Toll-like receptors: insights into their possible role in the pathogenesis of lyme neuroborreliosis

Borrelia species induce inflammasome activation and IL-17 production through a caspase-1-dependent mechanism

Borrelia burgdorferi spirochetes cause Lyme disease, which can result in severe clinical symptoms such as multiple joint inflammation and neurological disorders. IFN-γ and IL-17 have been suggested to play an important role in the host defense against Borrelia, and in the immunopathology of Lyme disease. The caspase-1-dependent cytokine IL-1β has been linked to the generation of IL-17-producing T cells, whereas caspase-1-mediated IL-18 is crucial for IFN-γ production. In this study, we show by using knockout mice the role of inflammasome-activated caspase-1 in the regulation of cytokine responses by B. burgdorferi. Caspase-1-deficient cells showed significantly less IFN-γ and IL-17 production after Borrelia stimulation. A lack of IL-1β was responsible for the defective IL-17 production, whereas IL-18 was crucial for the IFN-γ production. Caspase-1-dependent IL-33 played no role in the Borrelia-induced production of IL-1β, IFN-γ or IL-17. In conclusion, we describe for the first time the role of the inflammasome-dependent caspase-1 activation of cytokines in the regulation of IL-17 production induced by Borrelia spp. As IL-17 has been implicated in the pathogenesis of chronic Lyme disease, these data suggest that caspase-1 targeting may represent a new immunomodulatory strategy for the treatment of complications of late stage Lyme disease.

An intravascular immune response to Borrelia burgdorferi involves Kupffer cells and iNKT cells

Here we investigate the dynamics of the hepatic intravascular immune response to a pathogen relevant to invariant natural killer T cells (iNKT cells). Immobilized Kupffer cells with highly ramified extended processes into multiple sinusoids could effectively capture blood-borne, disseminating Borrelia burgdorferi, creating a highly efficient surveillance and filtering system. After ingesting B. burgdorferi, Kupffer cells induced chemokine receptor CXCR3-dependent clustering of iNKT cells. Kupffer cells and iNKT cells formed stable contacts via the antigen-presenting molecule CD1d, which led to iNKT cell activation. An absence of iNKT cells caused B. burgdorferi to leave the blood and enter the joints more effectively. B. burgdorferi that escaped Kupffer cells entered the liver parenchyma and survived despite Ito cell responses. Kupffer cell-iNKT cell interactions induced a key intravascular immune response that diminished the dissemination of B. burgdorferi.

Microglia in infectious diseases of the central nervous system

Microglia are the resident macrophage population in the central nervous system (CNS) parenchyma and, as such, are poised to provide a first line of defense against invading pathogens. Microglia are endowed with a vast repertoire of pattern recognition receptors that include such family members as Toll-like receptors and phagocytic receptors, which collectively function to sense and eliminate microbes invading the CNS parenchyma. In addition, microglial activation elicits a broad range of pro-inflammatory cytokines and chemokines that are involved in the recruitment and subsequent activation of peripheral immune cells infiltrating the infected CNS. Studies from several laboratories have demonstrated the ability of microglia to sense and respond to a wide variety of pathogens capable of colonizing the CNS including bacterial, viral, and fungal species. This review will highlight the role of microglia in microbial recognition and the resultant antipathogen response that ensues in an attempt to clear these infections. Implications as to whether microglial activation is uniformly beneficial to the CNS or in some circumstances may exacerbate pathology will also be discussed.

Inflammation and central nervous system Lyme disease

Lyme disease, caused by the bacterium Borrelia burgdorferi, can cause multi-systemic signs and symptoms, including peripheral and central nervous system disease. This review examines the evidence for and mechanisms of inflammation in neurologic Lyme disease, with a specific focus on the central nervous system, drawing upon human studies and controlled research with experimentally infected rhesus monkeys. Directions for future human research are suggested that may help to clarify the role of inflammation as a mediator of the chronic persistent symptoms experienced by some patients despite antibiotic treatment for neurologic Lyme disease.

