Effect of cell seeding on neotissue formation in a tissue engineered trachea

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.

A facile 3D bio-fabrication of customized tubular scaffolds using solvent-based extrusion printing for tissue-engineered tracheal grafts

Tracheal implantation remains a major therapeutic challenge due to the unavailability of donors and the lack of biomimetic tubular grafts. Fabrication of biomimetic tracheal scaffolds of suitable materials with matched rigidity, enhanced flexibility and biocompatibility has been a major challenge in the field of tracheal reconstruction. In this study, customized tubular grafts made up of FDA-approved polycaprolactone ( PCL ) and polyurethane ( PU ) were fabricated using a novel solvent-based extrusion 3D printing. The printed scaffolds were investigated by various physical, thermal, and mechanical characterizations such as contact angle measurement, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), radial compression, longitudinal compression, and cyclic radial compression. In this study, the native goat trachea was used as a reference for the fabrication of different types of scaffolds (cylindrical, bellow-shaped, and spiral-shaped). The mechanical properties of the goat trachea were also compared to find suitable formulations of PCL / PU . Spiral-shaped scaffolds were found to be an ideal shape based on longitudinal compression and torsion load maintaining clear patency. To check the long-term implantation, in vitro degradation test was performed for all the 3D printed scaffolds and it was found that blending of PU with PCL reduced the degradation behavior. The printed scaffolds were further evaluated for biocompatibility assay, live/dead assay, and cell adhesion assay using bone marrow-derived human mesenchymal stem cells (hMSCs). From biomechanical and biological assessments, PCL 70 / PU 30 of spiral-shaped scaffolds could be a suitable candidate for the development of tracheal regenerative applications.

Isolation of multipotent progenitor cells from pleura and pericardium for tracheal tissue engineering purposes

Tissue engineering (TE) of long tracheal segments is conceptually appealing for patients with inoperable tracheal pathology. In tracheal TE, stem cells isolated from bone marrow or adipose tissue have been employed, but the ideal cell source has yet to be determined. When considering the origin of stem cells, cells isolated from a source embryonically related to the trachea may be more similar. In this study, we investigated the feasibility of isolating progenitor cells from pleura and pericard as an alternative cells source for tracheal tissue engineering. Porcine progenitor cells were isolated from pleura, pericard, trachea and adipose tissue and expanded in culture. Isolated cells were characterized by PCR, RNA sequencing, differentiation assays and cell survival assays and were compared to trachea and adipose‐derived progenitor cells. Progenitor‐like cells were successfully isolated and expanded from pericard and pleura as indicated by gene expression and functional analyses. Gene expression analysis and RNA sequencing showed a stem cell signature indicating multipotency, albeit that subtle differences between different cell sources were visible. Functional analysis revealed that these cells were able to differentiate towards chondrogenic, osteogenic and adipogenic lineages. Isolation of progenitor cells from pericard and pleura with stem cell features is feasible. Although functional differences with adipose‐derived stem cells were limited, based on their gene expression, pericard‐ and pleura‐derived stem cells may represent a superior autologous cell source for cell seeding in tracheal tissue engineering.

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.

Applications of Electrospinning for Tissue Engineering in Otolaryngology

OBJECTIVE

In tissue engineering, biomaterials create a 3D scaffold for cell-to-cell adhesion, proliferation and tissue formation. Because of their similarity to extracellular matrix and architectural adaptability, nanofibers are of particular interest in tissue engineering. Electrospinning is a well-documented technique for nanofiber production for tissue engineering scaffolds. Here we present literature on the applications of electrospinning in the field of otolaryngology.

REVIEW METHODS

A PubMed database search was performed to isolate articles published about applications of electrospun nanofibers for tissue engineering in otolaryngology. Study design, size, material tested, site of application within the head and neck, and outcomes were obtained for each study.

RESULTS

Almost all data on electrospinning in otolaryngology was published in the last 6 years (84%), highlighting its novelty. A total of 25 pre-clinical studies were identified: 9 in vitro studies, 5 in vivo animal studies, and 11 combination studies. Sites of application included: tracheal reconstruction (n = 16), tympanic membrane repair (n = 3), cranial nerve regeneration (n = 3), mastoid osteogenesis (n = 1) and ear/nose chondrogenesis (n = 2).

