Structure-based design of chimeric antigens for multivalent protein vaccines

Semiautomated design and soluble expression of a chimeric antigen TbpAB01 from Glaesserella parasuis

Pathogenic bacterial membrane proteins (MPs) are a class of vaccine and antibiotic development targets with widespread clinical application. However, the inherent hydrophobicity of MPs poses a challenge to fold correctly in living cells. Herein, we present a comprehensive method to improve the soluble form of MP antigen by rationally designing multi-epitope chimeric antigen (ChA) and screening two classes of protein-assisting folding element. The study uses a homologous protein antigen as a functional scaffold to generate a ChA possessing four epitopes from transferrin-binding protein A of Glaesserella parasuis. Our engineered strain, which co-expresses P17 tagged-ChA and endogenous chaperones groEL-ES, yields a 0.346 g/L highly soluble ChA with the property of HPS-positive serum reaction. Moreover, the protein titer of ChA reaches 4.27 g/L with >90% soluble proportion in 5-L bioreactor, which is the highest titer reported so far. The results highlight a timely approach to design and improve the soluble expression of MP antigen in industrially viable applications.

Computational structure-based approach to study chimeric antigens using a new protein scaffold displaying foreign epitopes

The identification and recombinant production of functional antigens and/or epitopes of pathogens represent a crucial step for the development of an effective protein-based vaccine. Many vaccine targets are outer membrane proteins anchored into the lipidic bilayer through an extended hydrophobic portion making their recombinant production challenging. Moreover, only the extracellular loops, and not the hydrophobic regions, are naturally exposed to the immune system. In this work, the Domain 3 (D3) from Group B Streptococcus (GBS) pilus 2a backbone protein has been identified and engineered to be used as a scaffold for the display of extracellular loops of two Neisseria gonorrhoeae membrane proteins (PorB.1b and OpaB). A computational structure-based approach has been applied to the design of both the scaffold and the model antigens. Once identified the best D3 engineerable site, several different chimeric D3 displaying PorB.1b and OpaB extracellular loops were produced as soluble proteins. Each molecule has been characterized in terms of solubility, stability, and ability to correctly display the foreign epitope. This antigen dissection strategy allowed the identification of most immunogenic extracellular loops of both PorB.1b and OpaB gonococcal antigens. The crystal structure of chimeric D3 displaying PorB.1b immunodominant loop has been obtained confirming that the engineerization did not alter the predicted native structure of this epitope. Taken together, the reported data suggest that D3 is a novel protein scaffold for epitope insertion and display, and a valid alternative to the production of whole membrane protein antigens. Finally, this work describes a generalized computational structure-based approach for the identification, design, and dissection of epitopes in target antigens through chimeric proteins.

In Vitro Pre-Clinical Evaluation of a Gonococcal Trivalent Candidate Vaccine Identified by Transcriptomics

Gonorrhea, a sexually transmitted disease caused by Neisseria gonorrhoeae, poses a significant global public health threat. Infection in women can be asymptomatic and may result in severe reproductive complications. Escalating antibiotic resistance underscores the need for an effective vaccine. Approaches being explored include subunit vaccines and outer membrane vesicles (OMVs), but an ideal candidate remains elusive. Meningococcal OMV-based vaccines have been associated with reduced rates of gonorrhea in retrospective epidemiologic studies, and with accelerated gonococcal clearance in mouse vaginal colonization models. Cross-protection is attributed to shared antigens and possibly cross-reactive, bactericidal antibodies. Using a Candidate Antigen Selection Strategy (CASS) based on the gonococcal transcriptome during human mucosal infection, we identified new potential vaccine targets that, when used to immunize mice, induced the production of antibodies with bactericidal activity against N. gonorrhoeae strains. The current study determined antigen recognition by human sera from N. gonorrhoeae-infected subjects, evaluated their potential as a multi-antigen (combination) vaccine in mice and examined the impact of different adjuvants (Alum or Alum+MPLA) on functional antibody responses to N. gonorrhoeae. Our results indicated that a stronger Th1 immune response component induced by Alum+MPLA led to antibodies with improved bactericidal activity. In conclusion, a combination of CASS-derived antigens may be promising for developing effective gonococcal vaccines.

