Assembling multiple xenoprotective transgenes in pigs

Comparison of Multiple Strategies for Precision Transgene Knock-In in Gallus gallus Genome via Microhomology-Mediated End Joining

Precision exogenous gene knock-in is an attractive field for transgenic Gallus gallus (chicken) generation. In this article, we constructed multiple Precise Integration into Target Chromosome (PITCh) plasmid systems mediated by microhomology-mediated end-joining (MMEJ) for large-fragment integration in DF-1 cells and further assess the possibility of GAPDH (glyceraldehyde-3-phosphate dehydrogenase) as a genomic safe harbor for chickens. We designed three targeted sgRNAs for the all-in-one plasmid at the 3'UTR of GAPDH near the stop codon. The donor-plasmid-carrying microhomology arms correspond to sgRNA and EGFP fragments in the forward and reverse directions. MMEJ-mediated EGFP insertion can be efficiently expressed in DF-1 cells. Moreover, the differences between the forward and reverse fragments indicated that promoter interference does affect the transfection efficiency of plasmids and cell proliferation. The comparison of the 20 bp and 40 bp microhomology arms declared that the short one has higher knock-in efficiency. Even though all three different transgene insertion sites in GAPDH could be used to integrate the foreign gene, we noticed that the G2-20R-EGFP cell reduced the expression of GAPDH, and the G3-20R-EGFP cell exhibited significant growth retardation. Taken together, G1, located at the 3'UTR of GAPDH on the outer side of the last base of the terminator, can be a candidate genomic safe harbor (GSH) loci for the chicken genome. In addition, deleted-in-azoospermia-like (DAZL) and actin beta (ACTB) site-specific gene knock-in indicated that MMEJ has broad applicability and high-precision knock-in efficiency for genetically engineered chickens.

Transitioning of renal transplant pathology from allograft to xenograft and tissue engineering pathology: Are we prepared?

Currently, the most feasible and widely practiced option for patients with end-stage organ failure is the transplantation of part of or whole organs, either from deceased or living donors. However, organ shortage has posed and is still posing a big challenge in this field. Newer options being explored are xenografts and engineered/bioengineered tissues/organs. Already small steps have been taken in this direction and sooner or later, these will become a norm in this field. However, these developments will pose different challenges for the diagnosis and management of problems as compared with traditional allografts. The approach to pathologic diagnosis of dysfunction in these settings will likely be significantly different. Thus, there is a need to increase awareness and prepare transplant diagnosticians to meet this future challenge in the field of xenotransplantation/ regenerative medicine. This review will focus on the current status of transplant pathology and how it will be changed in the future with the emerging scenario of routine xenotransplantation.

Xenotransplantation becoming reality

To bridge the gap between organ demand and supply, xenotransplantation has long been considered as a realistic option for end-stage organ failure. Early this year this promise became reality for David Bennett Sr., the first patient whose own failing heart was replaced with a xeno-pig heart. To get here has been a rollercoaster ride of physiological hurdles seemingly impossible to overcome, technological breakthroughs and ethical and safety concerns. It started in 1984, with Stephanie Fae Beauclair, also known as baby Fae, receiving a baboon heart, which allowed her to survive for another 30 days. For ethical reasons primate work was soon abandoned in favour of the pig. But increased phylogenetic distance also brought with it an increased immunological incompatibility. It has been the development of ever more sophisticated genetic engineering tools, which brought down the physiological barriers, enabled humanisation of porcine organs and helped addressing safety concerns. This renewed the confidence in xenotransplantation, brought new funding opportunities and resulted finally in the first in human trial.

The advancements, challenges, and future implications of the CRISPR/Cas9 system in swine research

Clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (CRISPR/Cas9) genome editing technology has dramatically influenced swine research by enabling the production of high-quality disease-resistant pig breeds, thus improving yields. In addition, CRISPR/Cas9 has been used extensively in pigs as one of the tools in biomedical research. In this review, we present the advancements of the CRISPR/Cas9 system in swine research, such as animal breeding, vaccine development, xenotransplantation, and disease modeling. We also highlight the current challenges and some potential applications of the CRISPR/Cas9 technologies.

