation barriers and reported the cell-type expression patterns of 10 genes associated with pig-to-human immunobiological incompatibility and coagulation dysregulation. We also investigate the conserved transcription factors (TFs) and their candidate target genes and constructed five conserved TF regulatory networks in the main cell types shared by pig and human lungs. Finally, we present a comprehensive and openly accessible online platform, ScdbLung. Our scRNA-seq atlas of the domestic pig lung and ScdbLung database can guide pig lung research and clinical applicability.

Immunogenetics of xenotransplantation

Xenotransplantation may become the highly desired solution to close the gap between the availability of donated organs and number of patients on the waiting list. In recent years, enormous progress has been made in the development of genetically engineered donor pigs. The introduced genetic modifications showed to be efficient in prolonging xenograft survival. In this review, we focus on the type of immune responses that may target xeno-organs after transplantation and promising immunogenetic modifications that show a beneficial effect in ameliorating or eliminating harmful xenogeneic immune responses. Increasing histocompatibility of xenografts by eliminating genetic discrepancies between species will pave their way into clinical application.

Porcine genome engineering for xenotransplantation

The extreme shortage of human donor organs for treatment of patients with end-stage organ failures is well known. Xenotransplantation, which might provide unlimited organ supply, is a most promising strategy to solve this problem. Domestic pigs are regarded as ideal organ-source animals owing to similarity in anatomy, physiology and organ size to humans as well as high reproductive capacity and low maintenance cost. However, several barriers, which include immune rejection, inflammation and coagulative dysfunctions, as well as the cross-species transmission risk of porcine endogenous retrovirus, blocked the pig-to-human xenotransplantation. With the rapid development of genome engineering technologies and the potent immunosuppressive medications in recent years, these barriers could be eliminated through genetic modification of pig genome together with the administration of effective immunosuppressants. A number of candidate genes involved in the regulation of immune response, inflammation and coagulation have been explored to optimize porcine xenograft survival in non-human primate recipients. PERV inactivation in pigs has also been accomplished to firmly address the safety issue in pig-to-human xenotransplantation. Many encouraging preclinical milestones have been achieved with some organs surviving for years. Therefore, the clinical trials of some promising organs, such as islet, kidney and heart, are aimed to be launched in the near future.

Kidney xenotransplantation: Recent progress in preclinical research

Kidney transplantation is the most effective treatment for end-stage renal disease, but is limited by the increasing shortage of deceased and living human donor kidneys. Xenotransplantation using pig organs provides the possibility to resolve the issue of organ supply shortage and is regarded as the next great medical revolution. In the past five years, there have been sequential advances toward the prolongation of life-supporting pig kidney xenograft survival in non-human primates, with the longest survival being 499 days. This progress is due to the growing availability of pigs with multi-layered genetic modifications to overcome the pathobiological barriers and the application of a costimulation blockade-based immunosuppressive regimen. These encouraging results bring the hope to initiate the clinical trials of pig kidney transplantation in the near future. In this review, we summarized the latest advances regarding pig kidney xenotransplantation in preclinical models to provide a basis for future investigation and potential clinical translation.

Extensive germline genome engineering in pigs

The clinical applicability of porcine xenotransplantation-a long-investigated alternative to the scarce availability of human organs for patients with organ failure-is limited by molecular incompatibilities between the immune systems of pigs and humans as well as by the risk of transmitting porcine endogenous retroviruses (PERVs). We recently showed the production of pigs with genomically inactivated PERVs. Here, using a combination of CRISPR-Cas9 and transposon technologies, we show that pigs with all PERVs inactivated can also be genetically engineered to eliminate three xenoantigens and to express nine human transgenes that enhance the pigs' immunological compatibility and blood-coagulation compatibility with humans. The engineered pigs exhibit normal physiology, fertility and germline transmission of the 13 genes and 42 alleles edited. Using in vitro assays, we show that cells from the engineered pigs are resistant to human humoral rejection, cell-mediated damage and pathogenesis associated with dysregulated coagulation. The extensive genome engineering of pigs for greater compatibility with the human immune system may eventually enable safe and effective porcine xenotransplantation.

Improved production of GTKO/hCD55/hCD59 triple-gene-modified Diannan miniature pigs for xenotransplantation by recloning

Multiple genetic modification is necessary for successful xenotransplantation from pigs. However, multiple-genetically modified cells usually suffer from various drug selections and long-term in vitro culture, which have a poor performance for somatic cell nuclear transfer (SCNT) to produce genetically modified pigs. We used to generate GTKO/hCD55/hCD59 triple-gene modified pigs by using drug-selective cell lines for SCNT, but the majority of cloned pigs were transgenic-negative individuals. In this study, to improve the production efficiency of multiple genetically modified pigs, we performed the recloning process by using transgenic porcine fetal fibroblast cells. As a result, two fetuses expressing hCD55 and hCD59 were obtained from 12 live-cloned fetuses, and one carrying high transgene expression was selected as a source of donor cells for recloning. Then we obtained 12 cloned piglets, all GTKO and carrying hCD55 and hCD59. Both hCD55 and hCD59 were expressed in fibroblast cells, but the expression levels of hCD55 and hCD59 were different among these piglets. Furthermore, piglet P5# had the highest expression of hCD55 and hCD59 in fibroblast cells than other piglets. Correspondingly, fibroblast cells of piglet P5# had significantly higher resistance against human serum-mediated cytolysis than those of piglet P11#. In conclusion, our results firstly provide support for improving efficiency of generating multiple genetically modified pig by recloning.

