COVID-19 and Circulating Biomarkers of Lung Fibrosis and Endothelial Dysfunction

Citation: Marilena Greco (2022) COVID-19 and Circulating Biomarkers of Lung Fibrosis and Endothelial Dysfunction, 21st Century Pathology, Volume 2 (3): 121

Abstract

COVID-19 disease is a primary respiratory infection that can be responsible for lung interstitial damage and progression to fibrosis, especially in hospitalized patients, often associated with an overwhelming inflammatory state. Endothelial involvement appears to predominate and to be responsible for multiorgan involvement and thromboembolic events. These features determined the new definition of SARS-CoV-2 infection as endotheliopathy. The association of sFLT-1 and CA 15-3 with endothelial damage and pulmonary fibrosis has been recently reported, this finding appears of great interest for the possibility of using serum biomarkers for categorizing COVID-19 patients and better supporting their clinical care and treatment.

Keywords:

COVID-19; Biomarkers; Endotheliopathy, Lung fibrosis

Introduction

SARS-CoV-2 causes COVID-19, a viral infection that can cause severe pneumonia, long-term adverse effects to several organ systems, and be potentially lethal. Since the infection was first reported at the end of 2019, it became pandemic and still, the entire world is facing widespread infection mobilizing diagnostics and biopharmaceutical research to develop diagnostic tests, antiviral drugs, and vaccines to help combat the disease. COVID-19 is a primary respiratory infection that is often complicated by immune-inflammatory reactions and by systemic involvement of the vasculature.

Pulmonary interstitial damage and progression to fibrosis of the lung is a typical finding in hospitalized patients with SAR-CoV-2 infection. Diffuse alveolar interstitial thickening, interstitial mononuclear inflammatory infiltrates and edema has been widely reported, and Computed Tomography (CT) Imaging characteristically displays ground-glass opacities.

Autoptic analysis of lung tissue of Covid-19 patients showed marked fibrotic lung parenchyma and airspace obliteration [1]. Moreover, vascular involvement, inflammatory and prothrombotic changes of the endothelium in SARS-CoV-2 infected patients lead to several complications ranging from thrombosis to pulmonary edema secondary to loss of barrier function. Endotheliopathy has been reported by several studies [2, 3]. It is still unclear whether the endothelial damage results from direct endothelial infection by SARS-CoV-2 or from the inflammatory response to the virus-mediated by cytokines, reactive oxygen species, and acute-phase reactants. Some data suggest that SARS-CoV-2 can infect vascular endothelial cells, as the presence of viral elements inside endothelial cells of several tissues has been shown in post-mortem analysis [4-6], whereas other studies found a relatively little expression of ACE2, the SARS-CoV-2 receptor, in endothelial cells [7] compared to airway cells with no evidence of the virus inside the cells [8-10]. It has been proposed that, although SARS-CoV-2 probably could not easily enter and replicate in the endothelium, viral components are able to stimulate endothelial activation, cytokine release, transcriptional activation of several pro-inflammatory genes, and activation of immune receptors on the endothelium [11-14]. Hence viral entry and replication appear not necessary to cause endothelial cell damage [15].

Although no specific COVID-19 serum marker is routinely used, the possibility to evaluate the clinical situation of the patient by common biomarker has been proposed in several studies, such as inflammatory markers, e.g., CRP, LDH, ferritin, TNF-?, IL-1, IL-6, IL-8, IL-10, coagulation parameters (e.g., D-dimer, prothrombin time, fibrinogen) or evaluation of withe blood cells count, lymphopenia, thrombocytopenia, Red Cell Distribution Width (RDW). However, no specific laboratory marker has been so far exclusively established for COVID-19 [16-18].

Recent results [18] were obtained in a cohort of 262 hospitalized SARS-CoV-2 patients with different grades of clinical severity of the disease-associated the variation of circulating levels of CA 15-3 and sFlt-1 to COVID-19 lung fibrosis and endothelial dysfunction, respectively, two of the main characteristics of SARS-CoV-2 mild/severe infection. CA 15-3, the known tumor marker for breast cancer, is an alternative marker for KL-6 (serum Krebs von den Lungen-6), a marker for fibrotic lung diseases already used to evaluate pulmonary interstitial damage, fibroblast activity, and progression to fibrosis of the lung; KL-6 and CA 15-3 recognize different epitopes on the same molecule MUC1, corresponding to different carbohydrate residues [19-20]. Analysis of CA 15-3 serum levels in severe and non-severe SARS-CoV-2 infected patients gave a receiver operating characteristic (ROC) with 0.958 AUC value and a cut-off value of 24.8 U/mL (88.4% Sensitivity and 91.8% Specificity) which significantly differentiated severe/critic pulmonary interstitial damage [18]. Due to the observed significant increase of CA 15-3 in severe or critical disease (43.2 or 110 U/mL, respectively) compared to mild disease (18.4 U/mL) and absence of lung fibrosis (14.5 U/mL), CA 15-3 has been proposed as a marker of pulmonary interstitial damage and progression to fibrosis of the lung in SARS-CoV-2 infection [18].

