Assessment of the potential contribution of the highly conserved C-terminal motif (C10) of Borrelia burgdorferi outer surface protein C in transmission and infectivity

Borreliella burgdorferi factor H-binding proteins are not required for serum resistance and infection in mammals

The causative agent of Lyme disease (LD), Borreliella burgdorferi, binds factor H (FH) and other complement regulatory proteins to its surface. B. burgdorferi B31 (type strain) encodes five FH-binding proteins (FHBPs): CspZ, CspA, and the OspE paralogs OspEBBN38, OspEBBL39, and OspEBBP38. This study assessed potential correlations between the production of individual FHBPs, FH-binding ability, and serum resistance using a panel of infectious B. burgdorferi clonal populations recovered from dogs. FHBP production was assessed in cultivated spirochetes and by antibody responses in naturally infected humans, dogs, and eastern coyotes (wild canids). FH binding specificity and sensitivity to dog and human serum were also assessed and compared. No correlation was observed between the production of individual FHBPs and FH binding with serum resistance, and CspA was determined to not be produced in animals. Notably, one or more clones isolated from dogs lacked CspZ or the OspE proteins (a finding confirmed by genome sequence determination) and did not bind FH derived from canines. The data presented do not support a correlation between FH binding and the production of individual FHBPs with serum resistance and infectivity. In addition, the limited number and polymorphic nature of cp32s in B. burgdorferi clone DRI85A that were identified through genome sequencing suggest no strict requirement for a defined set of these replicons for infectivity. This study reveals that the immune evasion mechanisms employed by B. burgdorferi are diverse, complex, and yet to be fully defined.

Direct Detection of Lyme Borrelia: Recent Advancement and Use of Aptamer Technology

Borrelia burgdorferi sensu lato (B. burgdorferi s.l.), which is predominantly spread by ticks, is the cause of Lyme disease (LD), also known as Lyme borreliosis, one of the zoonotic diseases affecting people. In recent years, LD has become more prevalent worldwide, even in countries with no prior records. Currently, Lyme Borrelia detection is achieved through nucleic acid amplification, antigen detection, microscopy, and in vitro culture. Nevertheless, these methods lack sensitivity in the early phase of the disease and, thus, are unable to confirm active infection. This review briefly discusses the existing direct detection methods of LD. Furthermore, this review also introduces the use of aptamer technology integrated with biosensor platforms to detect the Borrelia antigen. This aptamer technology could be explored using other biosensor platforms targeting whole Borrelia cells or specific molecules to enhance Borrelia detection in the future.

FtlA and FtlB Are Candidates for Inclusion in a Next-Generation Multiantigen Subunit Vaccine for Lyme Disease

Lyme disease (LD) is a tick-transmitted bacterial infection caused by Borreliella burgdorferi and other closely related species collectively referred to as the LD spirochetes. The LD spirochetes encode an uncharacterized family of proteins originally designated protein family twelve (PF12). In B. burgdorferi strain B31, PF12 consists of four plasmid-carried genes, encoding BBK01, BBG01, BBH37, and BBJ08. Henceforth, we designate the PF12 proteins family twelve lipoprotein (Ftl) A (FtlA) (BBK01), FtlB (BBG01), FtlC (BBH37), and FtlD (BBJ08). The goal of this study was to assess the potential utility of the Ftl proteins in subunit vaccine development. Immunoblot analyses of LD spirochete cell lysates demonstrated that one or more of the Ftl proteins are produced by most LD isolates during cultivation. The Ftl proteins were verified to be membrane associated, and nondenaturing PAGE revealed that FtlA, FtlB, and FtlD formed dimers, while FtlC formed hexamers. Analysis of serum samples from B. burgdorferi antibody (Ab)-positive client-owned dogs (n = 50) and horses (n = 90) revealed that a majority were anti-Ftl Ab positive. Abs to the Ftl proteins were detected in serum samples from laboratory-infected dogs out to 497 days postinfection. Anti-FtlA and FtlB antisera displayed potent complement-dependent Ab-mediated killing activity, and epitope localization revealed that the bactericidal epitopes reside within the N-terminal domain of the Ftl proteins. This study suggests that FtlA and FtlB are potential candidates for inclusion in a multivalent vaccine for LD.