Xenotransplantation: Current Status in Preclinical Research

The increasing life expectancy of humans has led to a growing numbers of patients with chronic diseases and end-stage organ failure. Transplantation is an effective approach for the treatment of end-stage organ failure; however, the imbalance between organ supply and the demand for human organs is a bottleneck for clinical transplantation. Therefore, xenotransplantation might be a promising alternative approach to bridge the gap between the supply and demand of organs, tissues, and cells; however, immunological barriers are limiting factors in clinical xenotransplantation. Thanks to advances in gene-editing tools and immunosuppressive therapy as well as the prolonged xenograft survival time in pig-to-non-human primate models, clinical xenotransplantation has become more viable. In this review, we focus on the evolution and current status of xenotransplantation research, including our current understanding of the immunological mechanisms involved in xenograft rejection, genetically modified pigs used for xenotransplantation, and progress that has been made in developing pig-to-pig-to-non-human primate models. Three main types of rejection can occur after xenotransplantation, which we discuss in detail: (1) hyperacute xenograft rejection, (2) acute humoral xenograft rejection, and (3) acute cellular rejection. Furthermore, in studies on immunological rejection, genetically modified pigs have been generated to bridge cross-species molecular incompatibilities; in the last decade, most advances made in the field of xenotransplantation have resulted from the production of genetically engineered pigs; accordingly, we summarize the genetically modified pigs that are currently available for xenotransplantation. Next, we summarize the longest survival time of solid organs in preclinical models in recent years, including heart, liver, kidney, and lung xenotransplantation. Overall, we conclude that recent achievements and the accumulation of experience in xenotransplantation mean that the first-in-human clinical trial could be possible in the near future. Furthermore, we hope that xenotransplantation and various approaches will be able to collectively solve the problem of human organ shortage.

Copy-number variation introduced by long transgenes compromises mouse male fertility independently of pachytene checkpoints

Long transgenes are often used in mammalian genetics, e.g., to rescue mutations in large genes. In the course of experiments addressing the genetic basis of hybrid sterility caused by meiotic defects in mice bearing different alleles of Prdm9, we discovered that introduction of copy-number variation (CNV) via two independent insertions of long transgenes containing incomplete Prdm9 decreased testicular weight and epididymal sperm count. Transgenic animals displayed increased occurrence of seminiferous tubules with apoptotic cells at 18 days postpartum (dpp) corresponding to late meiotic prophase I, but not at 21 dpp. We hypothesized that long transgene insertions could cause asynapsis, but the immunocytochemical data revealed that the adult transgenic testes carried a similar percentage of asynaptic pachytene spermatocytes as the controls. These transgenic spermatocytes displayed less crossovers but similar numbers of unrepaired meiotic breaks. Despite slightly increased frequency of metaphase I spermatocytes with univalent chromosome(s) and reduced numbers of metaphase II spermatocytes, cytological studies did not reveal increased apoptosis in tubules containing the metaphase spermatocytes, but found an increased percentage of tubules carrying apoptotic spermatids. Sperm counts of subfertile animals inversely correlated with the transcription levels of the Psmb1 gene encoded within these two transgenes. The effect of the transgenes was dependent on sex and genetic background. Our results imply that the fertility of transgenic hybrid animals is not compromised by the impaired meiotic synapsis of homologous chromosomes, but can be negatively influenced by the increased expression of the introduced genes.

Genome editing in large animals: current status and future prospects

Large animals (non-human primates, livestock and dogs) are playing important roles in biomedical research, and large livestock animals serve as important sources of meat and milk. The recently developed programmable DNA nucleases have revolutionized the generation of gene-modified large animals that are used for biological and biomedical research. In this review, we briefly introduce the recent advances in nuclease-meditated gene editing tools, and we outline these editing tools' applications in human disease modeling, regenerative medicine and agriculture. Additionally, we provide perspectives regarding the challenges and prospects of the new genome editing technology.

Generation of GTKO Diannan Miniature Pig Expressing Human Complementary Regulator Proteins hCD55 and hCD59 via T2A Peptide-Based Bicistronic Vectors and SCNT

Pig-to-human organ transplantation has drawn attention in recent years due to the potential use of pigs as an alternative source of human donor organs. While GGTA1 knockout (GTKO) can protect xenografts from hyperacute rejection, complement-dependent cytotoxicity might still contribute to this type of rejection. To prolong the xenograft survival, we utilized a T2A-mediated pCMV-hCD55-T2A-hCD59-Neo vector and transfected the plasmid into GTKO Diannan miniature pig fetal fibroblasts. After G418 selection combined with single-cell cloning culture, four colonies were obtained, and three of these were successfully transfected with the hCD55 and hCD59. One of the three colonies was selected as donor cells for somatic cell nuclear transfer (SCNT). Then, the reconstructed embryos were transferred into eight recipient gilts, resulting in four pregnancies. Three of the pregnant gilts delivered, yielding six piglets. Only one piglet carried hCD55 and hCD59 genetic modification. The expression levels of the GGTA1, hCD55, and hCD59 in the tissues and fibroblasts of the piglet were determined by q-PCR, fluorescence microscopy, immunohistochemical staining, and western blotting analyses. The results showed the absence of GGTA1 and the coexpression of the hCD55 and hCD59. However, the mRNA expression levels of hCD55 and hCD59 in the GTKO/hCD55/hCD59 pig fibroblasts were lower than that in human 293T cells, which may be caused by low copy number and/or CMV promoter methylation. Furthermore, we performed human complement-mediated cytolysis assays using human serum solutions from 0 to 60%. The result showed that the fibroblasts of this triple-gene modified piglet had greater survival rates than that of wild-type and GTKO controls. Taken together, these results indicate that T2A-mediated polycistronic vector system combined with SCNT can effectively generate multiplex genetically modified pigs, additional hCD55 and hCD59 expression on top of a GTKO genetic background markedly enhance the protective effect towards human serum-mediated cytolysis than those of GTKO alone. Thus, we suggest that GTKO/hCD55/hCD59 triple-gene-modified Diannan miniature pig will be a more eligible donor for xenotransplantation.

