MyD88 deficiency results in tissue-specific changes in cytokine induction and inflammation in interleukin-18-independent mice infected with Borrelia burgdorferi

Borrelia burgdorferi initiates early transcriptional re-programming in macrophages that supports long-term suppression of inflammation

Borrelia burgdorferi (Bb), the causative agent of Lyme disease, establishes a long-term infection and leads to disease manifestations that are the result of host immune responses to the pathogen. Inflammatory manifestations resolve spontaneously despite continued bacterial presence, suggesting inflammatory cells become less responsive over time. This is mimicked by in vitro repeated stimulations, resulting in tolerance, a phenotypic subset of innate immune memory. We performed comparative transcriptional analysis of macrophages in acute and memory states and identified sets of Tolerized, Hyper-Induced, Secondary-Induced and Hyper-Suppressed genes resulting from memory induction, revealing previously unexplored networks of genes affected by cellular re-programming. Tolerized gene families included inflammatory mediators and interferon related genes as would be predicted by the attenuation of inflammation over time. To better understand how cells mediate inflammatory hypo-responsiveness, we focused on genes that could mediate maintenance of suppression, such as Hyper-Induced genes which are up-regulated in memory states. These genes were notably enriched in stress pathways regulated by anti-inflammatory modulators. We examined one of the most highly expressed negative regulators of immune pathways during primary stimulation, Aconitate decarboxylase 1 (Acod1), and tested its effects during in vivo infection with Bb. As predicted by our in vitro model, we show its inflammation-suppressive downstream effects are sustained during in vivo long-term infection with Bb, with a specific role in Lyme carditis.

Immune stress suppresses innate immune signaling in preleukemic precursor B-cells to provoke leukemia in predisposed mice

The initial steps of B-cell acute lymphoblastic leukemia (B-ALL) development usually pass unnoticed in children. Several preclinical studies have shown that exposure to immune stressors triggers the transformation of preleukemic B cells to full-blown B-ALL, but how this takes place is still a longstanding and unsolved challenge. Here we show that dysregulation of innate immunity plays a driving role in the clonal evolution of pre-malignant Pax5+/- B-cell precursors toward leukemia. Transcriptional profiling reveals that Myd88 is downregulated in immune-stressed pre-malignant B-cell precursors and in leukemic cells. Genetic reduction of Myd88 expression leads to a significant increase in leukemia incidence in Pax5+/-Myd88+/- mice through an inflammation-dependent mechanism. Early induction of Myd88-independent Toll-like receptor 3 signaling results in a significant delay of leukemia development in Pax5+/- mice. Altogether, these findings identify a role for innate immunity dysregulation in leukemia, with important implications for understanding and therapeutic targeting of the preleukemic state in children.

Report of the Pathogenesis and Pathophysiology of Lyme Disease Subcommittee of the HHS Tick Borne Disease Working Group

An understanding of the pathogenesis and pathophysiology of Lyme disease is key to the ultimate care of patients with Lyme disease. To better understand the various mechanisms underlying the infection caused by Borrelia burgdorferi, the Pathogenesis and Pathophysiology of Lyme Disease Subcommittee was formed to review what is currently known about the pathogenesis and pathophysiology of Lyme disease, from its inception, but also especially about its ability to persist in the host. To that end, the authors of this report were assembled to update our knowledge about the infectious process, identify the gaps that exist in our understanding of the process, and provide recommendations as to how to best approach solutions that could lead to a better means to manage patients with persistent Lyme disease.

Macrophage mediated recognition and clearance of Borrelia burgdorferi elicits MyD88-dependent and -independent phagosomal signals that contribute to phagocytosis and inflammation

BACKGROUND

Macrophages play prominent roles in bacteria recognition and clearance, including Borrelia burgdorferi (Bb), the Lyme disease spirochete. To elucidate mechanisms by which MyD88/TLR signaling enhances clearance of Bb by macrophages, we studied wildtype (WT) and MyD88-/- Bb-stimulated bone marrow-derived macrophages (BMDMs).

RESULTS

MyD88-/- BMDMs exhibit impaired uptake of spirochetes but comparable maturation of phagosomes following internalization of spirochetes. RNA-sequencing of infected WT and MyD88-/- BMDMs identified a large cohort of differentially expressed MyD88-dependent genes associated with re-organization of actin and cytoskeleton during phagocytosis along with several MyD88-independent chemokines involved in inflammatory cell recruitment. We computationally generated networks which identified several MyD88-dependent intermediate proteins (Rhoq and Cyfip1) that are known to mediate inflammation and phagocytosis respectively.

CONCLUSION

Our findings show that MyD88 signaling enhances, but is not required, for bacterial uptake or phagosomal maturation and provide mechanistic insights into how MyD88-mediated phagosomal signaling enhances Bb uptake and clearance.

Immune Response to Borrelia: Lessons from Lyme Disease Spirochetes

The mammalian host responds to infection with Borrelia spirochetes through a highly orchestrated immune defense involving innate and adaptive effector functions aimed toward limiting pathogen burdens, minimizing tissue injury, and preventing subsequent reinfection. The evolutionary adaptation of Borrelia spirochetes to their reservoir mammalian hosts may allow for its persistence despite this immune defense. This review summarizes our current understanding of the host immune response to B. burgdorferi sensu lato, the most widely studied Borrelia spp. and etiologic agent of Lyme borreliosis. Pertinent literature will be reviewed with emphasis on in vitro, ex vivo and animal studies that influenced our understanding of both the earliest responses to B. burgdorferi as it enters the mammalian host and those that evolve as spirochetes disseminate and establish infection in multiple tissues. Our focus is on the immune response of inbred mice, the most commonly studied animal model of B. burgdorferi infection and surrogate for one of this pathogen's principle natural reservoir hosts, the white-footed deer mouse. Comparison will be made to the immune responses of humans with Lyme borreliosis. Our goal is to provide an understanding of the dynamics of the mammalian immune response during infection with B. burgdorferi and its relation to the outcomes in reservoir (mouse) and non-reservoir (human) hosts.

Genetic Background Amplifies the Effect of Immunodeficiency in Antibiotic Efficacy Against Borrelia burgdorferi

Unrecognized immunodeficiency has been proposed as a possible cause of failure of antibiotics to resolve symptoms of Lyme disease. Here, we examined the efficacy of doxycycline in different immunodeficient mice to identify defects that impair antibiotic treatment outcomes. We found that doxycycline had significantly lower efficacy in the absence of adaptive immunity, specifically B cells. This effect was most pronounced in immunodeficient C3H mice compared with C57BL/6 mice, suggesting a role for genetic background beyond immunodeficiency. Addition of a single dose of ceftriaxone to doxycycline treatment effectively cleared infection in C3H mice with severe combined immunodeficiency.

