A low level of CD16^pos monocytes in SARS-CoV-2 infected patients is a marker of severity

Severe forms of coronavirus disease 2019 (COVID-19) are characterized by an excessive production of inflammatory cytokines. Activated monocytes secrete high levels of cytokines. Human monocytes are divided into three major populations: conventional (CD14^posCD16^neg), non-classical (CD14^dimCD16^pos), and intermediate (CD14^posCD16^pos) monocytes. The aim of this study was to analyze whether the distribution of conventional (CD16^neg) and CD16^pos monocytes is different in patients with COVID-19 and whether the variations could be predictive of the outcome of the disease.

We performed a prospective study on 390 consecutive patients referred to the Emergency Unit, with a proven diagnosis of SARS-CoV 2 infection by RT-PCR. Using the CytoDiff™ reagent, an automated routine leukocyte differential, we quantified CD16^neg and CD16^pos monocytes.

In the entire population, median CD16^neg and CD16^pos monocyte levels (0.398 and 0.054×10⁹/L, respectively) were in the normal range [(0.3-0.7×10⁹/L) and (0.015-0.065×10⁹/L), respectively], but the 35 patients in the intensive care unit (ICU) had a significantly (p<0.001) lower CD16^pos monocyte count (0.018 × 10⁹/L) in comparison to the 70 patients who were discharged (0.064 × 10⁹/L) or were hospitalized in conventional units (0.058 × 10⁹/L). By ROC curve analysis, the ratio [absolute neutrophil count/CD16^pos monocyte count] was highly discriminant to identify patients requiring ICU hospitalization: with a cut-off 193.1, the sensitivity and the specificity were 74.3 and 81.8%, respectively (area under the curve=0.817).

Quantification of CD16^pos monocytes and the ratio [absolute neutrophil count/CD16^pos monocyte count] could constitute a marker of the severity of disease in COVID-19 patients.