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

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

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

Complement Evasion by Lyme Disease Spirochetes

The complement system is an ancient arm of the innate immune system that plays important roles in pathogen recognition and elimination. Upon activation by microbes, complement opsonizes bacterial surfaces, recruits professional phagocytes, and causes bacteriolysis. Borreliella species are spirochetal bacteria that are transmitted to vertebrate hosts via infected Ixodes ticks and are the etiologic agents of Lyme disease. Pathogens that traffic in blood and other body fluids, like Borreliella, have evolved means to evade complement. Lyme disease spirochetes interfere with complement by producing a small arsenal of outer-surface lipoproteins that bind host complement components and manipulate their native activities. Here we review the current landscape of complement evasion by Lyme disease spirochetes and provide an update on recent discoveries.

RNA-Seq-based analysis of changes in Borrelia burgdorferi gene expression linked to pathogenicity

Background

Lyme disease is a global public health problem caused by the spirochaete Borrelia burgdorferi. Our previous studies found differences in disease severity between B. burgdorferi B31- and B. garinii SZ-infected mice. We hypothesized that genes that are differentially expressed between Borrelia isolates encode bacterial factors that contribute to disease diversity.

Methods

The present study used high-throughput sequencing technology to characterize and compare the transcriptional profiles of B. burgdorferi B31 and B. garinii SZ cultured in vitro. Real-time quantitative RT-PCR was used to validate selected data from RNA-seq experiments.

Results

A total of 731 genes were differentially expressed between B. burgdorferi B31 and B. garinii SZ isolates, including those encoding lipoproteins and purine transport proteins. The fold difference in expression for B. garinii SZ versus B. burgdorferi B31 ranged from 22.07 to 1.01. Expression of the OspA, OspB and DbpB genes were significantly lower in B. garinii SZ compared to B. burgdorferi B31.

Conclusions

The results support the hypothesis that global changes in gene expression underlie differences in Borrelia pathogenicity. The findings also provide an empirical basis for studying the mechanism of action of specific genes as well as their potential usefulness for the diagnosis and management of Lyme disease.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-014-0623-2) contains supplementary material, which is available to authorized users.

Immunization with a Borrelia burgdorferi BB0172-derived peptide protects mice against lyme disease

Lyme disease is the most prevalent arthropod borne disease in the US and it is caused by the bacterial spirochete Borrelia burgdorferi (Bb), which is acquired through the bite of an infected Ixodes tick. Vaccine development efforts focused on the von Willebrand factor A domain of the borrelial protein BB0172 from which four peptides (A, B, C and D) were synthesized and conjugated to Keyhole Limpet Hemocyanin, formulated in Titer Max® adjuvant and used to immunize C3H/HeN mice subcutaneously at days 0, 14 and 21. Sera were collected to evaluate antibody responses and some mice were sacrificed for histopathology to evaluate vaccine safety. Twenty-eight days post-priming, protection was evaluated by needle inoculation of half the mice in each group with 10³ Bb/mouse, whereas the rest were challenged with 10⁵Bb/mouse. Eight weeks post-priming, another four groups of similarly immunized mice were challenged using infected ticks. In both experiments, twenty-one days post-challenge, the mice were sacrificed to determine antibody responses, bacterial burdens and conduct histopathology. Results showed that only mice immunized with peptide B were protected against challenge with Bb. In addition, compared to the other the treatment groups, peptide B-immunized mice showed very limited inflammation in the heart and joint tissues. Peptide B-specific antibody titers peaked at 8 weeks post-priming and surprisingly, the anti-peptide B antibodies did not cross-react with Bb lysates. These findings strongly suggest that peptide B is a promising candidate for the development of a new DIVA vaccine (Differentiate between Infected and Vaccinated Animals) for protection against Lyme disease.