Development of a Novel Backbone Cyclic Peptide Inhibitor of the Innate Immune TLR/IL1R Signaling Protein MyD88

MyD88 is a cytoplasmic adaptor protein that plays a central role in signaling downstream of the TLRs and the IL1R superfamily. We previously demonstrated that MyD88 plays a critical role in EAE, the murine model of multiple sclerosis, and showed that the MyD88 BB-loop decoy peptide RDVLPGT ameliorates EAE. We now designed and screened a library of backbone cyclized peptides based on the linear BB loop peptide, to identify a metabolically stable inhibitor of MyD88 that retains the binding properties of the linear peptide. We identified a novel cyclic peptide protein mimetic that inhibits inflammatory responses to TLR ligands, and NFκB activation in response to IL-1 activation. The inhibitor, c(MyD 4-4), is metabolically stable in comparison to the linear peptide, blocks MyD88 in a specific manner, and inhibits MyD88 function by preventing MyD88 dimerization. Finally, treatment of mice with c(MyD 4-4) reduced the severity of clinical disease in the murine EAE model of multiple sclerosis. Thus, modulation of MyD88-dependent signaling using c(MyD 4-4) is a potential therapeutic strategy to lower innate immune inflammation in autoimmune CNS disease.

MyD88 in Mycobacterium tuberculosis infection

MyD88 adaptor protein mediates numerous biologically important signal transduction pathways in innate immunity. MyD88 signaling fosters bacterial containment and is necessary to raise an adequate innate and acquired immune response to Mycobacterium tuberculosis (Mtb). The phagosome is a crucial cellular location not only for Mtb replication, but it is also where components of the Myddosome and inflammasome are recruited. Besides its function as a TLR-adaptor protein, MyD88 may help stabilizing cytosolic receptors that are recruited to the phagosome. MyD88 plays a critical role not only in the generation of an inflammatory response, but also in inducing regulatory signals to prevent excessive inflammation and cellular damage in the lung.

Lyme borreliosis

Lyme borreliosis is a tick-borne disease that predominantly occurs in temperate regions of the northern hemisphere and is primarily caused by the bacterium Borrelia burgdorferi in North America and Borrelia afzelii or Borrelia garinii in Europe and Asia. Infection usually begins with an expanding skin lesion, known as erythema migrans (referred to as stage 1), which, if untreated, can be followed by early disseminated infection, particularly neurological abnormalities (stage 2), and by late infection, especially arthritis in North America or acrodermatitis chronica atrophicans in Europe (stage 3). However, the disease can present with any of these manifestations. During infection, the bacteria migrate through the host tissues, adhere to certain cells and can evade immune clearance. Yet, these organisms are eventually killed by both innate and adaptive immune responses and most inflammatory manifestations of the infection resolve. Except for patients with erythema migrans, Lyme borreliosis is diagnosed based on a characteristic clinical constellation of signs and symptoms with serological confirmation of infection. All manifestations of the infection can usually be treated with appropriate antibiotic regimens, but the disease can be followed by post-infectious sequelae in some patients. Prevention of Lyme borreliosis primarily involves the avoidance of tick bites by personal protective measures.

Outer surface protein OspC is an antiphagocytic factor that protects Borrelia burgdorferi from phagocytosis by macrophages

Outer surface protein C (OspC) is one of the major lipoproteins expressed on the surface of Borrelia burgdorferi during tick feeding and the early phase of mammalian infection. OspC is required for B. burgdorferi to establish infection in both immunocompetent and SCID mice and has been proposed to facilitate evasion of innate immune defenses. However, the exact biological function of OspC remains elusive. In this study, we showed that the ospC-deficient spirochete could not establish infection in NOD-scid IL2rγ(null) mice that lack B cells, T cells, NK cells, and lytic complement. The ospC mutant also could not establish infection in anti-Ly6G-treated SCID and C3H/HeN mice (depletion of neutrophils). However, depletion of mononuclear phagocytes at the skin site of inoculation in SCID and C3H/HeN mice allowed the ospC mutant to establish infection in vivo. In phagocyte-depleted mice, the ospC mutant was able to colonize the joints and triggered neutrophilia during dissemination. Furthermore, we found that phagocytosis of green fluorescent protein (GFP)-expressing ospC mutant spirochetes by murine peritoneal macrophages and human THP-1 macrophage-like cells, but not in PMN-HL60, was significantly higher than parental wild-type B. burgdorferi strains, suggesting that OspC has an antiphagocytic property. In addition, overproduction of OspC in spirochetes also decreased the uptake of spirochetes by murine peritoneal macrophages. Together, our findings provide evidence that mononuclear phagocytes play a key role in clearance of the ospC mutant and that OspC promotes spirochetes' evasion of macrophages during early Lyme borreliosis.

Adaptor Protein-3-Mediated Trafficking of TLR2 Ligands Controls Specificity of Inflammatory Responses but Not Adaptor Complex Assembly

Innate immune engagement results in the activation of host defenses that produce microbe-specific inflammatory responses. A long-standing interest in the field of innate immunity is to understand how varied host responses are generated through the signaling of just a limited number of receptors. Recently, intracellular trafficking and compartmental partitioning have been identified as mechanisms that provide signaling specificity for receptors by regulating signaling platform assembly. We show that cytokine activation as a result of TLR2 stimulation occurs at different intracellular locations and is mediated by the phagosomal trafficking molecule adaptor protein-3 (AP-3). AP-3 is required for trafficking TLR2 purified ligands or the Lyme disease causing bacterium, Borrelia burgdorferi, to LAMP-1 lysosomal compartments. The presence of AP-3 is necessary for the activation of cytokines such as IL-6 but not TNF-α or type I IFNs, suggesting induction of these cytokines occurs from a different compartment. Lack of AP-3 does not interfere with the recruitment of TLR signaling adaptors TRAM and MyD88 to the phagosome, indicating that the TLR-MyD88 signaling complex is assembled at a prelysosomal stage and that IL-6 activation depends on proper localization of signaling molecules downstream of MyD88. Finally, infection of AP-3-deficient mice with B. burgdorferi resulted in altered joint inflammation during murine Lyme arthritis. Our studies further elucidate the effects of phagosomal trafficking on tailoring immune responses in vitro and in vivo.

Tick sialostatins L and L2 differentially influence dendritic cell responses to Borrelia spirochetes

Background

Transmission of pathogens by ticks is greatly supported by tick saliva released during feeding. Dendritic cells (DC) act as immunological sentinels and interconnect the innate and adaptive immune system. They control polarization of the immune response towards Th1 or Th2 phenotype. We investigated whether salivary cystatins from the hard tick Ixodes scapularis, sialostatin L (Sialo L) and sialostatin L2 (Sialo L2), influence mouse dendritic cells exposed to Borrelia burgdorferi and relevant Toll-like receptor ligands.

Methods

DCs derived from bone-marrow by GM-CSF or Flt-3 ligand, were activated with Borrelia spirochetes or TLR ligands in the presence of 3 μM Sialo L and 3 μM Sialo L2. Produced chemokines and IFN-β were measured by ELISA test. The activation of signalling pathways was tested by western blotting using specific antibodies. The maturation of DC was determined by measuring the surface expression of CD86 by flow cytometry.

