The versatile roles of antibodies in Borrelia infections

Adaptive immunity in Mus musculus influences the acquisition and abundance of Borrelia burgdorferi in Ixodes scapularis ticks

The Lyme disease spirochete Borrelia burgdorferi cycles between immature black-legged ticks (Ixodes scapularis) and vertebrate reservoir hosts, such as rodents. Larval ticks acquire spirochetes from infected hosts, and the resultant nymphs transmit the spirochetes to naïve hosts. This study investigated the impact of immunocompetence and host tissue spirochete load on host-to-tick transmission (HTT) of B. burgdorferi and the spirochete load inside immature I. scapularis ticks. Wild-type (WT) C57BL/6J mice and mice with severe combined immunodeficiency (SCID) were experimentally infected with B. burgdorferi. To measure HTT, WT and SCID mice were repeatedly infested with I. scapularis larvae, and ticks were sacrificed at three different developmental stages: engorged larvae, 1-month-old, and 12-month-old nymphs. The spirochete loads in immature ticks and mouse tissues were estimated using qPCR. In WT mice, HTT decreased from 90% to 65% over the course of the infection, whereas in the SCID mice, HTT was always 100%. Larvae that fed on SCID mice acquired a much larger dose of spirochetes compared to larvae that fed on WT mice. This difference in spirochete load persisted over tick development where nymphs that fed as larvae on SCID mice had significantly higher spirochete loads compared to their WT counterparts. HTT and the tick spirochete loads were strongly correlated with the mouse tissue spirochete loads. Our study shows that the host immune system (e.g., the presence of antibodies) influences HTT of B. burgdorferi and the spirochete load in immature I. scapularis ticks.IMPORTANCEThe tick-borne spirochete Borrelia burgdorferi causes Lyme disease in humans. This pathogen is maintained in nature by cycles involving black-legged ticks and wildlife hosts. The present study investigated the host factors that influence the transmission of B. burgdorferi from infected hosts to feeding ticks. We infected immunocompetent mice and immunocompromised mice (that cannot develop antibodies) with B. burgdorferi and repeatedly infested these mice with ticks. We determined the percentage of infected ticks and their spirochete loads. This percentage was 100% for immunocompromised mice but decreased from 90% to 65% over time (8 weeks) for immunocompetent mice. The tick spirochete load was much higher in ticks fed on immunocompromised mice compared to ticks fed on immunocompetent mice. In summary, the host immune system influences the transmission of B. burgdorferi from infected hosts to ticks and the spirochete loads in those ticks, which, in turn, determines the risk of Lyme disease for people.

Human B Cell Epitope Map of the Lyme Disease Vaccine Antigen, OspA

The Lyme disease (LD) vaccine formerly approved for use in the United States consisted of recombinant outer surface protein A (OspA) from Borrelia burgdorferi sensu stricto (ss), the bacterial genospecies responsible for the vast majority of LD in North America. OspA is an ∼30 kDa lipoprotein made up of 21 antiparallel β-strands and a C-terminal α-helix. In clinical trials, protection against LD following vaccination correlated with serum antibody titers against a single epitope near the C-terminus of OspA, as defined by the mouse monoclonal antibody (MAb), LA-2. However, the breadth of the human antibody response to OspA following vaccination remains undefined even as next-generation multivalent OspA-based vaccines are under development. In this report, we employed hydrogen exchange-mass spectrometry (HX-MS) to localize the epitopes recognized by a unique panel of OspA human MAbs, including four shown to passively protect mice against experimental B. burgdorferi infection and one isolated from a patient with antibiotic refractory Lyme arthritis. The epitopes grouped into three spatially distinct bins that, together, encompass more than half the surface-exposed area of OspA. The bins corresponded to OspA β-strands 8-10 (bin 1), 11-13 (bin 2), and 16-20 plus the C-terminal α-helix (bin 3). Bin 3 was further divided into sub-bins relative to LA-2's epitope. MAbs with complement-dependent borreliacidal activity, as well as B. burgdorferi transmission-blocking activity in the mouse model were found within each bin. Therefore, the resulting B cell epitope map encompasses functionally important targets on OspA that likely contribute to immunity to B. burgdorferi.

Host association of Borrelia burgdorferi sensu lato: A review

Borrelia burgdorferi sensu lato (Bbsl) is a bacterial species complex that includes the etiological agents of the most frequently reported vector-borne disease in the Northern hemisphere, Lyme borreliosis. It currently comprises > 20 named and proposed genospecies that use vertebrate hosts and tick vectors for transmission in the Americas and Eurasia. Host (and vector) associations influence geographic distribution and speciation in Bbsl, which is of particular relevance to human health. To target gaps in knowledge for future efforts to understand broad patterns of the Bbsl-tick-host system and how they relate to human health, the present review aims to give a comprehensive summary of the literature on host association in Bbsl. Of 465 papers consulted (404 after exclusion criteria were applied), 96 sought to experimentally establish reservoir competence of 143 vertebrate host species for Bbsl. We recognize xenodiagnosis as the strongest method used, however it is infrequent (20% of studies) probably due to difficulties in maintaining tick vectors and/or wild host species in the lab. Some well-established associations were not experimentally confirmed according to our definition (ex: Borrelia garinii, Ixodes uriae and sea birds). We conclude that our current knowledge on host association in Bbsl is mostly derived from a subset of host, vector and bacterial species involved, providing an incomplete knowledge of the physiology, ecology and evolutionary history of these interactions. More studies are needed on all host, vector and bacterial species globally involved with a focus on non-rodent hosts and Asian Bbsl complex species, especially with experimental research that uses xenodiagnosis and genomics to analyze existing host associations in different ecosystems.

Investigating BB0405 as a novel Borrelia afzelii vaccination candidate in Lyme borreliosis

BB0405 is a surface exposed Borrelia burgdorferi protein and its vaccination protected mice against B. burgdorferi infection. As BB0405 is highly conserved across different B. burgdorferi sensu lato species, we investigated whether vaccination with recombinant BB0405 or through intradermal bb0405 DNA tattoo vaccination could provide protection against different Borrelia species, specifically against Borrelia afzelii, the predominant B. burgdorferi sensu lato genospecies causing Lyme borreliosis across Eurasia. We immunized C3H/HeN mice with recombinant BB0405 or with a codon-optimized bb0405 DNA vaccine using the pVAC plasmid and immunized corresponding control groups mice with only adjuvant or empty vectors. We subsequently subjected these immunized mice to a tick challenge with B. afzelii CB43-infected Ixodes ricinus nymphs. Upon vaccination, recombinant BB0405 induced a high total IgG response, but bb0405 DNA vaccination did not elicit antibody responses. Both vaccine formulations did not provide protection against Borrelia afzelii strain CB43 after tick challenge. In an attempt to understand the lack of protection of the recombinant vaccine, we determined expression of BB0405 and showed that B. afzelii CB43 spirochetes significantly and drastically downregulate the expression of BB0405 protein at 37 °C compared to 33 °C, where as in B. burgdorferi B31 spirochetes expression levels remain unaltered. Vaccination with recombinant BB0405 was previously shown to protect against B. burgdorferi sensu stricto. Here we show that vaccination with either recombinant BB0405 (or non-immunogenic bb0405 DNA), despite being highly conserved among B. burgdorferi sl genospecies, does not provide cross-protection against B. afzelii, mostly likely due to downregulation of this protein in B. afzelii in the mammalian host.

