Human airway xenograft models of epithelial cell regeneration

Basic Science Perspective on Engineering and Modeling the Large Airways

The airway epithelium provides a physical and biochemical barrier playing a key role in protecting the lung from infiltration of pathogens and irritants and is, therefore, crucial in maintaining tissue homeostasis and regulating innate immunity. Due to continual inspiration and expiration of air during breathing, the epithelium is exposed to a plethora of environmental insults. When severe or persistent, these insults lead to inflammation and infection. The effectiveness of the epithelium as a barrier is reliant upon its capacity for mucociliary clearance, immune surveillance, and regeneration upon injury. These functions are accomplished by the cells that comprise the airway epithelium and the niche in which they reside. Engineering of new physiological and pathological models of the proximal airways requires the generation of complex structures comprising the surface airway epithelium, submucosal gland epithelium, extracellular matrix, and niche cells, including smooth muscle cells, fibroblasts, and immune cells. This chapter focuses on the structure-function relationships in the airways and the challenges of developing complex engineered models of the human airway.

Pathophysiology of Lung Disease and Wound Repair in Cystic Fibrosis

Cystic fibrosis (CF) is an autosomal recessive, life-threatening condition affecting many organs and tissues, the lung disease being the chief cause of morbidity and mortality. Mutations affecting the CF Transmembrane Conductance Regulator (CFTR) gene determine the expression of a dysfunctional protein that, in turn, triggers a pathophysiological cascade, leading to airway epithelium injury and remodeling. In vitro and in vivo studies point to a dysregulated regeneration and wound repair in CF airways, to be traced back to epithelial CFTR lack/dysfunction. Subsequent altered ion/fluid fluxes and/or signaling result in reduced cell migration and proliferation. Furthermore, the epithelial-mesenchymal transition appears to be partially triggered in CF, contributing to wound closure alteration. Finally, we pose our attention to diverse approaches to tackle this defect, discussing the therapeutic role of protease inhibitors, CFTR modulators and mesenchymal stem cells. Although the pathophysiology of wound repair in CF has been disclosed in some mechanisms, further studies are warranted to understand the cellular and molecular events in more details and to better address therapeutic interventions.

Tissue-engineered trachea: A review

Tracheal replacement is performed after resection of a portion of the trachea that was impossible to reconnect via direct anastomosis. A tissue-engineered trachea is one of the available options that offer many advantages compared to other types of graft. Fabrication of a functional tissue-engineered trachea for grafting is very challenging, as it is a complex organ with important components, including cartilage, epithelium and vasculature. A number of studies have been reported on the preparation of a graftable trachea. A laterally rigid but longitudinally flexible hollow cylindrical scaffold which supports cartilage and epithelial tissue formation is the key element. The scaffold can be prepared via decellularization of an allograft or fabricated using biodegradable or non-biodegradable biomaterials. Commonly, the scaffold is seeded with chondrocytes and epithelial cells at the outer and luminal surfaces, respectively, to hasten tissue formation and improve functionality. To date, several clinical trials of tracheal replacement with tissue-engineered trachea have been performed. This article reviews the formation of cartilage tissue, epithelium and neovascularization of tissue-engineered trachea, together with the obstacles, possible solutions and future. Furthermore, the role of the bioreactor for in vitro tracheal graft formation and recently reported clinical applications of tracheal graft were also discussed. Generally, although encouraging results have been achieved, however, some obstacles remain to be resolved before the tissue-engineered trachea can be widely used in clinical settings.

Treatment with placenta-derived mesenchymal stem cells mitigates development of bronchiolitis obliterans in a murine model

OBJECTIVE

Bone marrow-derived mesenchymal stem cells (MSCs) have shown therapeutic potential in acute lung injury. Recently, placenta-derived human mesenchymal stem cells (PMSCs) have shown similarities with bone marrow-derived MSCs in terms of regenerative capabilities and immunogenicity. This study investigates the hypothesis that treatment with PMSCs reduces the development of bronchiolitis obliterans in a murine heterotopic tracheal transplant model.

