The Treponema denticola FhbB Protein Is a Dominant Early Antigen That Elicits FhbB Variant-Specific Antibodies That Block Factor H Binding and Cleavage by Dentilisin

The leptospiral OmpA-like protein (Loa22) is a surface-exposed antigen that elicits bactericidal antibody against heterologous Leptospira

Leptospirosis is the most widespread zoonosis, affecting over 1 million humans each year, with more than 60,000 deaths worldwide. Leptospirosis poses a significant health threat to dogs, horses, cattle, and wildlife. The disease may be self-limiting or progress to a life-threatening multi-system disorder affecting the kidneys, liver, and lungs. Currently, bacterin vaccine formulations that consist of one or more laboratory-cultivated strains are used for prevention. However, the antibody response elicited by these vaccines is directed primarily at lipopolysaccharide and is generally serovar-specific. The development of broadly protective subunit vaccines for veterinary and human applications would be a significant step forward in efforts to combat this emerging and antigenically variable pathogen. This study assessed the properties and potential utility of the Leptospira Loa22 (Leptospira OmpA-like 22 kDa protein) protein as a vaccine antigen. Loa22 is a virulence factor that is predicted to transverse the outer membrane and present its N-terminal domain on the cell surface. This report demonstrates that diverse Leptospira strains express Loa22 in vitro and that the protein is antigenic during infection in dogs. Immunoblot and size exclusion chromatography revealed that Loa22 exists in monomeric and trimeric forms. Immunization of rats with recombinant Loa22 elicited bactericidal antibodies against diverse Leptospira strains. The immunodominant bactericidal epitopes were localized within the N-terminal domain using protein-blocking bactericidal assays. This study supports the utility of Loa22, or subfragments thereof, in developing a multivalent chimeric subunit vaccine to prevent leptospirosis and sheds new light on the cellular localization of Loa22.

ErpY-like Lipoprotein of Leptospira Outsmarts Host Complement Regulation by Acquiring Complement Regulators, Activating Alternative Pathways, and Intervening in the Membrane Attack Complex

The survival of pathogenic Leptospira in the host depends on its proficiency to circumvent the immune response. These pathogens evade the complement system in serum by enticing and amassing the serum complement regulators onto their surface. ErpY-like lipoprotein, a surface-exposed protein of Leptospira spp., is conserved in the pathogenic Leptospira serovars. The recombinant form of this protein interacts with multiple extracellular matrix (ECM) components and serum proteins such as soluble complement regulators factor H (FH) and factor I (FI). Here, we document that the supplementation of rErpY-like protein (10 μg/mL) in human serum inhibits complement-mediated bacterial cell lysis and augments the viability of Escherichia coli and saprophytic Leptospira biflexa by more than two-fold. Complement regulators FH and FI, when bound to rErpY-like protein, preserve their respective cofactor and protease activity and cleave the complement component C3b. The supplementation of rErpY-like protein (40 μg/mL) in serum ensued in an ∼90% reduction of membrane attack complex (C5b-9/MAC) deposition through the alternative pathway (AP) of complement activation. However, rErpY-like protein could moderately reduce (∼16%) MAC deposition in serum through the classical pathway (CP). In addition, the rErpY-like protein solely initiated the AP, suggesting its role in the rapid consumption and depletion of the complement components. Blocking the pathogenic Leptospira interrogans surface with anti-rErpY-like antibodies resulted in an increase in MAC formation on the bacterial surface, indicating a specific role of the ErpY-like lipoprotein in complement-mediated immune evasion. This study underscores the role of the ErpY-like lipoprotein of Leptospira in complement evasion.

Potential Treponema denticola-based periodontal vaccine to resolve a global public health challenge: a narrative literature review

INTRODUCTION

Periodontitis is a diseased condition of the gum which imposes considerable costs on healthcare systems. It progresses further beyond the inflammation of supportive tissues of the teeth, and the collateral damages are closely associated with Alzheimer's disease, cardiovascular disease, and diabetes mellitus.

AREAS COVERED

A comprehensive literature review was performed to summarize published studies in English during the period of 1990-2021 to discuss the rationales for developing periodontal vaccine, cost-effectiveness analyses on the prevention of periodontitis, Treponema denticola-based vaccine candidates, as well as immunological mechanisms in animal models.

