Genetic control of experimental lyme arthritis in the absence of specific immunity

Borrelia burgdorferi Engages Mammalian Type I IFN Responses via the cGAS-STING Pathway

Borrelia burgdorferi, the etiologic agent of Lyme disease, is a spirochete that modulates numerous host pathways to cause a chronic, multisystem inflammatory disease in humans. B. burgdorferi infection can lead to Lyme carditis, neurologic complications, and arthritis because of the ability of specific borrelial strains to disseminate, invade, and drive inflammation. B. burgdorferi elicits type I IFN (IFN-I) responses in mammalian cells and tissues that are associated with the development of severe arthritis or other Lyme-related complications. However, the innate immune sensors and signaling pathways controlling IFN-I induction remain unclear. In this study, we examined whether intracellular nucleic acid sensing is required for the induction of IFN-I to B. burgdorferi. Using fluorescence microscopy, we show that B. burgdorferi associates with mouse and human cells in culture, and we document that internalized spirochetes colocalize with the pattern recognition receptor cyclic GMP-AMP synthase (cGAS). Moreover, we report that IFN-I responses in mouse macrophages and murine embryonic fibroblasts are significantly attenuated in the absence of cGAS or its adaptor stimulator of IFN genes (STING), which function to sense and respond to intracellular DNA. Longitudinal in vivo tracking of bioluminescent B. burgdorferi revealed similar dissemination kinetics and borrelial load in C57BL/6J wild-type, cGAS-deficient, or STING-deficient mice. However, infection-associated tibiotarsal joint pathology and inflammation were modestly reduced in cGAS-deficient compared with wild-type mice. Collectively, these results indicate that the cGAS-STING pathway is a critical mediator of mammalian IFN-I signaling and innate immune responses to B. burgdorferi.

Pathogenesis of Borrelia burgdorferi and Babesia microti in TLR4-Competent and TLR4-dysfunctional C3H mice

Toll‐like receptors (TLRs) are a class of membrane‐spanning proteins of host cells. TLR2 and TLR4 are displayed on the surface of macrophages, neutrophils and dendritic cells and recognise structurally conserved microbial signatures defined as Pathogen associated molecular patterns (PAMPs). C3H mice are susceptible to tick‐borne pathogens; Lyme disease causing Borrelia burgdorferi that manifests arthritis and carditis and Apicomplexan protozoan, Babesia microti (Bm) that causes significant parasitemia associated with erythrocytopenia and haemoglobinuria. B. burgdorferi lacks typical TLR4 ligand lipopolysaccharides (LPS) and Bm TLR ligand(s) remain unknown. Only Borrelia lipoproteins that signal through TLR2 are established as PAMPs of these pathogens for TLR2/TLR4. Infection of C3H mice with each pathogen individually resulted in increase in the percentage of splenic B, T and FcR+ cells while their co‐infection significantly diminished levels of these cells and caused increased B. burgdorferi burden in the specific organs. The most pronounced inflammatory arthritis was observed in co‐infected C3H/HeJ mice. Parasitemia levels and kinetics of resolution of Bm in both mice strains were not significantly different. Transfected HEK293 cells showed pronounced signalling by B. burgdorferi through TLR2 and to some extent by TLR4 while Bm and infected erythrocytes did not show any response confirming our results in mice.

A murine model of Lyme disease demonstrates that Borrelia burgdorferi colonizes the dura mater and induces inflammation in the central nervous system

Lyme disease, which is caused by infection with Borrelia burgdorferi and related species, can lead to inflammatory pathologies affecting the joints, heart, and nervous systems including the central nervous system (CNS). Inbred laboratory mice have been used to define the kinetics of B. burgdorferi infection and host immune responses in joints and heart, however similar studies are lacking in the CNS of these animals. A tractable animal model for investigating host-Borrelia interactions in the CNS is key to understanding the mechanisms of CNS pathogenesis. Therefore, we characterized the kinetics of B. burgdorferi colonization and associated immune responses in the CNS of mice during early and subacute infection. Using fluorescence-immunohistochemistry, intravital microscopy, bacterial culture, and quantitative PCR, we found B. burgdorferi routinely colonized the dura mater of C3H mice, with peak spirochete burden at day 7 post-infection. Dura mater colonization was observed for several Lyme disease agents including B. burgdorferi, B. garinii, and B. mayonii. RNA-sequencing and quantitative RT-PCR showed that B. burgdorferi infection was associated with increased expression of inflammatory cytokines and a robust interferon (IFN) response in the dura mater. Histopathologic changes including leukocytic infiltrates and vascular changes were also observed in the meninges of infected animals. In contrast to the meninges, we did not detect B. burgdorferi, infiltrating leukocytes, or large-scale changes in cytokine profiles in the cerebral cortex or hippocampus during infection; however, both brain regions demonstrated similar changes in expression of IFN-stimulated genes as observed in peripheral tissues and meninges. Taken together, B. burgdorferi is capable of colonizing the meninges in laboratory mice, and induces localized inflammation similar to peripheral tissues. A sterile IFN response in the absence of B. burgdorferi or inflammatory cytokines is unique to the brain parenchyma, and provides insight into the potential mechanisms of CNS pathology associated with this important pathogen.

Treatment of Borrelia burgdorferi-Infected Mice with Apoptotic Cells Attenuates Lyme Arthritis via PPAR-γ

Infection of mice with Borrelia burgdorferi causes an inflammatory arthritis that peaks 3-4 wk postinfection and then spontaneously resolves. Although the recruitment of neutrophils is known to drive the development of arthritis, mechanisms of disease resolution remain unclear. Efficient clearance of apoptotic cells (AC) is likely an important component of arthritis resolution. In this article, we show the number of AC increases in the joints of B. burgdorferi-infected mice around day 21 postinfection and peaks around day 28. Injection of AC directly into the ankles of B. burgdorferi-infected mice limited ankle swelling but had no effect on spirochete clearance or arthritis severity scores. In vitro, addition of AC to bone marrow macrophage cultures decreased B. burgdorferi-induced TNF-α and KC and increased IL-10. In addition, phagocytosis of B. burgdorferi and neutrophil migration to LTB₄ were inhibited by AC. Exogenous AC caused an increase in peroxisome proliferator-activated receptor-γ (PPAR-γ) expression both in vitro and in vivo during B. burgdorferi infection. The PPAR-γ agonist rosiglitazone elicited similar changes in macrophage cytokine production and neutrophil migration as exogenous AC. Addition of the PPAR-γ antagonist GW 9662 abrogated the effects of AC in vitro. Injection of rosiglitazone directly into the tibiotarsal joints of B. burgdorferi-infected mice decreased ankle swelling and immune cell recruitment, similar to the injection of AC. These results suggest that clearance of AC plays a role in the resolution of inflammation during experimental Lyme arthritis through the activation of PPAR-γ. PPAR-γ agonists, such as rosiglitazone, may therefore be effective treatments for inducing arthritis resolution.

IL-10 Deficiency Reveals a Role for TLR2-Dependent Bystander Activation of T Cells in Lyme Arthritis

T cells predominate the immune responses in the synovial fluid of patients with persistent Lyme arthritis; however, their role in Lyme disease remains poorly defined. Using a murine model of persistent Lyme arthritis, we observed that bystander activation of CD4+ and CD8+ T cells leads to arthritis-promoting IFN-γ, similar to the inflammatory environment seen in the synovial tissue of patients with posttreatment Lyme disease. TCR transgenic mice containing monoclonal specificity toward non-Borrelia epitopes confirmed that bystander T cell activation was responsible for disease development. The microbial pattern recognition receptor TLR2 was upregulated on T cells following infection, implicating it as marker of bystander T cell activation. In fact, T cell-intrinsic expression of TLR2 contributed to IFN-γ production and arthritis, providing a mechanism for microbial-induced bystander T cell activation during infection. The IL-10-deficient mouse reveals a novel TLR2-intrinsic role for T cells in Lyme arthritis, with potentially broad application to immune pathogenesis.