Results

We determined the effect of cystatins on the production of chemokines in Borrelia-infected bone-marrow derived DC. The production of MIP-1α was severely suppressed by both cystatins, while IP-10 was selectively inhibited only by Sialo L2. As TLR-2 is a major receptor activated by Borrelia spirochetes, we tested whether cystatins influence signalling pathways activated by TLR-2 ligand, lipoteichoic acid (LTA). Sialo L2 and weakly Sialo L attenuated the extracellular matrix-regulated kinase (Erk1/2) pathway. The activation of phosphatidylinositol-3 kinase (PI3K)/Akt pathway and nuclear factor-κB (NF-κB) was decreased only by Sialo L2. In response to Borrelia burgdorferi, the activation of Erk1/2 was impaired by Sialo L2. Production of IFN-β was analysed in plasmacytoid DC exposed to Borrelia, TLR-7, and TLR-9 ligands. Sialo L, in contrast to Sialo L2, decreased the production of IFN-β in pDC and also impaired the maturation of these cells.

Conclusions

This study shows that DC responses to Borrelia spirochetes are affected by tick cystatins. Sialo L influences the maturation of DC thus having impact on adaptive immune response. Sialo L2 affects the production of chemokines potentially engaged in the development of inflammatory response. The impact of cystatins on Borrelia growth in vivo is discussed.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-015-0887-1) contains supplementary material, which is available to authorized users.

MicroRNA-146a provides feedback regulation of lyme arthritis but not carditis during infection with Borrelia burgdorferi

MicroRNAs have been shown to be important regulators of inflammatory and immune responses and are implicated in several immune disorders including systemic lupus erythematosus and rheumatoid arthritis, but their role in Lyme borreliosis remains unknown. We performed a microarray screen for expression of miRNAs in joint tissue from three mouse strains infected with Borrelia burgdorferi. This screen identified upregulation of miR-146a, a key negative regulator of NF-κB signaling, in all three strains, suggesting it plays an important role in the in vivo response to B. burgdorferi. Infection of B6 miR-146a-/- mice with B. burgdorferi revealed a critical nonredundant role of miR-146a in modulating Lyme arthritis without compromising host immune response or heart inflammation. The impact of miR-146a was specifically localized to the joint, and did not impact lesion development or inflammation in the heart. Furthermore, B6 miR-146a-/- mice had elevated levels of NF-κB-regulated products in joint tissue and serum late in infection. Flow cytometry analysis of various lineages isolated from infected joint tissue of mice showed that myeloid cell infiltration was significantly greater in B6 miR-146a-/- mice, compared to B6, during B. burgdorferi infection. Using bone marrow-derived macrophages, we found that TRAF6, a known target of miR-146a involved in NF-κB activation, was dysregulated in resting and B. burgdorferi-stimulated B6 miR-146a-/- macrophages, and corresponded to elevated IL-1β, IL-6 and CXCL1 production. This dysregulated protein production was also observed in macrophages treated with IL-10 prior to B. burgdorferi stimulation. Peritoneal macrophages from B6 miR-146a-/- mice also showed enhanced phagocytosis of B. burgdorferi. Together, these data show that miR-146a-mediated regulation of TRAF6 and NF-κB, and downstream targets such as IL-1β, IL-6 and CXCL1, are critical for modulation of Lyme arthritis during chronic infection with B. burgdorferi.

Innate immunity networks during infection with Borrelia burgdorferi

The recognition of Borrelia species represents a complex process in which multiple components of the immune system are involved. In this review, we summarize the interplay between the host innate system and Borrelia spp., from the recognition by pattern recognition receptors (PRRs) to the induction of a complex network of proinflammatory mediators. Several PRR families are crucial for recognition of Borrelia spp., including Toll-like receptors (TLRs) and Nucleotide Oligomerization Domain (NOD)-like receptors (NLRs). TLR-2 is crucial for the recognition of outer surface protein (Osp)A from Borrelia spp. and together with TLR8 mediates phagocytosis of the microorganism and production of type I interferons. Intracellular receptors such as TLR7, TLR8 and TLR9 on the one hand and the NLR receptor NOD2 on the other hand, represent the second major recognition system of Borrelia. PRR-dependent signals induce the release of pro-inflammatory cytokines such as interleukin-1 and T-helper-derived cytokines, which are thought to mediate the inflammation during Lyme disease. Understanding the regulation of host defense mechanisms against Borrelia has the potential to lead to the discovery of novel immunotherapeutic targets to improve the therapy against Lyme disease.

Dual role for Fcγ receptors in host defense and disease in Borrelia burgdorferi-infected mice

Arthritis in mice infected with the Lyme disease spirochete, Borrelia burgdorferi, results from the influx of innate immune cells responding to the pathogen in the joint and is influenced in part by mouse genetics. Production of inflammatory cytokines by innate immune cells in vitro is largely mediated by Toll-like receptor (TLR) interaction with Borrelia lipoproteins, yet surprisingly mice deficient in TLR2 or the TLR signaling molecule MyD88 still develop arthritis comparable to that seen in wild type mice after B. burgdorferi infection. These findings suggest that other, MyD88-independent inflammatory pathways can contribute to arthritis expression. Clearance of B. burgdorferi is dependent on the production of specific antibody and phagocytosis of the organism. As Fc receptors (FcγR) are important for IgG-mediated clearance of immune complexes and opsonized particles by phagocytes, we examined the role that FcγR play in host defense and disease in B. burgdorferi-infected mice. B. burgdorferi-infected mice deficient in the Fc receptor common gamma chain (FcεR_γ^−/− mice) harbored ~10 fold more spirochetes than similarly infected wild type mice, and this was associated with a transient increase in arthritis severity. While the elevated pathogen burdens seen in B. burgdorferi-infected MyD88^−/− mice were not affected by concomitant deficiency in FcγR, arthritis was reduced in FcεR_γ^−/−MyD88^−/− mice in comparison to wild type or single knockout mice. Gene expression analysis from infected joints demonstrated that absence of both MyD88 and FcγR lowers mRNA levels of proteins involved in inflammation, including Cxcl1 (KC), Xcr1 (Gpr5), IL-1beta, and C reactive protein. Taken together, our results demonstrate a role for FcγR-mediated immunity in limiting pathogen burden and arthritis in mice during the acute phase of B. burgdorferi infection, and further suggest that this pathway contributes to the arthritis that develops in B. burgdorferi-infected MyD88^−/− mice.

Phagosomal TLR signaling upon Borrelia burgdorferi infection

Internalization and degradation of live Bb within phagosomal compartments of monocytes, macrophages and dendritic cells (DCs), allows for the release of lipoproteins, nucleic acids and other microbial products, triggering a broad and robust inflammatory response. Toll-like receptors (TLRs) are key players in the recognition of spirochetal ligands from whole viable organisms (i.e., vita-PAMPs). Herein we will review the role of endosomal TLRs in the response to the Lyme disease spirochete.

