Meningococcal factor H binding protein as immune evasion factor and vaccine antigen

Bactericidal human monoclonal antibody 1B1 shows specificity for meningococcal factor H binding protein variant 2 and displaces human factor H

Thousands of disease cases and hundreds of deaths occur globally each year due to invasive meningococcal disease. Neisseria meningitidis serogroup B (MenB) is the leading cause of such disease in developed countries. Two vaccines, 4CMenB and MenB-fHbp, that protect against MenB are available and include one or two forms respectively of factor H binding protein (fHbp), a key protective antigen. Studies of circulating meningococci have identified over 1380 different fHbp amino acid sequences, which form three immunologically distinct clusters, termed variants 1, 2, and 3. Neither of the current vaccines contains a variant 2 antigen, which is less well characterized than fHbp variants 1 and 3. We characterized the interaction of fHbp variant 2 with humAb 1B1 using biochemical methods and live meningococcal assays. Further, we determined the crystal structure of the complex at 2.4 Å resolution, clearly revealing the epitope and providing the first detailed report of an antibody with distinct specificity for fHbp variant 2. Extensive mutagenesis and binding studies elucidated key hotspots in the interface. This combination of structural and functional studies provides a molecular explanation for the bactericidal potency and specificity of humAb 1B1 for fHbp variant 2. Our studies, focused on fHbp variant 2, expand the understanding of this previously under characterized group of the vast family of variants of fHbp, a virulence factor present on all meningococci. Moreover, the definition of a protective conformational epitope on fHbp variant 2 may support the design and development of novel variant 2-containing MenB vaccines affording greater breadth of protection.

Interaction of Neisseria meningitidis carrier and disease isolates of MenB cc32 and MenW cc22 with epithelial cells of the nasopharyngeal barrier

Neisseria meningitidis (Nm, the meningococcus) is considered an asymptomatic colonizer of the upper respiratory tract and a transient member of its microbiome. It is assumed that the spread of N. meningitidis into the bloodstream occurs via transcytosis of the nasopharyngeal epithelial barrier without destroying the barrier layer. Here, we used Calu-3 respiratory epithelial cells that were grown under air-liquid-interface conditions to induce formation of pseudostratified layers and mucus production. The number of bacterial localizations in the outer mucus, as well as cellular adhesion, invasion and transmigration of different carrier and disease N. meningitidis isolates belonging to MenB:cc32 and MenW:cc22 lineages was assessed. In addition, the effect on barrier integrity and cytokine release was determined. Our findings showed that all strains tested resided primarily in the outer mucus layer after 24 h of infection (>80%). Nonetheless, both MenB:cc32 and MenW:cc22 carrier and disease isolates reached the surface of the epithelial cells and overcame the barrier. Interestingly, we observed a significant difference in the number of bacteria transmigrating the epithelial cell barrier, with the representative disease isolates being more efficient to transmigrate compared to carrier isolates. This could be attributed to the capacity of the disease isolates to invade, however could not be assigned to expression of the outer membrane protein Opc. Moreover, we found that the representative meningococcal isolates tested in this study did not damage the epithelial barrier, as shown by TEER measurement, FITC-dextran permeability assays, and expression of cell-junction components.

Epitope Mapping of Human Polyclonal Antibodies to the fHbp Antigen of a Neisseria Meningitidis Vaccine by Hydrogen-Deuterium Exchange Mass Spectrometry (HDX-MS)

