| Summary: | A malfunction of the innate immune response in COVID-19 is associated with eosinopenia, particularly in more severe cases. This study tested the hypothesis that this eosinopenia is COVID-19 specific and is associated with systemic activation of eosinophils. Blood of 15 healthy controls and 75 adult patients with suspected COVID-19 at the ER were included before PCR testing and analyzed by point-of-care automated flow cytometry (CD10, CD11b, CD16, and CD62L) in the absence or presence of a formyl peptide (fNLF). Forty-five SARS-CoV-2 PCR positive patients were grouped based on disease severity. PCR negative patients with proven bacterial (<i>n</i> = 20) or other viral (<i>n</i> = 10) infections were used as disease controls. Eosinophils were identified with the use of the FlowSOM algorithm. Low blood eosinophil numbers (<100 cells/μL; <i>p</i> < 0.005) were found both in patients with COVID-19 and with other infectious diseases, albeit less pronounced. Two discrete eosinophil populations were identified in healthy controls both before and after activation with fNLF based on the expression of CD11b. Before activation, the CD11b<sup>bright</sup> population consisted of 5.4% (CI<sub>95%</sub> = 3.8, 13.4) of total eosinophils. After activation, this population of CD11b<sup>bright</sup> cells comprised nearly half the population (42.21%, CI<sub>95%</sub> = 35.9, 54.1). Eosinophils in COVID-19 had a similar percentage of CD11b<sup>bright</sup> cells before activation (7.6%, CI<sub>95%</sub> = 4.5, 13.6), but were clearly refractory to activation with fNLF as a much lower percentage of cells end up in the CD11b<sup>bright</sup> fraction after activation (23.7%, CI<sub>95%</sub> = 18.5, 27.6; <i>p</i> < 0.001). Low eosinophil numbers in COVID-19 are associated with refractoriness in responsiveness to fNLF. This might be caused by migration of fully functional cells to the tissue.
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