Co-transplantation of Vascularized Thymic Graft with Kidney in Pig-to-Nonhuman Primates for the Induction of Tolerance Across Xenogeneic Barriers

Pretransplant Screening for Prevention of Hyperacute Graft Loss in Pig-to-primate Kidney Xenotransplantation

BACKGROUND

Xenotransplantation using pig organs is now a clinical reality. However, the process for xenograft recipient screening lacks clarity and scientific rigor: no established thresholds exist to determine which levels of preformed antipig natural antibodies (Nabs) will be safe for clinical xenograft transplantation, and hyperacute rejection (HAR) or acute humoral xenograft rejection (AHXR), which still impacts pig-to-primate kidney xenograft survivals, may impede broader application of pig-to-human clinical xenograft transplantation.

METHODS

We retrospectively examined 28 cases of pig-to-baboon kidney xenotransplantation using GalTKO±human complement regulatory protein (hCRP)-transgenic (Tg) pig donors, as well as 6 cases of triple-KO multi-Tg (10GE) pig donors, and developed screening algorithms to predict risk of HAR/AHXR based on recipient antipig Nab levels. Preformed Nabs were evaluated using both complement-dependent cytotoxicity and antibody (IgM and IgG) binding flow-cytometry assays.

RESULTS

High complement-dependent cytotoxicity was associated with HAR/AHXR as expected. However, we also found that high levels of IgG were independently associated with HAR/AHXR, and we developed 2 indices to interpret and predict the risk of IgG-mediated HAR/AHXR.

CONCLUSIONS

Based on the data in this study, we have established a new 2-step screening, which will be used for future clinical kidney xenotransplantation trials.

Milestones on the path to clinical pig organ xenotransplantation

Progress in pig organ xenotransplantation has been made largely through (1) genetic engineering of the organ-source pig to protect its tissues from the human innate immune response, and (2) development of an immunosuppressive regimen based on blockade of the CD40/CD154 costimulation pathway to prevent the adaptive immune response. In the 1980s, after transplantation into nonhuman primates (NHPs), wild-type (genetically unmodified) pig organs were rejected within minutes or hours. In the 1990s, organs from pigs expressing a human complement-regulatory protein (CD55) transplanted into NHPs receiving intensive conventional immunosuppressive therapy functioned for days or weeks. When costimulation blockade was introduced in 2000, the adaptive immune response was suppressed more readily. The identification of galactose-α1,3-galactose as the major antigen target for human and NHP anti-pig antibodies in 1991 allowed for deletion of expression of galactose-α1,3-galactose in 2003, extending pig graft survival for up to 6 months. Subsequent gene editing to overcome molecular incompatibilities between the pig and primate coagulation systems proved additionally beneficial. The identification of 2 further pig carbohydrate xenoantigens allowed the production of 'triple-knockout' pigs that are preferred for clinical organ transplantation. These combined advances enabled the first clinical pig heart transplant to be performed and opened the door to formal clinical trials.

Mechanisms and strategies to promote cardiac xenotransplantation

End stage heart failure is a terminal disease, and the only curative therapy is orthotopic heart transplantation. Due to limited organ availability, alternative strategies have received intense interest for treatment of patients with advanced heart failure. Recent studies using gene-edited porcine organs suggest that cardiac xenotransplantation may provide a future source of organs. In this review, we highlight the historical milestones for cardiac xenotransplantation and the gene editing strategies designed to overcome immunological barriers, which have culminated in a recent cardiac pig-to-human xenotransplant. We also discuss recent results of studies on the engineering of human-porcine chimeric organs that may provide an alternative and complementary strategy to overcome some of the major immunological barriers to producing a new source of transplantable organs.

Genetically engineered pig kidney transplantation in a brain-dead human subject

In September 2021, a kidney (with donor-specific thymic tissue) from an α1, 3-galactosyltransferase gene-knockout (GTKO) pig was transplanted into the groin (with anastomoses to the femoral vessels) of a brain-dead subject by a surgical team at New York University Langone Health (NYU). It was reported to function immediately, passing urine and excreting creatinine. The experiment was terminated after 54 h and, during this period, the kidney did not show macroscopic features of rejection. Does this experiment provide information not available to us previously and does it move the field forward to clinical trials? The information provided was very limited, but the following points are worthy of note. (i) Numerous in vivo studies in nonhuman primates have predicted that the pig kidney would function immediately. (ii) Numerous in vitro studies have predicted that a GTKO pig kidney would not be rejected within the first few days after transplantation into a human subject. (iii) GTKO kidneys are not optimal for clinical transplantation, and the transplantation of a triple-knockout (TKO) pig kidney would have been more relevant. (iv) There was no purpose in transplanting a "thymokidney" without pre-transplant conditioning therapy and follow-up for several months. (v) Because the native kidneys were retained, it is difficult to determine whether the function of the graft was sufficient to support life. (vi) The experiment was announced to the media rather than published in a peer-reviewed medical journal (although hopefully this will follow), suggesting that it was primarily carried out to gain attention to the great potential of xenotransplantation (and/or possibly to NYU). In this respect the experiment was successful. Because of the very limited period of time for which a brain-dead subject can be maintained in a metabolically and hemodynamically stable state, the value of experiments in such subjects will remain very limited. It is hoped that any future similar experiments will be planned to be more relevant to the clinical situation. Nevertheless, the report has stimulated public attention towards xenotransplantation which, unless there is an adverse response to what some might consider to be a bizarre experiment, should be of significant benefit to future progress.