Detection of Borrelia burgdorferi antigens in tissues and plasma during early infection in a mouse model

Borrelia burgdorferi is the causative agent of Lyme borreliosis, which is the most common tick-borne human disease in Europe and North America. Currently, the diagnosis of Lyme borreliosis is based on serological tests allowing indirect detection of anti-Borrelia antibodies produced by patients. Their main drawback is a lack of sensitivity in the early phase of disease and an incapacity to prove an active infection. Direct diagnostic tests are clearly needed. The objectives of this study were to produce tools allowing sensitive detection of potential circulating Borrelia antigens and to evaluate them in a mouse model. We focused on two potential early bacterial makers, the highly variable OspC protein and the conserved protein FlaB. High-affinity monoclonal antibodies were produced and used to establish various immunoassays and western blot detection. A very good limit of detection for OspC as low as 17 pg/mL of sample was achieved with SPIE-IA. In infected mice, we were able to measure OspC in plasma with a mean value of 10 ng/mL at 7 days post-inoculation. This result suggests that OspC could be a good blood marker for diagnosis of Lyme borreliosis and that the tools developed during this study could be very useful.

Lyme Disease Pathogenesis

Lyme disease Borrelia are obligately parasitic, tick- transmitted, invasive, persistent bacterial pathogens that cause disease in humans and non-reservoir vertebrates primarily through the induction of inflammation. During transmission from the infected tick, the bacteria undergo significant changes in gene expression, resulting in adaptation to the mammalian environment. The organisms multiply and spread locally and induce inflammatory responses that, in humans, result in clinical signs and symptoms. Borrelia virulence involves a multiplicity of mechanisms for dissemination and colonization of multiple tissues and evasion of host immune responses. Most of the tissue damage, which is seen in non-reservoir hosts, appears to result from host inflammatory reactions, despite the low numbers of bacteria in affected sites. This host response to the Lyme disease Borrelia can cause neurologic, cardiovascular, arthritic, and dermatologic manifestations during the disseminated and persistent stages of infection. The mechanisms by which a paucity of organisms (in comparison to many other infectious diseases) can cause varied and in some cases profound inflammation and symptoms remains mysterious but are the subjects of diverse ongoing investigations. In this review, we provide an overview of virulence mechanisms and determinants for which roles have been demonstrated in vivo, primarily in mouse models of infection.

Human and Veterinary Vaccines for Lyme Disease

Lyme disease (LD) is an emerging zoonotic infection that is increasing in incidence in North America, Europe, and Asia. With the development of safe and efficacious vaccines, LD can potentially be prevented. Vaccination offers a cost-effective and safe approach for decreasing the risk of infection. While LD vaccines have been widely used in veterinary medicine, they are not available as a preventive tool for humans. Central to the development of effective vaccines is an understanding of the enzootic cycle of LD, differential gene expression of Borrelia burgdorferi in response to environmental variables, and the genetic and antigenic diversity of the unique bacteria that cause this debilitating disease. Here we review these areas as they pertain to past and present efforts to develop human, veterinary, and reservoir targeting LD vaccines. In addition, we offer a brief overview of additional preventative measures that should employed in conjunction with vaccination.

Antigen Engineering Approaches for Lyme Disease Vaccines

Increasing rates of Lyme disease necessitate preventive measures such as immunization to mitigate the risk of contracting the disease. At present, there is no human Lyme disease vaccine available on the market. Since the withdrawal of the first and only licensed Lyme disease vaccine based on lipidated recombinant OspA, vaccine and antigen research has aimed to overcome its risks and shortcomings. Replacement of the putative cross-reactive T-cell epitope in OspA via mutation or chimerism addresses the potential risk of autoimmunity. Multivalent approaches in Lyme disease vaccines have been pursued to address sequence heterogeneity of Lyme borreliae antigens and to induce a repertoire of functional antibodies necessary for efficient heterologous protection. This Review summarizes recent antigen engineering strategies that have paved the way for the development of next generation vaccines against Lyme disease, some of which have reached clinical testing. Bioconjugation methods that incorporate antigens to self-assembling nanoparticles for immune response potentiation are also discussed.

