Recombinant bacterial lipoproteins as vaccine candidates

Combinations but Not a Single PlpE Epitope Induces Host Protective Immunity against Pasteurella multocida

Pasteurella multocida is the causative agent of a wide range of diseases (pasteurellosis) and a zoonotic pathogen in humans. Recombinant subunit vaccines are hot spots in recent pasteurellosis vaccine development. A chimeric vaccine is also constructed for rabbit hemorrhagic disease virus (RHDV) protective antigen VP60 chimeric with fragments of Pasteurella multocida protective antigen PlpE. The protective efficacy of the chimeric vaccine against P. multocida is not as high as that of PlpE, and the reason is not well known. In this study, we analyzed the linear B-cell epitopes of PlpE and then assessed the protective efficacy of these epitopes and their combinations. It was found that the immunodominant region of PlpE was mainly located in the region between the 21st to the 185th amino acids from the N terminus. Overlapping peptide scanning results demonstrated that this region contained six nonoverlapping epitopes, and epitope E was the predominant epitope. Chimeric protein antigens were constructed of single nonoverlapping PlpE epitopes or their combinations chimeric with the RHDV VP60 P domain. Immunization with recombinant antigen chimeric with a single PlpE epitope exhibited poor immunoprotection, whereas immunization with recombinant antigen chimeric with PlpE epitope combinations (epitopes A and E; epitopes C and E; epitopes A, C, and E; and epitopes B, D, and F) exhibited significant immunoprotection. In a word, P. multocida protective antigen PlpE contained six nonoverlapping linear B-cell epitopes, and combinations but not a single epitope induced host protective immunity. Our work will give help for future chimeric vaccine design.

Signal sequence contributes to the immunogenicity of Pasteurella multocida lipoprotein E

Recombinant Pasterurella multocida lipoprotein E (PlpE) has been shown to protect against fowl cholera. This study aimed to determine if the signal sequence may contribute to the antigenicity and protective efficacy of recombinant PlpE. A small antigenic domain of PlpE (termed truncated PlpE, tPlpE) was constructed with (SP-tPlpE) or without (tPlpE) the signal sequence and evaluated in vitro and in vivo. In vitro, the HEK-Bule hTLR2 Cells were used to evaluate the activation of NF-kB in the test associated with the stimulation of the SP-tPlpE and tPlpE proteins. When chickens were immunized, compared to the tPlpE vaccine group, the SP-tPlpE group showed higher antibody levels and enhanced CD4⁺ T cell response. In a challenge test, the SP-tPlpE group showed a survival rate of 87.5% (n = 8), compared to 25% for the tPlpE group. It is confirmed that the inclusion of the native signal sequence enhanced protective efficacy against fowl cholera and may act as a vaccine adjuvant. The short SP-tPlpE construct is amenable to further vaccine engineering and has potential to be developed as a fowl cholera vaccine.

Lipidation of Haemophilus influenzae Antigens P6 and OMP26 Improves Immunogenicity and Protection against Nasopharyngeal Colonization and Ear Infection

Nontypeable Haemophilus influenzae (NTHi) causes respiratory infections that lead to high morbidity and mortality worldwide, encouraging development of effective vaccines. To achieve a protective impact on nasopharyngeal (NP) colonization by NTHi, enhanced immunogenicity beyond that achievable with recombinant-protein antigens is likely to be necessary. Adding a lipid moiety to a recombinant protein would enhance immunogenicity through Toll-like receptor 2 signaling of antigen-presenting cells and Th17 cell response in the nasal-associated lymphoid tissue (NALT). We investigated effects of lipidation (L) of recombinant proteins P6 and OMP26 compared to nonlipidated (NL) P6 and OMP26 and as fusion constructs (L-OMP26ϕNL-P6 and L-P6ϕNL-OMP26) in a mouse model. After intraperitoneal or intranasal vaccination, antibody responses were compared and protection from NP colonization and middle ear infection were assessed. L-P6 and L-OMP26 induced approximately 10- to 100-fold-higher IgG antibody levels than NL-P6 and NL-OMP26. Fusion constructs significantly increased IgG antibody to both target proteins, even though only one of the proteins was lipidated. NP colonization and middle ear bullae NTHi density was 1 to 4 logs lower following vaccination with L-P6 and L-OMP26 than with NL-P6 and NL-OMP26. Fusion constructs also resulted in a 1- to 3-log-lower NTHi density following vaccination. NALT cells from mice vaccinated with lipidated protein constructs had higher levels of interleukin-17 (IL-17), IL-22, and CD4⁺ T-cell memory. Passive transfer of sera from L-OMP26ϕNL-P6-vaccinated mice to recipient infant mice reduced NP colonization and ear bulla NTHi density. We conclude that L-P6, L-OMP26, and fusion constructs generate enhanced antibody responses and protection from NP colonization and middle ear infection by NTHi in mice.

