Thyroid hormone: relevance to xenotransplantation

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.

Cardiac and Pulmonary Histopathology in Baboons Following Genetically-Engineered Pig Orthotopic Heart Transplantation

BACKGROUND Although improving, survival after pig orthotopic heart transplantation (OHTx) in baboons has been mixed and largely poor. The causes for the high incidence of early failure remain uncertain. MATERIAL AND METHODS We have carried out pig OHTx in 4 baboons. Two died or were euthanized within hours, and 2 survived for 3 and 8 months, respectively. There was evidence of a significant 'cytokine storm' in the immediate post-OHTx period with the elevations in IL-6 correlating closely with the final outcome. RESULTS All 4 baboons demonstrated features suggestive of respiratory dysfunction, including increased airway resistance, hypoxia, and tachypnea. Histopathological observations of pulmonary infiltration by neutrophils and, notably, eosinophils within vessels and in the perivascular and peribronchiolar space, with minimal cardiac pathology, suggested a role for early lung acute inflammation. In one, features suggestive of transfusion-related acute lung injury were present. The 2 longer-term survivors died of (i) a cardiac dysrhythmia with cellular infiltration around the conducting tissue (at 3 months), and (ii) mixed cellular and antibody-mediated rejection (at 8 months). CONCLUSIONS These initial findings indicate a potential role of acute lung injury early after OHTx. If this response can be prevented, increased survival may result, providing an opportunity to evaluate the factors affecting long-term survival.

What Therapeutic Regimen Will Be Optimal for Initial Clinical Trials of Pig Organ Transplantation?

We discuss what therapeutic regimen might be acceptable/successful in the first clinical trial of genetically engineered pig kidney or heart transplantation. As regimens based on a calcineurin inhibitor or CTLA4-Ig have proved unsuccessful, the regimen we administer to baboons is based on induction therapy with antithymocyte globulin, an anti-CD20 mAb (Rituximab), and cobra venom factor, with maintenance therapy based on blockade of the CD40/CD154 costimulation pathway (with an anti-CD40 mAb), with rapamycin, and a corticosteroid. An anti-inflammatory agent (etanercept) is administered for the first 2 wk, and adjuvant therapy includes prophylaxis against thrombotic complications, anemia, cytomegalovirus, and pneumocystis. Using this regimen, although antibody-mediated rejection certainly can occur, we have documented no definite evidence of an adaptive immune response to the pig xenograft. This regimen could also form the basis for the first clinical trial, except that cobra venom factor will be replaced by a clinically approved agent, for example, a C1-esterase inhibitor. However, none of the agents that block the CD40/CD154 pathway are yet approved for clinical use, and so this hurdle remains to be overcome. The role of anti-inflammatory agents remains unproven. The major difference between this suggested regimen and those used in allotransplantation is the replacement of a calcineurin inhibitor with a costimulation blockade agent, but this does not appear to increase the complications of the regimen.

Immunological selection and monitoring of patients undergoing pig kidney transplantation

Pig kidney xenotransplantation has the potential to alleviate the current shortage of deceased and living human organs and provide patients with end-stage renal disease with a greater opportunity for long-term survival and a better quality of life. In recent decades, advances in the genetic engineering of pigs and in immunosuppressive therapy have permitted the resolution of many historical obstacles to the success of pig kidney transplantation in nonhuman primates. Pig kidney xenotransplantation may soon be translated to the clinic. Given the potential risks of kidney xenotransplantation, particularly of immunologic rejection of the graft, potential patients must be carefully screened for inclusion in the initial clinical trials and immunologically monitored diligently post-transplantation. We provide an overview of the immunological methods we believe should be used to (i) screen potential patients for the first clinical trials to exclude those with a higher risk of rejection, and (ii) monitor patients with a pig kidney graft to determine their immunological response to the graft.

