Heterotopic tracheal transplantation with omentum wrapping in the abdominal position preserves functional and structural integrity of a human tracheal allograft

A Contemporary Review of Trachea, Nose, and Ear Cartilage Bioengineering and Additive Manufacturing

The complex structure, chemical composition, and biomechanical properties of craniofacial cartilaginous structures make them challenging to reconstruct. Autologous grafts have limited tissue availability and can cause significant donor-site morbidity, homologous grafts often require immunosuppression, and alloplastic grafts may have high rates of infection or displacement. Furthermore, all these grafting techniques require a high level of surgical skill to ensure that the reconstruction matches the original structure. Current research indicates that additive manufacturing shows promise in overcoming these limitations. Autologous stem cells have been developed into cartilage when exposed to the appropriate growth factors and culture conditions, such as mechanical stress and oxygen deprivation. Additive manufacturing allows for increased precision when engineering scaffolds for stem cell cultures. Fine control over the porosity and structure of a material ensures adequate cell adhesion and fit between the graft and the defect. Several recent tissue engineering studies have focused on the trachea, nose, and ear, as these structures are often damaged by congenital conditions, trauma, and malignancy. This article reviews the limitations of current reconstructive techniques and the new developments in additive manufacturing for tracheal, nasal, and auricular cartilages.

Dynamic flow for efficient partial decellularization of tracheal grafts: A preliminary rabbit study

Objective

Bioengineered tracheal grafts are a potential solution for the repair of long-segment tracheal defects. A recent advancement is partially decellularized tracheal grafts (PDTGs) which enable regeneration of host epithelium and retain viable donor chondrocytes for hypothesized benefits to mechanical properties. We propose a novel and tunable 3D-printed bioreactor for creating large animal PDTG that brings this technology closer to the bedside.

Methods

Conventional agitated immersion with surfactant and enzymatic activity was used to partially decellularize New Zealand white rabbit (Oryctolagus cuniculus) tracheal segments (n = 3). In parallel, tracheal segments (n = 3) were decellularized in the bioreactor with continuous extraluminal flow of medium and alternating intraluminal flow of surfactant and medium. Unprocessed tracheal segments (n = 3) were also collected as a control. The grafts were assessed using the H&E stain, tissue DNA content, live/dead assay, Masson's trichrome stain, and mechanical testing.

Results

Conventional processing required 10 h to achieve decellularization of the epithelium and submucosa with poor chondrocyte viability and mechanical strength. Using the bioreactor reduced processing time by 6 h and resulted in chondrocyte viability and mechanical strength similar to that of native trachea.

Conclusion

Large animal PDTG created using our novel 3D printed bioreactor is a promising approach to efficiently produce tracheal grafts. The bioreactor offers flexibility and adjustability favorable to creating PDTG for clinical research and use. Future research includes optimizing flow conditions and transplantation to assess post-implant regeneration and mechanical properties.

Level of Evidence

NA.

De-Epithelialized Viable Tracheal Allotransplantation Without Immunosuppressants: 5-Year Follow-Up

OBJECTIVE

Tracheal transplantation could be a better option for patients with long segmental laryngotracheal stenosis or defects, but the need for immunosuppressants limits its widespread use due to the antigenicity of the tracheal epithelium. Chemically treated or cryopreserved nonviable tracheal allografts have no immunogenicity but lead to necrosis and stenosis in long-term outcomes. The present report describes the 5-year outcomes of de-epithelialized viable tracheal allotransplantation without immunosuppressants in a patient with severe laryngotracheal stenosis.

METHODS

The recipient was a 47-year-old female with relapsing polychondritis affecting the larynx and cervical trachea and producing a 5 cm long stenosis that could not be repaired using resection and anastomosis. A tracheal allograft was obtained from a 45-year-old male donor and treated with a combination of 3% sodium dodecyl sulfate (SDS) and organ preservation solution for 138 hours. The allograft was revascularized by heterotopical implantation in the infrahyoid muscles of the recipient for 3 months and then transplantation to the laryngotracheal defect with a split-thickness skin graft sutured to the lumen and a silicon T-tube. No immunosuppressants were used postoperatively.

RESULTS

The allograft was de-epithelialized, and most of the cartilage rings remained viable after the treatment. The allograft was revascularized, viable, and mechanically stable after 3 months of heterotopic implantation. No apparent signs of rejection or destruction were observed. The T-tube was removed, and the internal lining of the allograft was repopulated 4 months after orthotopic transplantation, despite the skin graft necrotizing at 2 weeks. Endoscopy and computed tomography showed a patent airway 5 years after orthotopic transplantation. The patient was able to resume her usual quality of life.

CONCLUSION

The present study demonstrates that transplantation of the de-epithelialized viable tracheal allograft without immunosuppressants is safe and promising for patients with long laryngotracheal stenosis or defects, especially for those with malignant tumor resections.

Extended autologous tracheal replacement by a novel pedicled thoraco-chondro-costal flap: a cadaveric proof of concept

OBJECTIVES

Our aim was to report an anatomic model of an autologous flap based on the internal thoracic blood supply: the pedicled thoraco-chondro-costal flap; and establish the feasibility of various types of extended tracheal replacement with this novel flap, according to a newly proposed topographic classification.

METHODS

In a cadaveric model, a cervicotomy combined with median sternotomy was performed. The incision was extended laterally to expose the chest wall. The internal thoracic pedicle was freed from its origin down to the upper limit of the delineated flap to be elevated. The perichondria and adjacent periostea were incised longitudinally to remove cartilages and adjacent rib segments, preserving perichondria and periostea. A full-thickness quadrangular chest wall flap pedicled on internal thoracic vessels was then elevated and shaped into a neo conduit to replace the trachea with the pleura as an inner lining.

