Airway vascular changes after lung transplant: potential contribution to the pathophysiology of bronchiolitis obliterans syndrome

Role of transforming growth factor-β in airway remodelling in bronchiolitis obliterans

Airway remodelling is the main pathological mechanism of bronchiolitis obliterans (BO). Several studies have found that transforming growth factor-β (TGF-β) expression is increased in BO during airway remodelling, where it plays an important role in various biological processes by binding to its receptor complex to activate multiple signalling proteins and pathways. This review examines the role of TGF-β in airway remodelling in BO and its potential as a therapeutic target, highlighting the mechanisms of TGF-β activation and signalling, cellular targets of TGF-β actions, and research progress in TGF-β signalling and TGF-β-mediated processes.

Immune processes in the pathogenesis of chronic lung allograft dysfunction: identifying the missing pieces of the puzzle

Lung transplantation is the optimal treatment for selected patients with end-stage chronic lung diseases. However, chronic lung allograft dysfunction remains the leading obstacle to improved long-term outcomes. Traditionally, lung allograft rejection has been considered primarily as a manifestation of cellular immune responses. However, in reality, an array of complex, interacting and multifactorial mechanisms contribute to its emergence. Alloimmune-dependent mechanisms, including T-cell-mediated rejection and antibody-mediated rejection, as well as non-alloimmune injuries, have been implicated. Moreover, a role has emerged for autoimmune responses to lung self-antigens in the development of chronic graft injury. The aim of this review is to summarise the immune processes involved in the pathogenesis of chronic lung allograft dysfunction, with advanced insights into the role of innate immune pathways and crosstalk between innate and adaptive immunity, and to identify gaps in current knowledge.

Lung Transplantation with Controlled Donation after Circulatory Death Donors

Purpose: Utilization of donation after circulatory death (DCD) donors has the potential to decrease donor shortage in lung transplantation (LTx). This study reviews the long-term outcome of LTx from DCD donors.

Methods: We included all consecutive DCD (Maastricht Category III) and all donations after brain death (DBD) donor lung transplants at our Center performed between January 2012 and February 2017. Data were analyzed comparing the two groups in regard of survival after LTx as primary outcome.

Results: Median withdrawal to cardiac arrest time was 17 min (interquartile range [IQR]: 11.5–20.5). Median cardiac arrest to cold perfusion was 32 min (IQR: 24.5–36.5). Primary graft dysfunction (PGD) grade 3 at T72 occurred in three recipients. Chronic lung allograft dysfunction (CLAD) led to death in two cases. In DCD group, there was no 90-day mortality. In DCD, group 1- and 3-year survival rates were 100% and 80%. In DBD group, 1- and 3-year survival rates were 85% and 69% (p = 0.4).

Conclusions: Our report confirmed the comparable outcome from DCD donors compared with DBD donors. Utility of DCD donors is a safe option to overcome donor shortage.

Airway remodelling in the transplanted lung

Following lung transplantation, fibrotic remodelling of the small airways has been recognized for almost 5 decades as the main correlate of chronic graft failure and a major obstacle to long-term survival. Mainly due to airway fibrosis, pulmonary allografts currently show the highest attrition rate of all solid organ transplants, with a 5-year survival rate of 58 % on a worldwide scale. The observation that these morphological changes are not just the hallmark of chronic rejection but rather represent a manifestation of a multitude of alloimmune-dependent and -independent injuries was made more recently, as was the discovery that chronic lung allograft dysfunction manifests in different clinical phenotypes of respiratory impairment and corresponding morphological subentities. Although recent years have seen considerable advances in identifying and categorizing these subgroups on the basis of clinical, functional and histomorphological changes, as well as susceptibility to medicinal treatment, this process is far from over. Since the actual pathophysiological mechanisms governing airway remodelling are still only poorly understood, diagnosis and therapy of chronic lung allograft dysfunction presents a major challenge to clinicians, radiologists and pathologists alike. Here, we review and discuss the current state of the literature on chronic lung allograft dysfunction and shed light on classification systems, corresponding clinical and morphological changes, key cellular players and underlying molecular pathways, as well as on emerging diagnostic and therapeutic approaches.

