USP21 and COVID-19: In contrast to COVID-19(+) TV, highly expressed in the COVID-19(-) PU control group included MHC class II antigen processing (HLA-DRB3/4, HLA-DPB1, HLA-DRA, CIITA, and CD74), mast cell degranulation (TPSAB1/B2), B cell activation (CXCL13, BLNK, IL-6) and histone modification (USP21, HIST1H4E).