Complement regulator-acquiring surface proteins of Borrelia burgdorferi: Structure, function and regulation of gene expression

Borrelia burgdorferi, the etiological agent of Lyme disease, exploits an array of strategies to establish infection and to overcome host innate and adaptive immune responses. One key borrelial immune escape mechanism involves the inactivation of host complement attack through acquisition of human immune regulators factor H (CFH), factor H-like protein 1 (FHL1), factor H-related protein 1 (CFHR1), CFHR2, and/or CFHR5. Binding of these host proteins is primarily mediated by bacterial surface-exposed proteins that have been collectively referred to as complement regulator-acquiring surface proteins, or CRASPs. Different strains of B. burgdorferi produce as many as 5 different CRASP molecules that comprise 3 distinct, genetically unrelated groups. Depending on bacterial genetic composition, different combinations of these proteins can be found on the borrelial outer surface. The 3 groups differ in their gene location, gene regulatory mechanisms, expression patterns during the tick-mammal infection cycle, protein sequence and structure as well as binding affinity for complement regulators and other serum proteins. These attributes influence the proteins' abilities to contribute to complement resistance of this emerging human pathogen. In this review, we focus on the current knowledge on structure, function, and gene regulation of these B. burgdorferi infection-associated proteins.

The OspE-related proteins inhibit complement deposition and enhance serum resistance of Borrelia burgdorferi, the lyme disease spirochete

Borrelia burgdorferi, the Lyme disease spirochete, binds the host complement inhibitors factor H (FH) and FH-like protein 1 (FHL-1). Binding of FH/FHL-1 by the B. burgdorferi proteins CspA and the OspE-related proteins is thought to enhance resistance to serum-mediated killing. While previous reports have shown that CspA confers serum resistance in B. burgdorferi, it is unclear whether the OspE-related proteins are relevant in B. burgdorferi serum resistance when OspE is expressed on the borrelial surface. To assess the role of the OspE-related proteins, we overexpressed them in a serum-sensitive CspA mutant strain. OspE overexpression enhanced serum resistance of the CspA-deficient organisms. Furthermore, FH was more efficiently bound to the B. burgdorferi surface when OspE was overexpressed. Deposition of complement components C3 and C5b-9 (the membrane attack complex), however, was reduced on the surface of the OspE-overexpressing strain compared to that on the CspA mutant strain. These data demonstrate that OspE proteins expressed on the surface of B. burgdorferi bind FH and protect the organism from complement deposition and subsequent serum-mediated destruction.

Identification and functional characterisation of Complement Regulator Acquiring Surface Protein-1 of serum resistant Borrelia garinii OspA serotype 4

Background

B. burgdorferi sensu lato (sl) is the etiological agent of Lyme borreliosis in humans. Spirochetes have adapted themselves to the human immune system in many distinct ways. One important immune escape mechanism for evading complement activation is the binding of complement regulators Factor H (CFH) or Factor H-like protein1 (FHL-1) to Complement Regulator-Acquiring Surface Proteins (CRASPs).

Results

We demonstrate that B. garinii OspA serotype 4 (ST4) PBi resist complement-mediated killing by binding of FHL-1. To identify the primary ligands of FHL-1 four CspA orthologs from B. garinii ST4 PBi were cloned and tested for binding to human CFH and FHL-1. Orthologs BGA66 and BGA71 were found to be able to bind both complement regulators but with different intensities. In addition, all CspA orthologs were tested for binding to mammalian and avian CFH. Distinct orthologs were able to bind to CFH of different animal origins.

Conclusions

B. garinii ST4 PBi is able to evade complement killing and it can bind FHL-1 to membrane expressed proteins. Recombinant proteins BGA66 can bind FHL-1 and human CFH, while BGA71 can bind only FHL-1. All recombinant CspA orthologs from B. garinii ST4 PBi can bind CFH from different animal origins. This partly explains the wide variety of animals that can be infected by B. garinii.

CspA-mediated binding of human factor H inhibits complement deposition and confers serum resistance in Borrelia burgdorferi

Borrelia burgdorferi has developed efficient mechanisms for evading the innate immune response during mammalian infection and has been shown to be resistant to the complement-mediated bactericidal activity of human serum. It is well recognized that B. burgdorferi expresses multiple lipoproteins on its surface that bind the human complement inhibitors factor H and factor H-like protein 1 (FH/FHL-1). The binding of FH/FHL-1 on the surface of B. burgdorferi is thought to enhance its ability to evade serum-mediated killing during the acute phase of infection. One of the key B. burgdorferi FH/FHL-1 binding proteins identified thus far was designated CspA. While it is known that CspA binds FH/FHL-1, it is unclear how the interaction between CspA and FH/FHL-1 specifically enhances serum resistance. To better understand how CspA mediates serum resistance in B. burgdorferi, we inactivated cspA in a virulent strain of B. burgdorferi. An affinity ligand blot immunoassay and indirect immunofluorescence revealed that the CspA mutant does not efficiently bind human FH to its surface. Consistent with the lack of FH binding, the CspA mutant was also highly sensitive to killing by human serum. Additionally, the deposition of complement components C3, C6, and C5b-9 was enhanced on the surface of the CspA mutant compared to that of the wild-type strain. The combined data lead us to conclude that the CspA-mediated binding of human FH confers serum resistance by directly inhibiting complement deposition on the surface of B. burgdorferi.

