Сytokine expression and production in severe cases of SARS-CoV-2 infection

Most respiratory viral infections proceed in mild form including COVID-19. Gowever, some patients experience severe systemic inflammation, tissue damage, acute respiratory distress syndrome, and cytokine storm with potentially lethal outcomes. The cytokines have been thought to play an important role in immunopathology of viral infection. However, an excessive immune response, manifesting as massive release of pro-inflammatory cytokines, may cause immune damage in the body. Production and release of the cytokines in healthy individuals presumes a significant balance of inflammatory and homeostatic factors. Meanwhile, in the case of COVID-19 disease, uncontrolled increased production of cytokines often occurs with fatal consequences for patients. The aim of this work was to study the level of IL-1β, IL-18 and TNFα gene expression, as well as production of these cytokines at the level of mucous membranes of the upper respiratory tract, in particular in oral cavity, in patients with severe COVID-19 disease.

The present study included patients who recovered from severe COVID-19. The control group consisted of conditionally healthy individuals. Expression levels of the IL-1β, IL-18, and TNFα genes were determined by RT-PCR. The levels of IL-1β, IL-18 and TNFα protein production were determined by multiplex enzyme immunoassay.

The expression levels of IL-1β, IL-18 were reduced at the onset of the disease, as well as in the midpoint of the COVID-19 disease, but increased on the 30^th day. The protein production of these cytokines was also reduced in the first days from the onset of the disease. The levels of pro-inflammatory TNFα cytokine was high at the onset of the disease. The level of TNFα production at the onset of the disease was also higher relative to the control group. Subsequently, the TNFα gene expression levels decreased upon progression of the disease.

Thus, the increased expression level of pro-inflammatory cytokines may be explained by the fact that the S protein of the SARS-CoV-2 virus induces increased expression of these cytokines in human monocytes. Meanwhile, appropriate protein levels remain low, especially on day 1 from the onset of the disease. Thus, one may conclude that the virus triggers pyropotosis, however, within 15-30 days from the onset of the disease, when viral replication is already minimal.