Lung transplantation, ex-vivo reconditioning and regeneration: state of the art and perspectives

Recovery from cold-induced mitochondrial fission in endothelial cells requires reconditioning temperatures of ≥ 25◦C

Mitochondrial integrity and function constitute a prerequisite for cellular function and repair processes. We have previously shown that mitochondria of different cell types exhibit pronounced fragmentation under hypothermic conditions. This fission, accompanied by a decline of cellular ATP content, showed reversibility at 37◦C. However, it is unclear whether other temperatures as currently discussed for reconditioning of organs allow this reconstitution of mitochondria. Therefore, we here study in a model of cultured porcine aortic endothelial cells how different rewarming temperatures affect mitochondrial re-fusion and function. After 48 h cold incubation of endothelial cells in Krebs-Henseleit buffer with glucose (5 mM) and deferoxamine (1 mM) at 4◦C pronounced mitochondrial fission was observed. Following 2 h rewarming in cell culture medium, marked fission was still present after rewarming at 10◦ or 15◦C. At 21◦C some re-fusion was visible, which became more marked at 25◦C. Networks of tubular mitochondria similar to control cells only re-appeared at 37◦C. ATP content decreased at 4◦C from 3.6 ± 0.4 to 1.6 ± 0.4 nmol/106 cells and decreased even further when rewarming cells to 10◦ and 15◦C. Values after rewarming at 21◦C were similar to the values before rewarming while ATP gradually increased at higher rewarming temperatures. Metabolic activity dropped to 5 ± 11% of control values during 4◦C incubation and recovered with increasing temperatures to 36 ± 10% at 25◦C and 78 ± 17% at 37◦C. Integrity of monolayers, largely disturbed at 4◦C (large gaps between endothelial cells; cell injury ≤ 1%), showed partial recovery from 15◦C upwards, complete recovery at 37◦C. Endothelial repair processes (scratch assay) at 25◦C were clearly inferior to those at 37◦C. These data suggest that reconditioning temperatures below 21◦C are not optimal with regard to reconstitution of mitochondrial integrity and function. For this goal, temperatures of at least 25◦C appear required, with 30◦C being superior and 37◦C yielding the best results.

Specialized Donor Care Facility Model and Advances in Management of Thoracic Organ Donors

BACKGROUND

Donor hearts and lungs are more susceptible to the inflammatory physiologic changes that occur after brain death. Prior investigations have shown that protocolized management of potential organ donors can rehabilitate donor organs that are initially deemed unacceptable. In this review we discuss advances in donor management models with particular attention to the specialized donor care facility model. In addition we review specific strategies to optimize donor thoracic organs and improve organ yield in thoracic transplantation.

METHODS

We performed a literature review by searching the PubMed database for medical subject heading terms associated with organ donor management models. We also communicated with our local organ procurement organization to gather published and unpublished information first-hand.

RESULTS

The specialized donor care facility model has been shown to improve the efficiency of organ donor management and procurement while reducing costs and minimizing travel and its associated risks. Lung protective ventilation, recruitment of atelectatic lung, and hormone therapy (eg, glucocorticoids and triiodothyronine/thyroxine) are associated with improved lung utilization rates. Stroke volume-based resuscitation is associated with improved heart utilization rates, whereas studies evaluating hormone therapy (eg, glucocorticoids and triiodothyronine/thyroxine) have shown variable results.

CONCLUSIONS

Lack of high-quality prospective evidence results in conflicting practices across organ procurement organizations, and best practices remain controversial. Future studies should focus on prospective, randomized investigations to evaluate donor management strategies. The specialized donor care facility model fosters a collaborative environment that encourages academic inquiry and is an ideal setting for these investigations.

Mesenchymal Stromal/Stem Cells and Their Products as a Therapeutic Tool to Advance Lung Transplantation

Lung transplantation (LTx) has become the gold standard treatment for end-stage respiratory failure. Recently, extended lung donor criteria have been applied to decrease the mortality rate of patients on the waiting list. Moreover, ex vivo lung perfusion (EVLP) has been used to improve the number/quality of previously unacceptable lungs. Despite the above-mentioned progress, the morbidity/mortality of LTx remains high compared to other solid organ transplants. Lungs are particularly susceptible to ischemia-reperfusion injury, which can lead to graft dysfunction. Therefore, the success of LTx is related to the quality/function of the graft, and EVLP represents an opportunity to protect/regenerate the lungs before transplantation. Increasing evidence supports the use of mesenchymal stromal/stem cells (MSCs) as a therapeutic strategy to improve EVLP. The therapeutic properties of MSC are partially mediated by secreted factors. Hence, the strategy of lung perfusion with MSCs and/or their products pave the way for a new innovative approach that further increases the potential for the use of EVLP. This article provides an overview of experimental, preclinical and clinical studies supporting the application of MSCs to improve EVLP, the ultimate goal being efficient organ reconditioning in order to expand the donor lung pool and to improve transplant outcomes.