Comparison of spleen transcriptomes of two wild rodent species reveals differences in the immune response against Borrelia afzelii

Different host species often differ considerably in susceptibility to a given pathogen, but the causes of such differences are rarely known. The natural hosts of the tick-transmitted bacterium Borrelia afzelii, which is one of causative agents of Lyme borreliosis in humans, include a variety of small mammals like voles and mice. Previous studies have shown that B. afzelii-infected bank voles (Myodes glareolus) have about ten times higher bacterial load than infected yellow-necked mice (Apodemus flavicollis), indicating that these two species differ in resistance. In this study, we compared the immune response to B. afzelii infection in these host species by using RNA sequencing to quantify gene expression in spleen. Gene set enrichment analysis (GSEA) showed that several immune pathways were down-regulated in infected animals in both bank voles and yellow-necked mice. Moreover, IFNα response was up-regulated in B. afzelii-infected yellow-necked mice, while IL6 signaling and the complement pathway were down-regulated in infected bank voles; differences in regulation of these three pathways between bank voles and yellow-necked mice could thus contribute to the difference in resistance to B. afzelii between the species. This study provides knowledge of gene expression induced by a zoonotic pathogen in its natural host, and possible species-specific regulation of immune responses associated with resistance.

Analysis of host cell binding specificity mediated by the Tp0136 adhesin of the syphilis agent Treponema pallidum subsp. pallidum

Background

Syphilis affects approximately 11 million people each year globally, and is the third most prevalent sexually transmitted bacterial infection in the United States. Inability to independently culture and genetically manipulate Treponema pallidum subsp. pallidum, the causative agent of this disease, has hindered our understanding of the molecular mechanisms of syphilis pathogenesis. Here, we used the non-infectious and poorly adherent B314 strain of the Lyme disease-causing spirochete, Borrelia burgdorferi, to express two variants of a known fibronectin-binding adhesin, Tp0136, from T. pallidum SS14 and Nichols strains. Using this surrogate system, we investigated the ability of Tp0136 in facilitating differential binding to mammalian cell lines offering insight into the possible role of this virulence factor in colonization of specific tissues by T. pallidum during infection.

Principal findings

Expression of Tp0136 could be detected on the surface of B. burgdorferi by indirect immunofluorescence assay using sera from a secondary syphilis patient that does not react with intact B314 spirochetes transformed with the empty vector. Increase in Tp0136-mediated adherence of B314 strain to human epithelial HEK293 cells was observed with comparable levels of binding exhibited by both Tp0136 alleles. Adherence of Tp0136-expressing B314 was highest to epithelial HEK293 and C6 glioma cells. Gain in binding of B314 strain expressing Tp0136 to purified fibronectin and poor binding of these spirochetes to the fibronectin-deficient cell line (HEp-2) indicated that Tp0136 interaction with this host receptor plays an important role in spirochetal attachment to mammalian cells. Furthermore, preincubation of these cell lines with fibronectin-binding peptide from Staphylococcus aureus FnbA-2 protein significantly inhibited binding of B314 expressing Tp0136.

Conclusions

Our results show that Tp0136 facilitates differential level of binding to cell lines representing various host tissues, which highlights the importance of this protein in colonization of human organs by T. pallidum and resulting syphilis pathogenesis.

Syphilis is one of the most prevalent sexually transmitted infections that affect millions of people around the world. The causative bacterium, Treponema pallidum subsp. pallidum, can be transmitted from mother to fetus during maternal infection, resulting in adverse pregnancy outcomes. Although timely treatment of syphilis is highly effective, untreated infection causes late syphilis that affects virtually every organ and leads to serious clinical manifestations. Therefore, syphilis remains a serious healthcare problem. T. pallidum cannot be grown in laboratory using traditional methods, which has slowed the progress in understanding this pathogen biology and pathogenesis. We employed a novel approach of using a related bacterium, Borrelia burgdorferi, to express Tp0136 protein from two different T. pallidum isolates to study the function of this protein. This strategy enabled us to demonstrate the ability of this protein to bind to fibronectin and laminin receptors present on the surface of various host cells. We showed that Tp0136 facilitates binding to only those host cells that produce fibronectin. In addition, we found that Tp0136-mediated binding is not equivalent in all host cell types, suggesting that the protein could help in colonization of specific human organs and tissues during infection by T. pallidum.

Age-Related Differential Stimulation of Immune Response by Babesia microti and Borrelia burgdorferi During Acute Phase of Infection Affects Disease Severity

Lyme disease is the most prominent tick-borne disease with 300,000 cases estimated by CDC every year while ~2,000 cases of babesiosis occur per year in the United States. Simultaneous infection with Babesia microti and Borrelia burgdorferi are now the most common tick-transmitted coinfections in the U.S.A., and they are a serious health problem because coinfected patients show more intense and persisting disease symptoms. B. burgdorferi is an extracellular spirochete responsible for systemic Lyme disease while B. microti is a protozoan that infects erythrocytes and causes babesiosis. Immune status and spleen health are important for resolution of babesiosis, which is more severe and even fatal in the elderly and splenectomized patients. Therefore, we investigated the effect of each pathogen on host immune response and consequently on severity of disease manifestations in both young, and 30 weeks old C3H mice. At the acute stage of infection, Th1 polarization in young mice spleen was associated with increased IFN-γ and TNF-α producing T cells and a high Tregs/Th17 ratio. Together, these changes could help in the resolution of both infections in young mice and also prevent fatality by B. microti infection as observed with WA-1 strain of Babesia. In older mature mice, Th2 polarization at acute phase of B. burgdorferi infection could play a more effective role in preventing Lyme disease symptoms. As a result, enhanced B. burgdorferi survival and increased tissue colonization results in severe Lyme arthritis only in young coinfected mice. At 3 weeks post-infection, diminished pathogen-specific antibody production in coinfected young, but not older mice, as compared to mice infected with each pathogen individually may also contribute to increased inflammation observed due to B. burgdorferi infection, thus causing persistent Lyme disease observed in coinfected mice and reported in patients. Thus, higher combined proinflammatory response to B. burgdorferi due to Th1 and Th17 cells likely reduced B. microti parasitemia significantly only in young mice later in infection, while the presence of B. microti reduced humoral immunity later in infection and enhanced tissue colonization by Lyme spirochetes in these mice even at the acute stage, thereby increasing inflammatory arthritis.

Comparison of the lifetime host-to-tick transmission between two strains of the Lyme disease pathogen Borrelia afzelii

Background

Transmission from the vertebrate host to the arthropod vector is a critical step in the life-cycle of any vector-borne pathogen. How the probability of host-to-vector transmission changes over the duration of the infection is an important predictor of pathogen fitness. The Lyme disease pathogen Borrelia afzelii is transmitted by Ixodes ricinus ticks and establishes a chronic infection inside rodent reservoir hosts. The present study compares the temporal pattern of host-to-tick transmission between two strains of B. afzelii.

Methods

Laboratory mice were experimentally infected via tick bite with one of two strains of B. afzelii: A3 and A10. Mice were repeatedly infested with pathogen-free larval Ixodes ricinus ticks over a period of 4 months. Engorged larval ticks moulted into nymphal ticks that were tested for infection with B. afzelii using qPCR. The proportion of infected nymphs was used to characterize the pattern of host-to-tick transmission over time.