METHODS

A murine heterotopic tracheal transplant model was used to study the continuum from acute to chronic rejection. In the treatment groups, PMSCs or PMSC-conditioned medium (PMSCCM) were injected either locally or intratracheally into the allograft. Phosphate-buffered saline (PBS) or blank medium was injected in the control groups. Tracheal luminal obliteration was assessed on sections stained with hematoxylin and eosin. Infiltration of inflammatory and immune cells and epithelial progenitor cells was assessed using immunohistochemistry and densitometric analysis.

RESULTS

Compared with injection of PBS, local injection of PMSCs significantly reduced luminal obliteration at 28 days after transplantation (P = .015). Intratracheal injection of PMSCs showed similar results to local injection of PMSCs compared with injection of PBS and blank medium (P = .022). Tracheas treated with PMSC/PMSCCM showed protection against the loss of epithelium on day 14, with an increase in P63+CK14+ epithelial progenitor cells and Foxp3+ regulatory T cells. In addition, injection of PMSCs and PMSCCM significantly reduced the number of neutrophils and CD3+ T cells on day 14.

CONCLUSIONS

This study demonstrates that treatment with PMSCs is protective against the development of bronchiolitis obliterans in an heterotopic tracheal transplant model. These results indicate that PMSCs could provide a novel therapeutic option to reduce chronic rejection after lung transplant.

Prevention of the second stage of epithelial loss is a potential novel treatment for bronchiolitis obliterans

OBJECTIVES

Loss of epithelial cells is one of the key factors that lead to airway fibrosis. Loss of epithelial cells may decrease the barrier to host cell infiltration into the lumen, allowing deposition of extracellular matrix, with subsequent obliteration of the airway. The objective of this study was to determine whether injection of epithelial cells/progenitor cells from the recipient into the lumen of the donor trachea could prevent bronchiolitis obliterans (BO) in a mouse heterotopic tracheal transplantation (HTT) model.

METHODS

A major histocompatibility complex class I and class II mismatch of mouse HTT model of BO was used. Epithelial cells from recipient mice were isolated and reinjected into the lumen of the allografts on day 3 after transplantation. Rag-1 knock-out and isografts were also performed as controls. The grafts were analyzed by immunohistochemistry and densitometric analysis.

RESULTS

The results demonstrated that tracheal epithelium was lost by day 3, regenerated between 3 to 7 days, and was lost again in all allografts, but not in the isografts or in Rag-1 knock-out groups by day 12. The reconstituted epithelium was donor originated on day 7 based on green fluorescent protein staining. Furthermore, with the injection of recipient cells into the tracheal lumen, loss of the epithelium was not observed and the luminal obliteration was significantly less in the allografts.

CONCLUSIONS

Injection of recipient epithelial cells prevents the second phase of epithelial loss and significantly decreases BO development in an HTT model. Clinically, the use of injected recipient epithelial cells could be a novel treatment for BO.

Human IL-3/GM-CSF knock-in mice support human alveolar macrophage development and human immune responses in the lung

Mice with a functional human immune system have the potential to allow in vivo studies of human infectious diseases and to enable vaccine testing. To this end, mice need to fully support the development of human immune cells, allow infection with human pathogens, and be capable of mounting effective human immune responses. A major limitation of humanized mice is the poor development and function of human myeloid cells and the absence of human immune responses at mucosal surfaces, such as the lung. To overcome this, we generated human IL-3/GM-CSF knock-in (hIL-3/GM-CSF KI) mice. These mice faithfully expressed human GM-CSF and IL-3 and developed pulmonary alveolar proteinosis because of elimination of mouse GM-CSF. We demonstrate that hIL-3/GM-CSF KI mice engrafted with human CD34(+) hematopoietic cells had improved human myeloid cell reconstitution in the lung. In particular, hIL-3/GM-CSF KI mice supported the development of human alveolar macrophages that partially rescued the pulmonary alveolar proteinosis syndrome. Moreover, human alveolar macrophages mounted correlates of a human innate immune response against influenza virus. The hIL-3/GM-CSF KI mice represent a unique mouse model that permits the study of human mucosal immune responses to lung pathogens.

