TRAM is required for TLR2 endosomal signaling to type I IFN induction

J Stack, SL Doyle, DJ Connolly, LS Reinert… - The Journal of …, 2014 - journals.aai.org
J Stack, SL Doyle, DJ Connolly, LS Reinert, KM O'Keeffe, RM McLoughlin, SR Paludan
The Journal of Immunology, 2014journals.aai.org
Detection of microbes by TLRs on the plasma membrane leads to the induction of
proinflammatory cytokines such as TNF-α, via activation of NF-κB. Alternatively, activation of
endosomal TLRs leads to the induction of type I IFNs via IFN regulatory factors (IRFs). TLR4
signaling from the plasma membrane to NF-κB via the Toll/IL-1R (TIR) adaptor protein
MyD88 requires the TIR sorting adaptor Mal, whereas endosomal TLR4 signaling to IRF3
via the TIR domain–containing adaptor-inducing IFN-β (TRIF) requires the TRIF-related …
Abstract
Detection of microbes by TLRs on the plasma membrane leads to the induction of proinflammatory cytokines such as TNF-α, via activation of NF-κB. Alternatively, activation of endosomal TLRs leads to the induction of type I IFNs via IFN regulatory factors (IRFs). TLR4 signaling from the plasma membrane to NF-κB via the Toll/IL-1R (TIR) adaptor protein MyD88 requires the TIR sorting adaptor Mal, whereas endosomal TLR4 signaling to IRF3 via the TIR domain–containing adaptor-inducing IFN-β (TRIF) requires the TRIF-related adaptor molecule (TRAM). Similar to TLR4 homodimers, TLR2 heterodimers can also induce both proinflammatory cytokines and type I IFNs. TLR2 plasma membrane signaling to NF-κB is known to require MyD88 and Mal, whereas endosomal IRF activation by TLR2 requires MyD88. However, it was unclear whether TLR2 requires a sorting adaptor for endosomal signaling, like TLR4 does. In this study, we show that TLR2-dependent IRF7 activation at the endosome is both Mal-and TRAM-dependent, and that TRAM is required for the TLR2-dependent movement of MyD88 to endosomes following ligand engagement. TRAM interacted with both TLR2 and MyD88, suggesting that TRAM can act as a bridging adapter between these two molecules. Furthermore, infection of macrophages lacking TRAM with herpes viruses or the bacterium Staphylococcus aureus led to impaired induction of type I IFN, indicating a role for TRAM in TLR2-dependent responses to human pathogens. Our work reveals that TRAM acts as a sorting adaptor not only for TLR4, but also for TLR2, to facilitate signaling to IRF7 at the endosome, which explains how TLR2 is capable of causing type I IFN induction.
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