Detection and quantification of Lyme spirochetes using sensitive and specific molecular beacon probes

Background

Lyme disease, caused by Borrelia burgdorferi, affects a large number of people in both the USA and Europe. The mouse is a natural host for this spirochete and is widely used as a model system to study Lyme pathogenesis mechanisms. Since disease manifestations often depend upon the spirochete burden in a particular tissue, it is critical to accurately measure the bacterial number in infected tissues. The current methods either lack sensitivity and specificity (SYBR Green), or require independent analysis of samples in parallel to quantitate host and bacterial DNA (TaqMan). We have developed a novel molecular beacon-based convenient multiplex real-time quantitative PCR assay to identify and detect small numbers of B. burgdorferi in infected mouse tissues.

Results

We show here that molecular beacons are effective, sensitive and specific probes for detecting and estimating wide-ranging numbers of B. burgdorferi in the presence of mouse DNA. In our assays, the spirochete recA and the mouse nidogen gene amplicons were detected simultaneously using molecular beacons labeled with different fluorophores. We further validated the application of these probes by quantifying the wild-type strain and bgp-defective mutant of B. burgdorferi. The bgp-defective mutant shows a ten-fold reduction in the level of spirochetes present in various tissues.

Conclusion

The high sensitivity and specificity of molecular beacons makes them superior probes for the detection of small numbers of B. burgdorferi. Furthermore, the use of molecular beacons can be expanded for the simultaneous detection and quantification of multiple pathogens in the infected hosts, including humans, and in the arthropod vectors.

sodA is essential for virulence of Borrelia burgdorferi in the murine model of Lyme disease

Borrelia burgdorferi, the causative agent of Lyme disease, has a limited set of genes to combat oxidative/nitrosative stress encountered in its tick vector or mammalian hosts. We inactivated the gene encoding for superoxide dismutase A (sodA, bb0153), an enzyme mediating the dismutation of superoxide anions and examined the in vitro and in vivo phenotype of the mutant. There were no significant differences in the in vitro growth characteristics of the sodA mutant compared with the control strains. Microscopic analysis of viability of spirochaetes revealed greater percentage of cell death upon treatment of sodA mutant with superoxide generators compared with its controls. Infectivity analysis in C3H/HeN mice following intradermal needle inoculation of 10(3) or 10(5) spirochaetes per mouse revealed complete attenuation of infectivity for the sodA mutant compared with control strains at 21 days post infection. The sodA mutant was more susceptible to the effects of activated macrophages and neutrophils, suggesting that its in vivo phenotype is partly due to the killing effects of activated immune cells. These studies indicate that SodA plays an important role in combating oxidative stress and is essential for the colonization and dissemination of B. burgdorferi in the murine model of Lyme disease.

Molecular and serological evidence of Borrelia burgdorferi sensu lato in wild rodents in the Czech Republic

The aim of the present study was to determine the frequency and spatial distribution of the Borrelia species in wild rodents in the Czech Republic. In total, 293 muscle tissue samples and 106 sera from 293 wild rodents captured in North Bohemia and North-East and South Moravia were examined for the presence of Borrelia spp. and antibodies. Muscle samples were investigated with real-time polymerase chain reaction (PCR) with a recA primer set, with DNA quantification and melting curve analysis, and with restriction fragment length polymorphism (RFLP) analysis of the 5S-23S rDNA intergenic spacer. Infection with Borrelia burgdorferi sensu lato was found in 16.4% of the muscle samples. The most abundant genospecies was Borrelia afzelii (11.3%), followed by Borrelia burgdorferi sensu stricto (4.8%) and Borrelia garinii (0.7%). Borrelia infection was more frequently observed in Clethrionomys glareolus than in Apodemus spp. Sera were analyzed with an enzyme-linked immunosorbent assay (ELISA) test, yielding the total seropositivity rates of 24.5% for anti-Borrelia IgM antibodies and 25.5% for IgG antibodies. Total seroprevalence was higher in Apodemus spp. than in C. glareolus. In conclusion, our data indicate that in the Czech Republic small wild rodents can serve as hosts for B. burgdorferi s. s. as well as for B. afzelii.