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.

Cytokine-inducible promoters to drive dynamic transgene expression: The "Smart Graft" strategy

BACKGROUND

Ubiquitous expression of T-cell regulatory transgenes such as the cytotoxic T lymphocyte-associated antigen 4 (CTLA4) or the high-affinity variant LEA29Y improves xeno graft survival. Such donor pigs are however immunocompromised and susceptible to infection. Continous high expression of CTLA4 or LEA29Y in the graft could also compromise the health status of recipients. The novel "Smart Graft" strategy is likely to avoid these problems by controlling the expression of T-cell regulatory transgenes as and when required.

METHODS

Candidate promoters inducible by inflammatory cytokines were identified by in silico screening for potential NF-κB binding sites. Basal promoter levels and responsiveness to TNFα and IL1ß were quantified by expression of secreted embryonic alkaline phosphatase in cultured cells. Promoters were modified to increase responsiveness by removing regulatory elements or adding SP-1 or NF-κB binding sites and again tested in vitro. The most promising promoters were then assessed in vivo. Porcine cells expressing inducible Renilla luciferase constructs were transplanted into immunodeficient NOD-Scid-IL2 receptor gamma^null (NSG) mice. Following engraftment, the recipient's immune system was reconstituted by splenocyte transfer raising an immune response to the porcine xenograft. The resulting induction of promoter activity was detected by in vivo bioimaging.

RESULTS

Three human (hTNFAIP1, hVCAM1 and hCCL2), and one porcine promoter (pA20) were chosen for in vitro tests. In all experiments, the semi-synthetic and inducible ELAM promoter as well as the CAG promoter were used as references. In contrast to hTNFAIP1 and hVCAM1 the ELAM, hCCL2 and pA20 promoters showed significant induction after cytokine challenge. The hCCL2 and pA20 promoters were further optimized, resulting in increased responsiveness to TNFα and IL1ß. Cytokine-dependent upregulation of promoter activity was tested in vivo, where the ELAM and the optimized hCCL2 promoters showed a 2-fold upregulation, while one of the improved A20 promoters showed almost 10-fold upregulation. Our results also revealed more than 4-fold cytokine inducibility of the CAG promoter.

CONCLUSION

This is the first in vivo comparison of existing and newly designed cytokine-inducible promoters. Optimization of promoter structure resulted in almost 10-fold inducibility of promoter activity. Such a rapid and dynamically regulated response to inflammation and cell damage could reduce initial graft rejection, making the "Smart Graft" approach a useful means of modulating the expression of immune regulatory transgenes to avoid deleterious effects on porcine and human health. Expressing transgenes in this fashion could provide a safer organ for transplantation.

The potential role of 3D-bioprinting in xenotransplantation

PURPOSE OF REVIEW

To review the impact of a new technology, 3D-bioprinting, in xenotransplantation research.

RECENT FINDINGS

Genetically engineered pigs, beginning with human (h) CD55-transgenic and Gal-knockout pigs, have improved the outcomes of xenotransplantation research. Today, there are more than 30 different genetically engineered pigs either expressing human gene(s) or lacking pig gene(s). CRIPSR/cas9 technology has facilitated the production of multigene pigs (up to nine genes in a single pig), which lack multiple pig xenoantigens, and express human transgenes, such as hCD46, hCD55, hThrombomodulin, hCD39, etc. Although recent studies in nonhuman primates (NHPs) have demonstrated prolonged survival after life-supporting pig kidney, heart, and islet xenotransplantation, researchers have difficulty determining the best genetic combination to test in NHPs because of a potential greater than 100 000 genetic combinations. 3D-bioprinting of genetically engineered pig cells: is superior to 2D in-vitro testing, enables organ-specific testing, helps to understand differences in immunogenicity between organs, and is faster and cheaper than testing in NHPs. Moreover, 3D-bioprinted cells can be continuously perfused in a bioreactor, controlling for all variables, except the studied variable.