Superior adjuvanticity of the genetically fused D1 domain of Neisseria meningitides Ag473 lipoprotein among three Toll-like receptor ligands

Toll-like receptor (TLR) ligands have emerged as the attractive adjuvant for subunit vaccines. However, selection of TLR ligands needs to be rationally chosen on the basis of antigen and adjuvant properties. In the present study, we expressed the Ag473 lipoprotein from Neisseria meningitides, flagellin FlaB from Vibrio vulnificus and heat shock protein 70 from Mycobacterium tuberculosis (mHsp70) in Escherichia coli as single proteins and fusion proteins with VP2 protein of infectious bursal disease virus (IBDV). Both cellular and humoral adjuvanticities of the three TLR ligands were compared by immunization of mice in two different ways. Among the three co-administered TLR ligands, recombinant Ag473 lipoprotein exhibited the highest cellular and humoral adjuvanticities, including promotion of IL-4, IL-12, IFN-γ and IBDV VP2-specific antibody production. Among the three genetically fused TLR ligands, fusion with Ag473 D1 domain exhibited the highest cellular and humoral adjuvanticities. Overall, the adjuvanticities of genetically fused TRL ligands were significantly higher than that of co-administered TLR ligands. Fusion with Ag473 D1 domain exhibited superior adjuvanticity among the three TLR ligands delivered in two different ways.

A novel gonorrhea vaccine composed of MetQ lipoprotein formulated with CpG shortens experimental murine infection

Bacterial surface lipoproteins are emerging as attractive vaccine candidates due to their biological importance and the feasibility of their large-scale production for vaccine manufacturing. The global prevalence of gonorrhea, resistance to antibiotics, and serious consequences to reproductive and neonatal health necessitate development of effective vaccines. Reverse vaccinology identified the surface-displayed L-methionine binding lipoprotein MetQ (NGO2139) and its homolog GNA1946 (NMB1946) as gonococcal and meningococcal vaccine candidates, respectively. Here, we assessed the suitability of MetQ for inclusion in a gonorrhea vaccine by examining MetQ conservation, its function inNeisseria gonorrhoeae (Ng) pathogenesis, and its ability to induce protective immune responses using a female murine model of lower genital tract infection. In-depth bioinformatics, phylogenetics and mapping the most prevalent Ng polymorphic amino acids to the GNA1946 crystal structure revealed remarkable MetQ conservation: ~97% Ng isolates worldwide possess a single MetQ variant. Mice immunized with rMetQ-CpG (n = 40), a vaccine containing a tag-free version of MetQ formulated with CpG, exhibited robust, antigen-specific antibody responses in serum and at the vaginal mucosae including IgA. Consistent with the activity of CpG as a Th1-stimulating adjuvant, the serum IgG1/IgG2a ratio of 0.38 suggested a Th1 bias. Combined data from two independent challenge experiments demonstrated that rMetQ-CpG immunized mice cleared infection faster than control animals (vehicle, p < 0.0001; CpG, p = 0.002) and had lower Ng burden (vehicle, p = 0.03; CpG, p < 0.0001). We conclude rMetQ-CpG induces a protective immune response that accelerates bacterial clearance from the murine lower genital tract and represents an attractive component of a gonorrhea subunit vaccine.

Lipoproteins in Gram-Positive Bacteria: Abundance, Function, Fitness

When one thinks of the Gram+ cell wall, the peptidoglycan (PG) scaffold in particular comes to mind. However, the cell wall also consists of many other components, for example those that are covalently linked to the PG: the wall teichoic acid and the cell wall proteins tethered by the sortase. In addition, there are completely different molecules that are anchored in the cytoplasmic membrane and span the cell wall. These are lipoteichoic acids and bacterial lipoproteins (Lpp). The latter are in the focus of this review. Lpp are present in almost all bacteria. They fulfill a wealth of different tasks. They represent the window to the outside world by recognizing nutrients and incorporating them into the bacterial cell via special transport systems. Furthermore, they perform very diverse and special tasks such as acting as chaperonin, as cyclomodulin, contributing to invasion of host cells or uptake of plasmids via conjugation. All these functions are taken over by the protein part. Nevertheless, the lipid part of the Lpp plays an as important role as the protein part. It is the released lipoproteins and derived lipopeptides that massively modulate our immune system and ultimately play an important role in immune tolerance or non-tolerance. All these varied activities of the Lpp are considered in this review article.