Borrelia burgdorferi infection induces lipid mediator production during Lyme arthritis

Experimental Lyme arthritis provides a mouse model for exploring the development of pathology following infection of C3H mice with Borrelia burgdorferi. Infected mice develop a reliable inflammatory arthritis of the ankle joint with severity that typically peaks around two to three weeks post-infection and then undergoes spontaneous resolution. This makes experimental Lyme arthritis an excellent model for investigating the mechanisms that drive both the development and resolution phases of inflammatory disease. Eicosanoids are powerful lipid mediators of inflammation and are known to regulate multiple aspects of inflammatory processes. While much is known about the role of eicosanoids in regulating immune responses during autoimmune disease and cancer, relatively little is known about their role during bacterial infection. In this review, we discuss the role of eicosanoid biosynthetic pathways in mediating inflammatory responses during bacterial infection using experimental Lyme arthritis as a model system. We point out the critical role eicosanoids play in disease development and highlight surprising differences between sterile autoimmune responses and those occurring in response to bacterial infection. These differences should be kept in mind when designing therapies and treatments for inflammatory diseases.

Temporal Role for MyD88 in a Model of Brucella-Induced Arthritis and Musculoskeletal Inflammation

Brucella spp. are facultative intracellular Gram-negative bacteria that cause the zoonotic disease brucellosis, one of the most common global zoonoses. Osteomyelitis, arthritis, and musculoskeletal inflammation are common focal complications of brucellosis in humans; however, wild-type (WT) mice infected systemically with conventional doses of Brucella do not develop these complications. Here we report C57BL/6 WT mice infected via the footpad with 10³ to 10⁶ CFU of Brucella spp. display neutrophil and monocyte infiltration of the joint space and surrounding musculoskeletal tissue. Joint inflammation is detectable as early as 1 day postinfection and peaks 1 to 2 weeks later, after which WT mice are able to slowly resolve inflammation. B and T cells were dispensable for the onset of swelling but required for resolution of joint inflammation and infection. At early time points, MyD88^-/- mice display decreased joint inflammation, swelling, and proinflammatory cytokine levels relative to WT mice. Subsequently, swelling of MyD88^-/- joints surpassed WT joint swelling, and resolution of joint inflammation was prolonged. Joint bacterial loads in MyD88^-/- mice were significantly greater than those in WT mice by day 3 postinfection and at all time points thereafter. In addition, MyD88^-/- joint inflammatory cytokine levels on day 3 and beyond were similar to WT levels. Collectively these data demonstrate MyD88 signaling mediates early inflammatory responses in the joint but also contributes to subsequent clearance of Brucella and resolution of inflammation. This work also establishes a mouse model for studying Brucella-induced arthritis, musculoskeletal complications, and systemic responses, which will lead to a better understanding of focal complications of brucellosis.

T Cells Exacerbate Lyme Borreliosis in TLR2-Deficient Mice

Infection of humans with the spirochete, Borrelia burgdorferi, causes Lyme borreliosis and can lead to clinical manifestations such as arthritis, carditis, and neurological conditions. Experimental infection of mice recapitulates many of these symptoms and serves as a model system for the investigation of disease pathogenesis and immunity. Innate immunity is known to drive the development of Lyme arthritis and carditis, but the mechanisms driving this response remain unclear. Innate immune cells recognize B. burgdorferi surface lipoproteins primarily via toll-like receptor (TLR)2; however, previous work has demonstrated TLR2^−/− mice had exacerbated disease and increased bacterial burden. We demonstrate increased CD4 and CD8 T cell infiltrates in B. burgdorferi-infected joints and hearts of C3H TLR2^−/− mice. In vivo depletion of either CD4 or CD8 T cells reduced Borrelia-induced joint swelling and lowered tissue spirochete burden, whereas depletion of CD8 T cells alone reduced disease severity scores. Exacerbation of Lyme arthritis correlated with increased production of CXCL9 by synoviocytes, and this was reduced with CD8 T cell depletion. These results demonstrate T cells can exacerbate Lyme disease pathogenesis and prolong disease resolution possibly through dysregulation of inflammatory responses and inhibition of bacterial clearance.

CXCR2 Mediates Brucella-Induced Arthritis in Interferon γ-Deficient Mice

BACKGROUND

Brucella species are facultative intracellular gram-negative bacteria that cause brucellosis, a common global zoonosis. Infection of the joints is the most common focal complication of brucellosis in humans. The purpose of this study was to identify mediators of focal inflammation during brucellosis.

METHODS

Wild-type (WT) mice are naturally resistant to Brucella infection; therefore, we infected anti-interferon γ (IFN-γ)-treated, or IFN-γ(-/-) mice with Brucella to induce osteoarticular and musculoskeletal inflammation, as we previously described. Mice were infected intraperitoneally with Brucella melitensis, and the clinical course of disease, histopathologic changes, and cytokine levels were compared among groups.

RESULTS

Rag1(-/-) mice (B- and T-cell deficient) and µMT(-/-) mice (B-cell deficient) developed paw inflammation at a similar rate and severity as WT mice following infection with B. melitensis and treatment with anti-IFN-γ. Joints from B. melitensis-infected IFN-γ(-/-) mice had markedly increased levels of CCR2 and CXCR2 ligands. While anti-IFN-γ-treated CCR2(-/-) and WT mice behaved similarly, anti-IFN-γ-treated CXCR2(-/-) or IFN-γ(-/-)/CXCR2(-/-) mice had strikingly reduced focal swelling relative to anti-IFN-γ-treated WT or IFN-γ(-/-) mice, respectively. Additionally, neutrophil recruitment was dependent on CXCR2.

CONCLUSIONS

Adaptive immune cells and CCR2 are dispensable, while CXCR2 is necessary for Brucella-induced focal neutrophil recruitment and inflammation.

Infection of Interleukin 17 Receptor A-Deficient C3H Mice with Borrelia burgdorferi Does Not Affect Their Development of Lyme Arthritis and Carditis

Recently, a number of studies have reported the presence of interleukin 17 (IL-17) in patients with Lyme disease, and several murine studies have suggested a role for this cytokine in the development of Lyme arthritis. However, the role of IL-17 has not been studied using the experimental Lyme borreliosis model of infection of C3H mice with Borrelia burgdorferi. In the current study, we investigated the role of IL-17 in the development of experimental Lyme borreliosis by infecting C3H mice devoid of the common IL-17 receptor A subunit (IL-17RA) and thus deficient in most IL-17 signaling. Infection of both C3H and C3H IL-17RA(-/-) mice led to the production of high levels of IL-17 in the serum, low levels in the heart tissue, and no detectable IL-17 in the joint tissue. The development and severity of arthritis and carditis in the C3H IL-17RA(-/-) mice were similar to what was seen in wild-type C3H mice. In addition, development of antiborrelia antibodies and clearance of spirochetes from tissues were similar for the two mouse strains. These results demonstrate a limited role for IL-17 signaling through IL-17RA in the development of disease following infection of C3H mice with B. burgdorferi.

Forward genetic approaches for elucidation of novel regulators of Lyme arthritis severity

Patients experiencing natural infection with Borrelia burgdorferi display a spectrum of associated symptoms and severity, strongly implicating the impact of genetically determined host factors in the pathogenesis of Lyme disease. Herein, we provide a summary of the host genetic factors that have been demonstrated to influence the severity and chronicity of Lyme arthritis symptoms, and a review of the resources available, current progress, and added value of a forward genetic approach for identification of novel genetic regulators.

