TRAM is required for TLR2 endosomal signaling to type I IFN induction.
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.
School of Biochemistry and Immunology, Trinity Biomedical Sciences Institute, Trinity College Dublin, Dublin 2, Ireland;
Julianne
J
Stack
School of Medicine, Trinity College Dublin, Dublin 2, Ireland; The National Children's Research Centre, Our Lady's Children's Hospital, Crumlin, Dublin 12, Ireland; and.
Sarah L
SL
Doyle
School of Biochemistry and Immunology, Trinity Biomedical Sciences Institute, Trinity College Dublin, Dublin 2, Ireland;
Dympna J
DJ
Connolly
Department of Biomedicine, Aarhus University, DK-8000 Aarhus C, Denmark.
Line S
LS
Reinert
School of Biochemistry and Immunology, Trinity Biomedical Sciences Institute, Trinity College Dublin, Dublin 2, Ireland;
Kate M
KM
O'Keeffe
School of Biochemistry and Immunology, Trinity Biomedical Sciences Institute, Trinity College Dublin, Dublin 2, Ireland;
Rachel M
RM
McLoughlin
Department of Biomedicine, Aarhus University, DK-8000 Aarhus C, Denmark.
Søren R
SR
Paludan
School of Biochemistry and Immunology, Trinity Biomedical Sciences Institute, Trinity College Dublin, Dublin 2, Ireland; agbowie@tcd.ie.
Andrew G
AG
Bowie
D048868:Adaptor Proteins, Signal Transducing / Q000235:genetics / Q000378:metabolism*; D002460:Cell Line; D004705:Endocytosis; D011992:Endosomes / Q000378:metabolism*; D054884:Host-Pathogen Interactions; D006801:Humans; D007113:Immunity, Innate; D050839:Interferon Regulatory Factor-7 / Q000378:metabolism; D007370:Interferon Type I / Q000096:biosynthesis*; D016899:Interferon-beta / Q000096:biosynthesis; D042541:Intracellular Space / Q000378:metabolism; D008562:Membrane Glycoproteins / Q000378:metabolism; D053594:Myeloid Differentiation Factor 88 / Q000378:metabolism; D010455:Peptides / Q000494:pharmacology; D011485:Protein Binding; D021381:Protein Transport; D017472:Receptors, Interleukin-1 / Q000378:metabolism; D015398:Signal Transduction; D051195:Toll-Like Receptor 2 / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D051197:Toll-Like Receptor 4 / Q000737:chemistry / Q000378:metabolism
D048868:Adaptor Proteins, Signal Transducing; D050839:Interferon Regulatory Factor-7; D007370:Interferon Type I; D008562:Membrane Glycoproteins; D053594:Myeloid Differentiation Factor 88; D010455:Peptides; D017472:Receptors, Interleukin-1; C480228:TICAM2 protein, human; C434984:TIRAP protein, human; D051195:Toll-Like Receptor 2; D051197:Toll-Like Receptor 4; D016899:Interferon-beta