TMPRSS11E-dward Scissorhands? Proteolysis Potentiates Lung Inflammation during Pathogen Insult

The lung blood–gas barrier, or alveolar–capillary membrane, is only 1–2 μm thick but performs the herculean task of ensuring orderly gas exchange in the context of both direct injury and extrapulmonary insults (1). There is growing appreciation of consequential injury to the delicate blood–gas barrier that can occur because of host-derived or pathogen-mediated protease derangements (2–5). One family of proteases that may be crucial to lung biology is the transmembrane serine proteases, which are widely expressed on lung epithelial cells (6, 7). Because of the coronavirus disease (COVID-19) pandemic, there has been increased attention to TMPRSS2 (transmembrane serine protease 2), which is essential for cleavage of the severe acute respiratory syndrome (SARS) coronavirus spike protein to facilitate host cell entry via the angiotensin-converting enzyme 2 (ACE2) receptor (8). However, there are numerous transmembrane serine proteases with wide-ranging biological functions.

TMPRSS11E (transmembrane serine protease 11E), which is also known as DESC1 (differentially expressed in squamous cell carcinoma 1), is one such protease with poorly characterized biological roles (9). Others have reported that TMPRSS11E contributes to β coronavirus and influenza viral entry into human cells (6), and it sensitizes human tumor cells to apoptosis by cleaving EGFR (epidermal growth factor receptor) (10). In the context of this knowledge gap, in this issue of the Journal Zhang and colleagues (pp. 406–416) provide an important contribution to our understanding of TMPRSS11E biology in the lung (11). Building on prior work demonstrating that TMPRSS11E is lightly expressed in noninjured human lung (6), the authors establish the relevance of TMPRSS11E to human lung disease by demonstrating significantly increased protein concentrations in BAL fluid from patients with acute respiratory distress syndrome due to pneumonia compared with patients undergoing evaluation for solitary pulmonary nodules. The authors further demonstrate increased TMPRS11E transcription and translation in both lung epithelial cells and lung macrophages secondary to direct injury from LPS or extrapulmonary insult via cecal ligation and puncture. Having established that TMPRS11E is relevant during both murine and human lung injury, the authors then turned their attention to understanding its biological role. Using both plasmid-mediated overexpression and direct installation of TMPRS11E catalytic domain peptide, they demonstrate that TMPRS11E potentiates the release of inflammatory cytokines IL-1β, IL-6, and TNF-α in human lung epithelial cell lines and in the LPS-injured mouse lung. Furthermore, they demonstrate increased histologic lung injury in response to LPS or the viral mimic and toll-like receptor 3 agonist, polyinosinic:polycytidylic acid (Poly I:C), with TMPRSS11E overexpression. Importantly, the authors use a mutant construct control, S372A, that switches serine to alanine at amino acid 372 to inactivate the catalytic triad of histidine, aspartate, and serine that is conserved in the serine protease family (10). The S372A mutant plasmid, or gene silencing of TMPRS11E with shRNA, eliminated excess inflammatory cytokine release and attenuated histologic injury, pointing to TMPRS11E serine protease activity as the mediator of its biological effect. To further define a mechanism for potentiation of inflammation by TMPRSS11E, the authors demonstrate its ability to cleave a fluorogenic PAR-1 (protease activating receptor 1) substrate with efficacy similar to thrombin, a well-known proteolytic activator of PAR-1 (12). Furthermore, inhibition of serine proteolysis with camostat and inhibition of PAR-1 using vorapaxar decreased inflammatory cytokine transcription during direct LPS-mediated lung injury. Together, these findings suggest that TMPRSS11E potentiates lung inflammation during pathogen-mediated injury via proteolytic activation of PAR-1.

One major limitation of this study is that it is unclear whether proteolytic activity by TMPRS11E in the lung contributes to breakdown of the blood–gas barrier, as only the inflammatory cytokine and histologic injury domains of experimental lung injury recognized by a recent American Thoracic Society workshop report are satisfied by the presented data (13). Specifically, there are no direct measurements of alveolar–capillary barrier alteration in the presented data (13). However, this limitation is substantially mitigated by perhaps the most significant finding of the paper: the authors demonstrate that both plasmid-mediated overexpression and direct intranasal installation of the catalytic domain peptide increase mortality during a mouse model of extrapulmonary lung injury induced by cecal ligation and puncture. Mortality increased in a dose-dependent fashion with the catalytic domain peptide and was abrogated by gene silencing in the lung with shRNA. These data suggest a novel and important potential role for TMPRSS11E proteolytic potentiation of lung injury during extrapulmonary insults such as sepsis.

Despite the intriguing results provided by Zhang and colleagues, several important questions remain. Although the authors confirmed that TMPRSS11E can potentiate inflammation through the well-known role of serine protease action on PAR-1 (5, 12, 14), it is unclear whether TMPRS11E can also modulate additional pathways of importance during lung injury, such as EGFR signaling and cell death (10, 15, 16). In addition, further work is needed to determine whether TMPRSS11E increases disruption of the blood–gas barrier during direct and/or extrapulmonary insults, which would be helpful to understand the translational potential of these findings particularly given the striking survival benefit of TMPRS11E knockdown in the cecal ligation and puncture model. Regarding the translational potential of this work, the growing spectrum of potential therapeutics for transmembrane serine proteases (17) provides an opportunity to test in relevant preclinical translational models (18) whether therapeutic modulation of TMPRS11E may be of benefit during direct or extrapulmonary lung injury.

Through their thorough and fascinating work, Zhang and colleagues add another important brick to the wall of evidence building our understanding of proteolysis as an important biologic mechanism during pathogen-mediated acute lung injury.