SUMMARY

3D-bioprinting can help in the study of the impact of specific genes (human or pig) in xenotransplantation in a rapid, inexpensive, and reliable way.

Reproductive technologies in swine

This chapter highlights the importance of reproductive technologies that are applied to porcine breeds. Nowadays the porcine industry, part of a high technological and specialized sector, offers high-quality protein food. The development of the swine industry is founded in the development of breeding/genetics, nutrition, animal husbandry, and animal health. The implementation of reproductive technologies in swine has conducted to levels of productivity never reached before. In addition, the pig is becoming an important species for biomedicine. The generation of pig models for human disease, xenotransplantation, or production of therapeutic proteins for human medicine has in fact generated a growing field of interest.

Importance of the Major Histocompatibility Complex (Swine Leukocyte Antigen) in Swine Health and Biomedical Research

In pigs, the major histocompatibility complex (MHC), or swine leukocyte antigen (SLA) complex, maps to Sus scrofa chromosome 7. It consists of three regions, the class I and class III regions mapping to 7p1.1 and the class II region mapping to 7q1.1. The swine MHC is divided by the centromere, which is unique among mammals studied to date. The SLA complexspans between 2.4 and 2.7 Mb, depending on haplotype, and encodes approximately 150 loci, with at least 120 genes predicted to be functional. Here we update the whole SLA complex based on the Sscrofa11.1 build and annotate the organization for all recognized SLA genes and their allelic sequences. We present SLA nomenclature and typing methods and discuss the expression of SLA proteins, as well as their role in antigen presentation and immune, disease, and vaccine responses. Finally, we explore the role of SLA genes in transplantation and xenotransplantation and their importance in swine biomedical models.

Immortalization of porcine hepatocytes with a α-1,3-galactosyltransferase knockout background

BACKGROUND

In vivo pig liver xenotransplantation preclinical trials appear to have poor efficiency compared to heart or kidney xenotransplantation because of xenogeneic rejection, including coagulopathy, and particularly thrombocytopenia. In contrast, ex vivo pig liver (wild type) perfusion systems have been proven to be effective in "bridging" liver failure patients until subsequent liver allotransplantation, and transgenic (human CD55/CD59) modifications have even prolonged the duration of pig liver perfusion. Despite the fact that hepatocyte cell lines have also been proposed for extracorporeal blood circulation in conditions of acute liver failure, porcine hepatocyte cell lines, and the GalT-KO background in particular, have not been developed and applied in this field. Herein, we established immortalized wild-type and GalT-KO porcine hepatocyte cell lines, which can be used for artificial liver support systems, cell transplantation, and even in vitro studies of xenotransplantation.

METHODS

Primary hepatocytes extracted from GalT-KO and wild-type pigs were transfected with SV40 LT lentivirus to establish immortalized GalT-KO porcine hepatocytes (GalT-KO-hep) and wild-type porcine hepatocytes (WT). Hepatocyte biomarkers and function-related genes were assessed by immunofluorescence, periodic acid-Schiff staining, indocyanine green (ICG) uptake, biochemical analysis, ELISA, and RT-PCR. Furthermore, the tumorigenicity of immortalized cells was detected. In addition, a complement-dependent cytotoxicity (CDC) assay was performed with GalT-KO-hep and WT cells. Cell death and viability rates were assessed by flow cytometry and CCK-8 assay.

RESULTS

GalT-KO and wild-type porcine hepatocytes were successfully immortalized and maintained the characteristics of primary porcine hepatocytes, including albumin secretion, ICG uptake, urea and glycogen production, and expression of hepatocyte marker proteins and specific metabolic enzymes. GalT-KO-hep and WT cells were confirmed as having no tumorigenicity. In addition, GalT-KO-hep cells showed less apoptosis and more viability than WT cells when exposed to complement and xenogeneic serum.

CONCLUSIONS

Two types of immortalized cell lines of porcine hepatocytes with GalT-KO and wild-type backgrounds were successfully established. GalT-KO-hep cells exhibited higher viability and injury resistance against a xenogeneic immune response.