Results

Both strains of B. afzelii followed a similar pattern of host-to-tick transmission. Transmission decreased from the acute to the chronic phase of the infection by 16.1 and 29.3% for strains A3 and A10, respectively. Comparison between strains found no evidence of a trade-off in transmission between the acute and chronic phase of infection. Strain A10 had higher lifetime fitness and established a consistently higher spirochete load in nymphal ticks than strain A3.

Conclusion

Quantifying the relationship between host-to-vector transmission and the age of infection in the host is critical for estimating the lifetime fitness of vector-borne pathogens.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1929-z) contains supplementary material, which is available to authorized users.

CD40 Ligand Deficient C57BL/6 Mouse Is a Potential Surrogate Model of Human X-Linked Hyper IgM (X-HIGM) Syndrome for Characterizing Immune Responses against Pathogens

Individuals with X-HIGM syndrome fail to express functional CD40 ligand; consequently they cannot mount effective protective antibody responses against pathogenic bacteria. We evaluated, compared, and characterized the humoral immune response of wild type (WT) and C57-CD40L deficient (C57-CD40L^−/−) mice infected with Citrobacter rodentium. Basal serum isotype levels were similar for IgM and IgG3 among mice, while total IgG and IgG2b concentrations were significantly lower in C57-CD40L^−/− mice compared with WT. Essentially IgG1 and IgG2c levels were detectable only in WT mice. C57-CD40L^−/− animals, orally inoculated with 2 × 10⁹ CFU, presented several clinical manifestations since the second week of infection and eventually died. In contrast at this time point no clinical manifestations were observed among C57-CD40L^−/− mice infected with 1 × 10⁷ CFU. Infection was subclinical in WT mice inoculated with either bacterial dose. The serum samples from infected mice (1 × 10⁷ CFU), collected at day 14 after infection, had similar C. rodentium-specific IgM titres. Although C57-CD40L^−/− animals had lower IgG and IgG2b titres than WT mice, C57-CD40L^−/− mice sera displayed complement-mediated bactericidal activity against C. rodentium. C. rodentium-infected C57-CD40L^−/− mice are capable of producing antibodies that are protective. C57-CD40L^−/− mouse is a useful surrogate model of X-HIGM syndrome for studying immune responses elicited against pathogens.

Hypercholesterolemia and ApoE deficiency result in severe infection with Lyme disease and relapsing-fever Borrelia

The Lyme disease (Borrelia burgdorferi) and relapsing-fever (Borrelia hispanica) agents have distinct infection courses, but both require cholesterol for growth. They acquire cholesterol from the environment and process it to form cholesterol glycolipids that are incorporated onto their membranes. To determine whether higher levels of serum cholesterol could enhance the organ burdens of B. burgdorferi and the spirochetemia of B. hispanica in laboratory mice, apolipoprotein E (apoE)-deficient and low-density lipoprotein receptor (LDLR)-deficient mice that produce large amounts of serum cholesterol were infected with both spirochetes. Both apoE- and LDLR-deficient mice infected with B. burgdorferi had an increased number of spirochetes in the joints and inflamed ankles compared with the infected wild-type (WT) mice, suggesting that mutations in cholesterol transport that result in high serum cholesterol levels can affect the pathogenicity of B. burgdorferi. In contrast, elevated serum cholesterol did not lead to an increase in the spirochetemia of B. hispanica. In the LDLR-deficient mice, the course of infection was indistinguishable from the WT mice. However, infection of apoE-deficient mice with B. hispanica resulted in a longer spirochetemia and increased mortality. Together, these results argue for the apoE deficiency, and not hypercholesterolemia, as the cause for the increased severity with B. hispanica. Serum hyperlipidemias are common human diseases that could be a risk factor for increased severity in Lyme disease.

Selective association of outer surface lipoproteins with the lipid rafts of Borrelia burgdorferi

Borrelia burgdorferi contains unique cholesterol-glycolipid-rich lipid rafts that are associated with lipoproteins. These complexes suggest the existence of macromolecular structures that have not been reported for prokaryotes. Outer surface lipoproteins OspA, OspB, and OspC were studied for their participation in the formation of lipid rafts. Single-gene deletion mutants with deletions of ∆ospA, ∆ospB, and ∆ospC and a spontaneous gene mutant, strain B313, which does not express OspA and OspB, were used to establish their structural roles in the lipid rafts. All mutant strains used in this study produced detergent-resistant membranes, a common characteristic of lipid rafts, and had similar lipid and protein slot blot profiles. Lipoproteins OspA and OspB but not OspC were shown to be associated with lipid rafts by transmission electron microscopy. When the ability to form lipid rafts in live B. burgdorferi spirochetes was measured by fluorescence resonance energy transfer (FRET), strain B313 showed a statistically significant lower level of segregation into ordered and disordered membrane domains than did the wild-type and the other single-deletion mutants. The transformation of a B313 strain with a shuttle plasmid containing ospA restored the phenotype shared by the wild type and the single-deletion mutants, demonstrating that OspA and OspB have redundant functions. In contrast, a transformed B313 overexpressing OspC neither rescued the FRET nor colocalized with the lipid rafts. Because these lipoproteins are expressed at different stages of the life cycle of B. burgdorferi, their selective association is likely to have an important role in the structure of prokaryotic lipid rafts and in the organism’s adaptation to changing environments.

Lipid rafts are cholesterol-rich clusters within the membranes of cells. Lipid rafts contain proteins that have functions in sensing the cell environment and transmitting signals. Although selective proteins are present in lipid rafts, little is known about their structural contribution to these domains. Borrelia burgdorferi, the agent of Lyme disease, has lipid rafts, which are novel structures in bacteria. Here, we have shown that the raft-associated lipoproteins OspA and OspB selectively contribute to lipid rafts. A similar but non-raft-associated lipoprotein, OspC, cannot substitute for the role of OspA and OspB. In this study, we have demonstrated that lipoprotein association with lipid rafts is selective, further suggesting a functional adaptation to different stages of the spirochete life cycle. The results of this study are of broader importance and can serve as a model for other bacteria that also possess cholesterol in their membranes and, therefore, may share lipid raft traits with Borrelia.

Efficient B cell responses to Borrelia hermsii infection depend on BAFF and BAFFR but not TACI

T cell-independent antibody responses develop rapidly, within 3 to 4 days, and are critical for preventing blood-borne pathogens from evolving into life-threatening infections. The interaction of BAFF, also known as BLyS, with its receptors BAFFR and TACI on B cells is critical for B cell homeostasis and function. Using a synthetic polysaccharide antigen, it has previously been shown that TACI is critical for T cell-independent antibody responses. To examine the role of BAFFR and TACI in T cell-independent antibody responses to an active infection, we utilized the Borrelia hermsii infection system. In this infection system, T cell-independent responses mediated by the B1b cell subset are critical for controlling bacteremia. We found that B1b cells express BAFFR and TACI and that the surface expression of both receptors is upregulated on B1b cells following exposure to whole B. hermsii cells. Surprisingly, we found that TACI(-/-) mice are not impaired either in specific antibody responses to B. hermsii or in controlling B. hermsii bacteremia. In contrast, TACI-deficient mice immunized with heat-killed type 3 serotype pneumococcus cells are impaired in generating pneumococcal polysaccharide-specific responses and succumb to challenge with live type 3 serotype pneumococcus, indicating that TACI is required for T cell-independent antibody responses to bacterial-associated polysaccharides. Although we have found that TACI is dispensable for controlling B. hermsii infection, mice deficient in BAFFR or BAFF exhibit impairment in B. hermsii-specific IgM responses and clearance of bacteremia. Collectively, these data indicate a disparity in the roles for TACI and BAFFR in primary T cell-independent antibody responses to bacterial pathogens.