The role of eicosanoids in experimental Lyme arthritis

Experimental Lyme arthritis is an inflammatory arthritis caused by infection of mice with the spirochete, Borrelia burgdorferi. It recapitulates many of the disease parameters seen in human patients with Lyme arthritis, and thus serves as a model system for the investigation of disease pathogenesis. While much progress has been made in defining components of the immune response to Borrelia infection, an overall understanding of the host response leading to arthritis resistance or susceptibility remains elusive. In this review, we will focus on recent advancements of our understanding of the roles of eicosanoids as inflammatory mediators in the regulation of experimental Lyme arthritis. Eicosanoids, such as PGE₂ and LTB₄, are powerful regulators of inflammatory responses and thus may be important mediators of Lyme arthritis.

Pathogenic analysis of Borrelia garinii strain SZ isolated from Northeastern China

Background

Various genospecies of Borrelia burgdorferi sensu lato (s.l.) have been identified from patients and animals worldwide. Genospecies-related dissemination of disease has been reported. The present study aimed to elucidate the pathogenicity of infections caused by B. garinii SZ isolated in China. B. burgdorferi B31 and B. afzelii BO23 were used for comparison.

Methods

Spirochete load in blood and tissue samples of infected mice were measured by minor groove binder-based real-time polymerase chain reaction. The kinetics of spirochete dissemination and disease severity were assessed in BALB/c mice.

Results

The pattern of bacterial load differed between the three genospecies. The B. garinii SZ strain is highly pathogenic and can trigger multi-system pathological damage in mice.

Conclusions

Spirochete dissemination, persistence, tissue tropism and disease severity varied significantly, suggesting that different genospecies may play an important role in the pathogenicity and development of clinical diseases.

Spirochete antigens persist near cartilage after murine Lyme borreliosis therapy

An enigmatic feature of Lyme disease is the slow resolution of musculoskeletal symptoms that can continue after treatment, with some patients developing an inflammatory arthritis that becomes refractory to antibiotic therapy. Using intravital microscopy and the mouse model of Lyme borreliosis, we observed that Borrelia burgdorferi antigens, but not infectious spirochetes, can remain adjacent to cartilage for extended periods after antibiotic treatment. B. burgdorferi was not recovered by culture or xenodiagnosis with ticks after antibiotic treatment of WT mice and all but one of the immunodeficient mice with heightened pathogen burden due to impaired TLR responsiveness. Amorphous GFP+ deposits were visualized by intravital microscopy in the entheses of antibiotic-treated mice infected with GFP-expressing spirochetes and on the ear cartilage surface in sites where immunofluorescence staining detected spirochete antigens. Naive mice were not infected by tissue transplants from antibiotic-treated mice even though transplants contained spirochete DNA. Tissue homogenates from antibiotic-treated mice induced IgG reactive with B. burgdorferi antigens after immunization of naive mice and stimulated TNF-α production from macrophages in vitro. This is the first direct demonstration that inflammatory B. burgdorferi components can persist near cartilaginous tissue after treatment for Lyme disease. We propose that these deposits could contribute to the development of antibiotic-refractory Lyme arthritis.

Hamster and murine models of severe destructive Lyme arthritis

Arthritis is a frequent complication of infection in humans with Borrelia burgdorferi. Weeks to months following the onset of Lyme borreliosis, a histopathological reaction characteristic of synovitis including bone, joint, muscle, or tendon pain may occur. A subpopulation of patients may progress to a chronic, debilitating arthritis months to years after infection which has been classified as severe destructive Lyme arthritis. This arthritis involves focal bone erosion and destruction of articular cartilage. Hamsters and mice are animal models that have been utilized to study articular manifestations of Lyme borreliosis. Infection of immunocompetent LSH hamsters or C3H mice results in a transient synovitis. However, severe destructive Lyme arthritis can be induced by infecting irradiated hamsters or mice and immunocompetent Borrelia-vaccinated hamsters, mice, and interferon-gamma- (IFN-γ-) deficient mice with viable B. burgdorferi. The hamster model of severe destructive Lyme arthritis facilitates easy assessment of Lyme borreliosis vaccine preparations for deleterious effects while murine models of severe destructive Lyme arthritis allow for investigation of mechanisms of immunopathology.

Interleukin-35 enhances Lyme arthritis in Borrelia-vaccinated and -infected mice

Interleukin-35 (IL-35) has been reported to inhibit the production of interleukin-17 (IL-17) as a means of preventing arthritis and other inflammatory diseases. We previously showed that treatment of Borrelia-vaccinated and -infected mice with anti-IL-17 antibody at the time of infection prevented the development of arthritis. The anti-IL-17 antibody-treated mice lacked the extensive tissue damage, such as bone and cartilage erosion, that occurred in the tibiotarsal joints of untreated Borrelia-vaccinated and -infected control mice. We hypothesized that IL-35 would reduce the severity of arthritis by suppressing the production of IL-17 in Borrelia-vaccinated and -infected mice. Here, we show that administration of recombinant IL-35 (rIL-35) to Borrelia-vaccinated and -infected mice augments the development of severe arthritis compared to the results seen with untreated control mice. Borrelia-vaccinated and -infected mice treated with rIL-35 had significantly (P < 0.05) greater hind paw swelling and histopathological changes from day 4 through day 10 than non-rIL-35-treated Borrelia-vaccinated and -infected mice. In addition, the treatment with IL-35 only slightly decreased the production of IL-17 in Borrelia-primed immune cells and did not prevent the development of borreliacidal antibody. Our data do not support a role for IL-35 as a potential therapeutic agent to reduce inflammation in Lyme arthritis.

Significant differences between the Borrelia-infection and Borrelia-vaccination and -infection models of Lyme arthritis in C3H/HeN mice

The immunological events leading to the development of Lyme arthritis in humans are partially understood. Much of this information has been gained by studying the course of infection of naïve or vaccinated mice with Borrelia burgdorferi. However, the Borrelia-vaccination and -infection model has not been described using the organismal parameters commonly used in the widely accepted Borrelia-infection model. This is the first comparison between the Borrelia-infection and the Borrelia-vaccination and -infection models of arthritis. Borrelia-vaccinated and -infected C3H/HeN mice develop acute inflammation comparable to that of nonvaccinated, Borrelia-infected C3H/HeN mice. The duration and severity of arthritis in Borrelia-vaccinated and -infected mice was slightly increased compared with Borrelia-infected mice. Significantly, Borrelia-vaccinated and -infected C3H/HeN mice produce interleukin-17 (IL-17), while Borrelia-infected mice that had not been previously vaccinated do not. Neutralization of IL-17 in Borrelia-vaccinated and -infected C3H/HeN mice decreased the severity of arthritis, although not to the degree we observed previously in C57BL/6 mice. Collectively, these findings show that the Borrelia-vaccination and -infection model of Lyme arthritis incorporates elements of adaptive immunity that likely have relevance to human disease, but may not be observed in Borrelia-infected C3H/HeN mice.