Microglia are mediators of Borrelia burgdorferi-induced apoptosis in SH-SY5Y neuronal cells

Inflammation has long been implicated as a contributor to pathogenesis in many CNS illnesses, including Lyme neuroborreliosis. Borrelia burgdorferi is the spirochete that causes Lyme disease and it is known to potently induce the production of inflammatory mediators in a variety of cells. In experiments where B. burgdorferi was co-cultured in vitro with primary microglia, we observed robust expression and release of IL-6 and IL-8, CCL2 (MCP-1), CCL3 (MIP-1α), CCL4 (MIP-1β) and CCL5 (RANTES), but we detected no induction of microglial apoptosis. In contrast, SH-SY5Y (SY) neuroblastoma cells co-cultured with B. burgdorferi expressed negligible amounts of inflammatory mediators and also remained resistant to apoptosis. When SY cells were co-cultured with microglia and B. burgdorferi, significant neuronal apoptosis consistently occurred. Confocal microscopy imaging of these cell cultures stained for apoptosis and with cell type-specific markers confirmed that it was predominantly the SY cells that were dying. Microarray analysis demonstrated an intense microglia-mediated inflammatory response to B. burgdorferi including up-regulation in gene transcripts for TLR-2 and NFκβ. Surprisingly, a pathway that exhibited profound changes in regard to inflammatory signaling was triggering receptor expressed on myeloid cells-1 (TREM1). Significant transcript alterations in essential p53 pathway genes also occurred in SY cells cultured in the presence of microglia and B. burgdorferi, which indicated a shift from cell survival to preparation for apoptosis when compared to SY cells cultured in the presence of B. burgdorferi alone. Taken together, these findings indicate that B. burgdorferi is not directly toxic to SY cells; rather, these cells become distressed and die in the inflammatory surroundings generated by microglia through a bystander effect. If, as we hypothesized, neuronal apoptosis is the key pathogenic event in Lyme neuroborreliosis, then targeting microglial responses may be a significant therapeutic approach for the treatment of this form of Lyme disease.

Lyme disease, which is transmitted to humans through the bite of a tick, is currently the most frequently reported vector-borne illness in the northern hemisphere. Borrelia burgdorferi is the bacterium that causes Lyme disease and it is known to readily induce inflammation within a variety of infected tissues. Many of the neurological signs and symptoms that may affect patients with Lyme disease have been associated with B. burgdorferi-induced inflammation in the central nervous system (CNS). In this report we investigated which of the primary cell types residing in the CNS might be functioning to create the inflammatory environment that, in addition to helping clear the pathogen, could simultaneously be harming nearby neurons. We report findings that implicate microglia, a macrophage cell type in the CNS, as the key responders to infection with B. burgdorferi. We also present evidence indicating that this organism is not directly toxic to neurons; rather, a bystander effect is generated whereby the inflammatory surroundings created by microglia in response to B. burgdorferi may themselves be toxic to neuronal cells.

Toll-like receptors 1 and 2 heterodimers alter Borrelia burgdorferi gene expression in mice and ticks

Borrelia burgdorferi, the agent of Lyme disease, is recognized by Toll-like receptor (TLR) 1 and 2 heterodimers. Microarray analysis of in vivo B. burgdorferi gene expression in murine skin showed that several genes were altered in TLR1/2-deficient animals compared with wild-type mice. For example, expression of bbe21 (a gene involved in B. burgdorferi lp25 plasmid maintenance) and bb0665 (a gene encoding a glycosyl transferase) were higher in TLR1/2-deficient mice than in control animals. In contrast, messenger RNA levels for bb0731 (a spoJ-like gene) and bba74 (a gene encoding a periplasmic protein) were lower in TLR1/2-deficient mice than in wild-type animals. The expression profiles of some of these genes were altered similarly in B. burgdorferi-infected ticks fed on control or TLR1/2-deficient mice. Quantitative reverse-transcription polymerase chain reaction analysis supported the microarray analysis and suggested that spirochete gene expression is altered by the milieu created by specific host TLRs, both in the murine host and in the arthropod vector.

