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

Long-Term Chondrocyte Retention in Partially Decellularized Tracheal Grafts

OBJECTIVE

Decellularized tracheal grafts possess the biological cues necessary for tissue regeneration. However, conventional decellularization approaches to target the removal of all cell populations including chondrocytes lead to a loss of mechanical support. We have created a partially decellularized tracheal graft (PDTG) that preserves donor chondrocytes and the mechanical properties of the trachea. In this study, we measured PDTG chondrocyte retention with a murine microsurgical model.

STUDY DESIGN

Murine in vivo time-point study.

SETTING

Research Institute affiliated with Tertiary Pediatric Hospital.

METHODS

PDTG was created using a sodium dodecyl sulfate protocol. Partially decellularized and syngeneic grafts were orthotopically implanted into female C57BL/6J mice. Grafts were recovered at 1, 3, and 6 months postimplant. Pre- and postimplant grafts were processed and analyzed via quantitative immunofluorescence. Chondrocytes (SOX9+, DAPI+) present in the host and graft cartilage was evaluated using ImageJ.

RESULTS

Partial decellularization resulted in the maintenance of gross tracheal architecture with the removal of epithelial and submucosal structures on histology. All grafts demonstrated SOX9+ chondrocytes throughout the study time points. Chondrocytes in PDTG were lower at 6 months compared to preimplant and syngeneic controls.

CONCLUSION

PDTG retained donor graft chondrocytes at all time points. However, PDTG exhibits a reduction in chondrocytes at 6 months. The impact of these histologic changes on cartilage extracellular matrix regeneration and repair remains unclear.

Successful Early Neovascularization in Composite Tracheal Grafts

OBJECTIVE

Long-segment tracheal defects require tissue replacement for successful reconstruction. Rapid revascularization is imperative to maintain graft function. We previously showed that partially decellularized tracheal grafts (PDTG) and composite tracheal grafts (CTG; PDTG supported by a 3-dimensionally printed external splint) regenerate respiratory epithelium and may support the regeneration of endothelial cells (CD31+). However, the capability of graft endothelial cells to organize or contribute to tracheal revascularization remains unclear. In this study, we quantified endothelial cells (CD31+) and neovessel formation in PDTG and CTG. We hypothesize that PDTG and CTG support tracheal neovascularization to a similar extent as surgical (syngeneic tracheal graft [STG]) and native trachea (NT) controls.

STUDY DESIGN

The animal study, a randomized control trial.

SETTING

Center for Regenerative Medicine, Nationwide Children's Hospital.

METHODS

PDTG was created via an established decellularization protocol. Segmental tracheal reconstruction was performed with STG, PDTG, or CTG using a mouse microsurgical model. NT was used as a nonsurgical control. At 1 month, mice were euthanized, grafts harvested, sectioned, and stained with CD31 and hematoxylin and eosin. Neovessel formation was quantified by the number of formed blood vessels in the lamina propria and vessel size (vessel/graft area, mm2 ).

RESULTS

Decellularization eliminated all endothelial cells and there were no perfused vessels at implantation. At 1 month, PDTG and CTG supported neovessel formation with tubular vessels lined with endothelial cells. There was no difference in the number or size of vessels compared to controls.

CONCLUSION

PDTG and CTG support tracheal endothelial cell regeneration and neovessel formation. Future directions to assess the function, kinetics, and distribution of graft neovessels are needed.

Partial decellularization eliminates immunogenicity in tracheal allografts

There is currently no suitable autologous tissue to bridge large tracheal defects. As a result, no standard of care exists for long-segment tracheal reconstruction. Tissue engineering has the potential to create a scaffold from allografts or xenografts that can support neotissue regeneration identical to the native trachea. Recent advances in tissue engineering have led to the idea of partial decellularization that allows for the creation of tracheal scaffolds that supports tracheal epithelial formation while preserving mechanical properties. However, the ability of partial decellularization to eliminate graft immunogenicity remains unknown, and understanding the immunogenic properties of partially decellularized tracheal grafts (PDTG) is a critical step toward clinical translation. Here, we determined that tracheal allograft immunogenicity results in epithelial cell sloughing and replacement with dysplastic columnar epithelium and that partial decellularization creates grafts that are able to support an epithelium without histologic signs of rejection. Moreover, allograft implantation elicits CD8+ T-cell infiltration, a mediator of rejection, while PDTG did not. Hence, we establish that partial decellularization eliminates allograft immunogenicity while creating a scaffold for implantation that can support spatially appropriate airway regeneration.

Regeneration of tracheal neotissue in partially decellularized scaffolds

Extensive tracheal injury or disease can be life-threatening but there is currently no standard of care. Regenerative medicine offers a potential solution to long-segment tracheal defects through the creation of scaffolds that support the generation of healthy neotissue. We developed decellularized tracheal grafts (PDTG) by removing the cells of the epithelium and lamina propria while preserving donor cartilage. We previously demonstrated that PDTG support regeneration of host-derived neotissue. Here, we use a combination of microsurgical, immunofluorescent, and transcriptomic approaches to compare PDTG neotissue with the native airway and surgical controls. We report that PDTG neotissue is composed of native tracheal cell types and that the neoepithelium and microvasculature persisted for at least 6 months. Vascular perfusion of PDTG was established within 2 weeks and the graft recruited multipotential airway stem cells that exhibit normal proliferation and differentiation. Hence, PDTG neotissue recapitulates the structure and function of the host trachea and has the potential to regenerate.

Successful tracheal regeneration using biofabricated autologous analogues without artificial supports

Tracheas have a tubular structure consisting of cartilage rings continuously joined by a connective tissue membrane comprising a capillary network for tissue survival. Several tissue engineering efforts have been devoted to the design of scaffolds to produce complex structures. In this study, we successfully fabricated an artificial materials-free autologous tracheal analogue with engraftment ability by combining in vitro cell self-aggregation technique and in-body tissue architecture. The cartilage rings prepared by aggregating chondrocytes on designated culture grooves that induce cell self-aggregation were alternately connected to the connective tissues to form tubular tracheal analogues by subcutaneous embedding as in-body tissue architecture. The tracheal analogues allogeneically implanted into the rat trachea matured into native-like tracheal tissue by covering of luminal surfaces by the ciliated epithelium with mucus-producing goblet cells within eight months after implantation, while maintaining their structural integrity. Such autologous tracheal analogues would provide a foundation for further clinical research on the application of tissue-engineered tracheas to ensure their long-term functionality.

[The bronchial arteries: a small but vital contribution to lung perfusion after lung transplantation]

Blood supply to the lungs is carried out by the pulmonary and bronchial-arterial system. The bronchial-arterial vessels are involved in supplying the small airways all the way up to the terminal bronchioles. The bronchial-arterial system is also necessary for the regulation of airway temperature, humidity and mucociliary clearance. Chronic ischaemia of the small airways due to damage or injury to bronchial arterial supply increases the risk of fibrosis of the small airways (bronchiolitis obliteration), especially in lung transplantation (LTx). Although survival after LTx has improved over time, it is, with a 5-year survival rate of only 50 to 60%, still significantly worse than that of other organ transplants. It is likely that bronchial arterial revascularisation at the time of LTx plays an important transplant-preserving function.