EXPERT OPINION

Preventive strategies against periodontitis may halt the onset of gum inflammation itself and the consequent chronic diseases. Considering the multi-microbial condition of periodontitis, an ideal periodontal vaccine should target multiple pathological pathways. Preventive approaches compared to surgical treatments evidently have significant impact on the healthcare budget and long-term health of the individuals in different communities. Despite many advances in periodontal vaccine research, there are still significant hurdles to overcome in developing a vaccine. Investment in research and development activities on key periodontal pathogens including Treponema denticola and Porphyromonas gingivalis in the foreseeable future is a worthy and cost-effective approach for the policymakers to prevent deleterious impacts of periodontitis.

The role of properdin and Factor H in disease

The complement system consists of three pathways (alternative, classical, and lectin) that play a fundamental role in immunity and homeostasis. The multifunctional role of the complement system includes direct lysis of pathogens, tagging pathogens for phagocytosis, promotion of inflammatory responses to control infection, regulation of adaptive cellular immune responses, and removal of apoptotic/dead cells and immune complexes from circulation. A tight regulation of the complement system is essential to avoid unwanted complement-mediated damage to the host. This regulation is ensured by a set of proteins called complement regulatory proteins. Deficiencies or malfunction of these regulatory proteins may lead to pro-thrombotic hematological diseases, renal and ocular diseases, and autoimmune diseases, among others. This review focuses on the importance of two complement regulatory proteins of the alternative pathway, Factor H and properdin, and their role in human diseases with an emphasis on: (a) characterizing the main mechanism of action of Factor H and properdin in regulating the complement system and protecting the host from complement-mediated attack, (b) describing the dysregulation of the alternative pathway as a result of deficiencies, or mutations, in Factor H and properdin, (c) outlining the clinical findings, management and treatment of diseases associated with mutations and deficiencies in Factor H, and (d) defining the unwanted and inadequate functioning of properdin in disease, through a discussion of various experimental research findings utilizing in vitro, mouse and human models.

The Borrelia burgdorferi Adenylate Cyclase, CyaB, Is Important for Virulence Factor Production and Mammalian Infection

Borrelia burgdorferi, the causative agent of Lyme disease, traverses through vastly distinct environments between the tick vector and the multiple phases of the mammalian infection that requires genetic adaptation for the progression of pathogenesis. Borrelial gene expression is highly responsive to changes in specific environmental signals that initiate the RpoS regulon for mammalian adaptation, but the mechanism(s) for direct detection of environmental cues has yet to be identified. Secondary messenger cyclic adenosine monophosphate (cAMP) produced by adenylate cyclase is responsive to environmental signals, such as carbon source and pH, in many bacterial pathogens to promote virulence by altering gene regulation. B. burgdorferi encodes a single non-toxin class IV adenylate cyclase (bb0723, cyaB). This study investigates cyaB expression along with its influence on borrelial virulence regulation and mammalian infectivity. Expression of cyaB was specifically induced with co-incubation of mammalian host cells that was not observed with cultivated tick cells suggesting that cyaB expression is influenced by cellular factor(s) unique to mammalian cell lines. The 3′ end of cyaB also encodes a small RNA, SR0623, in the same orientation that overlaps with bb0722. The differential processing of cyaB and SR0623 transcripts may alter the ability to influence function in the form of virulence determinant regulation and infectivity. Two independent cyaB deletion B31 strains were generated in 5A4-NP1 and ML23 backgrounds and complemented with the cyaB ORF alone that truncates SR0623, cyaB with intact SR0623, or cyaB with a mutagenized full-length SR0623 to evaluate the influence on transcriptional and posttranscriptional regulation of borrelial virulence factors and infectivity. In the absence of cyaB, the expression and production of ospC was significantly reduced, while the protein levels for BosR and DbpA were substantially lower than parental strains. Infectivity studies with both independent cyaB mutants demonstrated an attenuated phenotype with reduced colonization of tissues during early disseminated infection. This work suggests that B. burgdorferi utilizes cyaB and potentially cAMP as a regulatory pathway to modulate borrelial gene expression and protein production to promote borrelial virulence and dissemination in the mammalian host.