RESULTS

Various types of extended non-circumferential and full-circumferential tracheal replacements were achieved with this composite flap. No anastomosis tension was noticed despite the absence of release manoeuvres.

CONCLUSIONS

This model could represent a suitable autologous tracheal substitute, which is long, longitudinally flexible and eventually transversely rigid. No microsurgical vascular anastomoses are required. The technique is reproducible. The perichondria and periostea would regenerate vascularized neo-cartilaginous rings, potentially decreasing the need for long-term stenting. The inner pleural lining could potentially transform into ciliated epithelium as shown in previous preclinical studies.

Tracheal Tissue Engineering: Principles and State of the Art

Patients affected by long-segment tracheal defects or stenoses represent an unsolved surgical issue, since they cannot be treated with the conventional surgery of tracheal resection and consequent anastomosis. Hence, different strategies for tracheal replacement have been proposed (synthetic materials, aortic allografts, transplantation, autologous tissue composites, and tissue engineering), each with advantages and drawbacks. Tracheal tissue engineering, on the other hand, aims at recreating a fully functional tracheal substitute, without the need for the patient to receive lifelong immunosuppression or endotracheal stents. Tissue engineering approaches involve the use of a scaffold, stem cells, and humoral signals. This paper reviews the main aspects of tracheal TE, starting from the choice of the scaffold to the type of stem cells that can be used to seed the scaffold, the methods for their culture and expansion, the issue of graft revascularization at the moment of in vivo implantation, and experimental models of tracheal research. Moreover, a critical insight on the state of the art of tracheal tissue engineering is also presented.

Human tracheal transplantation: A systematic review of case reports

BACKGROUND

Patients with long-segment airway stenosis not amenable to conventional surgery may benefit from tracheal transplantation. However, this procedure has been only anecdotally reported, and its indications, techniques, and outcomes have not been extensively reviewed.

METHODS

We conducted a systematic Literature search to identify all original articles reporting attempts at tracheal transplantation in humans.

RESULTS

Of 699 articles found by the initial search, 11 were included in the systematic review, describing 14 cases of tracheal transplantation. Patients underwent transplantation for benign stenosis in nine cases, and for malignancies in five cases. In 12 cases blood supply to the trachea was provided by wrapping the graft in a vascularized recipient's tissue, while in 2 cases the trachea was directly transplanted as a vascularized composite allograft. The transplantation procedure was aborted before orthotopic transplantation in two patients. Among the remaining 12 patients, there was 1 operative mortality, while 4 patients experienced complications. Immunosuppressants drugs were administered to the majority of patients postoperatively, and only one group of authors attempted their withdrawal, in five patients. At the end of follow-up, all 11 patients surviving the operation were alive, but 2 had a recurrent tracheal stenosis requiring an airway appliance for breathing.

CONCLUSION

Human tracheal transplantation is still at an embryonic phase. Studies available in the Literature report different surgical techniques, and information on long-term outcomes is still limited. Future research is needed in order to understand the clinical value of this procedure.

Orthotopic Ferret Tracheal Transplantation Using a Recellularized Bioengineered Graft Produces Functional Epithelia

Tracheal grafts may be necessary to bridge long-segment defects after curative resection for airway obstructions. Bioengineered grafts have emerged as an appealing option, given the possibilities of altering the histologic and cellular profile of the conduit. We previously designed a bioreactor capable of luminally decellularizing and recellularizing a ferret trachea with surface airway epithelia (SAE) basal cells (BCs), and we sought to assess the fate of these grafts when transplanted in an orthotopic fashion. As adjuncts to the procedure, we investigated the use of a vascular endothelial growth factor (VEGF)-laden hydrogel and of immunosuppression (IS) in graft revascularization and viability. IS was shown to limit early graft revascularization, but this effect could be counteracted with VEGF supplementation. Submucosal gland (SMG) loss was shown to be inevitable regardless of the revascularization strategy. Lastly, the bioengineered tracheas survived one month after transplant with differentiation of our implanted BCs that then transitioned into a recipient-derived functional epithelium. The work presented in this manuscript has important implications for future cellular and regenerative therapies.

The History of Engineered Tracheal Replacements: Interpreting the Past and Guiding the Future

The development of a tracheal graft to replace long-segment defects has thwarted clinicians and engineers alike for over 100 years. To better understand the challenges facing this field today, we have consolidated all published reports of engineered tracheal grafts used to repair long-segment circumferential defects in humans, from the first in 1898 to the most recent in 2018, totaling 290 clinical cases. Distinct trends emerge in the types of grafts used over time, including repair using autologous fascia, rigid tubes of various inert materials, and pretreated cadaveric allografts. Our analysis of maximum clinical follow-up, as a proxy for graft performance, revealed that the Leuven protocol has a significantly longer clinical follow-up time than all other methods of airway reconstruction. This method involves transplanting a cadaveric tracheal allograft that is first prevascularized heterotopically in the recipient. We further quantified graft-related causes of mortality, revealing failure modes that have been resolved, and those that remain a hurdle, such as graft mechanics. Finally, we briefly summarize recent preclinical work in tracheal graft development. In conclusion, we synthesized top clinical care priorities and design criteria to inform and inspire collaboration between engineers and clinicians toward the development of a functional tracheal replacement graft. Impact statement The field of tracheal engineering has floundered in recent years due to multiple article retractions. However, with recent advances in biofabrication and tissue analysis techniques, the field remains ripe for advancement through collaboration between engineers and clinicians. With a long history of clinical application of tracheal replacements, engineered tracheas are arguably the regenerative technology with the greatest potential for translation. This work describes the many phases of engineered tracheal replacements that have been applied in human patients over the past 100 years with the goal of carrying forward critical lessons into development of the next generation of engineered tracheal graft.