Genetically Modified Pigs as Organ Donors for Xenotransplantation

The growing shortage of available organs is a major problem in transplantology. Thus, new and alternative sources of organs need to be found. One promising solution could be xenotransplantation, i.e., the use of animal cells, tissues and organs. The domestic pig is the optimum donor for such transplants. However, xenogeneic transplantation from pigs to humans involves high immune incompatibility and a complex rejection process. The rapid development of genetic engineering techniques enables genome modifications in pigs that reduce the cross-species immune barrier.

Early barriers to neonatal porcine islet engraftment in a dual transplant model

Porcine islet xenografts have the potential to provide an inexhaustible source of islets for β cell replacement. Proof-of-concept has been established in nonhuman primates. However, significant barriers to xenoislet transplantation remain, including the poorly understood instant blood-mediated inflammatory reaction and a thorough understanding of early xeno-specific immune responses. A paucity of data exist comparing xeno-specific immune responses with alloislet (AI) responses in primates. We recently developed a dual islet transplant model, which enables direct histologic comparison of early engraftment immunobiology. In this study, we investigate early immune responses to neonatal porcine islet (NPI) xenografts compared with rhesus islet allografts at 1 hour, 24 hours, and 7 days. Within the first 24 hours after intraportal infusion, we identified greater apoptosis (caspase 3 activity and TUNEL [terminal deoxynucleotidyl transferase dUTP nick end labeling])-positive cells) of NPIs compared with AIs. Macrophage infiltration was significantly greater at 24 hours compared with 1 hour in both NPI (wild-type) and AIs. At 7 days, IgM and macrophages were highly specific for NPIs (α1,3-galactosyltransferase knockout) compared with AIs. These findings demonstrate an augmented macrophage and antibody response toward xenografts compared with allografts. These data may inform future immune or genetic manipulations required to improve xenoislet engraftment.

Strong xenoprotective function by single-copy transgenes placed sequentially at a permissive locus

BACKGROUND

Multiple xenoprotective transgenes are best grouped at a single locus to avoid segregation during breeding and simplify production of donor animals.

METHODS

We used transgene stacking to place a human CD55 transgene adjacent to a human heme oxygenase 1 construct at the porcine ROSA26 locus. A transgenic pig was analyzed by PCR, RT-PCR, droplet digital PCR, immunohistochemistry, immunofluorescence, and flow cytometry. Resistance to complement-mediated cell lysis and caspase 3/7 activation were determined in vitro.

RESULTS

The ROSA26 locus was retargeted efficiently, and animals were generated by nuclear transfer. RNA and protein analyses revealed abundant expression in all organs analyzed, including pancreatic beta cells. Transgenic porcine kidney fibroblasts were almost completely protected against complement-mediated lysis and showed reduced caspase 3/7 activation.

CONCLUSION

Step-by-step placement enables highly expressed single-copy xenoprotective transgenes to be grouped at porcine ROSA26.

Xenotransplantation: back to the future?

The field of xenotransplantation has fluctuated between great optimism and doubts over the last 50 years. The initial clinical attempts were extremely ambitious but faced technical and ethical issues that prompted the research community to go back to preclinical studies. Important players left the field due to perceived xenozoonotic risks and the lack of progress in pig-to-nonhuman-primate transplant models. Initial apparently unsurmountable issues appear now to be possible to overcome due to progress of genetic engineering, allowing the generation of multiple-xenoantigen knockout pigs that express human transgenes and the genomewide inactivation of porcine endogenous retroviruses. These important steps forward were made possible by new genome editing technologies, such as CRISPR/Cas9, allowing researchers to precisely remove or insert genes anywhere in the genome. An additional emerging perspective is the possibility of growing humanized organs in pigs using blastocyst complementation. This article summarizes the current advances in xenotransplantation research in nonhuman primates, and it describes the newly developed genome editing technology tools and interspecific organ generation.

Detection of recombinant human lactoferrin and lysozyme produced in a bitransgenic cow

Lactoferrin and lysozyme are 2 glycoproteins with great antimicrobial activity, being part of the nonspecific defensive system of human milk, though their use in commercial products is difficult because human milk is a limited source. Therefore, many investigations have been carried out to produce those proteins in biological systems, such as bacteria, yeasts, or plants. Mammals seem to be more suitable as expression systems for human proteins, however, especially for those that are glycosylated. In the present study, we developed a bicistronic commercial vector containing a goat β-casein promoter and an internal ribosome entry site fragment between the human lactoferrin and human lysozyme genes to allow the introduction of both genes into bovine adult fibroblasts in a single transfection. Embryos were obtained by somatic cell nuclear transfer, and, after 6 transferences to recipients, 3 pregnancies and 1 viable bitransgenic calf were obtained. The presence of the vector was confirmed by fluorescent in situ hybridization of skin cells. At 13 mo of life and after artificial induction of lactation, both recombinant proteins were found in the colostrum and milk of the bitransgenic calf. Human lactoferrin concentration in the colostrum was 0.0098 mg/mL and that in milk was 0.011 mg/mL; human lysozyme concentration in the colostrum was 0.0022 mg/mL and that in milk was 0.0024 mg/mL. The molar concentration of both human proteins revealed no differences in protein production of the internal ribosome entry site upstream and downstream protein. The enzymatic activity of lysozyme in the transgenic milk was comparable to that of human milk, being 6 and 10 times higher than that of bovine lysozyme present in milk. This work represents an important step to obtain multiple proteins or enhance single protein production by using animal pharming and fewer regulatory and antibiotic-resistant foreign sequences, allowing the design of humanized milk with added biological value for newborn nutrition and development. Transgenic animals can offer a unique opportunity to the dairy industry, providing starting materials suitable to develop specific products with high added value.