Diversity of the Lyme Disease Spirochetes and its Influence on Immune Responses to Infection and Vaccination

The Lyme disease spirochetes are a highly diverse group of bacteria with unique biological properties. Their ability to cycle between ticks and mammals requires that they adapt to variable and constantly changing environmental conditions. Outer surface protein C is an essential virulence determinant that has received considerable attention in vaccine and diagnostic assay development. Knowledge of OspC diversity, its antigenic determinants, and its production patterns throughout the enzootic cycle, as well as in the laboratory setting, is essential for understanding immune responses induced by infection or vaccination.

Analysis of the antigenic determinants of the OspC protein of the Lyme disease spirochetes: Evidence that the C10 motif is not immunodominant or required to elicit bactericidal antibody responses

As Ixodes ticks spread to new regions, the incidence of Lyme disease (LD) in companion animals and humans will increase. Preventive strategies for LD in canines center on vaccination and tick control (acaricides). Both subunit and bacterin based LD veterinary vaccines are available. Outer surface protein C (OspC), a potent immunogen and dominant early antigen, has been demonstrated to elicit protective antibody (Ab) responses. However, a single OspC protein elicits a relatively narrow range of protection. There are conflicting reports as to whether the immunodominant epitopes of OspC reside within variable or conserved domains. A detailed understanding of the antigenic determinants of OspC is essential for understanding immune responses to this essential virulence factor and vaccinogen. Here, we investigate the contribution of the conserved C-terminal C10 motif in OspC triggered Ab responses. Using a panel of diverse recombinant full length OspC proteins and their corresponding C10 deletion variants (OspCΔC10), we demonstrate that the C10 motif does not significantly contribute to immunization or infection induced Ab responses in rabbits, rats, canines, horses and non-human primates. Furthermore, the C10 motif is not required to trigger potent bactericidal Ab responses. This study provides insight into the antigenic structure of OspC. The results enhance our understanding of immune responses that develop during infection or upon vaccination and have implications for interpretation of LD diagnostic assays that employ OspC.

Enhanced Protective Immunogenicity of Homodimeric Borrelia burgdorferi Outer Surface Protein C

Lyme borreliosis is caused by tick-transmitted spirochetes of the Borrelia burgdorferi sensu lato group and is the most common vector-borne disease in the United States and Europe. Outer surface protein C (OspC) is a 23-kDa outer surface lipoprotein expressed during spirochete transmission from the tick to the vertebrate host. In a previous study, we found that immunization with a recombinant disulfide-bridged dimeric form of OspC (D-OspC) stimulates increased antibody responses relative to immunization with commonly employed monomeric OspC. Here, we report that mice immunized with dimeric OspC proteins also exhibited enhanced protection against infection with the cognate B. burgdorferi strain. Mice were protected by four immunizations containing as little as 100 ng of dimeric OspC, suggesting that this form of the protein can induce protective immunity within a dose range reasonable for a human or veterinary vaccine. In contrast, monomeric OspC was only partially protective at much higher doses. IgG subclass analysis revealed that D-OspC-immunized animals mainly possessed anti-OspC-IgG1. In contrast, infected animals develop anti-OspC restricted to the IgG3 isotype. A subset of antibodies generated by dimeric OspC immunization did not recognize the monomeric variant, indicating that unique epitopes exist on the dimeric form. Moreover, monoclonal antibodies that recognized only dimeric OspC protected mice from B. burgdorferi challenge, whereas another monoclonal that recognized both immunogens was not protective. These studies suggest that this dimeric OspC presents distinctive epitopes that generate antibodies protective against B. burgdorferi infection and could be a useful vaccine component.

Antibody profiling of canine IgG responses to the OspC protein of the Lyme disease spirochetes supports a multivalent approach in vaccine and diagnostic assay development

OspC performs essential functions during the enzootic cycle of the Lyme disease (LD) spirochetes. In this study, the specificity of antibody (Ab) responses to OspC was profiled to define the antigenic determinants during infection and after vaccination. Several OspC variants or 'types' were screened with serum from SNAP4Dx C6 positive dogs and with serum from rabbits hyperimmunized with OspC proteins. The OspC type-specific nature of the Ab response revealed that variable domains of OspC are immunodominant during infection and upon vaccination. To assess the potential of OspC to elicit Ab in the context of a bacterin vaccine, OspC production in strains cultivated in vitro was assessed. Immunoblot and indirect immunofluorescent antibody analyses demonstrated that production is low and that only a subset of cells actively produces OspC in vitro, raising questions about the potential of bacterin vaccines to stimulate significant anti-OspC Ab responses. The specificity of the OspC Ab response in experimentally infected mice over time was assessed to determine if domains shielded in the OspC homodimer become accessible and stimulate Ab production as infection progresses. The results demonstrate that the OspC Ab response remains focused on surface exposed variable regions of the protein throughout infection. In contrast to some earlier studies, it is concluded that conserved domains of OspC, including the C7 or C10 domain, do not elicit significant Ab responses during infection or upon vaccination. Collectively, the results indicate that OspC diversity must be considered in vaccine design and in the interpretation of diagnostic assays that employ OspC as a diagnostic antigen.