Custodiol-MP for ex vivo lung perfusion - A comparison in a porcine model of donation after circulatory determination of death

INTRODUCTION

Ex vivo lung perfusion (EVLP) is an established technique to evaluate and eventually recondition lungs prior to transplantation. Custodiol-MP (C-MP) solution is a new solution, designed for clinical machine perfusion, that has been used for kidneys. The aim of this study was to compare the effects of EVLP with Custodiol-MP on lung functional outcomes to the gold standard of EVLP with Steen Solution™.

MATERIAL AND METHODS

In a porcine EVLP model of DCDD (Donation after Circulatory Determination of Death), lungs were perfused with Steen Solution™ (SS, n = 7) or Custodiol-MP solution supplemented with 55 g/l albumin (C-MP, n = 8). Lungs were stored cold for 4 h in low potassium dextran solution and subsequently perfused ex vivo for 4 h. During EVLP pulmonary gas exchange, activities of lactate dehydrogenase (LDH) and alkaline phosphatase (AP) as well as levels of lactate in the perfusate were recorded hourly.

RESULTS

Oxygenation capacity differed significantly between groups (averaged over 4 h: SS 274 ± 178 mmHg; C-MP 284 ± 151 mmHg p = 0.025). Lactate dehydrogenase activities and lactate concentrations were significantly lower in Custodiol-MP perfused lungs.In a porcine model of DCDD with 4 h of EVLP the use of modified Custodiol-MP as perfusion solution was feasible. The use of C-MP showed at least comparable lung functional outcomes to the use of Steen SolutionTM. Furthermore C-MP perfusion resulted in significantly lower lactate dehydrogenase activity and lactate levels in the perfusate and higher oxygenation capacity.

Ischemia-Reperfusion Injury in Lung Transplantation

Lung transplantation has been established worldwide as the last treatment for end-stage respiratory failure. However, ischemia–reperfusion injury (IRI) inevitably occurs after lung transplantation. The most severe form of IRI leads to primary graft failure, which is an important cause of morbidity and mortality after lung transplantation. IRI may also induce rejection, which is the main cause of mortality in recipients. Despite advances in donor management and graft preservation, most donor grafts are still unsuitable for transplantation. Although the pulmonary endothelium is the primary target site of IRI, the pathophysiology of lung IRI remains incompletely understood. It is essential to understand the mechanism of pulmonary IRI to improve the outcomes of lung transplantation. Therefore, we reviewed the state-of-the-art in the management of pulmonary IRI after lung transplantation. Recently, the ex vivo lung perfusion (EVLP) system has been clinically introduced worldwide. Various promising therapeutic strategies for the protection of the endothelium against IRI, including EVLP, inhalation therapy with therapeutic gases and substances, fibrinolytic treatment, and mesenchymal stromal cell therapy, are awaiting clinical application. We herein review the latest advances in the field of pulmonary IRI in lung transplantation.

Evidence of Air Trapping During Ex Vivo Lung Perfusion: A Swine Experimental Lung Imaging and Mechanics Study

Ex vivo lung perfusion (EVLP) allows the ventilation and perfusion of lungs to evaluate their viability for transplantation. The aim of this study is to compare the mechanical, morphologic and functional properties of lungs during EVLP with values obtained in vivo to guide a safe mechanical ventilation strategy. Lungs from 5 healthy pigs were studied in vivo and during 4 hours of EVLP. Lung compliance, airway resistance, gas exchange, and hemodynamic parameters were collected at positive end-expiratory pressure (PEEP) of 5 cm H₂O. Computed tomography was performed at PEEP 0, PEEP 5, and total lung capacity (TLC). Lung pressure-volume (PV) curves were performed from PEEP 0 to TLC. Lung compliance decreased during EVLP (53 ± 5 mL/cm H₂O vs 29 ± 7 mL/cm H₂O, P < .05), and the PV curve showed a lower inflection point. Gas content (528 ± 118 mL vs 892 ± 402 mL at PEEP 0) and airway resistance (25 ± 5 vs 44 ± 9 cmH₂O/L∗s^-1, P < .05) were higher during EVLP. Alveolar dead space (5% ± 2% vs 17% ± 6%, P < .05) and intrapulmonary shunt (9% ± 2% vs 28% ± 13%, P < .05) increased ex vivo compared to in vivo, while the partial pressure of oxygen to inspired oxygen fraction ratio (PO₂/FiO₂) did not differ (468 ± 52 mm Hg vs 536 ± 14 mm Hg). In conclusion, during EVLP lungs show signs of air trapping and bronchoconstriction, resulting in low compliance and increased alveolar dead space. Intrapulmonary shunt is high despite oxygenation levels acceptable for transplantation.