Example of real-time quantitative reverse transcription-PCR (Q-RT-PCR) analysis of bacterial gene expression during mammalian infection: Borrelia burgdorferi in mouse tissues

This unit provides a chronological in-depth description of all protocols needed for quantitative reverse transcription-PCR (Q-RT-PCR) analysis of Borrelia burgdorferi gene expression within infected mouse tissues. Specifically, this unit discusses the extraction of RNA from infected mouse tissues, removal of contaminating genomic DNA from the purified RNA, preparation of cDNA and genomic DNA standards, LightCycler-based PCR, and a relative quantification analysis of the cDNA. Q-RT-PCR as a highly relevant and powerful tool used to detect gene expression by bacteria within mammalian host tissues. It is also an invaluable technique used to measure and assess the small differences in expression that can exist among a set of genes, whereas this type of analysis is not feasible using less-sensitive techniques such as indirect immunofluorescence analysis or qualitative RT-PCR. Although the protocols described herein are tailored for B. burgdorferi, they are broadly applicable to other microbes.

Borrelia burgdorferi expression of the bba64, bba65, bba66, and bba73 genes in tissues during persistent infection in mice

Borrelia burgdorferi, the etiological agent of Lyme disease in humans, is vectored between mammalian hosts in nature by Ixodes ticks. The organism adapts to diverse environments encountered throughout the enzootic cycle by differentially expressing essential gene products to survive the specialized conditions, whether in ticks or warm-blooded hosts. However, little is known regarding the identity and/or function of B. burgdorferi genes expressed during colonization of tissues during mammalian infection. Experimental evidence has shown that a group of genes (formerly classified as paralogous gene family 54) contiguously localized on the 54-kilobase linear plasmid of B. burgdorferi, are among the most highly regulated by in vitro conditions resembling mammalian infection. In this study, we employed quantitative reverse transcription-PCR to measure temporal gene expression of a subset of this B. burgdorferi gene family (bba64, bba65, bba66, and bba73) in tissues during chronic murine infection. The goal was to gain insight into the role of these genes in infectivity and pathogenesis by identifying when the genes are induced and whether they are expressed in specific target tissues. B. burgdorferi bba64, bba65, bba66, and bba73 expression was measured from infected mouse tissues relative to expression in in vitro culture conditions at specific times post-infection. bba64 expression was highly upregulated in bladder, heart, and spleen tissues throughout the infection period, contrasting with the sharp downregulation previously observed in ear tissues. bba65, bba66, and bba73 demonstrated upregulated differential expression in various tissues over 1 year post-infection. These results suggest an essential role for these genes in borrelial survival, persistence, and/or pathogenesis.

Borrelia burgdorferi complement regulator-acquiring surface proteins (BbCRASPs): Expression patterns during the mammal-tick infection cycle

Host complement is widely distributed throughout mammalian body fluids and can be activated immediately as part of the first line of defense against invading pathogens. The agent of Lyme disease, Borrelia burgdorferi sensu lato (s.l.), is naturally resistant to that innate immune defense system of its hosts. One resistance mechanism appears to involve binding fluid-phase regulators of complement to distinct borrelial outer surface molecules known as CRASPs (complement regulator acquiring surface proteins). Using sensitive molecular biology techniques, expression patterns of all three classes of genes encoding the CRASPs of B. burgdorferi sensu stricto (BbCRASPs) have been analyzed throughout the natural tick-mammal infection cycle. Each class shows a different expression profile in vivo and the results are summarized herein. Studies on the expression of B. burgdorferi genes using animal models of infection have advanced our knowledge on the ability of the causative agent to circumvent innate immune defenses, the contributions of CRASPs to spirochete infectivity, and the pathogenesis of Lyme disease.

