Xenotransplantation: an overview of the field

Moral approval of xenotransplantation in Egypt: associations with religion, attitudes towards animals and demographic factors

Xenotransplantation has great potential as an alternative to alleviate the shortage of organs for donation. However, given that the animal most suited for xenotransplantation is the pig, there are concerns that people in Muslim countries may be more hesitant to morally approve of these procedures. In this study, the moral approval of xenotransplantation was assessed in a group of 895 participants in Egypt. The results showed that religiosity itself does not predict moral approval of xenotransplantation, but religious identity does, as Muslims are less likely to approve of xenotransplantation than Christians. However, the strongest predictor of moral approval of xenotransplantation was gender, with women displaying less approval. A partial mediating factor in this association was concern for animal welfare. Based on these results, some implications for public policy are discussed.

Generation of heart and vascular system in rodents by blastocyst complementation

Generating organs from stem cells through blastocyst complementation is a promising approach to meet the clinical need for transplants. In order to generate rejection-free organs, complementation of both parenchymal and vascular cells must be achieved, as endothelial cells play a key role in graft rejection. Here, we used a lineage-specific cell ablation system to produce mouse embryos unable to form both the cardiac and vascular systems. By mouse intraspecies blastocyst complementation, we rescued heart and vascular system development separately and in combination, obtaining complemented hearts with cardiomyocytes and endothelial cells of exogenous origin. Complemented chimeras were viable and reached adult stage, showing normal cardiac function and no signs of histopathological defects in the heart. Furthermore, we implemented the cell ablation system for rat-to-mouse blastocyst complementation, obtaining xenogeneic hearts whose cardiomyocytes were completely of rat origin. These results represent an advance in the experimentation towards the in vivo generation of transplantable organs.

One-Step In Vitro Generation of ETV2-Null Pig Embryos

Simple Summary

One of the latest goals in regenerative medicine is to use pluripotent stem cells to generate whole organs in vivo through the blastocyst complementation technique. This method consists of the microinjection of pluripotent stem cells into preimplantation embryos that have been genetically modified to ablate the development of a target organ. By taking advantage of the spatiotemporal clues present in the developing embryo, pluripotent stem cells are able to colonize the empty developmental niche and create the missing organ. Combining human pluripotent stem cells with genetically engineered pig embryos, it would be possible to obtain humanized organs that could be used for transplantation, and, therefore, solve the worldwide issue of insufficient availability of transplantable organs. As endothelial cells play a critical role in xenotransplantation rejection in all organs, in this study, we optimized a protocol to generate a vascular-disabled preimplantation pig embryo using the CRISPR/Cas9 system. This protocol could be used to generate avascular embryos for blastocyst complementation experiments and work towards the generation of rejection-free humanized organs in pigs.

Abstract

Each year, tens of thousands of people worldwide die of end-stage organ failure due to the limited availability of organs for use in transplantation. To meet this clinical demand, one of the last frontiers of regenerative medicine is the generation of humanized organs in pigs from pluripotent stem cells (PSCs) via blastocyst complementation. For this, organ-disabled pig models are needed. As endothelial cells (ECs) play a critical role in xenotransplantation rejection in every organ, we aimed to produce hematoendothelial-disabled pig embryos targeting the master transcription factor ETV2 via CRISPR-Cas9-mediated genome modification. In this study, we designed five different guide RNAs (gRNAs) against the DNA-binding domain of the porcine ETV2 gene, which were tested on porcine fibroblasts in vitro. Four out of five guides showed cleavage capacity and, subsequently, these four guides were microinjected individually as ribonucleoprotein complexes (RNPs) into one-cell-stage porcine embryos. Next, we combined the two gRNAs that showed the highest targeting efficiency and microinjected them at higher concentrations. Under these conditions, we significantly improved the rate of biallelic mutation. Hence, here, we describe an efficient one-step method for the generation of hematoendothelial-disabled pig embryos via CRISPR-Cas9 microinjection in zygotes. This model could be used in experimentation related to the in vivo generation of humanized organs.

"Religious Belief": An Undervalued Ethical Inclusion Criterion for Clinical Trials on Bone Grafting Procedures

The aim of the present review was to assess randomized controlled trials (RCTs) on bone grafting procedures that included religious belief as an eligibility criterion. Indexed databases were searched up to and including February 2019 using different search strategies. In strategy 1, the following terms were used: (a) belief; (b) bone graft; (c) faith; (d) inclusion; (e) exclusion; (f) eligibility; (g) criteria; (h) randomized clinical trial; (i) religion; and (j) xenograft. In strategy 2, the following terms were used in addition to those used in strategy 1: Xenografts AND oral surgery OR xenografts AND maxillofacial OR xenografts AND dental implants. These searches were filtered using the terms "Randomized clinical trial" and "human studies". The initial search yielded 3932 studies. Filtration of results using the terms "Randomized clinical trial" and "human studies" showed 0 studies. Evaluation of patients' religious beliefs seems to be undervalued in RCTs related to the placement of xenografts. This is an essential and ethical criterion that should be taken into consideration prior to inclusion of participants and signing the informed consent form for RCTs related to the placement of bone grafts.