Expression of human CD47 in pig glomeruli prevents proteinuria and prolongs graft survival following pig-to-baboon xenotransplantation

BACKGROUND

Nephrotic syndrome is a common complication of pig-to-baboon kidney xenotransplantation (KXTx) that adversely affects outcomes. We have reported that upregulation of CD80 and down-regulation of SMPDL-3b in glomeruli have an important role in the development of proteinuria following pig-to-baboon KXTx. Recently we found induced expression of human CD47 (hCD47) on endothelial cells and podocytes isolated from hCD47 transgenic (Tg) swine markedly reduced phagocytosis by baboon and human macrophages. These observations led us to hypothesize that transplanting hCD47 Tg porcine kidneys could overcome the incompatibility of the porcine CD47-baboon SIRPα interspecies ligand-receptor interaction and prevent the development of proteinuria following KXTx.

METHODS

Ten baboons received pig kidneys with vascularized thymic grafts (n = 8) or intra-bone bone marrow transplants (n = 2). Baboons were divided into three groups (A, B, and C) based on the transgenic expression of hCD47 in GalT-KO pigs. Baboons in Group A received kidney grafts with expression of hCD47 restricted to glomerular cells (n = 2). Baboons in Group B received kidney grafts with high expression of hCD47 on both glomerular and tubular cells of the kidneys (n = 4). Baboons in Group C received kidney grafts with low/no glomerular expression of hCD47, and high expression of hCD47 on renal tubular cells (n = 4).

RESULTS

Consistent with this hypothesis, GalT-KO/hCD47 kidney grafts with high expression of hCD47 on glomerular cells developed minimal proteinuria. However, high hCD47 expression in all renal cells including renal tubular cells induced an apparent destructive inflammatory response associated with upregulated thrombospondin-1. This response could be avoided by a short course of weekly anti-IL6R antibody administration, resulting in prolonged survival without proteinuria (mean 170.5 days from 47.8 days).

CONCLUSION

Data showed that transgenic expression of hCD47 on glomerular cells in the GalT-KO donor kidneys can prevent xenograft nephropathy, a significant barrier for therapeutic applications of xenotransplantation. The ability to prevent nephrotic syndrome following KXTx overcomes a critical barrier for future clinical applications of KXTx.

Progress towards xenogenic tolerance

PURPOSE OF REVIEW

To describe the most recent progress towards tolerance in xenotransplantation.

RECENT FINDINGS

Mixed chimerism and thymic transplantation have been used to promote tolerance in xenotransplantation models. Intra-bone bone marrow transplantation is a recent advance for mixed chimerism, which promotes longer lasting chimerism and early graft function of subsequent organ transplantation. The hybrid thymus, an advancement to the vascularized thymokidney and vascularized thymic lobe, is being developed to allow for both donor and recipient T-cell selection in the chimeric thymus, encouraging tolerance to self and donor while maintaining appropriate immune function. Regulatory T cells show promise to promote tolerance by suppressing effector T cells and by supporting mixed chimerism. Monoclonal antibodies such as anti-CD2 may promote tolerance through suppression of CD2+ effector and memory T cells whereas Tregs, which express lower numbers of CD2, are relatively spared and might be used to promote tolerance.

SUMMARY

These findings contribute major advances to tolerance in xenotransplantation. A combination of many of these mechanisms will likely be needed to have long-term tolerance maintained without the use of immunosuppression.

Antibody reactivity with new antigens revealed in multi-transgenic triple knockout pigs may cause early loss of pig kidneys in baboons

BACKGROUND

Recent advances in gene editing technology have enabled the production of multi-knockout (KO) and transgenic pigs in order to overcome immunologic barriers in xenotransplantation (XTx). However, the genetic manipulations required to produce these changes may have the unintended consequence of producing or revealing neoantigens reactive with natural antibodies present in baboons. In this study, we examined whether the neoantigens that develop in multi-transgenic (mTg) GalT, Cytidine monophospho-N-acetylneuraminic acid hydroxylase (CMAH), β-1,4-N-acetyl-galactosaminyl transferase 2 (B4) KO pigs can cause rejection of xenografts in baboons.

METHODS

Five baboons that had <35% cytotoxicity against GalT-KO peripheral blood mononuclear cells (PBMCs) in a pre-screening assay received pig kidneys and vascularized thymic grafts (VT + K) from multi-transgenic hCD47, human thrombomodulin (hTBM), human endothelial protein C receptor (EPCR) with/without hCD46 and hCD55 with GalT-KO/NeuGC-KO/B4-KO (mTg Tri-KO) swine. In order to further examine the effects of anti-donor non-Gal natural antibody (nAb), anti-pig preformed IgM and IgG nAb binding against the GalT-KO PBMCs was compared with the donor-type PBMCs using donor pretransplant sera as well as 5 additional naïve baboon sera by flow cytometric analysis.

RESULTS

Five baboons that received VT + K grafts had stable renal function in the first 11 days (serum creatinine < 1.5 mg/dL). Two of the five baboons had higher binding of preformed IgG to mTg Tri-KO PBMCs than to GalT-KO PBMCs (mTg Tri-KO > GalT-KO), and they rejected their grafts at POD 20. In contrast, the other three baboons demonstrated either mTg Tri-KO = GalT-KO or mTg Tri-KO < GalT-KO, and they maintained renal function 43, 52, and 154 days without rejection. Among 10 baboon sera, two had less antibody binding against PBMCs that were syngeneic to the mTg Tri-KO than against GalT-KO PBMCs (mTg Tri-KO < GalT-KO); three had similar binding to mTg Tri-KO and GalT-KO PBMCs (mTg Tri-KO = GalT-KO); and five had higher binding to m Tg Tri-KO than to GalT-KO PBMCs (mTg Tri-KO > GalT-KO).

CONCLUSIONS

These data suggest that neoantigens associated with mTg Tri-KO promote acute xenograft rejection in a pig-to-baboon VT + K XTx model. The screening assays may be useful to select "safe" recipients to receive mTg Tri-KO kidneys.