AbstractAcute Respiratory Distress Syndrome (ARDS) is an inflammatory disorder in which the integrity of the alveolar epithelial-capillary endothelial barrier breaks down, facilitating the accumulation of a protein-rich oedema fluid within the alveoli. Despite years of clinical trials, no effective pharmacological therapy exists for the condition. Its management therefore remains largely supportive. Low tidal volume ventilation is a central component of supportive care, but may result in the development of hypercapnic acidosis (HCA). While it was once believed that HCA exerts protective effects in such an environment, more recent data are controversial. However, much of these data have been derived from in vitro experiments using cell types which are of limited relevance to the pathophysiology of ARDS. The first aim of this project was therefore to determine the effects of HCA on the inflammatory and reparative responses of primary human pulmonary microvascular endothelial cells (HPMECs) and primary human small airway epithelial cells (SAECs) – two of the primary cell types most relevant to the pathophysiology of ARDS – in an in vitro model of the condition. Inflammatory responses and wound repair by HPMECs and SAECs were attenuated in HCA, demonstrating its beneficial, but also potentially detrimental effects. This is the first time that the effects of HCA on the reparative response of HPMECs and SAECs has been demonstrated in an inflammatory environment.
Efforts to identify an effective therapy for ARDS are ongoing. Mesenchymal Stem Cells (MSCs) are among the promising candidates currently in early-phase clinical trials. However, no data are currently available on how HCA might influence the therapeutic efficacy of such a cell-based therapy. Identification of any alterations to the MSC therapeutic capacity could aid decision-making in the emerging era of stratified medicine and may facilitate future development of a more effective cellbased therapy. The second aim of this project was therefore to determine the influence of HCA on the biology and therapeutic potential of MSCs in an in vitro model of ARDS. MSCs promoted SAEC wound repair in normocapnia, but this effect was lost in HCA. This is the first time that the therapeutic capacity of MSCs in HCA – an environmental factor of great importance in ARDS – has been reported.
The third and final aim of the project was to elucidate the mechanism behind the effects of HCA on the inflammatory and reparative responses of HPMECs and SAECs, and on the therapeutic potential of MSCs. Surprisingly, NFκB activation was not altered by HCA, as it has been in many previous reports. However, HCA was, for the first time, associated with significant attenuation of mitochondrial membrane potential in all three cell types, highlighting a generalised effect of HCA on cell biology. Functional mitochondria were required for the therapeutic potential of MSCs to enhance SAEC wound repair. Mitochondrial transfer from MSCs to SAECs was observed in both normocapnia and HCA. These data support the concept that MSCs transfer functional mitochondria to SAECs to promote wound closure, but that mitochondria transferred in HCA are less functional, resulting in attenuation of the therapeutic capacity of MSCs. This is the first demonstration that the transfer of functional mitochondria is responsible and required for the therapeutic effects of MSCs on repair of the distal lung epithelium, revealing a novel mechanism for the effects of MSCs in ARDS.
|Date of Award||Jul 2018|
|Supervisor||Anna Krasnodembskaya (Supervisor), Danny McAuley (Supervisor) & Cecilia O'Kane (Supervisor)|