Activation of Human Monocytes by Live Borrelia burgdorferi Generates TLR2-Dependent and -Independent Responses Which Include Induction of IFN-β

Abstract

It is widely believed that innate immune responses to Borrelia burgdorferi (Bb) are primarily triggered by the spirochete's outer membrane lipoproteins signaling through cell surface TLR1/2. We recently challenged this notion by demonstrating that phagocytosis of live Bb by peripheral blood mononuclear cells (PBMCs) elicited greater production of proinflammatory cytokines than did equivalent bacterial lysates. Using whole genome microarrays, we show herein that, compared to lysates, live spirochetes elicited a more intense and much broader transcriptional response involving genes associated with diverse cellular processes; among these were IFN-β and a number of interferon-stimulated genes (ISGs), which are not known to result from TLR2 signaling. Using isolated monocytes, we demonstrated that cell activation signals elicited by live Bb result from cell surface interactions and uptake and degradation of organisms within phagosomes. As with PBCMs, live Bb induced markedly greater transcription and secretion of TNF-α, IL-6, IL-10 and IL-1β in monocytes than did lysates. Secreted IL-18, which, like IL-1β, also requires cleavage by activated caspase-1, was generated only in response to live Bb. Pro-inflammatory cytokine production by TLR2-deficient murine macrophages was only moderately diminished in response to live Bb but was drastically impaired against lysates; TLR2 deficiency had no significant effect on uptake and degradation of spirochetes. As with PBMCs, live Bb was a much more potent inducer of IFN-β and ISGs in isolated monocytes than were lysates or a synthetic TLR2 agonist. Collectively, our results indicate that the enhanced innate immune responses of monocytes following phagocytosis of live Bb have both TLR2-dependent and -independent components and that the latter induce transcription of type I IFNs and ISGs.

Author Summary

Lyme disease is a tick-borne infectious disorder caused by the spirochetal pathogen Borrelia burgdorferi (Bb). Innate immune responses to Bb are thought to be triggered by the spirochete's outer membrane lipoproteins signaling through cell surface toll-like receptors (TLR1/2). Using a whole genome microarray technique, we showed that live spirochetes elicited a more intense and broader immune response in human peripheral blood mononuclear cells (PBMCs) than could be explained simply by TLR1/2 cell surface stimulation. Of particular interest, live Bb also uniquely induced transcription of type I interferons. In similarly stimulated isolated human monocytes, live Bb generated a greater production of pro- and anti-inflammatory cytokines (TNF-α, IL-6, IL-10 and IL-1β), as well as interferon-β (IFN-β). Secreted IL-18, which like IL-1β requires cytosolic cleavage of its inactive form by activated caspase-1, was generated only in response to live Bb. The cytosolic responses occurred despite evidence that phagocytosed spirochetes were rapidly degraded in phagosomal vacuoles, and unable to escape unscathed into the cell cytosol. We conclude that the innate immune signals generated in human monocytes by phagocytosed spirochetes allow the host to control the bacterium through a number of non-exclusive pathways, that are both TLR2-dependent and -independent, and include a type I interferon response.