In the absence of NA pretreatment, TMPRSS2 enabled UCD infection, which remained limited at 12 hpi but showed extensive infection and increased syncytia formation by 48 hpi For UG-FH8, TMPRSS2 had no significant effect at 12 and 24 hpi compared to the mock group but promoted infection at 48 hpi, as indicated by fewer remaining cells. Here, XK is linked to infection.