Combining extracorporeal life support and cell therapy in critical illness: mesenchymal stem cell therapy and extracorporeal membrane oxygenation in the Acute Respiratory Distress syndrome

The Acute Respiratory Distress Syndrome (ARDS) is perhaps the defining condition of Intensive Care Medicine. However, despite more than a half-century of study we are yet to develop an effective treatment. In the same period, we have made substantial progress in developing the supportive care that we provide to these patients. A prime example is the use of extracorporeal membrane oxygenation (ECMO) as a tool for bridging those with the severest of disease to recovery. However, these interventions merely buy time. The translation of an effective therapy is crucial to reducing the unacceptably high mortality associated with ARDS. Recently, mesenchymal stem (or stromal) cells (MSCs) have shown promise as a novel intervention. The immunomodulatory properties of MSCs have the potential to treat ARDS in a fashion which overcomes the hurdles associated with conventional pharmacological approaches, all of which have failed to date. While the translational journey of MSCs for ARDS has reached the clinical phase, few data exist to describe the interaction between MSCs and ECMO. This is a deficiency, given that those who require ECMO are among the sickest and therefore in the greatest need of a proven therapy. This thesis details a programme of study designed to address this gap in our understanding.The aims of this thesis are outlined in Chapter 1. In brief, this thesis aims to; (1) synthesise the currently available evidence (Chapter 2), (2) describe the unique physiological environment of ECMO and its effect on the host immune response (Chapter 3), (3) assess the safety and efficacy of MSCs in an ex-vivo model of ECMO (Chapter 4), (4) develop a high-fidelity model of severe ARDS and ECMO as the basis for a trial in large animals (Chapters 5 and 6), and (5) conduct a controlled trial of a clinical-grade MSC in a large animal model (Chapter 7).Specifically, Chapter 4 details a series of experiments which employ an ex-vivo model of ECMO. This model incorporates a bench-top simulation using a commercial ECMO device and fresh whole human blood. A total of 14 simulations were performed. In 4 circuits, 40 x 10^6 induced pluripotent stem cell (iPSC) derived human MSCs were added and in a further 4, 20 x 10^6 iPSC-derived hMSCs. The remaining six served as controls. The first published manuscript in this chapter describes the safety outcomes associated with this study and the second the immune response. Based on these data, we report the first ex-vivo demonstration of hMSC adherence to the fibres of an ECMO membrane oxygenator (MO). Furthermore, we describe the ability of adherent hMSCs to adversely affect the function of a MO. In the second report, despite the tendency of hMSCs to interfere with the MO, we describe their ability to upregulate the expression of TGF-B1.In Chapters 5 and 6 the steps taken to design a clinically relevant model of severe ARDS and ECMO are described. Chapter 5 reports the results of a systematic review of existing animal models of ARDS and veno-venous ECMO. These data highlight a number of key deficiencies in the design and reporting of existing models. Chapter 6 details the results of a series of experiments in sheep which characterise two novel models of experimental ARDS – ‘double hit’ intravenous (i.v.) or intratracheal (i.t.) E. coli lipopolysaccharide (LPS) and intravenous oleic acid (OA). A total of 19 animals are included; OA (n=7), OA + i.v. LPS (n=5), and OA + i.t. LPS (n=7). These data describe the ability of all three methods of injury to induce severe ARDS (PaO2 /FiO2 <100 mmHg). However, we report rapid improvements in oxygenation among animals injured with OA alone. Furthermore, these data describe key differences in pulmonary mechanics, lung histopathology, and host-immune responses between models. Using unsupervised machine learning techniques, we also report preliminary evidence which may indicate the presence of distinct sub-phenotypes in experimental ARDS, similar to those found in prior studies of clinical cohorts.Finally, in Chapter 7, we report the results of a controlled trial of a clinical-grade iPSCderived MSC in a 24-hour sheep model of severe ARDS and ECMO. Here, we assigned animals to receive either 3 x 10^8 endobronchially delivered iPSC-derived hMSCs (n=7) or sham (n=7). In this study we report; (1) the failure of hMSCs to improve oxygenation, (2) an improvement in the histopathological severity of ARDS in treated animals, (3) and the replication of our previous finding that hMSCs adhere to and disturb the function of commercial membrane oxygenators.In conclusion, this thesis describes a series of experiments comprehensively characterising the safety and efficacy of MSCs in the context of ARDS and ECMO. These findings highlight important and previously unknown safety concerns centred on the adverse interaction between MSCs and ECMO.