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.Infection of humans with the Lyme disease spirochete, Borrelia burgdorferi, results in a characteristic pattern of skin lesions, arthritis, carditis, and neurologic abnormalities that can resolve over time despite the incomplete eradication of the pathogen (12). In the murine model of Lyme borreliosis, spirochetes inoculated into the skin disseminate within days to infect all organ systems, but disease is primarily manifested in the joints and heart (8). The severity of inflammation peaks at these sites about 2 to 4 weeks after infection and then regresses in the presence of B. burgdorferi-specific adaptive immune responses. For this animal model, disease is believed to reflect the innate immune response to spirochetes because histopathology reveals mainly neutrophils and macrophages within inflamed joints and hearts, respectively, and arises in the absence of adaptive immunity (11,26,33,35). B. burgdorferi infections of severe combined immunodeficiency (SCID) and rag-deficient mice, which lack both T and B cells (11,26,33,35

Borrelia burgdorferi strains exhibit various degrees of infectivity and pathogenicity in mammals, which may be due to their relative ability to evade initial host immunity. Innate immune cells recognize B. burgdorferi by Toll-like receptors (TLRs) that use the intracellular molecule MyD88 to mediate effector functions. To determine whether impaired TLR signaling enhances Ixodes scapularis acquisition of B. burgdorferi, we fed nymphs on wild-type (WT) and MyD88 ؊/؊ mice previously infected with two clinical isolates of B. burgdorferi, BL206, a high-virulence strain, and B348, an attenuated strain. Seventy-three percent of the nymphs that fed on BL206-infected WT mice and 40% of the nymphs that fed on B348-infected WT mice acquired B. burgdorferi, whereas 100% of the nymphs that fed on MyD88 ؊/؊ mice became infected, irrespective of B. burgdorferi strain. Ticks that acquired infection after feeding on MyD88؊/؊ mice harbored more spirochetes than those that fed on WT mice, as assessed by quantitative PCR for B. burgdorferi DNA. Vector transmission of BL206 and B348 was also enhanced when MyD88 ؊/؊ mice were the blood meal hosts, with the mean pathogen burden at the skin inoculation site significantly higher than levels in WT mice. These results show that the absence of MyD88 facilitates passage of both low-and high-infectivity B. burgdorferi strains between the tick vector and the mammal and enhances the infectivity of a low-infectivity B. burgdorferi strain.Lyme disease due to infection with the spirochete Borrelia burgdorferi is the most common vector-borne disease in the United States (4, 19). The infection can be acquired when humans serve as incidental blood meal hosts for Ixodes scapularis or Ixodes pacificus ticks, the vectors for B. burgdorferi, and manifests first with localized skin infection and a characteristic skin rash called erythema migrans (EM) in 70 to 80% of cases. In untreated infection, spirochetes can disseminate widely to infect any organ system, with disease primarily found in the skin, heart, joints, and nervous system.The severity of Lyme disease is related to infection duration before treatment (19), host genetic factors (2,20), and the infectivity and pathogenicity of B. burgdorferi strains (23,24,28). Recently, restriction fragment length polymorphism analysis of the 16S-23S rRNA spacer has allowed the classification of B. burgdorferi sensu stricto isolated from Lyme disease patients into three rRNA gene spacer restriction fragment length polymorphism genotypes (RSTs) that correlate with disease severity (11-13). RST1 isolates are more likely to be found in patients with signs of disseminated infection, including multiple EM lesions and positive blood cultures, whereas RST3 isolates are associated with localized disease (28). In the mouse model of Lyme borreliosis, RST1 isolates exhibit earlier dissemination, achieve higher pathogen loads, and cause more severe disease compared to similar infection with RST3 isolates (23, 24). RST3 isolates can be further subdivided into RST3A and RST3B...

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 1␤ (IL-1␤) 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 apoptosisassociated 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.

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 (FcεRγ−/− 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 FcεRγ−/−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.