TICAM-1 and TICAM-2: toll-like receptor adapters that participate in induction of type 1 interferons

Toll-like receptor (TLR) 3 and 4 mediate the expression of many genes, including NF-κB- and interferon-regulatory factor (IRF)-3/interferon (IFN)-inducible genes, in macrophages and dendritic cells (DCs) in response to their ligand stimuli, polyI:C and lipopolysaccharide (LPS). Toll-IL-1 receptor homology domain (TIR)-containing adapter molecule 1 (TICAM-1) facilitates expression of IFN-inducible genes via TLR3. Although MyD88 and Mal/TIRAP adapters function downstream of TLR4, they barely induce IFN-β. In addition, DC maturation as well as IFN-β induction are largely independent of MyD88 and Mal/TIRAP. TICAM-1 is the functional adapter for both TLR3 and TLR4 that induces type 1 IFN and MyD88-independent DC maturation. In LPS-mediated TLR4 activation, a complex of TICAM-1 and an additional TLR4-binding adapter serves as the adapter. We named this TLR4-TICAM-1-bridging adapter TICAM-2. Our results reveal the details of MyD88-independent pathways which separately recruit the distinct adapters downstream of TLR3 and TLR4 and variations of the TLR output are in part regulated by the two additional adapters in DCs.