IL-6 and IL-18 cytokine traps in COVID-19

Cytokines are mediators of immunity that regulate inflammation. Intensity of inflammatory process is strongly dependent on the cytokine type and duration of its effect. Interleukin 6 (IL-6) and interleukin 18 (IL-18) play an important role in the initiation and progression of inflammation. Cytokines regulate the inflammatory process in different ways by inducing or inhibiting inflammatory reactions. Functional activity of cytokines is limited by trap molecules whose levels determine initiation of protective or pathological effects of interleukins. Soluble glycoprotein sgp130 functions as a trap for IL-6, while IL-18 is controlled by IL-18 binding protein (IL-18BP). High IL-6 and IL-18 levels were recorded in COVID-19 patients, being associated with unfavorable outcome of the disease. Our objective was to compare sgp130 and IL-18BP levels in patients with different degrees of COVID-19. Retrospective study included 74 COVID-19 patients (40 men and 34 women) aged 63±14 years. The patients were assigned to groups according to severity of lung damage. Group 1 included patients without lung damage; group 2, patients with moderate pneumonia (< 50% lung damage); group 3, patients with severe pneumonia (> 50% lung damage). Plasma levels of cytokines and their trap molecules were determined by quantitative immunoenzyme assay. IL-6 and IL-18 plasma concentrations increased with COVID-19 severity. Ambiguous changes were recorded for their traps. Plasma levels of sgp130 were lower in patients with moderate pneumonia than in patients without lung damage. In patients with severe pneumonia sgp130 plasma concentrations were higher than those in patients with mild pneumonia, being similar to those in patients without lung damage. In contrast to sgp130, IL-18BP levels decreased with COVID-19 severity. Thus, an increase in IL-6 and IL-18 levels parallel to COVID-19 severity is accompanied by ambiguous changes in the levels of their trap molecules. The ratio between the levels of IL-6 and IL-18 and their traps reflects the degree of COVID-19 severity.