The kinetics and pattern of tracheal allograft re-epithelialization

Case Report: Human Tracheal Transplantation Undergoes Progressive Reepithelialization Over Time

OBJECTIVES

Tracheal transplantation is an ideal option for the reconstruction of long-segment circumferential tracheal defects. Our group performed the first successful vascularized single-staged tracheal transplantation in January 2021. Although a rigid biocompatible structure is necessary for a functioning tracheal replacement, the importance of ciliated epithelium, which allows for critical mucociliary clearance, is now being appreciated. Here, we examined the histological changes of the first single-staged human tracheal transplant from serial endoscopic biopsies.

METHODS

Biopsies of the tracheal mucosa were serially obtained since the time of the tracheal transplantation. Samples were examined via hematoxylin and eosin, electron microscopy, and immunohistochemistry.

RESULTS

One week after transplantation, there is loss of ciliated epithelium and seromucinous cells, with only a basal layer of epithelium remaining. By 2 weeks, however, the epithelium begins to recover, albeit differently depending on the location of the biopsy. Near the site of tracheal anastomosis, there is epithelial proliferation, with the appearance of early ciliated cells. However, in the midgraft, there appears to be evidence of squamous metaplasia. Over time, however, normal ciliated epithelium and mucous cells appear without signs of chronic inflammation.

CONCLUSIONS

Critically, the tracheal allograft regained normal appearing respiratory epithelium after initial ischemic injury. The histologic differences at the midgraft versus anastomosis may suggest unique mechanisms of reepithelialization. At the recipient-donor interface, there may be a faster direct migration of recipient-derived epithelial cells, in line with preclinical studies. The midgraft, in contrast, responds with epithelial proliferation from the donor basal cells or dedifferentiated mucous cells.

LEVEL OF EVIDENCE

N/A Laryngoscope, 134:2664-2671, 2024.

Human Tracheal Transplantation

Long-segment tracheal airway defects may be congenital or result from burns, trauma, iatrogenic intubation damage, or tumor invasion. Although airway defects <6 cm in length may be reconstructed using existing end-to-end reconstructive techniques, defects >6 cm continue to challenge surgeons worldwide. The reconstruction of long-segment tracheal defects has long been a reconstructive dilemma, and these defects are associated with significant morbidity and mortality. Many of these defects are not compatible with life or require a permanent extended-length tracheostomy that is fraught with complications including mucus plugging and tracheoesophageal fistula. Extensive circumferential tracheal defects require a reconstructive technique that provides a rigid structure able to withstand the inspiratory pressures, a structure that will biologically integrate, and contain functional ciliated epithelium to allow for normal mucociliary clearance. Tracheal transplantation has been considered the reconstructive "Holy Grail;" however, there has been a long-held scientific dogma that revascularization of the trachea was not possible. This dogma stifled research to achieve single-staged vascularized tracheal transplantation and prompted the introduction of many creative and inventive alternatives. Throughout history, alloplastic material, nonvascularized allografts, and homografts have been used to address this dilemma. However, these techniques have largely been unsuccessful. The recent introduction of a technique for single-staged vascularized tracheal transplantation may offer a solution to this dilemma and potentially a solution to management of the fatal tracheoesophageal fistula.

Pre-epithelialized cryopreserved tracheal allograft for neo-trachea flap engineering

Background: Tracheal reconstruction presents a challenge because of the difficulty in maintaining the rigidity of the trachea to ensure an open lumen and in achieving an intact luminal lining that secretes mucus to protect against infection. Methods: On the basis of the finding that tracheal cartilage has immune privilege, researchers recently started subjecting tracheal allografts to "partial decellularization" (in which only the epithelium and its antigenicity are removed), rather than complete decellularization, to maintain the tracheal cartilage as an ideal scaffold for tracheal tissue engineering and reconstruction. In the present study, we combined a bioengineering approach and a cryopreservation technique to fabricate a neo-trachea using pre-epithelialized cryopreserved tracheal allograft (ReCTA). Results: Our findings in rat heterotopic and orthotopic implantation models confirmed that tracheal cartilage has sufficient mechanical properties to bear neck movement and compression; indicated that pre-epithelialization with respiratory epithelial cells can prevent fibrosis obliteration and maintain lumen/airway patency; and showed that a pedicled adipose tissue flap can be easily integrated with a tracheal construct to achieve neovascularization. Conclusion: ReCTA can be pre-epithelialized and pre-vascularized using a 2-stage bioengineering approach and thus provides a promising strategy for tracheal tissue engineering.

Vascularized Tracheal Transplantation: A Twenty Month Follow Up

BACKGROUND

Tracheal transplantation has been considered the ideal option for the reconstruction of long-segment circumferential tracheal defects. Our group developed a technique for vascularized single-staged tracheal transplantation. Twenty months ago, we performed the first-in-human tracheal transplantation. Herein, we report a twenty-month follow-up.

