The Lung Transplantation Research Program| Victor Laubach
The main focus of this program is on understanding the molecular and cellular mechanisms of lung injury relevant to lung transplantation and particularly to chronic allograft rejection. Another focus is on improving the quality of marginal lungs though a novel technique, called Ex-Vivo Lung Perfusion (EVLP).
Importance of this Research Program
Lung transplantation currently is the preferred treatment option for a variety of end-stage pulmonary diseases. Remarkable progress has occurred through refinement in technique and improved understanding of transplant immunology and microbiology. Despite these improvements donor shortages and chronic lung allograft rejection continue to plague the field and prevent it from reaching its full potential. Chronic rejection of the lung allograft is currently the major hurdle limiting long-term survival. To date prevention of known risk factors and treatment strategies have not lessened the devastating toll this process has on lung transplant survival.
What is Chronic Allograft Rejection?
Chronic allograft rejection or bronchiolitis obliterans (BO) remains the leading cause of morbidity and mortality in lung transplant recipients after the first three months, and is the primary reason why the 5- and 10-yr survival rates of lung allograft recipients are only 50% and 26%, respectively post transplantation. BO is the histological finding of chronic rejection in the lung allograft. The pathogenesis of BO although not completely understood is probably a result of multiple hits of injury and inflammation followed temporally by the development of allospecific immune responses to graft antigens. The immune response leads to further allograft injury from direct cell-to-cell cytotoxicty as well as from secretion of cytokines and chemokines. The cycle of injury, inflammation and immune activation culminate in repair, remodeling and excessive fibro-proliferation. As the lung, unlike other solid organ transplants, is constantly exposed to the external environment via the main airways, injury can occur repeatedly from allo-independent factors (for example viral infections, reflux). These events can act synergistically with allo-dependent factors (acute rejection) to compound injury. The complex interplay between injury, inflammation, repair and immunity likely explain why increasing immunosuppression alone does not effectively mitigate progression of BO.
Current Research Projects
Adenosine Signaling in Lung Transplantation Injury and Rejection
Based on the complex etiology of BO, a multi-pronged approach that affects the inflammatory and innate as well as the adaptive immune responses to injury would be most effective in mitigating BO. Ischemia-reperfusion injury is linked to the subsequent development of BO. Furthermore, early post-transplant elevation of proinflammatory mediators has been shown to be associated with alloimmunity and BO. However, unlike other solid organ transplants it is likely that frequent and repetitive subclinical injury occurs in the lung allograft since it is constantly exposed to the external environment. Thus limiting ischemia-reperfusion injury may lessen the development of BO, but likely will not alone eliminate it. The same holds true with the prevention of the other risk factors (acute rejection/viral infections/reflux). A treatment strategy that is multifaceted in prevention of inflammation and immunity is likely needed.
Adenosine and it four receptors are a critical part of the physiological negative-feedback mechanism for limitation and termination of tissue-specific and systemic inflammatory responses. Adenosine and its receptors also play roles in the adaptive immune response and in wound healing and fibrosis. One of the main projects in our laboratory is characterizing the role of adenosine, and its various receptors in chronic allograft dysfunction. We have shown the adenosine A2A receptor is important in chronic allograft injury and that agonists to this receptor can attenuate this injury (Lau and colleagues, Ann Thorac Surg. 88(4):1071-8).
Figure 1 shows allograft tracheas from mice that have been treated with adenosine A2A receptor agonists compared to no treatment controls and trachea allografts into A2A R knock-out mice. The allografts transplanted into A2A R knock-out recipients experienced the most inflammation and subsequent luminal obliteration, and adenosine A2A R agonist (ATL in figure) treated allografts showed protection from inflammation and luminal obliteration.
Cross Talk between Coagulation and Inflammation in Lung Acute Injury
It is clear the two powerful biologic systems, coagulation and inflammation, interact and contribute to the severity of the response. It is well documented that increased coagulation and impaired fibrinolysis play an important role in the pathogenesis of the various forms of acute lung injury. One active area of research in our laboratory is on the cross-talk between the coagulation and inflammatory systems in ischemia-reperfusion injury, which is a type of acute lung injury and is a significant risk factor in the future development of BO. Experiments in our laboratory have already shown that lung ischemia-reperfusion injury triggers fibrin deposition in the murine lungs and fibrin creates a proinflammatory environment. Preventing fibrin deposition with the use of plasminogen activator inhibitor-1 (PAI-1) knock-out mice (see Figure 2) reduces ischemia-reperfusion injury and inflammation (see Figure 3). This finding may lead to novel treatment strategies for ischemia-reperfusion (JTCVS, 137(5):1241-1248, 2009).