5-Lipoxygenase-deficient mice infected with Borrelia burgdorferi develop persistent arthritis

The enzyme 5-lipoxygenase (5-LO) catalyzes the conversion of arachidonic acid into the leukotrienes, which are critical regulators of inflammation and inflammatory diseases, such as asthma and arthritis. Although leukotrienes are present in the synovial fluid of Lyme disease patients, their role in the development of Lyme arthritis has not been determined. In the current study, we used a murine model of Lyme arthritis to investigate the role 5-LO products might have in the development of this inflammatory disease. After infection of Lyme arthritis-susceptible C3H/HeJ mice with Borrelia burgdorferi, mRNA expression of 5-LO and 5-LO-activating protein was induced in the joints, and the 5-LO product leukotriene B(4) was produced. Using C3H 5-LO-deficient mice, we demonstrated that 5-LO activity was not necessary for the induction of Lyme arthritis, but that its deficiency resulted in earlier joint swelling and an inability to resolve arthritis as demonstrated by sustained arthritis pathology through day 60 postinfection. Although production of anti-Borrelia IgG was decreased in 5-LO-deficient mice, bacterial clearance from the joints was unaffected. Phagocytosis of B. burgdorferi and efferocytosis of apoptotic neutrophils was defective in macrophages from 5-LO-deficient mice, and uptake of opsonized spirochetes by neutrophils was reduced. These results demonstrate that products of the 5-LO metabolic pathway are not required for the development of disease in all models of arthritis and that caution should be used when targeting 5-LO as therapy for inflammatory diseases.

Invariant NKT cell development requires a full complement of functional CD3 zeta immunoreceptor tyrosine-based activation motifs

Invariant NKT (iNKT) cells regulate early immune responses to infections, in part because of their rapid release of IFN-gamma and IL-4. iNKT cells are proposed to reduce the severity of Lyme disease following Borrelia burgdorferi infection. Unlike conventional T cells, iNKT cells express an invariant alphabeta TCR that recognizes lipids bound to the MHC class I-like molecule, CD1d. Furthermore, these cells are positively selected following TCR interactions with glycolipid/CD1d complexes expressed on CD4+CD8+ thymocytes. Whereas conventional T cell development can proceed with as few as 4/10 CD3 immunoreceptor tyrosine-based activation motifs (ITAMs), little is known about the ITAM requirements for iNKT cell selection and expansion. We analyzed iNKT cell development in CD3 zeta transgenic lines with various tyrosine-to-phenylalanine substitutions (YF) that eliminated the functions of the first (YF1,2), third (YF5,6), or all three (YF1-6) CD3 zeta ITAMs. iNKT cell numbers were significantly reduced in the thymus, spleen, and liver of all YF mice compared with wild type mice. The reduced numbers of iNKT cells resulted from significant reductions in the expression of the early growth response 2 and promyelocytic leukemia zinc finger transcription factors. In the mice with few to no iNKT cells, there was no difference in the severity of Lyme arthritis compared with wild type controls, following infections with the spirochete B. burgdorferi. These findings indicate that a full complement of functional CD3 zeta ITAMs is required for effective iNKT cell development.

Molecular Mimicry toBorrelia burgdorferi: Pathway to Autoimmunity?

Lyme borreliosis is due to infection with the tick-borne spirochete Borrelia burgdorferi, and is associated with persistent infection unless treated with antibiotics. The persistent nature of infection by B. burgdorferi can lead to development of chronic disease, as found in patients infected before recognition of the effectiveness of antibiotic therapy. Much speculation has surrounded the possibility that autoimmune mechanisms are involved in chronic symptoms. In most cases, involvement of autoimmunity in Lyme disease has not received experimental support. The exception is in a small group of patients with chronic arthritis whose abnormal joint symptoms persist after apparent elimination of the bacteria. In this review, the evidence supporting autoimmune mechanisms in Lyme disease will be discussed.

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.

Adenoviral delivery of interleukin-10 fails to attenuate experimental Lyme disease

Production of interleukin-10 (IL-10) by C57BL/6 mice following infection with Borrelia burgdorferi has been proposed as a mechanism whereby resistance to the development of experimental Lyme arthritis is maintained. In the current study, we sought to determine the role of IL-10 during infection of arthritis- and carditis-susceptible C3H mice. Infection of C3H IL-10(-/-) mice led to increased joint swelling and arthritis severity scores over those of wild-type C3H mice. Measurement of B. burgdorferi numbers in joints or disseminated tissues indicated a more efficient clearance of spirochetes in the absence of IL-10, similar to that reported in C57BL/6 IL-10(-/-) mice. However, in contrast to previous in vitro work, infection of C3H IL-10(-/-) mice led to decreased in vivo expression of the cytokines KC, IL-1beta, IL-4, and IL-12p70 in the infected joints. Finally, adenoviral expression of IL-10 in the infected joints of C3H mice was unable to modulate the development of severe Lyme arthritis and had no effect on spirochete clearance or Borrelia-specific antibody production. Development of Lyme carditis appeared to be independent of modulation by IL-10. These results suggest that IL-10 limits the development of joint inflammation in both arthritis-resistant and -susceptible mouse strains infected with B. burgdorferi and that increased IL-10 production cannot rescue genetic susceptibility to development of pathology in this model.

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.

Arthritis develops but fails to resolve during inhibition of cyclooxygenase 2 in a murine model of Lyme disease

OBJECTIVE

Recent studies have implicated products of cyclooxygenase 2 (COX-2) in not only induction but also resolution of the inflammatory response; however, the contribution of COX-2 products to the in vivo response to infection is unknown. The aim of this study was to determine the contribution of COX-2 to temporal regulation of the inflammatory response to infection in a murine model of Lyme arthritis.

METHODS

Experimental Lyme disease was induced in both arthritis-resistant DBA/2J and arthritis-susceptible C3H/HeJ mice by inoculation in the hind footpads with Borrelia burgdorferi. COX-2 inhibitors were administered daily, and their effect on arthritis pathology was assessed at various time points postinfection. The COX-2 deficiency was also backcrossed onto both DBA and C3H backgrounds to confirm the findings from COX-2 inhibitor-treated mice.

RESULTS

In COX-2 inhibitor-treated or COX-2-/- C3H mice, arthritis developed normally but did not resolve. Cessation of COX-2 inhibitor treatment on day 14 postinfection did not induce resolution of arthritis, indicating an early onset for the molecular mechanisms governing resolution. The lack of resolution of arthritis correlated with altered COX-2 and cytosolic phospholipase A2 messenger RNA levels in the joints of C3H mice. In addition, the proresolution lipid molecule 15-deoxy-Delta12,14-prostaglandin J2 was produced in response to B burgdorferi infection, and its production was attenuated by the inhibition of COX-2.

CONCLUSION

Our results demonstrate that early production of COX-2 products is necessary for resolution of the inflammatory arthritis induced by Borrelia infection, and that COX-2 inhibition may result in prolonged inflammatory states, possibly by inhibition of proresolution eicosanoids.

Gene expression profiling provides insights into the pathways involved in inflammatory arthritis development: murine model of Lyme disease

The spirochete Borrelia burgdorferi, the etiologic agent of Lyme disease, causes severe subacute arthritis in susceptible inbred mouse strains, such as C3H/HeN, but only mild arthritis in resistant strains such as C57BL/6. The degree of Lyme arthritis severity is controlled in part by host genetics and several quantitative trait loci have been identified which contribute to this regulation. In addition, the anti-inflammatory cytokine IL-10 assumes an important role in the control of arthritis in C57BL/6 mice. However, the identification of genes and signaling pathways that dictate arthritis severity has remained elusive. In an attempt to elucidate such genes and pathways, the power of microarray analysis was combined with information gleaned from gene manipulation models. As a result of this approach, two novel gene profiles were identified: an IFN-inducible profile in arthritis-susceptible C3H and IL-10(-/-) mice, and an epidermal/differentiation profile in C57BL/6 mice. Application of this information to TLR2(-/-) mice, which also develop severe arthritis, indicated that they also upregulated IFN-responsive genes. These results provided new insight into the regulation of Lyme arthritis development and illustrated the utility of combining gene expression analyses with genetically manipulated mouse models in unraveling mechanisms underlying specific disease processes.