Human immune system mice: current potential and limitations for translational research on human antibody responses

It has recently become possible to generate chimeric mice durably engrafted with many components of the human immune system (HIS mice). We have characterized the maturation and function of the B cell compartment of HIS mice. The antibody response of HIS mice to T cell-dependent B cell antigens is limited, and contributing factors may be the general immaturity of the B cell compartment, infrequent helper T cells selected on human MHC class II antigens, and incomplete reconstitution of secondary lymphoid organs and their microenvironments. In contrast, HIS mice generate protective antibody responses to the bacterium Borrelia hermsii, which acts as a T cell-independent antigen in mice, but do not respond to purified polysaccharide antigens (PPS). We speculate that the anti-B. hermsii response of HIS mice is derived from an abundant B cell subset that may be analogous to B1 B cells in mice. We suggest that failure of HIS mice to respond to PPS is due to the lack of a B cell subset that may originate from adult bone marrow and is highly dependent on human interleukin-7 for development.

Of ticks, mice and men: understanding the dual-host lifestyle of Lyme disease spirochaetes

In little more than 30 years, Lyme disease, which is caused by the spirochaete Borrelia burgdorferi, has risen from relative obscurity to become a global public health problem and a prototype of an emerging infection. During this period, there has been an extraordinary accumulation of knowledge on the phylogenetic diversity, molecular biology, genetics and host interactions of B. burgdorferi. In this Review, we integrate this large body of information into a cohesive picture of the molecular and cellular events that transpire as Lyme disease spirochaetes transit between their arthropod and vertebrate hosts during the enzootic cycle.

Characteristics of Borrelia hermsii infection in human hematopoietic stem cell-engrafted mice mirror those of human relapsing fever

Rodents are natural reservoirs for a variety of species of Borrelia that cause relapsing fever (RF) in humans. The murine model of this disease recapitulates many of the clinical manifestations of the human disease and has revealed that T cell-independent antibody responses are required to resolve the bacteremic episodes. However, it is not clear whether such protective humoral responses are mounted in humans. We examined Borrelia hermsii infection in human hematopoietic stem cell-engrafted nonobese diabetic/SCID/IL-2Rγ(null) mice: "human immune system mice" (HISmice). Infection of these mice, which are severely deficient in lymphoid and myeloid compartments, with B. hermsii resulted in persistent bacteremia. In contrast, this infection in HISmice resulted in recurrent episodes of bacteremia, the hallmark of RF. The resolution of the primary episode of bacteremia was concurrent with the generation of B. hermsii-specific human IgM. Remarkably, HISmice generated antibody responses to the B. hermsii outer-membrane protein Factor H binding protein A. Sera from humans infected by B. hermsii have a similar reactivity, and studies in mice have shown that this response is generated by the B1b cell subset. HISmice contain several B-cell subsets, including those with the phenotype CD20(+)CD27(+)CD43(+)CD70(-), a proposed human equivalent of mouse B1 cells. Reduction of B cells by administration of anti-human CD20 antibody resulted in diminished anti-B. hermsii responses and persistent bacteremia in HISmice. These data indicate that analysis of B. hermsii infection in HISmice will serve as a model in which to study the cellular and molecular mechanisms involved in controlling human RF.

B1b lymphocyte-derived antibodies control Borrelia hermsii independent of Fcα/μ receptor and in the absence of host cell contact

The critical role of IgM in controlling pathogen burden has been demonstrated in a variety of infection models. In the murine model of Borrelia hermsii infection, IgM is necessary and sufficient for the rapid clearance of bacteremia. Convalescent, but not naïve, B1b cells generate a specific IgM response against B. hermsii, but the mechanism of IgM-mediated protection is unknown. Here, we show that neither Fcα/μR, a high-affinity receptor for IgM, nor IgM-dependent complement activation is required for controlling B. hermsii. Bacteria in diffusion chambers with a pore size impermeable to cells were killed when diffusion chambers were implanted into either convalescent or passively immunized mice. Furthermore, adoptively transferred convalescent B1b cells in Rag1(-/-) mice produced specific IgM that also cleared B. hermsii in diffusion chambers independent of complement. These results demonstrate that IgM-mediated clearance of B. hermsii does not require opsonophagocytosis and indicate that a mechanism for in vivo B1b cell-mediated protection is through the generation of bactericidal IgM.

ELISA-based measurement of antibody responses and PCR-based detection profiles can distinguish between active infection and early clearance of Borrelia burgdorferi

Borrelia burgdorferi is a spirochetal bacterium that causes Lyme disease. These studies address whether current research methods using either ELISA to detect seroconversion to B. burgdorferi antigens or PCR quantification of bacterial DNA within tissues can accurately distinguish between a productive infection versus a B. burgdorferi exposure that is rapidly cleared by the innate responses. Mice receiving even minimal doses of live B. burgdorferi produced significantly more B. burgdorferi-specific IgM and IgG than groups receiving large inocula of heat-killed bacteria. Additionally, sera from mice injected with varied doses of killed B. burgdorferi recognized unique borrelial antigens compared to mice infected with live B. burgdorferi. Intradermal injection of killed B. burgdorferi resulted in rapid DNA clearance from skin, whereas DNA was consistently detected in skin inoculated with viable B. burgdorferi. These data indicate that both ELISA-based serological analyses and PCR-based methods of assessing B. burgdorferi infection clearly distinguish between an established infection with live bacteria and exposure to large numbers of bacteria that are promptly cleared by the innate responses.

New antibody weapons against an old foe

Antibodies have been used in a diagnostic capacity for many diseases and for identifying serotypes within single species of pathogens, notably between the multiple capsular polysaccharide serotypes of Streptococcus pneumoniae. For many years, the functions of antibodies in infection were thought to be limited to the opsonization of microorganisms followed by phagocytosis and to the fixing of complement. The thought that antibodies could have other functions has emerged only recently. The study by Yano and coworkers from the laboratory of Liise-anne Pirofski published in mBio [M. Yano, S. Gohil, J. R. Coleman, C. Manix, and L.-A. Pirofski, mBio 2(5):e00176-11, 2011] identifies one mechanism whereby nonopsonic antibodies enhance the transformation competence of two S. pneumoniae serotypes, which leads to an increase in genetic exchange and bacterial variability with a resulting population reduction through fratricide. These new and revealing antibody functions will add another chapter to the burgeoning story of the diversity and versatility of the immune response to bacteria.

