Lung transplantation is the only therapeutic option for patients suffering from end-stage lung disease aiming to increase quality and expectancy of life. However, due to the limited number of donor organs eligible for lung transplantation, only a limited number of patients can benefit from this therapeutic option.
The use of "marginal donor lungs" that do not fulfill the standard selection criteria for donor lungs was shown to be valuable option to increase the number of available organs; however, using these lungs may bear an increased risk for graft dysfunction after lung transplantation. Thus the accurate evaluation of these potentially damaged organs prior to transplantation becomes critical to avoid unwanted outcomes. As shown recently, ex-vivo lung perfusion (EVLP) is an appropriate tool to assess donor lungs before transplantation. After procurement, donor lungs are mounted in a specially designed circuit to perfuse and ventilate them at physiologic and protective conditions. Instead of cold ischemic preservation, resulting in a highly slowed down metabolism of the graft, EVLP preserves the organ at body temperature before transplant and allows assessing lung function in a well defined environment outside the body. Since lungs remain metabolically active the concept of normothermic lung preservation also bears the potential to serve for ex-vivo drug delivery to recondition donor lungs for transplantation.
The goal of this thesis is to assess EVLP as a platform to deliver therapeutic agents for repair of damaged lung grafts and to prepare them for transplantation.
In a first step we have established a novel experimental rodent EVLP model to assess and treat donor lung grafts mimicking the clinical setting. This model allows us to keep rodent lungs in physiologic conditions over 4 hours and to assess quantitatively and qualitatively donor lung damage related to typical graft injuries including bacterial contamination or warm ischemia (WI).
WI is an important risk factor for ischemia-reperfusion injury (IRI), known to result in primary graft dysfunction in clinical practice.
a) One of the key processes involved in IRI are the formation of reactive oxygen/nitrogen species (ROS/RNS) and the activation of poly (ADP-ribose) polymerase (PARP). Therefore we sought to investigate whether rat lungs obtained after extended warm ischemia could be reconditioned during EVLP using the inhibitors of ROS/RNS or PARP.
b) Another known effect of WI resulting in IRI is the up-regulation of the Nuclear factor-kappa B (NF-κB), a family of transcription factors, playing a critical role in the inflammatory response. We therefore studied the potential of ex-vivo inhibition of NF-κB pathway to reduce WI induced lung damage.
c) Tissue damage due to IRI is triggered by the release of various inflammatory cytokines. Sevoflurane, a volatile anesthetic, recognized to affect the release of cytokines in-vitro, was therefore administrated using the EVLP platform and its effect on WI induced lung injury was determined.
We found that all three approaches of ex-vivo lung therapy improved the functional status of damaged lung grafts mounted in the EVLP circuit with significant attenuation of WI induced lung inflammation and tissue injury.
In a further step we have assessed if EVLP can reduce the bacterial load of streptococcus pneumoniae infected donor lungs, and if this affects lung function. We have found that in our experimental setting ex-vivo antibiotic treatment reduces the bacterial load without having a relevant effect on the impaired functional status of infected lungs.
Subsequently we have developed a new experimental model of EVLP followed by unilateral lung transplantation. We than assessed the effects of donor lung reconditioning by EVLP on lung function during blood recirculation in the recipient.
We found that transplantation of damaged lung grafts undergoing sham EVLP displayed severe dysfunction after transplantation. Pharmacological inhibition of PARP during ex-vivo perfusion of injured lungs resulted in significantly reduced IRI and excellent initial lung function after transplantation.
We conclude that EVLP bears the potential to be used as a platform to treat donor lungs before transplantation: (1) to repair preexisting donor lung injuries and (2) to prime the lung to attenuate deleterious effects of blood reperfusion after transplantation. Once translated to clinical practice this may become a pivotal strategy to expand the donor pool with donor grafts initially considered inappropriate to transplant.