Endoscopic high-resolution autofluorescence imaging and OCT of pulmonary vascular networks

High-resolution imaging from within airways may allow new methods for studying lung disease. In this work, we report an endoscopic imaging system capable of high-resolution autofluorescence imaging (AFI) and optical coherence tomography (OCT) in peripheral airways using a 0.9 mm diameter double-clad fiber (DCF) catheter. In this system, AFI excitation light is coupled into the core of the DCF, enabling tightly focused excitation light while maintaining efficient collection of autofluorescence emission through the large diameter inner cladding of the DCF. We demonstrate the ability of this imaging system to visualize pulmonary vasculature as small as 12 μm in vivo.

Differential cytokine, chemokine and growth factor expression in phenotypes of chronic lung allograft dysfunction

BACKGROUND

Chronic lung allograft dysfunction is a heterogeneous entity limiting long-term survival after lung transplantation. Different clinical phenotypes (bronchiolitis obliterans syndrome [BOS]-neutrophilic BOS-restrictive allograft syndrome [RAS]) have been identified but the mechanisms remain elusive.

METHODS

In this study, we measured 34 different cytokines, chemokines, and growth factors in bronchoalveolar lavage fluid of 20 stable patients, 20 patients suffering from non-neutrophilic BOS, 17 from neutrophilic BOS, and 20 from RAS using classic enzyme-linked immunosorbent assay and multiplex technology.

RESULTS

Total cell count and % neutrophils were elevated in neutrophilic BOS and RAS compared to stable and non-neutrophilic BOS patients, whereas also the % eosinophils was elevated at diagnosis of RAS. Levels of interleukin (IL)-1β (P<0.01), IL-1Rα (P<0.001), IL-6 (P<0.001), IL-8/CXCL8 (P<0.001), IP-10/CXCL10 (P<0.05), MCP-1/CCL2 (P<0.05), macrophage inflammatory protein (MIP)-1α/CCL3 (P<0.001), MIP-1β/CCL4, and vascular endothelial growth factor (VEGF; P<0.05) were differentially regulated in RAS compared to stable, whereas in neutrophilic BOS IL-1β (P<0.001), IL-1Rα (P<0.01), IL-7 (P<0.05), IL-8/CXCL8 (P<0.001), MCP-3/CXCCL7 (P<0.05) and MIP-1α/CCL-3 (P<0.05) were significantly upregulated compared to stable patients. We could not detect any differences between non-neutrophilic BOS and stable patients. Interestingly, bronchoalveolar lavage IL-6, interferon gamma-induced protein (IP)-10/CXCL10 and interferon-inducible T-cell alpha chemoattractant/chemokine (C-X-C motif) ligand 11 (ITAC/CXCL10) were associated with survival after diagnosis in RAS patients.

CONCLUSION

There were major differences in cytokine and chemokine expression in our different study groups. Especially IL-6, but also IP-10/CXCL10, and VEGF may be interesting mediators in RAS.

Bronchoalveolar lavage as a tool to predict, diagnose and understand bronchiolitis obliterans syndrome

Bronchiolitis obliterans syndrome (BOS), a condition of irreversible small airway fibrosis, is the principal factor limiting long-term survival after lung transplantation. Bronchoscopy and bronchoalveolar lavage (BAL), techniques central to lung transplant clinical practice, provide a unique opportunity to interrogate the lung allograft during BOS development and identify potential disease mechanisms or biomarkers. Over the past 20 years, numerous studies have evaluated the BAL cellular composition, cytokine profiles and protein constituents in lung transplant recipients with BOS. To date, however, no summative evaluation of this literature has been reported. We developed and applied objective criteria to qualitatively rank the strength of associations between BAL parameters and BOS in order to provide a comprehensive and systematic assessment of the literature. Our analysis indicates that several BAL parameters, including neutrophil count, interleukin-8, alpha defensins and MMP-9, demonstrate highly replicable associations with BOS. Additionally, we suggest that considerable opportunity exists to increase the knowledge gained from BAL analyses in BOS through increased sample sizes, covariant adjustment and standardization of the BAL technique. Further efforts to leverage analysis of BAL constituents in BOS may offer great potential to provide additional in-depth and mechanistic insights into the pathogenesis of this complex disease.

