Real-Time Measurement of Cell Mechanics as a Clinically Relevant Readout of an In Vitro Lung Fibrosis Model Established on a Bioinspired Basement Membrane

Lung fibrosis, one of the major post-COVID complications, is a progressive and ultimately fatal disease without a cure. Here, an organ- and disease-specific in vitro mini-lung fibrosis model equipped with noninvasive real-time monitoring of cell mechanics is introduced as a functional readout. To establish an intricate multiculture model under physiologic conditions, a biomimetic ultrathin basement (biphasic elastic thin for air-liquid culture conditions, BETA) membrane (<1 µm) is developed with unique properties, including biocompatibility, permeability, and high elasticity (<10 kPa) for cell culturing under air-liquid interface and cyclic mechanical stretch conditions. The human-based triple coculture fibrosis model, which includes epithelial and endothelial cell lines combined with primary fibroblasts from idiopathic pulmonary fibrosis patients established on the BETA membrane, is integrated into a millifluidic bioreactor system (cyclic in vitro cell-stretch, CIVIC) with dose-controlled aerosolized drug delivery, mimicking inhalation therapy. The real-time measurement of cell/tissue stiffness (and compliance) is shown as a clinical biomarker of the progression/attenuation of fibrosis upon drug treatment, which is confirmed for inhaled Nintedanib-an antifibrosis drug. The mini-lung fibrosis model allows the combined longitudinal testing of pharmacodynamics and pharmacokinetics of drugs, which is expected to enhance the predictive capacity of preclinical models and hence facilitate the development of approved therapies for lung fibrosis.

Mechanical load and mechanical integrity of lung cells - experimental mechanostimulation of epithelial cell- and fibroblast-monolayers

Experimental mechanostimulation of soft biologic tissue is widely used to investigate cellular responses to mechanical stress or strain. Reactions on mechanostimulation are investigated in terms of morphological changes, inflammatory responses and apoptosis/necrosis induction on a cellular level. In this context, the analysis of the mechanical characteristics of cell-layers might allow to indicate patho-physiological changes in the cell-cell contacts. Recently, we described a device for experimental mechanostimulation that allows simultaneous measurement of the mechanical characteristics of cell-monolayers. Here, we investigated how cultivated lung epithelial cell- and fibroblast-monolayers behave mechanically under different amplitudes of biaxial distension. The cell monolayers were sinusoidally deflected to 5%, 10% or 20% surface gain and their mechanical properties during mechanostimulation were analyzed. With increasing stimulation amplitudes more pronounced reductions of cell junctions were observed. These findings were accompanied by a substantial loss of monolayer rigidity. Pulmonary fibroblast monolayers were initially stiffer but were stronger effected by the mechanostimulation compared to epithelial cell-monolayers. We conclude that, according to their biomechanical function within the pulmonary tissue, epithelial cells and fibroblasts differ with respect to their mechanical characteristics and tolerance of mechanical load.

A method to measure mechanical properties of pulmonary epithelial cell layers

The lung has a huge inner alveolar surface composed of epithelial cell layers. The knowledge about mechanical properties of lung epithelia is helpful to understand the complex lung mechanics and biomechanical interactions. Methods have been developed to determine mechanical indices (e.g., tissue elasticity) which are both very complex and in need of costly equipment. Therefore, in this study, a mechanostimulator is presented to dynamically stimulate lung epithelial cell monolayers in order to determine their mechanical properties based on a simple mathematical model. First, the method was evaluated by comparison to classical tensile testing using silicone membranes as substitute for biological tissue. Second, human pulmonary epithelial cells (A549 cell line) were grown on flexible silicone membranes and stretched at a defined magnitude. Equal secant moduli were determined in the mechanostimulator and in a conventional tension testing machine (0.49 ± 0.05 MPa and 0.51 ± 0.03 MPa, respectively). The elasticity of the cell monolayer could be calculated by the volume-pressure relationship resulting from inflation of the membrane-cell construct. The secant modulus of the A549 cell layer was calculated as 0.04 ± 0.008 MPa. These findings suggest that the mechanostimulator may represent an adequate device to determine mechanical properties of cell layers.

Biaxial distension of precision-cut lung slices

The mechanical forces acting on lung parenchyma during (mechanical) ventilation and its (patho)physiological consequences are currently under intense scrutiny. Several in vivo and cell culture models have been developed to study the pulmonary responses to mechanical stretch. While providing extremely useful information, these models do also suffer from limitations in being either too complex for detailed mechanical or mechanistic studies, or in being devoid of the full complexity present in vivo (e.g., different cell types and interstitial matrix). Therefore in the present study it was our aim to develop a new model, based on the biaxial stretching of precision-cut lung slices (PCLS). Single PCLS were mounted on a thin and flexible carrier membrane of polydimethylsiloxane (PDMS) in a bioreactor, and the membrane was stretched by applying varying pressures under static conditions. Distension of the membrane-PCLS construct was modeled via finite element simulation. According to this analysis, lung tissue was stretched by up to 38% in the latitudinal and by up to 44% in the longitudinal direction, resulting in alveolar distension similar to what has been described in intact lungs. Stretch for 5 min led to increased cellular calcium levels. Lung slices were stretched dynamically with a frequency of 15/min for 4 h without causing cell injury {3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) test; live/dead straining}. These findings suggest that stretching of PCLS on PDMS-membranes may represent a useful model to investigate lung stretch in intact lung tissue in vitro for several hours.

Characteristics of highly flexible PDMS membranes for long-term mechanostimulation of biological tissue

Measurement of mechanical properties of soft biological tissue remains a challenging task in mechanobiology. Recently, we presented a bioreactor for simultaneous mechanostimulation and analysis of the mechanical properties of soft biological tissue samples. In this bioreactor, the sample is stretched via deflection of a flexible membrane. It was found that the use of highly compliant membranes increases accuracy of measurements. Here, we describe the production process and characteristics of thin and flexible membranes of polydimethylsiloxane (PDMS) designed to improve the signal-to-noise ratio of our bioreactor. By a spin-coating process, PDMS membranes were built by polymerization of a two component elastomer. The influence of resin components proportion, rotation duration, and speed of the spinning were related to the membrane mechanics. Membranes of 22 mm inner diameter and 33 to 36 microm thickness at homogeneous profiles were produced. Isolated rat diaphragms served as biological tissue samples. Mechanical properties of the membranes remained constant during 24 h of mechanostimulation. In contrast, time- and strain-dependent mechanical properties of the diaphragms were found.