Targeting Interleukin-10 Restores Graft Microvascular Supply and Airway Epithelium in Rejecting Allografts

Interleukin-10 (IL-10) is a vital regulatory cytokine, which plays a constructive role in maintaining immune tolerance during an alloimmune inflammation. Our previous study highlighted that IL-10 mediated immunosuppression established the immune tolerance phase and thereby modulated both microvascular and epithelial integrity, which affected inflammation-associated graft malfunctioning and sub-epithelial fibrosis in rejecting allografts. Here, we further investigated the reparative effects of IL-10 on microvasculature and epithelium in a mouse model of airway transplantation. To investigate the IL-10 mediated microvascular and epithelial repair, we depleted and reconstituted IL-10, and monitored graft microvasculature, airway epithelium, and associated repair proteins. Our data demonstrated that both untreated control allografts and IL-10 (−) allografts showed a significant early (d6) increase in microvascular leakiness, drop-in tissue oxygenation, blood perfusion, and denuded airway epithelium, which is associated with loss of adhesion protein Fascin-1 and β-catenin on vascular endothelial cells at d10 post-transplantation. However, IL-10 (+) promotes early microvascular and airway epithelial repair, and a proportional increase in endothelial Fascin-1, and β-catenin at d10 post-transplantation. Moreover, airway epithelial cells also express a significantly higher expression of FOXJ1 and β-catenin in syngrafts and IL-10 (+) allografts as compared to IL-10 (−) and untreated controls at d10 post-transplantation. Collectively, these findings demonstrated that IL-10 mediated microvascular and epithelial changes are associated with the expression of FOXJ1, β-catenin, and Fascin-1 proteins on the airway epithelial and vascular endothelial cells, respectively. These findings establish a potential reparative modulation of IL-10 associated microvascular and epithelial repair, which could provide a vital therapeutic strategy to facilitate graft repair in clinical settings.

IL-10 Mediated Immunomodulation Limits Subepithelial Fibrosis and Repairs Airway Epithelium in Rejecting Airway Allografts

Interleukin-10 plays a vital role in maintaining peripheral immunotolerance and favors a regulatory immune milieu through the suppression of T effector cells. Inflammation-induced microvascular loss has been associated with airway epithelial injury, which is a key pathological source of graft malfunctioning and subepithelial fibrosis in rejecting allografts. The regulatory immune phase maneuvers alloimmune inflammation through various regulatory modulators, and thereby promotes graft microvascular repair and suppresses the progression of fibrosis after transplantation. The present study was designed to investigate the therapeutic impact of IL-10 on immunotolerance, in particular, the reparative microenvironment, which negates airway epithelial injury, and fibrosis in a mouse model of airway graft rejection. Here, we depleted and reconstituted IL-10, and serially monitored the phase of immunotolerance, graft microvasculature, inflammatory cytokines, airway epithelium, and subepithelial collagen in rejecting airway transplants. We demonstrated that the IL-10 depletion suppresses FOXP3+ Tregs, tumor necrosis factor-inducible gene 6 protein (TSG-6), graft microvasculature, and establishes a pro-inflammatory phase, which augments airway epithelial injury and subepithelial collagen deposition while the IL-10 reconstitution facilitates FOXP3+ Tregs, TSG-6 deposition, graft microvasculature, and thereby favors airway epithelial repair and subepithelial collagen suppression. These findings establish a potential reparative modulation of IL-10-associated immunotolerance on microvascular, epithelial, and fibrotic remodeling, which could provide a vital therapeutic option to rescue rejecting transplants in clinical settings.

Microvasculature in murine tracheal allografts after combined therapy with clopidogrel and everolimus

OBJECTIVES

Survival after lung transplantation is mainly limited by the development of chronic lung allograft dysfunction. Previous studies have suggested T-cell mediated proliferation and microvascular changes in experimental small airways models as potential therapeutic targets. The aim of this study was to assess microvascular changes in murine orthotopic tracheal allografts after treatment with everolimus alone or in combination with clopidogrel.

METHODS

C57Bl/6 (H-2b) donor tracheas were orthotopically transplanted into CBA (H-2k) recipients. Mice received daily injections of everolimus (0.05 mg/kg) alone or combined with clopidogrel (1 mg/kg). Twenty-eight days after transplantation, ratio of the thickness of tracheal epithelium and lamina propria was measured as an indicator for chronic rejection. Additionally, graft oxygenation and graft perfusion were detected on postoperative days 4, 10 and 28. Quantitative reverse transcription polymerase chain reaction analysis was used for gene expression analysis.

RESULTS

While syngeneic grafts showed a stable tissue pO2 and undisturbed microvascular perfusion, rejecting allografts had a drastic decline in both parameters as well as a flattened epithelium and an increased thickness of the lamina propria. Treatment with everolimus reduced allogeneic fibroproliferation, but had no protective effects on the microvasculature; polymerase chain reaction analysis indicated hypoxic stress and inflammation. Combining everolimus with clopidogrel improved microvascular integrity in the tracheal grafts, but had no synergistic effect in preventing obliterative bronchiolitis development.

CONCLUSIONS

These data demonstrate that the ability of everolimus to reduce the development of post-transplant obliterative bronchiolitis is not caused by microvascular protection and has no synergistic effects with clopidogrel in acute airway rejection.

Hyperbaric oxygen therapy to prevent central airway stenosis after lung transplantation

BACKGROUND

Central airway stenosis (CAS) is a severe airway complication after lung transplantation associated with bronchial ischemia and necrosis. We sought to determine whether hyperbaric oxygen therapy (HBOT), an established treatment for tissue ischemia, attenuates post-transplant bronchial injury.

METHODS

We performed a randomized, controlled trial comparing usual care with HBOT (2 atm absolute for 2 hours × 20 sessions) in subjects with extensive airway necrosis 4 weeks after transplantation. Endobronchial biopsies were collected at 4, 7, and 10 weeks after transplantation for a quantitative polymerase chain reaction. Coprimary outcomes were incidence of airway stenting and acute cellular rejection (ACR) at 1 year.

RESULTS

The trial was stopped after enrolling 20 subjects (n = 10 per group) after a pre-planned interim analysis showed no difference between usual care and HBOT groups in stenting (both 40%), ACR (70% and 40%, respectively), or CAS (40% and 60%, respectively). Time to first stent placement (median [interquartile range]) was significantly shorter in the HBOT group (150 [73-150] vs 186 [167-206] days, p < 0.05). HIF gene expression was significantly increased in donor tissues at 4, 7, and 10 weeks after transplantation but was not altered by HBOT. Subjects who developed CAS or required stenting had significantly higher HMOX1 and VEGFA expression at 4 weeks (both p < 0.05). Subjects who developed ACR had significant FLT1, TIE2, and KDR expression at 4 weeks (all p < 0.05).

CONCLUSIONS

Incidence of CAS is high after severe, established airway necrosis after transplantation. HBOT does not reduce CAS severity or stenting. Elevated HMOX1 and VEGFA expressions appear to associate with airway complications.

Spatial and Temporal Analysis of Host Cells in Tracheal Graft Implantation

OBJECTIVES/HYPOTHESIS

The ideal trachea replacement would be a living graft that is genetically identical to the host, avoiding the need for immunosuppression. We have developed a mouse model of syngeneic tracheal transplant that results in long-term survival without graft stenosis or delayed healing. To understand how host cells contribute to tracheal transplant integration, we quantified the populations of host cells in the graft and native trachea following implant.

