Matthew Tector

Evaluation of human and non-human primate antibody binding to pig cells lacking GGTA1/CMAH/β4GalNT2 genes.

Simultaneous inactivation of pig GGTA1 and CMAH genes eliminates carbohydrate xenoantigens recognized by human antibodies. The β4GalNT2 glycosyltransferase may also synthesize xenoantigens. To further characterize glycan‐based species incompatibilities, we examined human and non‐human primate antibody binding to cells derived from genetically modified pigs lacking these carbohydrate‐modifying genes.

Efficient generation of genetically distinct pigs in a single pregnancy using multiplexed single-guide RNA and carbohydrate selection.

Manipulating the pig genome to increase compatibility with human biology may facilitate the clinical application of xenotransplantation. Genetic modifications to pig cells have been made by sequential recombination in fetal fibroblasts and liver‐derived cells followed by cross‐breeding or somatic cell nuclear transfer. The generation of pigs for research or organ donation by these methods is slow, expensive and requires technical expertise. A novel system incorporating the bacterial nuclease Cas9 and single‐guide RNA targeting a 20 nucleotide site within a gene can be expressed from a single plasmid leading to a double‐strand break and gene disruption. Coexpression of multiple unique single‐guide RNA can modify several genetic loci in a single step. We describe a process for increasing the efficiency of selecting cells with multiple genetic modifications.

Pre‐transplant antibody screening and anti‐CD154 costimulation blockade promote long‐term xenograft survival in a pig‐to‐primate kidney transplant model

Xenotransplantation has the potential to alleviate the organ shortage that prevents many patients with end‐stage renal disease from enjoying the benefits of kidney transplantation. Despite significant advances in other models, pig‐to‐primate kidney xenotransplantation has met limited success. Preformed anti‐pig antibodies are an important component of the xenogeneic immune response. To address this, we screened a cohort of 34 rhesus macaques for anti‐pig antibody levels. We then selected animals with both low and high titers of anti‐pig antibodies to proceed with kidney transplant from galactose‐α1,3‐galactose knockout/CD55 transgenic pig donors. All animals received T‐cell depletion followed by maintenance therapy with costimulation blockade (either anti‐CD154 mAb or belatacept), mycophenolate mofetil, and steroid. The animal with the high titer of anti‐pig antibody rejected the kidney xenograft within the first week. Low‐titer animals treated with anti‐CD154 antibody, but not belatacept exhibited prolonged kidney xenograft survival (>133 and >126 vs. 14 and 21 days, respectively). Long‐term surviving animals treated with the anti‐CD154‐based regimen continue to have normal kidney function and preserved renal architecture without evidence of rejection on biopsies sampled at day 100. This description of the longest reported survival of pig‐to‐non‐human primate kidney xenotransplantation, now >125 days, provides promise for further study and potential clinical translation.

Creating Class I MHC–Null Pigs Using Guide RNA and the Cas9 Endonuclease

Pigs are emerging as important large animal models for biomedical research, and they may represent a source of organs for xenotransplantation. The MHC is pivotal to the function of the immune system in health and disease, and it is particularly important in infection and transplant rejection. Pigs deficient in class I MHC could serve as important reagents to study viral immunity as well as allograft and xenograft rejection. In this study, we report the creation and characterization of class I MHC knockout pigs using the Cas9 nuclease and guide RNAs. Pig fetal fibroblasts were genetically engineered using Cas9 and guide RNAs, and class I MHC− cells were then used as nuclear donors for somatic cell nuclear transfer. We produced three piglets devoid of all cell surface class I proteins. Although these animals have reduced levels of CD4−CD8+ T cells in peripheral blood, the pigs appear healthy and are developing normally. These pigs are a promising reagent for immunological research.

Reduced Binding of Human Antibodies to Cells From GGTA1/CMAH KO Pigs

Xenotransplantation using genetically modified pig organs could solve the donor organ shortage problem. Two inactivated genes that make humans unique from pigs are GGTA1 and CMAH, the products of which produce the carbohydrate epitopes, aGal and Neu5Gc that attract preformed human antibody. When the GGTA1 and CMAH genes were deleted in pigs, human antibody binding was reduced in preliminary analysis. We analyzed the binding of human IgM and IgG from 121 healthy human serum samples for binding to GGTA1 KO and GGTA1/CMAH KO peripheral blood mononuclear cells (PBMCs). We analyzed a sub population for reactivity toward genetically modified pig PBMCs as compared to chimpanzee and human PBMCs. Deletion of the GGTA1 and CMAH genes in pigs improved the crossmatch results beyond those observed with chimpanzees. Sorting the 121 human samples tested against the GGTA1/CMAH KO pig PBMCs did not reveal a distinguishing feature such as blood group, age or gender. Modification of genes to make pig carbohydrates more similar to humans has improved the crossmatch with human serum significantly.

Recent advances in genome editing and creation of genetically modified pigs.

The field of xenotransplantation is benefiting greatly from recent advances in genetic engineering. The efficiency and pace with which new model animals are being created has dramatically sped progress towards clinical relevance. Endonuclease-driven genome editing now allows for the efficient generation of targeted genetic alterations. Herein we review the available methods of genetic engineering that have been successfully employed to create genetically modified pigs.