Endogenous and exogenous stem cells: a role in lung repair and use in airway tissue engineering and transplantation

Rapid repair of the denuded alveolar surface after injury is a key to survival. The respiratory tract contains several sources of endogenous adult stem cells residing within the basal layer of the upper airways, within or near pulmonary neuroendocrine cell rests, at the bronchoalveolar junction, and within the alveolar epithelial surface, which contribute to the repair of the airway wall. Bone marrow-derived adult mesenchymal stem cells circulating in blood are also involved in tracheal regeneration. However, an organism is frequently incapable of repairing serious damage and defects of the respiratory tract resulting from acute trauma, lung cancers, and chronic pulmonary and airway diseases. Therefore, replacement of the tracheal tissue should be urgently considered. The shortage of donor trachea remains a major obstacle in tracheal transplantation. However, implementation of tissue engineering and stem cell therapy-based approaches helps to successfully solve this problem. To date, huge progress has been achieved in tracheal bioengineering. Several sources of stem cells have been used for transplantation and airway reconstitution in animal models with experimentally induced tracheal defects. Most tracheal tissue engineering approaches use biodegradable three-dimensional scaffolds, which are important for neotracheal formation by promoting cell attachment, cell redifferentiation, and production of the extracellular matrix. The advances in tracheal bioengineering recently resulted in successful transplantation of the world's first bioengineered trachea. Current trends in tracheal transplantation include the use of autologous cells, development of bioactive cell-free scaffolds capable of supporting activation and differentiation of host stem cells on the site of injury, with a future perspective of using human native sites as micro-niche for potentiation of the human body's site-specific response by sequential adding, boosting, permissive, and recruitment impulses.

Differentiation of xenografted human fetal lung parenchyma

The goal of this study was to characterize xenografted human fetal lung tissue with respect to developmental stage-specific cytodifferentiation. Human fetal lung tissue (pseudoglandular stage) was grafted either beneath the renal capsule or the skin of athymic mice (NCr-nu). Tissues were analyzed from 3 to 42 days post-engraftment for morphological alterations by light and electron microscopy (EM), and for surfactant protein mRNA and protein by reverse transcription-polymerase chain reaction (RT-PCR) and immunocytochemistry (ICC), respectively. The changes observed resemble those seen in human lung development in utero in many respects, including the differentiation of epithelium to the saccular stage. Each stage occurred over approximately one week in the graft in contrast to the eight weeks of normal in utero development. At all time points examined, all four surfactant proteins (SP-A, SP-B, SP-C, and SP-D) were detected in the epithelium by ICC. Lamellar bodies were first identified by EM in 14 day xenografts. By day 21, a significant increase in lamellar body expression was observed. Cellular proliferation, as marked by PCNA ICC and elastic fiber deposition resembled those of canalicular and saccular in utero development. This model in which xenografted lung tissue in different stages of development is available may facilitate the study of human fetal lung development and the impact of various pharmacological agents on this process.

Clinical trials in cystic fibrosis

In patients with cystic fibrosis (CF), clinical trials are of paramount importance. Here, the current status of drug development in CF is discussed and future directions highlighted. Methods for pre-clinical testing of drugs with potential activity in CF patients including relevant animal models are described. Study design options for phase II and phase III studies involving CF patients are provided, including required patient numbers, safety issues and surrogate end point parameters for drugs, tested for different disease manifestations. Finally, regulatory issues for licensing new therapies for CF patients are discussed, including new directives of the European Union and the structure of a European clinical trial network for clinical studies involving CF patients is proposed.

[Injury and epithelial wound healing: a pathophysiologic hypothesis for nasal and sinus polyposis]

Nasal polyposis (NP), asthma, and chronic bronchitis are chronic inflammatory diseases of the upper airways. They may be caused by injury to the respiratory epithelium in a chronic inflammatory environment. Several studies show that during NP nasal epithelial cells are involved in the overexpression of cytokines and growth factors. Among these, transforming growth factor beta1 (TGF-beta1) appears to play a major role in the genesis of NP. Differentiated respiratory epithelium, obtained from in vivo or in vitro models, is used to study wound healing in inflammatory environments, to elucidate the pathophysiology of NP, and to improve understanding and management of upper airway inflammatory diseases.