STUDY DESIGN

Tracheal transplant, tracheal replacement, regenerative medicine, animal model.

METHODS

Tracheal grafts were obtained from female C57BL/6 mice and orthotopically transplanted into syngeneic male recipients. Cohorts were euthanized on day 14, day 45, and day 90 post-transplantation. Host and graft tracheas were explanted and analyzed by histology. Male host cells were quantified using fluorescence in situ hybridization, and macrophages were quantified with immunofluorescence.

RESULTS

Evidence of host-derived cells was found in the midgraft at the earliest time point (14 days). Host-derived cells transiently increased in the graft on day 45 and were predominantly found in the submucosa. By day 90, the population of host-derived cells population declined to a similar level on day 14. Macrophage infiltration of host and graft tissue was observed at all time points and was greatest on day 90.

CONCLUSIONS

Tracheal graft integration occurs by way of subacute transient host-cell infiltration and is primarily inflammatory in nature. Host-cell contribution to the graft epithelium is limited. These data indicate that creation of living, nonimmunogenic tracheal graft could serve as a viable solution for long-segment tracheal defects.

LEVEL OF EVIDENCE

3 Laryngoscope, 131:E340-E345, 2021.

Hypoxia-induced complement dysregulation is associated with microvascular impairments in mouse tracheal transplants

BACKGROUND

Complement Regulatory Proteins (CRPs), especially CD55 primarily negate complement factor 3-mediated injuries and maintain tissue homeostasis during complement cascade activation. Complement activation and regulation during alloimmune inflammation contribute to allograft injury and therefore we proposed to investigate a crucial pathological link between vascular expression of CD55, active-C3, T cell immunity and associated microvascular tissue injuries during allograft rejection.

METHODS

Balb/c→C57BL/6 allografts were examined for microvascular deposition of CD55, C3d, T cells, and associated tissue microvascular impairments during rejection in mouse orthotopic tracheal transplantation.

RESULTS

Our findings demonstrated that hypoxia-induced early activation of HIF-1α favors a cell-mediated inflammation (CD4⁺, CD8⁺, and associated proinflammatory cytokines, IL-2 and TNF-α), which proportionally triggers the downregulation of CRP-CD55, and thereby augments the uncontrolled release of active-C3, and Caspase-3 deposition on CD31⁺ graft vascular endothelial cells. These molecular changes are pathologically associated with microvascular deterioration (low tissue O₂ and Blood flow) and subsequent airway epithelial injuries of rejecting allografts as compared to non-rejecting syngrafts.

CONCLUSION

Together, these findings establish a pathological correlation between complement dysregulation, T cell immunity, and microvascular associated injuries during alloimmune inflammation in transplantation.

Bronchiolitis obliterans syndrome and restrictive allograft syndrome after lung transplantation: why are there two distinct forms of chronic lung allograft dysfunction?

Bronchiolitis obliterans syndrome (BOS) had been considered to be the representative form of chronic rejection or chronic lung allograft dysfunction (CLAD) after lung transplantation. In BOS, small airways are affected by chronic inflammation and obliterative fibrosis, whereas peripheral lung tissue remains relatively intact. However, recognition of another form of CLAD involving multiple tissue compartments in the lung, termed restrictive allograft syndrome (RAS), raised a fundamental question: why there are two phenotypes of CLAD? Increasing clinical and experimental data suggest that RAS may be a prototype of chronic rejection after lung transplantation involving both cellular and antibody-mediated alloimmune responses. Some cases of RAS are also induced by fulminant general inflammation in lung allografts. However, BOS involves alloimmune responses and the airway-centered disease process can be explained by multiple mechanisms such as external alloimmune-independent stimuli (such as infection, aspiration and air pollution), exposure of airway-specific autoantigens and airway ischemia. Localization of immune responses in different anatomical compartments in different phenotypes of CLAD might be associated with lymphoid neogenesis or the de novo formation of lymphoid tissue in lung allografts. Better understanding of distinct mechanisms of BOS and RAS will facilitate the development of effective preventive and therapeutic strategies of CLAD.

Deconstructing tissue engineered trachea: Assessing the role of synthetic scaffolds, segmental replacement and cell seeding on graft performance

The ideal construct for tracheal replacement remains elusive in the management of long segment airway defects. Tissue engineered tracheal grafts (TETG) have been limited by the development of graft stenosis or collapse, infection, or lack of an epithelial lining. We applied a mouse model of orthotopic airway surgery to assess the impact of three critical barriers encountered in clinical applications: the scaffold, the extent of intervention, and the impact of cell seeding and characterized their impact on graft performance. First, synthetic tracheal scaffolds electrospun from polyethylene terephthalate / polyurethane (PET/PU) were orthotopically implanted in anterior tracheal defects of C57BL/6 mice. Scaffolds demonstrated complete coverage with ciliated respiratory epithelium by 2 weeks. Epithelial migration was accompanied by macrophage infiltration which persisted at long term (>6 weeks) time points. We then assessed the impact of segmental tracheal implantation using syngeneic trachea as a surrogate for the ideal tracheal replacement. Graft recovery involved local upregulation of epithelial progenitor populations and there was no evidence of graft stenosis or necrosis. Implantation of electrospun synthetic tracheal scaffold for segmental replacement resulted in respiratory distress and required euthanasia at an early time point. There was limited epithelial coverage of the scaffold with and without seeded bone marrow-derived mononuclear cells (BM-MNCs). We conclude that synthetic scaffolds support re-epithelialization in orthotopic patch implantation, syngeneic graft integration occurs with focal repair mechanisms, however epithelialization in segmental synthetic scaffolds is limited and is not influenced by cell seeding. STATEMENT OF SIGNIFICANCE: The life-threatening nature of long-segment tracheal defects has led to clinical use of tissue engineered tracheal grafts in the last decade for cases of compassionate use. However, the ideal tracheal reconstruction using tissue-engineered tracheal grafts (TETG) has not been clarified. We addressed the core challenges in tissue engineered tracheal replacement (re-epithelialization and graft patency) by defining the role of cell seeding with autologous bone marrow-derived mononuclear cells, the mechanism of respiratory epithelialization and proliferation, and the role of the inflammatory immune response in regeneration. This research will facilitate comprehensive understanding of cellular regeneration and neotissue formation on TETG, which will permit targeted therapies for accelerating re-epithelialization and attenuating stenosis in tissue engineered airway replacement.

Calcineurin inhibitors augment endothelial-to-mesenchymal transition by enhancing proliferation in association with cytokine-mediated activation

Calcineurin Inhibitors (CNIs) are routinely used for immunosuppression following solid organ transplantation. However, the prolonged use of these agents lead to organ fibrosis which limits their efficacy. CNIs induce TGFβ expression, which is reported to augment endothelial-to-mesenchymal transition (EndMT), but their role in this process is not known. In these studies, we find that the CNIs FK506 and cyclosporine (CsA) are potent to increase endothelial cell (EC) proliferation using established in vitro assays (P < 0.05). Furthermore, using phosphokinase arrays, we find that each CNI activates the MAPK and Akt/mTOR signaling pathways, and that pharmacological inhibition of each pathway targets CNI-induced proliferative responses (P < 0.001). EndMT was evaluated by FACS for N-cadherin and CD31 expression and by qPCR for the expression of α-smooth muscle actin, N-cadherin and Snail. We find that CNIs do not directly induce dedifferentiation, while TGFβ and hypoxia induce EndMT in small numbers of EC. In contrast, the treatment of EC with the inflammatory cytokine TNFα was potent to elicit an EndMT response, and its effects were most notably in EC following proliferation/doubling. Taken together, these observations suggest that CNIs elicit proliferative responses, which enhance EndMT in association with local inflammation. The clinical implications of these findings are that anti-proliferative therapeutics have high potential to target the initiation of this EndMT response.