Meningococcal factor H-binding protein: implications for disease susceptibility, virulence, and vaccines

Neisseria meningitidis is a human-adapted pathogen that causes meningitis and sepsis worldwide. N. meningitidis factor H-binding protein (fHbp) provides a mechanism for immune evasion by binding human complement factor H (CFH) to protect it from complement-mediated killing. Here, we discuss features of fHbp which enable it to engage human CFH (hCFH), and the regulation of fHbp expression. Studies of host susceptibility and bacterial genome-wide association studies (GWAS) highlight the importance of the interaction between fHbp and CFH and other complement factors, such as CFHR3, on the development of invasive meningococcal disease (IMD). Understanding the basis of fHbp:CFH interactions has also informed the design of next-generation vaccines as fHbp is a protective antigen. Structure-informed refinement of fHbp vaccines will help to combat the threat posed by the meningococcus, and accelerate the elimination of IMD.

Pathogenic Neisseria Bind the Complement Protein CFHR5 via Outer Membrane Porins

Neisseria meningitidis and Neisseria gonorrhoeae are important human pathogens that have evolved to bind the major negative regulator of the complement system, complement factor H (CFH). However, little is known about the interaction of pathogens with CFH-related proteins (CFHRs) which are structurally similar to CFH but lack the main complement regulatory domains found in CFH. Insights into the role of CFHRs have been hampered by a lack of specific reagents. We generated a panel of CFHR-specific monoclonal antibodies and demonstrated that CFHR5 was bound by both pathogenic Neisseria spp. We showed that CFHR5 bound to PorB expressed by both pathogens in the presence of sialylated lipopolysaccharide and enhanced complement activation on the surface of N. gonorrhoeae. Our study furthered our understanding of the interactions of CFHRs with bacterial pathogens and revealed that CFHR5 bound the meningococcus and gonococcus via similar mechanisms.

Principles and practical applications of structure-based vaccine design

Viral proteins fold into a variety of structures as they perform their functions. Structure-based vaccine design aims to exploit knowledge of an antigen’s architecture to stabilize it in a vulnerable conformation. We summarize the general principles of structure-based vaccine design, with a focus on the major types of sequence modifications: proline, disulfide, cavity-filling, electrostatic and hydrogen-bond substitution, as well as domain deletion. We then review recent applications of these principles to vaccine-design efforts across five viral families: Coronaviridae, Orthomyxoviridae, Paramyxoviridae, Pneumoviridae, and Filoviridae. Outstanding challenges include continued application of proven design principles to pathogens of interest, as well as development of new strategies for those pathogens that resist traditional techniques.

Progress in the Development of Structure-Based Vaccines

The immune response elicited by vaccines against microorganisms makes it the most successful medical interventions against infectious diseases. Conventional vaccines have limitations in inducing immunity against many types of pathogenic microorganism. The genetic diversity of microorganisms, coupled with the high degree of sequence variability in antigenic proteins, presents a challenge to developing broadly effective conventional vaccines. Atomic-resolution structure determination is crucial for understanding antigenic protein function. Cryo-electron microscopy, nuclear magnetic resonance spectroscopy coupled with bioinformatics provide three-dimensional structure of the antigenic proteins and provide a wealth of information about the organization of individual atoms and their chemical makeup. The atomic detail information of proteins offers enormous potential to rationally engineer proteins to enhance their properties and act as effective immunogens to induce immunity. The observation that whole protein antigens are not necessarily essential for inducing immunity has led to the emergence "structural vaccinology." Structure-based vaccines are designed on the rationale that protective epitopes should be sufficient to induce immune responses and provide protection against pathogens. In 2013 we published a review on structure-based vaccines (Thomas and Luxon. Expert Rev Vaccines 12 1301-11, 2013). This review states the progress in development of structure-based vaccines since the first review.