A host-restricted viral vector for antigen-specific immunization against Lyme disease pathogen

Newcastle disease virus (NDV) is an avian virus that is attenuated in primates and is a potential vaccine vector for human use. We evaluated NDV as a vector for expressing selected antigens of the Lyme disease pathogen Borrelia burgdorferi. A series of recombinant NDVs were generated that expressed intracellular or extracellular forms of two B. burgdorferi antigens: namely, the basic membrane protein A (BmpA) and the outer surface protein C (OspC). Expression of the intracellular and extracellular forms of these antigens was confirmed in cultured chicken cells. C3H or Balb/C mice that were immunized intranasally with the NDV vectors mounted vigorous serum antibody responses against the NDV vector, but failed to mount a robust response against either the intracellular or extracellular forms of BmpA or OspC. By contrast, a single immunization of hamsters with the NDV vectors via the intranasal, intramuscular, or intraperitoneal route resulted in rapid and rigorous antibody responses against the intracellular or extracellular forms of BmpA and OspC. When groups of hamsters were separately inoculated with various NDV vectors and challenged with B. burgdorferi (10⁸ cells/animal), immunization with vector expressing either intracellular or extracellular BmpA was associated with a significant reduction of the pathogen load in the joints. Taken together, our studies highlighted the importance of NDV as vaccine vector that can be used for simple yet effective immunization of hosts against bacterial infections including Lyme disease.

Cholesterol lipids of Borrelia burgdorferi form lipid rafts and are required for the bactericidal activity of a complement-independent antibody

Borrelia burgdorferi, the agent of Lyme disease, is unusual as it contains free cholesterol and cholesterol glycolipids. It is also susceptible to complement-independent bactericidal antibodies, such as CB2, a monoclonal IgG1 against outer surface protein B (OspB). We find that the bactericidal action of CB2 requires the presence of cholesterol glycolipids and cholesterol. Ultrastructural, biochemical, and biophysical analysis revealed that the bacterial cholesterol glycolipids exist as lipid raft-like microdomains in the outer membrane of cultured and mouse-derived B. burgdorferi and in model membranes from B. burgdorferi lipids. The order and size of the microdomains are temperature sensitive and correlate with the bactericidal activity of CB2. This study demonstrates the existence of cholesterol-containing lipid raft-like microdomains in a prokaryote, and we suggest that the temperature dependence of B. burgdorferi lipid raft organization may have significant implications in the transmission cycle of the spirochetes which are exposed to a range of temperatures.

Toll-like receptor 2 deficiency results in impaired antibody responses and septic shock during Borrelia hermsii infection

Overwhelming bacteremia is a leading cause of death. To understand the mechanisms involved in protective antibody and pathological inflammatory responses during bacteremia, we have been studying the murine model of Borrelia hermsii infection. Toll-like receptor (TLR) signaling plays an important role in generating the rapid anti-B. hermsii antibody responses required for the resolution of bacteremia. Using NF-κB reporter assays, we found that B. hermsii activates TLR2 and TLR9. However, TLR2(-/-) TLR9(-/-) mice exhibited an impairment in anti-B. hermsii antibody responses similar to that of TLR2(-/-) mice. Moreover, the impairment in the antibody responses of TLR2(-/-) mice or TLR2(-/-) TLR9(-/-) mice coincides with an order-of-magnitude-higher bacteremia, and death results from septic shock, as evidenced by a dysregulated systemic cytokine response and characteristic organ pathology. Since TLR2 appears to be the major extracellular sensor stimulated by B. hermsii, we hypothesized that during elevated bacteremia the activation of intracellular sensors of bacteria triggers dysregulated inflammation in TLR2(-/-) mice. Indeed, blocking the internalization of B. hermsii prevented the induction of inflammatory cytokine responses in TLR2-deficient cells. Furthermore, we found that B. hermsii activates the cytoplasmic sensor nucleotide-binding oligomerization domain 2 (NOD2). Macrophages deficient in both TLR2 and NOD2 have impaired cytokine responses to B. hermsii compared to cells lacking TLR2 alone, and B. hermsii-infected TLR2(-/-) NOD2(-/-) mice exhibited improved survival compared to TLR2(-/-) mice. These data demonstrate that TLR2 is critical for protective immunity and suggest that, during heightened bacteremia, recognition of bacterial components by intracellular sensors can lead to pathological inflammatory responses.

Characterization of unique regions of Borrelia burgdorferi surface-located membrane protein 1

The pathogen of Lyme disease, Borrelia burgdorferi, produces a putative surface protein termed "surface-located membrane protein 1" (Lmp1). Lmp1 has been shown previously to assist the microbe in evasion of host-acquired immune defenses and in the establishment of persistent infection of mammals. Here, we show that Lmp1 is an integral membrane protein with surface-exposed N-terminal, middle, and C-terminal regions. During murine infection, antibodies recognizing these three protein regions were produced. Separate immunization of mice with each of the discrete regions exerted differential effects on spirochete survival during infection. Notably, antibodies against the C-terminal region primarily interfered with B. burgdorferi persistence in the joints, while antibodies specific to the N-terminal region predominantly affected pathogen levels in the heart, including the development of carditis. Genetic reconstitution of lmp1 deletion mutants with the lmp1 N-terminal region significantly enhanced its ability to resist the bactericidal effects of immune sera and also was observed to increase pathogen survival in vivo. Taken together, the combined data suggest that the N-terminal region of Lmp1 plays a distinct role in spirochete survival and other parts of the protein are related to specific functions corresponding to pathogen persistence and tropism during infection that is displayed in an organ-specific manner. The findings reported here underscore the fact that surface-exposed regions of Lmp1 could potentially serve as vaccine targets or antigenic regions that could alter the course of natural Lyme disease.

Phylogenetic analysis of a virulent Borrelia species isolated from patients with relapsing fever

Multilocus sequence analysis (MLSA) was used to clarify the taxonomic status of a virulent Borrelia organism previously isolated from patients with relapsing fever and from ticks in Spain that is designated the Spanish relapsing fever (SRF) Borrelia. This species has been used extensively in experimental infection models because of its continued virulence. Seven genes were amplified to analyze the phylogenetic relationships among several Spanish isolates of SRF Borrelia and other relapsing fever Borrelia species. The genes targeted in this study included rrs and flaB, which have commonly been used in phylogenetic studies; the rrf-rrl intergenic spacer (IGS), which is highly discriminatory; and four additional genes, p66, groEL, glpQ, and recC, which are located on the chromosome and which have therefore evolved in a clonal way. The species included in this study were Borrelia duttonii, B. recurrentis, B. crocidurae, and B. hispanica as Old World Borrelia species and B. turicatae and B. hermsii as New World Borrelia species. The results obtained by MLSA of the SRF Borrelia on the basis of 1% of the genomic sequence data analyzed confirmed that the SRF Borrelia isolates are B. hispanica. However, the prototype isolates of B. hispanica used in this study have an uncertain history and display unique phenotypic characteristics that are not shared with the SRF Borrelia. Therefore, we propose to use strain SP1, isolated from a relapsing fever patient in 1994 in southern Spain, as the type strain for B. hispanica.