Responses of the human airway epithelium transcriptome to in vivo injury

To identify genes participating in human airway epithelial repair, we used bronchoscopy and brushing to denude the airway epithelium of healthy individuals, sequentially sampled the same region 7 and 14 days later, and assessed gene expression by Affymetrix microarrays with TaqMan RT-PCR confirmation. Histologically, the injured area was completely covered by a partially redifferentiated epithelial layer after 7 days; by 14 days the airway epithelium was very similar to the uninjured state. At day 7 compared with resting epithelium, there were substantial differences in gene expression pattern, with a distinctive airway epithelial "repair transcriptome" of actively proliferating cells in the process of redifferentiation. The repair transcriptome at 7 days was dominated by cell cycle, signal transduction, metabolism and transport, and transcription genes. Interestingly, the majority of differentially expressed cell cycle genes belonged to the G2 and M phases, suggesting that the proliferating cells were relatively synchronized 1 wk following injury. At 14 days postinjury, the expression profile was similar to that of resting airway epithelium. These observations provide a baseline of the functional gene categories participating in the process of normal human airway epithelial repair that can be used in future studies of injury and repair in airway epithelial diseases.

Régénération de l′épithélium respiratoire lésé

The airway surface epithelium is frequently injured by microorganisms and viruses due to its permanent contact with the external medium. Following injury, the epithelium is able to repair itself and regenerate through several mechanisms including spreading and migration of basal cells, cell proliferation and differentiation. The cellular and molecular factors involved in wound repair and epithelial regeneration interact closely, implying the participation of cytoskeleton proteins and integrins receptors, matrix metalloproteinases and their inhibitors as well as cytokines and growth factors secreted by airway epithelial and mesenchymal cells. The spatio-temporal modulation of the pro-inflammatory cytokines such as IL-8, and MMPs (MMP-9 and -7) during the different steps of regeneration suggests that the matrix and secretory environment are markedly involved in these mechanisms and that their dysregulation may induce remodeling of the airway mucosa. A better knowledge of the mechanisms involved in airway epithelium regeneration may pave the way to regenerative therapeutics allowing the reconstitution of a functional airway mucosa in numerous respiratory diseases such as cystic fibrosis, chronic obstructive pulmonary diseases and bronchiolitis.

Pre-clinical and clinical endpoint assays for cystic fibrosis gene therapy

The credibility and hence value of pre-clinical and clinical cystic fibrosis gene therapy studies depend on the assays used to evaluate gene transfer. Awareness of assay suitability, sensitivity and variability is therefore crucial to the design of experimental programmes. Here, we review the assays that are in use to assess the efficacy of gene transfer in pre-clinical and clinical CF gene therapy research, highlight their weaknesses and suggest possible new strategies that may help to overcome current limitations.

Lentivirus-mediated gene transfer to the respiratory epithelium: a promising approach to gene therapy of cystic fibrosis

Gene therapy of cystic fibrosis (CF) lung disease needs highly efficient delivery and long-lasting complementation of the CFTR (cystic fibrosis transmembrane conductance regulator) gene into the respiratory epithelium. The development of lentiviral vectors has been a recent advance in the field of gene transfer and therapy. These integrating vectors appear to be promising vehicles for gene delivery into respiratory epithelial cells by virtue of their ability to infect nondividing cells and mediate long-term persistence of transgene expression. Studies in human airway tissues and animal models have highlighted the possibility of achieving gene expression by lentiviral vectors, which outlasted the normal lifespan of the respiratory epithelium, indicating targeting of a 'stem cell' compartment. Modification of the paracellular permeability and pseudotyping with heterologous envelopes are the strategies currently used to overcome the paucity of specific viral receptors on the apical surface of airway epithelial cells and to reach the basolateral surface receptors. Preclinical studies on CF mice, demonstrating complementation of the CF defect, offer hope that lentivirus gene therapy can be translated into an effective treatment of CF lung disease. Besides a direct targeting of the stem/progenitor niche(s) in the CF airways, an alternative approach may envision homing of hematopoietic stem cells engineered to express the CFTR gene by lentiviral vectors. In the context of lentivirus-mediated CFTR gene transfer to the CF airways, biosafety aspects should be of primary concern.

