Stem Cells, Cell Therapies and Bioengineering in Lung Biology and Diseases 2023

Repair and regeneration of a diseased lung using stem cells or bioengineered tissues is an exciting therapeutic approach for a variety of lung diseases and critical illnesses. Over the past decade increasing evidence from preclinical models suggests that cells, which are not normally resident in the lung can be utilized to modulate immune responses after injury, but there have been challenges in translating these promising findings to the clinic. In parallel, there has been a surge in bioengineering studies investigating the use of artificial and acellular lung matrices as scaffolds for three-dimensional lung or airway regeneration, with some recent attempts of transplantation in large animal models. The combination of these studies with those involving stem cells, induced pluripotent stem cell derivatives, and/or cell therapies is a promising and rapidly developing research area. These studies have been further paralleled by significant increases in our understanding of the molecular and cellular events by which endogenous lung stem and/or progenitor cells arise during lung development and participate in normal and pathologic remodeling after lung injury. For the 2023 Stem Cells, Cell Therapies, and Bioengineering in Lung Biology and Diseases Conference, scientific symposia were chosen to reflect the most cutting-edge advances in these fields. Sessions focused on the integration of "-omics" technologies with function, the influence of immune cells on regeneration, and the role of the extracellular matrix in regeneration. The necessity for basic science studies to enhance fundamental understanding of lung regeneration and to design innovative translational studies was reinforced throughout the conference.

Airway disease decreases the therapeutic potential of epithelial stem cells

BACKGORUND

Tissue-engineered tracheal grafts (TETG) can be recellularized by the host or pre-seeded with host-derived cells. However, the impact of airway disease on the recellularization process is unknown.

METHODS

In this study, we determined if airway disease alters the regenerative potential of the human tracheobronchial epithelium (hTBE) obtained by brushing the tracheal mucosa during clinically-indicated bronchoscopy from 48 pediatric and six adult patients.

RESULTS

Our findings revealed that basal cell recovery and frequency did not vary by age or region. At passage 1, all samples produced enough cells to cellularize a 3.5 by 0.5 cm2 graft scaffold at low cell density (~ 7000 cells/cm2), and 43.75% could cellularize a scaffold at high cell density (~ 100,000 cells/cm2). At passage 2, all samples produced the number of cells required for both recellularization models. Further evaluation revealed that six pediatric samples (11%) and three (50%) adult samples contained basal cells with a squamous basal phenotype. These cells did not form a polarized epithelium or produce differentiated secretory or ciliated cells. In the pediatric population, the squamous basal cell phenotype was associated with degree of prematurity (< 28 weeks, 64% vs. 13%, p = 0.02), significant pulmonary history (83% vs. 34%, p = 0.02), specifically with bronchopulmonary dysplasia (67% vs. 19%, p = 0.01), and patients who underwent previous tracheostomy (67% vs. 23%, p = 0.03).

CONCLUSIONS

In summary, screening high-risk pediatric or adult population based on clinical risk factors and laboratory findings could define appropriate candidates for airway reconstruction with tracheal scaffolds.

LEVEL OF EVIDENCE

Level III Cohort study.

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.

Biobanked tracheal basal cells retain the capacity to differentiate

Objective

While airway epithelial biorepositories have established roles in the study of bronchial progenitor stem (basal) cells, the utility of a bank of tracheal basal cells from pediatric patients, who have or are suspected of having an airway disease, has not been established. In vitro study of these cells can enhance options for tracheal restoration, graft design, and disease modeling. Development of a functional epithelium in these settings is a key measure. The aim of this study was the creation a tracheal basal cell biorepository and assessment of recovered cells.

Methods

Pediatric patients undergoing bronchoscopy were identified and endotracheal brush (N = 29) biopsies were collected. Cells were cultured using the modified conditional reprogramming culture (mCRC) method. Samples producing colonies by day 14 were passaged and cryopreserved. To explore differentiation potential, cells were thawed and differentiated using the air-liquid interface (ALI) method.

Results

No adverse events were associated with biopsy collection. Of 29 brush biopsies, 16 (55%) were successfully cultured to passage 1/cryopreserved. Samples with higher initial cell yields were more likely to achieve this benchmark. Ten unique donors were then thawed for analysis of differentiation. The average age was 2.2 ± 2.2 years with five donors (50%) having laryngotracheal pathology. Nine donors (90%) demonstrated differentiation capacity at 21 days of culture, as indicated by detection of ciliated cells (ACT+) and mucous cells (MUC5B+).

Conclusion

Pediatric tracheal basal cells can be successfully collected and cryopreserved. Recovered cells retain the ability to differentiate into epithelial cell types in vitro.

Level of Evidence

Level 3.

Assemblies of JAG1 and JAG2 determine tracheobronchial cell fate in mucosecretory lung disease

Mucosecretory lung disease compromises airway epithelial function and is characterized by goblet cell hyperplasia and ciliated cell hypoplasia. Goblet and ciliated cell types are derived from tracheobronchial stem/progenitor cells via a Notch-dependent mechanism. Although specific arrays of Notch receptors regulate cell fate determination, the function of the ligands Jagged1 (JAG1) and JAG2 is unclear. This study examined JAG1 and JAG2 function using human air-liquid-interface cultures that were treated with γ-secretase complex (GSC) inhibitors, neutralizing peptides/antibodies, or WNT/β-catenin pathway antagonists/agonists. These experiments revealed that JAG1 and JAG2 regulated cell fate determination in the tracheobronchial epithelium; however, their roles did not adhere to simple necessity and sufficiency rules. Biochemical studies indicated that JAG1 and JAG2 underwent posttranslational modifications that resulted in generation of a JAG1 C-terminal peptide and regulated the abundance of full-length JAG2 on the cell surface. GSC and glycogen synthase kinase 3 were implicated in these posttranslational events, but WNT agonist/antagonist studies and RNA-Seq indicated a WNT-independent mechanism. Collectively, these data suggest that posttranslational modifications create distinct assemblies of JAG1 and JAG2, which regulate Notch signal strength and determine the fate of tracheobronchial stem/progenitor cells.

Tissue-engineered composite tracheal grafts create mechanically stable and biocompatible airway replacements

We tested composite tracheal grafts (CTG) composed of a partially decellularized tracheal graft (PDTG) combined with a 3-dimensional (3D)-printed airway splint for use in long-segment airway reconstruction. CTG is designed to recapitulate the 3D extracellular matrix of the trachea with stable mechanical properties imparted from the extraluminal airway splint. We performed segmental orthotopic tracheal replacement in a mouse microsurgical model. MicroCT was used to measure graft patency. Tracheal neotissue formation was quantified histologically. Airflow dynamic properties were analyzed using computational fluid dynamics. We found that CTG are easily implanted and did not result in vascular erosion, tracheal injury, or inflammation. Graft epithelialization and endothelialization were comparable with CTG to control. Tracheal collapse was absent with CTG. Composite tracheal scaffolds combine biocompatible synthetic support with PDTG, supporting the regeneration of host epithelium while maintaining graft structure.

Tracheal Macrophages During Regeneration and Repair of Long-Segment Airway Defects

OBJECTIVES/HYPOTHESIS

Tissue-engineered tracheal grafts (TETGs) offer a potential solution for repair of long-segment airway defects. However, preclinical and clinical TETGs have been associated with chronic inflammation and macrophage infiltration. Macrophages express great phenotypic heterogeneity (generally characterized as classically activated [M1] vs. alternatively activated [M2]) and can influence tracheal repair and regeneration. We quantified and characterized infiltrating host macrophages using mouse microsurgical tracheal replacement models.

STUDY DESIGN

Translational research, animal model.

METHODS

We assessed macrophage infiltration and phenotype in animals implanted with syngeneic tracheal grafts, synthetic TETGs, or partially decellularized tracheal scaffolds (DTSs).