Replacement of a 5-cm intrathoracic trachea with a tissue-engineered prosthesis in a canine model

BACKGROUND

Critical obstacles must be addressed before clinical application of artificial tracheas. The major complications of long tracheal replacement include anastomotic dehiscence and stenosis owing to poor vascularity and incomplete re-epithelialization. The objective of this report was to clarify whether pre-incubation of the prosthesis in the omentum could be applicable for reconstruction of a long segment of the intrathoracic trachea in a canine model.

METHODS

The framework of an artificial trachea was fabricated from a polypropylene mesh tube and coated with 1% neutral atelocollagen inside and outside the lumen. The prosthesis was placed in the omentum of nine healthy male beagle dogs for 3 weeks. Then, the pedicled prosthesis was used to replace a 50 mm long section of intrathoracic trachea. Results were evaluated bronchoscopically, macroscopically, and histologically.

RESULTS

After 3 weeks of abdominal incubation, the prostheses were incorporated into the host tissue. None of the dogs showed dehiscence of the anastomosis or infection of the prostheses during the postoperative period. Seven of the nine dogs survived for more than 1 year. One dog died of a bowel obstruction resulting from a diaphragmatic hernia 3 months after replacement, and another died due to reasons unrelated to the prosthesis at 6 months. Bronchoscopic examination revealed no stenosis or dehiscence, and microscopic examination of all dogs showed that the luminal surface was covered by newly regenerated connective tissue and respiratory epithelium.

CONCLUSIONS

Pedicled omentum-prosthesis complexes may allow successful reconstruction of a long segment of the intrathoracic trachea.

The current status and outlook of trachea transplantation

PURPOSE OF REVIEW

The trachea is an enigmatic organ due to its complex morphology. Although circumferential tracheal defects are extremely difficult to repair with autologous tissue or with an allotransplant, the trachea has been touted as the first organ that could be regenerated. This review provides a comprehensive evaluation of the published evidence in tracheal tissue replacement surgery.

RECENT FINDINGS

In recent years, reports of successful tracheal regeneration have attracted great interest. Despite descriptions of the trachea as a perhaps uniquely regeneratable tissue since 2008, critical reporting provided insights into the more complex realities of tracheal regeneration attempts and led to the retraction of some articles making tracheal regeneration claims. Allotransplantation of the trachea is hindered by numerous difficult obstacles. The most promising approach developed thus far for difficult-to-repair patch airway defects is tracheal allotransplantation, which allows for tapering and withdrawal of immunosuppressive therapy.

SUMMARY

Restoration of a long-segment circumferential tracheal defect remains an unmet challenge. Future clinical studies require thoroughly documented visual evidence of outcomes to reduce confusion surrounding tracheal replacement and to prevent future scandals like those seen previously in the tracheal regeneration story. VIDEO ABSTRACT: http://links.lww.com/COOT/A6.

De-Epithelialized Heterotopic Tracheal Allografts without Immunosuppressants in Dogs: Long-Term Results for Cartilage Viability and Structural Integrity

OBJECTIVES

Reconstruction of long segmental tracheal defects is difficult because no ideal tracheal substitutes are currently available. Tracheal allotransplantation maintains cartilage and epithelium viability but requires immunosuppression because of epithelial immunogenicity. We aimed to obtain an epithelium-decellularized allograft that maintains cartilage viability and to evaluate long-term outcomes of such allografts implanted on dog backs without immunosuppressants.

METHODS

Twenty-five tracheas harvested from mongrel dogs were used to explore the period of epithelium decellularization by combined use of 1% sodium dodecyl sulfate and an organ preservation solution and to assess the chondrocyte viability and immunogenicity of the tracheas after decellularization. Sixteen epithelium-decellularized tracheal allografts and 10 fresh tracheal segments (6 cm long) were implanted in 26 beagles for durations of 10 days and 1, 3, 6, and 12 months. Macroscopic and microscopic examinations were used to evaluate the morphology, viability, and immune rejection of the allografts. Safranin-O staining was used to detect glycosaminoglycans.

RESULTS

The epithelium disappeared after 24 hours of decellularization. At 72 hours, almost no nuclei remained in the mucosa, while the mean survival rate of chondrocytes was 88.1%. Histological analysis demonstrated that the allograft retained intact tracheal rings and viable cartilage after heterotopic implantation for 1 year, with no immunological rejection. There were no significant differences in the glycosaminoglycan contents among the implanted epithelium-decellularized allografts.

CONCLUSIONS

Epithelium-decellularized tracheal allografts with chondrocyte viability can be achieved by combined use of a detergent and organ preservation solution, which showed satisfactory cartilage viability and structural integrity after long-term heterotopic transplantation. Further studies on orthotopic transplantation are needed to assess the feasibility of allografts in reconstructing long segmental tracheal defects.