IMPLICATIONS FOR PRACTICE

Tissue engineering is a burgeoning field, with recent innovative applications in the field of otolaryngology. Electrospun nanofibers specifically have relevant applications in the field of otolaryngology, due in part to their similarity to native extracellular matrix, with emerging areas of interest being tympanic membrane repair, cranial nerve regeneration and tracheal reconstruction.

Tissue engineering applications in otolaryngology-The state of translation

While tissue engineering holds significant potential to address current limitations in reconstructive surgery of the head and neck, few constructs have made their way into routine clinical use. In this review, we aim to appraise the state of head and neck tissue engineering over the past five years, with a specific focus on otologic, nasal, craniofacial bone, and laryngotracheal applications. A comprehensive scoping search of the PubMed database was performed and over 2000 article hits were returned with 290 articles included in the final review. These publications have addressed the hallmark characteristics of tissue engineering (cellular source, scaffold, and growth signaling) for head and neck anatomical sites. While there have been promising reports of effective tissue engineered interventions in small groups of human patients, the majority of research remains constrained to in vitro and in vivo studies aimed at furthering the understanding of the biological processes involved in tissue engineering. Further, differences in functional and cosmetic properties of the ear, nose, airway, and craniofacial bone affect the emphasis of investigation at each site. While otolaryngologists currently play a role in tissue engineering translational research, continued multidisciplinary efforts will likely be required to push the state of translation towards tissue-engineered constructs available for routine clinical use.

LEVEL OF EVIDENCE

NA.

Biomedical grafts for tracheal tissue repairing and regeneration "Tracheal tissue engineering: an overview"

Airway system is a vital part of the living being body. Trachea is the upper respiratory portion that connects nostril and lungs and has multiple functions such as breathing and entrapment of dust/pathogen particles. Tracheal reconstruction by artificial prosthesis, stents, and grafts are performed clinically for the repairing of damaged tissue. Although these (above-mentioned) methods repair the damaged parts, they have limited applicability like small area wounds and lack of functional tissue regeneration. Tissue engineering helps to overcome the above-mentioned problems by modifying the traditional used stents and grafts, not only repair but also regenerate the damaged area to functional tissue. Bioengineered tracheal replacements are biocompatible, nontoxic, porous, and having 3D biomimetic ultrastructure with good mechanical strength, which results in faster and better tissue regeneration. Till date, the bioengineered tracheal replacements studies have been going on preclinical and clinical levels. Besides that, still many researchers are working at advance level to make extracellular matrix-based acellular, 3D printed, cell-seeded grafts including living cells to overcome the demand of tissue or organ and making the ready to use tracheal reconstructs for clinical application. Thus, in this review, we summarized the tracheal tissue engineering aspects and their outcomes.

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

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

Electrospun scaffolds limit the regenerative potential of the airway epithelium

Objective

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

Study Design

Experimental.

Methods

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

Results

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

Conclusions

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

Level of Evidence

NA

Trachea Engineering Using a Centrifugation Method and Mouse-Induced Pluripotent Stem Cells

The outcomes of tracheal transplantation for the treatment of airway stenosis are unsatisfactory. We investigated the feasibility of regeneration of the trachea using a rat decellularized tracheal scaffold and mouse-induced pluripotent stem (iPS) cells for in vivo transplantation. The rat trachea was first decellularized using a detergent/enzymatic treatment method. We successfully established a centrifugation method that can transplant cells onto the luminal surface of the decellularized rat tracheal scaffold circumferentially. Two types of mouse iPS cells were differentiated into definitive endoderm cells and transplanted onto the luminal surface of the decellularized tracheal matrix scaffold using this centrifugation method. For in vivo study, normal rat tracheas, no-cell rat tracheal scaffolds, or rat tracheal scaffolds recellularized with rat tracheal epithelial cells (EGV-4T) were orthotopically transplanted on F344 rats, and rat tracheal scaffolds recellularized with mouse iPS cells were transplanted on F344/NJc1-rnu/rnu rats. Rats transplanted with no-cell scaffolds or scaffolds recellularized with EGV-4T survived for 1 month, although airway stenosis was observed. One of the F344/NJc1-rnu/rnu rats transplanted with rat trachea regenerated using mouse iPS cells survived over 5 weeks. Histological analysis indicated the cause of death was airway stenosis due to colonic cellular proliferation of undifferentiated iPS cells. Re-epithelialization with numerous ciliated epithelial cells was observed in one of the rats transplanted with trachea bioengineered using iPS cells. In this study, we present a simple and efficient tracheal tissue engineering model using a centrifugation method in a small-animal model. Tissue-engineered trachea using decellularized tracheal scaffolds and iPS cells is potentially applicable for tracheal transplantation.