Recognition of Borrelia burgdorferi, the Lyme disease spirochete, by TLR7 and TLR9 induces a type I IFN response by human immune cells

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-alpha, IFN-beta, and IRF7, and protein concentrations of IFN-alpha, were significantly elevated relative to those in unstimulated PBMCs. The induction of IFN-alpha 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-alpha protein expression and transcription of IFN-induced genes. Simultaneous inhibition of both TLR7 and TLR9 completely abrogated IFN-alpha induction. The IFN-alpha-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.

Possible role of glial cells in the onset and progression of Lyme neuroborreliosis

BACKGROUND

Lyme neuroborreliosis (LNB) may present as meningitis, cranial neuropathy, acute radiculoneuropathy or, rarely, as encephalomyelitis. We hypothesized that glia, upon exposure to Borrelia burgdorferi, the Lyme disease agent, produce inflammatory mediators that promote the acute cellular infiltration of early LNB. This inflammatory context could potentiate glial and neuronal apoptosis.

METHODS

We inoculated live B. burgdorferi into the cisterna magna of rhesus macaques and examined the inflammatory changes induced in the central nervous system (CNS), and dorsal root nerves and ganglia (DRG).

RESULTS

ELISA of the cerebrospinal fluid (CSF) showed elevated IL-6, IL-8, CCL2, and CXCL13 as early as one week post-inoculation, accompanied by primarily lymphocytic and monocytic pleocytosis. In contrast, onset of the acquired immune response, evidenced by anti-B. burgdorferi C6 serum antibodies, was first detectable after 3 weeks post-inoculation. CSF cell pellets and CNS tissues were culture-positive for B. burgdorferi. Histopathology revealed signs of acute LNB: severe multifocal leptomeningitis, radiculitis, and DRG inflammatory lesions. Immunofluorescence staining and confocal microscopy detected B. burgdorferi antigen in the CNS and DRG. IL-6 was observed in astrocytes and neurons in the spinal cord, and in neurons in the DRG of infected animals. CCL2 and CXCL13 were found in microglia as well as in endothelial cells, macrophages and T cells. Importantly, the DRG of infected animals showed significant satellite cell and neuronal apoptosis.

CONCLUSION

Our results support the notion that innate responses of glia to B. burgdorferi initiate/mediate the inflammation seen in acute LNB, and show that neuronal apoptosis occurs in this context.

Activation of human monocytes by live Borrelia burgdorferi generates TLR2-dependent and -independent responses which include induction of IFN-beta

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.

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.

The antibiotics doxycycline and minocycline inhibit the inflammatory responses to the Lyme disease spirochete Borrelia burgdorferi

Tetracyclines moderate inflammatory responses of various etiologies. We hypothesized that tetracyclines, in addition to their antimicrobial function, could exert control over the inflammation elicited by Borrelia burgdorferi. To model systemic effects, we used the human monocytic cell line THP-1; to model effects in the central nervous system, we used rhesus monkey brain astrocytes and microglia. Cells were stimulated with live or sonicated B. burgdorferi or with the lipoprotein outer surface protein A in the presence of increasing concentrations of doxycycline or minocycline. Both antibiotics significantly reduced the production of tumor necrosis factor-alpha, interleukin (IL)-6, and IL-8 in a dose-dependent manner in all cell types. Microarray analyses of the effect of doxycycline on gene transcription in spirochete-stimulated monocytes revealed that the NFKB and CHUK (alias, IKKA) genes were down-regulated. Functionally, phosphorylation of IkappaBalpha and binding of NF-kappaB to target DNA were both reduced in these cells. Our results suggest that tetracyclines may have a dual therapeutic effect in Lyme disease.