Preservation of Microvascular Integrity in Murine Orthotopic Tracheal Allografts by Clopidogrel

BACKGROUND

Survival after lung transplantation is mainly limited by the development of chronic lung allograft dysfunction (CLAD). The aim of this study was to investigate if platelet inhibition by clopidogrel has a functionally relevant influence on the microvascular integrity of orthotopic tracheal allografts as an anatomic basis for the development of CLAD.

METHODS

We orthotopically transplanted C57Bl/6 (H-2) tracheas into CBA.J (H-2) recipients who afterwards received clopidogrel (1 mg/kg). Morphometric analysis was performed by measuring epithelial height in proportion to thickness of the lamina propria (epithelium-lamina propria ratio). Tissue oxygenation was determined using a fluorescence quenching technique, and graft perfusion monitoring was performed by laser Doppler flowmetry and lectin-binding assay. Immunohistochemistry was used for detection of CD31 and inducible nitric oxide synthase while iron deposition was shown with Prussian blue reaction. Quantitative reverse transcription polymerase chain reaction analysis was used for gene expression analysis.

RESULTS

Isografts maintained good oxygenation and perfusion throughout the experiment, while both were drastically reduced in allografts. Treatment with clopidogrel attenuated graft hypoxia and reduced loss of perfusion. Additionally, clopidogrel led to increased epithelium-lamina propria ratio while iron deposition was impaired. Gene expression analysis revealed elevated levels of angiogenic vascular endothelial growth factor in the clopidogrel group. Improved endothelial function was shown by immunohistochemistry (CD31, inducible nitric oxide synthase).

CONCLUSIONS

Continuous administration of clopidogrel significantly improved tissue oxygenation, limited microvascular leakiness, and prevented airway ischemia. These data demonstrate that clopidogrel ameliorates microvascular injury during acute airway rejection, which is a known predisposing factor for the development of CLAD.

Chemokine signaling axis between endothelial and myeloid cells regulates development of pulmonary hypertension associated with pulmonary fibrosis and hypoxia

Pulmonary hypertension complicates the care of many patients with chronic lung diseases (defined as Group 3 pulmonary hypertension), yet the mechanisms that mediate the development of pulmonary vascular disease are not clearly defined. Despite being the most prevalent form of pulmonary hypertension, to date there is no approved treatment for patients with disease. Myeloid-derived suppressor cells (MDSCs) and endothelial cells in the lung express the chemokine receptor CXCR2, implicated in the evolution of both neoplastic and pulmonary vascular remodeling. However, precise cellular contribution to lung disease is unknown. Therefore, we used mice with tissue-specific deletion of CXCR2 to investigate the role of this receptor in Group 3 pulmonary hypertension. Deletion of CXCR2 in myeloid cells attenuated the recruitment of polymorphonuclear MDSCs to the lungs, inhibited vascular remodeling, and protected against pulmonary hypertension. Conversely, loss of CXCR2 in endothelial cells resulted in worsened vascular remodeling, associated with increased MDSC migratory capacity attributable to increased ligand availability, consistent with analyzed patient sample data. Taken together, these data suggest that CXCR2 regulates MDSC activation, informing potential therapeutic application of MDSC-targeted treatments.

iPSC-derived MSC therapy induces immune tolerance and supports long-term graft survival in mouse orthotopic tracheal transplants

Background

Lung transplantation is a life-saving surgical replacement of diseased lungs in patients with end-stage respiratory malfunctions. Despite remarkable short-term recovery, long-term lung survival continues to face several major challenges, including chronic rejection and severe toxic side effects due to global immunosuppression. Stem cell-based immunotherapy has been recognized as a crucial immunoregulatory regimen in various preclinical and clinical studies. Despite initial therapeutic outcomes, conventional stem cells face key limitations. The novel Cymerus™ manufacturing facilitates production of a virtually limitless supply of consistent human induced pluripotent stem cell (iPSC)-derived mesenchymal stem cells, which could play a key role in selective immunosuppression and graft repair during rejection.

Methods

Here, we demonstrated the impact of iPSC-derived human MSCs on the development of immune tolerance and long-term graft survival in mouse orthotopic airway allografts. BALB/c → C57BL/6 allografts were reconstituted with iPSC-derived MSCs (2 million/transplant/at d0), and allografts were examined for regulatory T cells (Tregs), oxygenation, microvascular blood flow, airway epithelium, and collagen deposition during rejection.

Results

We demonstrated that iPSC-derived MSC treatment leads to significant increases in hTSG-6 protein, followed by an upregulation of mouse Tregs and IL-5, IL-10, and IL-15 cytokines, which augments graft microvascular blood flow and oxygenation, and thereby maintained a healthy airway epithelium and prevented the subepithelial deposition of collagen at d90 post transplantation.

Conclusions

Collectively, these data confirmed that iPSC-derived MSC-mediated immunosuppression has potential to establish immune tolerance and rescue allograft from sustained hypoxic/ischemic phase, and subsequently limits long-term airway epithelial injury and collagen progression, which therapeutically warrant a study of Cymerus iPSC-derived MSCs as a potential management option for immunosuppression in transplant recipients.

Diagnosis, Pathophysiology and Experimental Models of Chronic Lung Allograft Rejection

Chronic rejection is the Achilles heel of modern lung transplantation, characterized by a slow, progressive decline in allograft function. Clinically, this manifests as obstructive disease, restrictive disease, or a mixture of the 2 depending on the underlying pathology. The 2 major phenotypes of chronic rejection include bronchiolitis obliterans syndrome and restrictive allograft syndrome. The last decade of research has revealed that each of these phenotypes has a unique underlying pathophysiology which may require a distinct treatment regimen for optimal control. Insights into the intricate alloimmune pathways contributing to chronic rejection have been gained from both large and small animal models, suggesting directions for future research. In this review, we explore the pathological hallmarks of chronic rejection, recent insights gained from both clinical and basic science research, and the current state of animal models of chronic lung rejection.

The Bronchial Arterial Circulation in Lung Transplantation: Bedside to Bench to Bedside, and Beyond

Chronic allograft dysfunction (CLAD) remains a major complication, causing the poor survival after lung transplantation (Tx). Although strenuous efforts have been made at preventing CLAD, surgical approaches for lung Tx have not been updated over the last 2 decades. The bronchial artery (BA), which supplies oxygenated blood to the airways and constitutes a functional microvasculature, has occasionally been revascularized during transplants, but this technique did not gain popularity and is not standard in current lung Tx protocols, despite the fact that a small number of studies have shown beneficial effects of BA revascularization on limiting CLAD. Also, recent basic and clinical evidence has demonstrated the relationship between microvasculature damage and CLAD. Thus, the protection of the bronchial circulation and microvasculature in lung grafts may be a key factor to overcome CLAD. This review revisits the history of BA revascularization, discusses the role of the bronchial circulation in lung Tx, and advocates for novel bronchial-arterial-circulation sparing approaches as a future direction for overcoming CLAD. Although there are some already published review articles summarizing the surgical techniques and their possible contribution to outcomes in lung Tx, to the best of our knowledge, this review is the first to elaborate on bronchial circulation that will contribute to prevent CLAD from both scientific and clinical perspectives: from bedside to bench to bedside, and beyond.