METHODS

The recipient presented with a 9.0 cm airway tracheal stenosis and complete cricoid stenosis. The patient previously failed six major conventional surgical procedures. Prior to transplantation, the patient's airway was maintained with an extended tracheostomy and stent. The patient experienced repeated life-threatening airway obstruction because of mucous plugging and obstructive granulation tissue. In January 2020 the patient underwent a single-staged tracheal transplantation treated with triple-therapy immunosuppression. Organ rejection was monitored with endoscopic tracheoscopy, narrow-band imaging, free-cell DNA assessment, and histological and cytogenetic analysis of tracheal biopsies. Mucociliary function was assessed with dye motility studies.

RESULTS

Twenty months following transplantation, there has been no evidence of acute or chronic rejection. Since day 60, there has been no detectable free cell DNA, a surrogate marker for immune-mediated allograft rejection. Fluorescence in situ hybridization (FISH) cytogenetics demonstrated that the donor trachea was repopulated with recipient epithelium establishing a chimeric allograft. Narrow-band imaging demonstrates a well-vascularized epithelial mucosa and bronchoscopic biopsies demonstrate normal ciliated epithelial architecture and viable epithelial lining with functional ciliated epithelium. The patient has resumed a normal life without a stent or tracheostomy and has returned to work.

CONCLUSIONS

Twenty months after single-staged vascularized tracheal transplantation, the trachea is functional and the patient breathes without the need for a tracheostomy or stent. Single-staged long-segment tracheal transplantation is a viable option for tracheal defects that are not amenable to conventional reconstructive techniques.

LEVEL OF EVIDENCE

4 Laryngoscope, 2022.

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.

A Novel Bioreactor for Reconstitution of the Epithelium and Submucosal Glands in Decellularized Ferret Tracheas

Tracheal grafts introduce the possibility to treat airway pathologies that require resection. While there has been success with engraftment of the surface airway epithelium (SAE) onto decellularized tracheas, there has been minimal advancement in regenerating the submucosal glands (SMGs). We designed a cost-effective open-system perfusion bioreactor to investigate the engraftment potential of ferret SAEs and murine myoepithelial cells (MECs) on a partly decellularized ferret trachea with the goal of creating a fully functional tracheal replacement. An air-liquid interface was also arranged by perfusing humidified air through the lumen of a recellularized conduit to induce differentiation. Our versatile bioreactor design was shown to support the successful partial decellularization and recellularization of ferret tracheas. The decellularized grafts maintained biomechanical integrity and chondrocyte viability, consistent with other publications. The scaffolds supported SAE basal cell engraftment, and early differentiation was observed once an air-liquid interface had been established. Lastly, MEC engraftment was sustained, with evidence of diffuse SMG reconstitution. This model will help shed light on SMG regeneration and basal cell differentiation in vitro for the development of fully functional tracheal grafts before transplantation.

Single-stage long-segment tracheal transplantation

Tracheal transplantation has been envisioned as a viable option for reconstruction of long-segment tracheal defects. We report the first human single-stage long-segment tracheal transplantation. Narrow-band imaging and bronchoscopic biopsies demonstrate allograft vascularization and viable epithelial lining. The recipient was immunosuppressed with Tacrolimus, Mycophenolate mofetil, and corticosteroids. Six months after transplantation, the trachea is both functional and the patient is breathing without the need of a tracheostomy or stent.

Partial Decellularization for Segmental Tracheal Scaffold Tissue Engineering: A Preliminary Study in Rabbits

In this study, we developed a new procedure for the rapid partial decellularization of the harvested trachea. Partial decellularization was performed using a combination of detergent and sonication to completely remove the epithelial layers outside of the cartilage ring. The post-decellularized tracheal segments were assessed with vital staining, which showed that the core cartilage cells remarkably remained intact while the cells outside of the cartilage were no longer viable. The ability of the decellularized tracheal segments to evade immune rejection was evaluated through heterotopic implantation of the segments into the chest muscle of rabbits without any immunosuppressive therapy, which demonstrated no evidence of severe rejection or tissue necrosis under H&E staining, as well as the mechanical stability under stress-pressure testing. Finally, orthotopic transplantation of partially decellularized trachea with no immunosuppression treatment resulted in 2 months of survival in two rabbits and one long-term survival (2 years) in one rabbit. Through evaluations of posttransplantation histology and endoscopy, we confirmed that our partial decellularization method could be a potential method of producing low-immunogenic cartilage scaffolds with viable, functional core cartilage cells that can achieve long-term survival after in vivo transplantation.

State of the art of clinical applications of Tissue Engineering in 2021

Tissue engineering (TE) was introduced almost 30 years ago as a potential technique for regenerating human tissues. However, despite promising laboratory findings, the complexity of the human body, scientific hurdles, and lack of persistent long-term funding still hamper its translation towards clinical applications. In this report, we compile an inventory of clinically applied TE medical products relevant to surgery. A review of the literature, including articles published within the period from 1991 to 2020, was performed according to the PRISMA protocol, using databanks PubMed, Cochrane Library, Web of Science, and Clinicaltrials.gov. We identified 1039 full-length articles as eligible; due to the scarcity of clinical, randomised, controlled trials and case studies, we extended our search towards a broad surgical spectrum. Forty papers involved clinical TE studies. Amongst these, 7 were related to TE protocols for cartilage applied in the reconstruction of nose, ear, and trachea. Nine papers reported TE protocols for articular cartilage, 9 for urological purposes, 7 described TE strategies for cardiovascular aims, and 8 for dermal applications. However, only two clinical studies reported on three-dimensional (3D) and functional long-lasting TE constructs. The concept of generating 3D TE constructs and organs based on autologous molecules and cells is intriguing and promising. The first translational tissue-engineered products and techniques have been clinically implemented. However, despite the 30 years of research and development in this field, TE is still in its clinical infancy. Multiple experimental, ethical, budgetary, and regulatory difficulties hinder its rapid translation. Nevertheless, the first clinical applications show great promise and indicate that the translation towards clinical medical implementation has finally started.