Identification of a specific surface epitope of OmpC for Escherichia coli O157:H7 with protein topology facilitated affinity mass spectrometry

Abstract

The goal of this work was to identify the target protein and epitope of a previously reported Escherichia coli O157:H7 (ECO157)–specific monoclonal antibody (mAb) 2G12. mAb 2G12 has shown high specificity for the recovery and detection of ECO157. To achieve this goal, the target protein was first separated by two-dimensional gel electrophoresis (2-DE) and located by Western blot (WB). The protein spots were identified to be the outer membrane protein (Omp) C by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF–MS). After that, the target protein was purified by immunoaffinity chromatography (IAC) and subjected to in situ enzymatic cleavage of the vulnerable peptides. Eight eluted peptides of OmpC identified by liquid chromatography–tandem mass spectrometry (LC–MS/MS) were further mapped onto the homologous protein structure of E. coli OmpC (2IXX). The topology of OmpC showed that three peptides had extracellular loops. Epitope mapping with overlapping peptide library and sequence homology analysis revealed that the epitope consisted of a specific peptide, “LGVING,” and an adjacent conservative peptide, “TQTYNATRVGSLG.” Both peptides loop around the overall structure of the epitope. To test the availability of the epitope when ECO157 was grown under different osmolarity, pH, and nutrition levels, the binding efficacy of mAb 2G12 with ECO157 grown in these conditions was evaluated. Results further demonstrated the good stability of this epitope under potential stressful environmental conditions. In summary, this study revealed that mAb 2G12 targeted one specific and one conservative extracellular loop (peptide) of the OmpC present on ECO157, and the epitope was stable and accessible on ECO157 cells grown in different environment.

Key points

• OmpC is the target of a recently identified ECO157-specific mAb 2G12.

• Eight peptides were identified from the OmpC by using LC–MS/MS.

• The specificity of mAb 2G12 is mainly determined by the “LGVING” peptide.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00253-021-11511-8.

Stability Assessment of Four Chimeric Proteins for Human Chagas Disease Immunodiagnosis

The performance of an immunoassay relies on antigen-antibody interaction; hence, antigen chemical stability and structural integrity are paramount for an efficient assay. We conducted a functional, thermostability and long-term stability analysis of different chimeric antigens (IBMP), in order to assess effects of adverse conditions on four antigens employed in ELISA to diagnose Chagas disease. ELISA-based immunoassays have served as a model for biosensors development, as both assess molecular interactions. To evaluate thermostability, samples were heated and cooled to verify heat-induced denaturation reversibility. In relation to storage stability, the antigens were analyzed at 25 °C at different moments. Long-term stability tests were performed using eight sets of microplates sensitized. Antigens were structurally analyzed through circular dichroism (CD), dynamic light scattering, SDS-PAGE, and functionally evaluated by ELISA. Data suggest that IBMP antigens are stable, over adverse conditions and for over a year. Daily analysis revealed minor changes in the molecular structure. Functionally, IBMP-8.2 and IBMP-8.3 antigens showed reactivity towards anti-T. cruzi antibodies, even after 72 h at 25 °C. Long-term stability tests showed that all antigens were comparable to the control group and all antigens demonstrated stability for one year. Data suggest that the antigens maintained their function and structural characteristics even in adverse conditions, making them a sturdy and reliable candidate to be employed in future in vitro diagnostic tests applicable to different models of POC devices, such as modern biosensors in development.

Rational Vaccine Design in Times of Emerging Diseases: The Critical Choices of Immunological Correlates of Protection, Vaccine Antigen and Immunomodulation

Vaccines are the most effective medical intervention due to their continual success in preventing infections and improving mortality worldwide. Early vaccines were developed empirically however, rational design of vaccines can allow us to optimise their efficacy, by tailoring the immune response. Establishing the immune correlates of protection greatly informs the rational design of vaccines. This facilitates the selection of the best vaccine antigens and the most appropriate vaccine adjuvant to generate optimal memory immune T cell and B cell responses. This review outlines the range of vaccine types that are currently authorised and those under development. We outline the optimal immunological correlates of protection that can be targeted. Finally we review approaches to rational antigen selection and rational vaccine adjuvant design. Harnessing current knowledge on protective immune responses in combination with critical vaccine components is imperative to the prevention of future life-threatening diseases.