RESULTS

Macrophage infiltration was observed following tracheal replacement with syngeneic trachea. Both M1 and M2 macrophages were present in native trachea and increased during early tracheal repair (P = .014), with an M1/M2 ratio of 0.48 ± 0.15. In contrast, orthotopic implantation of synthetic TETGs resulted in a shift to M1 predominant macrophage phenotype with an increased M1/M2 ratio of 1.35 ± 0.41 by 6 weeks following implant (P = .035). Modulation of the synthetic scaffold with the addition of polyglycolic acid (PGA) resulted in a reduction of M1/M2 ratio due to an increase in M2 macrophages (P = .006). Using systemic macrophage depletion, the M1/M2 ratio reverted to native values in synthetic TETG recipients and was associated with an increase in graft epithelialization. Macrophage ratios seen in DTSs were similar to native values.

CONCLUSIONS

M1 and M2 macrophages are present during tracheal repair. Poor epithelialization with synthetic TETG is associated with an elevation of the M1/M2 ratio. Macrophage phenotype can be altered with scaffold composition and host-directed systemic therapies. DTSs exhibit M1/M2 ratios similar to those seen in native trachea and syngeneic tracheal replacement.

LEVEL OF EVIDENCE

NA Laryngoscope, 132:737-746, 2022.

Assessment of Beta-2 Microglobulin Gene Edited Airway Epithelial Stem Cells as a treatment for Sulfur Mustard Inhalation

Respiratory system damage is the primary cause of mortality in individuals who are exposed to vesicating agents including sulfur mustard (SM). Despite these devastating health complications, there are no fielded therapeutics that are specific for such injuries. Previous studies reported that SM inhalation depleted the tracheobronchial airway epithelial stem cell (TSC) pool and supported the hypothesis, TSC replacement will restore airway epithelial integrity and improve health outcomes for SM-exposed individuals. TSC express Major Histocompatibility Complex (MHC-I) transplantation antigens which increases the chance that allogeneic TSC will be rejected by the patient’s immune system. However, previous studies reported that Beta-2 microglobulin (B2M) knockout cells lacked cell surface MHC-I and suggested that B2M knockout TSC would be tolerated as an allogeneic graft. This study used a Cas9 ribonucleoprotein (RNP) to generate B2M-knockout TSC, which are termed Universal Donor Stem Cells (UDSC). Whole genome sequencing identified few off-target modifications and demonstrated the specificity of the RNP approach. Functional assays demonstrated that UDSC retained their ability to self-renew and undergo multilineage differentiation. A preclinical model of SM inhalation was used to test UDSC efficacy and identify any treatment-associated adverse events. Adult male Sprague-Dawley rats were administered an inhaled dose of 0.8 mg/kg SM vapor which is the inhaled LD₅₀ on day 28 post-challenge. On recovery day 2, vehicle or allogeneic Fisher rat UDSC were delivered intravenously (n = 30/group). Clinical parameters were recorded daily, and planned euthanasia occurred on post-challenge days 7, 14, and 28. The vehicle and UDSC treatment groups exhibited similar outcomes including survival and a lack of adverse events. These studies establish a baseline which can be used to further develop UDSC as a treatment for SM-induced airway disease.

Repeated injury promotes tracheobronchial tissue stem cell attrition

Chronic lung disease has been attributed to stem cell aging and/or exhaustion. We investigated these mechanisms using mouse and human tracheobronchial tissue‐specific stem cells (TSC). In mouse, chromatin labeling and flow cytometry demonstrated that naphthalene (NA) injury activated a subset of TSC. These activated TSC continued to proliferate after the epithelium was repaired and a clone study demonstrated that ~96% of activated TSC underwent terminal differentiation. Despite TSC attrition, epithelial repair after a second NA injury was normal. The second injury accelerated proliferation of previously activated TSC and a nucleotide‐label retention study indicated that the second injury recruited TSC that were quiescent during the first injury. These mouse studies indicate that (a) injury causes selective activation of the TSC pool; (b) activated TSC are predisposed to further proliferation; and (c) the activated state leads to terminal differentiation. In human TSC, repeated proliferation also led to terminal differentiation and depleted the TSC pool. A clone study identified long‐ and short‐lived TSC and showed that short‐lived TSC clones had significantly shorter telomeres than their long‐lived counterparts. The TSC pool was significantly depleted in dyskeratosis congenita donors, who harbor mutations in telomere biology genes. The remaining TSC had short telomeres and short lifespans. Collectively, the mouse and human studies support a model in which epithelial injury increases the biological age of the responding TSC. When applied to chronic lung disease, this model suggests that repeated injury accelerates the biological aging process resulting in abnormal repair and disease initiation.

Regeneration of partially decellularized tracheal scaffolds in a mouse model of orthotopic tracheal replacement

Decellularized tracheal scaffolds offer a potential solution for the repair of long-segment tracheal defects. However, complete decellularization of trachea is complicated by tracheal collapse. We created a partially decellularized tracheal scaffold (DTS) and characterized regeneration in a mouse model of tracheal transplantation. All cell populations except chondrocytes were eliminated from DTS. DTS maintained graft integrity as well as its predominant extracellular matrix (ECM) proteins. We then assessed the performance of DTS in vivo. Grafts formed a functional epithelium by study endpoint (28 days). While initial chondrocyte viability was low, this was found to improve in vivo. We then used atomic force microscopy to quantify micromechanical properties of DTS, demonstrating that orthotopic implantation and graft regeneration lead to the restoration of native tracheal rigidity. We conclude that DTS preserves the cartilage ECM, supports neo-epithelialization, endothelialization and chondrocyte viability, and can serve as a potential solution for long-segment tracheal defects.

Airway epithelial stem cell chimerism in cystic fibrosis lung transplant recipients

BACKGROUND

The conducting airway epithelium is repaired by tissue specific stem cells (TSC). In response to mild/moderate injury, each TSC repairs a discrete area of the epithelium. In contrast, severe epithelial injury stimulates TSC migration and expands the stem cell's reparative domain. Lung transplantation (LTx) can cause a moderate/severe airway injury and the remodeled airway contains a chimeric mixture of donor and recipient cells. These studies supported the hypothesis, LTx stimulates TSC migration resulting in epithelial chimerism. We tested this hypothesis in cystic fibrosis (CF) LTx patients.

METHODS

Airway mucosal injury was quantified using bronchoscopic imaging and a novel grading system. Bronchial brushing was used to recover TSC from 10 sites in the recipient and allograft airways. TSC chimerism was quantified by short tandem repeat analysis. TSC self-renewal and differentiation potential were assayed using the clone forming cell frequency and air-liquid-interface methods. Electrophysiology was used to determine if TSC chimerism altered epithelial ion channel activity.

RESULTS

LTx caused a mild to moderate airway mucosal injury. Donor and recipient TSC were identified in 91% of anastomotic sites and 93% of bronchial airways. TSC chimerism did not alter stem cell self-renewal or differentiation potential. The frequency of recipient TSC was proportional to CF Transmembrane Conductance Regulator (CFTR)-dependent ion channel activity and 33% of allograft regions were at risk for abnormal CFTR activity.

CONCLUSIONS

LTx in CF patients stimulates bidirectional TSC migration across the anastomoses. TSC chimerism may alter ion homeostasis and compromise the host defense capability of the allograft airway epithelium.