Borrelia burgdorferi complement regulator-acquiring surface protein 2 (CspZ) as a serological marker of human Lyme disease

Serological diagnosis of Lyme disease may be complicated by antigenic differences between infecting organisms and those used as test references. Accordingly, it would be helpful to include antigens whose sequences are well conserved by a broad range of Lyme disease spirochetes. In the present study, line blot analyses were performed using recombinant complement regulator-acquiring surface protein 2 (BbCRASP-2) from Borrelia burgdorferi sensu stricto strain B31 and serum samples from human Lyme disease patients from throughout the United States and Germany. The results indicated that a large proportion of the patients had produced antibodies recognizing recombinant BbCRASP-2. In addition, Lyme disease spirochetes isolated from across North America and Europe were found to contain genes encoding proteins with high degrees of similarity to the B. burgdorferi type strain B31 BbCRASP-2, consistent with the high percentage of serologically positive patients. These data indicate that BbCRASP-2 may be valuable for use in a widely effective serological assay.

Coordinated expression of Borrelia burgdorferi complement regulator-acquiring surface proteins during the Lyme disease spirochete's mammal-tick infection cycle

The Lyme disease spirochete, Borrelia burgdorferi, is largely resistant to being killed by its hosts' alternative complement activation pathway. One possible resistance mechanism of these bacteria is to coat their surfaces with host complement regulators, such as factor H. Five different B. burgdorferi outer surface proteins having affinities for factor H have been identified: complement regulator-acquiring surface protein 1 (BbCRASP-1), encoded by cspA; BbCRASP-2, encoded by cspZ; and three closely related proteins, BbCRASP-3, -4, and -5, encoded by erpP, erpC, and erpA, respectively. We now present analyses of the recently identified BbCRASP-2 and cspZ expression patterns throughout the B. burgdorferi infectious cycle, plus novel analyses of BbCRASP-1 and erp-encoded BbCRASPs. Our results, combined with data from earlier studies, indicate that BbCRASP-2 is produced primarily during established mammalian infection, while BbCRASP-1 is produced during tick-to-mammal and mammal-to-tick transmission stages but not during established mammalian infection, and Erp-BbCRASPs are produced from the time of transmission from infected ticks into mammals until they are later acquired by other feeding ticks. Transcription of cspZ and synthesis of BbCRASP-2 were severely repressed during cultivation in laboratory medium relative to mRNA levels observed during mammalian infection, and cspZ expression was influenced by culture temperature and pH, observations which will assist identification of the mechanisms employed by B. burgdorferi to control expression of this borrelial infection-associated protein.

Analysis of the RpoS regulon in Borrelia burgdorferi in response to mammalian host signals provides insight into RpoS function during the enzootic cycle

Borrelia burgdorferi (Bb) adapts to its arthropod and mammalian hosts by altering its transcriptional and antigenic profiles in response to environmental signals associated with each of these milieus. In studies presented here, we provide evidence to suggest that mammalian host signals are important for modulating and maintaining both the positive and negative aspects of mammalian host adaptation mediated by the alternative sigma factor RpoS in Bb. Although considerable overlap was observed between genes induced by RpoS during growth within the mammalian host and following temperature-shift, comparative microarray analyses demonstrated unequivocally that RpoS-mediated repression requires mammalian host-specific signals. A substantial portion of the in vivo RpoS regulon was uniquely upregulated within dialysis membrane chambers, further underscoring the importance of host-derived environmental stimuli for differential gene expression in Bb. Expression profiling of genes within the RpoS regulon by quantitative reverse transcription polymerase chain reaction (qRT-PCR) revealed a level of complexity to RpoS-dependent gene regulation beyond that observed by microarray, including a broad range of expression levels and the presence of genes whose expression is only partially dependent on RpoS. Analysis of Bb-infected ticks by qRT-PCR established that expression of rpoS is induced during the nymphal blood meal but not within unfed nymphs or engorged larvae. Together, these data have led us to postulate that RpoS acts as a gatekeeper for the reciprocal regulation of genes involved in the establishment of infection within the mammalian host and the maintenance of spirochetes within the arthropod vector.