Efficient generation of B2m-null pigs via injection of zygote with TALENs

Donor major histocompatibility complex class I (MHC I) molecules are the main targets of the host immune response after organ allotransplantation. Whether and how MHC I-deficiency of pig donor tissues affects rejection after xenotransplantation has not been assessed. Beta2-microglobulin (B2M) is indispensable for the assembly of MHC I receptors and therefore provides an effective target to disrupt cell surface MHC I expression. Here, we report the one-step generation of mutant pigs with targeted disruptions in B2m by injection of porcine zygotes with B2m exon 2-specific TALENs. After germline transmission of mutant B2m alleles, we obtained F1 pigs with biallelic B2m frameshift mutations. F1 pigs lacked detectable B2M expression in tissues derived from the three germ layers, and their lymphocytes were devoid of MHC I surface receptors. Skin grafts from B2M deficient pigs exhibited remarkably prolonged survival on xenogeneic wounds compared to tissues of non-mutant littermates. Mutant founder pigs with bi-allelic disruption in B2m and B2M deficient F1 offspring did not display visible abnormalities, suggesting that pigs are tolerant to B2M deficiency. In summary, we show the efficient generation of pigs with germline mutations in B2m, and demonstrate a beneficial effect of donor MHC I-deficiency on xenotransplantation.

Efficient production of multi-modified pigs for xenotransplantation by 'combineering', gene stacking and gene editing

Xenotransplantation from pigs could alleviate the shortage of human tissues and organs for transplantation. Means have been identified to overcome hyperacute rejection and acute vascular rejection mechanisms mounted by the recipient. The challenge is to combine multiple genetic modifications to enable normal animal breeding and meet the demand for transplants. We used two methods to colocate xenoprotective transgenes at one locus, sequential targeted transgene placement - ‘gene stacking’, and cointegration of multiple engineered large vectors - ‘combineering’, to generate pigs carrying modifications considered necessary to inhibit short to mid-term xenograft rejection. Pigs were generated by serial nuclear transfer and analysed at intermediate stages. Human complement inhibitors CD46, CD55 and CD59 were abundantly expressed in all tissues examined, human HO1 and human A20 were widely expressed. ZFN or CRISPR/Cas9 mediated homozygous GGTA1 and CMAH knockout abolished α-Gal and Neu5Gc epitopes. Cells from multi-transgenic piglets showed complete protection against human complement-mediated lysis, even before GGTA1 knockout. Blockade of endothelial activation reduced TNFα-induced E-selectin expression, IFNγ-induced MHC class-II upregulation and TNFα/cycloheximide caspase induction. Microbial analysis found no PERV-C, PCMV or 13 other infectious agents. These animals are a major advance towards clinical porcine xenotransplantation and demonstrate that livestock engineering has come of age.

Dual islet transplantation modeling of the instant blood-mediated inflammatory reaction

Islet xenotransplantation is a potential treatment for diabetes without the limitations of tissue availability. Although successful experimentally, early islet loss remains substantial and attributed to an instant blood-mediated inflammatory reaction (IBMIR). This syndrome of islet destruction has been incompletely defined and characterization in pig-to-primate models has been hampered by logistical and statistical limitations of large animal studies. To further investigate IBMIR, we developed a novel in vivo dual islet transplant model to precisely characterize IBMIR as proof-of-concept that this model can serve to properly control experiments comparing modified xenoislet preparations. WT and α1,3-galactosyltransferase knockout (GTKO) neonatal porcine islets were studied in nonimmunosuppressed rhesus macaques. Inert polyethylene microspheres served as a control for the effects of portal embolization. Digital analysis of immunohistochemistry targeting IBMIR mediators was performed at 1 and 24 h after intraportal islet infusion. Early findings observed in transplanted islets include complement and antibody deposition, and infiltration by neutrophils, macrophages and platelets. Insulin, complement, antibody, neutrophils, macrophages and platelets were similar between GTKO and WT islets, with increasing macrophage infiltration at 24 h in both phenotypes. This model provides an objective and internally controlled study of distinct islet preparations and documents the temporal histology of IBMIR.

Xenotransplantation: immunological hurdles and progress toward tolerance

The discrepancy between organ need and organ availability represents one of the major limitations in the field of transplantation. One possible solution to this problem is xenotransplantation. Research in this field has identified several obstacles that have so far prevented the successful development of clinical xenotransplantation protocols. The main immunologic barriers include strong T-cell and B-cell responses to solid organ and cellular xenografts. In addition, components of the innate immune system can mediate xenograft rejection. Here, we review these immunologic and physiologic barriers and describe some of the strategies that we and others have developed to overcome them. We also describe the development of two strategies to induce tolerance across the xenogeneic barrier, namely thymus transplantation and mixed chimerism, from their inception in rodent models through their current progress in preclinical large animal models. We believe that the addition of further beneficial transgenes to Gal knockout swine, combined with new therapies such as Treg administration, will allow for successful clinical application of xenotransplantation.