Correction of Airway Stem Cells: Genome Editing Approaches for the Treatment of Cystic Fibrosis

Cystic fibrosis (CF) is an autosomal recessive disease caused by variations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. Although CF affects multiple organs, the primary cause of mortality is respiratory failure resulting from poor clearance of hyperviscous secretions and subsequent airway infection. Recently developed CFTR modulators provide significant therapeutic benefit to the majority of CF individuals. However, treatments directed at the underlying cause are needed for the ∼7% of CF patients who are not expected to be responsive to these modulators. Genome editing can restore the native CFTR genetic sequence and function to mutant cells, representing an approach to establish durable physiologic CFTR correction. Although editing the CFTR gene in various airway cell types may transiently restore CFTR activity, effort is focused on editing airway basal stem/progenitor cells, since their correction would allow appropriate and durable expression of CFTR in stem cell-derived epithelial cell types. Substantial progress has been made to directly correct airway basal cells in vitro, theoretically enabling transplantation of autologous corrected cells to regenerate an airway with CFTR functional cells. Another approach to create autologous, gene-edited airway basal cells is derivation of CF donor-specific induced pluripotent stem cells, correction of the CFTR gene, and subsequent directed differentiation to airway basal cells. Further work is needed to translate these advances by developing effective transplantation methods. Alternatively, gene editing in vivo may enable CFTR correction. However, this approach will require robust delivery methods ensuring that basal cells are efficiently targeted and corrected. Recent advances in gene editing-based therapies provide hope that the genetic underpinning of CF can be durably corrected in airway epithelial stem cells, thereby preventing or treating lung disease in all people with CF.

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.

Mathematical modelling identifies the role of adaptive immunity as a key controller of respiratory syncytial virus in cotton rats

Respiratory syncytial virus (RSV) is a common virus that can have varying effects ranging from mild cold-like symptoms to mortality depending on the age and immune status of the individual. We combined mathematical modelling using ordinary differential equations (ODEs) with measurement of RSV infection kinetics in primary well-differentiated human bronchial epithelial cultures in vitro and in immunocompetent and immunosuppressed cotton rats to glean mechanistic details that underlie RSV infection kinetics in the lung. Quantitative analysis of viral titre kinetics in our mathematical model showed that the elimination of infected cells by the adaptive immune response generates unique RSV titre kinetic features including a faster timescale of viral titre clearance than viral production, and a monotonic decrease in the peak RSV titre with decreasing inoculum dose. Parameter estimation in the ODE model using a nonlinear mixed effects approach revealed a very low rate (average single-cell lifetime > 10 days) of cell lysis by RSV before the adaptive immune response is initiated. Our model predicted negligible changes in the RSV titre kinetics at early times post-infection (less than 5 dpi) but a slower decay in RSV titre in immunosuppressed cotton rats compared to that in non-suppressed cotton rats at later times (greater than 5 dpi) in silico. These predictions were in excellent agreement with the experimental results. Our combined approach quantified the importance of the adaptive immune response in suppressing RSV infection in cotton rats, which could be useful in testing RSV vaccine candidates.

Protein Abundance Determination: An Optimized Western Blot Workflow

We report that the quantitative western blot (qWB) analysis requires a target protein-specific approach, and we provide a workflow that streamlines development of this process. First, the optimal primary antibody dilution is determined. Blots containing 15 μg total protein per lane are probed with the primary antibody at three concentrations and a secondary antibody concentration that is defined by the manufacturer. The lowest primary antibody concentration that detects a discrete band at the correct molecular weight is used in the remaining two steps. Secondly, the optimal protein load is determined. Blots containing 3.75 to 60 μg protein per lane are probed using the antibody concentrations defined in step 1. A target protein band intensity vs. protein load plot is used to determine the linear dynamic range (LDR) for the target protein. The midpoint of the LDR is defined as the optimal protein load. Finally, an appropriate loading control (LC) is identified. We found that the LDR for β-actin, a commonly used LC, exhibited a narrow range, 3.75 to 15 μg. In contrast, the total protein assessed by a Ponceau staining method exhibited a broader LDR, 3.75 to 60 μg. Thus, the total protein is used as a LC. We conclude that the sensitivity and accuracy of the qWB method is dependent on the use of an optimal: 1) primary antibody dilution; 2) total protein load; 3) and LC. Our workflow simplifies the identification of these values.

Electrospun scaffolds limit the regenerative potential of the airway epithelium

Objective

Significant morbidity and mortality are associated with clinical use of synthetic tissue‐engineered tracheal grafts (TETG). Our previous work focused on an electrospun polyethylene terephthalate and polyurethane (PET/PU) TETG that was tested in sheep using a long‐segment tracheal defect model. We reported that graft stenosis and limited epithelialization contributed to graft failure. The present study determined if the epithelialization defect could be attributed to: 1) postsurgical depletion of native airway basal stem/progenitor cells; 2) an inability of the PET/PU‐TETG to support epithelial migration; or 3) compromised basal stem/progenitor cell proliferation within the PET/PU environment.

Study Design

Experimental.

Methods

Basal stem/progenitor cell frequency in sheep that underwent TETG implantation was determined using the clone‐forming cell frequency (CFCF) method. A novel migration model that mimics epithelial migration toward an acellular scaffold was developed and used to compare epithelial migration toward a control polyester scaffold and the PET/PU scaffold. Basal stem/progenitor cell proliferation within the PET/PU scaffold was evaluated using the CFCF assay, doubling‐time analysis, and mitotic cell quantification.

Results

We report that TETG implantation did not decrease basal stem/progenitor cell frequency. In contrast, we find that epithelial migration toward the PET/PU scaffold was significantly less extensive than migration toward a polyester scaffold and that the PET/PU scaffold did not support basal stem/progenitor cell proliferation.

Conclusions

We conclude that epithelialization of a PET/PU scaffold is compromised by poor migration of native tissue‐derived epithelial cells and by a lack of basal stem/progenitor cell proliferation within the scaffold.

Level of Evidence

NA

Factors Influencing Poor Outcomes in Synthetic Tissue-Engineered Tracheal Replacement

OBJECTIVES

Humans receiving tissue-engineered tracheal grafts have experienced poor outcomes ultimately resulting in death or the need for graft explantation. We assessed the performance of the synthetic scaffolds used in humans with an ovine model of orthotopic tracheal replacement, applying standard postsurgical surveillance and interventions to define the factors that contributed to the complications seen at the bedside.

STUDY DESIGN

Large animal model.

SETTING

Pediatric academic research institute.

SUBJECTS AND METHODS

Human scaffolds were manufactured with an electrospun blend of polyethylene terephthalate and polyurethane reinforced with polycarbonate rings. They were seeded with autologous bone marrow-derived mononuclear cells and implanted in sheep. Animals were evaluated with routine bronchoscopy and fluoroscopy. Endoscopic dilation and stenting were performed to manage graft stenosis for up to a 4-month time point. Grafts and adjacent native airway were sectioned and evaluated with histology and immunohistochemistry.

RESULTS

All animals had signs of graft stenosis. Three of 5 animals (60%) designated for long-term surveillance survived until the 4-month time point. Graft dilation and stent placement resolved respiratory symptoms and prolonged survival. Necropsy demonstrated evidence of infection and graft encapsulation. Granulation tissue with signs of neovascularization was seen at the anastomoses, but epithelialization was never observed. Acute and chronic inflammation of the native airway epithelium was observed at all time points. Architectural changes of the scaffold included posterior wall infolding and scaffold delamination.

CONCLUSIONS

In our ovine model, clinically applied synthetic tissue-engineered tracheas demonstrated infectious, inflammatory, and mechanical failures with a lack of epithelialization and neovascularization.

Seeding and Implantation of a Biosynthetic Tissue-engineered Tracheal Graft in a Mouse Model

Treatment options for congenital or secondary long segment tracheal defects have historically been limited due to an inability to replace functional tissue. Tissue engineering holds great promise as a potential solution with its ability to integrate cells and signaling molecules into a 3-dimensional scaffold. Recent work with tissue engineered tracheal grafts (TETGs) has seen some success but their translation has been limited by graft stenosis, graft collapse, and delayed epithelialization. In order to investigate the mechanisms driving these issues, we have developed a mouse model for tissue engineered tracheal graft implantation. TETGs were constructed using electrospun polymers polyethylene terephthalate (PET) and polyurethane (PU) in a mixture of PET and PU (20:80 percent weight). Scaffolds were then seeded using bone marrow mononuclear cells isolated from 6-8 week-old C57BL/6 mice by gradient centrifugation. Ten million cells per graft were seeded onto the lumen of the scaffold and allowed to incubate overnight before implantation between the third and seventh tracheal rings. These grafts were able to recapitulate the findings of stenosis and delayed epithelialization as demonstrated by histological analysis and lack of Keratin 5 and Keratin 14 basal epithelial cells on immunofluorescence. This model will serve as a tool for investigating cellular and molecular mechanisms involved in host remodeling.

