Alveolar Fluid Clearance in Pathologically Relevant Conditions: In Vitro and In Vivo Models of Acute Respiratory Distress Syndrome

Abstract

Critically ill patients with respiratory failure from acute respiratory distress syndrome (ARDS) have reduced ability to clear alveolar edema fluid. This reduction in alveolar fluid clearance (AFC) contributes to the morbidity and mortality in ARDS. Thus, it is important to understand why AFC is reduced in ARDS in order to design targeted therapies. In this review, we highlight experiments that have advanced our understanding of ARDS pathogenesis, with particular reference to the alveolar epithelium. First, we review how vectorial ion transport drives the clearance of alveolar edema fluid in the uninjured lung. Next, we describe how alveolar edema fluid is less effectively cleared in lungs affected by ARDS and describe selected in vitro and in vivo experiments that have elucidated some of the molecular mechanisms responsible for the reduced AFC. Finally, we describe one potential therapy that targets this pathway: bone marrow-derived mesenchymal stem (stromal) cells (MSCs). Based on preclinical studies, MSCs enhance AFC and promote the resolution of pulmonary edema and thus may offer a promising cell-based therapy for ARDS.

Keywords: acute respiratory distress syndrome; alveolar fluid clearance; mesenchymal stem (stromal) cells; pulmonary edema; vectorial ion transport.

Figure 1

Alveolar fluid clearance pathways. Shown are the interstitial, capillary, and alveolar compartments, with pulmonary edema fluid in the alveolus. Both type I (yellow) and type II (orange) alveolar cells are involved in transepithelial ion transport. Sodium (Na⁺) is transported across the apical side of the type I and type II cells through the epithelial sodium channel (ENaC), and then across the basolateral side via the sodium/potassium ATPase pump (Na/K-ATPase). Chloride (Cl⁻) is transported via the cystic fibrosis transmembrane conductance regulator (CFTR) channel or by a paracellular route. Additional cation channels also transport ions across the alveolar epithelium (not shown). This vectorial ion transport creates an osmotic gradient that drives the clearance of fluid. Specifically, water (H₂O) moves down the osmotic gradient through aquaporin channels, such as aquaporin 5 (AQP5) or via an intracellular route (not shown).

Figure 2

In vitro model of polarized human alveolar type II epithelial cells. In 2006, Fang et al. developed an in vitro model of the polarized human alveolar epithelial surface, which has been used in multiple subsequent studies of alveolar fluid clearance (AFC) (32). To create this model, type II alveolar epithelial cells are isolated from human donor lungs and cultured on a collagen-I coated 24-well plate where they formed tight monolayers. Pulmonary edema fluid (pink), which contains water (H₂O), sodium ions (Na⁺), chloride ions (Cl⁻), as well as other ions and proteins, is mixed with ¹³¹I-albumin (yellow circles) and introduced to the apical compartment. Pulmonary edema fluid is able to cross the alveolar cell monolayer, but ¹³¹I-albumin cannot cross, so it is possible to calculate AFC by measuring the change in ¹³¹I-albumin concentration between the apical and basal compartments.