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.

Evaluation of changes in cartilage viability in detergent-treated tracheal grafts for immunosuppressant-free allotransplantation in dogs

OBJECTIVES

The first tissue-engineered clinical tracheal transplant prepared using the detergent-enzymatic method resulted in graft stenosis, possibly from detergent-enzymatic method-induced graft non-viability. We reported on the transplantation of de-epithelialized tracheal allografts while maintaining cartilage viability in dogs. No lethal stenosis occurred in allografts. Herein, on the basis of previous experimentation, we assessed cartilage viability in detergent-treated cartilages.

METHODS

Six canine tracheal grafts were treated with detergent [1% t-octylphenoxypolyethoxyethanol (Triton X-100)] before transplantation. The histoarchitecture was evaluated, and the viable chondrocytes ratio was calculated. Glycosaminoglycan was detected using safranin-O staining. Collagen II was tested using immunohistochemistry.

RESULTS

The epithelium was completely removed in 5 grafts. Compared with fresh tracheas, the viable chondrocyte ratio was significantly reduced in the de-epithelialized grafts (100 vs 54.70 ± 8.56%; P < 0.001). Image analysis revealed that the mean optical density of glycosaminoglycan (0.363 ± 0.027 vs 0.307 ± 0.012; P = 0.007) and collagen II (0.115 ± 0.013 vs 0.092 ± 0.011; P = 0.028) was decreased. The observation period ranged from 91 to 792 days. No stenosis occurred in 5 allografts; moderate stenosis developed in 1 allograft during the 4th week after surgery. The chondrocyte nuclei almost completely disappeared. Both glycosaminoglycan (0.307 ± 0.012 vs 0.164 ± 0.104; P = 0.044) and collagen II (0.092 ± 0.011 vs 0.068 ± 0.022; P = 0.022) were significantly degraded.

CONCLUSIONS

This study demonstrated successful tracheal transplantation; about 50% of the viable chondrocytes were retained in the cartilage that could prevent development of a lethal stenosis in tracheal grafts.

Engineered Tissue-Stent Biocomposites as Tracheal Replacements

Here we report the creation of a novel tracheal construct in the form of an engineered, acellular tissue-stent biocomposite trachea (TSBT). Allogeneic or xenogeneic smooth muscle cells are cultured on polyglycolic acid polymer-metal stent scaffold leading to the formation of a tissue comprising cells, their deposited collagenous matrix, and the stent material. Thorough decellularization then produces a final acellular tubular construct. Engineered TSBTs were tested as end-to-end tracheal replacements in 11 rats and 3 nonhuman primates. Over a period of 8 weeks, no instances of airway perforation, infection, stent migration, or erosion were observed. Histological analyses reveal that the patent implants remodel adaptively with native host cells, including formation of connective tissue in the tracheal wall and formation of a confluent, columnar epithelium in the graft lumen, although some instances of airway stenosis were observed. Overall, TSBTs resisted collapse and compression that often limit the function of other decellularized tracheal replacements, and additionally do not require any cells from the intended recipient. Such engineered TSBTs represent a model for future efforts in tracheal regeneration.

Tissue-Engineered Tracheal Replacement in a Child: A 4-Year Follow-Up Study

In 2010, a tissue-engineered trachea was transplanted into a 10-year-old child using a decellularized deceased donor trachea repopulated with the recipient's respiratory epithelium and mesenchymal stromal cells. We report the child's clinical progress, tracheal epithelialization and costs over the 4 years. A chronology of events was derived from clinical notes and costs determined using reference costs per procedure. Serial tracheoscopy images, lung function tests and anti-HLA blood samples were compared. Epithelial morphology and T cell, Ki67 and cleaved caspase 3 activity were examined. Computational fluid dynamic simulations determined flow, velocity and airway pressure drops. After the first year following transplantation, the number of interventions fell and the child is currently clinically well and continues in education. Endoscopy demonstrated a complete mucosal lining at 15 months, despite retention of a stent. Histocytology indicates a differentiated respiratory layer and no abnormal immune activity. Computational fluid dynamic analysis demonstrated increased velocity and pressure drops around a distal tracheal narrowing. Cross-sectional area analysis showed restriction of growth within an area of in-stent stenosis. This report demonstrates the long-term viability of a decellularized tissue-engineered trachea within a child. Further research is needed to develop bioengineered pediatric tracheal replacements with lower morbidity, better biomechanics and lower costs.