Induction of distinct neurologic disease manifestations during relapsing fever requires T lymphocytes

Relapsing fever borreliosis is a multisystemic infection characterized primarily by bacteremia but can extend to the CNS. The incidence of CNS disease manifestations in humans depends on the infecting relapsing fever Borrelia species. In the murine model of Borrelia hermsii infection we found high incidence of distinct signs of CNS disease that ranged from a flaccid tail to complete paralysis of hind limbs. Infiltration of large number of T cells into the spinal cord of B. hermsii-infected mice and the upregulation of MHC class II and CD80 on infiltrating macrophages and on microglial cells suggested a role for T cell and Ag-presenting cell interactions in this pathogenesis. Indeed, B. hermsii infection did not induce CNS disease manifestations in T cell-deficient mice (TCR-beta x delta(-/-)), although it resulted in bacteremia comparable to wild-type (Wt) level. Moreover, the infiltration of immune cells into the spinal cord of TCR-beta x delta(-/-) mice was reduced and the resident microglial cells were not activated. Histopathological analysis of lumbar sections of the spinal cord confirmed severe inflammation in Wt but not in TCR-beta x delta(-/-) mice. Induction of CNS disease was dependent on the B. hermsii strain as well as on the ability of the host to control bacteremia. Mice that are impaired in controlling B. hermsii, such as CD14(-/-) mice, exhibited more severe CNS disease than Wt mice. This study demonstrates that distinct neurologic disease manifestations develop during relapsing fever and that T cells play a critical role in the induction of neuropathogenesis.

Concomitant infection decreases the malaria burden but escalates relapsing fever borreliosis

About 500 million cases of malaria occur annually. However, a substantial number of patients who actually have relapsing fever (RF) Borrelia infection can be misdiagnosed with malaria due to similar manifestations and geographic distributions of the two diseases. More alarmingly, a high prevalence of concomitant infections with malaria and RF Borrelia has been reported. Therefore, we used a mouse model to study the effects of such mixed infection. We observed a 21-fold increase in spirochete titers, whereas the numbers of parasitized erythrocytes were reduced 15-fold. This may be explained by polarization of the host immune response toward the intracellular malaria parasite, resulting in unaffected extracellular spirochetes and hosts that succumb to sepsis. Mixed infection also resulted in severe malaria anemia with low hemoglobin levels, even though the parasite counts were low. Overall, coinfected animals had a higher fatality rate and shorter time to death than those with either malaria or RF single infection. Furthermore, secondary malaria infection reactivated a quiescent RF brain infection, which is the first evidence of a clinically and biologically relevant cue for reactivation of RF Borrelia infection. Our study highlights the importance of investigating concomitant infections in vivo to elucidate the immune responses that are involved in the clinical outcome.

IgM in microbial infections: taken for granted?

Much has been learned about the structure, function, and production of IgM, since the antibody's initial characterization. It is widely accepted that IgM provides a first line of defense during microbial infections, prior to the generation of adaptive, high-affinity IgG responses that are important for long-lived immunity and immunological memory. Although IgM responses are commonly used as a measure of exposure to infectious diseases, it is perhaps surprising that the role of and requirement for IgM in many microbial infections has not been well explored in vivo. This is in part due to the lack of capabilities, until relatively recently, to evaluate the requirement for IgM in the absence of coincident IgG responses. Such evaluations are now possible, using gene-targeted mouse strains that produce only IgM, or isotype-switched IgG. A number of studies have revealed that IgM, produced either innately, or in response to antigen challenge, plays an important and perhaps under appreciated role in many microbial infections. Moreover, the characterization of the roles of various B cell subsets, in the production of IgM, and in host defense, has revealed important and divergent roles for B-1a and B-1b cells. This review will highlight studies in which IgM, in its own right, has been found to play an important role, not only in early immunity, but also in long-term protection, against a variety of microbial pathogens. Observations that long-lived IgM responses can be generated in vivo suggest that it may be feasible to target IgM production as part of vaccination strategies.

The bactericidal effect of a complement-independent antibody is osmolytic and specific to Borrelia

A complement-independent bactericidal IgG1 against the OspB of Borrelia burgdorferi increased the permeability of the outer membrane through the creation of openings of 2.8 - 4.4 nm, resulting in its osmotic lysis. Cryo-electron microscopy and tomography demonstrated that exposure to the antibody causes the formation of outer membrane projections and large breaks which may precede the increase in permeability of the outer membrane. The bactericidal effect of this antibody is not transferable to Escherichia coli expressing rOspB on its outer membrane. Additionally, the porin P66, the only protein that coprecipitated with OspB, is dispensable for the bactericidal mechanism.

Roles for phagocytic cells and complement in controlling relapsing fever infection

Relapsing fever spirochetes, such as Borrelia hermsii, proliferate to high levels in their hosts' bloodstream until production of IgM against borrelial surface proteins promotes bacterial clearance. The mechanisms by which B. hermsii survives in host blood, as well as the immune mediators that control this infection, remain largely unknown. It has been hypothesized that B. hermsii is naturally resistant to killing by the alternative pathway of complement activation as a result of its ability to bind factor H, a host complement regulator. However, we found that Cfh(-/-) mice were infected to levels identical to those seen in wild-type mice. Moreover, only a small minority of B. hermsii in the blood of wild-type mice had detectable levels of factor H adhered to their outer surfaces. In vitro, complement was found to play a statistically significant role in antibody-mediated inactivation of B. hermsii, although in vivo studies indicated that complement is not essential for host control of B. hermsii. Depletion of mphi and DC from mice had significant impacts on B. hermsii infection, and depleted mice were unable to control bloodstream infections, leading to death. Infection studies using muMT indicated a significant antibody-independent role for mphi and/or DC in host control of relapsing fever infection. Together, these findings indicate mphi and/or DC play a critical role in the production of B. hermsii-specific IgM and for antibody-independent control of spirochete levels.

Local production of IFN-gamma by invariant NKT cells modulates acute Lyme carditis

The Lyme disease spirochete Borrelia burgdorferi is the only known human pathogen that directly activates invariant NKT (iNKT) cells. The number and activation kinetics of iNKT cells vary greatly among different strains of mice. We now report the role of the iNKT cell response in the pathogenesis of Lyme disease using C57BL/6 mice, a strain with optimal iNKT cell activation that is resistant to the development of spirochetal-induced inflammation. During experimental infection of B6 mice with B. burgdorferi, iNKT cells localize to the inflamed heart where they are activated by CD1d-expressing macrophages. Activation of iNKT cells in vivo results in the production of IFN-gamma, which we demonstrate ameliorates the severity of murine Lyme carditis by at least two mechanisms. First, IFN-gamma enhances the recognition of B. burgdorferi by macrophages, leading to increased phagocytosis of the spirochete. Second, IFN-gamma activation of macrophages increases the surface expression of CD1d, thereby facilitating further iNKT activation. Collectively, our data demonstrate that in the resistant background, B6, iNKT cells modulate the severity of murine Lyme carditis through the action of IFN-gamma, which appears to self-renew through a positive feedback loop during infection.

A chromosomally encoded virulence factor protects the Lyme disease pathogen against host-adaptive immunity

Borrelia burgdorferi, the bacterial pathogen of Lyme borreliosis, differentially expresses select genes in vivo, likely contributing to microbial persistence and disease. Expression analysis of spirochete genes encoding potential membrane proteins showed that surface-located membrane protein 1 (lmp1) transcripts were expressed at high levels in the infected murine heart, especially during early stages of infection. Mice and humans with diagnosed Lyme borreliosis also developed antibodies against Lmp1. Deletion of lmp1 severely impaired the pathogen's ability to persist in diverse murine tissues including the heart, and to induce disease, which was restored upon chromosomal complementation of the mutant with the lmp1 gene. Lmp1 performs an immune-related rather than a metabolic function, as its deletion did not affect microbial persistence in immunodeficient mice, but significantly decreased spirochete resistance to the borreliacidal effects of anti-B. burgdorferi sera in a complement-independent manner. These data demonstrate the existence of a virulence factor that helps the pathogen evade host-acquired immune defense and establish persistent infection in mammals.