The Leptospiral General Secretory Protein D (GspD), a secretin, elicits complement-independent bactericidal antibody against diverse Leptospira species and serovars

Leptospirosis, the most common zoonotic infection worldwide, is a multi-system disorder affecting the kidney, liver, and lungs. Infections can be asymptomatic, self-limiting or progress to multi-organ system failure and pulmonary hemorrhage. The incidence of canine and human leptospirosis is steadily increasing worldwide. At least sixty-four Leptospira species and several hundred lipopolysaccharide-based serovars have been defined. Preventive vaccines are available for use in veterinary medicine and limited use in humans in some countries. All commercially available vaccines are bacterin formulations that consist of a combination of laboratory cultivated strains of different lipopolysaccharide serotypes. The development of a broadly protective subunit vaccine would represent a significant step forward in efforts to combat leptospirosis in humans, livestock, and companion animals worldwide. Here we investigate the potential of General secretory protein D (GspD; LIC11570), a secretin, to serve as a possible antigen in a multi-valent vaccine formulation. GspD is conserved, expressed in vitro, antigenic during infection and elicits antibody with complement independent bactericidal activity. Importantly, antibody to GspD is bactericidal against diverse Leptospira species of the P1 subclade. Epitope mapping localized the bactericidal epitopes to the N-terminal N0 domain of GspD. The data within support further exploration of GspD as a candidate for inclusion in a next generation multi-protein subunit vaccine.

Hijacking Factor H for Complement Immune Evasion

The complement system is an essential player in innate and adaptive immunity. It consists of three pathways (alternative, classical, and lectin) that initiate either spontaneously (alternative) or in response to danger (all pathways). Complement leads to numerous outcomes detrimental to invaders, including direct killing by formation of the pore-forming membrane attack complex, recruitment of immune cells to sites of invasion, facilitation of phagocytosis, and enhancement of cellular immune responses. Pathogens must overcome the complement system to survive in the host. A common strategy used by pathogens to evade complement is hijacking host complement regulators. Complement regulators prevent attack of host cells and include a collection of membrane-bound and fluid phase proteins. Factor H (FH), a fluid phase complement regulatory protein, controls the alternative pathway (AP) both in the fluid phase of the human body and on cell surfaces. In order to prevent complement activation and amplification on host cells and tissues, FH recognizes host cell-specific polyanionic markers in combination with complement C3 fragments. FH suppresses AP complement-mediated attack by accelerating decay of convertases and by helping to inactivate C3 fragments on host cells. Pathogens, most of which do not have polyanionic markers, are not recognized by FH. Numerous pathogens, including certain bacteria, viruses, protozoa, helminths, and fungi, can recruit FH to protect themselves against host-mediated complement attack, using either specific receptors and/or molecular mimicry to appear more like a host cell. This review will explore pathogen complement evasion mechanisms involving FH recruitment with an emphasis on: (a) characterizing the structural properties and expression patterns of pathogen FH binding proteins, as well as other strategies used by pathogens to capture FH; (b) classifying domains of FH important in pathogen interaction; and (c) discussing existing and potential treatment strategies that target FH interactions with pathogens. Overall, many pathogens use FH to avoid complement attack and appreciating the commonalities across these diverse microorganisms deepens the understanding of complement in microbiology.

Development of an FhbB based chimeric vaccinogen that elicits antibodies that block Factor H binding and cleavage by the periopathogen Treponema denticola

Treponema denticola is a proteolytic anaerobic spirochete and key contributor to periodontal disease of microbial etiology. As periodontal disease develops and progresses, T. denticola thrives in the hostile environment of the subgingival crevice by exploiting the negative regulatory activity of the complement protein, factor H (FH). FH bound to the cell surface receptor, FhbB (FH binding protein B), is competent to serve as a cofactor for the Factor I mediated-cleavage of the opsonin C3b. However, bound FH is ultimately cleaved by the T. denticola protease, dentilisin. As the T. denticola population expands, the rate of FH cleavage may exceed its rate of replenishment leading to local FH depletion and immune dysregulation culminating in tissue and ligament destruction and tooth loss. The goal of this study was to develop a T. denticola FhbB based-vaccine antigen that can block FH binding and cleavage and kill cells via antibody-mediated bactericidal activity. Tetra (FhbB-ch4) and pentavalent fhbB (FhbB-ch5) chimerics were engineered to have attenuated FH binding ability. The chimerics were immunogenic and elicited high-titer bactericidal and agglutinating antibody. Anti-Fhb-ch4 antisera blocked FH binding and cleavage by the T. denticola protease, dentilisin, in a dose dependent manner. Precedent for the use of FH binding proteins comes from the successful development of two FDA approved vaccines for type B Neiserria meningitidis. This study is the first to extend this approach to the development of a preventive or therapeutic vaccine (or monoclonal Ab) for periodontal disease.

