USP18 and infection: PHEV significantly upregulated ISG15 expression (>1.5 log2FC) at all three time points, and it appears ISG15 does interact with HERC5, IFIT1, PKR, TRIM25, STAT1, and USP18, as their expression was also enhanced (>1.0 log2FC) at various timepoints during ALI-PRECs infection (Tables S1–S3).