In ARDS with COVID-19, circulating alveolar epithelial and endothelial injury markers were markedly elevated, but not in the case of non-ARDS COVID-19, indicating that alveolar barrier tissue injury is a hallmark of COVID-19 ARDS pathogenesis. Interestingly, time courses of changes in alveolar epithelial and in endothelial injury markers were different from each other. The observed serum sRAGE level was highest on admission in most patients with ARDS. In contrast, ANG-2 level reached a peak at later time points. These data suggest that the alveolar epithelial injury in the COVID-19 ARDS already reached the maximum level before hospital admission. Meanwhile, the endothelial injury seems to continue to deteriorate for several days after admission. Therefore, it is expected that there is a potential therapeutic time window to alleviate the endothelial injury even after hospitalization. Moreover, the ANG-2 levels were significantly correlated with the P/F ratios and the SP-D levels, suggesting endothelial injury, rather than alveolar epithelial injury, might be a main contributor that leads to deterioration of alveolar permeability during ARDS with COVID-19.
Several previous reports evaluating lung tissues obtained from patients who died from severe COVID-19 revealed diffused alveolar damage accompanied by epithelial and endothelial injuries [4, 5] along with the presence of viral RNA in these cells [6, 7]. Moreover, several studies demonstrated the blood levels of endothelial injury markers at admission were significantly increased[18–23]. However, the underlying mechanisms of alveolar epithelial and endothelial injury has not been fully elucidated. SARS-CoV-2 can infect alveolar type 1 and 2 epithelial cells and endothelial cells, and causes multiple cellular responses, which are potentially cytotoxic. On the other hand, inflammatory responses caused by SARS-CoV-2, rather than the SARS-CoV-2 infection itself to the targeted cells, may be the main driver of the alveolar tissue injuries[27, 28]. The present study clearly demonstrates the difference in the peak timing of epithelial and endothelial injury markers, indicating that there are distinct mechanisms for injury to each type of cell. Future studies to clarify each of the injury mechanisms, especially focusing the endothelial injury, can be a promising approach to find efficacious therapeutic targets for COVID-19 ARDS.
Only a small number of studies have evaluated the temporal kinetics of circulating alveolar epithelial and endothelial injury markers during ARDS caused by aetiologies other than COVID-19. Two reports have previously demonstrated that sRAGE levels in the blood of ARDS patients with bacterial sepsis peak on the initial day after admission[29, 30]. Thus, based on previous findings as well as our results, alveolar epithelial injury seems to occur during the initial phase of ARDS progression irrespective of the aetiology. On the other hand, the time course of circulating ANG-2 levels during ARDS has not been previously reported. Kumpers et al. have reported that circulating ANG-2 levels in patients with bacterial sepsis, 72 hours after intensive care unit admission, increased from the baseline level only in the non-survivors, not in the survivors. In our study, the ANG-2 levels increased from baseline in most patients, including survivors, suggesting that SARS-CoV-2 induces more severe endothelial injury than other aetiologies for injury, such as bacterial infection. Several reports have demonstrated that lung vascular thrombosis is a pathological feature of ARDS caused by SARS-CoV-2[4, 5, 32]. Moreover, the occurrence of systemic thrombotic complications in patients with COVID-19 has been reported. Endothelial injury caused by SARS-CoV-2 infection might be the basal mechanism of thrombosis formation.
Circulating markers of alveolar tissue injury can be easily measured in clinical settings. Our data suggest that initial levels of RAGE can be a prognostic marker of COVID-19. Moreover, patterns of these tissue injury markers might be useful for predicting or evaluating the responses to treatment for ARDS in the case of COVID-19. Recently, it has been shown that circulating biomarkers can distinguish sub-phenotypes among patients with ARDS[17, 35]. It is possible that ARDS caused by COVID-19 also has several sub-phenotypes, and different treatment strategies for each sub-phenotype might be necessary. Further studies to evaluate the utility of these circulating markers are, thus, warranted.
There are some limitations to our study. First, the sample size was small because of the factor of availability of daily serum samples from patients with COVID-19 ARDS. This may have affected the statistical strength of our analyses. It is, hence, necessary to confirm our results using samples from large cohorts to obtain insights into the pathogenesis of alveolar tissue injury during COVID-19. Second, because of the observational nature of the study design, treatments for COVID-19 were not standardized. Therefore, it is possible that the treatment strategies might affect the kinetics of circulating alveolar tissue injury markers.