Antibody profiling of a Borreliella burgdorferi (Lyme disease) C6 antibody positive, symptomatic Rottweiler and her pups

Lyme disease (LD) is a tick-transmitted disease caused by Borreliella burgdorferi (Bb). Temporal studies of maternal antibody (Ab) profiles in Bb infected pregnant dogs and their pups have not been conducted. In this study, Ab profiles of a client-owned Bb C6 Ab positive Rottweiler and her nine pups were assessed. The dam presented with lameness 12 days prior to parturition and was C6 Ab positive with a Quant C6 Ab concentration of 237U/mL. Treatment with amoxicillin was initiated and 11 days later nine pups were delivered. Screening of the sera from the dam and pups against Bb cell lysates and a panel of antigens revealed similar immunoreactivity profiles. While antigen-specific IgG and IgM reactivity persisted in the dam for at least 7 months, a rapid decline in IgG specific for BBA36, BBK53, BB0238, BBA73 and outer surface protein (Osp) E in the pups occurred between days 29 and 52 post-parturition. In contrast, Ab specific for DbpA and the diagnostic antigens VlsE (C6) and OspF, remained elevated in the pups. Sera from the dam displayed potent complement-dependent bactericidal activity against Bb. Sera from the pups was also bactericidal but primarily through a complement-independent mechanism. Lastly, single dose vaccination of the dam at day 51 post-parturition with a LD subunit vaccine consisting of OspA and an OspC chimeritope triggered a broad anti-OspC Ab response indicative of an anamnestic response. Although this study focused on a single case, these findings add to our knowledge of maternal Ab profiles and will aid the interpretation of serological assays in pups delivered by a Bb C6 Ab positive dog.

Glutathione catabolism by Treponema denticola impacts its pathogenic potential

Treponema denticola is a spirochete that is etiologic for periodontal diseases. This bacterium is one of two periodontal pathogens that have been shown to have a complete three step enzymatic pathway (GTSP) that catabolizes glutathione to H2S. This pathway may contribute to the tissue pathology seen in periodontitis since diseased periodontal pockets have lower glutathione levels than healthy sites with a concomitant increase in H2S concentration. In order to be able to demonstrate that glutathione catabolism by the GTSP is critical for the pathogenic potential of T. denticola, allelic replacement mutagenesis was used to make a deletion mutant (Δggt) in the gene encoding the first enzyme in the GTSP. The mutant cannot produce H2S from glutathione since it lacks gamma-glutamyltransferase (GGT) activity. The hemolytic and hemoxidation activities of wild type T. denticola plus glutathione are reduced to background levels with the Δggt mutant and the mutant has lost the ability to grow aerobically when incubated with glutathione. The Δggt bacteria with glutathione cause less cell death in human gingival fibroblasts (hGFs) in vitro than do wild type T. denticola and the levels of hGF death correlate with the amounts of H2S produced. Importantly, the mutant spirochetes plus glutathione make significantly smaller lesions than wild type bacteria plus glutathione in a mouse back lesion model that assesses soft tissue destruction, a major symptom of periodontal diseases. Our results are the first to prove that T. denticola thiol-compound catabolism by its gamma-glutamyltransferase can play a significant role in the in the types of host tissue damage seen in periodontitis.

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.

The major outer sheath protein forms distinct conformers and multimeric complexes in the outer membrane and periplasm of Treponema denticola