Tissue-Engineered Tracheal Replacement in a Child: A 4-Year Follow-Up Study

In 2010, a tissue-engineered trachea was transplanted into a 10-year-old child using a decellularized deceased donor trachea repopulated with the recipient's respiratory epithelium and mesenchymal stromal cells. We report the child's clinical progress, tracheal epithelialization and costs over the 4 years. A chronology of events was derived from clinical notes and costs determined using reference costs per procedure. Serial tracheoscopy images, lung function tests and anti-HLA blood samples were compared. Epithelial morphology and T cell, Ki67 and cleaved caspase 3 activity were examined. Computational fluid dynamic simulations determined flow, velocity and airway pressure drops. After the first year following transplantation, the number of interventions fell and the child is currently clinically well and continues in education. Endoscopy demonstrated a complete mucosal lining at 15 months, despite retention of a stent. Histocytology indicates a differentiated respiratory layer and no abnormal immune activity. Computational fluid dynamic analysis demonstrated increased velocity and pressure drops around a distal tracheal narrowing. Cross-sectional area analysis showed restriction of growth within an area of in-stent stenosis. This report demonstrates the long-term viability of a decellularized tissue-engineered trachea within a child. Further research is needed to develop bioengineered pediatric tracheal replacements with lower morbidity, better biomechanics and lower costs.

Patient-specific carbon nanocomposite tracheal prosthesis

PURPOSE

Surgical removal of the trachea is the current gold standard for treating severe airway carcinoma and stenosis. Resection of 6 cm or more of the trachea requires a replacement graft due to anastomotic tension. The high failure rates of current grafts are attributed to a mismatching of mechanical properties and slow epithelium formation on the inner lumen surface. There is also a current lack of tracheal prostheses that are closely tailored to the patient's anatomy.

METHODS

We propose the development of a patient-specific, artificial trachea made of carbon nanotubes and poly-di-methyl-siloxane (CNT-PDMS) composite material. Computational simulations and finite element analysis were used to study the stress behavior of the designed implant in a patient-specific, tracheal model.

RESULTS

Finite element studies indicated that the patient-specific carbon nanocomposite prosthesis produced stress distributions that are closer to that of the natural trachea. In vitro studies conducted on the proposed material have demonstrated its biocompatibility and suitability for sustaining tracheal epithelial cell proliferation and differentiation. In vivo studies done in porcine models showed no adverse side effects or breathing difficulties, with complete regeneration of the epithelium in the prosthesis lumen within 2 weeks.

CONCLUSIONS

This paper highlights the potential of a patient-specific CNT-PDMS graft as a viable airway replacement in severe tracheal carcinoma.

The need for a new animal model for chronic rejection after lung transplantation

The single most important cause of late mortality after lung transplantation is obliterative bronchiolitis (OB), clinically characterized by a decrease in lung function and morphologically by characteristic changes. Recently, new insights into its pathogenesis have been acquired: risk factors have been identified and the use of azithromycin showed a dichotomy with at least 2 different phenotypes of bronchiolitis obliterans syndrome (BOS). It is clear that a good animal model is indispensable to further dissect and unravel the pathogenesis of BOS. Many animal models have been developed to study BOS but, so far, none of these models truly mimics the human situation. Looking at the definition of BOS, a good animal model implies histological OB lesions, possibility to measure lung function, and airway inflammation. This review sought to discuss, including pros and cons, all potential animal models that have been developed to study OB/BOS. It has become clear that a new animal model is needed; recent developments using an orthotopic mouse lung transplantation model may offer the answer because it mimics the human situation. The genetic variants among this species may open new perspectives for research into the pathogenesis of OB/BOS.

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.

Repair and regeneration of tracheal surface epithelium and submucosal glands in a mouse model of hypoxic-ischemic injury

BACKGROUND AND OBJECTIVE

The heterotopic syngeneic tracheal transplant mouse model is an acute hypoxic-ischemic injury model that undergoes complete repair and regeneration. We hypothesized that the repair and regeneration process of the surface epithelium and submucosal glands would occur in a reproducible pattern that could be followed by the expression of specific markers of epithelial cell types.

METHODS

We used the syngeneic heterotopic tracheal transplant model to develop a temporal and spatial map of cellular repair and regeneration by examining the tracheal grafts at post-transplant days 1, 3, 5, 7, 10 and 14. We used pulsed BrdU and immunofluorescent staining to identify and follow proliferating and repairing cell populations.

RESULTS

We confirmed the reproducibility of the injury and repair in the model and we found a distinct sequence of reappearance of the various stem/progenitor and differentiated cell populations of the tracheal surface epithelium and submucosal glands. In the initial phase, the basal and duct cells that survived the injury proliferated to re-epithelialize the basement membrane with K5 and K14 expressing cells. Then these cells proliferated further and differentiated to restore the function of the epithelium. During this repair process, TROP-2 marked all repairing submucosal gland tubules and ducts. Non-CCSP-expressing serous cells were found to differentiate 4-5 days before Clara, mucus and ciliated cells.