In Vivo Imaging Demonstrates That Borrelia burgdorferi ospC Is Uniquely Expressed Temporally and Spatially throughout Experimental Infection

Borrelia burgdorferi is a spirochetal bacterium transmitted by the Ixodes tick that causes Lyme disease in humans due to its ability to evade the host immune response and disseminate to multiple immunoprotective tissues. The pathogen undergoes dynamic genetic alterations important for adaptation from the tick vector to the mammalian host, but little is known regarding the changes at the transcriptional level within the distal tissues they colonize. In this study, B. burgdorferi infection and gene expression of the essential virulence determinant ospC was quantitatively monitored in a spatial and temporal manner utilizing reporter bioluminescent borrelial strains with in vivo and ex vivo imaging. Although expressed from a shuttle vector, the PospC-luc construct exhibited a similar expression pattern relative to native ospC. Bacterial burden in skin, inguinal lymph node, heart, bladder and tibiotarsal joint varied between tissues and fluctuated over the course of infection possibly in response to unique cues of each microenvironment. Expression of ospC, when normalized for changes in bacterial load, presented unique profiles in murine tissues at different time points. The inguinal lymph node was infected with a significant B. burgdorferi burden, but showed minimal ospC expression. B. burgdorferi infected skin and heart induced expression of ospC early during infection while the bladder and tibiotarsal joint continued to display PospC driven luminescence throughout the 21 day time course. Localized skin borrelial burden increased dramatically in the first 96 hours following inoculation, which was not paralleled with an increase in ospC expression, despite the requirement of ospC for dermal colonization. Quantitation of bioluminescence representing ospC expression in individual tissues was validated by qRT-PCR of the native ospC transcript. Taken together, the temporal regulation of ospC expression in distal tissues suggests a role for this virulence determinant beyond early infection.

First arrived takes all: inhibitory priority effects dominate competition between co-infecting Borrelia burgdorferi strains

Background

Within-host microbial communities and interactions among microbes are increasingly recognized as important factors influencing host health and pathogen transmission. The microbial community associated with a host is indeed influenced by a complex network of direct and indirect interactions between the host and the lineages of microbes it harbors, but the mechanisms are rarely established. We investigated the within-host interactions among strains of Borrelia burgdorferi, the causative agent of Lyme disease, using experimental infections in mice. We used a fully crossed-design with three distinct strains, each group of hosts receiving two sequential inoculations. We used data from these experimental infections to assess the effect of coinfection on bacterial dissemination and fitness (by measuring the transmission of bacteria to xenodiagnostic ticks) as well as the effect of coinfection on host immune response compared to single infection.

Results

The infection and transmission data strongly indicate a competitive interaction among B. burgdorferi strains within a host in which the order of appearance of the strain is the main determinant of the competitive outcome. This pattern is well described by the classic priority effect in the ecological literature. In all cases, the primary strain a mouse was infected with had an absolute fitness advantage primarily since it was transmitted an order of magnitude more than the secondary strain. The mechanism of exclusion of the secondary strain is an inhibition of the colonization of mouse tissues, even though 29% of mice showed some evidence of infection by secondary strain. Contrary to expectation, the strong and specific adaptive immune response evoked against the primary strain was not followed by production of immunoglobulins after the inoculation of the secondary strain, neither against strain-specific antigen nor against antigens common to all strains. Hence, the data do not support a major role of the immune response in the observed priority effect.

Conclusion

The strong inhibitory priority effect is a dominant mechanism underlying competition for transmission between coinfecting B. burgdorferi strains, most likely through resource exploitation. The observed priority effect could shape bacterial diversity in nature, with consequences in epidemiology and evolution of the disease.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-015-0381-0) contains supplementary material, which is available to authorized users.