CD28 deficiency exacerbates joint inflammation upon Borrelia burgdorferi infection, resulting in the development of chronic Lyme arthritis

Lyme disease, caused by the tick-borne spirochete Borrelia burgdorferi (Bb), is a multisystem illness, affecting many organs, such as the heart, the nervous system, and the joints. Months after Bb infection, approximately 60% of patients experience intermittent arthritic attacks, a condition that in some individuals progresses to chronic joint inflammation. Although mice develop acute arthritis in response to Bb infection, the joint inflammation clears after 2 wk, despite continuous infection, only very rarely presenting with chronic Lyme arthritis. Thus, the lack of an animal system has so far prevented the elucidation of this persistent inflammatory process that occurs in humans. In this study, we report that the majority of Bb-infected CD28-/- mice develop chronic Lyme arthritis. Consistent with observations in chronic Lyme arthritis patients, the infected mutant, but not wild-type mice present recurring monoarticular arthritis over an extended time period, as well as anti-outer surface protein A of Bb serum titers. Furthermore, we demonstrate that anti-outer surface protein A Abs develop in these mice only after establishment of chronic Lyme arthritis. Thus, the Bb-infected CD28-/- mice provide a murine model for studying chronic Lyme arthritis.

T cell infiltration is associated with increased Lyme arthritis in TLR2-/- mice

C57BL/6 mice deficient in TLR2 develop more severe arthritis following infection with Borrelia burgdorferi than do wild-type C57BL/6 mice, and this increase is suppressed by the simultaneous presence of the scid mutation. This suggested a requirement for lymphocytes in the development of subacute Lyme arthritis in TLR2(-/-) mice, a feature not commonly associated with this arthritis. The increased pathology of B. burgdorferi-infected TLR2(-/-) mice was also accompanied by an increase in mononuclear cell infiltration. In this study, T cells were found to be responsible for the increase in mononuclear cells in infected TLR2(-/-) C3H mice. Accordingly, transcripts for the IFN-inducible T cell chemokines, CXCL9 and CXCL10, were greatly enhanced in joint tissue from TLR2(-/-) mice, as were transcripts for a prototypical IFN-inducible gene IFN-gamma-induced GTPase (igtp). Treatment of murine synovial cells with sonicated B. burgdorferi resulted in induction of transcripts for chemokines and other IFN-inducible genes, irrespective of the presence of TLR2. The presence of T lymphocytes greatly enhanced the transcriptional response of synovial cells. These results suggest that the increased inflammatory cell infiltration in TLR2(-/-) C3H mice is the result of localized overproduction of T cell attracting chemokines.

Bb2Bb3 regulation of murine Lyme arthritis is distinct from Ncf1 and independent of the phagocyte nicotinamide adenine dinucleotide phosphate oxidase

Several quantitative trait loci regulating murine Lyme arthritis severity have been mapped, including a highly significant linkage found on chromosome 5, termed Bb2Bb3. Within this region, the Ncf1 gene of the phagocyte nicotinamide adenine dinucleotide phosphate (NADPH) oxidase has recently been identified as a major regulator of arthritis severity in rodent models of rheumatoid arthritis, an effect attributed to protective properties of reactive oxygen species. To assess the role of Ncf1 in Lyme arthritis, we introgressed Bb2Bb3 from severely arthritic C3H/He mice onto mildly arthritic C57BL/6 mice. This increased Lyme arthritis severity, whereas the reciprocal transfer conferred protection from disease. A single nucleotide polymorphism was identified in the Ncf1 gene that did not influence the protein sequence or expression of Ncf1. Although polymorphonuclear leukocytes from C57BL/6 mice generated a greater oxidative burst than polymorphonuclear leukocytes from C3H/He mice, studies with the Bb2Bb3 congenic mice demonstrated this difference was not linked to Ncf1 alleles. Furthermore, Lyme arthritis severity was not altered in mice lacking either the Ncf1 or Gp91phox subunits of the NADPH oxidase complex. Together, these results argue that Ncf1 is not a candidate gene for regulation of Lyme arthritis and reveal Lyme arthritis to be independent of NADPH oxidase activity, distinguishing it from other models of rheumatoid arthritis.

Relative contributions of innate and acquired host responses to bacterial control and arthritis development in Lyme disease

TLR2(-/-)/scid double-mutant mice were infected with B. burgdorferi to assess the relative importance of acquired and innate host defenses. Although spirochete levels at 4 weeks were lower in TLR2(-/-) mice than in TLR2(-/-)/scid mice, the increased arthritis severity of TLR2 (Toll-like receptor 2)-deficient mice was reduced by the presence of the scid mutation.

Induction of host matrix metalloproteinases by Borrelia burgdorferi differs in human and murine lyme arthritis

Matrix metalloproteinases (MMPs) are induced from host tissues in response to Borrelia burgdorferi. Upregulation of MMPs may play a role in the dissemination of the organism through extracellular matrix tissues, but it can also result in destructive pathology. Although mice are a well-accepted model for Lyme arthritis, there are significant differences compared to human disease. We sought to determine whether MMP expression could account for some of these differences. MMP expression patterns following B. burgdorferi infection were analyzed in primary human chondrocytes, synovial fluid samples from patients with Lyme arthritis, and cartilage tissue from Lyme arthritis-susceptible and -resistant mice by using a gene array, real-time PCR, an enzyme-linked immunosorbent assay, and immunohistochemistry. B. burgdorferi infection significantly induced transcription of MMP-1, -3, -13, and -19 from primary human chondrocyte cells. Transcription of MMP-10 and tissue inhibitor of metalloprotease 1 was increased with B. burgdorferi infection, but protein expression was only minimally increased. The synovial fluid levels of MMPs from patients with high and low spirochete burdens were consistent with results seen in the in vitro studies. B. burgdorferi-susceptible C3H/HeN mice infected with B. burgdorferi showed induction of MMP-3 and MMP-19 but no other MMP or tissue inhibitor of metalloprotease. As determined by immunohistochemistry, MMP-3 expression was increased only in chondrocytes near the articular surface. The levels of MMPs were significantly lower in the more Lyme arthritis-resistant BALB/c and C57BL/6 mice. Differences between human and murine Lyme arthritis may be related to the lack of induction of collagenases, such MMP-1 and MMP-13, in mouse joints.

Research opportunities on human neuroborreliosis

A workshop, sponsored by the National Institutes of Health, was convened in September 2001 to evaluate the current knowledge in neurological Lyme disease. The meeting was centered into discussion of both clinical and basic aspects of the disease. Participants included researchers from the fields of infectious diseases, neurology, rheumatology, autoimmunity and basic immunology, largely but not exclusively focused on Lyme disease. This report summarizes the presentations made at the meeting.

Treatment of mice with the neutrophil-depleting antibody RB6-8C5 results in early development of experimental lyme arthritis via the recruitment of Gr-1- polymorphonuclear leukocyte-like cells

Recently, we demonstrated that blocking the entry of neutrophils into Borrelia burgdorferi-infected joints in mice deficient in the chemokine receptor CXCR2 prevented the development of experimental Lyme arthritis. Neutrophils were marginalized in blood vessels at the site of infection but could not enter the joint tissue. In the present study, we treated both genetically arthritis-resistant DBA/2J (DBA) and arthritis-susceptible C3H/HeJ (C3H) mice with the neutrophil-depleting monoclonal antibody RB6-8C5 (RB6) to determine the effect on arthritis development. Surprisingly, both DBA and C3H mice treated with RB6 developed arthritis at 1 week postinfection, approximately 1 week earlier than the control-treated C3H mice. The early development of arthritis in the RB6-treated mice was accompanied by an influx into the joints of cells with ring-shaped polymorphonuclear leukocyte (PMN) cell morphology that were negative for the Gr-1 neutrophil maturation marker. RB6 treatment of mice also resulted in increased numbers of B. burgdorferi cells in the joints at 7 days postinfection and earlier expression of the chemokines KC and monocyte chemoattractant protein 1 in the joints compared to control-treated animals. Together, these results suggest that recruitment of neutrophils or PMN-like cells into an infected joint is a key requirement for Lyme arthritis development and that altered recruitment of these cells into the joints of arthritis-resistant mice can exacerbate the development of pathology.