A perspective on the potential detrimental role of inflammation in pig orthotopic heart xenotransplantation

There is a critical shortage of deceased human donor organs for transplantation. The need is perhaps most acute in neonates and infants with life-threatening congenital heart disease, in whom mechanical support devices are largely unsuccessful. If orthotopic (life-supporting) heart transplantation (OHTx) were consistently successful in the genetically engineered pig-to-nonhuman primate (NHP) model, a clinical trial of bridging with a pig heart in such patients might be justified. However, the results of pig OHTx in NHPs have been mixed and largely poor. We hypothesise that a factor is the detrimental effects of the inflammatory response that is known to develop (a) during any surgical procedure that requires cardiopulmonary bypass, and (b) immediately after an NHP recipient is exposed to a pig xenograft. We suggest that the combination of these two inflammatory responses has a direct detrimental effect on pig heart graft function, but also, and possibly of more importance, on recipient baboon pulmonary function, which further impacts survival of the pig heart graft. In addition, the inflammatory response almost certainly adversely impacts the immune response to the graft. If our hypothesis is correct, the potential steps that could be taken to reduce the inflammatory response or its effects (with varying degrees of efficacy) include (a) white blood cell filtration, (b) complement depletion or inactivation, (c) immunosuppressive therapy, (d) high-dose corticosteroid therapy, (e) cytokine/chemokine-targeted therapy, (f) ultrafiltration or CytoSorb hemoperfusion, (g) reduction in the levels of endogenous catecholamines, (h) triiodothyronine therapy and (i) genetic engineering of the organ-source pig. Prevention of the inflammatory response, or attenuation of its effects, by judicious anti-inflammatory therapy may contribute not only to early survival of the recipient of a genetically engineered pig OHTx, but also to improved long-term pig heart graft survival. This would open the possibility of initiating a clinical trial of genetically engineered pig OHTx as a bridge to allotransplantation.

Update and breakthrough in cardiac xenotransplantation

PURPOSE OF REVIEW

Considerable advancements have been made in the field of cardiac xenotransplantation in the recent years, achieving prolonged survival of the life-supporting cardiac xenograft and paving the way toward first clinical implications.

RECENT FINDINGS

The combination of genetic modifications and novel immunosuppression with costimulation blockade, as well as supporting therapy with antiinflammatory treatment, growth prevention, and adaptation of the heart procurement system to reduce myocardial ischemia and reperfusion injury improves the overall cardiac xenograft function and overall survival in nonhuman primates. Through the newly identified xenoantigens and novel gene-editing techniques, further genetic modification of the porcine xenografts should be explored, to ensure clinical safety.

SUMMARY

With continuous progress in all fields of cardiac xenotransplantation, first clinical use in humans seems accomplishable. To ensure the clinical safety and to conform to the ethical regulations, further investigation of the infectious and immunological implications on humans should be explored prior to first clinical use. The first clinical use of cardiac xenotransplantation will be limited to only highly selected patients.

Euthyroid sick syndrome in paediatric and adult patients requiring extracorporeal circulatory support and the role of thyroid hormone supplementation: a review

Non-thyroid disorders may modify thyroid hormone metabolism, resulting in an 'euthyroid sick syndrome'. Studies determining the association of cardiopulmonary bypass to thyroid function showed changes in line with this euthyroid sick syndrome. In some cases, cardiovascular dysfunction after cardiac surgery with cardiopulmonary bypass is comparable to that noticed in hypothyroidism associated with low cardiac output and elevated systemic vascular resistance. Numerous lines of research have proposed that triiodothyronine can behave acutely as a positive inotropic and vasodilator agent. The aim of this review is to present an update on the current literature about in what clinical situations the use of thyroid supplementation during the perioperative period of extracorporeal circulation in the adult and paediatric populations may impact outcome to any appreciable degree. The contribution of thyroid function in patients undergoing a ventricular assist device implantation is additionally reviewed and future study directions are proposed. This is a narrative review, where the search strategy consisted on retrieving the articles through an extensive literature search performed using electronic databases from January 1978 up to September 2019. All controlled trials randomly allocating to perioperative thyroid hormone administration in children and adults undergoing extracorporeal circulation for cardiac surgery were considered. Thyroid hormone supplementation may be recommended particularly in selected paediatric sub-populations. There is currently no firm evidence regarding the benefits of routine use of thyroid hormone administration in cardiac adult patients. Further studies are required to assess the beneficial effect of thyroid hormone on patients with end-stage heart failure supported by ventricular assist devices.

