MyD88-Dependent and MyD88-Independent Pathways in Synergy, Priming, and Tolerance between TLR Agonists

Abstract

TLRs sense components of microorganisms and are critical host mediators of inflammation during infection. Different TLR agonists can profoundly alter inflammatory effects of one another, and studies suggest that the sequence of exposure to TLR agonists may importantly impact on responses during infection. We tested the hypothesis that synergy, priming, and tolerance between TLR agonists follow a pattern that can be predicted based on differential engagement of the MyD88-dependent (D) and the MyD88-independent (I) intracellular signaling pathways. Inflammatory effects of combinations of D and I pathway agonists were quantified in vivo and in vitro. Experiments used several D-specific agonists, an I-specific agonist (poly(I:C)), and LPS, which acts through both the D and I pathways. D-specific agonists included: peptidoglycan-associated lipoprotein, Pam3Cys, flagellin, and CpG DNA, which act through TLR2 (peptidoglycan-associated lipoprotein and Pam3Cys), TLR5, and TLR9, respectively. D and I agonists were markedly synergistic in inducing cytokine production in vivo in mice. All of the D-specific agonists were synergistic with poly(I:C) in vitro in inducing TNF and IL-6 production by mouse bone marrow-derived macrophages. Pretreatment of bone marrow-derived macrophages with poly(I:C) led to a primed response to subsequent D-specific agonists and vice versa, as indicated by increased cytokine production, and increased NF-κB translocation. Pretreatment with a D-specific agonist augmented LPS-induced IFN-β production. All D-specific agonists induced tolerance to one another. Thus, under the conditions studied here, simultaneous and sequential activation of both the D and I pathways causes synergy and priming, respectively, and tolerance is induced by agonists that act through the same pathway.

Visualization of IFNβ production by plasmacytoid versus conventional dendritic cells under specific stimulation conditions in vivo

“…Indeed, simultaneous targeting of the MyD88-dependent TLR9 pathway by CpG 1668 and the MyD88-independent TLR3 pathway by poly(I:C), synergistically induced IFN␤ expression in BMM⌽s. Similar effects were reported for IL-6 and TNF production after in vitro stimulation of murine M⌽s with poly(I:C) and B-type CpG ODN (22), or after in vivo challenge with the MyD88-independent TLR3 agonist poly(I:C) and the MyD88-dependent TLR2 agonist Pam3Cys (23). However, inhibitory effects on IFN␤ production were shown for poly(I:C) and CpG 2216 in BMM⌽s and CpG 2216 and R848 in BMDCs, respectively.…”

Induction of TLR Tolerance in Human Macrophages by Adiponectin: Does LPS Play a Role?

“…We found that LPS in the concentration range of 3-30 pg ⁄ ml, recapitulated some effects of adiponectin including attenuation of TNF induction by LPS after adiponectin pre-incubation (Fig. 1), heterologous desensitization of TLR signalling [35], as has been previously described for adiponectin by other authors [24] (Table 2), as well as direct induction of TNF-a and IL-6 and activation of p38 MAPK and nuclear factor-jB signalling (Figs. 2 and 3).…”

Prevention of autoimmune disease by induction of tolerance to Toll-like receptor 7

“…In a murine model of SLE, the genetic loss of TLR7 ameliorated disease, decreased lymphocyte activation and reduced serum IgG (16). These findings prompted us to hypothesize that repeated stimulation with a synthetic TLR7 agonist, at well tolerated low doses (10,17,18), might also restrain autoimmune inflammation.…”