The TIR-domain containing adaptor TRAM is required for TLR7 mediated RANTES production

Abstract

Toll-like receptor 7 (TLR7) plays a vital role in the immune response to ssRNA viruses such as human rhinovirus (HRV) and Influenza, against which there are currently no treatments or vaccines with long term efficacy available. Clearly, a more comprehensive understanding of the TLR7 signaling axis will contribute to its molecular targeting. TRIF related adaptor molecule (TRAM) plays a vital role in TLR4 signaling by recruiting TRIF to TLR4, followed by endosomal trafficking of the complex and initiation of IRF3 dependent type I interferon production as well as NF-kB dependent pro-inflammatory cytokine production. Towards understanding the molecular mechanisms that regulate TLR7 functionality, we found that TRAM-/- murine macrophages exhibited a transcriptional and translational impairment in TLR7 mediated RANTES, but not TNFa, production. Suppression of TRAM expression in human macrophages also resulted in an impairment in TLR7 mediated CCL5 and IFN-b, but not TNFa, gene induction. Furthermore, suppression of endogenous human TRAM expression in human macrophages significantly impaired RV16 induced CCL5 and IFNb, but not TNFa gene induction. Additionally, TRAM-G2A dose-dependently inhibited TLR7 mediated activation of CCL5, IFNb and IFNa reporter genes. TLR7-mediated phosphorylation and nuclear translocation of IRF3 was impaired in TRAM-/- cells. Finally, co-immunoprecip-itation studies indicated that TRAM physically interacts with MyD88 upon TLR7 stimulation, but not under basal conditions. Our results clearly demonstrate that TRAM plays a, hitherto unappreciated, role in TLR7 signaling through a novel signaling axis containing, but not limited to, MyD88, TRAM and IRF3 towards the activation of anti-viral immunity.