CCR2 and COVID-19: Substantiating our hypotheses on the cellular origin of cytokine production, scRNA-seq of PBMCs showed that classical monocytes were the main source of major COVID-19 mediating cytokines, including the monocyte chemoattractant CCL2 and its receptor CCR2, the neutrophil chemoattractant CXCL8, and TNF-α.