Antigen-antibody interactions play a key role in the immune response post vaccination and the mechanism of action of antibody-based biopharmaceuticals. 4CMenB is a multicomponent vaccine against Neisseria meningitidis serogroup B in which factor H binding protein (fHbp) is one of the key antigens. In this study, we use hydrogen/deuterium exchange mass spectrometry (HDX-MS) to identify epitopes in fHbp recognized by polyclonal antibodies (pAb) from two human donors (HDs) vaccinated with 4CMenB. Our HDX-MS data reveal several epitopes recognized by the complex mixture of human pAb. Furthermore, we show that the pAb from the two HDs recognize the same epitope regions. Epitope mapping of total pAb and purified fHbp-specific pAb from the same HD reveals that the two antibody samples recognize the same main epitopes, showing that HDX-MS based epitope mapping can, in this case at least, be performed directly using total IgG pAb samples that have not undergone Ab-selective purification. Two monoclonal antibodies (mAb) were previously produced from B-cell repertoire sequences from one of the HDs and used for epitope mapping of fHbp with HDX-MS. The epitopes identified for the pAb from the same HD in this study, overlap with the epitopes recognized by the two individual mAbs. Overall, HDX-MS epitope mapping appears highly suitable for simultaneous identification of epitopes recognized by pAb from human donors and to thus both guide vaccine development and study basic human immunity to pathogens, including viruses.

A design strategy to generate a SARS-CoV-2 RBD vaccine that abrogates ACE2 binding and improves neutralizing antibody responses

The structure-based design of antigens holds promise for developing vaccines with higher efficacy and improved safety profiles. We postulate that abrogation of host receptor interaction bears potential for the improvement of vaccines by preventing antigen-induced modification of receptor function as well as the displacement or masking of the immunogen. Antigen modifications may yet destroy epitopes crucial for antibody neutralization. Here, we present a methodology that integrates deep mutational scans to identify and score SARS-CoV-2 receptor binding domain variants that maintain immunogenicity, but lack interaction with the widely expressed host receptor. Single point mutations were scored in silico, validated in vitro, and applied in vivo. Our top-scoring variant receptor binding domain-G502E prevented spike-induced cell-to-cell fusion, receptor internalization, and improved neutralizing antibody responses by 3.3-fold in rabbit immunizations. We name our strategy BIBAX for body-inert, B-cell-activating vaccines, which in the future may be applied beyond SARS-CoV-2 for the improvement of vaccines by design.

An adenoviral-vectored vaccine confers seroprotection against capsular group B meningococcal disease

Adenoviral-vectored vaccines are licensed for prevention of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and Ebola virus, but, for bacterial proteins, expression in a eukaryotic cell may affect the antigen's localization and conformation or lead to unwanted glycosylation. Here, we investigated the potential use of an adenoviral-vectored vaccine platform for capsular group B meningococcus (MenB). Vector-based candidate vaccines expressing MenB antigen factor H binding protein (fHbp) were generated, and immunogenicity was assessed in mouse models, including the functional antibody response by serum bactericidal assay (SBA) using human complement. All adenovirus-based vaccine candidates induced high antigen-specific antibody and T cell responses. A single dose induced functional serum bactericidal responses with titers superior or equal to those induced by two doses of protein-based comparators, as well as longer persistence and a similar breadth. The fHbp transgene was further optimized for human use by incorporating a mutation abrogating binding to the human complement inhibitor factor H. The resulting vaccine candidate induced high and persistent SBA responses in transgenic mice expressing human factor H. The optimized transgene was inserted into the clinically relevant ChAdOx1 backbone, and this vaccine has now progressed to clinical development. The results of this preclinical vaccine development study underline the potential of vaccines based on genetic material to induce functional antibody responses against bacterial outer membrane proteins.

Emerging roles of SARS-CoV-2 Spike-ACE2 in immune evasion and pathogenesis

The COVID-19 pandemic, caused by SARS-CoV-2, has caused an estimated 5 billion infections and 20 million deaths by respiratory failure. In addition to the respiratory disease, SARS-CoV-2 infection has been associated with many extrapulmonary complications not easily explainable by the respiratory infection. A recent study showed that the SARS-CoV-2 spike protein, which mediates cell entry by binding to the angiotensin-converting enzyme 2 (ACE2) receptor, signals through ACE2 to change host cell behavior. In CD8⁺ T cells, spike-dependent ACE2-mediated signaling suppresses immunological synapse (IS) formation and impairs their killing ability, leading to immune escape of virus-infected cells. In this opinion article, we discuss the consequences of ACE2 signaling on the immune response and propose that it contributes to the extrapulmonary manifestations of COVID-19.