Advances in vaccine delivery systems against viral infectious diseases

Although vaccines are available for many infectious diseases, there are still unresolved infectious diseases that threaten global public health. In particular, the rapid spread of unpredictable, highly contagious viruses has recorded numerous infection cases and deaths, and has changed our lives socially or economically through social distancing and wearing masks. The pandemics of unpredictable, highly contagious viruses increase the ever-high social need for rapid vaccine development. Nanotechnologies may hold promise and expedite the development of vaccines against newly emerging infectious viruses. As potential nanoplatforms for delivering antigens to immune cells, delivery systems based on lipids, polymers, proteins, and inorganic nanomaterials have been studied. These nanoplatforms have been tested as a means to deliver vaccines not as a whole, but in the form of protein subunits or as DNA or mRNA sequences encoding the antigen proteins of viruses. This review covers the current status of nanomaterial-based delivery systems for viral antigens, with highlights on nanovaccines against recently emerging infectious viruses, such as severe acute respiratory syndrome coronavirus-2, Middle East respiratory syndrome coronavirus, and Zika virus.

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.

Comprehensive Bioinformatic Assessments of the Variability of Neisseria gonorrhoeae Vaccine Candidates

A protective vaccine is the only viable way to stop the spread of gonorrhea in the face of rising antibiotic resistance. However, the notorious phase and antigenic variation of Neisseria gonorrhoeae surface proteins remains one of the challenges in vaccine development. To facilitate vaccine advancement efforts, we carried out comprehensive bioinformatic analyses of sequence variation by comparing 34 gonorrhea antigen candidates among >5,000 clinical N. gonorrhoeae isolates deposited in the Neisseria PubMLST database. Eight protein antigens showed exceptional conservation by having a single allele variant distributed in >80% of isolates. An additional 18 vaccine candidates were represented by ≤3 alleles in >50% of N. gonorrhoeae isolates globally. Phylogenetic analyses highlighted closely related antigen variants and additionally showed that AniA and FetB were the closest between N. gonorrhoeae and N. meningitidis Up to 44% of N. meningitidis alleles for both antigens have premature stop codons, suggesting differential expression. Mapping polymorphisms to the available three-dimensional structures of 12 antigens revealed low-frequency surface polymorphisms. PorB and TbpB possessed numerous high-prevalence polymorphic sites. While TbpA was also highly variable, conserved loops were nonetheless identified. A high degree of sequence conservation, the distribution of a single antigen variant among N. gonorrhoeae strains globally, or low-frequency sequence polymorphisms in surface loops make ACP, AniA, BamA, BamE, MtrE, NspA, NGO0778, NGO1251, NGO1985, OpcA, PldA, Slam2, and ZnuD promising candidates for a gonorrhea vaccine. Finally, the commonly used N. gonorrhoeae FA1090 strain emerges as a vaccine prototype, as it carries antigen sequence types identical to the most broadly distributed antigen variants.IMPORTANCE Neisseria gonorrhoeae, the Gram-negative bacterium responsible for the sexually transmitted infection gonorrhea, is categorized as a high-priority pathogen for research and development efforts. N. gonorrhoeae's "superbug" status, its high morbidity, and the serious health impact associated with gonorrhea highlight the importance of vaccine development. One of the longstanding barriers to developing an effective vaccine against N. gonorrhoeae is the remarkable variability of surface-exposed antigens. In this report, we addressed this roadblock by applying extensive bioinformatic analyses to 34 gonorrhea antigen candidates among >5,000 clinical N. gonorrhoeae isolates. Our studies are important, as they reveal promising, conserved gonorrhea vaccine candidates and aid structural vaccinology. Moreover, these approaches are broadly applicable to other infectious diseases where surface antigen variability impedes successful vaccine design.

