The disease COVID-19 has developed into a worldwide pandemic. Hyperinflammation and high levels of several cytokines, for example, IL-6, are observed in severe COVID-19 cases. However, little is known about the cellular origin of these cytokines. Here, we investigated whether circulating leukocytes from patients with COVID-19 had spontaneous cytokine production. Patients with hyperinflammatory COVID-19 (n = 6) and sepsis (n = 3) were included at Skåne University Hospital, Sweden. Healthy controls were also recruited (n = 5). Cytokines were measured in COVID-19 and sepsis patients using an Immulite immunoassay system. PBMCs were cultured with brefeldin A to allow cytokine accumulation. In parallel, LPS was used as an activator. Cells were analyzed for cytokines and surface markers by flow cytometry. High levels of IL-6 and measurable levels of IL-8 and TNF, but not IL-1β, were observed in COVID-19 patients. Monocytes from COVID-19 patients had spontaneous production of IL-1β and IL-8 (P = 0.0043), but not of TNF and IL-6, compared to controls. No spontaneous cytokine production was seen in lymphocytes from either patients or controls. Activation with LPS resulted in massive cytokine production by monocytes from COVID-19 patients and healthy controls, but not from sepsis patients. Finally, monocytes from COVID-19 patients produced more IL-1β than from healthy controls (P = 0.0087) when activated. In conclusion, monocytes contribute partly to the ongoing hyperinflammation by production of IL-1β and IL-8. Additionally, they are responsive to further activation. This data supports the notion of IL-1β blockade in treatment of COVID-19. However, the source of the high levels of IL-6 remains to be determined.
© 2020 The Authors. Journal of Leukocyte Biology published by Wiley Periodicals LLC on behalf of Society for Leukocyte Biology.

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