Airway hypoxia in lung transplantation

Lung transplantation is a life-saving operation for patients with advanced lung disease. Pulmonary allografts eventually fail because of infection, thromboembolism, malignancy, airway complications, and chronic rejection, otherwise known as chronic lung allograft dysfunction (CLAD). Emerging evidence suggests that a highly-compromised airway circulation contributes to the evolution of airway complications and CLAD. There are two significant causes of poor perfusion and airway hypoxia in lung transplantation: an abnormal bronchial circulation which causes airway complications and microvascular rejection which induces CLAD. At the time of transplantation, the bronchial artery circulation, a natural component of the airway circulatory anatomy, is not surgically connected, and bronchi distal to the anastomosis become hypoxic. Subsequently, the bronchial anastomosis is left to heal under ischemic conditions. Still later, the extant microvessels in transplant bronchi are subjected to alloimmune insults that can further negatively impact pulmonary function. This review describes how airway tissue hypoxia evolves in lung transplantation, why depriving oxygenation in the bronchi and more distal bronchioles contributes to disease pathology and what therapeutic interventions are currently emerging to address these vascular injuries. Improving anastomotic vascular healing at the time of transplantation and preventing microvascular loss during acute rejection episodes are two steps that could limit airway hypoxia and improve patient outcomes.

Endothelial Hypoxia-Inducible Factor-2α Is Required for the Maintenance of Airway Microvasculature

BACKGROUND

Hypoxia-inducible factors (HIFs), especially HIF-1α and HIF-2α, are key mediators of the adaptive response to hypoxic stress and play essential roles in maintaining lung homeostasis. Human and animal genetics studies confirm that abnormal HIF correlates with pulmonary vascular pathology and chronic lung diseases, but it remains unclear whether endothelial cell HIF production is essential for microvascular health. The large airway has an ideal circulatory bed for evaluating histological changes and physiology in genetically modified rodents.

METHODS

The tracheal microvasculature of mice, with conditionally deleted or overexpressed HIF-1α or HIF-2α, was evaluated for anatomy, perfusion, and permeability. Angiogenic signaling studies assessed vascular changes attributable to dysregulated HIF expression. An orthotopic tracheal transplantation model further evaluated the contribution of individual HIF isoforms in airway endothelial cells.

RESULTS

The genetic deletion of Hif-2α but not Hif-1α caused tracheal endothelial cell apoptosis, diminished pericyte coverage, reduced vascular perfusion, defective barrier function, overlying epithelial abnormalities, and subepithelial fibrotic remodeling. HIF-2α promoted microvascular integrity in airways through endothelial angiopoietin-1/TIE2 signaling and Notch activity. In functional tracheal transplants, HIF-2α deficiency in airway donors accelerated graft microvascular loss, whereas HIF-2α or angiopoietin-1 overexpression prolonged transplant microvascular perfusion. Augmented endothelial HIF-2α in transplant donors promoted airway microvascular integrity and diminished alloimmune inflammation.

CONCLUSIONS

Our findings reveal that the constitutive expression of endothelial HIF-2α is required for airway microvascular health.

Bronchus-associated lymphoid tissue-resident Foxp3+ T lymphocytes prevent antibody-mediated lung rejection

Antibody-mediated rejection (AMR) is a principal cause of acute and chronic failure of lung allografts. However, mechanisms mediating this oftentimes fatal complication are poorly understood. Here, we show that Foxp3+ T cells formed aggregates in rejection-free human lung grafts and accumulated within induced bronchus-associated lymphoid tissue (BALT) of tolerant mouse lungs. Using a retransplantation model, we show that selective depletion of graft-resident Foxp3+ T lymphocytes resulted in the generation of donor-specific antibodies (DSA) and AMR, which was associated with complement deposition and destruction of airway epithelium. AMR was dependent on graft infiltration by B and T cells. Depletion of graft-resident Foxp3+ T lymphocytes resulted in prolonged interactions between B and CD4+ T cells within transplanted lungs, which was dependent on CXCR5-CXCL13. Blockade of CXCL13 as well as inhibition of the CD40 ligand and the ICOS ligand suppressed DSA production and prevented AMR. Thus, we have shown that regulatory Foxp3+ T cells residing within BALT of tolerant pulmonary allografts function to suppress B cell activation, a finding that challenges the prevailing view that regulation of humoral responses occurs peripherally. As pulmonary AMR is largely refractory to current immunosuppression, our findings provide a platform for developing therapies that target local immune responses.

DEPTOR at the Nexus of Cancer, Metabolism, and Immunity

DEP domain-containing mechanistic target of rapamycin (mTOR)-interacting protein (DEPTOR) is an important modulator of mTOR, a kinase at the center of two important protein complexes named mTORC1 and mTORC2. These highly studied complexes play essential roles in regulating growth, metabolism, and immunity in response to mitogens, nutrients, and cytokines. Defects in mTOR signaling have been associated with the development of many diseases, including cancer and diabetes, and approaches aiming at modulating mTOR activity are envisioned as an attractive strategy to improve human health. DEPTOR interaction with mTOR represses its kinase activity and rewires the mTOR signaling pathway. Over the last years, several studies have revealed key roles for DEPTOR in numerous biological and pathological processes. Here, we provide the current state of the knowledge regarding the cellular and physiological functions of DEPTOR by focusing on its impact on the mTOR pathway and its role in promoting health and disease.

Complement factor and T-cell interactions during alloimmune inflammation in transplantation

Complement factor and T-cell signaling during an effective alloimmune response plays a key role in transplant-associated injury, which leads to the progression of chronic rejection (CR). During an alloimmune response, activated complement factors (C3a and C5a) bind to their corresponding receptors (C3aR and C5aR) on a number of lymphocytes, including T-regulatory cells (Tregs), and these cell-molecular interactions have been vital to modulate an effective immune response to/from Th1-effector cell and Treg activities, which result in massive inflammation, microvascular impairments, and fibrotic remodeling. Involvement of the complement-mediated cell signaling during transplantation signifies a crucial role of complement components as a key therapeutic switch to regulate ongoing inflammatory state, and further to avoid the progression of CR of the transplanted organ. This review highlights the role of complement-T cell interactions, and how these interactions shunt the effector immune response during alloimmune inflammation in transplantation, which could be a novel therapeutic tool to protect a transplanted organ and avoid progression of CR.

