Lung transplantation is an accepted form of therapy for selected patients with endstage lung disease. However, despite stringent criteria for the selection of potential recipients, a far greater number of patients await transplantation compared with the availability of donor lungs. This imbalance results in significant mortality for patients on the lung transplant waiting list, and is a major limiting factor of lung transplantation as an effective form of therapy in endstage lung disease. Only 20% of all multiorgan cadaveric donors are used as lung donors worldwide (21
), and the vast majority of donor lungs for transplantation are obtained from heart-beating BD donors (21
). BD is associated with significant hemodynamic instability because of the well-studied autonomic storm associated with BD (23
), which results in increased alveolar capillary leakage and NPE, triggering a systemic and pulmonary inflammatory response in the donor (1
). The neurogenically induced hypertension with subsequent hypotension associated with the autonomic storm further amplifies the inflammatory responses that contribute to lung injury (1
The mechanism of lung injury in BD donors is poorly understood, and the inability to predict lung graft injury during donor selection is a root cause of pulmonary graft dysfunction (PGD) in lung transplant recipients. Furthermore, the current criteria of clinical history, physical examination, radiological findings, blood gas analysis, and bronchoscopic evaluation are inadequate in predicting PGD, whose incidence ranges from 15–75%, sometimes resulting in graft failure and mortality. Early lung injury appears to serve as a precursor for late graft failure. The attenuation of lung injury related to donor BD may therefore constitute a reasonable contributor in preventing subsequent injury to the graft.
S1P, a byproduct of sphingomyelin metabolism, is a potent vascular barrier–protective molecule (8
), and is produced by nearly all cell types but is particularly in abundance in platelets because of their lack of sphingosine lyases. S1P binds to the S1P-specific G-protein–coupled S1PR1 receptor on the endothelial cell surface, resulting in cytoskeletal rearrangement and reduced agonist-induced permeability (6
). S1PR1 receptors signal through Gi protein to promote recruitment into membrane lipid rafts, and Gi-coupled signaling to cytoskeletal elements via the small GTPase Ras-related protein family results in vascular maturation and decreased permeability (8
). We previously showed that S1P protects lungs from injury during ischemia reperfusion, ventilator-induced lung injury, and lung injury caused by endotoxin (6
). We also used S1P analogues such as 2-amino-2-(2-[4-octylphenyl]ethyl)-1,3-propanediol (or FTY720) (28
) and SEW-2871 (29
) as S1PR1 agonists to evaluate the protective effects of S1P in lung injury. Because endothelial cell barrier disruption is a common downstream phenotype of both ARDS and NPE, S1P may represent an excellent candidate to attenuate the lung injuries observed in these models.
In the present study, SEW-2871 produced substantial protective effects in BD-induced NPE, with reductions in BAL total protein content and BAL leukocytes and W/D weight lung ratios. Furthermore, SEW attenuated the BD-induced increases in BAL concentrations of the proinflammatory IL-6 and histological lung abnormalities. The effects of SEW-2871 on non–lung organ systems were not evaluated after BD, but are currently under study. SEW and S1P agonists improve endothelial capillary dysfunction in multiple vascular beds, and we speculate that SEW will improve vascular function in these organs as well. Due to early, profoundly injurious hemodynamic changes, we reasoned that injecting SEW-2871 15 minutes after the induction of BD would be beneficial, whereas its injection at later time points would have a very limited effect on lung injury. SEW-2871 was tested in a group of SD rats (n = 5) 2 hours after BD, but no physiological improvement was detected compared with the sham-treated group (Figure E3).
In this study, we also used genomic approaches to decipher the effects of BD on lung gene expression, as well as the effects of SEW on BD-mediated gene dysregulation. We noted that BD and SEW exert diverse impacts on gene expression. Among the 3,670 genes differentially expressed across the four experimental groups, the majority of genes (66%) displayed a reversal of BD-driven expression by SEW. The expression levels in a small set of 201 genes up-regulated by BD returned in the BD–SEW group to near-normal control levels, thus revealing the potential molecular mechanism underlying the therapeutic effects of SEW, along with the main biological function of these genes involved in immune and inflammatory responses.
In conclusion, we used a rat model of BD intimately related to lung transplantation, and investigated the potential therapeutic effects of SEW-2871 in the donor lung after BD, to imitate the clinical scenario. Our results indicate that the acute increases in ICP associated with BD produce significant inflammatory lung injury and increases in vascular permeability, which were attenuated by SEW-2871, an S1PR1 agonist. These findings are significant for the preservation of organs after BD in donor cadavers, to combat the universal shortage of suitable lungs for transplantation.