Immune modulation in xenotransplantation

The use of animals as donors of tissues and organs for xenotransplantations may help in meeting the increasing demand for organs for human transplantations. Clinical studies indicate that the domestic pig best satisfies the criteria of organ suitability for xenotransplantation. However, the considerable phylogenetic distance between humans and the pig causes tremendous immunological problems after transplantation, thus genetic modifications need to be introduced to the porcine genome, with the aim of reducing xenotransplant immunogenicity. Advances in genetic engineering have facilitated the incorporation of human genes regulating the complement into the porcine genome, knockout of the gene encoding the formation of the Gal antigen (α1,3-galactosyltransferase) or modification of surface proteins in donor cells. The next step is two-fold. Firstly, to inhibit processes of cell-mediated xenograft rejection, involving natural killer cells and macrophages. Secondly, to inhibit rejection caused by the incompatibility of proteins participating in the regulation of the coagulation system, which leads to a disruption of the equilibrium in pro- and anti-coagulant activity. Only a simultaneous incorporation of several gene constructs will make it possible to produce multitransgenic animals whose organs, when transplanted to human recipients, would be resistant to hyperacute and delayed xenograft rejection.

Expression analysis of combinatorial genes using a bi-cistronic T2A expression system in porcine fibroblasts

In pig-to-primate xenotransplantation, multiple transgenic pigs are required to overcome a series of transplant rejections. The generation of multiple transgenic pigs either by breeding or the introduction of several mono-cistronic vectors has been hampered by the differential expression patterns of the target genes. To achieve simultaneous expression of multiple genes, a poly-cistronic expression system using the 2A peptide derived from the Thosea asigna virus (T2A) can be considered an alternative choice. Before applying T2A expression system to pig generation, the expression patterns of multiple genes in this system should be precisely evaluated. In this study, we constructed several bi-cistronic T2A expression vectors, which combine target genes that are frequently used in the xenotransplantation field, and introduced them into porcine fibroblasts. The proteins targeted to the same or different subcellular regions were efficiently expressed without affecting the localization or expression levels of the other protein. However, when a gene with low expression efficiency was inserted into the upstream region of the T2A sequences, the expression level of the downstream gene was significantly decreased compared with the expression efficiency without the insertion. A small interfering RNA targeting one gene in this system resulted in the significant downregulation of both the target gene and the other gene, indicating that multiple genes combined into a T2A expression vector can be considered as a single gene in terms of transcription and translation. In summary, the efficient expression of a downstream gene can be achieved if the expression of the upstream gene is efficient.

Production of multiple transgenic Yucatan miniature pigs expressing human complement regulatory factors, human CD55, CD59, and H-transferase genes

The present study was conducted to generate transgenic pigs coexpressing human CD55, CD59, and H-transferase (HT) using an IRES-mediated polycistronic vector. The study focused on hyperacute rejection (HAR) when considering clinical xenotransplantation as an alternative source for human organ transplants. In total, 35 transgenic cloned piglets were produced by somatic cell nuclear transfer (SCNT) and were confirmed for genomic integration of the transgenes from umbilical cord samples by PCR analysis. Eighteen swine umbilical vein endothelial cells (SUVEC) were isolated from umbilical cord veins freshly obtained from the piglets. We observed a higher expression of transgenes in the transgenic SUVEC (Tg SUVEC) compared with the human umbilical vein endothelial cells (HUVEC). Among these genes, HT and hCD59 were expressed at a higher level in the tested Tg organs compared with non-Tg control organs, but there was no difference in hCD55 expression between them. The transgenes in various organs of the Tg clones revealed organ-specific and spatial expression patterns. Using from 0 to 50% human serum solutions, we performed human complement-mediated cytolysis assays. The results showed that, overall, the Tg SUVEC tested had greater survival rates than did the non-Tg SUVEC, and the Tg SUVEC with higher HT expression levels tended to have more down-regulated α-Gal epitope expression, resulting in greater protection against cytotoxicity. By contrast, several Tg SUVEC with low CD55 expression exhibited a decreased resistance response to cytolysis. These results indicated that the levels of HT expression were inversely correlated with the levels of α-Gal epitope expression and that the combined expression of hCD55, hCD59, and HT proteins in SUVECs markedly enhances a protective response to human serum-mediated cytolysis. Taken together, these results suggest that combining a polycistronic vector system with SCNT methods provides a fast and efficient alternative for the generation of transgenic large animals with multiple genetic modifications.

CD4 T cells mediate cardiac xenograft rejection via host MHC Class II

BACKGROUND

Previous studies have shown that acute CD4 T-cell-mediated cardiac allograft rejection requires donor major histocompatibility complex (MHC) Class II expression and can be independent of "indirect" antigen presentation. However, other studies suggested that indirect antigen presentation to CD4 T cells may play a primary role in cellular xenograft immunity. Thus, the relative roles of direct/indirect CD4 T cell reactivity against cardiac xenografts are unclear. In this study we set out to determine the role for indirect CD4 T cell reactivity in cardiac xenograft rejection.

METHODS

Rat hearts were transplanted heterotopically into wild-type and immunodeficient mice. Recipients were untreated, treated with depleting antibodies, or reconstituted with wild-type cells.

RESULTS

Antibody depletion confirmed that rat heart xenograft rejection in C57Bl/6 mice was CD4 T-cell-dependent. Also, heart xenografts survived long term in B6 MHC Class II (C2D)-deficient mice. Graft acceptance in C2D mice was not secondary to CD4 T cell deficiency alone, because transferred B6 CD4 T cells failed to trigger rejection in C2D hosts. Furthermore, purified CD4 T cells were sufficient for acute rejection of rat heart xenografts in immune-deficient B6rag1(-/-) recipients. Importantly, CD4 T cells did not reject rat hearts in C2Drag1(-/-) hosts, in contrast to results using cardiac allografts. "Direct" xenoreactive CD4 T cells were not sufficient to mediate rejection despite vigorous reactivity to rat stimulator cells in vitro.

