A mechanism for trif adaptor-biased signaling by toll-like receptor 4.

Host cells respond to bacterial lipid A through Toll-like receptor 4 (TLR4). Activation of TLR4 by lipid A triggers a response that involves two main adaptor proteins, MyD88 and TRIF. MyD88-dependent gene expression is associated with proinflammatory protein production, while TRIF-dependent gene expression is essential for optimal activation of adaptive immunity by antigen-presenting cells. Detoxified, monophosphoryl lipid A agonists (MPLA or synthetic MLA) were previously suggested to elicit TRIF-biased TLR4 signaling; that is, induction of weaker MyD88-associated gene expression but relatively intact TRIF-dependent gene expression when compared to fully active diphosphoryl lipid A (lipid A). In this work, we explored potential mechanisms by which monophosphoryl lipid A could induce TRIF-biased signaling in mouse cells. TRIF-dependent and MyD88-associated gene expression induced by both MPLA and lipid A was reduced to a similar extent by CD14 ablation, indicating that these two agonists do not differentially utilize CD14, despite this coreceptor’s primary role in directing the TRIF signaling pathway. In a second study, we demonstrated that the observation of TRIF-biased gene expression by sMLA was not because the agonist induced a TRIF-biased gene expression profile, but rather that TLR4 itself was TRIF- biased. The potencies of three different agonists were significantly higher for the induction of expression of TRIF-dependent genes than they were for induction of expression of MyD88-associated genes. Autocrine and paracrine signaling by type I interferons contributed to higher potency of TLR4 agonists for induction of TRIF-dependent gene expression because blocking the type I interferon receptor before agonist treatment diminished the effect. We propose that TLR4 is a prime target for vaccine adjuvants. The therapeutic window of TLR4 agonists may be inherently large due to the ease with which TRIF-dependent genes required for adaptive priming are activated relative to MyD88-dependent genes associated with toxicity.