Airway injury in an in vitro human epithelium-fibroblast model of diacetyl vapor exposure: diacetyl-induced basal/suprabasal spongiosis

Inhalation exposure to diacetyl (DA) is associated with obliterative bronchiolitis (OB) in workers and induces OB-like fibrotic airway lesions in rats. The pathogenesis of OB is poorly understood in part due to complex interactions between airway epithelial, mesenchymal and blood-derived inflammatory cells. DA-induced airway toxicity in the absence of recruited-inflammatory/immune cells was characterized using an air-liquid interface (ALI) model consisting of human airway epithelium with (Epi/FT) and without (Epi) a mesenchymal component. ALI cultures were exposed to 25 mM DA-derived vapors (using vapor cups) for 1 h on day 0, 2 and 4. In some experiments, the tissues were exposed to 2,3-hexanedione (Hex) which is structurally-similar, but much less fibrogenic than DA. Lactate dehydrogenase activity and day 6 histopathologic changes associated with epithelial injury, including basal/suprabasal spongiosis, were increased following exposure of Epi/FT tissues to DA but not control or Hex vapors. IL-1a, IL-6, IL-8, sIL-1Ra, TGFa, MCP-3 and TNFa proteins were increased following DA exposure of Epi/FT tissues; only IL-1a, IL-8, sIL-1Ra and TGFa were increased following exposure of Epi tissues. MMP-1, MMP-3 and TIMP-1 proteins were increased following DA exposure of Epi/FT tissues; whereas MMP-2, MMP-7 and TIMP-2 were decreased, and production was largely dependent upon the presence of sub-epithelial stromal matrix/fibroblasts. Hex-induced protein changes were minimal. This in vitro study demonstrated that exposure of human airways to DA vapors induced epithelial injury (with the histopathologic feature of basal/suprabasal spongiosis) and increased release of pro-inflammatory and pro-fibrotic cytokines/chemokines as well as MMPs/TIMPs in the absence of recruited-inflammatory cells.

C5a Blockade Increases Regulatory T Cell Numbers and Protects Against Microvascular Loss and Epithelial Damage in Mouse Airway Allografts

Microvascular injury during acute rejection has been associated with massive infiltration of CD4+ T effector cells, and the formation of complement products (C3a and C5a). Regulatory T cells (Tregs) are potent immunosuppressors of the adaptive immune system and have proven sufficient to rescue microvascular impairments. Targeting C5a has been linked with improved microvascular recovery, but its effects on the Treg and T effector balance is less well known. Here, we demonstrate the impact of C5a blockade on Treg induction and microvascular restoration in rejecting mouse airway allografts. BALB/c→C57BL/6 allografts were treated with a C5a-neutralizing l-aptamer (10 mg/kg, i.p. at d0 and every second day thereafter), and allografts were serially monitored for Treg infiltration, tissue oxygenation (tpO2), microvascular blood flow, and functional microvasculature between donor and recipients during allograft rejection. We demonstrated that C5a blocking significantly leads to enhanced presence of Tregs in the allograft, reinstates donor-recipient functional microvasculature, improves tpO2, microvascular blood flow, and epithelial repair, followed by an upregulation of IL-5, TGF-β, IL-10 vascular endothelial growth factor, and ANGPT1 gene expression, while it maintained a healthy epithelium and prevented subepithelial collagen deposition at d28 posttransplantation. Together, these data indicate that inhibition of C5a signaling has potential to preserve microvasculature and rescue allograft from a sustained hypoxic/ischemic phase, limits airway tissue remodeling through the induction of Treg-mediated immune tolerance. These findings may be useful in designing anti-C5a therapy in combination with existing immunosuppressive regimens to rescue tissue/organ rejection.

Microhemorrhage-associated tissue iron enhances the risk for Aspergillus fumigatus invasion in a mouse model of airway transplantation

Invasive pulmonary disease due to the mold Aspergillus fumigatus can be life-threatening in lung transplant recipients, but the risk factors remain poorly understood. To study this process, we used a tracheal allograft mouse model that recapitulates large airway changes observed in patients undergoing lung transplantation. We report that microhemorrhage-related iron content may be a major determinant of A. fumigatus invasion and, consequently, its virulence. Invasive growth was increased during progressive alloimmune-mediated graft rejection associated with high concentrations of ferric iron in the graft. The role of iron in A. fumigatus invasive growth was further confirmed by showing that this invasive phenotype was increased in tracheal transplants from donor mice lacking the hemochromatosis gene (Hfe^-/- ). The invasive phenotype was also increased in mouse syngrafts treated with topical iron solution and in allograft recipients receiving deferoxamine, a chelator that increases iron bioavailability to the mold. The invasive growth of the iron-intolerant A. fumigatus double-knockout mutant (ΔsreA/ΔcccA) was lower than that of the wild-type mold. Alloimmune-mediated microvascular damage and iron overload did not appear to impair the host's immune response. In human lung transplant recipients, positive staining for iron in lung transplant tissue was more commonly seen in endobronchial biopsy sections from transplanted airways than in biopsies from the patients' own airways. Collectively, these data identify iron as a major determinant of A. fumigatus invasive growth and a potential target to treat or prevent A. fumigatus infections in lung transplant patients.

Microvascular injury after lung transplantation

PURPOSE OF REVIEW

Airway microvessel injury following transplantation has been implicated in the development of chronic rejection. This review focuses on the most recent developments in the field describing preclinical and clinical findings that further implicate the loss of microvascular integrity as an important pathological event in the evolution of irreversible fibrotic remodeling.

RECENT FINDINGS

When lungs are transplanted, the airways appear vulnerable from the perspective of perfusion. Two vascular systems are lost, the bronchial artery and the lymphatic circulations, and the remaining vasculature in the airways expresses donor antigens susceptible to alloimmune-mediated injury via innate and adaptive immune mechanisms. Preclinical studies indicate the importance of hypoxia-inducible factor-1α in mediating microvascular repair and that hypoxia-inducible factor-1α can be upregulated to bolster endogenous repair.

SUMMARY

Airway microvascular injury is a feature of lung transplantation that limits short-term and long-term organ health. Although some problems are attributable to a missing bronchial artery circulation, another significant issue involves alloimmune-mediated injury to transplant airway microvessels. For a variety of reasons, bronchial artery revascularization surgery at the time of transplantation has not been widely adopted, and the current best hope for this era may be new medical approaches that offer protection against immune-mediated vascular injury or that promote microvascular repair.

Models of Lung Transplant Research: a consensus statement from the National Heart, Lung, and Blood Institute workshop

Lung transplantation, a cure for a number of end-stage lung diseases, continues to have the worst long-term outcomes when compared with other solid organ transplants. Preclinical modeling of the most common and serious lung transplantation complications are essential to better understand and mitigate the pathophysiological processes that lead to these complications. Various animal and in vitro models of lung transplant complications now exist and each of these models has unique strengths. However, significant issues, such as the required technical expertise as well as the robustness and clinical usefulness of these models, remain to be overcome or clarified. The National Heart, Lung, and Blood Institute (NHLBI) convened a workshop in March 2016 to review the state of preclinical science addressing the three most important complications of lung transplantation: primary graft dysfunction (PGD), acute rejection (AR), and chronic lung allograft dysfunction (CLAD). In addition, the participants of the workshop were tasked to make consensus recommendations on the best use of these complimentary models to close our knowledge gaps in PGD, AR, and CLAD. Their reviews and recommendations are summarized in this report. Furthermore, the participants outlined opportunities to collaborate and directions to accelerate research using these preclinical models.