The contribution of airway ischemia and vascular remodelling to the pathophysiology of bronchiolitis obliterans syndrome and chronic lung allograft dysfunction

PURPOSE OF REVIEW

Chronic allograft dysfunction continues to limit the enduring success of lung transplantation. Increasingly it is recognized that events very early post-transplant such as primary graft dysfunction can be linked to poor clinical outcomes at much later time points. In this article we review a number of the different processes that predispose the allograft to ischemia early post-transplant and explore how these events may contribute to obliterative bronchiolitis, the histological correlate of chronic lung allograft dysfunction.

RECENT FINDINGS

Allograft ischemia may arise during explantation (warm ischemia), at implantation (in the absence of bronchial arterial reanastomosis) or at later time points (small airway microvascular damage). We describe how allograft ischemia may result in a hypoxic inflammatory milieu within the lung allograft that is conducive to vascular remodelling and angiogenesis.

SUMMARY

Whilst the published literature for vascular remodelling in post-transplant obliterative bronchiolitis is not as extensive as that for asthma, a disease also characterized by airway pathology, there are clear parallels and shared pathophysiological pathways between the two diseases. An understanding of the complex interaction between ischemia, vascular remodelling and chronic lung allograft dysfunction may lead to the future development of therapeutic strategies that can unravel this association.

Lipid raft facilitated ligation of K-alpha1-tubulin by specific antibodies on epithelial cells: Role in pathogenesis of chronic rejection following human lung transplantation

Long term function of human lung allografts is hindered by development of chronic rejection manifested as Bronchiolitis Obliterans Syndrome (BOS). We have previously identified the development of antibodies (Abs) following lung transplantation to K-alpha1-tubulin (KAT), an epithelial surface gap junction cytoskeletal protein, in patients who develop BOS. However, the biochemical and molecular basis of the interactions and signaling cascades mediated by KAT Abs are yet to be defined. In this report, we investigated the biophysical basis of the epithelial cell membrane surface interaction between KAT and its specific Abs. Towards this, we analyzed the role of the lipid raft-domains in the membrane interactions which lead to cell signaling and ultimately increased growth factor expression. Normal human bronchial epithelial (NHBE) cells, upon specific ligation with Abs to KAT obtained either from the serum of BOS(+) patients or monoclonal KAT Abs, resulted in upregulation of growth factors VEGF, PDGF, and bFGF (6.4+/-1.1-, 3.2+/-0.9-, and 3.4+/-1.1-fold increase, respectively) all of which are important in the pathogenesis of BOS. To define the role for lipid raft in augmenting surface interactions, we analyzed the changes in the growth factor expression pattern upon depletion and enrichment with lipid raft following the ligation of the epithelial cell membranes with Abs specific for KAT. NHBE cells cultured in the presence of beta-methyl cyclodextran (betaMCD) had significantly reduced growth factor expression (1.3+/-0.3, vs betaMCD untreated being 6.4+/-1.1-fold increase) upon stimulation with KAT Abs. Depletion of cholesterol on NHBE cells upon treatment with betaMCD also resulted in decreased partitioning of caveolin in the membrane fraction indicating a decrease in raft-domains. In conclusion, our results demonstrate an important role for lipid raft-mediated ligation of Abs to KAT on the epithelial cell membrane, which results in the upregulation of growth factor cascades involved in the pathogenesis of BOS following human lung transplantation.