Cryostructuring of polymer systems. 47. Preparation of wide porous gelatin-based cryostructurates in sterilizing organic media and assessment of the suitability of thus formed matrices as spongy scaffolds for 3D cell culturing

New gelatin-based cryostructurates have been elaborated and tested as scaffolds for three-dimensional (3D) cell culturing. Scaffold preparation included dissolution of Type A gelatin in dimethylsulfoxide, freezing of such solution, cryoextraction of crystalline phase with cold ethanol, cross-linking of gelatin with carbodiimide in ethanol medium, treatment of the matrix with ethanolic solution of Tris and tanning of the matrix with formaldehyde dissolved in ethanol. The use of organic media during all the preparation stages ensured the sterility of the scaffolds. The matrices thus prepared were seeded with human adipose tissue multipotent mesenchymal stromal cells to confirm the biocompatibility of scaffolds and their possibility to provide necessary environment for the cell growth and differentiation. The cells attached onto the surface of the pore walls, proliferated and differentiated into osteogenic and adipogenic lineages. These results demonstrate that gelatin-based cryostructurates prepared in the sterility ensuring organic media can be used as scaffolds for tissue engineering purposes.

The Development of a Xenograft-Derived Scaffold for Tendon and Ligament Reconstruction Using a Decellularization and Oxidation Protocol

PURPOSE

To evaluate the biological, immunological, and biomechanical properties of a scaffold derived by architectural modification of a fresh-frozen porcine patella tendon using a decellularization protocol that combines physical, chemical, and enzymatic modalities.

METHODS

Porcine patellar tendons were processed using a decellularization and oxidation protocol that combines physical, chemical, and enzymatic modalities. Scaffolds (n = 88) were compared with native tendons (n = 70) using histologic, structural (scanning electron microscopy, porosimetry, and tensile testing), biochemical (mass spectrometry, peracetic acid reduction, DNA quantification, alpha-galactosidase [α-gal] content), as well as in vitro immunologic (cytocompatibility, cytokine induction) and in vivo immunologic nonhuman primate analyses.

RESULTS

A decrease in cellularity based on histology and a significant decrease in DNA content were observed in the scaffolds compared with the native tendon (P < .001). Porosity and pore size were increased significantly (P < .001). Scaffolds were cytocompatible in vitro. There was no difference between native tendons and scaffolds when comparing ultimate tensile load, stiffness, and elastic modulus. The α-gal xenoantigen level was significantly lower in the decellularized scaffold group compared with fresh-frozen, nondecellularized tissue (P < .001). The in vivo immunological response to implanted scaffolds measured by tumor necrosis factor-α and interleukin-6 levels was significantly (P < .001) reduced compared with untreated controls in vitro. These results were confirmed by an attenuated response to scaffolds in vivo after implantation in a nonhuman primate model.

CONCLUSIONS

Porcine tendon was processed via a method of decellularization and oxidation to produce a scaffold that possessed significantly less inflammatory potential than a native tendon, was biocompatible in vitro, of increased porosity, and with significantly reduced amounts of α-gal epitope while retaining tensile properties.

CLINICAL RELEVANCE

Porcine-derived scaffolds may provide a readily available source of material for musculoskeletal reconstruction and repair while eliminating concerns regarding disease transmission and the morbidity of autologous harvest.

Cancer immunotherapy for pancreatic cancer utilizing α-gal epitope/natural anti-Gal antibody reaction

Pancreatic ductal adenocarcinoma (PDAC) has the poorest prognosis of all malignancies and is largely resistant to standard therapy. Novel treatments against PDAC are desperately needed. Anti-Gal is the most abundant natural antibody in humans, comprising about 1% of immunoglobulins and is also naturally produced in apes and Old World monkeys. The anti-Gal ligand is a carbohydrate antigen called “α-gal epitopes” with the structure Galα1-3Galβ1-4GlcNAc-R. These epitopes are expressed as major carbohydrate antigens in non-primate mammals, prosimians, and New World monkeys. Anti-Gal is exploited in cancer vaccines to increase the immunogenicity of antigen-presenting cells (APCs). Cancer cells or PDAC tumor lysates are processed to express α-gal epitopes. Vaccination with these components results in in vivo opsonization by anti-Gal IgG in PDAC patients. The Fc portion of the vaccine-bound anti-Gal interacts with Fcγ receptors of APCs, inducing uptake of the vaccine components, transport of the vaccine tumor membranes to draining lymph nodes, and processing and presentation of tumor-associated antigens (TAAs). Cancer vaccines expressing α-gal epitopes elicit strong antibody production against multiple TAAs contained in PDAC cells and induce activation of multiple tumor-specific T cells. Here, we review new areas of clinical importance related to the α-gal epitope/anti-Gal antibody reaction and the advantages in immunotherapy against PDAC.