Cell Therapy for Cystic Fibrosis Lung Disease: Regenerative Basal Cell Amplification

The human airway epithelium is regenerated by basal cells. Thus, basal cell therapy has the potential to cure cystic fibrosis (CF) lung disease. We previously reported that the human basal cells repopulated the mouse airway epithelium after transplantation, and we estimated that 60 million cells would be needed to treat a human patient. To further develop cell therapy, we compared the proliferation potential of non‐CF and CF tissue‐derived bronchial basal cells. Three methods were used: regenerative cell frequency, burst size, and cell division frequency. Second, we used a serial passage strategy to determine if CF basal cells could be amplified to the estimated therapeutic dose. These studies evaluated that tissue‐derived bronchial basal cells and the basal cells that were recovered by brushing bronchial airways or the nasal respiratory epithelium. Finally, we used the limiting dilution method to isolate non‐CF and CF basal cell clones. The proliferation assays and the air‐liquid‐interface differentiation method were used to determine if cell amplification altered the proliferation and/or differentiation potential of clonal isolates. We demonstrate that: (a) non‐CF and CF basal cell proliferation is similar, (b) CF basal cells can be amplified to a therapeutic cell dose, and (c) amplified non‐CF and CF basal cell clones differentiate normally. Despite these encouraging findings, we also find that the cell amplification process depletes the regenerative basal cell pool. Analysis of basal cell clones indicates that serial passage selects for long‐lived basal cells and raise the possibility that prospective isolation of these stem‐like cells will improve the efficacy of cell replacement therapy. stem cells translational medicine2019;8:225&235

Regulation of Human Airway Epithelial Tissue Stem Cell Differentiation by β-Catenin, P300, and CBP

The wingless/integrase-1 (WNT)/β-catenin signaling pathway is active in several chronic lung diseases including idiopathic pulmonary fibrosis, asthma, and chronic obstructive pulmonary disease. Although this WNT/β-catenin pathway activity is associated with an increase in mucus cell frequency and a decrease in ciliated cell frequency, a cause and consequence relationship between signaling and cell frequency has not been established. We previously demonstrated that genetic stabilization of β-catenin inhibited differentiation of mouse bronchiolar tissue stem cells (TSC). This study determined the effect of β-catenin and its co-factors P300 (E1A-binding protein, 300 kDa) and cAMP response element binding (CREB)-binding protein (CBP) on human bronchial epithelial TSC differentiation to mucus and ciliated cells. We developed a modified air-liquid interface (ALI) culture system in which mucus and ciliated cell frequency is similar. These cultures were treated with the β-catenin agonist CHIR99021 (CHIR) and antagonists to β-catenin (XAV939), P300 (IQ1), and CBP (ICG001). We report that human TSC differentiation to mucus and ciliated cells can be divided into two stages, specification and commitment. CHIR treatment inhibited mucus and ciliated cell commitment while XAV939 treatment demonstrated that β-catenin was necessary for mucus and ciliated cell specification. Additional studies demonstrate that a β-catenin/P300 complex promotes mucus cell specification and that β-catenin interacts with either P300 or CBP to inhibit ciliated cell commitment. These data indicate that activation of β-catenin-dependent signaling in chronic lung disease leads to changes in mucus and ciliated cell frequency and that P300 and CBP tune the β-catenin signal to favor mucus cell differentiation. Stem Cells 2018;36:1905-12.

AR-13 reduces antibiotic-resistant bacterial burden in cystic fibrosis phagocytes and improves cystic fibrosis transmembrane conductance regulator function

BACKGROUND

There are no effective treatments for Burkholderia cenocepacia in patients with cystic fibrosis (CF) due to bacterial multi-drug resistance and defective host killing. We demonstrated that decreased bacterial killing in CF is caused by reduced macrophage autophagy due to defective cystic fibrosis transmembrane conductance regulator (CFTR) function. AR-12 is a small molecule autophagy inducer that kills intracellular pathogens such as Francisella. We evaluated the efficacy of AR-12 and a new analogue AR-13 in reducing bacterial burden in CF phagocytes.

METHODS

Human CF and non-CF peripheral blood monocyte-derived macrophages, neutrophils, and nasal epithelial cells were exposed to CF bacterial strains in conjunction with treatment with antibiotics and/or AR compounds.

RESULTS

AR-13 and not AR-12 had growth inhibition on B. cenocepacia and methicillin-resistantStaphylococcus aureus (MRSA) in media alone. There was a 99% reduction in MRSA in CF macrophages, 71% reduction in Pseudomonas aeruginosa in CF neutrophils, and 70% reduction in non-CF neutrophils using AR-13. Conversely, there was no reduction in B. cenocepacia in infected CF and non-CF macrophages using AR-13 alone, but AR-13 and antibiotics synergistically reduced B. cenocepacia in CF macrophages. AR-13 improved autophagy in CF macrophages and CF patient-derived epithelial cells, and increased CFTR protein expression and channel function in CF epithelial cells.

CONCLUSIONS

The novel AR-12 analogue AR-13, in combination with antibiotics, reduced antibiotic-resistant bacterial burden in CF phagocytes, which correlated with increased autophagy and CFTR expression. AR-13 is a novel therapeutic for patients infected with B. cenocepacia and other resistant organisms that lack effective therapies.

The wound healing capacity of undifferentiated and differentiated airway epithelial cells in vitro

INTRODUCTION

Congenital or acquired tracheal lesions alter airway epithelial structure and can lead to long-segment tracheal defects. Tissue engineered tracheal grafts (TETG) have the potential to cure such defects; however, clinical applications have been plagued with numerous complications including delayed graft epithelialization. The knowledge that epithelial cells migrate from native tissue to the TETG raises the possibility that TETG performance can be improved by increasing the rate of epithelialization.

OBJECTIVES

We developed a model that can be used quantify epithelial migration in clinically-relevant conditions.

METHODS

Existing histological analyses determined the differentiation status of the normal and injured human tracheal epithelium and were used to identify in vitro culture conditions that mimic these parameters. The classical scratch assay was adapted to permit analysis of migratory velocity as a function of differentiation status. Migration of undifferentiated (UD), partially-differentiated (PD), and well-differentiated (WD) epithelia was quantified.

RESULTS

The normal and injured epithelium can be modeled using human cells that are cultured using a modified air-liquid-interface culture system. PD cell cultures are similar to the remodeled epithelium; whereas; WD cultures are similar to the normal epithelium. Preliminary results indicate that PD cells migrate more rapidly than WD cells and that PD and WD cells migrate more rapidly than UD cells.

CONCLUSION

Pending verification of these results, we suggest that epithelial migration is significantly altered by differentiation status. Thus, efforts to improve TETG epithelialization should use model systems that faithfully-represent the differentiation state of the native tissue.

Do they stay or do they go? Acoustic monitoring of whale sharks at Ningaloo Marine Park, Western Australia

Whale sharks Rhincodon typus were monitored via acoustic transmitters at the northern end of Western Australia's Ningaloo Marine Park to establish the extent to which the species inhabits the region beyond the whale-shark ecotourism industry season, which usually extends from March to August in each year. Despite the vast majority (c. 98%) of photographic submissions of R. typus from Ningaloo Reef being between March and August, acoustic detections from the tagged R. typus at Ningaloo were recorded in all months of the year, but do not preclude the occurrence of extended absences. It is concluded that as a species, R. typus occurs year round at Ningaloo, where it generally remains in close proximity to the reef edge, but that some individuals move outside of the detection range of the array for extended periods.