Factors Influencing Poor Outcomes in Synthetic Tissue-Engineered Tracheal Replacement

OBJECTIVES

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

STUDY DESIGN

Large animal model.

SETTING

Pediatric academic research institute.

SUBJECTS AND METHODS

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

RESULTS

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

CONCLUSIONS

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

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

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

Mouse Model of Tracheal Replacement With Electrospun Nanofiber Scaffolds

OBJECTIVES

The clinical experience with tissue-engineered tracheal grafts (TETGs) has been fraught with graft stenosis and delayed epithelialization. A mouse model of orthotopic replacement that recapitulates the clinical findings would facilitate the study of the cellular and molecular mechanisms underlying graft stenosis.

METHODS

Electrospun nanofiber tracheal scaffolds were created using nonresorbable (polyethylene terephthalate + polyurethane) and co-electrospun resorbable (polylactide-co-caprolactone/polyglycolic acid) polymers (n = 10/group). Biomechanical testing was performed to compare load displacement of nanofiber scaffolds to native mouse tracheas. Mice underwent orthotopic tracheal replacement with syngeneic grafts (n = 5) and nonresorbable (n = 10) and resorbable (n = 10) scaffolds. Tissue at the anastomosis was evaluated using hematoxylin and eosin (H&E), K5+ basal cells were evaluated with the help of immunofluorescence testing, and cellular infiltration of the scaffold was quantified. Micro computed tomography was performed to assess graft patency and correlate radiographic and histologic findings with respiratory symptoms.

RESULTS

Synthetic scaffolds were supraphysiologic in compression tests compared to native mouse trachea ( P < .0001). Nonresorbable scaffolds were stiffer than resorbable scaffolds ( P = .0004). Eighty percent of syngeneic recipients survived to the study endpoint of 60 days postoperatively. Mean survival with nonresorbable scaffolds was 11.40 ± 7.31 days and 6.70 ± 3.95 days with resorbable scaffolds ( P = .095). Stenosis manifested with tissue overgrowth in nonresorbable scaffolds and malacia in resorbable scaffolds. Quantification of scaffold cellular infiltration correlated with length of survival in resorbable scaffolds (R² = 0.95, P = .0051). Micro computed tomography demonstrated the development of graft stenosis at the distal anastomosis on day 5 and progressed until euthanasia was performed on day 11.

CONCLUSION

Graft stenosis seen in orthotopic tracheal replacement with synthetic tracheal scaffolds can be modeled in mice. The wide array of lineage tracing and transgenic mouse models available will permit future investigation of the cellular and molecular mechanisms underlying TETG stenosis.

Quantification of tissue-engineered trachea performance with computational fluid dynamics

OBJECTIVES/HYPOTHESIS

Current techniques for airway characterization include endoscopic or radiographic measurements that produce static, two-dimensional descriptions. As pathology can be multilevel, irregularly shaped, and dynamic, minimal luminal area (MLA) may not provide the most comprehensive description or diagnostic metric. Our aim was to examine the utilization of computational fluid dynamics (CFD) for the purpose of defining airway stenosis using an ovine model of tissue-engineered tracheal graft (TETG) implantation.

STUDY DESIGN

Animal research model.

METHODS

TETGs were implanted into sheep, and MLA was quantified with imaging and endoscopic measurements. Graft stenosis was managed with endoscopic dilation and stenting when indicated. Geometries of the TETG were reconstructed from three-dimensional fluoroscopic images. CFD simulations were used to calculate peak flow velocity (PFV) and peak wall shear stress (PWSS). These metrics were compared to values derived from a quantitative respiratory symptom score.

RESULTS

Elevated PFV and PWSS derived from CFD modeling correlated with increased respiratory symptoms. Immediate pre- and postimplantation CFD metrics were similar, and implanted sheep were asymptomatic. Respiratory symptoms improved with stenting, which maintained graft architecture similar to dilation procedures. With stenting, baseline PFV (0.33 m/s) and PWSS (0.006 Pa) were sustained for the remainder of the study. MLA measurements collected via bronchoscopy were also correlated with respiratory symptoms. PFV and PWSS found via CFD were correlated (R²  = 0.92 and 0.99, respectively) with respiratory symptoms compared to MLA (R²  = 0.61).