Autologous tissue patch rich in stem cells created in the subcutaneous tissue

AIM: To investigate whether we could create natural autologous tissue patches in the subcutaneous space for organ repair.

METHODS: We implanted the following three types of inert foreign bodies in the subcutaneous tissue of rats to produce autologous tissue patches of different geometries: (1) a large-sized polyvinyl tube (L = 25 mm, internal diameter = 7 mm) sealed at both ends by heat application for obtaining a large flat piece of tissue patch for organ repair; (2) a fine polyvinyl tubing (L = 25 mm, internal diameter = 3 mm) for creating cylindrically shaped grafts for vascular or nerve repair; and (3) a slurry of polydextran particle gel for inducing a bladder-like tissue. Implantation of inert materials was carried out by making a small incision on one or either side of the thoracic-lumbar region of rats. Subcutaneous pockets were created by blunt dissection around the incision into which the inert bodies were inserted (1 or 2 per rat). The incisions were closed with silk sutures, and the animals were allowed to recover. In case of the polydextran gel slurry 5 mL of the slurry was injected in the subcutaneous space using an 18 gauge needle. After implanting the foreign bodies a newly regenerated encapsulating tissue developed around the foreign bodies. The tissues were harvested after 4-42 d of implantation and studied by gross examination, histology, and histochemistry for organization, vascularity, and presence of mesenchymal stem cells (MSCs) (CD271+CD34+ cells).

RESULTS: Implanting a large cylindrically shaped polyvinyl tube resulted in a large flat sheet of tissue that could be tailored to a specific size and shape for use as a tissue patch for repairing large organs. Implanting a smaller sized polyvinyl tube yielded a cylindrical tissue that could be useful for repairing nerves and blood vessels. This type of patch could be obtained in different lengths by varying the length of the implanted tube. Implanting a suspension of inert polydextran suspension gave rise to a bladder-like tissue that could be potentially used for repairing heart valves. Histologically, the three different types of tissue patches generated were organized similarly, consisting of three layers, increasing in thickness until day 14. The inner layer in contact with the inert material was avascular; a middle layer that was highly vascular and filled with matrix, and an outer layer consisting of loose connective tissue. MSCs identified as CD271+CD34+ cells were present in the medial layer and around major blood vessels at day 4 but absent at later time points. The early-harvested tissues, endowed with MSCs, could be used for tissue repair, while the later-harvested tissues, being less vascular but thicker and tougher, could be used as filler tissue for cosmetic purposes.

CONCLUSION: An autologous, vascularized tissue patch of desired shape and size can be created in the subcutaneous space by implanting different types of inert bodies.

Role of DNA methylation in expression and transmission of porcine endogenous retroviruses

Porcine endogenous retroviruses (PERV) represent a major safety concern in pig-to-human xenotransplantation. To date, no PERV infection of a xenograft recipient has been recorded; however, PERVs are transmissible to human cells in vitro. Some recombinants of the A and C PERV subgroups are particularly efficient in infection and replication in human cells. Transcription of PERVs has been described in most pig cells, but their sequence and insertion polymorphism in the pig genome impede identification of transcriptionally active or silenced proviral copies. Furthermore, little is known about the epigenetic regulation of PERV transcription. Here, we report on the transcriptional suppression of PERV by DNA methylation in vitro and describe heavy methylation in the majority of PERV 5' long terminal repeats (LTR) in porcine tissues. In contrast, we have detected sparsely methylated or nonmethylated proviruses in the porcine PK15 cells, which express human cell-tropic PERVs. We also demonstrate the resistance of PERV DNA methylation to inhibitors of methylation and deacetylation. Finally, we show that the high permissiveness of various human cell lines to PERV infection coincides with the inability to efficiently silence the PERV proviruses by 5'LTR methylation. In conclusion, we suggest that DNA methylation is involved in PERV regulation, and that only a minor fraction of proviruses are responsible for the PERV RNA expression and porcine cell infectivity.