Transplantation of Airway Epithelial Stem/Progenitor Cells: A Future for Cell-Based Therapy

Cell therapy has the potential to cure disease through replacement of malfunctioning cells. Although the tissue stem cell (TSC) is thought to be the optimal therapeutic cell, transplantation of TSC/progenitor cell mixtures has saved lives. We previously purified the mouse tracheobronchial epithelial TSCs and reported that in vitro amplification generated numerous TSCs. However, these cultures also contained TSC-derived progenitor cells and TSC repurification by flow cytometry compromised TSC self-renewal. These limitations prompted us to determine if a TSC/progenitor cell mixture would repopulate the injured airway epithelium. We developed a cell transplantation protocol and demonstrate that transplanted mouse and human tracheobronchial epithelial TSC/progenitor cell mixtures are 20-25% of airway epithelial cells, actively contribute to epithelial repair, and persist for at least 43 days. At 2 weeks after transplantation, TSCs/progenitor cells differentiated into the three major epithelial cell types: basal, secretory, and ciliated. We conclude that cell therapy that uses adult tracheobronchial TSCs/progenitor cells is an effective therapeutic option.

Airway Progenitor Clone Formation Is Enhanced by Y-27632-Dependent Changes in the Transcriptome

The application of conditional reprogramming culture (CRC) methods to nasal airway epithelial cells would allow more wide-spread incorporation of primary airway epithelial culture models into complex lung disease research. In this study, we adapted the CRC method to nasal airway epithelial cells, investigated the growth advantages afforded by this technique over standard culture methods, and determined the cellular and molecular basis of CRC cell culture effects. We found that the CRC method allowed the production of 7.1 × 10(10) cells after 4 passages, approximately 379 times more cells than were generated by the standard bronchial epithelial growth media (BEGM) method. These nasal airway epithelial cells expressed normal basal cell markers and could be induced to form a mucociliary epithelium. Progenitor cell frequency was significantly higher using the CRC method in comparison to the standard culture method, and progenitor cell maintenance was dependent on addition of the Rho-kinase inhibitor Y-27632. Whole-transcriptome sequencing analysis demonstrated widespread gene expression changes in Y-27632-treated basal cells. We found that Y-27632 treatment altered expression of genes fundamental to the formation of the basal cell cytoskeleton, cell-cell junctions, and cell-extracellular matrix (ECM) interactions. Importantly, we found that Y-27632 treatment up-regulated expression of unique basal cell intermediate filament and desmosomal genes. Conversely, Y-27632 down-regulated multiple families of protease/antiprotease genes involved in ECM remodeling. We conclude that Y-27632 fundamentally alters cell-cell and cell-ECM interactions, which preserves basal progenitor cells and allows greater cell amplification.

Impact of Donor Arterial Partial Pressure of Oxygen on Outcomes After Lung Transplantation in Adult Cystic Fibrosis Recipients

INTRODUCTION

Donor PaO2 levels are used for assessing organs for lung transplantation (LTx), but survival implications of PaO2 levels in adult cystic fibrosis (CF) patients receiving LTx are unclear.

METHODS

UNOS registry data spanning 2005-2013 were used to test for associations of donor PaO2 with patient survival and bronchiolitis obliterans syndrome (BOS) in adult (age ≥ 18 years) first-time LTx recipients diagnosed with CF.

RESULTS

The analysis included 1587 patients, of whom 1420 had complete data for multivariable Cox models. No statistically significant differences among donor PaO2 categories of ≤200, 201-300, 301-400, or >400 mmHg were found in univariate survival analysis (log-rank test p = 0.290). BOS onset did not significantly differ across donor PaO2 categories (Chi-square p = 0.480). Multivariable Cox models of patient survival supported the lack of difference across donor PaO2 categories. Interaction analysis found a modest difference in survival between the two top categories of donor PaO2 when examining patients with body mass index (BMI) in the lowest decile (≤16.5 kg/m(2)).

CONCLUSIONS

Donor PaO2 was not associated with survival or BOS onset in adult CF patients undergoing LTx. Notwithstanding statistically significant interactions between donor PaO2 and BMI, there was no evidence of post-LTx survival risk associated with donor PaO2 below conventional thresholds in any subgroup of adults with CF.

Chibby promotes ciliary vesicle formation and basal body docking during airway cell differentiation

Airway multiciliated epithelial cells play crucial roles in the mucosal defense system, but their differentiation process remains poorly understood. Mice lacking the basal body component Chibby (Cby) exhibit impaired mucociliary transport caused by defective ciliogenesis, resulting in chronic airway infection. In this paper, using primary cultures of mouse tracheal epithelial cells, we show that Cby facilitates basal body docking to the apical cell membrane through proper formation of ciliary vesicles at the distal appendage during the early stages of ciliogenesis. Cby is recruited to the distal appendages of centrioles via physical interaction with the distal appendage protein CEP164. Cby then associates with the membrane trafficking machinery component Rabin8, a guanine nucleotide exchange factor for the small guanosine triphosphatase Rab8, to promote recruitment of Rab8 and efficient assembly of ciliary vesicles. Thus, our study identifies Cby as a key regulator of ciliary vesicle formation and basal body docking during the differentiation of airway ciliated cells.

Regulation of trachebronchial tissue-specific stem cell pool size

Tissue-specific stem cell (TSC) number is tightly regulated in normal individuals but can change following severe injury. We previously showed that tracheobronchial epithelial TSC number increased after severe naphthalene (NA) injury and then returned to normal. This study focused on the fate of the supernumerary TSC and the signals that regulate TSC pool size. We used the Keratin 5-rTA/Histone 2B:green fluorescent protein (GFP) model to purify basal cells that proliferated infrequently (GFP(bright) ) or frequently (GFP(dim) ) after NA injury. Both populations contained TSC but TSCs were 8.5-fold more abundant in the GFP(bright) population. Interestingly, both populations also contained a unipotential basal progenitor (UPB), a mitotic basal cell subtype whose daughters were terminally differentiated basal cells. The ratio of TSC to UPB was 5:1 in the GFP(bright) population and 1:5 in the GFP(dim) population. These data suggested that TSC proliferation in vivo promoted TSC-to-UPB differentiation. To evaluate this question, we cloned TSC from the GFP(bright) and GFP(dim) populations and passaged the clones seven times. We found that TSC number decreased and UPB number increased at each passage. Reciprocal changes in TSC and UPB frequency were more dramatic in the GFP(dim) lineage. Gene expression analysis showed that β-catenin and Notch pathway genes were differentially expressed in freshly isolated TSC derived from GFP(bright) and GFP(dim) populations. We conclude that (a) TSC and UPB are members of a single lineage; (b) TSC proliferation in vivo or in vitro promotes TSC-to-UPB differentiation; and (c) an interaction between the β-catenin and Notch pathways regulates the TSC-to-UPB differentiation process.

Decellularized tracheal extracellular matrix supports epithelial migration, differentiation, and function

Tracheal loss is a source of significant morbidity for affected patients with no acceptable solution. Interest in engineering tracheal transplants has created a demand for small animal models of orthotopic tracheal transplantation. Here, we examine the use of a decellularized graft in a murine model of tracheal replacement. Fresh or decellularized tracheas harvested from age-matched female donor C57BL/6 mice were transplanted into syngeneic recipients. Tracheas were decellularized using repeated washes of water, 3% Triton X-100, and 3 M NaCl under cyclic pressure changes, followed by disinfection with 0.1% peracetic acid/4% ethanol, and terminal sterilization by gamma irradiation. Tracheas were explanted for immunolabeling at 1, 4, and 8 weeks following surgery. Video microscopy and computed tomography were performed to assess function and structure. Decellularized grafts supported complete reepithelialization by 8 weeks and motile cilia were observed. Cartilaginous portions of the trachea were maintained in mice receiving fresh transplants, but repopulation of the cartilage was not seen in mice receiving decellularized transplants. We observed superior postsurgical survival, weight gain, and ciliary function in mice receiving fresh transplants compared with those receiving decellularized transplants. The murine orthotopic tracheal transplant provides an appropriate model to assess the repopulation and functional regeneration of decellularized tracheal grafts.