Role of human and porcine MHC DRB1 alleles in determining the intensity of individual human anti-pig T-cell responses

BACKGROUND

Differences in quality and strength of immune responses between individuals are mainly due to polymorphisms in major histocompatibility complex (MHC) molecules. Focusing on MHC class-II, we asked whether the intensity of human anti-pig T-cell responses is influenced by genetic variability in the human HLA-DRB1 and/or the porcine SLA-DRB1 locus.

METHODS

ELISpot assays were performed using peripheral blood mononuclear cells (PBMCs) from 62 HLA-DRB1-typed blood donors as responder and the porcine B cell line L23 as stimulator cells. Based on the frequency of IFN-γ-secreting cells, groups of weak, medium, and strong responder individuals were defined. Mixed lymphocyte reaction (MLR) assays were performed to study the stimulatory capacity of porcine PBMCs expressing different SLA-DRB1 alleles.

RESULTS

Concerning the MHC class-II configuration of human cells, we found a significant overrepresentation of HLA-DRB1*01 alleles in the medium/strong responder group as compared to individuals showing weak responses to stimulation with L23 cells. Evaluation of the role of MHC class-II variability in porcine stimulators revealed that cells expressing SLA-DRB1*06 alleles triggered strong proliferation in approximately 70% of humans. Comparison of amino acid sequences indicated that strong human anti-pig reactivity may be associated with a high rate of similarity between human and pig HLA/SLA-DRB1 alleles.

CONCLUSION

Variability in human and porcine MHC determines the intensity of individual human anti-pig T-cell responses. MHC typing and cross-matching of prospective recipients of xenografts and donor pigs could be relevant to select for donor-recipient combinations with minimal anti-porcine immunity.

Application of new biotechnologies for improvements in swine nutrition and pork production

Meeting the increasing demands for high-quality pork protein requires not only improved diets but also biotechnology-based breeding to generate swine with desired production traits. Biotechnology can be classified as the cloning of animals with identical genetic composition or genetic engineering (via recombinant DNA technology and gene editing) to produce genetically modified animals or microorganisms. Cloning helps to conserve species and breeds, particularly those with excellent biological and economical traits. Recombinant DNA technology combines genetic materials from multiple sources into single cells to generate proteins. Gene (genome) editing involves the deletion, insertion or silencing of genes to produce: (a) genetically modified pigs with important production traits; or (b) microorganisms without an ability to resist antimicrobial substances. Current gene-editing tools include the use of zinc finger nuclease (ZFN), transcription activator-like effector nuclease (TALEN), or clustered regularly interspaced short palindromic repeats-associated nuclease-9 (CRISPR/Cas9) as editors. ZFN, TALEN, or CRISPR/Cas9 components are delivered into target cells through transfection (lipid-based agents, electroporation, nucleofection, or microinjection) or bacteriophages, depending on cell type and plasmid. Compared to the ZFN and TALEN, CRISPR/Cas9 offers greater ease of design and greater flexibility in genetic engineering, but has a higher frequency of off-target effects. To date, genetically modified pigs have been generated to express bovine growth hormone, bacterial phytase, fungal carbohydrases, plant and C. elagan fatty acid desaturases, and uncoupling protein-1; and to lack myostatin, α-1,3-galactosyltransferase, or CD163 (a cellular receptor for the "blue ear disease" virus). Biotechnology holds promise in improving the efficiency of swine production and developing alternatives to antibiotics in the future.