CONCLUSIONS

CFD is valid for informed interventions based on multilevel, complex airflow and airway characteristics. Furthermore, CFD may be used to evaluate TETG functionality.

LEVEL OF EVIDENCE

NA. Laryngoscope, E272-E279, 2018.

Designing a tissue-engineered tracheal scaffold for preclinical evaluation

OBJECTIVE

Recent efforts to tissue engineer long-segment tracheal grafts have been complicated by stenosis and malacia. It has been proposed that both the mechanical characteristics and cell seeding capacity of TETG scaffolds are integral to graft performance. Our aim was to design a tracheal construct that approximates the biomechanical properties of native sheep trachea and optimizes seeding with bone marrow derived mononuclear cells prior to preclinical evaluation in an ovine model.

METHODS

A solution of 8% polyethylene terephthalate (PET) and 3% polyurethane (PU) was prepared at a ratio of either 8:2 or 2:8 and electrospun onto a custom stainless steel mandrel designed to match the dimensional measurements of the juvenile sheep trachea. 3D-printed porous or solid polycarbonate C-shaped rings were embedded within the scaffolds during electrospinning. The scaffolds underwent compression testing in the anterior-posterior and lateral-medial axes and the biomechanical profiles compared to that of a juvenile ovine trachea. The most biomimetic constructs then underwent vacuum seeding with ovine bone marrow derived mononuclear cells. Fluorometric DNA assay was used to quantify scaffold seeding.

RESULTS

Both porous and solid rings approximated the biomechanics of the native ovine trachea, but the porous rings were most biomimetic. The load-displacement curve of scaffolds fabricated from a ratio of 2:8 PET:PU most closely mimicked that of native trachea in the anterior-posterior and medial-lateral axes. Solid C-ringed scaffolds had a greater cell seeding efficiency when compared to porous ringed scaffolds (Solid: 19 × 10⁴ vs. Porous: 9.6 × 10⁴ cells/mm³, p = 0.0098).

CONCLUSION

A long segment tracheal graft composed of 2:8 PET:PU with solid C-rings approximates the biomechanics of the native ovine trachea and demonstrates superior cell seeding capacity of the two prototypes tested. Further preclinical studies using this graft design in vivo would inform the rational design of an optimal TETG scaffold.

Autologous Cell Seeding in Tracheal Tissue Engineering

Purpose of Review

There is no consensus on the best technology to be employed for tracheal replacement. One particularly promising approach is based upon tissue engineering and involves applying autologous cells to transplantable scaffolds. Here, we present the reported pre-clinical and clinical data exploring the various options for achieving such seeding.

Recent Findings

Various cell combinations, delivery strategies, and outcome measures are described. Mesenchymal stem cells (MSCs) are the most widely employed cell type in tracheal bioengineering. Airway epithelial cell luminal seeding is also widely employed, alone or in combination with other cell types. Combinations have thus far shown the greatest promise. Chondrocytes may improve mechanical outcomes in pre-clinical models, but have not been clinically tested. Rapid or pre-vascularization of scaffolds is an important consideration. Overall, there are few published objective measures of post-seeding cell viability, survival, or overall efficacy.

Summary

There is no clear consensus on the optimal cell-scaffold combination and mechanisms for seeding. Systematic in vivo work is required to assess differences between tracheal grafts seeded with combinations of clinically deliverable cell types using objective outcome measures, including those for functionality and host immune response.

Electronic supplementary material

The online version of this article (10.1007/s40778-017-0108-2) contains supplementary material, which is available to authorized users.

The innate immune response to lower respiratory tract E. Coli infection and the role of the CCL2-CCR2 axis in neonatal mice

Neonates have greater morbidity/mortality from lower respiratory tract infections (LRTI) compared to older children. Lack of conditioning of the pulmonary immune system due to limited environmental exposures and/or infectious challenges likely contributes to the increase susceptibility in the neonate. In this study, we sought to gain insights into the nature and dynamics of the neonatal pulmonary immune response to LRTI using a murine model.