Broad vaccine protection against Neisseria meningitidis using factor H binding protein

Neisseria meningitidis, the causative agent of invasive meningococcal disease (IMD), is classified into different serogroups defined by their polysaccharide capsules. Meningococcal serogroups A, B, C, W, and Y are responsible for most IMD cases, with serogroup B (MenB) causing a substantial percentage of IMD cases in many regions. Vaccines using capsular polysaccharides conjugated to carrier proteins have been successfully developed for serogroups A, C, W, and Y. However, because the MenB capsular polysaccharide is poorly immunogenic, MenB vaccine development has focused on alternative antigens. The 2 currently available MenB vaccines (MenB-4C and MenB-FHbp) both include factor H binding protein (FHbp), a surface-exposed protein harboured by nearly all meningococcal isolates that is important for survival of the bacteria in human blood. MenB-4C contains a nonlipidated FHbp from subfamily B in addition to other antigens, including Neisserial Heparin Binding Antigen, Neisserial adhesin A, and outer membrane vesicles, whereas MenB-FHbp contains a lipidated FHbp from each subfamily (A and B). FHbp is highly immunogenic and a main target of bactericidal activity of antibodies elicited by both licensed MenB vaccines. FHbp is also an important vaccine component, in contrast to some other meningococcal antigens that may have limited cross-protection across strains, as FHbp-specific antibodies can provide broad cross-protection within each subfamily. Limited cross-protection between subfamilies necessitates the inclusion of FHbp variants from both subfamilies to achieve broad FHbp-based vaccine coverage. Additionally, immune responses to the lipidated form of FHbp have a superior cross-reactive profile to those elicited by the nonlipidated form. Taken together, the inclusion of lipidated FHbp variants from both FHbp subfamilies is expected to provide broad protection against the diverse disease-causing meningococcal strains expressing a wide range of FHbp sequence variants. This review describes the development of vaccines for MenB disease prevention, with a focus on the FHbp antigen.

Leveraging the wheat germ cell-free protein synthesis system to accelerate malaria vaccine development

Vaccines against infectious diseases have had great successes in the history of public health. Major breakthroughs have occurred in the development of vaccine-based interventions against viral and bacterial pathogens through the application of classical vaccine design strategies. In contrast the development of a malaria vaccine has been slow. Plasmodium falciparum malaria affects millions of people with nearly half of the world population at risk of infection. Decades of dedicated research has taught us that developing an effective vaccine will be time consuming, challenging, and expensive. Nevertheless, recent advancements such as the optimization of robust protein synthesis platforms, high-throughput immunoscreening approaches, reverse vaccinology, structural design of immunogens, lymphocyte repertoire sequencing, and the utilization of artificial intelligence, have renewed the prospects of an accelerated discovery of the key antigens in malaria. A deeper understanding of the major factors underlying the immunological and molecular mechanisms of malaria might provide a comprehensive approach to identifying novel and highly efficacious vaccines. In this review we discuss progress in novel antigen discoveries that leverage on the wheat germ cell-free protein synthesis system (WGCFS) to accelerate malaria vaccine development.

Immunization of mice with chimeric antigens displaying selected epitopes confers protection against intestinal colonization and renal damage caused by Shiga toxin-producing Escherichia coli

Shiga toxin-producing Escherichia coli (STEC) cause diarrhea and dysentery, which may progress to hemolytic uremic syndrome (HUS). Vaccination has been proposed as a preventive approach against STEC infection; however, there is no vaccine for humans and those used in animals reduce but do not eliminate the intestinal colonization of STEC. The OmpT, Cah and Hes proteins are widely distributed among clinical STEC strains and are recognized by serum IgG and IgA in patients with HUS. Here, we develop a vaccine formulation based on two chimeric antigens containing epitopes of OmpT, Cah and Hes proteins against STEC strains. Intramuscular and intranasal immunization of mice with these chimeric antigens elicited systemic and local long-lasting humoral responses. However, the class of antibodies generated was dependent on the adjuvant and the route of administration. Moreover, while intramuscular immunization with the combination of the chimeric antigens conferred protection against colonization by STEC O157:H7, the intranasal conferred protection against renal damage caused by STEC O91:H21. This preclinical study supports the potential use of this formulation based on recombinant chimeric proteins as a preventive strategy against STEC infections.