PLK1 Inhibition alleviates transplant-associated obliterative bronchiolitis by suppressing myofibroblast differentiation

Chronic allograft dysfunction (CAD) resulting from fibrosis is the major limiting factor for long-term survival of lung transplant patients. Myofibroblasts promote fibrosis in multiple organs, including the lungs. In this study, we identified PLK1 as a promoter of myofibroblast differentiation and investigated the mechanism by which its inhibition alleviates transplant-associated obliterative bronchiolitis (OB) during CAD. High-throughput bioinformatic analyses and experiments using the murine heterotopic tracheal transplantation model revealed that PLK1 is upregulated in grafts undergoing CAD as compared with controls, and that inhibiting PLK1 alleviates OB in vivo. Inhibition of PLK1 in vitro reduced expression of the specific myofibroblast differentiation marker α-smooth muscle actin (α-SMA) and decreased phosphorylation of both MEK and ERK. Importantly, we observed a similar phenomenon in human primary fibroblasts. Our results thus highlight PLK1 as a promising therapeutic target for alleviating transplant-associated OB through suppression of TGF-β1-mediated myofibroblast differentiation.

Regeneration of trachea graft with cartilage support, vascularization, and epithelization

The repair and functional reconstruction of long-segment tracheal defects is always a great challenge in the clinic. Finding an ideal substitute for tracheal transplantation is the only way to solve this problem. The current study proposed a series of novel strategies for constructing a bionic living trachea substitute. For the issue of tubular cartilage support, cartilage sheet technique based on high-density culture of chondrocytes was adopted to avoid the inflammatory reaction triggered by the materials and thus formed mature cartilage-like tissue in autologous goat model. For the issue of epithelialization, the autologous transplantation of oral mucosal epithelium was used to realize mucosa coverage of the constructed trachea lumen. Finally, the flat trapezius fascia flap with double blood supply was separated by microsurgical techniques to achieve stable pre-vascularization of both the regenerated cartilage and the grafted epithelium simultaneously. By integrating the above strategies, the vascularized and epithelialized tracheal substitute with tubular cartilage support was successfully constructed in a goat model. The reconstructed trachea possessed a multiple layer structure of muscle-cartilage-fascia-mucosa comparable to the native trachea, and thus might realize stable survival and long-term airway function maintenance, providing a promising tracheal substitute for the repair and permanent functional reconstruction of long-segment tracheal defects. STATEMENT OF SIGNIFICANCE: The repair of long-segment tracheal defects is always a great challenge in the clinic. Finding an ideal substitute for tracheal transplantation is the only way to solve this problem. In the current study, by technical integration of cartilage regeneration, microsurgery, and oral mucosa transplantation, a complex tracheal substitute with satisfactory vascularization, epithelialization, and tubular cartilage support was successfully constructed in a goat autologous model. The reconstructed trachea substitute possessed a multiple layer structure of muscle-cartilage-fascia-mucosa exactly similar to native trachea, and thus might realize stable survival and long-term airway function maintenance. The current study provides feasible strategies and ideal tracheal substitutes for permanent functional reconstruction of long-segmental trachea defects.

Successful immunosuppressant-free heterotopic transplantation of tracheal allografts in the pig

OBJECTIVES

It has been demonstrated that both heterotopic and orthotopic transplants of epithelium-denuded cryopreserved tracheal allografts are feasible in immunosuppressant-free rabbits. Validation of these results in large animals is required before considering clinical applications. We evaluated the viability, immune tolerance and strain properties of such tracheal allografts heterotopically transplanted in a pig model.

METHODS

Ten tracheal segments, 5 short (5 rings) and 5 long (10 rings), were obtained from male Landrace pigs. The tracheal segments were surgically denuded of their epithelium, then cryopreserved and stored in a tissue bank for 33 to 232 days. After thawing, tracheal segments stented with a silicone tube were wrapped in the omentum in 2 groups of 5 female recipients. The animals did not receive any immunosuppressive drugs. The animals were euthanized from Day 6 to Day 90 in both groups.

RESULTS

An effective revascularization of allografts regardless of length was observed. Lymphocyte infiltrate was shown in the early postoperative period and became non-significant after 30 days. Allografts displayed high levels of neoangiogenesis and viable cartilage rings with islets of calcification. Biomechanical measurements demonstrated strain properties similar to those of a fresh tracheal segment from Day 58.

CONCLUSIONS

Our results demonstrate the acceptability and satisfactory stiffness of epithelium-denuded cryopreserved tracheal allografts implanted in the omentum, despite the absence of immunosuppressive drugs. Since the omentum has the capability to reach the tracheal region, this approach should be investigated in the setting of orthotopic transplants in a pig model before considering clinical applications.

Tracheal replacement

Tracheal reconstruction is one of the greatest challenges in thoracic surgery when direct end-to-end anastomosis is impossible or after this procedure has failed. The main indications for tracheal reconstruction include malignant tumours (squamous cell carcinoma, adenoid cystic carcinoma), tracheoesophageal fistula, trauma, unsuccessful surgical results for benign diseases and congenital stenosis. Tracheal substitutes can be classified into five types: 1) synthetic prosthesis; 2) allografts; 3) tracheal transplantation; 4) tissue engineering; and 5) autologous tissue composite. The ideal tracheal substitute is still unclear, but some techniques have shown promising clinical results. This article reviews the advantages and limitations of each technique used over the past few decades in clinical practice. The main limitation seems to be the capacity for tracheal tissue regeneration. The physiopathology behind this has yet to be fully understood. Research on stem cells sparked much interest and was thought to be a revolutionary technique; however, the poor long-term results of this approach highlight that there is a long way to go in this research field. Currently, an autologous tissue composite, with or without a tracheal allograft, is the only long-term working solution for every aetiology, despite its technical complexity and setbacks.

[Long-term oucomes of tracheal transplantation: success and unsolved problems]

AIM

To analyze long-term outcomes of tracheal transplantation.