The pathogen of Lyme borreliosis, Borrelia burgdorferi, causes disease in many parts of the world, resulting in multi-system complications in infected humans and animals. The microbe produces certain antigens in response to host environments that potentially allow it to persist and cause disease. Here, we analyzed the expression of B. burgdorferi genes encoding potential membrane proteins in infected hosts and show that one of them, termed Lmp1, is dramatically expressed in infected mice, most prominently in cardiac tissue during early infection. Mice and humans diagnosed with Lyme borreliosis also develop antibodies against Lmp1. Deletion of lmp1 in an infectious isolate of B. burgdorferi impairs the pathogen's ability to persist in murine tissues, especially the heart, and to induce disease, which was reversed when the gene was inserted back into the chromosome of the mutant. Lmp1 performs an immune-related, rather than a metabolic, function as its deletion does not affect microbial persistence in immunodeficient mice, but decreases the spirochete's ability to resist the borreliacidal effects of anti-B. burgdorferi sera. These data identify the existence of a surface-located antigen of B. burgdorferi that helps the pathogen evade host-acquired immune defense and establish persistent infection and disease in mammals.

Inhibition of Salmonella enterica serovar typhimurium motility and entry into epithelial cells by a protective antilipopolysaccharide monoclonal immunoglobulin A antibody

Secretory immunoglobulin A (SIgA) antibodies directed against the O antigen of lipopolysaccharide (LPS) are the primary determinants of mucosal immunity to gram-negative enteric pathogens. However, the underlying mechanisms by which these antibodies interfere with bacterial colonization and invasion of intestinal epithelial cells are not well understood. In this study, we report that Sal4, a protective, anti-O5-specific monoclonal IgA, is a potent inhibitor of Salmonella enterica serovar Typhimurium flagellum-based motility. Using video light microscopy, we observed that Sal4 completely and virtually instantaneously "paralyzed" laboratory and clinical strains of serovar Typhimurium. Sal4-mediated motility arrest preceded and occurred independently of agglutination. Polyclonal anti-LPS IgG antibodies and F(ab)(2) fragments were as potent as was Sal4 at impeding bacterial motility, whereas monovalent Fab fragments were 5- to 10-fold less effective. To determine whether motility arrest can fully account for Sal4's protective capacity in vitro, we performed epithelial cell infection assays in which the requirement for flagellar motility in adherence and invasion was bypassed by centrifugation. Under these conditions, Sal4-treated serovar Typhimurium cells remained noninvasive, revealing that the monoclonal IgA, in addition to interfering with motility, has an effect on bacterial uptake into epithelial cells. Sal4 did not, however, inhibit bacterial uptake into mouse macrophages, indicating that the antibody interferes specifically with Salmonella pathogenicity island 1 (SPI-1)-dependent, but not SPI-1-independent, entry into host cells. These results reveal a previously unrecognized capacity of SIgA to "disarm" microbial pathogens on mucosal surfaces and prevent colonization and invasion of the intestinal epithelium.

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.

Bactericidal action of a complement-independent antibody against relapsing fever Borrelia resides in its variable region

A single chain variable fragment (scFv) of CB515, a complement-independent bactericidal monoclonal IgM against a relapsing fever Borrelia, was constructed to investigate the region wherein the unique bactericidal function resides. Monomeric CB515 scFv (26 kDa) was capable of binding its Ag on whole organisms and by immunoblot. This binding was shown to be species and serotype-specific to the 19 kDa variable small protein, recognized by its parent monoclonal IgM. A dose-dependent bactericidal effect of the CB515 scFv was detected by direct enumeration of spirochetes. Spirochetes incubated with the CB515 scFv before inoculation into mice grew into escape mutants, whereas spirochetes incubated with an irrelevant scFv developed as the original infecting serotype. This bactericidal effect, as seen at the ultrastructural level, was due to disruption of the outer membrane and to severe membrane blebbing eventually progressing to lysis. These results indicate that the variable region of CB515 is responsible for this bactericidal activity and that the constant region of the Ab is dispensable.

Interannual dynamics of antibody levels in naturally infected long-lived colonial birds

Little is known about the long-term persistence of specific antibodies (Ab) in natural host-parasite systems despite its potential epidemiological and ecological importance. In long-lived species, knowledge of the dynamics of individual immunological profiles can be important not only for interpreting serology results, but also for assessing transmission dynamics and the potential selective pressures acting on parasites. The aim of this paper was to investigate temporal variation in levels of specific Ab against the bacterium Borrelia burgdorferi sensu lato in adults of a long-lived colonial seabird, the Black-legged Kittiwake Rissa tridactyla. In wild populations, adults are naturally exposed each breeding season to a Borrelia vector, the tick Ixodes uriae. Breeding birds were captured during four consecutive breeding seasons, and parasite infestation quantified. Using enzyme-linked immunosorbent assay (ELISA) and immunoblots, we found that the immunological profiles of anti-Borrelia Ab were highly repeatable among years, reflecting the interannual persistence of Ab levels. We nevertheless also observed that year-to-year changes of Ab levels were related to exposure to ticks in the previous year. The long-term persistence of Ab levels may be an important mechanism of individual protection against future exposure to the microparasite. It will also affect the availability of susceptible hosts, and thus the transmission dynamics of the bacterium. These results illustrate the need to consider the dynamics of the immune response in order to better understand the evolutionary ecology of host-parasite interactions in natural populations.

Phagocytosis of Borrelia burgdorferi, the Lyme disease spirochete, potentiates innate immune activation and induces apoptosis in human monocytes

We have previously demonstrated that phagocytosed Borrelia burgdorferi induces activation programs in human peripheral blood mononuclear cells that differ qualitatively and quantitatively from those evoked by equivalent lipoprotein-rich lysates. Here we report that ingested B. burgdorferi induces significantly greater transcription of proinflammatory cytokine genes than do lysates and that live B. burgdorferi, but not B. burgdorferi lysate, is avidly internalized by monocytes, where the bacteria are completely degraded within phagolysosomes. In the course of these experiments, we discovered that live B. burgdorferi also induced a dose-dependent decrease in monocytes but not a decrease in dendritic cells or T cells and that the monocyte population displayed morphological and biochemical hallmarks of apoptosis. Particularly noteworthy was the finding that apoptotic changes occurred predominantly in monocytes that had internalized spirochetes. Abrogation of phagocytosis with cytochalasin D prevented the death response. Heat-killed B. burgdorferi, which was internalized as well as live organisms, induced a similar degree of apoptosis of monocytes but markedly less cytokine production. Surprisingly, opsonophagocytosis of Treponema pallidum did not elicit a discernible cell death response. Our combined results demonstrate that B. burgdorferi confined to phagolysosomes is a potent inducer of cytosolic signals that result in (i) production of NF-kappaB-dependent cytokines, (ii) assembly of the inflammasome and activation of caspase-1, and (iii) induction of programmed cell death. We propose that inflammation and apoptosis represent mutually reinforcing components of the immunologic arsenal that the host mobilizes to defend itself against infection with Lyme disease spirochetes.