Designs of Antigen Structure and Composition for Improved Protein-Based Vaccine Efficacy

Today, vaccinologists have come to understand that the hallmark of any protective immune response is the antigen. However, it is not the whole antigen that dictates the immune response, but rather the various parts comprising the whole that are capable of influencing immunogenicity. Protein-based antigens hold particular importance within this structural approach to understanding immunity because, though different molecules can serve as antigens, only proteins are capable of inducing both cellular and humoral immunity. This fact, coupled with the versatility and customizability of proteins when considering vaccine design applications, makes protein-based vaccines (PBVs) one of today's most promising technologies for artificially inducing immunity. In this review, we follow the development of PBV technologies through time and discuss the antigen-specific receptors that are most critical to any immune response: pattern recognition receptors, B cell receptors, and T cell receptors. Knowledge of these receptors and their ligands has become exceptionally valuable in the field of vaccinology, where today it is possible to make drastic modifications to PBV structure, from primary to quaternary, in order to promote recognition of target epitopes, potentiate vaccine immunogenicity, and prevent antigen-associated complications. Additionally, these modifications have made it possible to control immune responses by modulating stability and targeting PBV to key immune cells. Consequently, careful consideration should be given to protein structure when designing PBVs in the future in order to potentiate PBV efficacy.

Structure-Based Vaccine Antigen Design

Enabled by new approaches for rapid identification and selection of human monoclonal antibodies, atomic-level structural information for viral surface proteins, and capacity for precision engineering of protein immunogens and self-assembling nanoparticles, a new era of antigen design and display options has evolved. While HIV-1 vaccine development has been a driving force behind these technologies and concepts, clinical proof-of-concept for structure-based vaccine design may first be achieved for respiratory syncytial virus (RSV), where conformation-dependent access to neutralization-sensitive epitopes on the fusion glycoprotein determines the capacity to induce potent neutralizing activity. Success with RSV has motivated structure-based stabilization of other class I viral fusion proteins for use as immunogens and demonstrated the importance of structural information for developing vaccines against other viral pathogens, particularly difficult targets that have resisted prior vaccine development efforts. Solving viral surface protein structures also supports rapid vaccine antigen design and application of platform manufacturing approaches for emerging pathogens.

Engineering Magnetosomes for High-Performance Cancer Vaccination

A novel cancer vaccine is developed by using Fe₃O₄ magnetic nanoclusters (MNCs) as the core and cancer cell membranes decorated with anti-CD205 as the cloak. Because of the superparamagnetism and magnetization of MNCs, it is first achieved for the magnetic retention of vaccine in the lymph nodes with a magnetic resonance imaging (MRI) guide, which opened the time window for antigen uptake by dendritic cells (DCs). Meanwhile, the camouflaged cancer cell membranes serve as a reservoir of various antigens, enabling subsequent multiantigenic response. Additionally, the decorated anti-CD205 direct more vaccine into CD8⁺ DCs, facilitating the major histocompatibility complex (MHC) I cross-presentation. These unique advantages together lead to a great proliferation of T cells with superior clonal diversity and cytotoxic activity. As a result, potent prophylactic and therapeutic effects with few abnormalities are observed on five different tumor models. Therefore, such a cancer-derived magnetosome with the integration of various recent nanotechnologies successfully demonstrates its promise for safe and high-performance cancer vaccination.

Protein engineering and particulate display of B-cell epitopes to facilitate development of novel vaccines

Induction of antigen-specific humoral immunity is a correlate of protection for many diseases and remains a primary vaccine goal. Pathogens can evade such responses by limiting epitope access, by diversifying surface residues, or by keeping antigens in metastable conformations. B cells can target diverse epitopes on an antigen, but only a subset of which produce functional antibodies. Structure-based immunogen engineering can help overcome these hurdles by using structural information for targeted induction of particular antibodies while improving the overall vaccine immunogenicity. This review will cover recent progress in vaccine design, specifically focusing on strategies to stabilize antigens for optimal B-cell epitope exposure, engineer synthetic B-cell epitopes to induce antibodies with specific features and enhancement of vaccine potency through antigen presentation on multivalent particles.