MATERIAL AND METHODS

There were 1128 patients with cicatricial tracheal stenosis who have been operated at the Petrovsky Russian Research Center for Surgery and the Sechenov First Moscow State Medical University for the period 1963-2015.

RESULTS

Operations have become safer. Postoperative morbidity and mortality reduced from 41.4% (1963-1980) to 5.6% (2001-2015) and from 21.9% (1963-1980) to 0.5% (2001-2015), respectively. Tracheal transplantation was performed in 2 cases and fundamentally different tracheal structures were applied. Donor thyreotracheal complex with restored blood supply through thyroid vessels was used in the first case (2006). Perennial experimental trials preceded clinical application of this technique. In the second case (2010) we applied scientific results of foreign colleagues (cellular technologies and methods of regenerative medicine to create artificial trachea). Patients are still alive after 12 and 8 years, respectively. Restoration of blood supply of donor trachea is possible through thyroid collaterals. This technique is successful in long-term period. Tissue-engineered trachea cannot be considered true trachea due to no all tracheal components. However, such trachea provides air-conducting, evacuation and protective functions. Tracheomalacia requires further researches as one of the main problems of tracheal transplantation.

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.

Current Solutions for Long-Segment Tracheal Reconstruction

This article is a continuation of previous reviews about the appropriate method for long-segment tracheal reconstruction. We attempted to cover the most recent, successful and promising results of the different solutions for reconstruction that are rather innovative and suitable for imminent clinical application. Latest efforts to minimize the limitations associated with each method have been covered as well. In summary, autologous and allogenic tissue reconstruction of the trachea have been successful methods for reconstruction experimentally and clinically. Autologous tissues were best utilized clinically to enhance revascularization, whether as a definitive airway or as an adjunct to allografts or tissue-engineered trachea (TET). Allogenic tissue transplantation is, currently, the most suitable for clinical application, especially after elimination of the need for immunosuppressive therapy with unlimited supply of tissues. Similar results have been reported in many studies that used TET. However, clinical application of this method was limited to use as a salvage treatment in a few studies with promising results. These results still need to be solidified by further clinical and long-term follow-up reports. Combining different methods of reconstruction was often required to establish a physiological rather than an anatomical trachea and have shown superior outcomes.

Reconstruction of defects of the trachea

The trachea has a complex anatomy to fulfill its tasks. Its unique fibro-cartilaginous structure maintains an open conduit during respiration, and provides vertical elasticity for deglutition, mobility of the neck and speech. Blood vessels pierce the intercartilaginous ligaments to perfuse the ciliated epithelium, which ensures effective mucociliary clearance. Removal of a tracheal segment affected by benign or malignant disease requires airtight restoration of the continuity of the tube. When direct approximation of both tracheal ends is no longer feasible, a reconstruction is needed. This may occur in recurrent short-segment defects in a scarred environment, or in defects comprising more than half the length of the trachea. The resulting gap must be filled with vascularized tissue that restores the mucosal lining and supports the semi-rigid, semi-flexible framework of the trachea. For long-segment or circular defects, restoration of this unique biomechanical profile becomes even more important. Due to the inherent difficulty of creating such a tube, a tracheostomy or palliative stenting are often preferred over permanent reconstruction. To significantly improve and sustain quality of life of these patients, surgeons proposed innovative strategies for complex tracheal repair. In this review, we provide an overview of current clinical applications of tracheal repair using autologous and allogenic tissues. We look at recent advances in the field of tissue engineering, and the areas for improvement of these first human applications. Lastly, we highlight the focus of our research, in an effort to contribute to the development of optimized tracheal reconstructive techniques.

Tracheal replacement

Non-malignant and malignant obstruction of the tracheal airway causes significant morbidity and mortality. With increased use of artificial airways, benign and iatrogenic complications are increasing. A tracheal stenosis that is less than 5 cm in length can be resected with end-to-end anastomosis. Longer tracheal lesions can be treated in a palliative way by placement of a stent to secure airway lumen patency. The management of tracheal defects is an evolving field. Tracheal transplantation and tracheal regeneration may bring major treatment advances to cases with long-segment tracheal involvement. This review examines the current possibilities and future prospects in the area of tracheal transplantation and regeneration.

[Repeated tracheal resection for non-neoplastic restenosis]

Treatment of patients with recurrent cicatrical tracheal stenosis after previous circular tracheal resection is one of the most difficult problems in thoracic surgery at present time. In most cases repeated radical surgery as new resection is declined in favour of palliative treatment. It is often associated with lingering or perpetual preserving of T-shape or tracheostomy tube and respiratory tract stenting. Development of thoracic surgery last years permits to perform repeated tracheal resections with restoration of respiratory tract integrity by using of new tracheal anastomosis. For the last 4 years 6 such operations were performed with satisfactory immediate and remote results. Diagnostic algorithm before repeated surgery is similar to those before primary intervention. Special attention should be attended to state of remained parts of respiratory tract, degree and length of stenosis and tracheomalacia which may be result of divergence of edges of the primary anastomosis. Preserving of not less than 1/4 primary length of intact trachea with its satisfactory mobility is main condition for this surgery because it will permit to perform new anastomosis without high tension. Risk of postoperative complications after repeated operations is not higher than those after primary resection. But at present time these operations are in competence of small number of specialists and medical institutions with serious experience in thoracic surgery.

Biomechanical changes from long-term freezer storage and cellular reduction of tracheal scaffoldings

OBJECTIVES/HYPOTHESIS

To determine structural biomechanical changes in tracheal scaffolds resulting from cellular reduction and storage at -80(o) C.

