Xenogeneic Crosscirculation for Extracorporeal Recovery of Injured Human Lungs
Hozain AE, O’Neill JD, Pinezich MR, et al. Nat Med. 2020;26(7):1102–1113.
The shortage of suitable donor organs remains a primary limitation of solid organ transplantation. In lung transplantation, for example, only 15%–20% of donor lungs are utilized. Ex vivo lung perfusion (EVLP) has become an established approach to evaluate and rehabilitate marginal donor lungs to expand the donor pool.1 Current EVLP systems perfuse lungs at normothermic temperature with either cellular or acellular perfusate, using mechanical pumps in a constantly recirculating system. Obvious physiologic shortcomings include the clearance of waste products, lack of regulation of glucose metabolism, and lack of neuro hormonal support. As such, EVLP systems thus far have predominantly been used for short-term assessment of marginal lung grafts rather than platforms to support more prolonged periods of repair and regeneration.
In this study, Hozain et al2 demonstrated the feasibility of a novel xenogeneic crosscirculation approach to promote repair and recovery of injured human lung grafts. Human lung grafts are perfused continuously by blood from an anesthetized swine in a closed circuit. In essence, the swine serves as a bioreactor, providing complex metabolic support to promote recovery of the injured graft. Using this approach, the authors demonstrate significant improvements in tissue integrity and multiple parameters of lung function over the course of 24 h. As ex vivo organ perfusion technology continues to move toward applications of repair and regeneration, xenogeneic crosscirculation demonstrates the immense benefit of recapitulating physiologic conditions as closely as possible. Similar concepts were also highlighted recently by Eshmuminov et al,3 who demonstrated the ability to sustain a liver graft for 7 d ex vivo by incorporating several innovations to the perfusion platform, including automated glucose management and continuous dialysis.
The future of xenogeneic crosscirculation appears bright, although there are critical areas that require further investigation. In this study, the authors noted the presence of swine leukocytes in the graft after 24 h of crosscirculation. The immunologic implications of this observation following transplantation of such a graft remain unknown. Another significant issue to contend with going forward is the possibility of transmission of infection, particularly porcine endogeneous retroviruses. Of note, improving capacities to genetically engineer pigs for the use as crosscirculation hosts may be adequately used to address both potential issues. Translation of this approach to other organs will also require modification of hemodynamics, which may not be trivial. For crosscirculation of lung grafts, the authors demonstrated success using deoxygenated blood at relatively low flow rates. However, adequate perfusion of a liver graft will require approximately 5-fold the flow rate used in this study, including both venous and arterial blood, which may lead to hemodynamic instability of the swine over a prolonged period. Furthermore, whether the immunosuppression regimen used here for lung crosscirculation will be adequate for species-specific metabolic products of the liver graft to prevent immunological damage and preserve host stability during crosscirculation may also warrant further studies.
In summary, this interesting study demonstrates the feasibility of a highly innovative approach to resuscitating damaged lung grafts for transplantation. This laudable achievement will undoubtedly spur advances in vivo organ perfusion technology and regenerative medicine approaches going forward.
Senolytic CAR T Cells Reverse Senescence-associated Pathologies
Amor C, Feucht J, Leibold J, et al. Nature. Jun 2020.
Senescence refers to a form of cellular stress response that normally results in cell-cycle arrest of the senescent cells, typically accompanied by secretion of a complex cocktail of factors recruiting immune cells.1 However, aberrant accumulation of senescent cells promotes chronic inflammation, leading to organ fibrosis, cancer development, and a number of age-related diseases such as atherosclerosis and diabetes. Small molecule drugs called “senolytic agents,” many repurposed from anticancer drugs, have been experimented for selectively inducing apoptosis of senescent cells.2 However, these agents frequently carry significant side effects.