A Modular Vaccine Platform Combining Self-Assembled Peptide Cages and Immunogenic Peptides

Subunit vaccines use delivery platforms to present minimal antigenic components for immunization. The benefits of such systems include multivalency, self-adjuvanting properties, and more specific immune responses. Previously, the design, synthesis, and characterization of self-assembling peptide cages (SAGEs) have been reported. In these, de novo peptides are combined to make hubs that assemble into nanoparticles when mixed in aqueous solution. Here it is shown that SAGEs are nontoxic particles with potential as accessible synthetic peptide scaffolds for the delivery of immunogenic components. To this end, SAGEs functionalized with the model antigenic peptides tetanus toxoid632-651 and ovalbumin323-339 drive antigen-specific responses both in vitro and in vivo, eliciting both CD4+ T cell and B cell responses. Additionally, SAGEs functionalized with the antigenic peptide hemagglutinin518-526 from the influenza virus are also able to drive a CD8+ T cell response in vivo. This work demonstrates the potential of SAGEs to act as a modular scaffold for antigen delivery, capable of inducing and boosting specific and tailored immune responses.

An Overview of Neisseria meningitidis

Neisseria meningitidis (the meningococcus) is a member of the normal nasopharyngeal microbiome in healthy individuals, but can cause septicemia and meningitis in susceptible individuals. In this chapter we provide an overview of the disease caused by N. meningitidis and the schemes used to type the meningococcus. We also review the adhesions, virulence factors, and phase variable genes that enable it to successfully colonize the human host. Finally, we outline the history and current status of meningococcal vaccines and highlight the importance of continued molecular investigation of the epidemiology and the structural analysis of the antigens of this pathogen to aid future vaccine development.

Sequence Variation of Rare Outer Membrane Protein β-Barrel Domains in Clinical Strains Provides Insights into the Evolution of Treponema pallidum subsp. pallidum, the Syphilis Spirochete

In recent years, considerable progress has been made in topologically and functionally characterizing integral outer membrane proteins (OMPs) of Treponema pallidum subspecies pallidum, the syphilis spirochete, and identifying its surface-exposed β-barrel domains. Extracellular loops in OMPs of Gram-negative bacteria are known to be highly variable. We examined the sequence diversity of β-barrel-encoding regions of tprC, tprD, and bamA in 31 specimens from Cali, Colombia; San Francisco, California; and the Czech Republic and compared them to allelic variants in the 41 reference genomes in the NCBI database. To establish a phylogenetic framework, we used T. pallidum 0548 (tp0548) genotyping and tp0558 sequences to assign strains to the Nichols or SS14 clades. We found that (i) β-barrels in clinical strains could be grouped according to allelic variants in T. pallidum subsp. pallidum reference genomes; (ii) for all three OMP loci, clinical strains within the Nichols or SS14 clades often harbored β-barrel variants that differed from the Nichols and SS14 reference strains; and (iii) OMP variable regions often reside in predicted extracellular loops containing B-cell epitopes. On the basis of structural models, nonconservative amino acid substitutions in predicted transmembrane β-strands of T. pallidum repeat C (TprC) and TprD2 could give rise to functional differences in their porin channels. OMP profiles of some clinical strains were mosaics of different reference strains and did not correlate with results from enhanced molecular typing. Our observations suggest that human host selection pressures drive T. pallidum subsp. pallidum OMP diversity and that genetic exchange contributes to the evolutionary biology of T. pallidum subsp. pallidum They also set the stage for topology-based analysis of antibody responses to OMPs and help frame strategies for syphilis vaccine development.IMPORTANCE Despite recent progress characterizing outer membrane proteins (OMPs) of Treponema pallidum, little is known about how their surface-exposed, β-barrel-forming domains vary among strains circulating within high-risk populations. In this study, sequences for the β-barrel-encoding regions of three OMP loci, tprC, tprD, and bamA, in T. pallidum subsp. pallidum isolates from a large number of patient specimens from geographically disparate sites were examined. Structural models predict that sequence variation within β-barrel domains occurs predominantly within predicted extracellular loops. Amino acid substitutions in predicted transmembrane strands that could potentially affect porin channel function were also noted. Our findings suggest that selection pressures exerted within human populations drive T. pallidum subsp. pallidum OMP diversity and that recombination at OMP loci contributes to the evolutionary biology of syphilis spirochetes. These results also set the stage for topology-based analysis of antibody responses that promote clearance of T. pallidum subsp. pallidum and frame strategies for vaccine development based upon conserved OMP extracellular loops.