Final Report Summary - SERPINB1 (Role of serpinB1 in cellular homeostasis in the bone marrow and the lung)
Chronic inflammatory diseases are a prevalent cause of morbidity in the EU and are associated with important medical care costs and loss of working time. In the first place, inflammatory responses are vital to preserve host integrity and survival following infection with pathogens. However, genetic and environmental factors contribute to modify, enhance or sustain these responses leading inflammatory diseases. Excessive acute amplification can lead to severe disease and lethality such as in acute respiratory distress syndrome and septic shock. Chronic inflammation is associated with sustained release of cytokines and enzymes leading to altered antimicrobial responses, reduced organ function and tissue destruction such as in cystic fibrosis (CF), asthma or chronic obstructive pulmonary disease (COPD) and emphysema.
Neutrophils are short-lived leukocytes with vital functions in host defense. Neutrophils are massively recruited from the bone marrow, via the circulation, to tissue sites in response to danger signals. This immediate, life saving, inflammatory response can turn against the host if its intensity and duration are dysregulated leading to tissue injury and/or failed immune response to pathogens. The dual protective/pathologic role of neutrophils is mirrored by the functions of neutrophil serine proteases (NSPs), potent granule enzymes that hydrolyze other proteins. Indeed, NSPs carried in neutrophil granules are indiscriminate in their activity and have been shown to contribute to both protective and destructive inflammatory functions. These proteases directly kill pathogens and inactivate their toxins but they are also harmful by fueling the inflammatory response and by destroying extracellular matrix proteins and immune defense proteins.
Many studies have focused on neutrophil elastase, its targets and its regulation by plasma and secreted protease inhibitors such as α1-antitrypsin. Here, we investigated the regulation of NSPs by the little studied, but increasingly recognized, endogenous inhibitor serpinB1. SerpinB1 is a potent intracellular inhibitor of the NSPs (elastase, cathepsin G and proteinase-3) found of many cells. We found that serpinB1 has a cytoprotective function, where it prevents early neutrophil death both at inflammatory sites as well as in the bone marrow. Mice lacking serpinB1 have a severely reduced reserve of mature neutrophils in the bone marrow; they fail to clear Pseudomonas lung infection and have increased morbidity in influenza A virus infection. In these models, lack of serpinB1 is also associated with increased and sustained production of pro-inflammatory cytokines. A major achievement of this project was the discovery that cathepsin G induces neutrophil cell death. We showed that cathepsin G, but not the related NSP neutrophil elastase, induced neutrophil death upon granule membrane permeabilization.
While it is well established that deficiency in α1-antitrypsin, a potent plasma inhibitor of NSPs, is associated with early onset emphysema, the role of serpinB1 in the development of chronic lung disease is unknown. Using serpinB1 deficient mice, we investigated the function of serpinB1 in the development of emphysema in aging and in a model of chronic cigarette smoke exposure. While cigarette smoke induced significant induction of emphysema on smoke-exposed mice compared control mice, no difference was observed in the severity of emphysema between the two genotypes, suggesting that serpinB1 is not essential for regulating the extent of the protease-mediated damage following chronic smoke exposure.
Overall, our findings established an essential role for serpinB1 in neutrophil homeostasis and uncovered a novel cell death pathway initiated by cathepsin G and essentially regulated by serpinB1. In contrast, we found that serpinB1 does not play a pivotal role in protecting against lung injury induced by chronic exposure to cigarette smoke.
Another important achievement of this Marie Curie - International Reintegration Grant, was to help durably establish the laboratory of the applicant from the US back to the EU and associated countries (here, Switzerland). Funds from this Marie Curie fellowship contributed to support studies leading to the publication of eight articles and helped secure new and renewal funding from different funding bodies and thus providing support for the applicant’s career in the long term.