Borrelia burgdorferi elicited-IL-10 suppresses the production of inflammatory mediators, phagocytosis, and expression of co-stimulatory receptors by murine macrophages and/or dendritic cells

Borrelia burgdorferi (Bb) is a tick-borne spirochete that is the causative agent for Lyme disease. Our previous studies indicate that virulent Bb can potently enhance IL-10 production by macrophages (MØs) and that blocking IL-10 production significantly enhances bacterial clearance. We hypothesize that skin-associated APC types, such as MØs and dendritic cells (DCs) are potent producers of IL-10 in response to Bb, which may act in autocrine fashion to suppress APC responses critical for efficient Bb clearance. Our goal is to delineate which APC immune functions are dysregulated by Bb-elicited IL-10 using a murine model of Lyme disease. Our in vitro studies indicated that both APCs rapidly produce IL-10 upon exposure to Bb, that these levels inversely correlate with the production of many Lyme-relevant proinflammatory cytokines and chemokines, and that APCs derived from IL-10(-/-) mice produced greater amounts of these proinflammatory mediators than wild-type APCs. Phagocytosis assays determined that Bb-elicited IL-10 levels can diminish Bb uptake and trafficking by MØs, suppresses ROS production, but does not affect NO production; Bb-elicited IL-10 had little effect on phagocytosis, ROS, and NO production by DCs. In general, Bb exposure caused little-to-no upregulation of several critical surface co-stimulatory markers by MØs and DCs, however eliminating Bb-elicited IL-10 allowed a significant upregulation in many of these co-stimulatory receptors. These data indicate that IL-10 elicited from Bb-stimulated MØs and DCs results in decreased production of proinflammatory mediators and co-stimulatory molecules, and suppress phagocytosis-associated events that are important for mediating both innate and adaptive immune responses by APCs.

Experimental and natural infections in MyD88- and IRAK-4-deficient mice and humans

Most Toll-like-receptors (TLRs) and interleukin-1 receptors (IL-1Rs) signal via myeloid differentiation primary response 88 (MyD88) and interleukin-1 receptor-associated kinase 4 (IRAK-4). The combined roles of these two receptor families in the course of experimental infections have been assessed in MyD88- and IRAK-4-deficient mice for almost fifteen years. These animals have been shown to be susceptible to 46 pathogens: 27 bacteria, eight viruses, seven parasites, and four fungi. Humans with inborn MyD88 or IRAK-4 deficiency were first identified in 2003. They suffer from naturally occurring life-threatening infections caused by a small number of bacterial species, although the incidence and severity of these infections decrease with age. Mouse TLR- and IL-1R-dependent immunity mediated by MyD88 and IRAK-4 seems to be vital to combat a wide array of experimentally administered pathogens at most ages. By contrast, human TLR- and IL-1R-dependent immunity mediated by MyD88 and IRAK-4 seems to be effective in the natural setting against only a few bacteria and is most important in infancy and early childhood. The roles of TLRs and IL-1Rs in protective immunity deduced from studies in mutant mice subjected to experimental infections should therefore be reconsidered in the light of findings for natural infections in humans carrying mutations as discussed in this review.

TRIF mediates Toll-like receptor 2-dependent inflammatory responses to Borrelia burgdorferi

TRIF is an adaptor molecule important in transducing signals from intracellularly signaling Toll-like receptor 3 (TLR3) and TLR4. Recently, TLR2 was found to signal from intracellular compartments. Using a synthetic ligand for TLR2/1 heterodimers, as well as Borrelia burgdorferi, which is a strong activator of TLR2/1, we found that TLR2 signaling can utilize TRIF. Unlike TRIF signaling by other TLRs, TLR2-mediated TRIF signaling is dependent on the presence of another adaptor molecule, MyD88. However, unlike MyD88 deficiency, TRIF deficiency does not result in diminished control of infection with B. burgdorferi in a murine model of disease. This appears to be due to the effects of MyD88 on phagocytosis via scavenger receptors, such as MARCO, which are not affected by the loss of TRIF. In mice, TRIF deficiency did have an effect on the production of inflammatory cytokines, suggesting that regulation of inflammatory cytokines and control of bacterial growth may be uncoupled, in part through transduction of TLR2 signaling through TRIF.

Murine Borrelia arthritis is highly dependent on ASC and caspase-1, but independent of NLRP3

Introduction

The protein platform called the NOD-like-receptor -family member (NLRP)-3 inflammasome needs to be activated to process intracellular caspase-1. Active caspase-1 is able to cleave pro-Interleukin (IL)-1β, resulting in bioactive IL-1β. IL-1β is a potent proinflammatory cytokine, and thought to play a key role in the pathogenesis of Lyme arthritis, a common manifestation of Borrelia burgdorferi infection. The precise pathways through which B. burgdorferi recognition leads to inflammasome activation and processing of IL-1β in Lyme arthritis has not been elucidated. In the present study, we investigated the contribution of several pattern recognition receptors and inflammasome components in a novel murine model of Lyme arthritis.

Methods

Lyme arthritis was elicited by live B. burgdorferi, injected intra-articularly in knee joints of mice. To identify the relevant pathway components, the model was applied to wild-type, NLRP3-/-, ASC-/-, caspase-1-/-, NOD1-/-, NOD2-/-, and RICK-/- mice. As a control, TLR2-/-, Myd88-/- and IL-1R-/- mice were used. Peritoneal macrophages and bone marrow-derived macrophages were used for in vitro cytokine production and inflammasome activation studies. Joint inflammation was analyzed in synovial specimens and whole knee joints. Mann-Whitney U tests were used to detect statistical differences.

Results

We demonstrate that ASC/caspase-1-driven IL-1β is crucial for induction of B. burgdorferi-induced murine Lyme arthritis. In addition, we show that B. burgdorferi-induced murine Lyme arthritis is less dependent on NOD1/NOD2/RICK pathways while the TLR2-MyD88 pathway is crucial.

Conclusions

Murine Lyme arthritis is strongly dependent on IL-1 production, and B. burgdorferi induces inflammasome-mediated caspase-1 activation. Next to that, murine Lyme arthritis is ASC- and caspase-1-dependent, but NLRP3, NOD1, NOD2, and RICK independent. Also, caspase-1 activation by B. burgdorferi is dependent on TLR2 and MyD88. Based on present results indicating that IL-1 is one of the major mediators in Lyme arthritis, there is a rationale to propose that neutralizing IL-1 activity may also have beneficial effects in chronic Lyme arthritis.

Nod2 suppresses Borrelia burgdorferi mediated murine Lyme arthritis and carditis through the induction of tolerance

The internalization of Borrelia burgdorferi, the causative agent of Lyme disease, by phagocytes is essential for an effective activation of the immune response to this pathogen. The intracellular, cytosolic receptor Nod2 has been shown to play varying roles in either enhancing or attenuating inflammation in response to different infectious agents. We examined the role of Nod2 in responses to B. burgdorferi. In vitro stimulation of Nod2 deficient bone marrow derived macrophages (BMDM) resulted in decreased induction of multiple cytokines, interferons and interferon regulated genes compared with wild-type cells. However, B. burgdorferi infection of Nod2 deficient mice resulted in increased rather than decreased arthritis and carditis compared to control mice. We explored multiple potential mechanisms for the paradoxical response in in vivo versus in vitro systems and found that prolonged stimulation with a Nod2 ligand, muramyl dipeptide (MDP), resulted in tolerance to stimulation by B. burgdorferi. This tolerance was lost with stimulation of Nod2 deficient cells that cannot respond to MDP. Cytokine patterns in the tolerance model closely paralleled cytokine profiles in infected Nod2 deficient mice. We propose a model where Nod2 has an enhancing role in activating inflammation in early infection, but moderates inflammation after prolonged exposure to the organism through induction of tolerance.