ITIM-dependent negative signaling pathways for the control of cell-mediated xenogeneic immune responses

Xenotransplantation is an innovative field of research with the potential to provide us with an alternative source of organs to face the severe shortage of human organ donors. For several reasons, pigs have been chosen as the most suitable source of organs and tissues for transplantation in humans. However, porcine xenografts undergo cellular immune responses representing a major barrier to their acceptance and normal functioning. Innate and adaptive xenogeneic immunity is mediated by both the recognition of xenogeneic tissue antigens and the lack of inhibition due to molecular cross-species incompatibilities of regulatory pathways. Therefore, the delivery of immunoreceptor tyrosine-based inhibitory motif (ITIM)-dependent and related negative signals to control innate (NK cells, macrophages) and adaptive T and B cells might overcome cell-mediated xenogeneic immunity. The proof of this concept has already been achieved in vitro by the transgenic overexpression of human ligands of several inhibitory receptors in porcine cells resulting in their resistance against xenoreactivity. Consequently, several transgenic pigs expressing tissue-specific human ligands of inhibitory coreceptors (HLA-E, CD47) or soluble competitors of costimulation (belatacept) have already been generated. The development of these robust and innovative approaches to modulate human anti-pig cellular immune responses, complementary to conventional immunosuppression, will help to achieve long-term xenograft survival. In this review, we will focus on the current strategies to enhance negative signaling pathways for the regulation of undesirable cell-mediated xenoreactive immune responses.

A simple PCR-based strategy for estimating species-specific contributions in chimeras and xenografts

Many tissue-engineering approaches for repair and regeneration involve transplants between species. Yet a challenge is distinguishing donor versus host effects on gene expression. This study provides a simple molecular strategy to quantify species-specific contributions in chimeras and xenografts. Species-specific primers for reverse transcription quantitative real-time PCR (RT-qPCR) were designed by identifying silent mutations in quail, duck, chicken, mouse and human ribosomal protein L19 (RPL19). cDNA from different pairs of species was mixed in a dilution series and species-specific RPL19 primers were used to generate standard curves. Then quail cells were transplanted into transgenic-GFP chick and resulting chimeras were analyzed with species-specific primers. Fluorescence-activated cell sorting (FACS) confirmed that donor- and host-specific levels of RPL19 expression represent actual proportions of cells. To apply the RPL19 strategy, we measured Runx2 expression in quail-duck chimeras. Elevated Runx2 levels correlated with higher percentages of donor cells. Finally, RPL19 primers also discriminated mouse from human and chick. Thus, this strategy enables chimeras and/or xenografts to be screened rapidly at the molecular level.

Role of α-gal epitope/anti-Gal antibody reaction in immunotherapy and its clinical application in pancreatic cancer

Pancreatic cancer is one of the most common causes of death from cancer. Despite the availability of various treatment modalities, such as surgery, chemotherapy and radiotherapy, the 5-year survival remains poor. Although gemcitabine-based chemotherapy is typically offered as the standard care, most patients do not survive longer than 6 months. Therefore, new therapeutic approaches are needed. The α-gal epitope (Galα1-3Galβ1-4GlcNAc-R) is abundantly synthesized from glycoproteins and glycolipids in non-primate mammals and New World monkeys, but is absent in humans, apes and Old World monkeys. Instead, they produce anti-Gal antibody (Ab) (forming approximately 1% of circulating immunoglobulins), which specifically interacts with α-gal epitopes. Anti-Gal Ab can be exploited in cancer immunotherapy as vaccines that target antigen-presenting cells (APC) to increase their immunogenicity. Tumor cells or tumor cell membranes from pancreatic cancer are processed to express α-gal epitopes. Subsequent vaccination with such processed cell membranes results in in vivo opsonization by anti-Gal IgG in cancer patients. The interaction of the Fc portion of the vaccine-bound anti-Gal with Fcγ receptors of APC induces effective uptake of the vaccinating tumor cell membranes by the APC, followed by effective transport of the vaccinating tumor membranes to the regional lymph nodes, and processing and presentation of the tumor-associated antigens. Activation of tumor-specific B and T cells could elicit an immune response that in some patients is potent enough to eradicate the residual cancer cells that remain after completion of standard therapy. This review addresses these topics and new avenues of clinical importance related to this unique antigen/antibody system (α-gal epitope/anti-Gal Ab) and advances in immunotherapy in pancreatic cancer.

Bioreactor design for perfusion-based, highly-vascularized organ regeneration

Bioartificial or laboratory-grown organs is a growing field centered on developing replacement organs and tissues to restore body function and providing a potential solution to the shortage of donor organs for transplantation. With the entry of engineered planar tissues, such as bladder and trachea, into clinical studies, an increasing focus is being given to designing complex, three-dimensional solid organs. As tissues become larger, thicker and more complex, the vascular network becomes crucial for supplying nutrients and maintaining viability and growth of the neo-organ. Perfusion decellularization, the process of removing cells from an entire organ, leaves the matrix of the vascular network intact. Organ engineering requires a delicate process of decellularization, sterilization, reseeding with appropriate cells, and organ maturation and stimulation to ensure optimal development. The design of bioreactors to facilitate this sequence of events has been refined to the extent that some bioartificial organs grown in these systems have been transplanted into recipient animals with sustained, though limited, function. This review focuses on the state-of-art in bioreactor development for perfusion-based bioartificial organs and highlights specific design components in need of further refinement.