Epidermal growth factor receptor activity is necessary for mouse basal cell proliferation

ERB family receptors (EGFR, ERB-B2, ERB-B3, and ERB-B4) regulate epithelial cell function in many tissue types. In the human airway epithelium, changes in ERB receptor expression are associated with epithelial repair defects. However, the specific role(s) played by ERB receptors in repair have not been determined. We aimed to determine whether ERB receptors regulate proliferation of the tracheobronchial progenitor, the basal cell. Receptor tyrosine kinase arrays were used to evaluate ERB activity in normal and naphthalene (NA)-injured mouse trachea and in air-liquid interface cultures. Roles for epidermal growth factor (EGF), EGFR, and ERB-B2 in basal cell proliferation were evaluated in vitro. NA injury and transgenic expression of an EGFR-dominant negative (DN) receptor were used to evaluate roles for EGFR signaling in vivo. EGFR and ERB-B2 were active in normal and NA-injured trachea and were the only active ERB receptors detected in proliferating basal cells in vitro. EGF was necessary for basal cell proliferation in vitro. The EGFR inhibitor, AG1478, decreased proliferation by 99, and the Erb-B2 inhibitor, AG825, decreased proliferation by ∼66%. In vivo, EGFR-DN expression in basal cells significantly decreased basal cell proliferation after NA injury. EGF and EGFR are necessary for basal cell proliferation. The EGFR/EGFR homo- and the EGFR/ERB-B2 heterodimer account for ∼34 and 66%, respectively, of basal cell proliferation in vitro. Active EGFR is necessary for basal cell proliferation after NA injury. We conclude that EGFR activation is necessary for mouse basal cell proliferation and normal epithelial repair.

Human tracheobronchial basal cells. Normal versus remodeling/repairing phenotypes in vivo and in vitro

Human tracheobronchial epithelial (TBE) basal cells (BCs) function as progenitors in normal tissue. However, mechanistic studies are typically performed in vitro and frequently use BCs recovered from patients who die of nonrespiratory disease. It is not known whether the cadaveric epithelium (1) is undergoing homeostatic remodeling and/or repair, or (2) yields BC clones that represent homeostatic processes identified in tissue. We sought to compare the phenotype of TBE-BCs with that of BCs cultured under optimal clone-forming conditions. TBE pathology was evaluated using quantitative histomorphometry. The cultured BC phenotype was determined by fluorescence-activated cell sorter analysis. Clone organization and cell phenotype were determined by immunostaining. The cadaveric TBE is 20% normal. In these regions, BCs are keratin (K)-5(+) and tetraspanin CD151(+), and demonstrate a low mitotic index. In contrast, 80% of the cadaveric TBE exhibits homeostatic remodeling/repair processes. In these regions, BCs are K5(+)/K14(+), and a subset expresses tissue factor (TF). Passage 1 TBE cells are BCs that are K5(+)/TF(+), and half coexpress CD151. Optimal clone formation conditions use an irradiated NIH3T3 fibroblast feeder layer (American Type Culture Collection, Frederick, MD) and serum-supplemented Epicult-B medium (Stemcell Technologies, La Jolla, CA). The TF(+)/CD151(-) BC subpopulation is the most clonogenic BC subtype, and is enriched with K14(+) cells. TF(+)/CD151(-) BCs generate clones containing BCs that are K5(+)/Trp63(+), but K14(-)/CD151(-). TF(+) cells are limited to the clone edge. In conclusion, clonogenic human TBE BCs (1) exhibit a molecular phenotype that is a composite of the normal and remodeling/reparative BC phenotypes observed in tissue, and (2) generate organoid clones that contain phenotypically distinct BC subpopulations.

Chemosensory brush cells of the trachea. A stable population in a dynamic epithelium

Tracheal brush cells (BCs) are specialized epithelial chemosensors that use the canonical taste transduction cascade to detect irritants. To test whether BCs are replaced at the same rate as other cells in the surrounding epithelium of adult mice, we used 5-bromo-2'-deoxyuridine (BrdU) to label dividing cells. Although scattered BrdU-labeled epithelial cells are present 5-20 days after BrdU, no BCs are labeled. These data indicate that BCs comprise a relatively static population. To determine how BCs are generated during development, we injected 5-day-old mice with BrdU and found labeled BCs and non-BC epithelial cells 5 days after BrdU. During the next 60 days, the percentage of labeled BCs increased, whereas the percentage of other labeled cell types decreased. These data suggest that BCs are generated from non-BC progenitor cells during postnatal tracheal growth. To test whether the adult epithelium retains the capacity to generate BCs, tracheal epithelial cells were recovered from adult mice and grown in an air-liquid interface (ALI) culture. After transition to differentiation conditions, BCs are detected, and comprise 1% of the total cell population by Day 14. BrdU added to cultures before the differentiation of BCs was chased into BCs, indicating that the increase in BC density is attributable to the proliferation of a non-BC progenitor. We conclude that: (1) BCs are normally a static population in adult mice; (2) BC progenitors proliferate and differentiate during neonatal development; and (3) BCs can be regenerated from a proliferative population resident in adult epithelium.

Tissue factor signals airway epithelial basal cell survival via coagulation and protease-activated receptor isoforms 1 and 2

Tissue factor (TF) initiates the extrinsic coagulation cascade and is a high-affinity receptor for coagulation factor VII. TF also participates in protease-activated receptor (PAR)1 and PAR2 activation. Human epithelial basal cells were previously purified on the basis of TF expression. The purpose of this study was to determine if tracheobronchial epithelial basal cell-associated TF drives coagulation and/or activates PARs to promote basal cell functions. We used human tracheobronchial tissues to isolate human airway epithelial cells using specific cell surface markers by flow cytometry and studied TF expression by immunostaining. TF-dependent fibrin network formation was observed by confocal and scanning electron microscopy. TF knockdown was done using short hairpin RNA, and TF mRNA was measured using quantitative RT-PCR. We found that 97 ± 5% of first-passage human tracheobronchial epithelial cells were basal cells, and 100% of these basal cells expressed TF. Basal cell-associated TF was active, but TF activity was dependent on added extrinsic coagulation cascade factors. TF inhibition caused basal cell apoptosis and necrosis. This was due to two parallel but interdependent TF-regulated processes: failure to generate a basal cell-associated fibrin network and suboptimal PAR1 and PAR2 activity. The data indicate that membrane surface TF mediates airway epithelial basal cell attachment, which maintains cell survival and mitotic potential. The implications of these findings are discussed in the context of basal cell-associated TF activity in normal and injured tissues and of the potential for repair of airway epithelium in lung disease.

Wnt signaling in lung organogenesis

Reporter transgene, knockout, and misexpression studies support the notion that Wnt/beta-catenin signaling regulates aspects of branching morphogenesis, regional specialization of the epithelium and mesenchyme, and establishment of progenitor cell pools. As demonstrated for other foregut endoderm-derived organs, beta-catenin and the Wnt/beta-catenin signaling pathway contribute to control of cellular proliferation, differentiation and migration. However, the contribution of Wnt/beta-catenin signaling to these processes is shaped by other signals impinging on target tissues. In this review, we will concentrate on roles for Wnt/beta-catenin in respiratory system development, including segregation of the conducting airway and alveolar compartments, specialization of the mesenchyme, and establishment of tracheal asymmetries and tracheal glands.

