Preclinical life-supporting lung xenotransplantation – where do we stand?    

L Burdorf and RN Pierson III

Department of Surgery, University of Maryland School of Medicine and VA Maryland Health Care System, Baltimore, MD, USA

In contrast to steadily improving preclinical results for life-supporting heart or kidney xenotransplants, with outcomes measured in weeks or months, pig lung xenograft function in baboons is still measured in hours.  Here we summarize significant progress toward consistent long-term lung xenograft survival.

As is true of other organs, xenogenic lung injury is associated with activation of the coagulation- and complement cascade, neutrophil and platelet activation, endothelial cell activation, vascular leakage leading to trachea- and lung edema and an increase in pulmonary vascular resistance.  Our work suggests that possible explanations for the discrepancy in outcomes (increased vulnerability of the lung to rapid injury) might include the high number of pro-inflammatory pulmonary intravascular macrophages (PIMs), high levels of pro-coagulant von Willebrand factor (vWF), as well as to a fragile and anatomically vulnerable lung parenchyma based on rheology, thrombogenicity, and architecture.  We hypothesize that the lung must be more completely protected from insults, most of which are likely common to all xenografts, to achieve similar preclinical survival results.

In our prior reports, pig-to-baboon xenogenic lung transplantation using lungs from pigs with either hCD46 or GalTKO genetic modifications allowed inconsistent life-supporting function (median 21 min, range 0-225 min) while the combination of both genetic modifications (GalTKO.hCD46) was associated with up to 4 hours of life-support (median 240 min, range 0-240 min). Addition of coagulation pathway regulatory genes including human thrombomodulin (hTM) and the human endothelial protein-C receptor (hEPCR), evaluated in recent transplants of GalTKO.hCD46.hTM and GalTKO.hCD46.hEPCR transgenic lungs, consistently extended life-supporting function to up to 10 hours with sustained low pulmonary vascular resistance (PVR).  Desmopressin (DDAVP), given to the donor animal prior to organ procurement to deplete vWF, and anti-Glycoprotein Ib (aGPIb) Fab, blocking platelet binding and activation mediated by vWP-GPIb interaction, were used in these experiments, based on ex vivo perfusion data showing inhibition of initial platelet activation and sequestration. With the combination of DDAVP/aGPIb treatment and lungs from either GalTKO.hCD46.hTM or GalTKO.hCD46.hEPCR transgenic pigs, this phenomenon could be almost completely be prevented. While only about 50% of initial platelets remained in the blood at one hour after reperfusion with GalTKO-lungs, platelet counts remained at >90% in transplants using this combination. Because one of the main factors preventing clinical applications of xenogenic organs – not only lungs and livers, but also in some long-term heart recipients – has been the platelet sequestration, associated with coagulopathy, bleeding complications and death, this observation may have relevance beyond lung xenotransplantation.

Thus genetic engineering of pigs and the addition of clinically applicable drug treatments that successfully inhibit mechanisms known to cause xenogenic lung injury are associated with extended lung survival time.  Planned addition of further transgenes (tissue factor pathway inhibitor or CD39; knock-down of pig vWF) and depletion of PIMs should be expected to further advance lung xenotransplantation.