Genome-wide screening of lipoproteins in Actinobacillus pleuropneumoniae identifies three antigens that confer protection against virulent challenge

Actinobacillus pleuropneumoniae is an important veterinary pathogen that causes porcine pleuropneumonia. Lipoproteins of bacterial pathogens play pleiotropic roles in the infection process. In addition, many bacterial lipoproteins are antigenic and immunoprotective. Therefore, characterization of lipoproteins is a promising strategy for identification of novel vaccine candidates or diagnostic markers. We cloned 58 lipoproteins from A. pleuropneumoniae JL03 (serovar 3) and expressed them in Escherichia coli. Five proteins with strong positive signals in western blotting analysis were used to immunize mice. These proteins elicited significant antibody responses, and three of them (APJL_0922, APJL_1380 and APJL_1976) generated efficient immunoprotection in mice against lethal heterologous challenge with A. pleuropneumoniae 4074 (serovar 1), both in the active and passive immunization assays. Then immunogenicity of these three lipoproteins (APJL_0922, APJL_1380 and APJL_1976) were further tested in pigs. Results showed that these proteins elicited considerable humoral immune responses and effective protective immunity against virulent A. pleuropneumoniae challenge. Our findings suggest that these three novel lipoproteins could be potential subunit vaccine candidates.

Integrated Bioinformatic Analyses and Immune Characterization of New Neisseria gonorrhoeae Vaccine Antigens Expressed during Natural Mucosal Infection

There is an increasingly severe trend of antibiotic-resistant Neisseria gonorrhoeae strains worldwide and new therapeutic strategies are needed against this sexually-transmitted pathogen. Despite the urgency, progress towards a gonococcal vaccine has been slowed by a scarcity of suitable antigens, lack of correlates of protection in humans and limited animal models of infection. N. gonorrhoeae gene expression levels in the natural human host does not reflect expression in vitro, further complicating in vitro-basedvaccine analysis platforms. We designed a novel candidate antigen selection strategy (CASS), based on a reverse vaccinology-like approach coupled with bioinformatics. We utilized the CASS to mine gonococcal proteins expressed during human mucosal infection, reported in our previous studies, and focused on a large pool of hypothetical proteins as an untapped source of potential new antigens. Via two discovery and analysis phases (DAP), we identified 36 targets predicted to be immunogenic, membrane-associated proteins conserved in N. gonorrhoeae and suitable for recombinant expression. Six initial candidates were produced and used to immunize mice. Characterization of the immune responses indicated cross-reactive antibodies and serum bactericidal activity against different N. gonorrhoeae strains. These results support the CASS as a tool for the discovery of new vaccine candidates.

The Role of Lipoproteins in Mycoplasma-Mediated Immunomodulation

Mycoplasma infections, such as walking pneumonia or pelvic inflammatory diseases, are a major threat to public health. Despite their relatively small physical and genomic size, mycoplasmas are known to elicit strong host immune responses, generally inflammatory, while also being able to evade the immune system. The mycoplasma membrane is composed of approximately two-thirds protein and one-third lipid and contains several lipoproteins that are known to regulate host immune responses. Herein, the immunomodulatory effects of mycoplasma lipoproteins are reviewed. A better understanding of the immunomodulatory effects, both activating and evasive, of Mycoplasma surface lipoproteins will contribute to understanding mechanisms potentially relevant to mycoplasma disease vaccine development and treatment.

Efficient Uptake of Recombinant Lipidated Survivin by Antigen-Presenting Cells Initiates Antigen Cross-Presentation and Antitumor Immunity

Survivin is overexpressed in various types of human cancer, but rarely expressed in terminally differentiated adult tissues. Thus, survivin is a potential target antigen for a cancer vaccine. However, self-tumor-associated antigens are not highly immunogenic. Bacteria-derived lipoproteins can activate antigen-presenting cells through their toll-like receptors to enhance immune responses. In this context, lipidated survivin is an attractive candidate for cancer immunotherapy. In the present study, recombinant lipidated human survivin (LSur) was prepared from an Escherichia coli-based system. We investigated whether LSur is efficiently captured by antigen-presenting cells then facilitating effective induction of survivin cross-presentation and generation of immunity against cancer cells. Our results demonstrate that LSur, but not its non-lipidated counterpart, can activate mouse bone-marrow-derived-dendritic cells (BMDCs) to enhance cytokine (IL-6, TNF-α, and IL-12) secretion and costimulatory molecules (CD40, CD80, CD86, and MHC II) expression. However, the pathways involved in the capture of the recombinant lipidated antigen by antigen-presenting cells have not yet been elucidated. To this end, we employ various endocytosis inhibitors to study the effect on LSur internalization. We show that the internalization of LSur is suppressed by the inhibition of various routes of endocytosis. These results suggest that endocytosis of LSur by BMDCs can be mediated by multiple mechanisms. Furthermore, LSur is trafficked to the early endosome after internalization by BMDCs. These features of LSur are advantageous for cross-presentation and the induction of antitumor immunity. We demonstrate that immunization of C57BL/6 mice with LSur under treatment with exogenous adjuvant-free formulation induce survivin-specific CD8⁺ T-cell responses and suppress tumor growth. The antitumor responses are mediated by CD8⁺ cells. Our findings indicate that LSur is a potential candidate for stimulating protective antitumor immunity. This study suggests that lipidated tumor antigens may be a promising approach for raising a robust antitumor response in cancer immunotherapy.