Evidence for the important role of inflammation in xenotransplantation

There is increasing evidence of a sustained state of systemic inflammation after pig-to-nonhuman primate (NHP) xenotransplantation (that has been termed systemic inflammation in xenograft recipients [SIXR]). Increases in inflammatory markers, e.g., C-reactive protein, histones, serum amyloid A, D-dimer, cytokines, chemokines, and a decrease in free triiodothyronine, have been demonstrated in the recipient NHPs. The complex interactions between inflammation, coagulation, and the immune response are well-recognized, but the role of inflammation in xenograft recipients is not fully understood. The evidence suggests that inflammation can promote the activation of coagulation and the adaptive immune response, but the exact mechanisms remain uncertain. If prolonged xenograft survival is to be achieved, anti-inflammatory strategies (e.g., the administration of anti-inflammatory agents, and/or the generation of genetically-engineered organ-source pigs that are protected from the effect of inflammation) may be necessary to prevent, control, or negate the effect of the systemic inflammation that develops in xenograft recipients. This may allow for a reduction in the intensity of exogenous immunosuppressive therapy. If immunological tolerance to a xenograft is to be obtained, then control of inflammation may be essential.

Overcoming Coagulation Dysregulation in Pig Solid Organ Transplantation in Nonhuman Primates: Recent Progress

There has recently been considerable progress in the results of pig organ transplantation in nonhuman primates, largely associated with the availability of (i) pigs genetically engineered to overcome coagulation dysregulation, and (ii) novel immunosuppressive agents. The barriers of thrombotic microangiopathy and/or consumptive coagulation were believed to be associated with (i) activation of the graft vascular endothelial cells by a low level of antipig antibody binding and/or complement deposition and/or innate immune cell activity, and (ii) molecular incompatibilities between the nonhuman primate and pig coagulation-anticoagulation systems. The introduction of a human coagulation-regulatory transgene, for example, thrombomodulin, endothelial protein C receptor, into the pig vascular endothelial cells has contributed to preventing a procoagulant state from developing, resulting in a considerable increase in graft survival. In the heterotopic (non-life-supporting) heart transplant model, graft survival has increased from a maximum of 179 days in 2005 to 945 days. After life-supporting kidney transplantation, survival has been extended from 90 days in 2004 to 499 days. In view of the more complex coagulation dysfunction seen after pig liver and, particularly, lung transplantation, progress has been less dramatic, but the maximum survival of a pig liver has been increased from 7 days in 2010 to 29 days, and of a pig lung from 4 days in 2007 to 9 days. There is a realistic prospect that the transplantation of a kidney or heart, in combination with a conventional immunosuppressive regimen, will enable long-term recipient survival.

Xenotransplantation: past, present, and future

PURPOSE OF REVIEW

To review the progress in the field of xenotransplantation with special attention to most recent encouraging findings which will eventually bring xenotransplantation to the clinic in the near future.

RECENT FINDINGS

Starting from early 2000, with the introduction of galactose-α1,3-galactose (Gal)-knockout pigs, prolonged survival especially in heart and kidney xenotransplantation was recorded. However, remaining antibody barriers to non-Gal antigens continue to be the hurdle to overcome. The production of genetically engineered pigs was difficult requiring prolonged time. However, advances in gene editing, such as zinc finger nucleases, transcription activator-like effector nucleases, and most recently clustered regularly interspaced short palindromic repeats (CRISPR) technology made the production of genetically engineered pigs easier and available to more researchers. Today, the survival of pig-to-nonhuman primate heterotopic heart, kidney, and islet xenotransplantation reached more than 900, more than 400, and more than 600 days, respectively. The availability of multiple-gene pigs (five or six genetic modifications) and/or newer costimulation blockade agents significantly contributed to this success. Now, the field is getting ready for clinical trials with an international consensus.

