The synergy in cytokine production through MyD88-TRIF pathways is co-ordinated with ERK phosphorylation in macrophages.

Although specific single Toll-like receptor (TLR) ligands are known to drive the development of Th1 or Th2 immunity, the outcome of different combinations of TLR ligands on innate immunity is not well defined. Spatiotemporal dynamics are critical in determining the specificity of the immune response, but the mechanisms underlying combinatorial TLR stimulation remain unclear. Here, we tested pairwise combinations of TLR ligands separated by different time intervals for their effect on cytokine production in macrophages. We observed that stimulation via a combination of MyD88- and TRIF-utilizing adaptors leads to a highly synergistic cytokine response. On a timescale of 4-24 h, macrophages pretreated with poly(I:C) (TLR3 ligand) are cross-primed to a second stimulation with R848 (TLR7 ligand) and vice versa, and each condition exhibits different optimal time windows of synergistic response for each cytokine. We show that the synergy resulting from combinatorial stimuli (poly(I:C) and R848 is also regulated by the order and dosage of the TLR agonists. Secondary response genes, which depend on new protein synthesis for transcription, show greater synergy than primary response genes, and such enhancement is abolished when new protein synthesis is inhibited. Synergistic cytokine production appears concordant with sustained ERK phosphorylation, suggesting that the de novo factors act via inhibition of ERK dephosphorylation, for example, by the downregulation of dual specificity phosphatase 6. Taken together, our findings illustrate a checkpoint in the innate immune system, where the synchronization of timing of both MyD88 and TRIF pathways is required for a maximal cytokine response and potential memory effect in macrophages.

SARM: a novel Toll-like receptor adaptor, is functionally conserved from arthropod to human.

Sterile-alpha and Armadillo motif containing protein (SARM) was recently identified as the fifth member of the Toll-like receptor (TLR) adaptor family. Whilst the Caenorhabditis elegans SARM homologue, TIR-1, is crucial for efficient immune responses against bacterial infections, human SARM was demonstrated to function as a specific inhibitor of TRIF-dependent TLR signaling. The opposing role of SARM in C. elegans and human is intriguing, prompting us to seek clarification on the enigmatic function of SARM in an ancient species which relies solely on innate immunity for survival. Here, we report the discovery of a primitive but functional SARM (CrSARM) in the immune defense of a "living fossil", the horseshoe crab, Carcinoscorpius rotundicauda. CrSARM shares numerous signature motifs and displays significant homology with vertebrate and invertebrate SARM homologues. CrSARM downregulates TRIF-dependent TLR signaling suggesting the conservation of SARM function from horseshoe crab to human. During infection by Pseudomonas aeruginosa, CrSARM is rapidly upregulated within 3h and strongly repressed at 6h, coinciding with the timing of bacterial clearance, thus demonstrating its dynamic role in innate immunity. Furthermore, yeast-two-hybrid screening revealed several potential interaction partners of CrSARM implying the role of SARM in downregulating TLR signaling events. Altogether, our study shows that, although C. elegans SARM upregulates immune signaling, its disparate role as a suppressor of TLR signaling, specifically via TRIF and not MyD88, is well-conserved from horseshoe crab to human.