Acute respiratory distress syndrome (ARDS) is a serious condition induced by uncontrolled and excessive inflammatory responses causing direct or indirect injury to the lungs. The most common ARDS precipitants are bacterial and viral pneumonia and non-pulmonary sepsis, aspiration of gastric contents, pancreatitis, high-risk surgery, and others (Meyer et al., 2021). Symptoms of ARDS are alveolar and interstitial pulmonary edema, hypoxic respiratory failure, decreased pulmonary compliance, and increased pulmonary vascular permeability. ARDS pathogenesis includes damage to the alveolar epithelial cells and capillary endothelial cell barriers, and the exudation of protein-rich edema fluid from the alveolar lumen. It is thought that the infiltration of inflammatory cells, such as neutrophils and macrophages, into the lungs is an initial pathogenic process in ARDS (Thompson et al., 2017). Specifically, different stimuli such as viral infections or mechanical injury release damage-associated molecular patterns (DAMPs). High mobility group box protein 1 (HMGB1) is one of the key DAMP molecules released in the lungs during ARDS (Fink, 2002; Entezari et al., 2014). This shifts the lung endothelium towards a dysregulated, leaky state that attracts inflammatory cells, particularly neutrophils, monocytes, and T lymphocytes, to the lung parenchyma and alveolar space. These activated cells release large amounts of pro-inflammatory cytokines (TNF, IL-1β, IFN-γ, IL-17, IL-6) and other inflammatory mediators which then disrupt the integrity of alveolar epithelial cells and the pulmonary capillary endothelial cell barrier, leading to pulmonary edema and alveolar hemorrhage (Vassiliou et al., 2020; Sun et al., 2013).

Ethyl pyruvate (EP), a potent redox molecule that can prevent damage mediated by reactive oxygen species, is also a modulator of inflammation that acts by shaping immune cell activation and differentiation. We have previously reviewed the beneficial effects of EP in different autoimmune and chronic inflammatory diseases (Koprivica et al., 2022). EP was shown efficient in restraining experimental autoimmune encephalomyelitis (Miljković et al., 2015), type 1 diabetes (Koprivica et al., 2019), and experimental autoimmune myocarditis (Gajić et al., 2021). There are reports of the effects of EP that could be of importance in ARDS. Namely, EP was shown to dose-dependently inhibit TNF release from murine alveolar macrophages and chemokine release from murine respiratory epithelial cells in vitro (van Zoelen et al., 2007). Studies have shown that EP inhibits HMGB1 in different animal models of ARDS/acute lung injury (ALI) (Shang et al., 2009; Entezari et al., 2014). HMGB1 mediates leukocyte infiltration into the lungs and inhibition of extracellular HMGB1 and/or its accumulation in the airways by EP mitigated inflammatory ALI (Entezari et al., 2014).

It has already been suggested that EP could exert protective effects in the cytokine storm and ARDS in COVID-19 due to its anti-inflammatory properties (Tanwar et al., 2021; Fink, 2002). However, to the best of our knowledge, there has been no study on the effects of EP administration on pro-inflammatory cytokines and chemokines in ARDS. Specifically, we investigated the effects of EP administration on histopathological changes and HMGB1 expression in the lungs induced by polyriboinosinic-ribocytidylic acid (poly(I:C)), a synthetic double-stranded RNA analogue similar to viral RNAs. Poly(I:C) activates cells of innate and adaptive immunity leading to excessive release of pro-inflammatory cytokines, like interleukin (IL)-17, IL-6, and interferon (IFN)-γ, which contributes to tissue damage observed in ARDS (Li et al., 2012; Gurkan et al., 2011; Theron et al., 2005). Thus, the activation status of T lymphocytes and expression of appropriate cytokines and chemokines were examined in lungs and thoracic draining lymph nodes (DLN). In brief, this study aimed to investigate cellular and molecular targets of EP in the ARDS mouse model.