What Have We Learned From In Vitro Studies About Pig-to-primate Organ Transplantation?

International Xenotransplantation Association (IXA) Position Paper on Kidney Xenotransplantation

Porcine kidney xenotransplantation for end-stage renal disease (ESRD) has reached the stage of clinical testing following major advances in donor pig genetic modifications and effective immunosuppressive strategies through decades of rigorous translational research. Reports of pig kidney xenograft survival beyond 1 year post-transplant in nonhuman primate (NHP) models justify optimism for its potential as an alternative to allotransplantation. In the United States, experimental transplantations of genetically engineered (GE) porcine kidneys into brain-dead subjects and a small number of ESRD patients have shown no evidence of hyperacute rejection and adequate pig kidney function for up to several months. Here we discuss pre-clinical/clinical results, infectious disease, ethical, and regulatory considerations, and propose evidence-based recommendations. For initial clinical trials in kidney xenotransplantation, we make the following recommendations: (i) transplantation with organs from a triple knockout (TKO) donor pig, preferably with added human transgenes, (ii) an immunosuppressive regimen with induction therapy to deplete T (and possibly B) cells, and maintenance therapy based on a cluster of differentiation (CD)40/CD154 co-stimulation pathway blockade, (iii) the patient should be fully acceptable as a candidate for allotransplantation but should be unlikely ever to receive an allograft. Patients aged 60-69 years (extendable to 40-75 years, if one of the criteria mentioned below is present), of blood group B or O, and with diabetes are most at risk in this regard. Other patients who could be considered are (i) those who have lost two or more previous kidney allografts from recurrent disease in the graft, (ii) those with broad human leukocyte antigen (HLA)-reactivity but no evidence of anti-pig antibodies, including swine leukocyte antigen (SLA), and (iii) those with failing vascular access. Clinical pilot studies in carefully and highly selected patients with no alternative therapy will provide the foundation upon which to base subsequent formal expanded clinical trials.

International Xenotransplantation Association (IXA) Position Paper on Kidney Xenotransplantation

Porcine kidney xenotransplantation for end-stage renal disease (ESRD) has reached the stage of clinical testing following major advances in donor pig genetic modifications and effective immunosuppressive strategies through decades of rigorous translational research. Reports of pig kidney xenograft survival beyond 1 year posttranplant in nonhuman primate (NHP) models justify optimism for its potential as an alternative to allotransplantation. In the United States, experimental transplantations of genetically engineered (GE) porcine kidneys into brain-dead subjects and a small number of ESRD patients have shown no evidence of hyperacute rejection and adequate pig kidney function for up to several months. Here we discuss pre-clinical/clinical results, infectious disease, ethical, and regulatory considerations, and propose evidence-based recommendations. For initial clinical trials in kidney xenotransplantation, we make the following recommendations: (i) transplantation with organs from a triple knockout (TKO) donor pig, preferably with added human transgenes, (ii) an immunosuppressive regimen with induction therapy to deplete T (and possibly B) cells, and maintenance therapy based on a cluster of differentiation (CD)40/CD154 co-stimulation pathway blockade, (iii) the patient should be fully acceptable as a candidate for allotransplantation but should be unlikely ever to receive an allograft. Patients aged 60-69 years (extendable to 40-75 years, if one of the criteria mentioned below is present), of blood group B or O, and with diabetes are most at risk in this regard. Other patients who could be considered are (i) those who have lost two or more previous kidney allografts from recurrent disease in the graft, (ii) those with broad human leukocyte antigen (HLA)-reactivity but no evidence of anti-pig antibodies, including swine leukocyte antigen (SLA), and (iii) those with failing vascular access. Clinical pilot studies in carefully and highly selected patients with no alternative therapy will provide the foundation upon which to base subsequent formal expanded clinical trials.

The Evolution of Immunosuppressive Therapy in Pig-to-Nonhuman Primate Organ Transplantation

An overview is provided of the evolution of strategies towards xenotransplantation during the past almost 40 years, focusing on advances in gene-editing of the organ-source pigs, pre-transplant treatment of the recipient, immunosuppressive protocols, and adjunctive therapy. Despite initial challenges, including hyperacute rejection resulting from natural (preformed) antibody binding and complement activation, significant progress has been made through gene editing of the organ-source pigs and refinement of immunosuppressive regimens. Major steps were the identification and deletion of expression of the three known glycan xenoantigens on pig vascular endothelial cells, the transgenic expression of human "protective" proteins, e.g., complement-regulatory, coagulation-regulatory, and anti-inflammatory proteins, and the administration of an immunosuppressive regimen based on blockade of the CD40/CD154 T cell co-stimulation pathway. Efforts to address systemic inflammation followed. The synergy between gene editing and judicious immunomodulation appears to largely prevent graft rejection and is associated with a relatively good safety profile. Though there remains an incidence of severe or persistent proteinuria (nephrotic syndrome) in a minority of cases. This progress offers renewed hope for patients in need of life-saving organ transplants.