Myeloid differentiation antigen 88 deficiency impairs pathogen clearance but does not alter inflammation in Borrelia burgdorferi-infected mice

The spirochete Borrelia burgdorferi causes acute inflammation in mice that resolves with the development of pathogen-specific adaptive immunity. B. burgdorferi lipoproteins activate innate immune cells via Toll-like receptor 2 (TLR2), but TLR2-deficient mice are not resistant to B. burgdorferi-induced disease, suggesting the involvement of other TLRs or non-TLR mechanisms in the induction of acute inflammation. For this study, we used mice that were deficient in the intracellular adapter molecule myeloid differentiation antigen 88 (MyD88), which is required for all TLR-induced inflammatory responses, to determine whether the interruption of this pathway would alter B. burgdorferi-induced disease. Infected MyD88(-/-) mice developed carditis and arthritis, similar to the disease in wild-type (WT) mice analyzed at its peak (days 14 and 28) and during regression (day 45). MyD88(-/-) macrophages produced tumor necrosis factor alpha only when spirochetes were opsonized, suggesting a role for B. burgdorferi-specific antibody in disease expression. MyD88(-/-) mice produced stronger pathogen-specific Th2-dependent immunoglobulin G1 (IgG1) responses than did WT mice, and their IgM titers remained significantly elevated through 90 days of infection. Despite specific antibodies, the pathogen burden was 250-fold higher in MyD88(-/-) mice than in WT mice 45 days after infection; by 90 days of infection, the pathogen burden had diminished substantially in MyD88(-/-) mice, but it was still elevated compared to that in WT mice. The elevated pathogen burden may be explained in part by the finding that MyD88(-/-) peritoneal macrophages could ingest spirochetes but degraded them more slowly than WT macrophages. Our results show that MyD88-dependent signaling pathways are not required for B. burgdorferi-induced inflammation but are necessary for the efficient control of the pathogen burden by phagocytes.

Quantitative analysis of Borrelia burgdorferi gene expression in naturally (tick) infected mouse strains

Adaptation of Borrelia burgdorferi in the vector and vertebrate host is mediated by mechanisms that regulate differential expression of outer surface lipoproteins (Osps). In this study, real time PCR was applied to quantify tissue-specific expression of four linear plasmid (lp54)-encoded (ospA, zs7.a36, zs7.a66 zs7.a68) and one circular plasmid (cp26)-encoded (ospC) gene from B. burgdorferi sensu stricto, in a natural setting of tick-infected immunodeficient (C.B-17 SCID) and immunocompetent (BALB/c and AKR/OlaHsd) mice for up to 120 days post-infection (p.i.). Early during infection (day 30 p.i.) high numbers of spirochetes were found in the heart and joint, but not the ear and spleen tissues of disease-susceptible SCID mice. In disease-susceptible AKR mice spirochetes colonized the ear and joint tissues, but were undetectable in tissues of disease-resistant BALB/c mice. Later in infection (day 120 p.i.), spirochetes had expanded (approximately 1,000-fold) in all SCID tissues tested but were undetectable in AKR and BALB/c mice. Of the five genes analyzed, only zs7.a36 transcripts were detected in various tissues of all infected mouse strains, though at differing levels, whereas ospC transcripts were only found in tissue specimens of SCID mice. Furthermore, gene expression of ospC and zs7.a36 appears to be differentially regulated in distinct organs of individual mice. In contrast, transcripts for ospA, zs7.a66, and zs7.a68 were not detected in any of the mouse strains, independent of their immune status and/or the severity of their infection/inflammatory responses. Late during infection (day 120 p.i.), transcription of zs7.a36 and ospC was down-regulated in the tissues of SCID mice despite expansion of spirochetes. This type of quantitative analysis may be helpful to further disclose principles of pathogenesis of Lyme borreliosis and to design strategies for its therapeutic treatment.

Elucidation of Lyme arthritis

Before the first description of Lyme arthritis in 1976, patients with this disease were often thought to have juvenile or adult rheumatoid arthritis. It is now known that Lyme arthritis is caused by a tick-borne spirochete that disseminates to joints, where it induces marked pro-inflammatory responses. In most patients, the arthritis resolves with antibiotic treatment. However, in the United States, about 10% of patients with Lyme arthritis develop persistent synovitis, which lasts for months or even several years after the apparent eradication of the spirochete from the joint with antibiotic therapy. The elucidation of Lyme arthritis, from acute infection to chronic synovitis, might help in our understanding not only of this entity, but also of other forms of chronic inflammatory arthritis, including rheumatoid arthritis.

Inflammatory cytokine production predominates in early Lyme disease in patients with erythema migrans

In a study of cytokine production ex vivo by Borrelia burgdorferi-stimulated peripheral blood mononuclear cells from 27 patients with culture-positive erythema migrans, production of inflammatory cytokines predominated, particularly gamma interferon and, to a lesser degree, tumor necrosis factor alpha. In contrast, with the exception of interleukin-13, anti-inflammatory cytokine production was negligible. Thus, B. burgdorferi antigens in early Lyme disease often induce a strong inflammatory response.

An analysis of spirochete load, strain, and pathology in a model of tick-transmitted Lyme borreliosis

Four laboratory-grown, low-passage isolates of Borrelia burgdorferi sensu stricto, B31, JD-1, 910255, and N40, were incorporated into Ixodes scapularis ticks to examine the pathogenesis of these isolates in mice after tick transmission. All isolates induced multifocal, lymphoid nodular cystitis, subacute, multifocal, necrotizing myocarditis, and a localized periostitis and arthritis of the femorotibial joint 6-18 weeks after tick infestation. In terms of the number of mice that demonstrated pathology in bladder, heart, and joint, the highest incidence of lesions occurred 12 weeks after tick bite. Utilizing the Taqman quantitative polymerase chain reaction (q-PCR) fluorogenic detection technology to amplify a conserved region of the flagellin gene, a trend was demonstrated between the number of spirochetes in tissue with duration of pathology. The q-PCR assay developed for this study was sensitive and could reliably measure as few as 1 to 10 spirochetes in the target tissues tested. A higher percentage of B31- and N40-infected mice (92 and 100%, respectively) developed myocarditis than JD-1- or 910255-infected mice (67 and 46%, respectively) 12 weeks after tick bite. The amount of spirochetal DNA that could be amplified for heart at this time point was not statistically different between isolates, indicating a difference in virulence between B31 and N40 relative to JD-1 and 910225. N40-infected mice demonstrated a significantly higher spirochete load (an average of 1.23 spirochetes/mg of tissue, p = 0.045) in femorotibial joints 18 weeks after infection, with 60% of these mice maintaining lesions compared with those infected with B31 (13%), JD-1 (25%), or 910255 (50%), which averaged <0.5 spirochetes/mg of tissue. This mouse model of Lyme borreliosis, including the ability to monitor lesion development and spirochete load, can facilitate the testing of therapeutic regimens for the later stages of tick-transmitted Lyme disease and help investigate aspects of the immunopathogenesis of lesion development.