Recommendations for the assessment and reporting of multivariable logistic regression in transplantation literature

Multivariable logistic regression is an important method to evaluate risk factors and prognosis in solid organ transplant literature. We aimed to assess the quality of this method in six major transplantation journals. Eleven analytical criteria and four documentation criteria were analyzed for each selected article that used logistic regression. A total of 106 studies (6%) out of 1,701 original articles used logistic regression analyses from January 1, 2005 to January 1, 2006. The analytical criteria and their respective reporting percentage among the six journals were: Linearity (25%); Beta coefficient (48%); Interaction tests (19%); Main estimates (98%); Ovefitting prevention (84%); Goodness-of-fit (3.8%); Multicolinearity (4.7%); Internal validation (3.8%); External validation (8.5%). The documentation criteria were reported as follows: Selection of independent variables (73%); Coding of variables (9%); Fitting procedures (49%); Statistical program (65%). No significant differences were found among different journals or between general versus subspecialty journals with respect to reporting quality. We found that the report of logistic regression is unsatisfactory in transplantation journals. Because our findings may have major consequences for the care of transplant patients and for the design of transplant clinical trials, we recommend a practical solution for the use and reporting of logistic regression in transplantation journals.

Bronchiolitis obliterans in lung transplantation: the good, the bad, and the future

Lung transplantation remains the hope for many incurable pulmonary diseases, such as cystic fibrosis, pulmonary fibrosis, and chronic obstructive pulmonary disease. Remarkable progress has been made in improving outcomes, although the incidence of acute rejection remains more than 50% in the 1st year, and the 5-year graft survival is still less than 50% primarily because of the development of chronic rejection and graft dysfunction. Chronic rejection is characterized by the development of obliterative bronchiolitis in allografts and manifests as bronchiolitis obliterans syndrome in humans with no effective treatment. Previous studies support a role for alloreactive T cells in the development of bronchiolitis obliterans syndrome, but the specific mechanisms are unknown. One major stumbling block to research in the field of lung transplantation has been the lack of physiologic models to study the disease in the laboratory. We will review the current understanding of the immunology of the pathogenesis of obliterative bronchiolitis and will discuss exciting new advances from the laboratory as well as the implications for future research in lung transplantation.

Endoscopic narrow-band imaging-quantitative assessment of airway vascularity after lung transplantation

In lung transplant recipients, the submucosal vascular plexus of the airway wall potentially represents one of the key structures of graft injury. Narrow band imaging is a novel endoscope technique that allows visual enhancement of the mucosa vasculature. It was our aim to investigate the ability of narrow-band imaging in combination with computerized image analysis to quantitatively assess airway vascularity in lung transplant recipients. In consecutive lung transplant recipients, in addition to the routine procedures, optical analysis of the main carina (autologous tissue) and the upper lobe carina (allogeneic tissue) were performed. From every site, three representative pictures were chosen. A total of 63 bronchoscopies were analyzed. The intraclass correlation coefficient (measure for test-retest reliability) of the three measurements were 0.69 and 0.74 for the main carina and the upper lobe carina, respectively. A mixed linear regression revealed increased vascularity in autologous tissue of patients with cystic fibrosis (p=0.06) and decreased vascularity in allogeneic tissue with time after transplantation (p=0.09). Endoscopic narrow-band imaging (NBI) in combination with computerized image analysis allows consistent assessment of airway vascularity in vivo. In lung transplant recipients, there might be differences in airway vascularity in both autologous and allogeneic large airways.

Lymphatics in lung disease

The lymphatic circulation appears to be a vital component in lung biology in health and in disease. Animal models have established the role of the lymphatic circulation in neoplastic and inflammatory diseases of the lung, such as asthma and cancer, and allowed for the understanding of the molecular controls of lymphangiogenesis in normal lung development. Understanding the role of lymphatics in human lung disease appears likely to contribute to the understanding of the pathogenesis of disease and the development of novel therapeutic targets.

De novo production of K-alpha1 tubulin-specific antibodies: role in chronic lung allograft rejection

Lung transplantation is the treatment option for a variety of end-stage pulmonary diseases. Posttransplant development of Abs against donor HLA and non-HLA Ags have been associated with acute and chronic rejection of transplanted organs. Development of bronchiolitis obliterans syndrome (BOS) following lung transplantation has been correlated with de novo production of anti-donor-HLA Abs. However, only a portion of the patients with BOS demonstrate detectable anti-donor-HLA Abs. Airway epithelium is considered as a major target for lung allograft rejection. In this study we demonstrate that many BOS(+) patients (12 of 36) develop Abs reactive to epithelial cell Ag that are distinct from HLA. Furthermore, de novo production of antiepithelial cell Ab precedes clinical onset of BOS. N-terminal sequencing and blastx analysis as well as blocking with K-alpha1 tubulin-specific Ab identified the epithelial Ag as K-alpha1 tubulin. Binding of the de novo-produced anti-K-alpha1 tubulin Abs to the airway epithelial cells resulted in the increased expression of transcription factors (TCF5 and c-Myc), leading to increased expression of fibrogenic growth factors, activation of cell cycle signaling, and fibroproliferation, the central events in immunopathogenesis of BOS following human lung transplantation.