A reverse transcriptase-polymerase chain reaction assay of Borrelia burgdorferi 16S rRNA for highly sensitive quantification of pathogen load in a vector

We developed a real-time quantitative detection assay for the pathogen Borrelia burgdorferi, a Lyme borreliosis (LB) agent, using reverse transcription-polymerase chain reaction (RT-PCR) with primers and probe for a Borrelia genus-specific region of 16S ribosomal RNA. The standard curve of the assay was linear by semi-log plot over more than five orders of magnitude, and the detection limit of the assay was one thousandth of a single cell of B. burgdorferi. The minimum target level for detection using the RT-PCR assay for 16S RNA was 40-fold lower than the RT-PCR assay for messenger RNA of ospA, a highly expressed, plasmid-borne gene, and 1600-fold lower than the RT-PCR assay for messenger RNA of p66, a chromosome-borne gene of B. burgdorferi. The 16S rRNA assay was then applied in an experimental setting for monitoring the spirochetal load in B. burgdorferi-infected Ixodes scapularis ticks before and after they fed on Peromyscus leucopus mice immunized with recombinant OspA. Unfed infected ticks had a mean of 2,240 spirochetes per tick, and after feeding on non-immunized mice and engorgement, the mean number of spirochetes increased to 223,900 per tick. In contrast, there were either no or <or=7 spirochetes in ticks that had fed on OspA-immunized mice.

Evolving models of Lyme disease spirochete gene regulation

The spirochete Borrelia burgdorferi, the causative agent of Lyme disease (Lyme borreliosis), is well-adapted to maintain a natural cycle of alternately infecting vertebrates and blood-sucking ticks. During this cycle, B. burgdorferi interacts with a broad spectrum of vertebrate and arthropod tissues, acquires nutrients in diverse environments and evades killing by vertebrate and tick immune systems. The bacterium also senses when situations occur that necessitate transmission between hosts, such as when an infected tick is taking a blood meal from a potential host. To accurately accomplish the requirements necessary for survival in nature, B. burgdorferi must be keenly aware of its surroundings and respond accordingly. In this review, we trace studies performed to elucidate regulatory mechanisms employed by B. burgdorferi to control gene expression, and the development of models or "paradigms" to explain experimental results. Through comparisons of five borrelial gene families, it is readily apparent that each is controlled through a distinct mechanism. Furthermore, those results indicate that current models of interpreting in vitro data cannot accurately predict all aspects of B. burgdorferi environmental sensing and gene regulation in vivo.

Temporal expression analysis of the Borrelia burgdorferi paralogous gene family 54 genes BBA64, BBA65, and BBA66 during persistent infection in mice

Members of the Borrelia burgdorferi paralogous gene family 54 (pgf 54) are regulated by conditions simulating mammalian infection and are thought to be instrumental in borrelial host survival and pathogenesis. To explore the activities of these genes in vivo, a comprehensive analysis of pgf 54 genes BBA64, BBA65, and BBA66 was performed to assess the genetic stability, host antibody responses, and kinetics of gene expression in the murine model of persistent infection. DNA sequencing of pgf 54 genes obtained from re-isolates at 1 year postinfection demonstrated that all genes of this family are stable and do not undergo recombination to generate variant antigens during persistent infection. Antibodies against BBA64 and BBA66 appeared soon after infection and were detectable throughout the infection, suggesting that there was gene expression during infection. However, quantitative reverse transcription-PCR revealed that BBA64 gene expression was considerably decreased in Borrelia residing in the mouse ear tissue compared to the expression in cultured spirochetes by 20 days postinfection and that the levels of expression remained low throughout the infection. Conversely, transcription of the BBA65 and BBA66 genes was increased, and both of these genes were continuously expressed until 100 days postinfection; this was followed by periods of differential expression late in infection. The expression profile of the BBA64 gene suggests that this gene has an important role during tick-to-host transmission and early infection, whereas the expression profile of the BBA65 and BBA66 genes suggests that these genes have a role in persistent infection. The differential regulation of pgf 54 genes observed during infection may help confer a survival advantage during persistent infection, influencing mechanisms for B. burgdorferi dissemination, tissue tropism, or evasion of the adaptive immune response.

Vector seasonality, host infection dynamics and fitness of pathogens transmitted by the tick Ixodes scapularis

Fitness of tick-borne pathogens may be determined by the degree to which their infection dynamics in vertebrate hosts permits transmission cycles if infective and uninfected tick stages are active at different times of the year. To investigate this hypothesis we developed a simulation model that integrates the transmission pattern imposed by seasonally asynchronous nymphal and larval Ixodes scapularis ticks in northeastern North America, with a model of infection in white-footed mice (Peromyscus leucopus) reservoir hosts, using the bacteria Borrelia burgdorferi and Anaplasma phagocytophilum as examples. In simulations, survival of microparasites, their sensitivity to reduced rodent and tick abundance, and to 'dilution' by a reservoir-incompetent host depended on traits that allowed (i) highly efficient transmission from acutely-infected hosts, (ii) long-lived acute or 'carrier' host infections, and/or (iii) transmission amongst co-feeding ticks. Minimum values for transmission efficiency to ticks, and duration of host infectivity, necessary for microparasite persistence, were always higher when nymphal and larval ticks were seasonally asynchronous than when these instars were synchronous. Thus, traits influencing duration of host infectivity, transmission efficiency to ticks and co-feeding transmission are likely to be dominant determinants of fitness in I. scapularis-borne microparasites in northeastern North America due to abiotic forcings influencing I. scapularis seasonality.