Roles for β-catenin and doxycycline in the regulation of respiratory epithelial cell frequency and function

The expression of β-catenin-dependent genes can be increased through the Cre recombinase (Cre)-mediated elimination of the exon 3-encoded sequence. This mutant β-catenin is termed DE3, and promotes the expression of β-catenin-dependent genes. Our previous study used the DE3 model to demonstrate that persistent β-catenin activity inhibited bronchiolar Clara-to-ciliated cell differentiation. The present study was designed to evaluate the roles of β-catenin in regulating the tracheal progenitor cell hierarchy. However, initial experiments demonstrated that the tetracycline-responsive element-Cre transgene (TRE-Cre) was active in the absence of a reverse tetracycline transactivator driver or inducer, doxycycline (Dox). This spurious TRE-Cre transgene activity was not detected using the ROSA26-floxed STOP-LacZ reporter. To determine if the phenotype was a consequence of genotype or treatment with Dox, tracheal and lung specimens were evaluated using quantitative histomorphometric techniques. Analyses of uninduced mice demonstrated a significant effect of genotype on tracheal epithelial cell mass, involving basal, Clara-like cell types. The bronchial and bronchiolar Clara cell mass was also decreased. Paradoxically, an effect on ciliated cell mass was not detected. Activation of the β-catenin reporter transgene TOPGal demonstrated that β-catenin-dependent gene expression led to the genotype-dependent tracheal and bronchiolar phenotype. Comparative analyses of wild-type or keratin 14-rtTA(+/0)/TRE-cre(+/0)/DE3(+/+) mice receiving standard or Dox chow demonstrated an effect of treatment with Dox on basal, Clara-like, and Clara cell masses. We discuss these results in terms of cautionary notes and with regard to alterations of progenitor cell hierarchies in response to low-level injury.

Direct and indirect roles for β-catenin in facultative basal progenitor cell differentiation

The conducting airway epithelium is maintained and repaired by endogenous progenitor cells. Dysregulated progenitor cell proliferation and differentiation is thought to contribute to epithelial dysplasia in chronic lung disease. Thus modification of progenitor cell function is an attractive therapeutic goal and one that would be facilitated by knowledge of the molecular pathways that regulate their behavior. We modeled the human tracheobronchial epithelium using primary mouse tracheal epithelial cell cultures that were differentiated by exposure to the air-liquid-interface (ALI). A basal cell subset, termed facultative basal cell progenitors (FBP), initiate these cultures and are the progenitor for tracheal-specific secretory cells, the Clara-like cell, and ciliated cells. To test the hypothesis that β-catenin is necessary for FBP function, ALI cultures were generated from mice homozygous for the Ctnb(flox(E2-6)) allele. In this model, exons 2-6 of the β-catenin gene are flanked by LoxP sites, allowing conditional knockout of β-catenin. The β-catenin locus was modified through transduction with Adenovirus-5-encoding Cre recombinase. This approach generated a mosaic epithelium, comprised of β-catenin wild-type and β-catenin knockout cells. Dual immunostaining and quantitative histomorphometric analyses demonstrated that β-catenin played a direct role in FBP-to-ciliated cell differentiation and that it regulated cell-cell interactions that were necessary for FBP-to-Clara-like cell differentiation. β-catenin was also necessary for FBP proliferation and long-term FBP viability. We conclude that β-catenin is a critical determinant of FBP function and suggest that dysregulation of the β-catenin signaling pathway may contribute to disease pathology.

Antimicrobial activity of PLUNC protects against Pseudomonas aeruginosa infection

Epithelial antimicrobial activity may protect the lung against inhaled pathogens. The bactericidal/permeability-increasing protein family has demonstrated antimicrobial activity in vitro. PLUNC (palate, lung, and nasal epithelium associated) is a 25-kDa secreted protein that shares homology with bactericidal/permeability-increasing proteins and is expressed in nasopharyngeal and respiratory epithelium. The objective of this study was to determine whether PLUNC can limit Pseudomonas aeruginosa infection in mice. Transgenic mice (Scgb1a1-hPLUNC) were generated in which human PLUNC (hPLUNC) was directed to the airway epithelium with the Scgb1a1 promoter. The hPLUNC protein (hPLUNC) was detected in the epithelium throughout the trachea and bronchial airways and in bronchoalveolar lavage fluid. Bronchoalveolar lavage fluid from transgenic mice exhibited higher antibacterial activity than that from wild type littermates in vitro. After in vivo P. aeruginosa challenge, Scgb1a1-hPLUNC transgenic mice displayed enhanced bacterial clearance. This was accompanied by a decrease in neutrophil infiltration and cytokine levels. More importantly, the overexpressed hPLUNC in Scgb1a1-hPLUNC transgenic mouse airway significantly enhanced mouse survival against P. aeruginosa-induced respiratory infection. These data indicate that PLUNC is a novel antibacterial protein that likely plays a critical role in airway epithelium-mediated innate immune response.

β-catenin dosage is a critical determinant of tracheal basal cell fate determination

The purpose of this study was to determine whether β-catenin regulates basal cell fate determination in the mouse trachea. Analysis of TOPGal transgene reporter activity and Wnt/β-catenin pathway gene expression suggested a role for β-catenin in basal cell proliferation and differentiation after naphthalene-mediated Clara-like and ciliated cell depletion. However, these basal cell activities occurred simultaneously, limiting precise determination of the role(s) played by β-catenin. This issue was overcome by analysis of β-catenin signaling in tracheal air-liquid interface cultures. The cultures could be divided into two phases: basal cell proliferation and basal cell differentiation. A role for β-catenin in basal cell proliferation was indicated by activation of the TOPGal transgene on proliferation days 3 to 5 and by transient expression of Myc (alias c-myc). Another peak of TOPGal transgene activity was detected on differentiation days 2 to 10 and was associated with the expression of Axin 2. These results suggest a role for β-catenin in basal to ciliated and basal to Clara-like cell differentiation. Genetic stabilization of β-catenin in basal cells shortened the period of basal cell proliferation but had a minor effect on this process. Persistent β-catenin signaling regulated basal cell fate by driving the generation of ciliated cells and preventing the production of Clara-like cells.

MALDI imaging MS of phospholipids in the mouse lung

Lipid mediators are important in lung biochemistry and are derived from the enzymatic oxidation of arachidonic and docosahexaenoic acids, which are PUFAs that are present in phospholipids in cell membranes. In this study, MALDI imaging MS was used to determine the localization of arachidonate- and docosahexaenoate-containing phospholipids in mouse lung. These PUFA-containing phospholipids were determined to be uniquely abundant at the lining of small and large airways, which were unequivocally identified by immunohistochemistry. In addition, it was found that the blood vessels present in the lung were characterized by sphingomyelin molecular species, and lung surfactant phospholipids appeared evenly distributed throughout the lung parenchyma, indicating alveolar localization. This technique revealed unexpected high concentrations of arachidonate- and docosahexaenoate-containing phospholipids lining the airways in pulmonary tissue, which could serve as precursors of lipid mediators affecting airways biology.

Context-dependent differentiation of multipotential keratin 14-expressing tracheal basal cells

Multipotential (MP) differentiation is one characteristic of a tissue-specific stem cell (TSC). Lineage tracing of tracheobronchial basal cells after naphthalene (NA) injury or in the postnatal period demonstrated that basal cells were MP progenitors for Clara-like and ciliated cells. These studies, as well as reports of spatially restricted, label-retaining basal cells, and MP differentiation by human bronchial cells support the hypothesis that a TSC maintained and repaired the tracheobronchial epithelium. However, differences in basal cell phenotype (keratin [K] 5+ versus K14+), age (postnatal versus adult), health status (normal versus injured), and injury type (acid, detergent, NA) limited comparisons among studies and thus diminished the strength of the TSC argument. The finding that K14 was up-regulated after NA injury was a caveat to our previous analysis of reparative (r)K14-expressing cells (EC). Thus, the present study lineage traced steady-state (s)K14EC and evaluated differentiation potential in the normal and repairing epithelium. We showed that sK14EC were unipotential in the normal epithelium and MP after NA, sK14EC-dervied clones were not restricted to putative TSC niches, sK14EC cells were a direct progenitor for Clara-like and ciliated cells, MP-sK14EC clones accumulated over time, and sK14EC-derived Clara-like cells were progenitors for ciliated cells.

