Date Published: May 8, 2019
Publisher: Public Library of Science
Author(s): Janelle M. Veazey, Timothy J. Chapman, Timothy R. Smyth, Sara E. Hillman, Sophia I. Eliseeva, Steve N. Georas, Jian Fu.
The airway epithelial barrier is critical for preventing pathogen invasion and translocation of inhaled particles into the lung. Epithelial cells also serve an important sentinel role after infection and release various pro-inflammatory mediators that recruit and activate immune cells. Airway epithelial barrier disruption has been implicated in a growing number of respiratory diseases including viral infections. It is thought that when a pathogen breaks the barrier and gains access to the host tissue, pro-inflammatory mediators increase, which further disrupts the barrier and initiates a vicious cycle of leak. However, it is difficult to study airway barrier integrity in vivo, and little is known about relationship between epithelial barrier function and airway inflammation. Current assays of pulmonary barrier integrity quantify the leak of macromolecules from the vasculature into the airspaces (or “inside/out” leak). However, it is also important to measure the ease with which inhaled particles, allergens, or pathogens can enter the subepithelial tissues (or “outside/in” leak). We challenged mice with inhaled double stranded RNA (dsRNA) and explored the relationship between inside/out and outside/in barrier function and airway inflammation. Using wild-type and gene-targeted mice, we studied the roles of the dsRNA sensors Toll Like Receptor 3 (TLR3) and Melanoma Differentiation-Associated protein 5 (MDA5). Here we report that after acute challenge with inhaled dsRNA, airway barrier dysfunction occurs in a TLR3-dependent manner, whereas leukocyte accumulation is largely MDA5-dependent. We conclude that airway barrier dysfunction and inflammation are regulated by different mechanisms at early time points after exposure to inhaled dsRNA.
A crucial step in the innate immune response to viral infections is the sensing of viral nucleic acid intermediates by pattern recognition receptors (PRRs) [1, 2]. Double stranded RNA (dsRNA) is a particularly potent nucleic acid intermediate and activates different PRRs including Toll Like Receptor 3 (TLR3) and members of the RIG-I like receptor (RLR) family, including Melanoma Differentiation-Associated protein 5 (MDA5) . After binding to dsRNA, these PRRs induce the expression of interferons and cytokine genes, leading to tissue inflammation and anti-viral immunity [1, 3].
Activation of innate immunity by viral dsRNA involves multiple pattern recognition receptors, including TLR3 and MDA5. Emerging data suggest that these dsRNA sensors have compartmentalized roles in the development of tissue inflammation and anti-viral immunity, but it is currently unclear if they regulate distinct aspects of lung mucosal inflammation. We studied the acute response to inhalation challenge with polyI:C in TLR3-/- and MDA5-/- mice, compared with their wild-type counterparts. We measured airway inflammation (BAL neutrophils and CXCL1 levels) as well as barrier function, using assays of both inside/out as well as outside/in barrier integrity. We found that TLR3-/- and MDA5-/- have distinct roles in the acute response to inhaled polyI:C, promoting epithelial barrier disruption and neutrophilic airway inflammation, respectively. Here we consider the potential mechanisms for these observations and their implications for our understanding of respiratory virus-induced lung injury.