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

• Published in: The Journal of immunology (1950) Vol. 193; no. 12; pp. 6090 - 6102
• Main Authors: Stack, Julianne, Doyle, Sarah L, Connolly, Dympna J, Reinert, Line S, O'Keeffe, Kate M, McLoughlin, Rachel M, Paludan, Søren R, Bowie, Andrew G
• Format: Journal Article
• Language: English
• Published: United States 15.12.2014
• Subjects: Adaptor Proteins, Signal Transducing - genetics; Adaptor Proteins, Signal Transducing - metabolism; Cell Line; Endocytosis; Endosomes - metabolism; Host-Pathogen Interactions; Humans; Immunity, Innate; Interferon Regulatory Factor-7 - metabolism; Interferon Type I - biosynthesis; Interferon-beta - biosynthesis; Intracellular Space - metabolism; Membrane Glycoproteins - metabolism; Myeloid Differentiation Factor 88 - metabolism; Peptides - pharmacology; Protein Binding; Protein Transport; Receptors, Interleukin-1 - metabolism; Signal Transduction; Staphylococcus aureus; Toll-Like Receptor 2 - antagonists & inhibitors; Toll-Like Receptor 2 - genetics; Toll-Like Receptor 2 - metabolism; Toll-Like Receptor 4 - chemistry; Toll-Like Receptor 4 - metabolism
• ISSN: 0022-1767; 1550-6606
• DOI: 10.4049/jimmunol.1401605

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.