CONCLUSIONS

Taken together, our results show that CD4 T cells are both necessary and sufficient for acute cardiac xenograft rejection and that host MHC Class II is critical in this process.

Genetically modified pigs for biomedical research

During the last two decades, pigs have been used to develop some of the most important large animal models for biomedical research. Advances in pig genome research, genetic modification (GM) of primary pig cells and pig cloning by nuclear transfer, have facilitated the generation of GM pigs for xenotransplantation and various human diseases. This review summarizes the key technologies used for generating GM pigs, including pronuclear microinjection, sperm-mediated gene transfer, somatic cell nuclear transfer by traditional cloning, and somatic cell nuclear transfer by handmade cloning. Broadly used genetic engineering tools for porcine cells are also discussed. We also summarize the GM pig models that have been generated for xenotransplantation and human disease processes, including neurodegenerative diseases, cardiovascular diseases, eye diseases, bone diseases, cancers and epidermal skin diseases, diabetes mellitus, cystic fibrosis, and inherited metabolic diseases. Thus, this review provides an overview of the progress in GM pig research over the last two decades and perspectives for future development.

Infection barriers to successful xenotransplantation focusing on porcine endogenous retroviruses

Xenotransplantation may be a solution to overcome the shortage of organs for the treatment of patients with organ failure, but it may be associated with the transmission of porcine microorganisms and the development of xenozoonoses. Whereas most microorganisms may be eliminated by pathogen-free breeding of the donor animals, porcine endogenous retroviruses (PERVs) cannot be eliminated, since these are integrated into the genomes of all pigs. Human-tropic PERV-A and -B are present in all pigs and are able to infect human cells. Infection of ecotropic PERV-C is limited to pig cells. PERVs may adapt to host cells by varying the number of LTR-binding transcription factor binding sites. Like all retroviruses, they may induce tumors and/or immunodeficiencies. To date, all experimental, preclinical, and clinical xenotransplantations using pig cells, tissues, and organs have not shown transmission of PERV. Highly sensitive and specific methods have been developed to analyze the PERV status of donor pigs and to monitor recipients for PERV infection. Strategies have been developed to prevent PERV transmission, including selection of PERV-C-negative, low-producer pigs, generation of an effective vaccine, selection of effective antiretrovirals, and generation of animals transgenic for a PERV-specific short hairpin RNA inhibiting PERV expression by RNA interference.

Protective effects of different combinations of human MCP, DAF, and CD59 on complement-dependent cytolysis in NIH 3T3 cells

OBJECTIVES

To analyze the protective effects against complement-mediated cytolysis of the MCP, DAF, and CD59 human complement regulatory proteins, alone and in combination, on NIH 3T3 mouse fibroblast cells.

MATERIALS AND METHODS

We constructed 3 double and 3 single-human complement regulatory protein plasmids (pIRES-hMCP-hDAF, pIRES-hMCP-hCD59, pIRES-hDAF-hCD59, pIRES-A-hMCP, pIRES-B-hDAF, and pIRES-B-hCD59). The plasmids were transfected into NIH 3T3 cells, and stable transfectants were obtained by treatment with 200 kg/m3 G418 for 2 weeks. Normal human serum (50%) as a source of complement was added to the culture medium of stable transfectants. The 3-(4,5-dimethylthiazol-2-yl)- 2,5-diphenyltetrazolium bromide assay was used to analyze the protective ability of different human complement regulatory protein plasmids on complement-dependent cytolysis.

RESULTS

The viability of double-human complement regulatory protein stable transfectants was significantly higher than that of single-human complement regulatory protein stable transfectants (P < .05). Among the double-transfectants, cells expressing pIRES-hMCP-hDAF and pIRES-hMCPhCD59 survived better than cells expressing pIREShDAF- hCD59 (91.75% ± 3.30% and 84.88% ± 2.36% vs 66.19% ± 6.52%; P < .05). Among the single transfectants, cells expressing pIRES-A-hMCP or pIRES-B-hDAF survived better than cells expressing pIRES-B-hCD59 or pIRES empty vector (53.76% ± 3.84% and 56.32% ± 2.83% vs 43.28% ± 0.96% and 40.27% ± 1.11%; P < .05).

CONCLUSIONS

These results suggest that the MCP+DAF and MCP+CD59 combinations could be more effective than DAF+CD59 in protecting the NIH 3T3 cells from injury caused by complement-dependent cytolysis, whereas MCP or DAF alone is stronger than CD59 alone in inhibiting membrane attack complex formation.

Current status of immunomodulatory and cellular therapies in preclinical and clinical islet transplantation

Clinical islet transplantation is a β-cell replacement strategy that represents a possible definitive intervention for patients with type 1 diabetes, offering substantial benefits in terms of lowering daily insulin requirements and reducing incidences of debilitating hypoglycemic episodes and unawareness. Despite impressive advances in this field, a limiting supply of islets, inadequate means for preventing islet rejection, and the deleterious diabetogenic and nephrotoxic side effects associated with chronic immunosuppressive therapy preclude its wide-spread applicability. Islet transplantation however allows a window of opportunity for attempting various therapeutic manipulations of islets prior to transplantation aimed at achieving superior transplant outcomes. In this paper, we will focus on the current status of various immunosuppressive and cellular therapies that promote graft function and survival in preclinical and clinical islet transplantation with special emphasis on the tolerance-inducing capacity of regulatory T cells as well as the β-cells regenerative capacity of stem cells.