Responses of well-differentiated airway epithelial cell cultures from healthy donors and patients with cystic fibrosis to Burkholderia cenocepacia infection

Well-differentiated cultures established from airway epithelia of patients with cystic fibrosis (CF cultures) exhibited goblet cell hyperplasia, increased secretion of mucus, and higher basal levels of interleukin-8 than similarly cultured cells from healthy donors. Upon apical infection with low doses (10(4) to 10(5) CFU) of Burkholderia cenocepacia isolate BC7, the two cultures gave different responses. While normal cultures trapped the added bacteria in the mucus layer, killed and/or inhibited bacterial replication, and prevented bacterial invasion of the cells, CF cultures failed to kill and/or supported the growth of bacteria, leading to invasion of underlying epithelial cells, compromised transepithelial permeability, and cell damage. Depletion of the surface mucus layer prior to bacterial infection rendered the normal cultures susceptible to bacterial invasion, but the invading bacteria were mainly confined to vacuoles within the cells and appeared to be nonviable. In contrast, bacteria that invaded cells in CF cultures were found free in the cytoplasm surrounded by intermediate filaments and also between cells. Cultured CF airway epithelium was therefore more susceptible to infection than normal epithelium. This mimics CF tissue in vivo and illustrates differences in the way epithelia in CF patients and normal subjects handle bacterial infection. In addition, we found that the CF and normal cell cultures responded differently not only to isolate BC7 but also to isolates of other B. cepacia complex species. We therefore conclude that this cell culture model is suitable for investigation of B. cepacia complex pathogenesis in CF patients.

Breaking the species barrier: use of SCID mouse-human chimeras for the study of human infectious diseases

Mouse-human chimeras have become a novel way to model the interactions between microbial pathogens and human cells, tissues or organs. Diseases studied with human xenografts in severe combined immunodeficient (SCID) mice include Pseudomonas aeruginosa infection in cystic fibrosis, group A streptococci and impetigo, bacillary and amoebic dysentery, and AIDS. In many cases, disease in the human xenograft appears to accurately reproduce the disease in humans, providing a powerful model for identifying virulence factors, host responses to infection and the effects of specific interventions on disease. In this review, we summarize recent studies that have used mouse-human chimeras to understand the pathophysiology of specific bacterial and protozoan infections.

Targeted delivery of antiprotease to the epithelial surface of human tracheal xenografts

The cystic fibrosis (CF) lung is uniquely susceptible to Pseudomonas aeruginosa, and infection with this organism incites an intense, compartmentalized inflammatory response that leads to chronic airway obstruction and bronchiectasis. Neutrophils migrate into the airway, and released neutrophil elastase contributes to the progression of the lung disease characteristic of CF. We have developed a strategy that permits the delivery of antiproteases to the inaccessible CF airways by targeting the respiratory epithelium via the human polymeric immunoglobulin receptor (hpIgR). A fusion protein consisting of a single-chain Fv directed against secretory component, the extracellular portion of the pIgR, linked to human alpha1-antitrypsin is effectively ferried across human tracheal xenografts and delivers the antiprotease to the apical surface to a much greater extent than occurs by passive diffusion of human alpha1-antitrypsin alone. Targeted antiprotease delivery paralleled hpIgR expression in the respiratory epithelium in vivo and was not increased by escalating dose, so airway penetration was receptor-dependent, not dose-dependent. Thus, this approach provides us with the ability to deliver therapeutics, like antiproteases, specifically to the lumenal surface of the respiratory epithelium, within the airway surface fluid, where it will be in highest concentration at this site.