STUDY DESIGN

Laboratory-based study.

METHODS

Forty-four rabbit tracheal segments were separated into four treatment groups: untreated (group A, control), cellular-reduced (group B), storage at -80(o) C followed by cellular reduction (group C), and cellular-reduced followed by storage at -80(o) C (group D). Tracheal segments were subjected to uniaxial tension (n = 21) or compression (n = 23) using a universal testing machine to determine sutured tensile yield load and radial compressive strengths at 50% lumen occlusion. Mean differences among groups for tension and compression were compared by analysis of variance with post-hoc Tukey-Kramer test.

RESULTS

The untreated trachea (group A) demonstrated mean yield strength of 5.93 (± 1.65) N and compressive strength of 2.10 (± 0.51) N. Following treatment/storage, the tensile yield strength was not impaired (group B = 6.79 [± 1.58] N, C = 6.21 [± 1.40] N, D = 6.26 [± 1.18]; P > 0.10 each). Following cellular reduction, there was a significant reduction in compressive strength (group B = 0.44 N [± 0.13], P < 0.0001), but no further reduction due to storage (group C = 0.39 N [± 0.10]; P = 0.97 compared to group B).

CONCLUSION

The data suggest cellular reduction leads to loss of compressive strength. Freezing at -80°C (either before, or subsequent to cellular reduction) may be a viable storage method for tracheal grafts.

Immune tolerance of epithelium-denuded-cryopreserved tracheal allograft

OBJECTIVES

Animal and clinical studies have demonstrated the feasibility of tracheal allograft transplantation after a revascularization period in heterotopy, thus requiring immunosuppressive therapy. Given the key role of the respiratory epithelium in the immune rejection, we investigated the consequence of both epithelium denudation and cryopreservation in immune tolerance of tracheal allograft in a novel rabbit model.

METHODS

Five adult female New Zealand rabbits served as donors of tracheas that were denuded of their epithelium and then cryopreserved, and 13 males were used as recipients. Following graft wrap using a lateral thoracic fascial flap, allograft segments 20 mm in length with (n = 9) or without (n = 4) insertion of an endoluminal tube were implanted under the skin of the chest wall. The animals did not receive any immunosuppressive drugs. Sacrifices were scheduled up to 91 days. Macroscopic and microscopic examinations and detection of apoptotic cells by immunohistochemical staining (Apostain) were used to study the morphology, stiffness, viability and immune rejection of allografts.

RESULTS

There were no postoperative complications. Grafted composite allografts displayed satisfactory tubular morphology provided that an endoluminal tube was inserted. All rabbits were found to have an effective revascularization of their allograft and a mild non-specific inflammatory infiltrate with no significant lymphocyte infiltration. Cartilage rings showed early central calcification deposit, which increased over time, ensuring graft stiffness. Apoptosis events observed into the allograft cells were suggestive of minimal chronic rejection.

CONCLUSIONS

Our results demonstrated that the epithelium-denuded-cryopreserved tracheal allograft implanted in heterotopy displayed satisfactory morphology, stiffness and immune tolerance despite the absence of immunosuppressive drugs. This allograft with a fascial flap transferable to the neck should be investigated in the setting of tracheal replacement in rabbits. Similar studies need to be conducted in bigger mammals before considering clinical applications.

Airway transplantation

No definitive solution has been discovered for replacing long segments or the entire trachea in humans. Most of this challenge stems from the specific function and mechanics that are almost impossible to replicate except in the setting of an allotransplantation, which requires lifelong immunosuppressive medication. Recently, tissue engineering provided significant evidence concerning the next promising therapeutic alternative for tracheal replacement. Underlying mechanism and pathways of cell-surface interactions, cell migration, and differentiation are essential to understand the complexity of tracheal tissue regeneration. Tracheal replacement remains challenging but initial steps toward an ideal therapeutic concept have been made.

Tracheal replacement for primary tracheal cancer

PURPOSE OF REVIEW

To summarize the so far applied clinical methods of tracheal replacement, comparing pros and cons of conventional and tissue-engineered approaches.

RECENT FINDINGS

Several strategies have been most recently described to replace the trachea-like aortic homografts, allotransplantation, and tissue engineering. Allotransplantation requires lifelong immunosuppression and this may be ethically questioned being not a lifesaving procedure. Tissue-engineered tracheal transplantation has been clinically applied using biological or bioartificial tubular or bifurcated scaffolds reseeded with mesenchymal stromal cells, and bioactive molecules boosting regeneration and promoting neovascularization.

SUMMARY

Tracheal tissue engineering may be a promising alternative to conventional allotransplantation in adults and children. Different methods have been developed and are currently under active clinical investigation, and await long-term results.

Tracheal reconstruction with a composite graft: fascial flap-wrapped allogenic aorta with external cartilage-ring support

OBJECTIVES

Animal and clinical studies have demonstrated the feasibility of tracheal replacement by silicone-stented allogenic aortas. In clinical trials, however, this graft did not show mature cartilage regeneration into the grafts as was observed in animal models. To solve this issue, we investigated tracheal replacement with a composite graft based on a fascial flap-wrapped allogenic aorta with external cartilage-ring support in a rabbit model.

METHODS

Seven male 'Géant des Flandres' and 'New Zealand' rabbits served as donors of aortas and cartilage rings, respectively. Nineteen female 'New Zealand' rabbits were used as recipients. First, in nine animals, neoangiogenesis of the composite graft following a wrap using a pedicled lateral thoracic fascial flap and implantation under the skin of the chest wall was investigated. Animal sacrifice was scheduled at regular intervals up to 38 days. Second, 10 animals underwent tracheal replacement with the composite graft after a 7-to-9 day revascularization period, and were followed-up to death. Macroscopic and microscopic examinations were used to study the morphology, stiffness and viability of the construct.