Role of adrenomedullin in Lyme disease

Borrelia burgdorferi stimulates a strong inflammatory response during infection of a mammalian host. To understand the mechanisms of immune regulation employed by the host to control this inflammatory response, we focused our studies on adrenomedullin, a peptide produced in response to bacterial stimuli that exhibits antimicrobial activity and regulates inflammatory responses by modulating the expression of inflammatory cytokines. Specifically, we investigated the effect of B. burgdorferi on the expression of adrenomedullin as well as the ability of adrenomedullin to dampen host inflammatory responses to the spirochete. The concentration of adrenomedullin in the synovial fluid of untreated Lyme arthritis patients was elevated compared with that in control osteoarthritis patient samples. In addition, coculture with B. burgdorferi significantly increased the expression of adrenomedullin in RAW264.7 macrophages through MyD88-, phosphatidylinositol 3-kinase (PI3-K)-, and p38-dependent signaling cascades. Furthermore, the addition of exogenous adrenomedullin to B. burgdorferi-stimulated RAW264.7 macrophages resulted in a significant decrease in the induction of proinflammatory cytokines. Taken together, these results suggest that B. burgdorferi increases the production of adrenomedullin, which in turn negatively regulates the B. burgdorferi-stimulated inflammatory response.

The Lyme disease spirochete Borrelia burgdorferi utilizes multiple ligands, including RNA, for interferon regulatory factor 3-dependent induction of type I interferon-responsive genes

We recently discovered a critical role for type I interferon (IFN) in the development of murine Lyme arthritis. Borrelia burgdorferi-mediated induction of IFN-responsive genes by bone marrow-derived macrophages (BMDMs) was dependent upon a functional type I IFN receptor but independent of Toll-like receptor 2 (TLR2), TLR4, TLR9, and the adapter molecule MyD88. We now demonstrate that induction of the IFN transcriptional profile in B. burgdorferi-stimulated BMDMs occurs independently of the adapter TRIF and of the cytoplasmic sensor NOD2. In contrast, B. burgdorferi-induced transcription of these genes was dependent upon a rapid STAT1 feedback amplification pathway. IFN profile gene transcription was IRF3 dependent but did not utilize B. burgdorferi-derived DNA or DNase-sensitive ligands. Instead, IFN-responsive gene expression could be induced by B. burgdorferi-derived RNA. Interferon regulatory factor 3 (IRF3)-dependent IFN profile gene transcription was also induced by sonicated bacteria, by the lipoprotein OspA, and by factors released into the BSKII medium during culture of B. burgdorferi. The IFN-stimulatory activity of B. burgdorferi culture supernatants was not destroyed by nuclease treatment. Nuclease digestion also had no effect on IFN profile induction mediated by sonicated B. burgdorferi. Thus, B. burgdorferi-derived RNA, OspA, and non-nucleic acid ligands present in both sonicated bacteria and B. burgdorferi culture medium contribute to type I IFN-responsive gene induction. These findings suggest that B. burgdorferi invasion of joint tissue and the resultant type I IFN induction associated with Lyme arthritis development may involve multiple triggering ligands.

Neutralizing endogenous chemokines with small molecules. Principles and potential therapeutic applications

Regulation of cellular responses to external stimuli such as hormones, neurotransmitters, or cytokines is achieved through the control of all steps of the complex cascade starting with synthesis, going through maturation steps, release, distribution, degradation and/or uptake of the signalling molecule interacting with the target protein. One possible way of regulation, referred to as scavenging or neutralization of the ligand, has been increasingly studied, especially for small protein ligands. It shows innovative potential in chemical biology approaches as well as in disease treatment. Neutralization of protein ligands, as for example cytokines or chemokines can lead to the validation of signalling pathways under physiological or pathophysiological conditions, and in certain cases, to the development of therapeutic molecules now used in autoimmune diseases, chronic inflammation and cancer treatment. This review explores the field of ligand neutralization and tries to determine to what extent small chemical molecules could substitute for neutralizing antibodies in therapeutic approaches.

The role of toll-like receptors in host defenses and their relevance to dermatologic diseases

The family of toll-like receptors (TLRs) plays a central role in the cutaneous immune defense system. To date, different TLRs have been found on several major cell populations of the skin, such as keratinocytes, fibroblasts, antigen-presenting cells, and melanocytes. Activation of TLRs leads, via different intracellular signaling pathways, to the production of pro-inflammatory stimuli, and is considered a danger signal that should transform the skin in to the functional state of defense. However, TLRs have also been implicated in tissue homeostasis and renewal. Within the group of TLRs, two types have been identified: surface-expressed TLRs, which are predominantly active against bacterial cell wall compounds; and intracellular receptors, which preferentially recognize virus-associated pattern molecules. In addition, surface-expressed receptors trigger phagocytotic and maturation signals, while the intracellular TLRs lead to the induction of antiviral genes. Our review aims to outline the importance of TLRs in the pathogenesis of numerous skin diseases and the potential of TLR agonists as a treatment option for various skin diseases.

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.

Downstream signals for MyD88-mediated phagocytosis of Borrelia burgdorferi can be initiated by TRIF and are dependent on PI3K

We previously have shown that MyD88 is important for uptake of Borrelia burgdorferi by bone marrow derived macrophages (BMDMs). The mechanism by which MyD88 is involved in uptake of B. burgdorferi is currently is not well characterized. Here, we report that MyD88-mediated defect in the phagocytosis of B. burgdorferi can be complemented by TLR3/Toll/IL-1R domain-containing adaptor-inducing IFN-beta (TRIF) activation in BMDMs from MyD88(-/-) mice. This effect of TLR3/TRIF activation was not due to its induction of type I IFNs, suggesting instead a convergence of signaling pathways downstream of MyD88 and TRIF. To characterize signaling pathways involved in MyD88-mediated phagocytosis of B. burgdorferi, BMDMs were treated with specific inhibitors of MAPK, protein kinase C, JAK/STAT, or PI3K. Only inhibition of PI3K resulted in a significant decrease of B. burgdorferi uptake. Consistent with this, B. burgdorferi activation of MyD88 or TLR3/TRIF signaling resulted in increased activity of PI3K. Additionally, association of B. burgdorferi with actin-related protein (Arp2/3) complexes, which facilitate actin rearrangements during phagocytosis, was similarly reduced in MyD88(-/-) BMDMs and in BMDMs treated with a PI3K inhibitor. Taken together, these findings define an essential pathway whereby downstream signals from MyD88 or TRIF converge on PI3K, which triggers actin polymerization to initiate the phagocytosis of B. burgdorferi.

The caspase 1 inflammasome is not required for control of murine Lyme borreliosis

The contribution of the inflammasome to the development of immune responses and disease during infection with the Lyme disease spirochete, Borrelia burgdorferi, is not well defined. Host defense against the spirochete is severely impaired in mice deficient in the adaptor molecule myeloid differentiation antigen 88 (MyD88), which is required not only for Toll-like receptor-mediated responses but also for the production of the proforms of interleukin 1beta (IL-1beta) and IL-18. These cytokines are released in active forms after cleavage by the inflammasome-associated enzyme caspase 1. To investigate the contribution of the inflammasome to host defense against B. burgdorferi, we examined Lyme borreliosis in mice deficient in either caspase 1 or apoptosis-associated speck-like protein containing a C-terminal caspase recruitment domain (ASC), a molecule upstream of caspase 1 in the inflammasome signaling cascade. We found that caspase 1-deficient mice had a mild transient elevation in pathogen burden and a trend toward an increase in the prevalence of arthritis early in infection, but these differences resolved by day 14 postinfection. Caspase 1 deficiency had no effect on B. burgdorferi-induced humoral immunity, T-cell responses, or the abilities of macrophages to ingest and degrade spirochetes. The absence of the ASC protein had no effect on the control of the spirochete or the development of immune responses and disease. These findings reveal that the caspase 1 inflammasome is not critical to host defense against the extracellular pathogen Borrelia burgdorferi.