CONCLUSIONS

Improving our understanding of the reparative process of the airway epithelium will allow us to identify cell-specific mechanisms of repair that could be used as novel therapeutic approaches for abnormal repair leading to airway diseases.

Successful circumferential free tracheal transplantation in a large animal model

BACKGROUND

Long segment tracheobronchial stenoses are associated with high morbi-mortality rates and difficult treatment. Transplantation hasn't proved to be useful yet. Currently, the successful results achieved in small animal models couldn't be satisfactorily accomplished or extrapolated in large mammals. We aimed to evaluate the viability of orthotopic tracheal autoimplantation in an ovine model.

METHODS

All animals underwent tracheal transplantation of 4 cm (5-7 rings) of the cervical trachea and were divided randomly in two groups: isolated autoimplantation (Group A/6) and autoimplantation with omental wrapping (Group B/6). Clinical follow up and weekly bronchoscopical examinations were performed. The grafts were macroscopically, histologically, and bacteriologically analyzed.

RESULTS

In group A, four animals achieved their planed survival and were sacrificed up to 60 days after transplantation with viable grafts. In group B, only two sheep had successful results. Graft failure with infection, necrosis and severe stenosis was observed in the rest of the animals from both groups. Pseudomonas aeruginose was isolated in all cases. The main complication of the omental pedicle was vascular congestion and peritracheal hemorrhage.

CONCLUSIONS

Contrary to the data reported to date, we found that tracheal transplantation is viable in a large mammal like the sheep. The main complication observed in this animal model was graft infection. The use of an omental pedicle with the technique applied worsened the grafts survival. The encouraging results obtained in this investigation justify further research in order to manage graft infection, leading us to establish a suitable large animal model for allotransplantation.

Novel stem/progenitor cell population from murine tracheal submucosal gland ducts with multipotent regenerative potential

The airway epithelium is in direct contact with the environment and therefore constantly at risk for injury. Basal cells (BCs) have been found to repair the surface epithelium (SE), but the contribution of other stem cell populations to airway epithelial repair has not been identified. We demonstrated that airway submucosal gland (SMG) duct cells, in addition to BCs, survived severe hypoxic-ischemic injury. We developed a method to isolate duct cells from the airway. In vitro and in vivo models were used to compare the self-renewal and differentiation potential of duct cells and BCs. We found that only duct cells were capable of regenerating SMG tubules and ducts, as well as the SE overlying the SMGs. SMG duct cells are therefore a multipotent stem cell for airway epithelial repair This is of importance to the field of lung regeneration as determining the repairing cell populations could lead to the identification of novel therapeutic targets and cell-based therapies for patients with airway diseases.

Presence of a putative tumor-initiating progenitor cell population predicts poor prognosis in smokers with non-small cell lung cancer

Smoking is the most important known risk factor for the development of lung cancer. Tobacco exposure results in chronic inflammation, tissue injury, and repair. A recent hypothesis argues for a stem/progenitor cell involved in airway epithelial repair that may be a tumor-initiating cell in lung cancer and which may be associated with recurrence and metastasis. We used immunostaining, quantitative real-time PCR, Western blots, and lung cancer tissue microarrays to identify subpopulations of airway epithelial stem/progenitor cells under steady-state conditions, normal repair, aberrant repair with premalignant lesions and lung cancer, and their correlation with injury and prognosis. We identified a population of keratin 14 (K14)-expressing progenitor epithelial cells that was involved in repair after injury. Dysregulated repair resulted in the persistence of K14+ cells in the airway epithelium in potentially premalignant lesions. The presence of K14+ progenitor airway epithelial cells in NSCLC predicted a poor prognosis, and this predictive value was strongest in smokers, in which it also correlated with metastasis. This suggests that reparative K14+ progenitor cells may be tumor-initiating cells in this subgroup of smokers with NSCLC.

Suppression of the obliteration process by ventilation in a mouse orthotopic tracheal transplantation model

BACKGROUND

Obliterative airway disease (OAD) has been a major obstacle to long-term survival after lung or tracheal transplantations, but the role of airflow has not been examined in the orthotopic or the heterotopic tracheal transplantation models.

METHODS

Sixty mice were assigned to two experimental groups. Two C57BL/6 tracheal segments were surgically prepared and then orthotopically transplanted into allogeneic BALB/c recipients. In group A mice, both segments were left patent, whereas in group B mice, one of the donor tracheas was occluded with a silk knot to obstruct airflow. Histology, quantitative OAD measurements, electron microscopy, immunohistochemical staining, and apoptosis measurement of the epithelium were performed.

RESULTS

Gross examination at harvest showed patent lumens of all tracheal segments. Group A allografts (ventilating tracheas) showed a markedly higher proportion of ciliated epitheliums and less lymphocyte infiltration in the lamina propria, whereas the epithelium appeared metaplastic in group B, with a higher proportion of flattened attenuated epithelium and loss of the normal ciliate architecture. Quantitative morphometric measurements suggested more prominent OAD manifestations in the nonventilating allografts of group B than were present in group A, although recipient-derived epithelium was observed in all allografts under immunohistochemical staining. The apoptotic indexes of the epithelium were 12.1% in allografts with adequate ventilation (group A) and 66.2% in ventilation-occluded allotracheas (group B).

