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In View: Game Changer

Clearing the Antigenic Slate of Donor Organs Using Ex Vivo Perfusion

Shepherd, Hailey M. MD1; Bery, Amit I. MD2; Kreisel, Daniel MD, PhD1,3

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doi: 10.1097/TP.0000000000004182
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Pulmonary transplantation is generally avoided in the context of ABO incompatibility because of fear of preformed recipient antibodies binding foreign donor blood antigens, resulting in hyperacute or acute antibody-mediated rejection. Most documented cases of ABO-incompatible lung transplantation have thus far occurred in the setting of clerical errors with variable clinical outcomes. One widely publicized case of unintentional ABO-incompatible lung transplantation resulted in rapid development of severe rejection with graft dysfunction and ultimately death.1 Other unintentional cases have resulted in longer graft survival but required close monitoring of anti-ABO blood group antibody titers and utilization of aggressive postoperative immunosuppressive interventions including plasmapheresis, intravenous immunoglobulin, immunoadsorption, anti-CD20 antibody, and cyclophosphamide therapy.2-4 These recipient-centric desensitization strategies have since been utilized in 2 cases of intentional ABO-incompatible lung transplantation with acceptable short-term outcomes.5,6 Altogether, the rigorous immunosuppression required and the perilous clinical outcomes associated with intentional ABO-incompatible lung transplantation continue to limit its utility.

In addition to rising rates of waitlist mortality due to shortage of available donor organs, ABO incompatibilities limited access to donor organs. This disparity has persisted despite numerous changes to organ allocation policies over the last 2 decades, resulting in a significantly higher risk of waitlist mortality for patients with blood type O.7 Because of the paucity of available donors in the current climate of drastic need and the unfortunate risks associated with ABO-incompatible lung transplantation, the development of donor-centric strategies to safely expand the pool of universal donor organs and eliminate blood type disparities in organ availability is paramount.

In a recent issue of Science Translational Medicine, Wang et al8 reported remarkable findings demonstrating successful conversion of blood type A human donor lungs into universal donor type lungs by cleaving the A-antigen from graft endothelium using enzymatic ex vivo perfusion. The authors utilized 2 enzymes derived from the bacterium Flavonifractor plautii to cleave the N-acetyl-galactosamine residue (referred to as Azymes: FpGalNAc deacetylase and FpGalNase) from blood type A-antigen and convert it to H-antigen, effectively rendering treated tissues as blood type O. The authors first demonstrated that A-antigen could be effectively cleared from lung perfusate and excised human aortic tissue using even small doses of Azymes. Subsequently, blood type A1 human donor lungs deemed unsuitable for clinical transplantation were randomized to normothermic ex vivo lung perfusion (EVLP) using blood type O human plasma perfusate with or without the addition of Azymes. Remarkably, the authors found that within 1 h of Azyme treatment, A-antigen within the graft had been almost entirely depleted from the vascular endothelium and nearly 50% of the A-antigen had been cleaved from alveolar epithelium. Additionally, Azyme-treated lungs had significantly lower anti-A-antibody binding capacities and preserved lung physiology compared with untreated controls. This innovative approach holds substantial promise for re-engineering and personalizing donor organs for transplantation and may provide an elegant solution for mitigating the increased waitlist mortality for blood type O lung transplant candidates. Additionally, this approach proposes an exciting new niche for the utilization of EVLP technology in lung transplantation.

Several open questions remain before it will be possible to translate these findings to clinical transplantation: Will similar findings be observed with ex vivo perfusion when whole blood is used instead of cell-free plasma? Will antibody-mediated rejection be triggered in vivo because of the presence of other molecular and cellular physiologic cues not reproducible in an ex vivo perfusion model? How long can the donor blood type remain camouflaged? As astutely pointed out by the authors, endogenous glycosyltransferases installing ABO blood antigens will remain present within the donor tissues and would likely regenerate the cleaved donor antigens within a matter of hours. Once the reemergence of these donor blood antigens occurs, could the recipient immune responses be altered to promote accommodation of the donor graft and induce a state of ABO tolerance? Even if hyperacute and antibody-mediated rejection are effectively prevented, how will this strategy affect longer-term outcomes such as graft function and development of chronic lung allograft dysfunction? Additionally, it remains unclear whether the incomplete depletion of donor blood antigens elsewhere in the donor graft, such as alveolar epithelium, may result in adverse outcomes. To this end, alternative routes of enzyme administration (such as nebulization) may be necessary to achieve complete donor blood antigen depletion.

Incorporation of additional variables such as cellular perfusion and in vivo physiology remain pivotal to answering these questions and to further delineating the clinical relevance of this novel strategy. Indeed, evaluating antibody binding, lung physiology, and graft function after longer periods of perfusion with ABO-incompatible plasma and whole blood will be necessary. Additionally, utilization of transgenic mice that express human ABO blood antigens in transplant models may provide crucial insights regarding the re-expression kinetics of blood antigens following enzyme treatment and its associated long-term effects and posttransplant outcomes.9

Importantly, the development of donor-centric strategies to modify and personalize organs before transplantation could help provide other solutions to the worldwide donor organ shortage. For example, this technology could plausibly be expanded to allow cleavage of human leukocyte antigens, which remain a major barrier to transplantation for sensitized lung transplant candidates. One could also envision adding this approach to recent breakthroughs in transgenic modification of porcine organs to further advance the field of xenotransplantation.10 Undoubtedly, the possibility of using EVLP to clean the antigenic slate of excised donor lungs opens many exciting windows for expanding the pool of universal donor organs and beyond.

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