Novel B cell population producing functional IgG in the absence of membrane IgM expression

Surface expression of IgM is a characteristic feature of the development of most B cells. Only pre-B cells bearing functional IgM heavy chains mu chains) are selected for clonal expansion and differentiation. Cells lacking mu chains are normally eliminated. muMT mice carrying a deletion of the first exon coding for the transmembrane domain of the immunoglobulin mu chain gene were described as mice deficient for mature B cells, plasma cells and immunoglobulins in serum. In this study, we describe in muMT/BALB/c mice the presence of a novel B cell population, producing IgG, IgA and IgE in the absence of IgM membrane expression. Moreover, this small population of B cells is able to recognize antigens and to differentiate into plasma cells. These "non-conventional" mu(- / -) B cells produce functional immunoglobulins after immunization, undergo germinal center reactions, and maintain B cell memory. Our findings support the concept, that a small percentage of mu -non-expressing pre-B cells can escape elimination, switch to downstream immunoglobulin heavy chains and respond to antigens. It remains an open question how the reactivity of these B cells is regulated and in which extent such B cells play a role in physiological and pathological processes such as autoantibody production and autoimmunity.

[Rheumatologic manifestations of Lyme borreliosis]

PURPOSE

Lyme borreliosis is a multisystemic infection caused by the spirochaete Borrelia burgdorferi. In European endemic areas like northeast France, articular manifestations are, after neuroborreliosis, the most frequent extra-cutaneous features observed. Among the pathogenic species of Borrelia, Borrelia burgdorferi sensu stricto is the most frequently identified during Lyme arthritis, but others species also seem to be involved.

CURRENT KNOWLEDGE AND KEY POINTS

The diagnosis of Lyme arthritis is usually based on combined clinical data and serological laboratory tests. In atypical forms, detection of bacterial DNA could be useful. While mechanisms involved in acute Lyme arthritis are beginning to be better understood, the pathogenesis of chronic arthritis, which concerns about 10% of the patients, remains unknown. Two hypotheses are proposed to explain the prolonged evolution of the articular disease: a chronic persistence of Borrelia burgdorferi, which evades the host immune system within the joint and/or an autoimmune mechanism by molecular mimicry. The antibiotic therapy is codified in acute arthritis, but is not really adapted in chronic Lyme arthritis or post-Lyme syndrome.

FUTURE PROSPECTS AND PROJECTS

To prevent the disease, the vaccine available in the United States does not offer complete protection and is not useful in Europe since the species heterogeneity is important for the outer surface protein A. A better understanding of Lyme disease pathogenesis can subsequently lead to new therapeutic or preventive approaches.

Innate resistance to Babesia infection is influenced by genetic background and gender

Infection of severe combined immunodeficient mice with Babesia sp. strain WA1 was studied to assess the contributions of innate and adaptive immunity in resistance to acute babesiosis. The scid mutation showed little effect in genetically susceptible C3H mice and did not decrease the inherent resistance of C57BL/6 mice to the infection, suggesting that innate immunity plays a central role in determining the course of Babesia infection in these strains. In contrast, the scid mutation dramatically impaired resistance in moderately susceptible BALB/c mice, suggesting that acquired immunity may play an important secondary role. In comparison to their female counterparts, male mice of different genetic backgrounds showed increased resistance to the infection, indicating that the gender of the host may influence protection against babesiosis.

Genetic control of susceptibility to experimental Lyme arthritis is polygenic and exhibits consistent linkage to multiple loci on chromosome 5 in four independent mouse crosses

C3H/He mice infected with Borrelia burgdorferi develop severe arthritis and are high antibody responders, while infected C57BL/6 and BALB/c mice develop mild arthritis and less robust humoral responses. Genetic analysis using composite interval mapping (CIM) on reciprocal backcross populations derived from C3H/HeN and C57BL/6N or C3H/HeJ and BALB/cAnN mice identified 12 new quantitative trait loci (QTL) linked to 10 murine Lyme disease phenotypes. These QTL reside on chromosomes 1, 2, 4, 6, 7, 9, 10, 12, 14, 15, 16, and 17. A reanalysis of an F(2) intercross between C57BL/6N and C3H/HeN mice using CIM identified two new QTL on chromosomes 4 and 15 and confirmed the location of seven previously identified loci. Two or more experimental crosses independently verified six QTL controlling phenotypes after B. burgdorferi infection. Additionally, Bb2 on chromosome 5 was reproduced in four experimental populations and was linked to the candidate locus Cora1. Evidence of four distinct QTL residing within the 30-cM region of chromosome 5 encompassing the previously mapped Bb2 and Bb3 loci was shown by CIM. Interestingly, some alleles contributing to susceptibility to Lyme arthritis were derived from C57BL/6N and BALB/cAnN mice, showing that disease-resistant strains harbor susceptibility alleles.

Impact of genotypic variation of Borrelia burgdorferi sensu stricto on kinetics of dissemination and severity of disease in C3H/HeJ mice

Various genotypes of Borrelia burgdorferi sensu stricto have been previously identified among a large collection of isolates cultured from patients with Lyme disease in the United States. Furthermore, association of specific genotypes with hematogenous dissemination early in the disease course has been observed. The present study assessed kinetics of spirochete dissemination and disease severity in C3H/HeJ mice infected with two different genotypes of B. burgdorferi. Spirochete load in plasma and ear and other tissue samples of infected mice was measured by quantitative PCR, and these data were compared to those obtained by culture and histopathologic analysis. In mice infected with isolate BL206 (a type 1 strain), the peak number of spirochetes was observed in plasma between day 4 and 7, in heart and ear tissue on day 14, and in joints on day 28 postinoculation. There was a correlation between the peak number of spirochetes in plasma on day 4 or 7 and that in ear biopsy and joint specimens on day 14. By contrast, spirochete burdens in plasma of mice infected with isolate B356 (a type 3 strain) were 16- and 5-fold lower than those of BL206-infected mice on days 7 and 14 of infection, respectively. Similarly, approximately 6- and 13-fold fewer spirochetes were detected in the heart tissues of B356-infected mice compared to BL206-infected mice. Histopathologically, severe arthritis and aortitis were noted only in mice infected with isolate BL206. Spirochete dissemination and disease severity vary significantly in mice infected with distinct genotypes of B. burgdorferi, suggesting that genotypic differences in the infecting spirochetes play a key role in the pathogenesis and development of clinical disease.

Gamma interferon is not required for arthritis resistance in the murine Lyme disease model

Lyme arthritis is the most common complication following infection of human individuals with Borrelia burgdorferi sensu stricto. In mice, B. burgdorferi infection leads to arthritis of the tibiotarsal joints. Arthritis severity in mice is under host genetic control, as BALB/c mice developed mild arthritis but C3H/He mice developed severe disease following B. burgdorferi infection. To study the role of gamma interferon (IFN-gamma) in arthritogenesis, targeted mutant mice lacking the IFN-gamma receptor (IFN-gammaR) were infected by inoculation with B. burgdorferi. IFN-gammaR(-/-) and parental 129/SvEv mice developed mild arthritis of similar severity, as determined both by weekly tibiotarsal joint measurements and histopathology at 2 and 5 weeks postinfection. Both strains of mice had the same spirochetal burden in the joints, suggesting that the IFN-gammaR(-/-) mice were not impaired in controlling spirochetal expansion in vivo. The wild-type mice mounted a Th1 response, with a predominance of CD4(+) IFN-gamma(+) T cells observed by flow cytometry. In contrast, the IFN-gammaR(-/-) mice mounted a Th2 response, with a predominance of CD4(+) IL-4(+) T cells. As expected given their cytokine profile, the IFN-gammaR(-/-) mice produced fewer CD8(+) IFN-gamma(+) and MAC-1(+) IL-12(+) cells and less immunoglobulin G2a (IgG2a) than their wild-type counterparts. These results strongly suggest that IFN-gamma is not required for arthritis resistance or as part of an effective immune response against B. burgdorferi.