Role for Toll-like receptor 2 in the immune response to Streptococcus pneumoniae infection in mouse otitis media

Streptococcus pneumoniae is the most common pathogen associated with otitis media. To examine the role of Toll-like receptor 2 (TLR2) in host defense against Streptococcus pneumoniae infection in the middle ear, wild-type (WT; C57BL/6) and TLR2-deficient (TLR2(-/-)) mice were inoculated with Streptococcus pneumoniae (1 x 10(6) CFU) through the tympanic membrane. Nineteen of 37 TLR2(-/-) mice showed bacteremia and died within 3 days after the challenge, compared to only 4 of 32 WT mice that died. Of those that survived, more severe hearing loss in the TLR2(-/-) mice than in the WT mice was indicated by an elevation in auditory-evoked brain stem response thresholds at 3 or 7 days postinoculation. The histological pathology was characterized by effusion and tissue damage in the middle ear, and in the TLR2(-/-) mice, the outcome of infection became more severe at 7 days. At both 3 and 7 days postchallenge, the TLR2(-/-) mice had higher blood bacterial titers than the WT mice (P < 0.05), and typical bacteria were identified in the effusion from both ears of both mouse groups by acridine orange staining. Moreover, by 3 days postchallenge, the mRNA accumulation levels of NF-kappaB, tumor necrosis factor alpha, interleukin 1beta, MIP1alpha, Muc5ac, and Muc5b were significantly lower in the ears of TLR2(-/-) mice than in WT mice. In summary, TLR2(-/-) mice may produce relatively low levels of proinflammatory cytokines following pneumococcal challenge, thus hindering the clearance of bacteria from the middle ear and leading to sepsis and a high mortality rate. This study provides evidence that TLR2 is important in the molecular pathogenesis and host response to otitis media.

Matrix metalloproteinase 9 plays a key role in lyme arthritis but not in dissemination of Borrelia burgdorferi

Borrelia burgdorferi, the causative agent of Lyme arthritis, does not produce any exported proteases capable of degrading extracellular matrix despite the fact that it is able to disseminate from a skin insertion site to infect multiple organs. Prior studies have shown that B. burgdorferi induces the host protease, matrix metalloproteinase 9 (MMP-9), and suggested that the induction of MMP-9 may allow the organism to disseminate and produce local tissue destruction. We examined the role of MMP-9 in dissemination of B. burgdorferi and pathogenesis of Lyme arthritis. In a MMP-9(-/-) mouse model, MMP-9 was not required for the dissemination of the spirochete to distant sites. However, MMP-9(-/-) exhibited significantly decreased arthritis compared to wild-type mice. The decrease in arthritis was not due to an inability to control infection since the spirochete numbers in the joints were identical. Levels of inflammatory chemokines and cytokines were also similar in MMP-9(-/-) and wild-type mice. We examined whether decreased inflammation in MMP-9(-/-) mice may be the result of decreased production of neoattractants by MMP-9-dependent cleavage of collagen. MMP-9 cleavage of type I collagen results in increased monocyte chemoattraction. MMP-9 plays an important role in regulating inflammation in Lyme arthritis, potentially through the cleavage of type I collagen.

A critical role for type I IFN in arthritis development following Borrelia burgdorferi infection of mice

Gene expression analysis previously revealed a robust IFN-responsive gene induction profile that was selectively up-regulated in Borrelia burgdorferi-infected C3H mice at 1 wk postinfection. This profile was correlated with arthritis development, as it was absent from infected, mildly arthritic C57BL/6 mice. In this report we now demonstrate that profile induction in infected C3H scid mice occurs independently of B or T lymphocyte infiltration in the joint tissue. Additionally, type I IFN receptor-blocking Abs, but not anti-IFN-gamma Abs, dramatically reduced arthritis, revealing a critical but previously unappreciated role for type I IFN in Lyme arthritis development. Certain examined IFN-inducible transcripts were also significantly diminished within joint tissue of mice treated with anti-IFNAR1, whereas expression of other IFN-responsive genes was more markedly altered by anti-IFN-gamma treatment. These data indicate that induction of the entire IFN profile is not necessary for arthritis development. These findings further tie early type I IFN induction to Lyme arthritis development, a connection not previously made. Bone marrow-derived macrophages readily induced IFN-responsive genes following B. burgdorferi stimulation, and this expression required a functional type I IFN receptor. Strikingly, induction of these genes was independent of TLRs 2,4, and 9 and of the adapter molecule MyD88. These data demonstrate that the extracellular pathogen B. burgdorferi uses a previously unidentified receptor and a pathway traditionally associated with viruses and intracellular bacteria to initiate transcription of type I IFN and IFN-responsive genes and to initiate arthritis development.

Biology of infection with Borrelia burgdorferi

The spirochete Borrelia burgdorferi is a tick-borne obligate parasite whose normal reservoir is a variety of small mammals. Although infection of these natural hosts does not lead to disease, infection of humans can result in Lyme disease as a consequence of the human immunopathologic response to B burgdorferi. Consistent with the pathogenesis of Lyme disease, bacterial products that allow B burgdorferi to replicate and survive seem to be primarily what is required for the bacterium to cause disease in a susceptible host. This article describes the basic biology of B burgdorferi and reviews some of the bacterial components required for infection of and survival in the mammalian and tick hosts.

Distinct roles for MyD88 and Toll-like receptors 2, 5, and 9 in phagocytosis of Borrelia burgdorferi and cytokine induction

The contribution of Toll-like receptors (TLRs) to phagocytosis of Borrelia burgdorferi has not been extensively studied. We show that bone marrow-derived macrophages (BMDM) from MyD88(-/-) mice or Raw cells transfected with a dominant-negative MyD88 were unable to efficiently internalize B. burgdorferi. Knockouts of TLR2 and TLR9 or knockdown of TLR5 by small interfering RNA produced no defects in phagocytosis of B. burgdorferi. Production of inflammatory cytokines was greatly diminished in MyD88(-/-) BMDM but only partially affected in TLR2(-/-) BMDM or knockdown of TLR5 and unaffected in TLR9(-/-) BMDM. Cytochalasin D reduced cytokine induction, but not to the level of the MyD88(-/-) BMDM. Addition of cytochalasin D to TLR2(-/-) BMDM inhibited inflammatory responses to B. burgdorferi to the level of MyD88(-/-) BMDM, consistent with a role for TLR2 in both recognition of extracellular products and lysosomal sampling by TLR2 after processing of the organism. Cytochalasin D had no impact on cytokine production in cells undergoing TLR5 knockdown. These results suggest that MyD88, but not TLR2, TLR5, and TLR9, is important for the uptake of B. burgdorferi and that MyD88 affects inflammatory responses through both its effects on phagocytosis and its role in transducing signals from TLR2 and TLR5.