Stem Cells and Bronchiolitis Obliterans (BO) Development
Loss of epithelial cells is one of the key factors that lead to airway fibrosis. Loss of epithelial cells may decrease the barrier to host cell infiltration into the lumen, allowing deposition of extracellular matrix, with subsequent obliteration of the airway. We are trying to determine whether injection of epithelial cells/progenitor cells from the recipient into the lumen of the donor trachea could prevent bronchiolitis obliterans (BO) in a mouse heterotopic tracheal transplantation (HTT) model.
Bone marrow-derived mesenchymal stem cells (BM-MSCs) have shown therapeutic potential in acute lung injury. Recently, placental-derived human mesenchymal stem cells (PMSCs) have exhibited similarities with BM-MSCs in terms of regenerative capabilities and immunogenicity. We are testing whether treatment with PMSCs would reduce the development of bronchiolitis obliterans BO in a murine heterotopic tracheal transplant model.
mTOR Inhibition Prevent BO Development
We have found that fibrocytes, a circulating population of mesenchymal progenitor cells, are present in higher numbers in the circulation of patients destined to develop BO and that these cells contribute to the histologic lesions of BO suggesting their role as potential biomarkers for this process. We also found that migration of fibrocytes into lung allografts and contribution of the fibrocytes to BO depends on the mammalian target of rapamycin (mTOR). We are testing the mechanisms of mTOR inhibition (rapamycin) can prevent development of BO through: (1) affecting fibrocytes recruitment; (2) promoting epithelial progenitor cells and (3) altering regulatory B cells.
Increasing the Usage of Marginal Lung via Ex-vivo Lung Perfusion with a selective adenosine 2A receptor agonist, Regadenoson
Ischemia-reperfusion injury (IRI) remains the leading cause of early morbidity and mortality after lung transplantation and also predisposes to bronchiolitis obliterans, the major limitation to long-term survival after transplant. IRI is known clinically as (PGD) which entails diffuse alveolar damage and hypoxemia in the acute post-lung transplant period. Currently there are no preventative therapies for PGD. Furthermore because of the concern for PGD, surgeons are reluctant to use marginal donor lungs. In multi-organ donors the lungs are the least likely solid organ to be deemed transplantable.
The pathogenesis of IRI has historically been characterized by the recruitment and extravasation of neutrophils. However we have recently reported that one subset of CD4+ T cells, Type I invariant natural killer T (iNKT) cells, are key initiators of IRI via IL-17 production which drives the infiltration of neutrophils – the end effector cells for tissue injury. Selective agonists of the adenosine 2A receptor (A2AR), an anti-inflammatory G protein-coupled receptor, have been shown to inhibit the activity of most inflammatory cells (including neutrophils and iNKT cells). Our laboratory has extensive preclinical evidence demonstrating that 1) A2AR agonists potently prevent IRI after lung transplantation and 2) that use of A2AR agonist combined with ex-vivo lung perfusion (EVLP) enhances EVLP-mediated donor lung rehabilitation and successful transplantation. Preliminary data shows that A2AR agonist attenuates IRI largely by blocking the activation of iNKT cells.
Regadenoson (Lexiscan®) is a selective A2AR agonist drug that targets A2AR-bearing cells and is FDA approved as a pharmacologic stress agent indicated for radionuclide myocardial perfusion imaging. A recently completed NIH funded study successfully used a 12-hour regadenoson infusion safely and efficaciously in sickle cell adults with acute chest syndrome. The current proposal requests support fo*** r both: 1) a human phase I pilot trial evaluating the safety of regadenoson for the prevention of IRI after lung transplantation and 2) use of regadenoson in human EVLP as a novel strategy to recondition marginal donor lungs and limit the IRI seen in these lungs when transplanted. Regadenoson is an innovative therapy with great promise to attenuate IRI and thus improve both short- and long-term clinical outcomes.
Many lung transplant programs have increased their volume of lung transplants performed by accepting marginal donor criteria, but even with the use of these donors the need greatly outweighs the number of available lung donors. Recently, a novel, recent FDA-approved EVLP technique (Figure 4) has been developed that allows previously unusable (marginal) donor lungs to be rehabilitated, tested, and successfully used in recipients. EVLP has been shown to reduce the degree of IRI in marginal lungs and has the potential to vastly increase the donor lung supply. We are trying to combine the novel EVLP technique with the selective A2AR agonist, Regadenoson to improve quality of the marginal lungs. This could help further expand the donor pool and lessen PGD. Our long term goal is to improve the human marginal lungs quality and apply our experimental findings to clinical operation.
This work is funded by a relatively new RO1 NIH grant with Dr. Lau as the Principle Investigator. The NIH grant number is: R01 HL128492
For more information about the Lung Transplantation Research Program at the University of Virginia, please contact Dr. Lau:
Victor Laubach, PhD
Mark Roeser, MD
University of Virginia
PO Box 800679
Charlottesville, VA 22908