SUMMARY

Clinical trials in cellular or solid organ xenotransplantation are getting closer with convincing preclinical data from many centers. The next decade will show us new achievements and additional barriers in clinical xenotransplantation.

The impact of serum incubation time on IgM/IgG binding to porcine aortic endothelial cells

The results of the assay for measuring anti-non-Gal antibodies (which affect pig xenograft survival) in recipients are important. Serum incubation time and concentration may be important factors in the extent of antibody binding to the graft. The aim of this in vitro study was to determine the optimal incubation time and serum concentration for measuring anti-non-Gal antibody binding to porcine aortic endothelial cells (pAECs). Pooled human, naive, and sensitized baboon sera were incubated with wild-type, α1,3-galactosyltransferase gene-knockout (GTKO), and GTKO/human CD55 pAECs. IgM/IgG binding to pAECs after varying serum incubation times (0.5, 1, 2, and 3 hour) and concentrations (5, 10, 20, and 40 μL) was determined by flow cytometry. An increase in incubation time from 30 minutes to 2 hour was associated with increases in anti-non-Gal IgM/IgG binding to GTKO and GTKO/hCD55 pAECs of pooled human, naive and sensitized baboon sera (P<.05). Pooled human serum showed a significant increase in anti-non-Gal IgM (1.5 times) and a minimal increase in anti-non-Gal IgG antibody binding. IgM/IgG binding of sensitized baboon serum to GTKO pAECs after 2-hour incubation was 1.5 times and 2 times greater than after 30-minutes incubation, respectively, whereas naïve baboon sera showed minimal (non-significant) increase in anti-non-Gal IgM/IgG antibody binding. With 2-hour incubation, increasing the serum concentration from 5 μL to 20 μL significantly increased antibody binding to non-Gal antigens in pooled human and sensitized baboon serum. With naïve baboon serum, only IgG was significantly increased. Increasing the serum incubation time contributed to improve the sensitivity of detecting anti-non-Gal antibodies, without affecting cell viability in vitro.

Immunological and physiological observations in baboons with life-supporting genetically engineered pig kidney grafts

BACKGROUND

Genetically engineered pigs could provide a source of kidneys for clinical transplantation. The two longest kidney graft survivals reported to date have been 136 and 310 days, but graft survival >30 days has been unusual until recently.

METHODS

Donor pigs (n=4) were on an α1,3-galactosyltransferase gene-knockout (GTKO)/human complement regulatory protein (CD46) background (GTKO/CD46). In addition, the pigs were transgenic for at least one human coagulation regulatory protein. Two baboons received a kidney from a six-gene pig (GroupA) and two from a three-gene pig (GroupB). Immunosuppressive therapy was identical in all four cases and consisted of anti-thymoglobulin (ATG)+anti-CD20mAb (induction) and anti-CD40mAb+rapamycin+corticosteroids (maintenance). Anti-TNF-α and anti-IL-6R mAbs were administered to reduce the inflammatory response. Baboons were followed by clinical/laboratory monitoring of immune/coagulation/inflammatory/physiological parameters. At biopsy or euthanasia, the grafts were examined by microscopy.

RESULTS

The two GroupA baboons remained healthy with normal renal function >7 and >8 months, respectively, but then developed infectious complications. However, no features of a consumptive coagulopathy, eg, thrombocytopenia and reduction of fibrinogen, or of a protein-losing nephropathy were observed. There was no evidence of an elicited anti-pig antibody response, and histology of biopsies taken at approximately 4, 6, and 7 months and at necropsy showed no significant abnormalities. In contrast, both GroupB baboons developed features of a consumptive coagulopathy and required euthanasia on day 12.

CONCLUSIONS

The combination of (i) a graft from a specific six-gene genetically modified pig, (ii) an effective immunosuppressive regimen, and (iii) anti-inflammatory therapy prevented immune injury, a protein-losing nephropathy, and coagulation dysfunction for >7 months. Although the number of experiments is very limited, our impression is that expression of human endothelial protein C receptor (±CD55) in the graft is important if coagulation dysregulation is to be avoided.