RESULTS

There was one operative death after tracheal replacement. The first group of animals was found to have a satisfactory tubular morphology and stiffness of their construct associated with preserved histological structure of cartilages and moderate to severe aortic ischaemic lesions. In the group of rabbits having undergone tracheal replacement, the anatomical results were characterized by a discrepancy between the severity of ischaemic lesions involving both allogenic aorta and cartilage rings and the satisfactory biomechanical characteristics of the graft in 7 of 10 animals, probably due to cartilage calcification deposits associated with inflammatory scar tissue ensuring the stiffness of the construct.

CONCLUSIONS

Our investigations demonstrate the feasibility of the replacement of circumferential tracheal defects using our composite graft. Future experiments using therapeutic bronchoscopy tools are required to draw conclusions regarding the effectiveness of this tracheal substitute in the long-term.

The body as a living bioreactor: a feasibility study of pedicle flaps for tracheal transplantation

Reconstruction of long-segment tracheal stenosis remains problematic. Ex vivo transplantation of stem cell-derived tracheas has been established in humans using external tissue bioreactors. These bioreactors, however, are not widely accessible. Thus, we are developing a rotational flap-based "internal bioreactor" to allow in vivo stem cell engraftment in a pre-vascularized recipient bed. This muscle will also then serve as a carrier for the transplanted trachea during rotation into position for airway reconstruction. Herein, we present a study investigating the feasibility of two pedicle muscle flaps for implantation and subsequent tracheal transplantation. Trapezius and latissimus flaps were raised using established surgical techniques. The length and width of each flap, along with the distance from the pedicle takeoff to the trachea, were measured. The overall ability of the flaps to reach the trachea was assessed. Twelve flaps were raised in 5 fresh adult human cadavers. For the trapezius flap, averages were: flap length of 16.4 cm, flap width of 5.95 cm at the tip, and distance from the pedicle takeoff to the trachea of 11.1 cm. For the latissimus dorsi flap, averages were: flap length of 35.4 cm, flap width of 7.25 cm at the tip, and distance from the pedicle takeoff to the trachea of 27.3 cm. All flaps showed sufficient durability and rotational ability. Our results show that both trapezius and latissimus dorsi flaps can be transposed into the neck to allow tension-free closure of tracheal defects. For cervical tracheal transplantation, both flaps are equally adequate. We believe that trapezius and latissimus dorsi muscle flaps are potential tracheal implantation beds in terms of vascular supply, durability, and rotational ability.

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.

Carinal resection

Carinal resection is defined as the resection of tracheo-bronchial bifurcation, with or without lung parenchyma resection. It represents one of the most challenging areas of airway resection and reconstruction, basically due to the variability in the location and extent of the lesions. Main indications for this procedure are primary tumours of the carina or the distal trachea or, more frequently, bronchogenic carcinoma with carinal involvement. Very different approaches and reconstruction techniques have been experimentally and clinically described in the last 50 years, with some corner stone procedures in the history of modern thoracic surgery. Despite many technical and oncological difficulties encountered in this field, encouraging results have been reported in recent series, in particular an excellent 5-year survival rate of 50% in pN0 patients suffering form carinal infiltration form lung cancer. Several aspects of the multimodality approach to neoplastic carinal involvement still remain debatable like radio-chemotherapeutic approach instead of the extremely rare left carinal pneumonectomy as well as the role of induction treatments before embarking in such demanding procedures, according to the -pathological nodal status.

Tracheal allotransplantation after withdrawal of immunosuppressive therapy

Reconstruction of long-segment tracheal defects requires a vascularized allograft. We report successful tracheal allotransplantation after indirect revascularization of the graft in a heterotopic position. Immunosuppressive therapy was administered before the operation, and the tracheal allograft was wrapped in the recipient's forearm fascia. Once revascularization was achieved, the mucosal lining was replaced progressively with buccal mucosa from the recipient. At 4 months, the tracheal chimera was fully lined with mucosa, which consisted of respiratory epithelium from the donor and buccal mucosa from the recipient. After withdrawal of immunosuppressive therapy, the tracheal allograft was moved to its correct anatomical position with an intact blood supply. No treatment-limiting adverse effects occurred.

The history of human composite tissue allotransplantation

Restoration of amputations and disfigurement are represented in ancient mythology, but the modern history of composite tissue allotransplantation begins with World War II injuries that generated seminal immunologic experiments by Medawar and co-workers. These studies led to the first successful human allografts in the 1950s by Peacock with composite tissue and Murray and co-workers with solid organs. Pharmacologic immunosuppression brought rapid growth of solid organ transplantation over the next 50 years, but composite tissue transplantation virtually disappeared. This evolution was judged to be a consequence of the greater antigenicity of skin, which that was insurmountable by the available immunosuppression. In the mid-1990s, progress in immunosupression allowed skin-bearing grafts, led by successful hand transplants, which produced a renaissance in composite tissue allotransplantation. Since then, graft types have expanded to over 10, and graft numbers to over 150, with success rates that equal or exceed solid organs. The field has emerged as one of the most exciting in contemporary medicine, although accompanied by substantial challenges and controversy. This paper reviews the origins and progress of this field, assessing its potential for future evolution.