A review of pig liver xenotransplantation: Current problems and recent progress

Pig liver xenotransplantation appears to be more perplexing when compared to heart or kidney xenotransplantation, even though great progress has been achieved. The relevant molecular mechanisms involved in xenogeneic rejection, including coagulopathy, and particularly thrombocytopenia, are complex, and need to be systematically investigated. The deletion of expression of Gal antigens in the liver graft highlights the injurious impact of nonGal antigens, which continue to induce humoral rejection. Innate immunity, particularly mediated by macrophages and natural killer cells, interplays with inflammation and coagulation disorders. Kupffer cells and liver sinusoidal endothelial cells (LSECs) together mediate leukocyte, erythrocyte, and platelet sequestration and phagocytosis, which can be exacerbated by increased cytokine production, cell desialylation, and interspecies incompatibilities. The coagulation cascade is activated by release of tissue factor which can be dependent or independent of the xenoreactive immune response. Depletion of endothelial anticoagulants and anti-platelet capacity amplify coagulation activation, and interspecies incompatibilities of coagulation-regulatory proteins facilitate dysregulation. LSECs involved in platelet phagocytosis and transcytosis, coupled with hepatocyte-mediated degradation, are responsible for thrombocytopenia. Adaptive immunity could also be problematic in long-term liver graft survival. Currently, relevant evidence and study results of various genetic modifications to the pig donor need to be fully determined, with the aim of identifying the ideal transgene combination for pig liver xenotransplantation. We believe that clinical trials of pig liver xenotransplantation should initially be considered as a bridge to allotransplantation.

Emerging approaches and technologies in transplantation: the potential game changers

Newly emerging technologies are rapidly changing conventional approaches to organ transplantation. In the modern era, the key challenges to transplantation include (1) how to best individualize and possibly eliminate the need for life-long immunosuppression and (2) how to expand the donor pool suitable for human transplantation. This article aims to provide readers with an updated review of three new technologies that address these challenges. First, single-cell RNA sequencing technology is rapidly evolving and has recently been employed in settings related to transplantation. The new sequencing data indicate an unprecedented cellular heterogeneity within organ transplants, as well as exciting new molecular signatures involved in alloimmune responses. Second, sophisticated nanotechnology platforms provide a means of therapeutically delivering immune modulating reagents to promote transplant tolerance. Tolerogenic nanoparticles with regulatory molecules and donor antigens are capable of targeting host immune responses with tremendous precision, which, in some cases, results in donor-specific tolerance. Third, CRISPR/Cas9 gene editing technology has the potential to precisely remove immunogenic molecules while inserting desirable regulatory molecules. This technology is particularly useful in generating genetically modified pigs for xenotransplantation to solve the issue of the shortage of human organs. Collectively, these new technologies are positioning the transplant community for major breakthroughs that will significantly advance transplant medicine.

Will Genetic Engineering Carry Xenotransplantation of Pig Islets to the Clinic?

PURPOSE OF REVIEW

Porcine islets represent a potentially attractive beta-cell source for xenotransplantation into patients with type 1 diabetes, who are not eligible to islet allo-transplantation due to a lack of suitable human donor organs. Recent progress in genetic engineering/gene editing of donor pigs provides new opportunities to overcome rejection of xeno-islets, to improve their engraftment and insulin secretion capacity, and to reduce the risk for transmission of porcine endogenous retroviruses. This review summarizes the current issues and progress in islet xenotransplantation with special emphasis on genetically modified/gene edited donor pigs.

RECENT FINDINGS

Attempts to overcome acute rejection of xeno-islets, especially after intraportal transplantation into the liver, include the genetic elimination of specific carbohydrate antigens such as αGal, Neu5Gc, and Sd(a) for which humans and-in part-non-human primates have natural antibodies that bind to these targets leading to activation of complement and coagulation. A complementary approach is the expression of one or more human complement regulatory proteins (hCD46, hCD55, hCD59). Transgenic attempts to overcome cellular rejection of islet xenotransplants include the expression of proteins that inhibit co-stimulation of T cells. Expression of glucagon-like peptide-1 and M3 muscarinic receptors has been shown to increase the insulin secretion of virally transduced porcine islets in vitro and it will be interesting to see the effects of these modifications in transgenic pigs and islet products derived from them. Genome-wide inactivation of porcine endogenous retrovirus (PERV) integrants by mutating their pol genes using CRISPR/Cas9 is a recent approach to reduce the risk for PERV transmission by xeno-islets. Genetic engineering/gene editing of xeno-islet donor pigs facilitated major progress towards clinical islet xenotransplantation. The required set of genetic modifications will depend on the source of islets (fetal/neonatal vs. adult), the mode of delivery (encapsulated vs. free), and the transplantation site.