In the present study, Amor et al3 applied an entirely different approach to eliminate senescent cells. They took advantage of the chimeric antigen receptor (CAR) technology and generated cytotoxic T cells that specifically target senescent cells, identified by their characteristic upregulation of the cell surface urokinase-type plasminogen activator receptor (uPAR). In this study, cell surface uPAR was first identified and selected as a targetable senescent cell marker by overlapping RNA-sequencing databases derived from 3 independent murine models of senescence; this step was followed by excluding the expression of uPAR on vital tissues based on the Human Protein Atlas. CAR T cells with specificity to uPAR were then constructed and tested for their senolytic effects in vitro and in vivo. In vitro, uPAR CAR T cells effectively cytolysed human B cells transduced to express the target antigen uPAR as well as KP cells, senescent lung cancer cells that express endogenous uPAR. In vivo, uPAR CAR T cells effectively eliminated senescent cells in several established murine models: (1) oncogene-induced hepatocyte senescence by hepatic overexpression of NrasGL2V; (2) CCl4-induced liver fibrosis; and (3) nonalcoholic steatohepatitis. In the latter 2 models, uPAR CAR T cells did not only efficiently eliminate senescent cells but also reduced liver fibrosis and improved liver function. Interestingly, subsequently to an in vivo adoptive transfer, uPAR CAR T cells developed an effector memory phenotype at the site of their targets with little evidence of T-cell exhaustion.
Intrinsic to CAR T-cell therapy is the risk for cytokine-release syndrome. In the present study, the authors showed that it was possible to minimize this risk by reducing the dose of uPAR CAR T cells that was still effective for senolysis. However, the authors also observed a moderate infiltration of macrophages into the lungs even at the low uPAR CAR T-cell dose. The cause and consequence of such pulmonary macrophage infiltration warrant further investigation. On the other hand, it is also conceivable that uPAR CAR T cells may activate apoptosis-mediated immune tolerance.4 How these confounding processes may contribute to the ultimate efficacy of this therapeutic modality remains to be seen. If resolved, this form of therapy could be highly beneficial to cancer treatment and to numerous age-related diseases.
Relevant to the field of transplantation, organ donor is thought to significantly influence the donor organ quality. For instance, an increasing donor age significantly increases the Kidney Donor Profile Index, a calculated parameter commonly used to estimate the relative risk of kidney graft failure for that donor in comparison to the median. Whether or not senolytic CAR T cells can be used to “rejuvenate” organs from older donors will have a significant impact on our ability to expand the donor pool and shorten wait time for our transplant recipients. In this regard, it would be informative to determine the following: (1) in procured organs, whether uPAR remains an adequate target for senolysis; (2) whether CAR T cell–based senolytic therapies might also eliminate organ parenchymal cells that, albeit senescent, are vital to the function of the procured organ5; and (3) whether treatment of the donor or the organ during preservation would minimize unwanted side effects of this therapeutic modality.
In summary, the current study provides the proof-of-principle evidence of the feasibility of using senolytic CAR T cells to eliminate senescent cells, possibly reversing age-associated pathologies with relevance for organ transplantation.
1. Cypel M, Yeung JC, Liu M, et al. Normothermic ex vivo lung perfusion in clinical lung transplantation N Engl J Med. 2011; 364:1431–1440. doi: 10.1056/NEJMoa1014597
2. Hozain AE, O’Neill JD, Pinezich MR, et al. Xenogeneic cross-circulation for extracorporeal recovery of injured human lungs Nat Med. 2020; 26:1102–1113. doi: 10.1038/s41591-020-0971-8
3. Eshmuminov D, Becker D, Bautista Borrego L, et al. An integrated perfusion machine preserves injured human livers for 1 week Nat Biotechnol. 2020; 38:189–198. doi: 10.1038/s41587-019-0374-x
1. Gorgoulis V, Adams PD, Alimonti A, et al. Cellular senescence: defining a path forward Cell. 2019; 179:813–827. doi: 10.1016/j.cell.2019.10.005
2. Xu M, Pirtskhalava T, Farr JN, et al. Senolytics improve physical function and increase lifespan in old age Nat Med. 2018; 24:1246–1256. doi: 10.1038/s41591-018-0092-9
3. Amor C, Feucht J, Leibold J, et al. Senolytic CAR T cells reverse senescence-associated pathologies Nature. 2020; 583:127–132. doi: 10.1038/s41586-020-2403-9
4. Morioka S, Maueroder C, Ravichandran KS. Living on the edge: efferocytosis at the interface of homeostasis and pathology Immunity. 2019; 50:1149–1162. doi: 10.1016/j.immuni.2019.04.018
5. Hayek SS, Leaf DE, Tahhan AS, et al. Soluble urokinase receptor and acute kidney injury N Engl J Med. 2020; 382:416–426. doi: 10.1056/NEJMoa1911481