Common Criteria for Ex Vivo Lung Perfusion Have No Significant Impact on Posttransplant Outcomes

BACKGROUND

Although it is intense in health care resources, by facilitating assessment and reconditioning, ex vivo lung perfusion (EVLP) has the potential to expand the donor pool and improve lung transplant outcomes. However, inclusion criteria used in EVLP trials have not been validated.

METHODS

This retrospective study from 2014 to 2018 reviewed our local state-based donation organization donor records as well as subsequent recipient outcomes to explore the relation between EVLP indications used in clinical trials and recipient outcomes. The primary outcome was primary graft dysfunction grade 3 at 24 hours, with 30-day mortality and posttransplant survival time as secondary outcomes, compared with univariate and multivariate analysis.

RESULTS

From 705 lung donor referrals, 304 lung transplantations were performed (use rate of 42%); 212 of recipients (70%) met at least 1 of the commonly cited EVLP initiation criteria. There was no significant difference in primary graft dysfunction grade 3 or 30-day mortality between recipients with or without an EVLP indication (10.2% versus 7.8%, P = .51; and 2.4% versus 0%, P = .14, respectively). Multivariate analyses showed no significant relationship between commonly cited EVLP criteria and primary graft dysfunction grade 3 or survival time. Recipient outcomes were significantly associated with recipient diagnosis.

CONCLUSIONS

At least 1 commonly cited criterion for EVLP initiation was present in 70% of the transplanted donors, and yet it did not predict clinical results; acceptable outcomes were seen in both subgroups. To discover the true utility of EVLP beyond good clinical management and focus EVLP on otherwise unacceptable lungs, a reconsideration of EVLP inclusion criteria is required.

MSC Based Therapies-New Perspectives for the Injured Lung

Chronic lung diseases pose a tremendous global burden. At least one in four people suffer from severe pulmonary sequelae over the course of a lifetime. Despite substantial improvements in therapeutic interventions, persistent alleviation of clinical symptoms cannot be offered to most patients affected to date. Despite broad discrepancies in origins and pathomechanisms, the important disease entities all have in common the pulmonary inflammatory response which is central to lung injury and structural abnormalities. Mesenchymal stem cells (MSC) attract particular attention due to their broadly acting anti-inflammatory and regenerative properties. Plenty of preclinical studies provided congruent and convincing evidence that MSC have the therapeutic potential to alleviate lung injuries across ages. These include the disease entities bronchopulmonary dysplasia, asthma and the different forms of acute lung injury and chronic pulmonary diseases in adulthood. While clinical trials are so far restricted to pioneering trials on safety and feasibility, preclinical results point out possibilities to boost the therapeutic efficacy of MSC application and to take advantage of the MSC secretome. The presented review summarizes the most recent advances and highlights joint mechanisms of MSC action across disease entities which provide the basis to timely tackle this global disease burden.

NG2 as an Identity and Quality Marker of Mesenchymal Stem Cell Extracellular Vesicles

The therapeutic potential of mesenchymal stem cell (MSC) extracellular vesicles (EV) is currently under investigation in many pathological contexts. Both adult and perinatal MSC are being considered as sources of EV. Herein, we address antigen expression of cord blood and bone marrow MSC and released EV to define an identity and quality parameter of MSC EV as a medicinal product in the context of clinical applications. The research focuses on EV-shuttled neural/glial antigen 2 (NG2), which has previously been detected as a promising surface marker to distinguish perinatal versus adult MSC. Indeed, NG2 was significantly more abundant in cord blood than bone marrow MSC and MSC EV. Ultracentrifuge-isolated EV were then challenged for their pro-angiogenic properties on an xCELLigence system as quality control. NG2⁺ cord blood MSC EV, but not bone marrow MSC EV, promote bFGF and PDGF-AA proliferative effect on endothelial cells. Likewise, they successfully rescue angiostatin-induced endothelial cell growth arrest. In both cases, the effects are NG2-dependent. These results point at NG2 as an identity and quality parameter for cord blood MSC EV, paving the way for their clinical translation.

Noteworthy Literature Published in 2018 for Thoracic Organ Transplantation

Publications of note from 2018 are reviewed for the cardiothoracic transplant anesthesiologist. Strategies to expand the availability of donor organs were highlighted, including improved donor management, accumulating experience with increased-risk donors, ex vivo perfusion techniques, and donation after cardiac death. A number of reports examined posttransplant outcomes, including outcomes other than mortality, with new data-driven risk models. Use of extracorporeal support in cardiothoracic transplantation was a prominent theme. Major changes in adult heart allocation criteria were implemented, aiming to improve objectivity and transparency in the listing process. Frailty and prehabilitation emerged as targets of comprehensive perioperative risk mitigation programs.