Genetically engineered pig heart transplantation in non-human primates

BACKGROUND

Improvement in gene modifications of donor pigs has led to the prevention of early cardiac xenograft rejection and significantly prolonged cardiac xenograft survival in both heterotopic and orthotopic preclinical non-human primate (NHP) models. This progress formed the basis for FDA approval for compassionate use transplants in two patients.

METHODS

Based on our earlier report of 9-month survival of seven gene-edited (7-GE) hearts transplanted (life-supporting orthotopic) in baboons, we transplanted 10 gene-edited pig hearts into baboons (n = 4) using non-ischemic continuous perfusion preservation (NICP) and immunosuppression regimen based on co-stimulation blockade by anti-CD40 monoclonal antibody. This pivotal study expands on the 7-GE backbone, with 3 additional gene edits, using 10-GE pigs as donors to baboon recipients.

RESULTS

10 GE cardiac xenografts provide life-supporting function up to 225 days (mean 128 ± 36 days) in a non-human primate model. Undetectable or latent porcine cytomegalovirus (PCMV) does not influence cardiac xenograft survival in this study but still needs more exploration with a larger cohort. Xenograft histology demonstrates adipose (Fat) deposition (n = 1), chronic vasculopathy (n = 1), micro and macro thrombosis, and acute cellular rejection (n = 1).

CONCLUSIONS

These data demonstrate that 10 GE cardiac xenografts have variable cardiac xenograft survival in NHP due to perhaps presence of 4th antigen and require further study. However, these 10GE organs may be suitable for clinical cardiac xenotransplantation and have already been utilized in two human cases.

Use of Brain Death Recipients in Xenotransplantation: A Double-Edged Sword

Organ transplants are used to treat many end-stage diseases, but a shortage of donors means many patients cannot be treated. Xenogeneic organs have become an important part of filling the donor gap. Many current studies of kidney, heart, and liver xenotransplantation have used gene-edited pig organs on brain-dead recipients. However, the endocrine system, immune system, and nervous system of brain-dead people are changed, which are different from that of real patients transplanted, and the current research results of brain death (BD) recipients are also different. So there are drawbacks to using brain-dead people for xenotransplantation. In addition, although the policy requires the use of non-human primate (NHP) experiments as the research standard for xenotransplantation, there are still differences between NHP and humans in terms of immunity. Therefore, to better study xenotransplantation, new models may be needed in addition to NHP and brain-dead individuals. Humanized animal models or organoids may be able to fill this gap.

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.

The decedent model: A new paradigm for de-risking high stakes clinical trials like xenotransplantation

The first 2 living recipients of pig hearts died unexpectedly within 2 months, despite both recipients receiving what over 30 years of nonhuman primate (NHP) research would suggest were the optimal gene edits and immunosuppression to ensure success. These results prompt us to question how faithfully data from the NHP model translate into human outcomes. Before attempting any further heart xenotransplants in living humans, it is highly advisable to gain a more comprehensive understanding of why the promising preclinical NHP data did not accurately predict outcomes in humans. It is also unlikely that additional NHP data will provide more information that would de-risk a xenoheart clinical trial because these cases were based on the best practices from the most successful NHP results to date. Although imperfect, the decedent model offers a complementary avenue to determine appropriate treatment regimens to control the human immune response to xenografts and better understand the biologic differences between humans and NHP that could lead to such starkly contrasting outcomes. Herein, we explore the potential benefits and drawbacks of the decedent model and contrast it to the advantages and disadvantages of the extensive body of data generated in the NHP xenoheart transplantation model.

Incidence of serum antibodies to xenoantigens on triple-knockout pig cells in different human groups

BACKGROUND

Xenoantigens other than Gal, Neu5Gc, and Sda may be playing a role in pig graft rejection. We investigated the incidence of antibodies to unknown pig xenoantigen in different human groups.