Development of an FHbp-CTB holotoxin-like chimera and the elicitation of bactericidal antibodies against serogroup B Neisseria meningitidis

The Neisseria meningitidis factor H binding protein (FHbp) is an important virulence factor and vaccine antigen contained in both USA licensed serogroup B meningococcal vaccines. Recent studies in human factor H (hFH) transgenic mice suggest that hFH-FHbp interactions lower FHbp-elicited immunogenicity. To provide tools with which to characterize and potentially improve FHbp immunogenicity, we developed an FHbp-cholera holotoxin-like chimera vaccine expression system in Escherichia coli that utilizes cholera toxin B (CTB) as both a scaffold and adjuvant for FHbp. We developed FHbp-CTB chimeras using a wild-type (WT) FHbp and a low hFH-binding FHbp mutant R41S. Both chimeras bound to G_M1 ganglioside and were recognized by the FHbp-specific monoclonal antibody JAR4. The R41S mutant had greatly reduced hFH binding compared to the WT FHbp-CTB chimera. WT and R41S FHbp-CTB chimeric antigens were compared to equimolar amounts of FHbp admixed with CTB or FHbp alone in mouse immunogenicity studies. The chimeras were significantly more immunogenic than FHbp alone or mixed with CTB, and elicited bactericidal antibodies against a panel of MenB isolates. This study demonstrates a unique and simple method for studying FHbp immunogenicity. The chimeric approach may facilitate studies of other protein-based antigens targeting pathogenic Neisseria and lay groundwork for the development of new protein based vaccines against meningococcal and gonococcal disease.

Spirochetal Lipoproteins and Immune Evasion

Spirochetes are a major threat to public health. However, the exact pathogenesis of spirochetal diseases remains unclear. Spirochetes express lipoproteins that often determine the cross talk between the host and spirochetes. Lipoproteins are pro-inflammatory, modulatory of immune responses, and enable the spirochetes to evade the immune system. In this article, we review the modulatory effects of spirochetal lipoproteins related to immune evasion. Understanding lipoprotein-induced immunomodulation will aid in elucidating innate pathogenesis processes and subsequent adaptive mechanisms potentially relevant to spirochetal disease vaccine development and treatment.

Biotechnology approaches to produce potent, self-adjuvanting antigen-adjuvant fusion protein subunit vaccines

Traditional vaccination approaches (e.g. live attenuated or killed microorganisms) are among the most effective means to prevent the spread of infectious diseases. These approaches, nevertheless, have failed to yield successful vaccines against many important pathogens. To overcome this problem, methods have been developed to identify microbial components, against which protective immune responses can be elicited. Subunit antigens identified by these approaches enable the production of defined vaccines, with improved safety profiles. However, they are generally poorly immunogenic, necessitating their administration with potent immunostimulatory adjuvants. Since few safe and effective adjuvants are currently used in vaccines approved for human use, with those available displaying poor potency, or an inability to stimulate the types of immune responses required for vaccines against specific diseases (e.g. cytotoxic lymphocytes (CTLs) to treat cancers), the development of new vaccines will be aided by the availability of characterized platforms of new adjuvants, improving our capacity to rationally select adjuvants for different applications. One such approach, involves the addition of microbial components (pathogen-associated molecular patterns; PAMPs), that can stimulate strong immune responses, into subunit vaccine formulations. The conjugation of PAMPs to subunit antigens provides a means to greatly increase vaccine potency, by targeting immunostimulation and antigen to the same antigen presenting cell. Thus, methods that enable the efficient, and inexpensive production of antigen-adjuvant fusions represent an exciting mean to improve immunity towards subunit antigens. Herein we review four protein-based adjuvants (flagellin, bacterial lipoproteins, the extra domain A of fibronectin (EDA), and heat shock proteins (Hsps)), which can be genetically fused to antigens to enable recombinant production of antigen-adjuvant fusion proteins, with a focus on their mechanisms of action, structural or sequence requirements for activity, sequence modifications to enhance their activity or simplify production, adverse effects, and examples of vaccines in preclinical or human clinical trials.