Borrelia burgdorferi RNA induces type I and III interferons via Toll-like receptor 7 and contributes to production of NF-κB-dependent cytokines.

Borrelia burgdorferi elicits a potent cytokine response through activation of multiple signaling receptors on innate immune cells. Spirochetal lipoproteins initiate expression of NF-κB-dependent cytokines primarily via TLR2, whereas type I interferon (IFN) production is induced through the endosomal receptors TLR7 and TLR9 in human dendritic cells and TLR8 in monocytes. We demonstrate that DNA and RNA are the B. burgdorferi components that initiate a type I IFN response by human peripheral blood mononuclear cells (PBMCs). IFN-α protein and transcripts for IRF7, MX1, and OAS1 were induced by endosomal delivery of B. burgdorferi DNA, RNA, or whole-cell lysate, but not by lysate that had been treated with DNase and RNase. Induction of IFN-α and IFN-λ1, a type III IFN, by B. burgdorferi RNA or live spirochetes required TLR7-dependent signaling and correlated with significantly enhanced transcription and expression of IRF7 but not IRF3. Induction of type I and type III IFNs by B. burgdorferi RNA could be completely abrogated by a TLR7 inhibitor, IRS661. In addition to type I and type III IFNs, B. burgdorferi RNA contributed to the production of the NF-κB-dependent cytokines, IFN-γ, interleukin-10 (IL-10), IL-1β, IL-6, and tumor necrosis factor alpha (TNF-α), by human PBMCs. Collectively, these data indicate that TLR7-dependent recognition of RNA is pivotal for IFN-α and IFN-λ1 production by human PBMCs, and that RNA-initiated signaling contributes to full potentiation of the cytokine response generated during B. burgdorferi infection.