Reduced human platelet uptake by pig livers deficient in the asialoglycoprotein receptor 1 protein

The lethal thrombocytopenia that accompanies liver xenotransplantation is a barrier to clinical application. Human platelets are bound by the asialoglycoprotein receptor (ASGR) on pig sinusoidal endothelial cells and phagocytosed. Inactivation of the ASGR1 gene in donor pigs may prevent xenotransplantation‐induced thrombocytopenia.

Humoral Reactivity of Renal Transplant-Waitlisted Patients to Cells From GGTA1/CMAH/B4GalNT2, and SLA Class I Knockout Pigs

Background Antipig antibodies are a barrier to clinical xenotransplantation. We evaluated antibody binding of waitlisted renal transplant patients to 3 glycan knockout (KO) pig cells and class I swine leukocyte antigens (SLA). Methods Peripheral blood mononuclear cells from SLA identical wild type (WT), &agr;1, 3-galactosyltransferase (GGTA1) KO, GGTA1/ cytidine monophosphate-N-acetylneuraminic acid hydroxylase (CMAH) KO, and GGTA1/ CMAH /b1,4 N-acetylgalactosaminyl transferase (B4GalNT2) KO pigs were screened for human antibody binding using flow cytometric crossmatch (FCXM). Sera from 820 patients were screened on GGTA1/CMAH/B4GalNT2 KO cells and a subset with elevated binding was evaluated further. FCXM was performed on SLA intact cells and GGTA1/SLA class I KO cells after depletion with WT pig RBCs to remove cell surface reactive antibodies, but leave SLA antibodies. Lastly, human and pig reactive antibodies were eluted and tested for cross-species binding and reactivity to single-antigen HLA beads. Results Sequential glycan KO modifications significantly reduce antibody binding of waitlisted patients. Sera exhibiting elevated binding without reduction after depletion with WT RBCs demonstrate reduced binding to SLA class I KO cells. Human IgG, eluted from human and pig peripheral blood mononuclear cells, interacted across species and bound single-antigen HLA beads in common epitope-restricted patterns. Conclusions Many waitlisted patients have minimal xenoreactive antibody binding to the triple KO pig, but some HLA antibodies in sensitized patients cross-react with class I SLA. SLA class I is a target for genome editing in xenotransplantation.

Erythrocytes from GGTA1/CMAH knockout pigs: implications for xenotransfusion and testing in non‐human primates

Pig erythrocytes are potentially useful to solve the worldwide shortage of human blood for transfusion. Domestic pig erythrocytes, however, express antigens that are bound by human preformed antibodies. Advances in genetic engineering have made it possible to rapidly knock out the genes of multiple xenoantigens, namely galactose α1,3 galactose (aGal) and N‐glycolylneuraminic acid (Neu5Gc). We have recently targeted the GGTA1 and CMAH genes with zinc finger endonucleases resulting in double knockout pigs that no longer express aGal or Neu5Gc and attract significantly fewer human antibodies. In this study, we characterized erythrocytes from domestic and genetically modified pigs, baboons, chimpanzees, and humans for binding of human and baboon natural antibody, and complement‐mediated lysis.

Silencing Porcine CMAH and GGTA1 Genes Significantly Reduces Xenogeneic Consumption of Human Platelets by Porcine Livers.

Background A profound thrombocytopenia limits hepatic xenotransplantation in the pig-to-primate model. Porcine livers also have shown the ability to phagocytose human platelets in the absence of immune-mediated injury. Recently, inactivation of the porcine ASGR1 gene has been shown to decrease this phenomenon. Inactivating GGTA1 and CMAH genes has reduced the antibody-mediated barrier to xenotransplantation; herein, we describe the effect that these modifications have on xenogeneic consumption of human platelets in the absence of immune-mediated graft injury. Methods Wild type (WT), ASGR1−/−, GGTA1−/−, and GGTA1−/−CMAH−/− knockout pigs were compared for their xenogeneic hepatic consumption of human platelets. An in vitro assay was established to measure the association of human platelets with liver sinusoidal endothelial cells (LSECs) by immunohistochemistry. Perfusion models were used to measure human platelet uptake in livers from WT, ASGR1−/−, GGTA1−/−, and GGTA1−/− CMAH−/− pigs. Results GGTA1−/−, CMAH−/− LSECs exhibited reduced levels of human platelet binding in vitro when compared with GGTA1−/− and WT LSECs. In a continuous perfusion model, GGTA1−/− CMAH−/− livers consumed fewer human platelets than GGTA1−/− and WT livers. GGTA1−/− CMAH−/− livers also consumed fewer human platelets than ASGR1−/− livers in a single-pass model. Conclusions Silencing the porcine carbohydrate genes necessary to avoid antibody-mediated rejection in a pig-to-human model also reduces the xenogeneic consumption of human platelets by the porcine liver. The combination of these genetic modifications may be an effective strategy to limit the thrombocytopenia associated with pig-to-human hepatic xenotransplantation.