The same study [18] investigated circulating levels of sFlt-1 in the cohort of 262 hospitalized SARS-CoV-2 patients, a biomarker known for promoting endothelial dysfunction, particularly during preeclampsia [21], which has been previously associated with endothelial injury in bacterial sepsis [22]. Serum sFlt-1 is the soluble form of Vascular Endothelial Growth Factor Receptor (sVEGFR) and binds and antagonizes Vascular Endothelial Growth Factor (VEGF) and Placental Growth Factor (PlGF) signal [23]. Recently sFlt-1 has been correlated with endothelial dysfunction and organ failure in critically ill COVID-19 patients and reported as a potential early predictor of respiratory failure in COVID-19 patients [24-25]. According to Greco M, et al. (2021) [18] serum sFlt-1 appears associated with SARS-CoV-2 infection status with a decisive threshold value > 90.3 pg/mL, (AUC 0.902, Sensitivity 83.9%, Specificity 86.7%; control value 78.9 pg/mL) and with the extent, severity and poorest outcome of the disease in hospitalized patients. Values up to 200 pg/mL have been found after two weeks of hospitalization in COVID-19 patients who didn?t survive [18], and similar serum sFlt-1 concentrations have been previously observed in patients with bacterial sepsis [22].

Discussion

Several evidence address attention to the involvement of vascular complications and endotheliopathy in the setting of COVID-19, a primary respiratory illness in which vascular commitment leads to sequelae of complications ranging from thrombosis to pulmonary edema consequent to barrier dysfunction. The finding of dysregulation of endothelial markers [15, 18], like the soluble VEGF receptor sFlt-1, appears an easy tool for COVID-19 management. Several markers of endothelial inflammation have been identified in COVID-19, as recently reviewed [15], including Angpt-2, ICAM, VCAM1, E-selectin, P-selectin, VEGF-A, VEGFR-1. sFlt-1, has been found significantly increased in COVID-19 hospitalized patients and it seems to be a useful marker for the identification of patients with the poorest prognosis because of its dramatic increase in the serum of non-survivors patients. Furthermore, it is a common laboratory test, already used for preeclampsia testing, and therefore it can be rapidly and easily assayed in COVID-19 patients for the endothelial dysfunction assessment. In severe illness a crucial role is played by exaggerated immune response mediated by an array of cytokines and overactive immune response which are more destructive than that of the infection itself, resulting in massive and irreversible damage to the organs [26]. The endothelium plays a central role in the systemic response to SARS-CoV-2 infection and in the specific involvement of pulmonary vasculature [15]. Endotheliopathy in COVID-19 was observed from the beginning of the pandemic [19-20] and emerging data suggest the involvement of endotheliopathy-centered pathophysiology of the disease [27]. The endothelium both orchestrate and reacts to the progressive inflammatory response elicited by SARS-CoV-2 infection. In the activated endothelium pro-inflammatory pathway and upregulation of leukocytes adhesion molecules, including E-selectin, P-selectin, ICAM-1, and VCAM-1, led to massive recruitment of leukocyte, macrophages activation, DNA released from activated neutrophils and formation of neutrophil extracellular traps (NET) [15]. The uncontrolled release of cytokines and inflammatory status negatively affects endothelium leading to its dysfunction, loss of barrier function, and impaired angiogenesis and coagulation [5, 28]; vascular injury led to platelet activation, which partly explains the hypercoagulable states in many COVID-19 patients with severe disease [29]. Endothelial injury is likely responsible for pulmonary circulation thrombosis and consequential alveolar damage in COVID-19.

Interestingly, other common serum circulating biomarkers have been associated with the typical SARS-CoV-2 pulmonary interstitial damage and lung fibrosis. The recently proposed assay of CA 15-3 [18] is particularly noteworthy as an alternative to KL-6, previously reported as a highly expressed molecule by damaged alveolar type II cells and a predictive biomarker in severe COVID-19 [30]. CA 15-3 is well correlated with the grade of mild, severe, or critic pulmonary damage caused by SARS-CoV-2 infection, as categorized from lung ground-glass opacities, interstitial damage, and presence of fibrosis from chest CT scans in hospitalized COVID-19 patients. Moreover, SARS-CoV-2 patients hospitalized in the Intensive Care ward (ICU) show circulating levels of CA 15-3 40% higher than NON-ICU hospitalized patients (i.e. 42.6 vs 25.7 U/mL, and 14.5 U/mL in control patients) [18]. Since severely affected individuals show increased pulmonary fibrosis, the CT imaging data of interstitial thickening, irregular interface, and thick reticular patterns have been suggested as predictors of early COVID-19 fibrosis [31]. In this context, it appears of great interest and utility the possibility to use CA 15-3 as a circulating biomarker for lung fibrosis in SARS-CoV-2 infection.

The severity of the disease has been associated with lung infiltrating immune cells causing two well-defined features that are connected: lung hyper inflammation, which becomes systemic through the progression of the disease, and that is also directly linked to virus-induced endothelial damage [27, 32], and the consequent lung fibrosis. It?s of notice that, based on recent studies, both aspects can be investigated in COVID-19 patients using circulating serum variation of sFlt-1 and CA 15-3 thus allowing an easier follow-up of the disease progression.

Conclusion

COVID-19 is a disease that often poses several challenges to clinicians, due to its unpredictable clinical course. The identification of specific laboratory biomarkers for the known well-defined features of lung and endothelial damage could contribute to the prognosis and management of COVID-19 patients. Recent insight on the pathogenesis of SARS-CoV-2 infection highlighted the dramatic changes in endothelial phenotypes, its dysfunction, and loss of barrier function. The injury of the lung vascular bed determines progression to pneumopathy and fibrosis. The newly associated sFlt-1 and CA 15-3 to COVID-19 clinical alterations appear useful serum markers of, respectively, endothelial dysfunction and pulmonary fibrosis to be used in the prognosis and follow-up of hospitalized COVID-19 patients.