Fundamental processes in the evolutionary ecology of Lyme borreliosis

The evolutionary ecology of many emerging infectious diseases, particularly vector-borne zoonoses, is poorly understood. Here, we aim to develop a biological, process-based framework for vector-borne zoonoses, using Borrelia burgdorferi sensu lato (s.l.), the causative agent of Lyme borreliosis in humans, as an example. We explore the fundamental biological processes that operate in this zoonosis and put forward hypotheses on how extrinsic cues and intrinsic dynamics shape B. burgdorferi s.l. populations. Additionally, we highlight possible epidemiological parallels between B. burgdorferi s.l. and other vector-borne zoonotic pathogens, including West Nile virus.

Evidence that the BBA68 protein (BbCRASP-1) of the Lyme disease spirochetes does not contribute to factor H-mediated immune evasion in humans and other animals

BBA68 (BbCRASP-1) of the Lyme disease spirochetes binds human factor H (FH) and FH-like protein 1 (FHL-1). Here we assess transcription of the BBA68 gene and production of BBA68 in infected mice and humans using real-time reverse transcriptase PCR and immunoblotting. The species specificity of FH binding to BBA68 was also tested. The data suggest that BBA68 does not play an important role in immune evasion in animals.

Borrelia burgdorferi erp genes are expressed at different levels within tissues of chronically infected mammalian hosts

The spirochete Borrelia burgdorferi is the causative agent of Lyme disease and is transmitted to humans and other vertebrate hosts through the bites of ixodid ticks. B. burgdorferi Erp (OspE-F related lipoprotein) family members are encoded on members of the 32 kb circular plasmid-like prophage family (cp32s). Many Erp proteins serve as receptors for the complement inhibitory factor H molecules of numerous vertebrate hosts, providing one mechanism by which the bacteria potentially evade the innate immune system. Indirect immunofluorescence analyses (IFA) have demonstrated that Erp expression is temporally regulated throughout the mammal-tick infectious cycle, indicating that Erp proteins perform an important role (or even roles) during mammalian infection. However, it was not previously known whether Erp proteins are continually produced by B. burgdorferi throughout the course of mammalian infection. To address this issue, quantitative RT-PCR (q-RT-PCR) was utilized to assess erp transcription levels by bacteria within numerous different tissues of both mice and non-human primates (NHPs) chronically infected with B. burgdorferi. Q-RT-PCR results obtained using both animal models indicated that while the majority of erp genes were detectably transcribed during chronic infection, differences in expression levels were noted. These data strongly suggest that Erp proteins contribute to B. burgdorferi persistence within chronically infected host tissues, perhaps by protecting the bacteria from complement-mediated killing.

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.

Expression of Borrelia burgdorferi erp genes during infection of non-human primates

All examined isolates of the Lyme disease spirochete contain multiple operons encoding Erp outer membrane lipoproteins. Many Erp proteins have been demonstrated to bind the host complement regulator factor H, and may thereby help protect the bacteria from complement-mediated killing during mammalian infection. Consistent with that hypothesis, all Erp proteins are produced by Borrelia burgdorferi during transmission between tick vectors and mammalian hosts. The present study examined whether erp genes are also expressed by B. burgdorferi following establishment of mammalian infection. To that end, quantitative RT-PCR was utilized to assess erp transcription levels within different tissues of infected non-human primates, a model that closely mimics human Lyme disease. The majority of erp genes were detectably transcribed after more than 3 months of mammalian infection. Intriguingly, differences in expression levels were noted among the various erp loci. No significant differences in erp expression were apparent between examined tissues, which included central and peripheral nervous system tissue, skeletal muscle, bladder, skin and heart tissues. These data strongly suggest that Erp proteins are expressed by B. burgdorferi throughout infection of their vertebrate hosts.