Airway wall remodeling is a hallmark of asthma, characterized by subepithelial thickening and extracellular matrix (ECM) remodeling. Mechanical stress due to hyperresponsive smooth muscle cells may contribute to this remodeling, but its relevance in a threedimensional environment (where the ECM plays an important role in modulating stresses felt by cells) is unclear. To characterize the effects of dynamic compression in ECM remodeling in a physiologically relevant three-dimensional environment, a tissue-engineered human airway wall model with differentiated bronchial epithelial cells atop a collagen gel containing lung fibroblasts was used. Lateral compressive strain of 10 or 30% at 1 or 60 cycles per hour was applied using a novel straining device. ECM remodeling was assessed by immunohistochemistry and zymography. Dynamic strain, particularly at the lower magnitude, induced airway wall remodeling, as indicated by increased deposition of types III and IV collagen and increased secretion of matrix metalloproteinase-2 and -9. These changes paralleled increased myofibroblast differentiation and were fibroblast-dependent. Furthermore, the spatial pattern of type III collagen deposition correlated with that of myofibroblasts; both were concentrated near the epithelium and decreased diffusely away from the surface, indicating some epithelial control of the remodeling response. Thus, in a physiologically relevant three-dimensional model of the bronchial wall, dynamic compressive strain induced tissue remodeling that mimics many features of remodeling seen in asthma, in the absence of inflammation and dependent on epithelial-fibroblast signaling.Keywords: asthma; collagen; fibrosis; in vitro; myofibroblast Remodeling of the airway wall in asthma is characterized by thickening of the subepithelial lamina reticularis, increase in smooth muscle mass, fibroblast hyperplasia, mucus hypersecretion, edema, and angiogenesis (1-6). The subepithelial fibrosis that is often part of this remodeling is further characterized by the differentiation of fibroblasts to myofibroblasts, which are more contractile than fibroblasts and are likely to contribute to the alignment and stiffening of the airway matrix by secretion of types I, III, and V collagen (7-14). In addition, matrix metalloproteinases (MMPs) and their inhibitors (tissue inhibitors of metalloproteinases [TIMPs]), particularly MMP-2 and -9 and TIMP-1, are also increased in the airways in asthma, as they are key players in extracellular matrix (ECM) remodeling (15, 16). Evidence from clinical studies supports the concept that airway remodeling contributes to irreversible airflow obstruction and loss of pulmonary function over time in patients with chronic asthma (17, 18).The complex interplay of factors contributing to airway wall remodeling, which includes inflammatory and mechanical factors, is not well understood. Immune cells recruited to the inflamed airways release cytokines such as RANTES (CCL5), IL-13, TGF-␤ 1 , and cysteinyl leukotrienes C4, D4, and E4 (19-21) that c...

Recent studies have shown that mechanical forces on airway epithelial cells can induce upregulation of genes involved in airway remodeling in diseases such as asthma. However, the relevance of these responses to airway wall remodeling is still unclear since 1). mechanotransduction is highly dependent on environment (e.g., matrix and other cell types) and 2). inflammatory mediators, which strongly affect remodeling, are also present in asthma. To assess the effects of mechanical forces on the airway wall in a relevant three-dimensional inflammatory context, we have established a tissue culture model of the human airway wall that can be induced to undergo matrix remodeling. Our model contains differentiated human bronchial epithelial cells characterized by tight junctions, cilia formation, and mucus secretion atop a collagen gel embedded with human lung fibroblasts. We found that addition of activated eosinophils and the application of 50% strain to the same system increased the epithelial thickness compared with either condition alone, suggesting that mechanical strain affects airway wall remodeling synergistically with inflammation. This integrated model more closely mimics airway wall remodeling than single-cell, conditioned media, or even two-dimensional coculture systems and is relevant for examining the importance of mechanical strain on airway wall remodeling in an inflammatory environment, which may be crucial for understanding and treating pathologies such as asthma.

This protocol describes the setup, maintenance and characteristics of a tissue-engineered model of the human bronchial mucosa that can be used for basic physiology and pathophysiology studies. The model includes a well-differentiated epithelium with functional cilia, mucus secretion and subepithelial fibroblasts within type I collagen. The tissue is created within porous polymeric wells to prevent gel contraction and allow culture at the air-liquid interface. It requires at least 2 wk to be established and can be maintained thereafter for over 4 wk, with tissue differentiation moving towards a more physiologically relevant phenotype with increasing time in culture. Over time, the extracellular matrix also remodels, depositing proteins such as types III and IV collagen and fibronectin. Because it recapitulates many key anatomical and functional features of the airway wall, this model is well suited for a wide range of studies, including those on airway remodeling, transepithelial transport and inflammatory cell interactions with the mucosa. The entire protocol takes 4-6 wk, including cell expansion, depending on the extent of ciliogenesis desired.

Tomei AA, Choe MM, Swartz MA. Effects of dynamic compression on lentiviral transduction in an in vitro airway wall model. Am J Physiol Lung Cell Mol Physiol 294: L79-L86, 2008. First published November 16, 2007 doi:10.1152/ajplung.00062.2007.-Asthmatic patients are more susceptible to viral infection, and we asked whether dynamic strain on the airway wall (such as that associated with bronchoconstriction) would influence the rate of viral infection of the epithelial and subepithelial cells. To address this, we characterized the barrier function of a three-dimensional culture model of the bronchial airway wall mucosa, modified the culture conditions for optimization of ciliogenesis, and compared epithelial and subepithelial green fluorescent protein (GFP) transduction by a pWpts-GFP lentivirus, pseudotyped with VSV-G, under static vs. dynamic conditions. The model consisted of human lung fibroblasts, bronchial epithelial cells, and a type I collagen matrix, and after 21 days of culture at air liquid interface, it exhibited a pseudostratified epithelium comprised of basal cells, mucus-secreting cells, and ciliated columnar cells with beating cilia. Microparticle tracking revealed partial coordination of mucociliary transport among groups of cells. Slow dynamic compression of the airway wall model (15% strain at 0.1 Hz over 3 days) substantially enhanced GFP transduction of epithelial cells and underlying fibroblasts. Fibroblast-only controls showed a similar degree of transduction enhancement when undergoing dynamic strain, suggesting enhanced transport through the matrix. Tight junction loss in the epithelium after mechanical stress was observed by immunostaining. We conclude that dynamic compressive strain such as that associated with bronchoconstriction may promote transepithelial transport and enhance viral transgene delivery to epithelial and subepithelial cells. This finding has significance for asthma pathophysiology as well as for designing delivery strategies of viral gene therapies to the airways. mechanical stress; three-dimensional model; asthma; bronchoconstriction; ciliogenesis; human ASTHMA AND AIRWAY SUSCEPTIBILITY to viral infections have long been correlated clinically (2,9,16,26,36,61,62). Asthma is associated with recruitment of inflammatory cells, remodeling and thickening of the reticular basement membrane, subepithelial fibrosis, and airway smooth muscle hyperresponsiveness and hyperreactivity (reviewed in Refs. 7,17,19,21,23,25,48,70). The epithelial barrier function of asthmatic airways is often damaged (35), possibly increasing susceptibility to viral infection. Moreover, it has been shown that epithelial cells from asthmatic patients express a higher number of viral receptors on their surface (4, 62), which when activated exacerbate epithelial secretion of cytokines like granulocyte-macrophage colony-stimulating factor, interleukin (IL)-6, IL-8, IL-11, and RANTES that help maintain a persistent inflammatory state (46).Nevertheless, inflammatory mediators are only part of the story, as ...