Simultaneously Targeting Myofibroblast Contractility and Extracellular Matrix Cross-Linking as a Therapeutic Concept in Airway Fibrosis

Fibrosis after solid organ transplantation is considered an irreversible process and remains the major cause of graft dysfunction and death with limited therapies. This remodeling is characterized by aberrant accumulation of contractile myofibroblasts that deposit excessive extracellular matrix (ECM) and increase tissue stiffness. Studies demonstrate, however, that a stiff ECM itself promotes fibroblast-to-myofibroblast differentiation, stimulating further ECM production. This creates a positive feedback loop that perpetuates fibrosis. We hypothesized that simultaneously targeting myofibroblast contractility with relaxin and ECM stiffness with lysyl oxidase inhibitors could break the feedback loop, reversing established fibrosis. To test this, we used the orthotopic tracheal transplantation (OTT) mouse model, which develops robust fibrotic airway remodeling. Mice with established fibrosis were treated with saline, mono-, or combination therapies. Although monotherapies had no effect, combining these agents decreased collagen deposition and promoted re-epithelialization of remodeled airways. Relaxin inhibited myofibroblast differentiation and contraction in a matrix-stiffness-dependent manner through prostaglandin E₂ (PGE₂ ). Furthermore, the effect of combination therapy was lost in PGE₂ receptor knockout and PGE₂ -inhibited OTT mice. This study revealed the important synergistic roles of cellular contractility and tissue stiffness in the maintenance of fibrotic tissue and suggests a new therapeutic principle for fibrosis.

T regulatory cell mediated immunotherapy for solid organ transplantation: A clinical perspective

T regulatory cells (Tregs) play a vital role in suppressing heightened immune responses, and thereby promote a state of immunological tolerance. Tregs modulate both innate and adaptive immunity, which make them a potential candidate for cell-based immunotherapy to suppress uncontrolled activation of graft specific inflammatory cells and their toxic mediators. These grafts specific inflammatory cells (T effector cells) and other inflammatory mediators (Immunoglobulins, active complement mediators) are mainly responsible for graft vascular deterioration followed by acute/chronic rejection. Treg mediated immunotherapy is under investigation to induce allospecific tolerance in various ongoing clinical trials in organ transplant recipients. Treg immunotherapy is showing promising results but the key issues regarding Treg immunotherapy are not yet fully resolved including their mechanism of action, and specific Treg cell phenotype responsible for a state of tolerance. This review highlights the involvement of various subsets of Tregs during immune suppression, novelty of Tregs functions, effects on angiogenesis, emerging technologies for effective Treg expansion, plasticity and safety associated with clinical applications. Altogether this information will assist in designing single/combined Treg mediated therapies for successful clinical trials in solid organ transplantations.

FOXP3+ regulatory T cell ameliorates microvasculature in the rejection of mouse orthotopic tracheal transplants

Microvascular loss may be a root cause of chronic rejection in lung transplants, which leads to the bronchiolitis obliterans syndrome. Previous research implicates T regulatory cell (Treg) as a key component of immune modulation, however, Treg has never been examined as a reparative mediator to salvage microvasculature during transplantation. Here, we reconstituted purified Tregs in to allografts, and serially monitored allografts for tissue oxygenation, microvascular perfusion for four weeks. We demonstrated that Tregs reconstitution of allografts significantly improve tissue oxygenation, microvascular flow, epithelial repair, number of CD4⁺CD25^highFOXP3⁺ Tregs, followed by an upregulation of proinflammatory, angiogenic and regulatory genes, while prevented subepithelial deposition of CD4⁺T cells at d10, and collagen at d28 post-transplantation. Altogether, these findings concluded that Treg-mediated immunotherapy has potential to preserve microvasculature and rescue allograft from sustained hypoxic/ischemic phase, limits airway tissue remodeling, and therefore may be a useful therapeutic tool to prevent chronic rejection after organ transplantation.

The intragraft microenvironment as a central determinant of chronic rejection or local immunoregulation/tolerance

PURPOSE OF REVIEW

Chronic rejection is associated with persistent mononuclear cell recruitment, endothelial activation and proliferation, local tissue hypoxia and related biology that enhance effector immune responses. In contrast, the tumor microenvironment elicits signals/factors that inhibit effector T cell responses and rather promote immunoregulation locally within the tissue itself. The identification of immunoregulatory check points and/or secreted factors that are deficient within allografts is of great importance in the understanding and prevention of chronic rejection.

RECENT FINDINGS

The relative deficiency of immunomodulatory molecules (cell surface and secreted) on microvascular endothelial cells within the intragraft microenvironment, is of functional importance in shaping the phenotype of rejection. These regulatory molecules include coinhibitory and/or intracellular regulatory signals/factors that enhance local activation of T regulatory cells. For example, semaphorins may interact with endothelial cells and CD4 T cells to promote local tolerance. Additionally, metabolites and electrolytes within the allograft microenvironment may regulate local effector and regulatory cell responses.

SUMMARY

Multiple factors within allografts shape the microenvironment either towards local immunoregulation or proinflammation. Promoting the expression of intragraft cell surface or secreted molecules that support immunoregulation will be critical for long-term graft survival and/or alloimmune tolerance.

Aspergillus-related pulmonary diseases in lung transplantation

While lung transplantation is an attractive treatment option for many end stage lung diseases, the relatively high 5-year mortality continues to be a significant limiting factor. Among the foremost reasons for this is the eventual development of obstructive chronic lung allograft dysfunction. Infections, which the lung allograft is especially prone to, are a major risk factor. Specifically, the Aspergillus species cause a higher burden of disease among lung transplant recipients, due to unique risk factors, such as relative hypoxemia. However, these risk factors also provide unique opportunities for treatment and preventative strategies, as outlined in this review.

Pathology of diacetyl and 2,3-pentanedione airway lesions in a rat model of obliterative bronchiolitis

Inhalation of diacetyl vapors by workers has been associated with obliterative bronchiolitis (OB), a poorly understood fibroproliferative disease of the small airways. Significant insights into the pathogenesis of OB have been obtained through the use of a rat model. Inhalation exposure of rats to diacetyl or 2,3-pentanedione, a related flavoring agent, can cause severe injury to the airway epithelium and underlying basement membrane. Repeated exposure to diacetyl or 2,3-pentanedione leads to aberrant repair, fibroproliferation and partial to complete occlusion of the airway lumen. Fibroproliferative lesions in rat airways were found to include both intraluminal polyps and circumferential intramural lesions. Intraluminal polyps have been observed to form secondary attachments spanning the airway lumen causing increasing obstruction. These airway lesions in rats are accompanied by inflammation in the form of peribronchial and perivascular infiltrates of lymphocytes, eosinophils and neutrophils. Diacetyl-induced OB lesions in the rat are similar to OB lesions in humans and provide a good model for studying the pathogenesis of this disease.

Safety of hyperbaric oxygen therapy for management of central airway stenosis after lung transplant

BACKGROUND

Central airway stenosis (CAS) is common after lung transplantation and causes significant post-transplant morbidity. It is often preceded by extensive airway necrosis, related to airway ischemia. Hyperbaric oxygen therapy (HBOT) is useful for ischemic grafts and may reduce the development of CAS.

METHODS

The purpose of this study was to determine whether HBOT could be safely administered to lung transplant patients with extensive necrotic airway plaques. Secondarily, we assessed any effects of HBOT on the incidence and severity of CAS. Patients with extensive necrotic airway plaques within 1-2 months after lung transplantation were treated with HBOT along with standard care. These patients were compared with a contemporaneous reference group with similar plaques who did not receive HBOT.

RESULTS

Ten patients received HBOT for 18.5 (interquartile range, IQR 11-20) sessions, starting at 40.5 (IQR 34-54) days after transplantation. HBOT was well tolerated. Incidence of CAS was similar between HBOT-treated patients and reference patients (70% vs 87%, respectively; P=.34), but fewer stents were required in HBOT patients (10% vs 56%, respectively; P=.03).

