TLR and RLR Signaling Are Reprogrammed in Opposite Directions after Detection of Viral Infection

Abstract

Innate immune recognition of RNA is key for the initiation of immunity in response to viral infection. Although the factors controlling the detection of viral RNA by innate immune receptors in host cells are increasingly well understood, little is known about the dynamic changes in signaling after the initial triggering of these receptors. In this study, we report that preconditioning with the synthetic dsRNA polyinosinic-polycytidylic acid [poly(I:C)], a mimetic of viral RNA, rapidly reprograms murine APCs by simultaneously augmenting sensitivity of endosomal TLRs and inhibiting activation of RIG-I–like receptors (RLRs) in an IFN-β–dependent manner. These changes in receptor sensitivity were also seen in vivo after treatment of mice with poly(I:C). Mechanistically, the increased sensitivity of the TLR pathway was associated with elevated MAPK and NF-κB activity. The RLR response was inhibited downstream of TANK-binding kinase-1, resulting in decreased IFN regulatory factor 3 phosphorylation. Reprogramming of pattern-recognition receptor signaling also occurred after viral infection, because infection of host cells with Sendai virus or their exposure to supernatant from virus-infected cells induced the same changes in TLR and RLR sensitivity as poly(I:C). Thus, innate recognition of viral infection critically modifies responses to pattern-recognition receptor stimulation. These dynamic adaptations to infection may reinforce antiviral immunity and at the same time serve to limit pathological inflammation.

Reprogramming of TLR7 signaling enhances antitumor NK and cytotoxic T cell responses

“…Freshly isolated murine bone marrow cells, which comprise of a mixture of myeloid cells that are responsive to PRR stimulation, were stimulated twice at an interval of 24 h with poly(I:C), R848, or a sequential combination of the two stimuli. Stimulation with poly(I:C) alone did not result in strong cytokine secretion in these cells, as previously shown 17 (Fig. 1A).…”

“…[24][25][26] Here, we show that the sequential triggering first of a MyD88-independent pathway, and then of a MyD88-dependent pathway, leads to even higher Th1 responses than synergy relying on simultaneous stimulation, but with a different mechanism: We have demonstrated earlier that the viral activation of RLR reprograms the TLR7 receptor pathway and renders this pathway more sensitive to subsequent stimuli in a type I IFN-dependent manner. 17 A similar IFN-dependent reprogramming phenomenon has been described for NOD receptors following viral infection. 27 This contrasts with the synergy achieved by simultaneous activation of PRR pathways, which seems to be independent of type-I interferon.…”

“…27 This contrasts with the synergy achieved by simultaneous activation of PRR pathways, which seems to be independent of type-I interferon. 15,27,28 We have shown that the IFN-dependent TLR7 sensitization was mimicked by stimulation with poly(I:C) and was maximal after 8 to 24 h, 17 whereas synergistic poly(I:C)/TLR7 stimulation showed maximal efficiency when ligands were given 4 h apart. 15,28 We thus propose that PRR synergy and PRR reprogramming are independent and complementary phenomena to reinforce anti-viral responses by triggering Th1-type immunity.…”

“…16 We have recently demonstrated that poly(I:C) exposure, which mimics the immune response induced by viral infection, globally reprograms PRR pathways within 24 h, leading to a sensitization of TLR7 and simultaneously to a block in RLR signaling. 17 These findings highlight the importance of the timing and sequence of PRR stimulation for immunotherapeutic strategies. Indeed, we found earlier that repetitive stimulation of the TLR7 pathway led to unresponsiveness to TLR7 ligands and other MyD88-dependent agonists.…”

“…17 Microbead-based kits were used to isolate NK cells (STEM-CELL Technologies; British Columbia, Canada) or CD8 C OT-I and CD4 C OT-II T cells (Miltenyi Biotec; Bergisch Gladbach, Germany) from mouse spleens. Purity of isolated cells was routinely 85¡95%.…”