MPO and COVID-19: In terms of disease severity, MDSC frequency has been reported to be significantly higher in patients with severe COVID-19 when compared to subjects with mild disease (236), whilst single cell transcriptomics has revealed the presence of immature CD14+MPO+Ki67+HLA-DRlo suppressive monocytes and immature ARG1+CD101+S100A8/A9+ neutrophils only in patients with severe disease (237).