Borrelia burgdorferi is the spirochetal agent of Lyme disease, a multisystemic disorder characterized by inflammation. Using global transcriptional profiling, we characterized the response of human PBMCs exposed to B. burgdorferi in an ex vivo coculture system. The expression profiles induced by B. burgdorferi were marked by the intense up-regulation of IFN-responsive transcripts and transcripts involved in the JAK/STAT signaling pathway. Transcript levels of IFN-α, IFN-β, and IRF7, and protein concentrations of IFN-α, were significantly elevated relative to those in unstimulated PBMCs. The induction of IFN-α was completely dependent upon phagocytosis of B. burgdorferi. Addition of a soluble type I IFN receptor, B18R, did not abolish the induction of IFN-inducible genes, indicating that B. burgdorferi directly elicits enhanced expression of these genes independently of type I IFN feedback signaling. Inhibitors of either TLR7 or TLR9 significantly reduced B. burgdorferi-stimulated IFN-α protein expression and transcription of IFN-induced genes. Simultaneous inhibition of both TLR7 and TLR9 completely abrogated IFN-α induction. The IFN-α-producing populations in PBMCs were identified as plasmacytoid dendritic and CD14+CD11c+ cells. These results reveal a TLR7/9-dependent signaling pathway used by human PBMCs to initiate a type I IFN response to the extracellular bacterium B. burgdorferi.

Borrelia burgdorferi (Bb) is the extracellular bacterial agent of Lyme disease. Bb elicits type I interferon (IFN) in humans via Toll-like receptors 7 and 9. Bb clinical isolates with different pathogenic potential were tested for their ability to induce IFNs in human peripheral blood mononuclear cells (PBMCs). Disseminating Bb isolates induced significantly higher levels of both type I (IFN-α) and type III IFN (IFN-λ) than did non-disseminating Bb isolates. In contrast, there was no significant difference in type II IFN (IFN-γ) production, nor in levels of IL-2, 6, 8, 10. However, non-disseminating Bb isolates induced significantly higher levels of TNF-α and IL-1β. In addition, disseminating clinical isolates were able to directly induce multiple IFN-responsive genes; non-disseminating strains required IFN-feedback signaling. A mutant of Bb was identified that was unable to induce a robust IFN-α response when compared to wild-type (wt); IFN-γ levels were not affected. Wt and mutant Bb were GFP-tagged and assessed for their ability to be phagocytosed by human immune cell populations using flow cytometry and fluorescence microscopy. There were more wt than mutant Bb associated with (p=0.01) and internalized by (p=0.034) PBMCs. These findings suggest that disseminating and non-disseminating clinical isolates may be differentially phagocytosed by dendritic cells. Intriguingly, the differential uptake appears to be mediated by factor(s) encoded on a single linear plasmid of Bb.