Regulation of muscle growth by pathogen-associated molecules

Skeletal muscle demonstrates great plasticity in response to environmental and hormonal factors including pathogen-associated molecules, inflammatory cytokines, and growth factors. These signals impinge on muscle by forcing individual muscle fibers to either grow or atrophy. We recently demonstrated that skeletal muscle cells express multiple Toll-like receptors (TLR) that recognize bacterial cell wall components, such as lipopolysaccharide (LPS). Exposure of myocytes to LPS and other TLR ligands initiates an inflammatory response culminating in the autocrine production of cytokines and NO by NO synthase (NOS)2. The TLR signal through protein kinases that phosphorylate and promote the degradation of an inhibitory protein that normally retains the transcription factor, nuclear factor kappaB (NFkappaB), in the cytoplasm. Phosphorylation and degradation of the inhibitor of NFkappaB allows for translocation of NFkappaB to the nucleus and activation of inflammatory genes. Overexpression of a constitutively active inhibitor of NFkappaB kinase in skeletal muscle causes severe wasting, and we found that inhibitors of either the phosphorylation of IkappaB or its proteolytic degradation prevent TLR ligand-induced expression of cytokines and NOS2. The combination of LPS and interferon gamma dramatically enhances the magnitude and duration of LPS-stimulated NOS2 expression and reduces protein translation. Lipopolysaccharide and interferon gamma also downregulates signaling from the mammalian target of rapamycin, a kinase that directs changes in cell size. Inhibitors of NOS block the fall in muscle cell protein synthesis and restore translational signaling, indicating that activation of the NOS2-NO pathway is responsible for the observed decrease in muscle protein synthesis. Our work provides a molecular explanation for reduced muscle growth during infection. Muscle is largely self-sufficient because it expresses receptors, signaling pathways, and effectors to regulate its own size. Prolonged activation of NFkappaB and NOS2 have emerged as detrimental facets of the immune response in muscle. The interplay between inflammatory components and growth factor signaling clearly places muscle at the interface between growth and immunity.

c-Jun N-terminal kinase 1 is required for Toll-like receptor 1 gene expression in macrophages

The regulation of innate immune responses to pathogens occurs through the interaction of Toll-like receptors (TLRs) with pathogen-associated molecular patterns and the activation of several signaling pathways whose contribution to the overall innate immune response to pathogens is poorly understood. We demonstrate a mechanism of control of murine macrophage responses mediated by TLR1/2 heterodimers through c-Jun N-terminal kinase 1 (JNK1) activity. JNK controls tumor necrosis factor alpha production and TLR-mediated macrophage responses to Borrelia burgdorferi, the causative agent of Lyme disease, and the TLR1/TLR2-specific agonist PAM(3)CSK(4). JNK1, but not JNK2, activity regulates the expression of the tlr1 gene in the macrophage cell line RAW264.7, as well as in primary CD11b(+) cells. We also show that the proximal promoter region of the human tlr1 gene contains an AP-1 binding site that is subjected to regulation by the kinase and binds two complexes that involve the JNK substrates c-Jun, JunD, and ATF-2. These results demonstrate that JNK1 regulates the response to TLR1/2 ligands and suggest a positive feedback loop that may serve to increase the innate immune response to the spirochete.

Role of novel protein kinase C isoforms in Lyme arthritis

Inflammation caused by Borrelia burgdorferi infection occurs as a result of induction of pro-inflammatory cytokines from activation of multiple signalling pathways. It has previously been shown that mitogen-activated protein kinase (MAPK) and Janus kinase/signal transducer and activator of transcription signalling pathways are activated by B. burgdorferi in cultured human chondrocytes. Protein kinase C (PKC) signalling pathways are potential candidates that may control these downstream signalling pathways. Here we show that B. burgdorferi infection leads to phosphorylation and activation of novel PKC isoforms (PKC delta, epsilon, eta and theta) in a time-dependent manner. A specific inhibitor of novel PKC isoforms blocked the induction of pro-inflammatory molecules in response to B. burgdorferi infection as did transient transfection of novel PKC dominant-negative plasmids into chondrocytes. B. burgdorferi-induced p38 MAPK phosphorylation was also significantly inhibited by an inhibitor of novel PKC isoforms, suggesting that PKC activation occurs upstream of p38 activation. In vivo, administration of an inhibitor of classical and novel PKC isoforms to C3H/HeN mice infected with B. burgdorferi resulted in significantly reduced ankle inflammation and swelling. In conclusion, these data suggest that novel PKC isoforms are specifically activated by B. burgdorferi infection and this can contribute to the regulation of inflammation in vitro and in vivo.

Control of Borrelia burgdorferi-specific CD4+-T-cell effector function by interleukin-12- and T-cell receptor-induced p38 mitogen-activated protein kinase activity

Infection with Borrelia burgdorferi, the causative agent of Lyme disease, results in a Th1 response and proinflammatory cytokine production. Mice deficient for MKK3, an upstream activator of p38 mitogen-activated protein (MAP) kinase, develop a lower Th1 response and exhibit an impaired ability to produce proinflammatory cytokines upon infection with the spirochete. We investigated the contribution of p38 MAP kinase activity in gamma interferon (IFN-gamma) production in CD4+ T cells in response to specific antigen through T-cell receptor (TCR)- and interleukin-12 (IL-12)-mediated signals. The specific inhibition of p38 MAP kinase in T cells and the administration of a pharmacological inhibitor of the kinase during the course of infection with the spirochete resulted in reduced levels of IFN-gamma in the sera of infected mice. Our results also demonstrate that although p38 MAP kinase activity is not required for the differentiation of B. burgdorferi-specific CD4+ T cells, the production of IFN-gamma by Th1 effector cells is regulated by the kinase. Both TCR engagement and IL-12 induced the production of the Th1 cytokine through the activation of the p38 MAP kinase pathway. Thus, the inhibition of this pathway in vitro resulted in decreased levels of IFN-gamma during restimulation of B. burgdorferi-specific T cells in response to anti-CD3 and IL-12 stimulation. These results clarify the specific contribution of the p38 MAP kinase in the overall immune response to the spirochete and its role in the effector function of B. burgdorferi-specific T cells.

Role of aggrecanase 1 in Lyme arthritis

OBJECTIVE

Arthritis is one of the hallmarks of late-stage Lyme disease. Previous studies have shown that infection with Borrelia burgdorferi, the causative agent of Lyme disease, results in degradation of proteoglycans and collagen in cartilage. B burgdorferi do not appear to produce any exported proteases capable of digesting proteoglycans and collagen, but instead, induce and activate host proteases, such as matrix metalloproteinases (MMPs), which results in cartilage degradation. The role of aggrecanases in Lyme arthritis has not yet been determined. We therefore sought to delineate the contribution of aggrecanases to joint destruction in Lyme arthritis.

METHODS

We examined the expression patterns of aggrecanases 1 and 2 (ADAMTS 4 and 5, respectively) in B burgdorferi-infected primary human chondrocyte cell cultures, in synovial fluid samples from patients with active Lyme arthritis, and in the joints of mice by real-time quantitative reverse transcription-polymerase chain reaction and immunoblotting techniques. Bovine cartilage explants were used to determine the role of aggrecanases in B burgdorferi-induced cartilage degradation.