METHODS

We collected blood from TKO/hCD55 pigs (n = 3), and isolated PBMCs and RBCs. Serum samples were collected from (i) healthy human volunteers (n = 43), (ii) patients with end-stage renal disease (ESRD) (n = 87), (iii) the same patients after kidney allotransplantation (n = 50), and (iv) renal allotransplant recipients experiencing T cell-mediated rejection (allo-TCMR, n = 10). The sera were initially incubated with TKO/hCD55 pRBCs (1 × 108 cells) for 1 h to absorb anti-pig antibodies (except against SLA and possibly other antigens not expressed on pRBCs) and then the serum (absorbed or unabsorbed) was tested for antibody binding and complement-dependent cytotoxicity (CDC) to TKO/hCD55 pig PBMCs.

RESULTS

A significant reduction in IgM/IgG binding and CDC was observed in the absorbed sera. Serum obtained before and after renal allotransplantation showed no significant difference in IgM or IgG binding to, or in CDC of, TKO/hCD55 pig cells. IgM antibodies (but rarely IgG) against unknown xenoantigens expressed on TKO/hCD55 PBMCs, possibly against swine leukocyte antigens, were documented in healthy humans, patients with ESRD, and those with renal allografts undergoing acute T cell rejection. IgM (but not CDC) was higher in patients experiencing allo-TCMR.

CONCLUSION

Human sera contain IgM antibodies against unknown pig xenoantigens expressed on TKO/hCD55 pPBMCs. Although not confirmed in the present study, the targets for these antibodies may include swine leukocyte antigens.

Xenografts Show Signs of Concentric Hypertrophy and Dynamic Left Ventricular Outflow Tract Obstruction After Orthotopic Pig-to-baboon Heart Transplantation

BACKGROUND

Orthotopic cardiac xenotransplantation has seen substantial advancement in the last years and the initiation of a clinical pilot study is close. However, donor organ overgrowth has been a major hurdle for preclinical experiments, resulting in loss of function and the decease of the recipient. A better understanding of the pathogenesis of organ overgrowth after xenotransplantation is necessary before clinical application.

METHODS

Hearts from genetically modified ( GGTA1-KO , hCD46/hTBM transgenic) juvenile pigs were orthotopically transplanted into male baboons. Group I (control, n = 3) received immunosuppression based on costimulation blockade, group II (growth inhibition, n = 9) was additionally treated with mechanistic target of rapamycin inhibitor, antihypertensive medication, and fast corticoid tapering. Thyroid hormones and insulin-like growth factor 1 were measured before transplantation and before euthanasia, left ventricular (LV) growth was assessed by echocardiography, and hemodynamic data were recorded via a wireless implant.

RESULTS

Insulin-like growth factor 1 was higher in baboons than in donor piglets but dropped to porcine levels at the end of the experiments in group I. LV mass increase was 10-fold faster in group I than in group II. This increase was caused by nonphysiological LV wall enlargement. Additionally, pressure gradients between LV and the ascending aorta developed, and signs of dynamic left ventricular outflow tract (LVOT) obstruction appeared.

CONCLUSIONS

After orthotopic xenotransplantation in baboon recipients, untreated porcine hearts showed rapidly progressing concentric hypertrophy with dynamic LVOT obstruction, mimicking hypertrophic obstructive cardiomyopathy in humans. Antihypertensive and antiproliferative drugs reduced growth rate and inhibited LVOT obstruction, thereby preventing loss of function.

In vitro and in vivo immune assessments of genetically-engineered pig skin grafts in New World (squirrel) monkeys

Half a million patients in the USA alone require treatment for burns annually. Following an extensive burn, it may not be possible to provide sufficient autografts in a single setting. Genetic manipulations (GM) of pigs offer the possibility of reducing primate humoral and cellular rejection of pig skin xenografts and thus extending graft survival. We compared the survival of skin grafts from pigs with 9-GM with that of autografts and allografts in squirrel monkeys. Monitoring for rejection was by (1) macroscopic examination, (2) histopathological examination of skin biopsies, and (3) measurement of anti-monkey and anti-pig IgM and IgG antibodies. Autografts (n = 5) survived throughout the 28 days of follow-up without histopathological features of rejection. Median survival of allografts (n = 6) was 14 days and of pig xenografts (n = 12) 21 days. Allotransplantation was associated with an increase in anti-monkey IgM, but the anticipated subsequent rise in IgG had not yet occurred at the time of euthanasia. Pig grafts were associated with increases in anti-pig IgM and IgG. In all cases, histopathologic features of rejection were similar. 9-GM pig skin xenografts survive at least as long as monkey skin allografts (and trended to survive longer), suggesting that they are a realistic clinical option for the temporary treatment of burns. Although monkeys with pig skin grafts developed anti-pig IgM and IgG antibodies, these did not cross-react with monkey antigens, indicating that a primary 9-GM pig skin graft would not be detrimental to a subsequent monkey skin allograft.