CONCLUSIONS

This pilot study is the first to demonstrate HBOT safety in patients who develop necrotic airway plaques after lung transplantation. HBOT may reduce the need for airway stent placement in patients with CAS.

Hypoxic Gene Expression of Donor Bronchi Linked to Airway Complications after Lung Transplantation

RATIONALE

Central airway stenosis (CAS) after lung transplantation has been attributed in part to chronic airway ischemia; however, little is known about the time course or significance of large airway hypoxia early after transplantation.

OBJECTIVES

To evaluate large airway oxygenation and hypoxic gene expression during the first month after lung transplantation and their relation to airway complications.

METHODS

Subjects who underwent lung transplantation underwent endobronchial tissue oximetry of native and donor bronchi at 0, 3, and 30 days after transplantation (n = 11) and/or endobronchial biopsies (n = 14) at 30 days for real-time polymerase chain reaction of hypoxia-inducible genes. Patients were monitored for 6 months for the development of transplant-related complications.

MEASUREMENTS AND MAIN RESULTS

Compared with native endobronchial tissues, donor tissue oxygen saturations (Sto2) were reduced in the upper lobes (74.1 ± 1.8% vs. 68.8 ± 1.7%; P < 0.05) and lower lobes (75.6 ± 1.6% vs. 71.5 ± 1.8%; P = 0.065) at 30 days post-transplantation. Donor upper lobe and subcarina Sto2 levels were also lower than the main carina (difference of -3.9 ± 1.5 and -4.8 ± 2.1, respectively; P < 0.05) at 30 days. Up-regulation of hypoxia-inducible genes VEGFA, FLT1, VEGFC, HMOX1, and TIE2 was significant in donor airways relative to native airways (all P < 0.05). VEGFA, KDR, and HMOX1 were associated with prolonged respiratory failure, prolonged hospitalization, extensive airway necrosis, and CAS (P < 0.05).

CONCLUSIONS

These findings implicate donor bronchial hypoxia as a driving factor for post-transplantation airway complications. Strategies to improve airway oxygenation, such as bronchial artery re-anastomosis and hyperbaric oxygen therapy merit clinical investigation.

Translational implications of endothelial cell dysfunction in association with chronic allograft rejection

Advances in therapeutics have dramatically improved short-term graft survival, but the incidence of chronic rejection has not changed in the past 20 years. New insights into mechanism are sorely needed at this time and it is hoped that the development of predictive biomarkers will pave the way for the emergence of preventative therapeutics. In this review, we discuss a paradigm suggesting that sequential changes within graft endothelial cells (EC) lead to an intragraft microenvironment that favors the development of chronic rejection. Key initial events include EC injury, activation and uncontrolled leukocyte-induced angiogenesis. We propose that all of these early changes in the microvasculature lead to abnormal blood flow patterns, local tissue hypoxia, and an associated overexpression of HIF-1α-inducible genes, including vascular endothelial growth factor. We also discuss how cell intrinsic regulators of mTOR-mediated signaling within EC are of critical importance in microvascular stability and may thus have a role in the inhibition of chronic rejection. Finally, we discuss recent findings indicating that miRNAs may regulate EC stability, and we review their potential as novel non-invasive biomarkers of allograft rejection. Overall, this review provides insights into molecular events, genes, and signals that promote chronic rejection and their potential as biomarkers that serve to support the future development of interruption therapeutics.

Targeted complement inhibition and microvasculature in transplants: a therapeutic perspective

Active complement mediators play a key role in graft-versus-host diseases, but little attention has been given to the angiogenic balance and complement modulation during allograft acceptance. The complement cascade releases the powerful proinflammatory mediators C3a and C5a anaphylatoxins, C3b, C5b opsonins and terminal membrane attack complex into tissues, which are deleterious if unchecked. Blocking complement mediators has been considered to be a promising approach in the modern drug discovery plan, and a significant number of therapeutic alternatives have been developed to dampen complement activation and protect host cells. Numerous immune cells, especially macrophages, develop both anaphylatoxin and opsonin receptors on their cell surface and their binding affects the macrophage phenotype and their angiogenic properties. This review discusses the mechanism that complement contributes to angiogenic injury, and the development of future therapeutic targets by antagonizing activated complement mediators to preserve microvasculature in rejecting the transplanted organ.

Therapeutic lymphangiogenesis ameliorates established acute lung allograft rejection

Lung transplantation is the only viable option for patients suffering from otherwise incurable end-stage pulmonary diseases such as chronic obstructive pulmonary disease and idiopathic pulmonary fibrosis. Despite aggressive immunosuppression, acute rejection of the lung allograft occurs in over half of transplant recipients, and the factors that promote lung acceptance are poorly understood. The contribution of lymphatic vessels to transplant pathophysiology remains controversial, and data that directly address the exact roles of lymphatic vessels in lung allograft function and survival are limited. Here, we have shown that there is a marked decline in the density of lymphatic vessels, accompanied by accumulation of low-MW hyaluronan (HA) in mouse orthotopic allografts undergoing rejection. We found that stimulation of lymphangiogenesis with VEGF-C156S, a mutant form of VEGF-C with selective VEGFR-3 binding, alleviates an established rejection response and improves clearance of HA from the lung allograft. Longitudinal analysis of transbronchial biopsies from human lung transplant recipients demonstrated an association between resolution of acute lung rejection and decreased HA in the graft tissue. Taken together, these results indicate that lymphatic vessel formation after lung transplantation mediates HA drainage and suggest that treatments to stimulate lymphangiogenesis have promise for improving graft outcomes.

The Role of Lysophosphatidic Acid on Airway Epithelial Cell Denudation in a Murine Heterotopic Tracheal Transplant Model

Supplemental digital content is available in the text.

Background

Chronic rejection is the major leading cause of morbidity and mortality after lung transplantation. Obliterative bronchiolitis (OB), a fibroproliferative disorder of the small airways, is the main manifestation of chronic lung allograft rejection. However, there is currently no treatment for the disease. We hypothesized that lysophosphatidic acid (LPA) participates in the progression of OB. The aim of this study was to reveal the involvement of LPA on the lesion of OB.

Methods

Ki16198, an antagonist specifically for LPA₁ and LPA₃, was daily administered into the heterotopic tracheal transplant model mice at the day of transplantation. At days 10 and 28, the allografts were isolated and evaluated histologically. The messenger RNA levels of LPAR in microdissected mouse airway regions were assessed to reveal localization of lysophosphatidic acid receptors. The human airway epithelial cell was used to evaluate the mechanism of LPA-induced suppression of cell adhesion to the extracellular matrix (ECM).

Results

The administration of Ki16198 attenuated airway epithelial cell loss in the allograft at day 10. Messenger RNAs of LPA₁ and LPA₃ were detected in the airway epithelial cells of the mice. Lysophosphatidic acid inhibited the attachment of human airway epithelial cells to the ECM and induced cell detachment from the ECM, which was mediated by LPA₁ and Rho-kinase pathway. However, Ki16198 did not prevent obliteration of allograft at day 28.

Conclusions

The LPA signaling is involved in the status of epithelial cells by distinct contribution in 2 different phases of the OB lesion. This finding suggests a role of LPA in the pathogenesis of OB.