CONCLUSIONS

OAD severity and the epithelial repopulation process are closely related to the physiologic environment of airflow. Further research is warranted to explore the underlying mechanisms of this phenomenon.

Tissue inhibitor of metalloproteinase-1 moderates airway re-epithelialization by regulating matrilysin activity

Obliterative bronchiolitis (OB) is the histopathological finding in chronic lung allograft rejection. Mounting evidence suggests that epithelial damage drives the development of airway fibrosis in OB. Tissue inhibitor of metalloproteinase (TIMP)-1 expression increases in lung allografts and is associated with the onset of allograft rejection. Furthermore, in a mouse model of OB, airway obliteration is reduced in TIMP-1-deficient mice. Matrilysin (matrix metallproteinase-7) is essential for airway epithelial repair and is required for the re-epithelialization of airway wounds by facilitating cell migration; therefore, the goal of this study was to determine whether TIMP-1 inhibits re-epithelialization through matrilysin. We found that TIMP-1 and matrilysin co-localized in the epithelium of human lungs with OB and both co-localized and co-immunoprecipitated in wounded primary airway epithelial cultures. TIMP-1-deficient cultures migrated faster, and epithelial cells spread to a greater extent compared with wild-type cultures. TIMP-1 also inhibited matrilysin-mediated cell migration and spreading in vitro. In vivo, TIMP-1 deficiency enhanced airway re-epithelialization after naphthalene injury. Furthermore, TIMP-1 and matrilysin co-localized in airway epithelial cells adjacent to the wound edge. Our data demonstrate that TIMP-1 interacts with matrix metalloproteinases and regulates matrilysin activity during airway epithelial repair. Furthermore, we speculate that TIMP-1 overexpression restricts airway re-epithelialization by inhibiting matrilysin activity, contributing to a stereotypic injury response that promotes airway fibrosis via bronchiole airway epithelial damage and obliteration.

Animal models for bronchiolitis obliterans syndrome following human lung transplantation

Lung transplantation is the only viable treatment option that can improve survival and enhance the quality of life of patients with end-stage lung diseases such as emphysema, cystic fibrosis, idiopathic pulmonary fibrosis, and primary pulmonary hypertension. However, the long-term survival of lung allografts is still limited by the development of bronchiolitis obliterans syndrome (BOS), an irreversible condition unresponsive to therapy. BOS is the most significant cause of long-term morbidity and mortality after lung transplantation. Over the past decade, several animal models have been developed to investigate BOS. These are valuable to elucidate the immunologic and pathologic mechanisms that lead to BOS and to test treatment options for BOS. In this review, we discuss the advantages and disadvantages of different animal models and highlight work that has been done with each model.

Role of airway epithelial injury in murine orthotopic tracheal allograft rejection

BACKGROUND

Murine tracheal transplantation is a model used to study bronchiolitis obliterans syndrome, a major cause of morbidity and mortality after lung transplantation. Unlike murine heterotopic tracheal transplants, orthotopic transplantation does not cause luminal obliteration despite major histocompatibility antigen mismatch. Repopulation of the tracheal allografts with recipient-derived epithelium confers protection against luminal obliteration. The purpose of this study was to determine whether (1) orthotopic tracheal transplantation showed signs of allograft rejection, and (2) airway epithelial cell injury promoted orthotopic tracheal allograft rejection.

METHODS

Forty isogeneic (C57BL/6 to C57BL/6) and 40 allogeneic (BALB/c to C57BL/6) orthotopic tracheal transplants were performed. Damage to airway epithelial cells was induced by Sendai viral (SdV) infection and tracheal transplantation into non-reepithelializing matrix metalloproteinase-7 knockout (MMP7-KO) recipient mice. Percent fibrosis and lamina propria to cartilage ratio were calculated with computer assistance on harvested allografts.

RESULTS

Allografts showed significantly more intramural fibrosis compared with isografts at 30, 60, and 180 days after transplant without luminal occlusion. Tracheal allografts infected with SdV showed an increase in fibrosis and lamina propria to cartilage ratio compared with noninfected controls. Allografts retrieved from MMP7-KO recipients also showed a significant increase in fibrosis and lamina propria to cartilage ratio.

CONCLUSIONS

Although orthotopic tracheal transplantation does not cause luminal obliteration, it results in increased fibrosis in allografts. Damage to the respiratory epithelium by viral infection or defective reepithelialization after transplant as seen in MMP7-KO recipient mice leads to changes consistent with chronic allograft rejection, suggesting a role for epithelial injury in bronchiolitis obliterans syndrome development.

Laryngeal transplantation in 2005: a review

There is no good surgical, medical or prosthetic solution to the problems faced by those with a larynx whose function is irreversibly damaged by tumor or trauma. Over the past 10 years, the pace of research designed to establish laryngeal transplantation as a therapeutic option for these persons has increased steadily. The biggest milestone in this field was the world's first true laryngeal transplant performed in Cleveland, Ohio in 1998. The recipient's graft continues to function well, in many respects, even after 7 years. However, it has also highlighted the remaining barriers to full-scale clinical trials. Stimulated by these observations, several groups have accumulated data which point to answers to some of the outstanding questions surrounding functional reinnervation and immunomodulation. This review seeks to outline the progress achieved in this field by 2005 and to point the way forward for laryngeal transplantation research in the 21st century.

