Investigating the mechanism of TLR signaling crosstalk in mouse macrophages

Authors

• Bin Lin
• Iain Fraser

Abstract

It is well established that the TRIF- and MyD88-dependent signaling branches in the TLR4 pathway cooperate to induce a strong innate immune response. It is also known that certain toll-like receptor (TLR) ligand combinations can induce synergistic production of some cytokines. However, the molecular mechanisms underlying this signaling pathway cooperation remain elusive. We conducted a microarray analysis of mouse bone marrow-derived macrophages stimulated with all pairwise combinations of the TLR ligands LPS, Pam3CSK4, R848 and poly I:C to characterize synergy in transcript induction. We find that the crosstalk between the different TLR pathways is most prevalent in combinations that induce both the TRIF and the MyD88 pathways, which is essentially a mimic of the TLR4 pathway. Genes synergistically induced under dual TLR stimulation are mainly cytokines and immune effectors, including IL-6, IL-10, IL-1α, IL-1β, haptoglobin, lipocalin 2, and SOCS3. Based on these data, over 200 ‘synergy factor’ candidates were picked and targeted using RNAi screening, with IL-6 and IL-12p40 secretion as read outs. A total of 32 hits were identified in the primary screening, including well-known regulators of secondary response gene transcription such as Nfkbiz, Irak2, Tnfaip3 and Tnip1. A subset of the gene hits were selected for Fluidigm analysis to determine if they selectively affect different classes of TLR-induced inflammatory mediators. This analysis has identified a group of specific regulators of secondary response genes that may provide novel therapeutic targets for regulating TLR-driven outputs. This study was supported by the Intramural Research Program of NIAID, NIH.

Scientific Focus Area: Immunology

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