Use of the 2A peptide for generation of multi-transgenic pigs through a single round of nuclear transfer

Multiple genetic modifications in pigs can essentially benefit research on agriculture, human disease and xenotransplantation. Most multi-transgenic pigs have been produced by complex and time-consuming breeding programs using multiple single-transgenic pigs. This study explored the feasibility of producing multi-transgenic pigs using the viral 2A peptide in the light of previous research indicating that it can be utilized for multi-gene transfer in gene therapy and somatic cell reprogramming. A 2A peptide-based double-promoter expression vector that mediated the expression of four fluorescent proteins was constructed and transfected into primary porcine fetal fibroblasts. Cell colonies (54.3%) formed under G418 selection co-expressed the four fluorescent proteins at uniformly high levels. The reconstructed embryos, which were obtained by somatic cell nuclear transfer and confirmed to express the four fluorescent proteins evenly, were transplanted into seven recipient gilts. Eleven piglets were delivered by two gilts, and seven of them co-expressed the four fluorescent proteins at equivalently high levels in various tissues. The fluorescence intensities were directly observed at the nose, hoof and tongue using goggles. The results suggest that the strategy of combining the 2A peptide and double promoters efficiently mediates the co-expression of the four fluorescent proteins in pigs and is hence a promising methodology to generate multi-transgenic pigs by a single nuclear transfer.

Design of a real-time quantitative polymerase chain reaction to assess human complement regulatory protein gene expression in polytransgenic xenograft pigs

OBJECTIVE

To design a real-time quantitative polymerase chain reaction (q-PCR) to assess gene expression for hCD55, hCD59, and hCD46 in polytransgenic (PT) pigs used as xenograft donors for orthotopic liver xenotransplantation using a pig-to-baboon model.

MATERIALS AND METHODS

Three pairs of primers were designed using PrimerBlast and mRNA of hCD55, hCD59, and hCD46 sequences. Blood samples from five PT pigs (two males and three females) were used to isolated peripheral blood mononuclear cells (PBMCs) by means of Ficoll gradients. After DNAase digestion of isolated mRNA, we synthesized cDNA. Using SYBR-Green chemistry of q-PCR, we constructed a standard curve. Two wild-type (WT) pigs were used as negative controls, and PBMCs from two healthy human volunteers as positive controls. The amplicon length was assessed by means of agarose gel electrophoresis and PCR products, sequenced.

RESULTS

We observed amplification for hCD55, hCD59, and hCD46 in all samples from the five PT pigs except for hCD55 and hCD46 in one male PT pig. Neither the human samples nor the negative controls showed amplification. The expected amplicon length was confirmed; sequencing showed high homology with human mRNA for the three proteins and no match with any known pig sequence.

CONCLUSIONS

The q-PCR allowed detection of animals with the highest gene expression for hCD55, hCD59, and hCD46 for xenograft donors in transplantation experiments.

Reduction of N-glycolylneuraminic acid xenoantigen on human adipose tissue-derived stromal cells/mesenchymal stem cells leads to safer and more useful cell sources for various stem cell therapies

Adipose tissue is an attractive source for somatic stem cell therapy. Currently, human adipose tissue-derived stromal cells/mesenchymal stem cells (hADSCs/MSCs) are cultured with fetal bovine serum (FBS). Recently, however, not only human embryonic stem cell lines cultured on mouse feeder cells but also bone marrow-derived human MSCs cultured with FBS were reported to express N-glycolylneuraminic acid (Neu5Gc) xenoantigen. Human serum contains high titers of natural preformed antibodies against Neu5Gc. We studied the presence of Neu5Gc on hADSCs/MSCs cultured with FBS and human immune response mediated by Neu5Gc. Our data indicated that hADSCs/MSCs cultured with FBS expressed Neu5Gc and that human natural preformed antibodies could bind to hADSCs/MSCs. However, hADSCs/MSCs express complement regulatory proteins such as CD46, CD55, and CD59 and are largely resistant to complement-mediated cytotoxicity. hADSCs/MSCs cultured with FBS could be injured by antibody-dependent cell-mediated cytotoxicity mechanism. Further, human monocyte-derived macrophages could phagocytose hADSCs/MSCs cultured with FBS and this phagocytic activity was increased in the presence of human serum. Culturing hADSCs/MSCs with heat-inactivated human serum for a week could markedly reduce Neu5Gc on hADSCs/MSCs and prevent immune responses mediated by Neu5Gc, such as binding of human natural preformed antibodies, antibody-dependent cell-mediated cytotoxicity, and phagocytosis. Adipogenic and osteogenic differentiation potentials of hADSCs/MSCs cultured with heat-inactivated human serum were not less than that of those cultured with FBS. For stem cell therapies based on hADSCs/MSCs, hADSCs/MSCs that presented Neu5Gc on their cell surfaces after exposure to FBS should be cleaned up to be rescued from xenogeneic rejection.

Transgenic pigs for xenotransplantation: selection of promoter sequences for reliable transgene expression

PURPOSE OF REVIEW

Appropriate expression of immunomodulatory and anticoagulant proteins on endothelial cells is essential to prevent rejection of vascularized porcine organs after transplantation into primates. Here, we review the promoter sequences used for the establishment of transgenic pigs, as organ donors for xenotransplantation.