METHODS

Wildtype (WT) and Ccr2^-/- C57BL/6 neonatal and juvenile mice received E. coli or PBS by direct pharyngeal aspiration. Flow cytometry was used to measure immune cell dynamics and identify cytokine-producing cells. Real-time PCR and ELISA were used to measure cytokine/chemokine expression.

RESULTS

Innate immune cell recruitment in response to E. coli-induced LRTI was delayed in the neonatal lung compared to juvenile lung. Lung clearance of bacteria was also significantly delayed in the neonate. Ccr2^-/- neonates, which lack an intact CCL2-CCR2 axis, had higher mortality after E. coli challenged than Ccr2^+/+ neonates. A greater percentage of CD8⁺ T cells and monocytes from WT neonates challenged with E. coli produced TNF compared to controls.

CONCLUSION

The pulmonary immune response to E. coli-induced LRTI differed significantly between neonatal and juvenile mice. Neonates were more susceptible to increasing doses of E. coli and exhibited greater mortality than juveniles. In the absence of an intact CCL2-CCR2 axis, susceptibility to LRTI-induced mortality was further increased in neonatal mice. Taken together these findings underscore the importance of age-related differences in the innate immune response to LRTI during early stages of postnatal life.

Circumferential Three-Dimensional-Printed Tracheal Grafts: Research Model Feasibility and Early Results

BACKGROUND

Methods for tracheal graft research have presented persistent challenges to investigators, and three-dimensional (3D)-printed biosynthetic grafts offer one potential development platform. We aimed to develop an efficient research platform for customizable circumferential 3D-printed tracheal grafts and evaluate feasibility and early structural integrity with a large-animal model.

METHODS

Virtual 3D models of porcine subject tracheas were generated using preoperative computed tomography scans. Two designs were used to test graft customizability and the limits of the construction process. Designs I and II used 270-degree and 360-degree external polycaprolactone scaffolds, respectively, both encompassing a circumferential extracellular matrix collagen layer. The polycaprolactone scaffolds were made in a fused-deposition modeling 3D printer and customized to the recipient's anatomy. Design I was implanted in 3 pigs and design II in 2 pigs, replacing 4-ring tracheal segments. Data collected included details of graft construction, clinical outcomes, bronchoscopy, and gross and histologic examination.

RESULTS

The 3D-printed biosynthetic grafts were produced with high fidelity to the native organ. The fabrication process took 36 hours. Grafts were implanted without immediate complication. Bronchoscopy immediately postoperatively and at 1 week demonstrated patent grafts and appropriate healing. All animals lived beyond a predetermined 1-week survival period. Bronchoscopy at 2 weeks showed significant paraanastomotic granulation tissue, which, along with partial paraanastomotic epithelialization, was confirmed on pathology. Overall survival was 17 to 34 days.

CONCLUSIONS

We propose a rapid, reproducible, resource efficient method to develop various anatomically precise grafts. Further graft refinement and strategies for granulation tissue management are needed to improve outcomes.

Endoscopic management of tissue-engineered tracheal graft stenosis in an ovine model

OBJECTIVE

To evaluate the safety and efficacy of bronchoscopic interventions in the management of tissue-engineered tracheal graft (TETG) stenosis.

STUDY DESIGN

Animal research study.

METHODS

TETGs were constructed with seeded autologous bone marrow-derived mononuclear cells on a bioartificial graft. Eight sheep underwent tracheal resection and orthotopic implantation of this construct. Animals were monitored by bronchoscopy and fluoroscopy at 3 weeks, 6 weeks, 3 months, and 4 months. Bronchoscopic interventions, including dilation and stenting, were performed to manage graft stenosis. Postdilation measurements were obtained endoscopically and fluoroscopically.

RESULTS

Seven dilations were performed in six animals. At the point of maximal stenosis, the lumen measured 44.6 ± 8.4 mm² predilation and 50.7 ± 14.1 postdilation by bronchoscopy (P = 0.3517). By fluoroscopic imaging, the airway was 55.9 ± 12.9 mm² predilation and 65.9 ± 22.4 mm² postdilation (P = 0.1303). Stents were placed 17 times in six animals. Pre- and poststenting lumen sizes were 62.8 ± 38.8 mm² and 80.1 ± 54.5 mm² by bronchoscopy (P = 0.6169) and 77.1 ± 38.9 mm² and 104 ± 60.7 mm² by fluoroscopy (P = 0.0825). Mortality after intervention was 67% with dilation and 0% with stenting (P = 0.0004). The average days between bronchoscopy were 8 ± 2 for the dilation group and 26 ± 17 in the stenting group (P = 0.05). One hundred percent of dilations and 29% of stent placements required urgent follow-up bronchoscopy (P = 0.05).

