SPDL1 and COVID-19: Further analyses confirmed a dysfunctional PD-1/PD-L1 immune checkpoint axis in patients who recovered from COVID-19, as revealed by lower plasma levels of sPD-L1, as well as by increased expression of miR–15a-5p — a previously reported PD1-targeting miRNA with inhibitory effects — and by higher PD1 promoter methylation occurring in CD8+ T cells as compared with healthy controls.