Bone morphogenetic protein-2 stimulation of cartilage regeneration in canine tracheal graft

BACKGROUND

Graft stenosis is among the most serious post-surgical complications that can occur after tracheal transplantation. Typically, stenosis is caused by resorption of tracheal cartilage. Bone morphogenetic protein-2 (BMP-2) is efficient at stimulating bone or cartilage regeneration. In this study, BMP-2 is tested for its effects on stimulation of cartilage regeneration in tracheal transplantation.

METHODS

For tracheal autotransplantation, 24 mongrel dogs were divided equally into four groups and BMP-2 was injected between the cartilage rings at doses of 1, 3, 5 or 7 mg. For tracheal allotransplantation, 12 mongrel dogs were divided equally into two groups. One group received 5 mg of BMP-2 per graft, and the other received collagen only as a control. The grafts were harvested after 4 weeks and subjected to pathologic analysis. The diameter of the graft lumen and areas of new cartilage regeneration were measured.

RESULTS

Regenerated cartilage areas were found in both the injected area and around the perichondrium. The areas of regenerated cartilage, as well as the diameter of the tracheal lumen, increased significantly with increasing concentrations of BMP-2. Five milligrams per milliliter was the most effective dose of BMP-2 in this study.

CONCLUSIONS

BMP-2 can significantly stimulate cartilage regeneration in tracheal grafts and also can be used to prevent stenosis after tracheal transplantation.

Engineered Liver-Like Tissue on a Capillarized Matrix for Applied Research

Liver tissue that is functional and viable for several weeks in vitro represents an auspicious test system for basic and applied research. In this study, a coculture system for hepatocytes (HCs) and microvascular endothelial cells (mECs) was generated applying tissue-engineering techniques, establishing the basis for a new bioartificial liver in vitro model. Porcine mECs were seeded on a decellularized porcine jejunal segment with preserved vascular structures. Porcine HCs were seeded onto this vascularized scaffold, and the resulting coculture was maintained for 3 weeks in vitro. Tissue morphology and differentiation was monitored using histology and immunohistochemistry. Tissue metabolism was monitored using daily assessment of urea and lactate production. HC monolayer cultures served as controls. The 2-stage seeding procedure resulted in a 3-dimensional coculture system harboring HC cell clusters in multiple cell layers lining the generated mEC-seeded capillary structures. It was viable for 3 weeks, and HCs maintained their morphology and differentiation. Biochemical testing revealed stable metabolic activity of the tissue culture. In contrast, HCs cultured in monolayer showed morphological dedifferentiation and an unfavorable metabolic state. Our mEC-HC coculture represents a new approach toward a functional bioartificial liver-like tissue applicable as a test system for basic and applied research.

Allogreffes de langue, de trachée, de paroi abdominale, d'utérus, de pénis. Précisions sur d'autres applications de l'allotransplantation de tissus composites vascularisés chez l'homme

The first hand and face allografts opened a new era in medicine history: a time when allotransplantation and reconstructive surgery coupled their principles. Their success and their development made composite tissue allotransplantation (CTA) a clinical reality for our speciality. Although still recent and limited, experience from this new surgical practice will widen with feedback from the first clinical cases and with experience gained from more clinical cases, more anatomical areas, more type of allografts, more surgical techniques, more immunosuppressive regimens. Tongue, trachea, abdominal wall, uterus, penis allotransplantations have been performed, contemporarily. Whatever the future and the benefits for the selected patients might have been, reports from these - un- and misknown - cases contribute to a better knowledge of CTA, its therapeutic potential, its limits, its challenges.

[Scared atrophy of the cervical part of trachea as an effect of prolongated intubation]

Tracheal intubation is presently one of the basic medical procedures. It is connected with many different complications. One of them is tracheal stenosis, which occurs in 6-21 percent of patients after intubation of the trachea. In contrast to this high frequency of tracheal stenosis we didn't find any publications about complete atrophy of a big part of trachea after prolongated intubation and we describe a first case of such complication. The reasons and the possibilities of treatment in such situation are discussed.

Tracheal replacements: part 1

In this review, we summarize the history of tracheal reconstruction and replacement as well as progress in current tracheal substitutes. In Part 1, we cover the historical highlights of grafts, flaps, tube construction, and tissue transplants and address the progress made in tracheal stenting as a means of temporary tracheal support. This is followed in Part 2 by an analysis of solid and porous tracheal prostheses in experimental and clinical trials. We conclude Part 2 with a summary of recent efforts toward generating a bioengineered trachea. Finally, we provide an algorithm on the spectrum of options available for tracheal replacement.

The Potential of Bioartificial Tissues in Oncology Research and Treatment

This review article addresses the relevance and potential of bioartificial tissues in oncologic research and therapy and reconstructive oncologic surgery. In order to translate the findings from basic cellular research into clinical applications, cell-based models need to recapitulate both the 3D organization and multicellular complexity of an organ but at the same time accommodate systematic experimental intervention. Here, tissue engineering, the generation of human tissues and organs in vitro, provides new perspectives for basic and applied research by offering 3D tissue cultures resolving fundamental obstacles encountered in currently applied 2D and 3D cell culture systems. Tissue engineering has already been applied to create replacement structures for reconstructive surgery. Applied in vitro, these complex multicellular 3D tissue cultures mimic the microenvironment of human tissues. In contrast to the currently available cell culture systems providing only limited insight into the complex interactions in tissue differentiation, carcinogenesis, angiogenesis and the stromal reaction, the more realistic (micro)environment afforded by the bioartificial tissuespecific 3D test systems may accelerate the progress in design and development of cancer therapies.