CONCLUSION

Dilation has limited efficacy for managing TETG stenosis, whereas stenting has a more lasting clinical effect.

LEVEL OF EVIDENCE

NA. Laryngoscope, 127:2219-2224, 2017.

Clinical Translation of Tissue Engineered Trachea Grafts

OBJECTIVE

To provide a state-of-the-art review discussing recent achievements in tissue engineered tracheal reconstruction.

DATA SOURCES AND REVIEW METHODS

A structured PubMed search of the current literature up to and including October 2015. Representative articles that discuss the translation of tissue engineered tracheal grafts (TETG) were reviewed.

CONCLUSIONS

The integration of a biologically compatible support with autologous cells has resulted in successful regeneration of respiratory epithelium, cartilage, and vascularization with graft patency, although the optimal construct composition has yet to be defined. Segmental TETG constructs are more commonly complicated by stenosis and delayed epithelialization when compared to patch tracheoplasty.

IMPLICATIONS FOR PRACTICE

The recent history of human TETG recipients represents revolutionary proof of principle studies in regenerative medicine. Application of TETG remains limited to a compassionate use basis; however, defining the mechanisms of cartilage formation, epithelialization, and refinement of in vivo regeneration will advance the translation of TETG from the bench to the bedside.

Tissue Engineered Airways: A Prospects Article

An ideal tracheal scaffold must withstand luminal collapse yet be flexible, have a sufficient degree of porosity to permit vascular and cellular ingrowth, but also be airtight and must facilitate growth of functional airway epithelium to avoid infection and aid in mucocilliary clearance. Finally, the scaffold must also be biocompatible to avoid implant rejection. Over the last 40 years, efforts to design and manufacture the airway have been undertaken worldwide but success has been limited and far apart. As a result, tracheal resection with primary repair remains the Gold Standard of care for patients presenting with airway disorders and malignancies. However, the maximum resectable length of the trachea is restricted to 30% of the total length in children or 50% in adults. Attempts to provide autologous grafts for human application have also been disappointing for a host of different reasons, including lack of implant integration, insufficient donor organs, and poor mechanical strength resulting in an unmet clinical need. The two main approaches researchers have taken to address this issue have been the development of synthetic scaffolds and the use of decellularized organs. To date, a number of different decellularization techniques and a variety of materials, including polyglycolic acid (PGA) and nanocomposite polymers have been explored. The findings thus far have shown great promise, however, there remain a significant number of caveats accompanying each approach. That being said, the possibilities presented by these two approaches could be combined to produce a highly successful, clinically viable hybrid scaffold. This article aims to highlight advances in airway tissue engineering and provide an overview of areas to explore and utilize in accomplishing the aim of developing an ideal tracheal prosthesis. J. Cell. Biochem. 117: 1497-1505, 2016. © 2016 Wiley Periodicals, Inc.

Improved cellular infiltration in electrospun fiber via engineered porosity

Small pore sizes inherent to electrospun matrices can hinder efficient cellular ingrowth. To facilitate infiltration while retaining its extracellular matrix-like character, electrospinning was combined with salt leaching to produce a scaffold having deliberate, engineered delaminations. We made elegant use of a specific randomizing component of the electrospinning process, the Taylor Cone and the falling fiber beneath it, to produce a uniform, well-spread distribution of salt particles. After 3 weeks of culture, up to 4 mm of cellular infiltration was observed, along with cellular coverage of up to 70% within the delaminations. To our knowledge, this represents the first observation of extensive cellular infiltration of electrospun matrices. Infiltration appears to be driven primarily by localized proliferation rather than coordinated cellular locomotion. Cells also moved from the salt-generated porosity into the surrounding electrospun fiber matrix. Given that the details of salt deposition (amount, size, and number density) are far from optimized, the result provides a convincing illustration of the ability of mammalian cells to interact with appropriately tailored electrospun matrices. These layered structures can be precisely fabricated by varying the deposition interval and particle size conceivably to produce in vivo-like gradients in porosity such that the resulting scaffolds better resemble the desired final structure.