PIWIL2 and infection: Then, Ex/Mv were isolated and purified from htNSC-PIWIL2 KO versus control htNSCs, and the same dose of these Ex/Mv (3.5 μg Ex/Mv for 200,000 hACE2-A549 cells) was used to treat hACE2-A549 cells upon infection of MOI 1.0 SARS-CoV-2.