The major outer sheath protein (MOSP) is a prominent constituent of the cell envelope of Treponema denticola (TDE) and one of its principal virulence determinants. Bioinformatics predicts that MOSP consists of N- and C-terminal domains, MOSP^N and MOSP^C. Biophysical analysis of constructs refolded in vitro demonstrated that MOSP^C, previously shown to possess porin activity, forms amphiphilic trimers, while MOSP^N forms an extended hydrophilic monomer. In TDE and E. coli expressing MOSP with a PelB signal sequence (PelB-MOSP), MOSP^C is OM-embedded and surface-exposed, while MOSP^N resides in the periplasm. Immunofluorescence assay, surface proteolysis, and novel cell fractionation schemes revealed that MOSP in TDE exists as outer membrane (OM) and periplasmic trimeric conformers; PelB-MOSP, in contrast, formed only OM-MOSP trimers. Although both conformers form hetero-oligomeric complexes in TDE, only OM-MOSP associates with dentilisin. Mass spectrometry (MS) indicated that OM-MOSP interacts with proteins in addition to dentilisin, most notably, oligopeptide-binding proteins (OBPs) and the β-barrel of BamA. MS also identified candidate partners for periplasmic MOSP, including TDE1658, a spirochete-specific SurA/PrsA ortholog. Collectively, our data suggest that MOSP destined for the TDE OM follows the canonical BAM pathway, while formation of a stable periplasmic conformer involves an export-related, folding pathway not present in E. coli.

Identification of a defined linear epitope in the OspA protein of the Lyme disease spirochetes that elicits bactericidal antibody responses: Implications for vaccine development

The lipoprotein OspA is produced by the Lyme disease spirochetes primarily in unfed ticks. OspA production is down-regulated by the blood meal and it is not produced in mammals except for possible transient production during late stage infection in patients with Lyme arthritis. Vaccination with OspA elicits antibody (Ab) that can target spirochetes in the tick midgut during feeding and inhibit transmission to mammals. OspA was the primary component of the human LYMErix^™ vaccine. LYMErix^™ was available from 1998 to 2002 but then pulled from the market due to declining sales as a result of unsubstantiated concerns about vaccination induced adverse events and poor efficacy. It was postulated that a segment of OspA that shares sequence similarity with a region in human LFA-1 and may trigger putative autoimmune events. While evidence supporting such a link has not been demonstrated, most efforts to move forward with OspA as a vaccine component have sought to eliminate this region of concern. Here we identify an OspA linear epitope localized within OspA amino acid residues 221–240 (OspA_221–240) that lacks the OspA region suggested to elicit autoimmunity. A peptide consisting of residues 221–240 was immunogenic in mice. Ab raised against OspA_221–240 peptide surface labeled B. burgdorferi in IFAs and displayed potent Ab mediated-complement dependent bactericidal activity. BLAST analyses identified several variants of OspA_221–240 and a closely related sequence in OspB. It is our hypothesis that integration of the OspA_221–240 epitope into a multivalent-OspC based chimeric epitope based vaccine antigen (chimeritope) could result in a subunit vaccine that protects against Lyme disease through synergistic mechanisms.

Gene Regulation, Two Component Regulatory Systems, and Adaptive Responses in Treponema Denticola

The oral microbiome consists of a remarkably diverse group of 500-700 bacterial species. The microbial etiology of periodontal disease is similarly complex. Of the ~400 bacterial species identified in subgingival plaque, at least 50 belong to the genus Treponema. As periodontal disease develops and progresses, T. denticola transitions from a low to high abundance species in the subgingival crevice. Changes in the overall composition of the bacterial population trigger significant changes in the local physical, immunological and physiochemical conditions. For T. denticola to thrive in periodontal pockets, it must be nimble and adapt to rapidly changing environmental conditions. The purpose of this chapter is to review the current understanding of the molecular basis of these essential adaptive responses, with a focus on the role of two component regulatory systems with global regulatory potential.

Complement Immune Evasion by Spirochetes

The complement system plays an important role in the innate and acquired immune response against pathogens. A sophisticated network of activating and regulating proteins allows the distinction between intact and damaged host and non-host surfaces such as bacteria and other parasites. Non-host structures trigger the alternative pathway which may lead to their elimination by phagocytosis or cell lysis. In addition, complement proteins such as C1q, mannose binding lectin (MBL), and ficolins act as pathogen pattern-recognition molecules. Biological functions such as opsonization, activation of B lymphocytes and production of antibodies, degranulation of mast cells and basophils, and cell lysis that are important for elimination of microorganisms are dependent on complement activation. However, several pathogens including spirochetes have developed several specialized mechanisms to evade the complement system, thereby contributing to survival in the host. In this review, we give a brief overview of complement activation and regulation, and discuss in detail the strategies used by spirochetes from the genera Borrelia, Leptospira, and Treponema to overcome complement activation.