RECENT FINDINGS

Transgenic pigs were produced using viral, chicken, mouse, human, and porcine promoter sequences with ubiquitous or cell type-specific activity. In addition to the expression of human complement regulatory proteins, which were efficient to prevent hyperacute rejection of pig-to-primate xenografts, novel transgenes, targeting cellular rejection mechanisms, abnormal-blood coagulation, or the risk of viral transmission, have been published or announced in preliminary reports.

SUMMARY

Accurate spatiotemporal expression of immunomodulatory and anticoagulant proteins on the endothelial cells of transgenic pigs is required for the successful xenotransplantation of vascularized organs into primates. Targeting transgene expression specifically to the cells critical for xenograft rejection may eliminate potential side effects of ubiquitous expression. Comparison of regulatory sequences from various species indicates that carefully selected porcine promoter sequences may be beneficial to achieve this aim.

Cell-based therapeutic approaches for Parkinson's disease: progress and perspectives

Motor dysfunctions in Parkinson's disease are believed to be primarily due to the degeneration of dopaminergic neurons located in the substantia nigra pars compacta. Because a single-type cell population is depleted, Parkinson's disease is considered a primary target for cell replacement-based therapeutic strategies. Extensive studies have confirmed transplantation of donor neurons could be beneficial, yet identifying an alternative cell source is clearly essential. Human embryonic stem cells (hESCs) have been proposed as a renewable source of dopaminergic neurons for transplantation in Parkinson's disease; other potential sources could include neural stem cells (hNSCs) and adult mesenchymal stem cells (hMSCs). However, numerous difficulties avert practical application of stem cell-based therapeutic approaches for the treatment of Parkinson's disease. Among the latter, ethical, safety (including xeno- and tumor formation-associated risks) and technical issues stand out. This review aims to provide a balanced and updated outlook on various issues associated with stem cells in regard to their potential in the treatment of Parkinson's disease. Essential features of the individual stem cell subtypes, principles of available differentiation protocols, transplantation, and safety issues are discussed extensively.

Synergistic effects of alpha-1,2-fucosyltransferase, DAF, and CD59 in suppression of xenogenic immunological responses

BACKGROUND

Previous studies showed that alpha-1,2-fucosyltransferase (HT), decay accelerating factor (DAF), and CD59 have an inhibitory effect on the immunological rejection of xenogenic transplantation.

METHODS

To investigate their possible synergistic effects in suppression of heterogeneic transplantation, we produced transgenic mouse lines expressing human HT, DAF, and/or CD59 by the standard pronuclear injection approach. PCR and Southern blot were used to identify the transgenic founder lines. Flow cytometry confirmed the high-level expression of HT, DAF, or CD59 in the transgenic mice.

RESULTS

The deposition of IgM, C3c, or C9 in the cardiac vascular endothelial cells of the HT, HT/CD59, and/or DAF multiple positive transgenic mice was markedly decreased. The survival time and function of the hearts of the co-transgenic mice were significantly longer and higher than that of the single HT-positive transgenic mice (P < 0.05).

CONCLUSION

The mice co-expressing HT/DAF or HT/CD59 could resist the hyperacute rejection better than those expressing HT alone. It is feasible to use HT and C-reactive proteins co-transgenic tissues to resist hyperacute rejection and xenograft rejection.

Transgenic mice designed to express human alpha-1,2-fucosyltransferase in combination of human DAF and CD59 to avoid xenograft rejection

The expression of human alpha-1,2-fucosyltransferase (HT) or complement regulatory proteins has been proved as an strategy to overcome hypercute rejection in discordant xenogeneic organ transplantation. In this study, we examined whether peripheral blood mononuclear cells (PBMCs) from polytransgenic mice expressing the human HT, and complement regulatory proteins (DAF and CD59), can provide more effective protection against xenograft rejection. Transgenic mice were produced by co-injection of gene constructs for human HT, DAF and/or CD59. Flow Cytometry (FCM) was used to screen the positive transgenic mice. PBMCs from transgenic mice were incubated with 15% human serum to evaluate natural antibody binding, complement activation and expression of adhesion molecules. Three transgenes were strongly expressed in PBMCs of transgenic mice, and HT expression significantly reduced expression of the major xenoepitope galactose-alpha-1,3-galactose (alpha-Gal). Functional studies with PBMCs showed that co-expression of HT and DAF or CD59 markedly increased their resistance to human serum-mediated cytolysis when compared with single transgenic PBMCs. Moreover, the combined expression of triple transgenes in PBMCs led to the greatest protection against human serum-mediated cytolysis, avoided hyperacute rejection and reduced expression of adhesion molecules. Strong co-expression of triple transgenes was completely protected from xenograft hyperacute rejection and partially inhibited acute vascular rejection. The studies suggest that engineering mice to express triple molecules represents an critical step toward prolonging xenograft survival and might be more suitable for xenotransplantation.

Growth characteristics of human adenoviruses on porcine cell lines

Human adenoviruses (hAdV) have been recognized as a highly prevalent virus family causing severe disease in immunocompromised patients. In xenotransplantation the xenograft therefore will be exposed to these viruses, which in case of its infection might contribute to posttransplant complications. To evaluate the susceptibility of porcine cells for hAdV, we infected the porcine cell line POEK with seven serotypes representing all six hAdV species. Additionally, a second porcine cell line (ST) was infected with two serotypes. Viral replication of serotypes varied: porcine cells were fully permissive for serotypes 1, 4 and 17, semi-permissive for 11 and 21, and non-permissive for 31 and 40. Furthermore, we demonstrated the interaction of serotype 1 with the porcine homologue of the coxsackie-adenovirus receptor, the receptor used by many hAdV serotypes for cell attachment. Thus, various adenovirus types of different hAdV species may be capable of infecting different porcine tissue types.