Tissue inhibitor of metalloproteinase-1 deficiency abrogates obliterative airway disease after heterotopic tracheal transplantation

Obliterative bronchiolitis (OB) is a major cause of allograft dysfunction after lung transplantation and is thought to result from immunologically mediated airway epithelial destruction and luminal fibrosis. Matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs) have been implicated in the regulation of lung inflammation, airway epithelial repair, and extracellular matrix remodeling and therefore may participate in the pathogenesis of OB. The goals of this study were to determine the expression profiles of MMPs and TIMPs and the role of TIMP-1 in the development of airway obliteration using the murine heterotopic tracheal transplant model of OB. We demonstrate the selective induction of MMP-3, MMP-9, MMP-12, and TIMP-1 in a temporally restricted manner in tracheal allografts compared with isografts. In contrast, the expression of MMP-7, TIMP-2, and TIMP-3 was decreased in allografts relative to isografts during the period of graft rejection. TIMP-1 protein localized to epithelial, mesenchymal, and inflammatory cells in the tracheal grafts in a temporally and spatially restricted manner. Using TIMP-1-deficient mice, we demonstrate that the absence of TIMP-1 in the donor trachea or the allograft recipient reduced luminal obliteration and increased re-epithelialization in the allograft compared with wild-type control at 28 d after transplantation. Our findings provide direct evidence that TIMP-1 contributes to the development of airway fibrosis in the heterotopic tracheal transplant model, and suggest a potential role for this proteinase inhibitor in the pathogenesis of OB in patients with lung transplant.

Role of CXCR2/CXCR2 ligands in vascular remodeling during bronchiolitis obliterans syndrome

Angiogenesis and vascular remodeling support fibroproliferative processes; however, no study has addressed the importance of angiogenesis during fibro-obliteration of the allograft airway during bronchiolitis obliterans syndrome (BOS) that occurs after lung transplantation. The ELR(+) CXC chemokines both mediate neutrophil recruitment and promote angiogenesis. Their shared endothelial cell receptor is the G-coupled protein receptor CXC chemokine receptor 2 (CXCR2). We found that elevated levels of multiple ELR(+) CXC chemokines correlated with the presence of BOS. Proof-of-concept studies using a murine model of BOS not only demonstrated an early neutrophil infiltration but also marked vascular remodeling in the tracheal allografts. In addition, tracheal allograft ELR(+) CXC chemokines were persistently expressed even in the absence of significant neutrophil infiltration and were temporally associated with vascular remodeling during fibro-obliteration of the tracheal allograft. Furthermore, in neutralizing studies, treatment with anti-CXCR2 Abs inhibited early neutrophil infiltration and later vascular remodeling, which resulted in the attenuation of murine BOS. A more profound attenuation of fibro-obliteration was seen when CXCR2(-/-) mice received cyclosporin A. This supports the notion that the CXCR2/CXCR2 ligand biological axis has a bimodal function during the course of BOS: early, it is important for neutrophil recruitment and later, during fibro-obliteration, it is important for vascular remodeling independent of neutrophil recruitment.

Community acquired respiratory viral infections after lung transplantation: clinical features and long-term consequences

Community acquired respiratory viruses (CARVs) are increasingly recognized as serious threats to lung transplant recipients. While CARVs such as respiratory syncytial virus, parainfluenza, influenza, and adenovirus usually cause self-limited illnesses in immunocompetent subjects, infections in the transplant recipient can be dramatic. As transplant recipients live longer and diagnostic methods improve, the burden of CARVs will undoubtedly increase. Because of limited therapeutic options, some patients may succumb to CARV infections, while many survivors develop chronic allograft dysfunction. Recognition of this latter phenomenon has implicated CARVs in the pathogenesis of bronchiolitis obliterans.

Simultaneous LFA-1 and CD40 ligand antagonism prevents airway remodeling in orthotopic airway transplantation: implications for the role of respiratory epithelium as a modulator of fibrosis

Airway remodeling is a prominent feature of certain immune-mediated lung diseases such as asthma and chronic lung transplant rejection. Under conditions of airway inflammation, the respiratory epithelium may serve an important role in this remodeling process. Given the proposed role of respiratory epithelium in nonspecific injury models, we investigated the respiratory epithelium in an immune-specific orthotopic airway transplant model. MHC-mismatched tracheal transplants in mice were used to generate alloimmune-mediated airway lesions. Attenuation of this immune injury and alteration of antidonor reactivity were achieved by the administration of combined anti-LFA-1/anti-CD40L mAbs. By contrast, without immunotherapy, transplanted airways remodeled with a flattening of respiratory epithelium and significant subepithelial fibrosis. Unopposed alloimmune injury for 10 days was associated with subsequent epithelial transformation and subepithelial fibrosis that could not be reversed with immunotherapy. The relining of donor airways with recipient-derived epithelium was delayed with immunotherapy resulting in partially chimeric airways by 28 days. Partial chimerism was sufficient to prevent luminal fibrosis. However, epithelial chimerism was also associated with airway remodeling. Therefore, there appears to be an intimate relationship between the morphology and level of chimerism of the respiratory epithelium and the degree of airway remodeling following alloimmune injury.