IL-10 deficiency promotes increased Borrelia burgdorferi clearance predominantly through enhanced innate immune responses

Borrelia burgdorferi is capable of persistently infecting a variety of hosts despite eliciting potent innate and adaptive immune responses. Preliminary studies indicated that IL-10-deficient (IL-10(-/-)) mice exhibit up to 10-fold greater clearance of B. burgdorferi from target tissues compared with wild-type mice, establishing IL-10 as the only cytokine currently known to have such a significant effect on spirochetal clearance. To further delineate these IL-10-mediated immune effects, kinetic studies indicated that spirochete dissemination to target tissues is similar in both wild-type and IL-10(-/-) mouse strains, and that enhanced clearance of B. burgdorferi in IL-10(-/-) mice is correlated with increased B. burgdorferi-specific Ab as early as 2 wk postinfection. Immunoblot analysis indicated that Abs produced by infected IL-10(-/-) and wild-type mice recognize similar ranges of spirochetal Ags. Immune sera from IL-10(-/-) and wild-type mice also exhibited similar bactericidal activity in vitro, and passive transfer of these immune sera into B. burgdorferi-infected SCID mice caused similar reductions of bacterial numbers in target tissues. Infectious dose studies indicated that 8-fold more B. burgdorferi were needed to efficiently infect naive IL-10(-/-) mice, suggesting these animals possess higher innate barriers to infection. Moreover, macrophages derived from IL-10(-/-) mice exhibit enhanced proinflammatory responses to B. burgdorferi stimulation compared with wild-type controls, and these responses are not significantly affected by the presence of immune serum. These findings confirm that B. burgdorferi clearance by innate immune responses is more efficient in the absence of IL-10, and these activities are not directly related to increased levels of B. burgdorferi-specific Ab.

Possibilities for relapsing fever reemergence

Increasing globalization may pave the way for reemergence of relapsing fever.

Relapsing fever Borrelia infections have attracted little attention in recent years; however, where endemic, these infections still result in considerable illness and death. Despite the marked antimicrobial drug susceptibility of these organisms, therapy is often delayed through lack of clinical suspicion. With increasing travel, infections may be imported, through exotic relapsing fever infection or through resurgence of infected disease vectors. Although louseborne relapsing fever is now geographically limited, it was once of global importance. The possibility for reemergence was recently highlighted by the probable reemergence of louseborne relapsing fever in homeless persons from France. Host limitations enforced through louseborne transmission are less applicable for the tickborne forms of relapsing fever. Although the latter have reduced potential for epidemic spread, they have the ability to infect diverse hosts, thus establishing reservoirs of infection and presenting greater challenges for their control.

Hepatitis B virus capsid-like particles can display the complete, dimeric outer surface protein C and stimulate production of protective antibody responses against Borrelia burgdorferi infection

Hepatitis B virus capsid-like particles (CLPs), icosahedral assemblies formed by 90 or 120 core protein dimers, hold promise as immune-enhancing vaccine carriers for heterologous antigens. Insertions into the immunodominant c/e1 B cell epitope, a surface-exposed loop, are especially immunogenic. However, display of whole proteins, desirable to induce multispecific and possibly neutralizing antibody responses, can be restrained by an unsuitable structure of the foreign protein and by its propensity to undergo homomeric interactions. Here we analyzed CLP formation by core fusions with two distinct variants of the dimeric outer surface lipoprotein C (OspC) of the Lyme disease agent Borrelia burgdorferi. Although the topology of the termini in the OspC dimer does not match that of the insertion sites in the carrier dimer, both fusions, coreOspCa and coreOspCb, efficiently formed stable CLPs. Electron cryomicroscopy clearly revealed the surface disposition of the OspC domains, possibly with OspC dimerization occurring across different core protein dimers. In mice, both CLP preparations induced high-titered antibody responses against the homologous OspC variant, but with substantial cross-reactivity against the other variant. Importantly, both conferred protection to mice challenged with B. burgdorferi. These data show the principal applicability of hepatitis B virus CLPs for the display of dimeric proteins, demonstrate the presence in OspC of hitherto uncharacterized epitopes, and suggest that OspC, despite its genetic variability, may be a valid vaccine candidate.

Identification of Borrelia burgdorferi outer surface proteins

Several Borrelia burgdorferi outer surface proteins have been identified over the past decade that are up-regulated by temperature- and/or mammalian host-specific signals as this spirochete is transmitted from ticks to mammals. Given the potential role(s) that these differentially up-regulated proteins may play in B. burgdorferi transmission and Lyme disease pathogenesis, much attention has recently been placed on identifying additional borrelial outer surface proteins. To identify uncharacterized B. burgdorferi outer surface proteins, we previously performed a comprehensive gene expression profiling analysis of temperature-shifted and mammalian host-adapted B. burgdorferi. The combined microarray analyses revealed that many genes encoding known and putative outer surface proteins are down-regulated in mammalian host-adapted B. burgdorferi. At the same time, however, several different genes encoding putative outer surface proteins were found to be up-regulated during the transmission and infection process. Among the putative outer surface proteins identified, biochemical and surface localization analyses confirmed that seven (Bb0405, Bb0689, BbA36, BbA64, BbA66, BbA69, and BbI42) are localized to the surface of B. burgdorferi. Furthermore, enzyme-linked immunosorbent assay analysis using serum from tick-infested baboons indicated that all seven outer surface proteins identified are immunogenic and that antibodies are generated against all seven during a natural infection. Specific antibodies generated against all seven of these surface proteins were found to be bactericidal against B. burgdorferi, indicating that these newly identified outer surface proteins are prime candidates for analysis as second-generation Lyme disease vaccinogens.

Complement Regulator-Acquiring Surface Protein 1 Imparts Resistance to Human Serum inBorrelia burgdorferi

Factor H and factor H-like protein 1 (FH/FHL-1) are soluble serum proteins that negatively regulate the alternative pathway of complement. It is now well recognized that many pathogenic bacteria, including Borrelia burgdorferi, bind FH/FHL-1 on their cell surface to evade complement-mediated destruction during infection. Recently, it was suggested that B. burgdorferi open reading frame bbA68, known as complement regulator-acquiring surface protein 1 (CRASP-1), encodes the major FH/FHL-1-binding protein of B. burgdorferi. However, because several other proteins have been identified on the surface of B. burgdorferi that also can bind FH/FHL-1, it is presently unclear what role CRASP-1 plays in serum resistance. To examine the contribution of CRASP-1 in serum resistance, we generated a B. burgdorferi mutant that does not express CRASP-1. The B. burgdorferi CRASP-1 mutant, designated B31cF-CRASP-1, was found to be as susceptible to human serum as a wild-type strain of Borrelia garinii 50 known to be sensitive to human serum. To further examine the role of CRASP-1 in serum resistance, we also created a shuttle vector that expresses CRASP-1 from the native B. burgdorferi gene, which was designated pKFSS-1::CRASP-1. When the pKFSS-1::CRASP-1 construct was transformed into the B. burgdorferi B31cF-CRASP-1 mutant, wild-type levels of serum resistance were restored. Additionally, when pKFSS-1::CRASP-1 was transformed into the serum-sensitive B. garinii 50 isolate, human serum resistance was imparted on this strain to a level indistinguishable from wild-type B. burgdorferi. The combined data led us to conclude that CRASP-1 expression is necessary for B. burgdorferi to resist killing by human serum.