The Potential of Lung Epithelium Specific Proteins as Biomarkers for COVID-19-Associated Lung Injury

Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) infection was first reported in Wuhan, China, and was declared a pandemic by the World Health Organization (WHO) on 20 March 2020. The respiratory system is the major organ system affected by COVID-19. Numerous studies have found lung abnormalities in patients with COVID-19, including shortness of breath, respiratory failure, and acute respiratory distress syndrome. The identification of lung-specific biomarkers that are easily measurable in serum would be valuable for both clinicians and patients with such conditions. This review is focused on the pneumoproteins and their potential to serve as biomarkers for COVID-19-associated lung injury, including Krebs von den Lungen-6 (KL-6), surfactant proteins (SP-A, SP-B, SP-C, SP-D), and Clara cell secretory protein (CC16). The current findings indicate the aforementioned pneumoproteins may reflect the severity of pulmonary manifestations and could serve as potential biomarkers in COVID-19-related lung injury.

Fibrosis and Damage Markers in Occupational Interstitial Lung Diseases

The investigation of biomarkers for fibrosis and lung tissue damage is very important for the diagnosis and prognosis of interstitial lung disease, as well as treatment strategies. The biomarkers secreted by Type II pneumocytes and cells of the bronchiolar epithelium are most reflective of the formation of pulmonary fibrosis and the degree of damage to the lung tissue. The levels of Krebs von den Lungen-6 (KL-6), alveolomucin (mucin-antigen 3EG5), and Clara cell secretory protein in occupational interstitial lung diseases (e.g., exogenous alveolitis, pneumoconiosis), caused by exposure to organic and inorganic factors, were analysed in dependence of disease activity phase. The level of alveolomucin in pneumoconiosis and in occupational alveolitis remission may reflect the extent of pulmonary fibrosis, which is a prognostic sign of the outcome of the disease. Higher specificity and lower sensitivity of alveolomucin compared to KL-6 can be used as a screening test for exogenous alveolitis. KL-6 and alveolomucin are more useful biomarkers than Clara cell secretory protein for diagnosis, exacerbation, and progression of occupational alveolitis.

Fibrosis and Damage Markers in Occupational Interstitial Lung Diseases

The investigation of biomarkers for fibrosis and lung tissue damage is very important for the diagnosis and prognosis of interstitial lung disease, as well as treatment strategies. The biomarkers secreted by Type II pneumocytes and cells of the bronchiolar epithelium are most reflective of the formation of pulmonary fibrosis and the degree of damage to the lung tissue. The levels of Krebs von den Lungen-6 (KL-6), alveolomucin (mucin-antigen 3EG5), and Clara cell secretory protein in occupational interstitial lung diseases (e.g., exogenous alveolitis, pneumoconiosis), caused by exposure to organic and inorganic factors, were analysed in dependence of disease activity phase. The level of alveolomucin in pneumoconiosis and in occupational alveolitis remission may reflect the extent of pulmonary fibrosis, which is a prognostic sign of the outcome of the disease. Higher specificity and lower sensitivity of alveolomucin compared to KL-6 can be used as a screening test for exogenous alveolitis. KL-6 and alveolomucin are more useful biomarkers than Clara cell secretory protein for diagnosis, exacerbation, and progression of occupational alveolitis.