Reepithelialized orthotopic tracheal allografts expand memory cytotoxic T lymphocytes but show no evidence of chronic rejection

BACKGROUND

Acute rejection of mouse tracheal allografts is characterized by infiltration of the lamina propria with CD4+/CD8+ T cells that leads to the destruction of the epithelium and luminal obliteration. The donor epithelium is progressively replaced by recipient-derived epithelium. Once allograft reepithelialization has occurred, immunosuppression can be withdrawn without inciting acute rejection. We hypothesize that reepithelialization will also prevent chronic rejection of the trachea after withdrawal of immunosuppression.

METHODS

BALB/c tracheal grafts were transplanted orthotopically into allogeneic C57BL/6 recipients. Allografted mice were nonimmunosuppressed for 10 or 100 days or immunosuppressed with cyclosporine A continuously for 50 days and then withdrawn from immunosuppression for an additional 50 days. In addition, grafts from this group were then heterotopically retransplanted into isogenic C57BL/6 or allogeneic BALB/c recipients to assess their immunogenicity.

RESULTS

Cyclosporine A-treated mice showed no signs of chronic rejection or priming of cellular immunity as measured by proliferation and cytokine secretion in a mixed leukocyte reaction. However, there was a notable expansion of memory CD8+ T cells specific for donor major histocompatibility complex. When these tracheal allografts were retransplanted heterotopically into C57BL/6 or BALB/c, they demonstrated reduced responses toward BALB/c and primed responses toward C57BL/6, respectively. These results suggest that the grafts express a chimeric phenotype consisting of both BALB/c and C57BL/6 antigens.

CONCLUSION

These observations suggest that long-term withdrawal of immunosuppression does not lead to chronic tracheal rejection even in the presence of alloantigen specific cytotoxic T-lymphocyte responses and that the reepithelialized grafts may contain donor elements that impact the generation of immunity.

Induction of mast cell activation and CC chemokine responses in remodeling tracheal allografts

Activated mast cells release stored and newly synthesized mediators that influence the caliber and responsiveness of inflamed airways. In this work, we show that alloimmune-mediated mechanisms induce mast cell activation and expression of CC chemokines in remodeling rat tracheal allografts. Decreased expression of rat mast cell protease (RMCP) I and II, in concert with tryptase release in tracheal allografts, identified degranulation of stored serine proteases as an early mast cell response to allotransplantation. Transient upregulation of c-Kit expression occurred in a synchronous manner, suggesting that c-Kit receptor signaling controls mast cell responses. Increased expression of CC chemokine ligand (CCL) 2 and CCL3 by RMCP I-positive cells identified mast cells as epithelial and mesenchymal sources of chemoattractant chemokines in allograft airways. Cyclosporin A immunosuppression both attenuated and delayed these changes in mast cell phenotypes. Incubation of rat basophil leukemia 2H3 cells with CCL2 or CCL3 decreased surface c-Kit expression, an effect blocked by protease inhibitors. By controlling surface receptor availability, CC chemokines may regulate c-Kit signaling via a novel proteolytic mechanism. These data suggest that targeting alloimmune responses and restoring quiescence of mast cells may attenuate the development of fibroproliferative and obstructive distortions of bronchiolar architecture in lung allografts.

Airway epithelium is the primary target of allograft rejection in murine obliterative airway disease

Murine heterotopic tracheal allografts develop obliterative airway disease (OAD), a suitable model of chronic lung allograft rejection. This model, however, fails to account for the behavior of the allograft when adjacent to recipient airway tissues, particularly the epithelium. This study was performed to determine the immunologic role of the epithelium in development of OAD. BALB/c (H2d) tracheal allografts were transplanted orthotopically into C57BL/6 (H2b) mice and harvested 14-150 days post-transplantation. The phenotype of the allograft epithelium after orthotopic transplantation was determined with immunofluorescent staining. Orthotopic BALB/c tracheal allografts harvested at 28 days were re-transplanted heterotopically into BALB/c or C57BL/6 mice, harvested after 28 days, and assessed for OAD. Orthotopic allografts displayed mild cellular infiltration, no fibrosis and preserved epithelium at 28 days post-transplant. The presence of recipient-derived epithelium within the allograft was demonstrated with immunofluorescent staining at day 14. Significantly, BALB/c orthotopic allografts re-transplanted heterotopically into BALB/c mice developed OAD by day 28, whereas BALB/c orthotopic allografts re-transplanted heterotopically into C57BL/6 mice did not. Repopulation of orthotopic tracheal allografts with recipient-derived epithelium confers a protective effect against OAD after heterotopic re-transplantation. This indicates that the airway epithelium plays a crucial role in OAD development.