Host-pathogen interactions promoting inflammatory Lyme arthritis: use of mouse models for dissection of disease processes

Recent studies have confirmed the infectious and inflammatory nature of arthritis induced by Borrelia burgdorferi, or Lyme arthritis. This arthritis is directed by the presence of the bacteria in joint tissue, and is mediated through activation of the Toll-like receptor 2 (TLR2) signaling pathways by borrelial lipoproteins. Several host genes regulate the severity of arthritis, possibly by regulating the balance of pro- and anti-inflammatory responses.

Resistance to Lyme disease in decorin-deficient mice

Microbial adhesion to the host tissue represents an early, critical step in the pathogenesis of most infectious diseases. BORRELIA: burgdorferi, the causative agent of Lyme disease (LD), expresses two surface-exposed decorin-binding adhesins, DbpA and DbpB. A decorin-deficient (Dcn(-/-)) mouse was recently developed and found to have a relatively mild phenotype. We have now examined the process of experimental LD in Dcn(-/-) mice using both needle inoculation and tick transmission of spirochetes. When exposed to low doses of the infective agent, Dcn(-/-) mice had fewer Borrelia-positive cultures from most tissues analyzed than did Dcn(+/+) or Dcn(+/-) mice. When the infection dose was increased, similar differences were not observed in most tissues but were seen in bacterial colonization of joints and the extent of Borreila-induced arthritis. Quantitative PCR demonstrated that joints harvested from Dcn(-/-) mice had diminished Borrelia numbers compared with issues harvested from Dcn(+/+) controls. Histological examination also revealed a low incidence and severity of arthritis in Dcn(-/-) mice. Conversely, no differences in the numbers of Borreila-positive skin cultures were observed among the different genotypes regardless of the infection dose. These differences, which were observed regardless of genetic background of the mice (BALB/c or C3H/HeN) or method of infection, demonstrate the importance of decorin in the pathogenesis of LD.

Modulation of cytokine release in ex vivo-stimulated blood from borreliosis patients

In lipopolysaccharide-stimulated blood from 71 late-stage borreliosis patients, the ex vivo cytokine release capacity of tumor necrosis factor alpha (TNF-alpha) and gamma interferon (IFN-gamma) was reduced to 28% +/- 5% and to 31% +/- 5% (P < or = 0.001), respectively, compared to that of 24 healthy controls. White blood cell counts were normal in both groups. To investigate direct interactions between the pathogen and the immune cells, blood from healthy controls was exposed in vitro to live or heat-killed Borrelia or to Borrelia lysate. Compared to the pattern induced by bacterial endotoxins, a reduced release of TNF-alpha and IFN-gamma and an enhanced secretion of interleukin-10 and granulocyte colony-stimulating factor was found. In blood from 10 borreliosis patients stimulated with Borrelia lysate, TNF-alpha formation was decreased to 31% +/- 14% and IFN-gamma formation was decreased to 8% +/- 3% (P < or = 0.001) compared to the cytokine response of blood from healthy controls (n = 24). We propose to consider anti-inflammatory changes in the blood cytokine response capacity elicited by Borrelia as a condition that might favor the persistence of the spirochete.

Interleukin-4 (IL-4) and IL-13 signaling pathways do not regulate Borrelia burgdorferi-induced arthritis in mice: IgG1 is not required for host control of tissue spirochetes

Previous studies have suggested that interleukin-4 (IL-4) has a protective effect in host defense to Borrelia burgdorferi infection, both in limiting the severity of arthritis and in controlling spirochete numbers in tissues, and a mapping study revealed suggestive linkage to a cluster of genes on mouse chromosome 11, including the genes for IL-4 and IL-13. In contrast, other studies have questioned the importance of IL-4. In this study the involvement of IL-4 in murine Lyme disease was examined in C57BL/6J and BALB/cJ mice with targeted disruptions in the IL-4 gene, the IL-4Ralpha chain gene, or both. A spectrum of arthritis severity was seen in BALB/cJ mice, and ablation of IL-4, IL-4Ralpha, or both had no effect on the overall severity of arthritis as determined by joint swelling and histopathology. Wild-type C57BL/6J mice exhibited mild to moderate arthritis, and ablation of IL-4 again had no effect on arthritis severity. IL-4- and IL-4Ralpha-deficient mice produced extremely low levels of immunoglobulin G1 (IgG1) and showed increased production of IgG2b. This shift in immunoglobulin isotype had no effect on the host's ability to control spirochete growth in either strain of mouse, as determined by PCR detection of B. burgdorferi DNA from heart and ankle tissues. In summary, the IL-4-IL-4Ralpha pathway, including IL-13 signaling, neither limits arthritis severity nor is required for control of spirochete growth during B. burgdorferi infection of mice. Furthermore, the IgG1 isotype is not required to control B. burgdorferi cell numbers in tissues. These findings suggest the host defense against B. burgdorferi infection is not dependent on the Th1-Th2 paradigm of T-cell responses.

Abnormal Stat activation, hematopoietic homeostasis, and innate immunity in c-fes-/- mice

The c-fes protooncogene encodes a nonreceptor tyrosine kinase (Fes) implicated in cytokine receptor signal transduction, neutrophil survival, and myeloid differentiation. To determine the role of Fes in embryonic development and hematopoiesis, we engineered a null mutation of the murine c-fes locus. c-fes-/- mice are viable but not born in the expected Mendelian ratios. Live born c-fes-/- mice exhibit lymphoid/myeloid homeostasis defects, compromised innate immunity, and increased Stat activation in response to GM-CSF and IL-6 signaling. Therefore, increased cytokine responsiveness in the absence of Fes leads to abnormal myeloid proliferation and functional defects in the macrophage lineage.

Bone-marrow chimeras reveal hemopoietic and nonhemopoietic control of resistance to experimental Lyme arthritis

Both genetic resistance and susceptibility to development of experimental Lyme arthritis are mediated by the innate immune response. To determine whether this process is mainly controlled by hemopoietic or nonhemopoietic cells, we created bone marrow (BM) chimeric mice between arthritis-resistant DBA/2J (DBA) and arthritis-susceptible C3H/HeJ (C3H) mice and infected them with Borrelia burgdorferi. Both sets of BM chimeric mice, C3H donors into DBA recipients (C-->D) and DBA donors into C3H recipients (D-->C), as well as DBA sham chimeric mice (D-->D) were resistant to the development of experimental Lyme arthritis as measured by ankle swelling and arthritis severity scores. Only the C3H sham chimeric mice (C-->C) developed severe arthritis. These results indicate that independent and nonoverlapping mechanisms exist in hemopoietic and nonhemopoietic cellular compartments that can provide protection against arthritic pathology.

Lyme disease

Lyme disease (LD) is the most common tick-borne disease in the US. The overall trend has been an average annual increase in cases since surveillance was initiated by the Centers for Disease Control and Prevention in 1982. To date, 10 different Borrelia species have been described within the Borrelia burgdorferi sensu lato complex, although only Borrelia burgdorferi sensu strico, Borrelia garinii, and Borrelia afzelii have been associated with human disease. Ixodes ticks often carry more than one potential pathogen, and co-infection with B. burgdorferi and other organisms have been reported. Recent findings suggested that maintenance cycles of other tick-borne pathogens may be different than those for B. burgdorferi. A better understanding of the pathogenesis of Lyme arthritis has provided clues about the mechanisms responsible for variation in clinical expression of the disease. Results of therapeutic trials in Lyme neuroborreliosis are likely to have an impact upon treatment recommendations. A long term follow-up study of children treated for LD indicated that the prognosis is excellent in most cases. A safe vaccine for the prevention of LD has been approved in adults. Preliminary data suggested that the vaccine is safe and immunogenic in children.