We have previously shown that mutations of CD14 or TLR4 impair type I interferon (IFN) production and macrophage survival during infection with vesicular stomatitis virus (VSV). We now report that VSV glycoprotein G (gpG) is essential for the induction of a previously unrecognized CD14/TLR4-dependent response pathway in which the adapter TRAM has predominant importance, absent any need for MyD88 or Mal, and with only a partial requirement for TRIF. Downstream of TRAM, IRF7 activation leads to a type I IFN response. The pathway is utilized by myeloid dendritic cells (mDCs) and macrophages rather than plasmacytoid DCs. This new mode of TLR4 signal transduction, which does not stimulate NF-κB activation, reveals the importance of viral protein recognition by mDCs and shows that TLR4 can drive qualitatively different events within the cell in response to different ligands.
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