A single cell functions as a tissue-specific stem cell and the in vitro niche-forming cell

Tissue-specific stem cell (TSC) behavior is determined by the stem cell niche. However, delineation of the TSC-niche interaction requires purification of both entities. We reasoned that the niche could be defined by the location of the TSC. We demonstrate that a single CD49f(bright)/Sca1(+)/ALDH(+) basal cell generates rare label-retaining cells and abundant label-diluting cells. Label-retaining and label-diluting cells were located in the rimmed domain of a unique clone type, the rimmed clone. The TSC property of self-renewal was tested by serial passage at clonal density and analysis of clone-forming cell frequency. A single clone could be passaged up to five times and formed only rimmed clones. Thus, rimmed clone formation was a cell-intrinsic property. Differentiation potential was evaluated in air-liquid interface cultures. Homogenous cultures of rimmed clones were highly mitotic but were refractory to standard differentiation signals. However, rimmed clones that were cocultured with unfractionated tracheal cells generated each of the cell types found in the tracheal epithelium. Thus, the default niche is promitotic: Multipotential differentiation requires adaptation of the niche. Because lung TSCs are typically evaluated after injury, the behavior of CD49f(bright)/Sca1(+)/ALDH(+) cells was tested in normal and naphthalene-treated mice. These cells were mitotically active in the normal and repaired epithelium, their proliferation rate increased in response to injury, and they retained label for 34 days. We conclude that the CD49f(bright)/Sca1(+)/ALDH(+) tracheal basal cell is a TSC, that it generates its own niche in vitro, and that it participates in tracheal epithelial homeostasis and repair.

Tracheal Basal cells: a facultative progenitor cell pool

Analysis of lineage relationships in the naphthalene-injured tracheal epithelium demonstrated that two multipotential keratin 14-expressing cells (K14ECs) function as progenitors for Clara and ciliated cells. These K14EC were distinguished by their self-renewal capacity and were hypothesized to reside at the stem and transit amplifying tiers of a tissue-specific stem cell hierarchy. In this study, we used gene expression and histomorphometric analysis of the steady-state and naphthalene-injured trachea to evaluate the predictions of this model. We found that the steady-state tracheal epithelium is maintained by two progenitor cell pools, secretory and basal cells, and the latter progenitor pool is further divided into two subsets, keratin 14-negative and -positive. After naphthalene-mediated depletion of the secretory and ciliated cell types, the two basal cell pools coordinate to restore the epithelium. Both basal cell types up-regulate keratin 14 and generate a broadly distributed, abundant, and highly mitotic cell pool. Furthermore, basal cell proliferation is associated with generation of differentiated Clara and ciliated cells. The uniform distribution of basal cell progenitors and of their differentiated progeny leads us to propose that the hierarchical organization of tracheal reparative cells be revised to include a facultative basal cell progenitor pool.

Clara cell: progenitor for the bronchiolar epithelium

Clara cells were first described as a morphologically distinct cell type by Kolliker in 1881, but they take their name from the seminal study of human and rabbit bronchioles by Max Clara in 1937. Since their discovery, Clara cells have been identified as central players in protecting the airway from environmental exposures. The diverse functions of Clara cells in lung homeostasis include roles in xenobiotic metabolism, immune system regulation, and progenitor cell activity. Recent identification of a sub-population of Clara cells as a bronchiolar tissue-specific stem cell and a potential tumor initiating cell has focused the attention of cell and molecular biologists on the Clara cell and its behavior under normal and disease conditions.

Dysfunctional cystic fibrosis transmembrane conductance regulator inhibits phagocytosis of apoptotic cells with proinflammatory consequences

Cystic fibrosis (CF) is caused by mutated CF transmembrane conductance regulator (CFTR) and is characterized by robust airway inflammation and accumulation of apoptotic cells. Phagocytosis of apoptotic cells (efferocytosis) is a pivotal regulator of inflammation, because it prevents postapoptotic necrosis and actively suppresses release of a variety of proinflammatory mediators, including IL-8. Because CF is associated with accumulation of apoptotic cells, inappropriate levels of IL-8, and robust inflammation, we sought to determine whether CFTR deficiency specifically impairs efferocytosis and its regulation of inflammatory mediator release. Here we show that CFTR deficiency directly interferes with efferocytosis by airway epithelium, an effect that is not due to altered binding of apoptotic cells to epithelial cells or altered expression of efferocytosis receptors. In contrast, expression of RhoA, a known negative regulator of efferocytosis, is substantially increased in CFTR-deficient cells, and inhibitors of RhoA or its downstream effector Rho kinase normalize efferocytosis in these cells. Impaired efferocytosis appears to be mediated through an amiloride-sensitive ion channel, because amiloride restores phagocytic competency in CFTR-deficient cells. Finally, ineffective efferocytosis in CFTR-deficient cells appears to have proinflammatory consequences, because apoptotic cells enhance IL-8 release by these cells, but not by wild-type controls. Therefore, in CF, dysregulated efferocytosis may lead to accumulation of apoptotic cells and impaired regulation of the inflammatory response and, ultimately, may suggest a new therapeutic target.

Clara cells attenuate the inflammatory response through regulation of macrophage behavior

Chronic lung diseases are marked by excessive inflammation and epithelial remodeling. Reduced Clara cell secretory function and corresponding decreases in the abundance of the major Clara cell secretory protein (CCSP) are characteristically seen in these disease states. We sought to define the impact of Clara cell and CCSP depletion on regulation of the lung inflammatory response. We used chemical and genetic mouse models of Clara cell and CCSP deficiency (CCSP(-/-)) coupled with Pseudomonas aeruginosa LPS elicited inflammation. Exposure of Clara cell-depleted or CCSP(-/-) mice to LPS resulted in augmented inflammation as assessed by polymorphonuclear leukocyte recruitment to the airspace. Gene expression analysis and pathway modeling of the CCSP(-/-) inflammatory response implicated increased TNF-alpha signaling. Consistent with this model was the demonstration of significantly elevated TNF-alpha in airway fluid of LPS-stimulated CCSP(-/-) mice compared with similarly exposed wild-type mice. Increased LPS-elicited TNF-alpha production was also observed in cultured lung macrophages from CCSP(-/-) mice compared with wild-type mice. We demonstrate that macrophages from Clara cell-depleted and CCSP(-/-) mice displayed increased Toll-like receptor 4 surface expression. Our results provide evidence that Clara cells can attenuate inflammation through regulation of macrophage behavior, and suggest that epithelial remodeling leading to reduced Clara cell secretory function is an important factor that increases the intensity of lung inflammation in chronic lung disease.

beta-Catenin is not necessary for maintenance or repair of the bronchiolar epithelium

Signaling by Wnt/beta-catenin regulates self-renewal of tissue stem cells in the gut and, when activated in the embryonic bronchiolar epithelium, leads to stem cell expansion. We have used transgenic and cell type-specific knockout strategies to determine roles for beta-catenin-regulated gene expression in normal maintenance and repair of the bronchiolar epithelium. Analysis of TOPGal transgene activity detected beta-catenin signaling in the steady-state and repairing bronchiolar epithelium. However, the broad distribution and phenotype of signaling cells precluded establishment of a clear role for beta-catenin in the normal or repairing state. Necessity of beta-catenin signaling was tested through Cre-mediated deletion of Catnb exons 2-6 in airway epithelial cells. Functional knockout of beta-catenin had no impact on expression of Clara cell differentiation markers, mitotic index, or sensitivity of these cells to the Clara cell-specific toxicant, naphthalene. Repair of the naphthalene-injured airway proceeded with establishment of focal regions of beta-catenin-null epithelium. The size of regenerative epithelial units, mitotic index, and restoration of the ciliated cell population did not vary between wild-type and genetically modified mice. Thus, beta-catenin was not necessary for maintenance or efficient repair of the bronchiolar epithelium.