Natural killer cells and lung transplantation, roles in rejection, infection, and tolerance

Despite improvements in surgical technique, organ preservation, immunosuppression, and management of infection, the long term survival following lung transplantation remains low, mainly due to immune mediated complications such as acute and chronic rejection. Almost all immunosuppressive agents used in the prophylaxis and treatment of rejection following lung transplantation are targets of T cell maturation, function or proliferation, which in theory should cause sufficient disruption of the adaptive immune system to prevent graft rejection. However the five year survival rate of only 50% suggests this is not the case. More recent evidence suggests that NK cells may play a significant role in immune processes following lung transplantation. This article reviews the literature on the potential function of NK cells in rejection, infection, malignancy and tolerance following lung transplantation.

Angiogenesis: a potentially critical part of remodelling in chronic airway diseases?

Angiogenesis is a prominent feature of the structural tissue remodelling that occurs in the chronic airway diseases of asthma, Bronchiolitis Obliterans Syndrome (BOS, post-lung transplantation), and in smoking-related Chronic Obstructive Pulmonary Disease (COPD). For each, we have explored the relationship between angiogenesis and underlying chronic inflammatory processes--are the hypervascular changes secondary to inflammation, or do they occur in parallel? What are the likely growth factors which stimulate the angiogenic process? We discuss the relationships that have been studied between angiogenesis and the physiological impairment of airflow obstruction. The pattern that emerges is complex and variable. In asthma, there is strong evidence to suggest that Vascular Endothelial Growth Factor (VEGF) and its receptor system is upregulated in the airway. Local production of VEGF has also been implicated as a major driver of angiogenesis in the airway component of COPD, though paradoxically emphysema seems to be due to lack of VEGF in the lung parenchyma. In BOS, the evidence suggests that VEGF is lacking in the airway: other mediators and especially C-X-C chemokines such as Interleukin (IL)-8, are likely to be more important in angiogenesis. The physiological consequences of angiogenesis are likely to be important in asthma (especially during acute episodes of deterioration), and probably also in COPD, although data is equivocal. In BOS, increased airway vascularity appears to occur early, but is not progressive. In terms of therapy, evidence for anti-angiogenic effectiveness is strongest for Inhaled Corticosteroid (ICS) and Long Acting Beta-Agonists (LABA) in asthma.

Microvascular destruction identifies murine allografts that cannot be rescued from airway fibrosis

Small airway fibrosis (bronchiolitis obliterans syndrome) is the primary obstacle to long-term survival following lung transplantation. Here, we show the importance of functional microvasculature in the prevention of epithelial loss and fibrosis due to rejection and for the first time, relate allograft microvascular injury and loss of tissue perfusion to immunotherapy-resistant rejection. To explore the role of alloimmune rejection and airway ischemia in the development of fibroproliferation, we used a murine orthotopic tracheal transplant model. We determined that transplants were reperfused by connection of recipient vessels to donor vessels at the surgical anastomosis site. Microcirculation through the newly formed vascular anastomoses appeared partially dependent on VEGFR2 and CXCR2 pathways. In the absence of immunosuppression, the microvasculature in rejecting allografts exhibited vascular complement deposition, diminished endothelial CD31 expression, and absent perfusion prior to the onset of fibroproliferation. Rejecting grafts with extensive endothelial cell injury were refractory to immunotherapy. After early microvascular loss, neovascularization was eventually observed in the membranous trachea, indicating a reestablishment of graft perfusion in established fibrosis. One implication of this study is that bronchial artery revascularization at the time of lung transplantation may decrease the risk of subsequent airway fibrosis.