RESULTS

ADAMTS-4, but not ADAMTS-5, was induced in human chondrocytes infected with B burgdorferi. The active forms of ADAMTS-4 were increased in synovial fluid samples from patients with active Lyme arthritis and were elevated in the joints of mice infected with B burgdorferi. Using cartilage explant models of Lyme arthritis, it appeared that the cleavage of aggrecan was predominantly mediated by "aggrecanases" rather than MMPs.

CONCLUSION

The induction of ADAMTS-4 by B burgdorferi results in the cleavage of aggrecan, which may be an important first step that leads to permanent degradation of cartilage.

Recruitment of macrophages and polymorphonuclear leukocytes in Lyme carditis

Lyme arthritis, caused by the spirochete Borrelia burgdorferi, can be recurrent or prolonged, whereas Lyme carditis is mostly nonrecurring. A prominent difference between arthritis and carditis is the differential representation of phagocytes in these lesions: polymorphonuclear leukocytes (PMN) are more prevalent in the joint, and macrophages predominate in the heart lesion. We have previously shown differential efficiency of B. burgdorferi clearance by PMN and macrophages, and we now investigate whether these functional differences at the cellular level may contribute to the observed differences in organ-specific pathogenesis. When we infected mice lacking the neutrophil chemokine receptor (CXCR2(-/-) mice) with spirochetes, we detected fewer PMN in joints and less-severe arthritis. Here we have investigated the effects of the absence of the macrophage chemokine receptor CCR2 on the development and resolution of Lyme carditis in resistant (C57BL/6J [B6]) and sensitive (C3H/HeJ [C3H]) strains of mice. In B6 CCR2(-/-) mice, although inflammation in hearts is mild, we detected an increased burden of B. burgdorferi compared to that in wild-type (WT) mice, suggesting reduced clearance in the absence of macrophages. In contrast, C3H CCR2(-/-) mice have severe inflammation but a decreased B. burgdorferi burden compared to that in WT C3H mice both at peak disease and during resolution. Histopathologic examination of infected hearts revealed that infected C3H CCR2(-/-) animals have an increased presence of PMN, suggesting compensatory mechanisms of B. burgdorferi clearance in the hearts of infected C3H CCR2(-/-) mice. The more efficient clearance of B. burgdorferi from hearts by CCR2(-/-) versus WT C3H mice suggests a natural defect in the recruitment or function of macrophages in C3H mice, which may contribute to the sensitivity of this strain to B. burgdorferi infection.

p38 mitogen-activated protein kinase controls NF-kappaB transcriptional activation and tumor necrosis factor alpha production through RelA phosphorylation mediated by mitogen- and stress-activated protein kinase 1 in response to Borrelia burgdorferi antigens

The interaction of Borrelia burgdorferi, the causative agent of Lyme borreliosis, with phagocytic cells induces the activation of NF-kappaB and the expression of proinflammatory cytokines including tumor necrosis factor alpha (TNF-alpha). B. burgdorferi-induced TNF-alpha production is also dependent on the activation of p38 mitogen-activated protein (MAP) kinase. The specific contribution of these signaling pathways to the response of phagocytic cells to the spirochete and the molecular mechanisms underlying this response remain unresolved. We now show that p38 MAP kinase activity regulates the transcriptional activation of NF-kappaB in response to spirochetal lysate stimulation of phagocytic cells. The regulation occurs at the nuclear level and is independent of the translocation of the transcription factor to the nucleus or its capacity to bind to specific DNA target sequences. In RAW264.7 cells, p38alpha MAP kinase regulates the phosphorylation of NF-kappaB RelA. p38 MAP kinase phosphorylates the nuclear kinase mitogen- and stress-activated protein kinase 1 (MSK1). MSK1 in turn phosphorylates the transcriptionally active subunit of NF-kappaB, RelA. The repression of MSK1 expression with small interfering RNA results in reduced RelA phosphorylation and a significant decrease in the production of TNF-alpha in response to B. burgdorferi lysates. Overall, these results clarify the contribution of the signaling pathways that are activated in response to the interaction of spirochetes with phagocytic cells to TNF-alpha production. Our results situate p38 MAP kinase activity as a central regulator of the phagocytic proinflammatory response through MSK1-mediated transcriptional activation of the transcription factor NF-kappaB.

Dual role of MyD88 in rapid clearance of relapsing fever Borrelia spp

Relapsing fever Borrelia spp. undergo antigenic variation, achieve high levels in blood, and require rapid production of immunoglobulin M (IgM) for clearance. MyD88-deficient mice display defective clearance of many pathogens; however, the IgM response to persistent infection is essentially normal. Therefore, MyD88(-/-) mice provided a unique opportunity to study the effect of nonantibody, innate host defenses to relapsing fever Borrelia. Infected MyD88(-/-) mice harbored extremely high levels of B. hermsii in the blood compared to wild-type littermates. In the comparison of MyD88(-/-) mice and B- and T-cell-deficient scid mice, two features stood out: (i) bacterial numbers in blood were at least 10-fold greater in MyD88(-/-) mice than scid mice, even though the production of IgM still occurred in MyD88(-/-) mice; and (ii) many of the MyD88(-/-) mice were able to exert partial clearance, although with delayed kinetics relative to wild-type mice, a feature not seen in scid mice. Further analysis revealed a delay in the IgM response to lipoproteins expressed by the original inoculum; however, by 6 days of infection antibodies were produced in MyD88(-/-) mice that could clear spirochetemia in scid mice. While these results indicated that the production of IgM was delayed in MyD88(-/-) mice, they also point to a second, antibody-independent role for MyD88 signaling in host defense to relapsing fever Borrelia. This second defect was apparent only when antibody levels were limiting.

Identification of a TLR-independent pathway for Borrelia burgdorferi-induced expression of matrix metalloproteinases and inflammatory mediators through binding to integrin alpha 3 beta 1

Borrelia burgdorferi stimulates a robust inflammatory response at sites of localization. Binding of borrelial lipoproteins to TLR-2 is one pathway important in the host response to B. burgdorferi. However, while TLR-2 is clearly important in control of infection, inflammation is actually worsened in the absence of TLR-2 or the shared TLR adapter molecule, MyD88, suggesting that there are alternative pathways regulating inflammation. Integrins are cell surface receptors that play an important role in cell to cell communications and that can activate inflammatory signaling pathways. In this study, we report for the first time that B. burgdorferi binds to integrin alpha(3)beta(1) and that binding of B. burgdorferi to this integrin results in induction of proinflammatory cytokines, chemokines, and end-effector molecules such as matrix metalloproteinases in primary human chondrocyte cells. Expression of these same molecules is not affected by the absence of MyD88 in murine articular cartilage, suggesting that the two pathways act independently in activating host inflammatory responses to B. burgdorferi. B. burgdorferi-induced alpha(3) signaling is mediated by JNK, but not p38 MAPK. In summary, we have identified a new host receptor for B. burgdorferi, integrin alpha(3)beta(1); binding of B. burgdorferi to integrin alpha(3)beta(1) results in the release of inflammatory mediators and is proposed as a TLR-independent pathway for activation of the innate immune response by the organism.