TLR4 and infection: To investigate whether modulation of TLR4 expression influences HMPV-induced responses, we examined TLR4 mRNA levels and observed that, while TLR4 expression in MDMs was not significantly affected by HMPV infection, TLR4 mRNA in THP-1 cells was significantly reduced at 6 and 9 h post-infection and subsequently upregulated at 20 h (Figures 1A, B, right panels).