The Host-Pathogen Interactions and Epicellular Lifestyle of Neisseria meningitidis

Neisseria meningitidis is a gram-negative diplococcus and a transient commensal of the human nasopharynx. It shares and competes for this niche with a number of other Neisseria species including N. lactamica, N. cinerea and N. mucosa. Unlike these other members of the genus, N. meningitidis may become invasive, crossing the epithelium of the nasopharynx and entering the bloodstream, where it rapidly proliferates causing a syndrome known as Invasive Meningococcal Disease (IMD). IMD progresses rapidly to cause septic shock and meningitis and is often fatal despite aggressive antibiotic therapy. While many of the ways in which meningococci survive in the host environment have been well studied, recent insights into the interactions between N. meningitidis and the epithelial, serum, and endothelial environments have expanded our understanding of how IMD develops. This review seeks to incorporate recent work into the established model of pathogenesis. In particular, we focus on the competition that N. meningitidis faces in the nasopharynx from other Neisseria species, and how the genetic diversity of the meningococcus contributes to the wide range of inflammatory and pathogenic potentials observed among different lineages.

Structural characterization of a cross-protective natural chimera of factor H binding protein from meningococcal serogroup B strain NL096

Invasive meningococcal disease can cause fatal sepsis and meningitis and is a global health threat. Factor H binding protein (fHbp) is a protective antigen included in the two currently available vaccines against serogroup B meningococcus (MenB). FHbp is a remarkably variable surface-exposed meningococcal virulence factor with over 1300 different amino acid sequences identified so far. Based on this variability, fHbp has been classified into three variants, two subfamilies or nine modular groups, with low degrees of cross-protective activity. Here, we report the crystal structure of a natural fHbp cross-variant chimera, named variant1-2,3.x expressed by the MenB clinical isolate NL096, at 1.2 Å resolution, the highest resolution of any fHbp structure reported to date. We combined biochemical, site-directed mutagenesis and computational biophysics studies to deeply characterize this rare chimera. We determined the structure to be composed of two adjacent domains deriving from the three variants and determined the molecular basis of its stability, ability to bind Factor H and to adopt the canonical three-dimensional fHbp structure. These studies guided the design of loss-of-function mutations with potential for even greater immunogenicity. Moreover, this study represents a further step in the understanding of the fHbp biological and immunological evolution in nature. The chimeric variant1-2,3.x fHbp protein emerges as an intriguing cross-protective immunogen and suggests that identification of such naturally occurring hybrid proteins may result in stable and cross-protective immunogens when seeking to design and develop vaccines against highly variable pathogens.

Epitope Mapping of Polyclonal Antibodies by Hydrogen-Deuterium Exchange Mass Spectrometry (HDX-MS)

Epitope mapping of antibodies (Abs) is crucial for understanding adaptive immunity, as well as studying the mode of action of therapeutic antibodies and vaccines. Especially insights into the binding of the entire polyclonal antibody population (pAb) raised upon vaccination would be of unique value to vaccine development. However, very few methods for epitope mapping can tolerate the complexity of a pAb sample. Here we show how hydrogen-deuterium exchange mass spectrometry (HDX-MS) can be used to map epitopes recognized by pAb samples. Our approach involves measuring the HDX of the antigen in absence or presence of varied amounts of pAbs, as well as dissociating additives. We apply the HDX-MS workflow to pAbs isolated from rabbit immunized with factor H-binding protein (fHbp), a Neisseria meningitidis vaccine antigen. We identify four immunogenic regions located on the N- and C-terminal region of fHbp and provide insights into the relative abundance and avidity of epitope binding Abs present in the sample. Overall, our results show that HDX-MS can provide a unique and relatively fast method for revealing the binding impact of the entire set of pAbs present in blood samples after vaccination. Such information provides a rare view into effective immunity and can guide the design of improved vaccines against viruses or bacteria.

Lipoproteins in Gram-negative bacteria: new insights into their biogenesis, subcellular targeting and functional roles

Bacterial lipoproteins are globular proteins anchored to a membrane by a lipid moiety. By discovering new functions carried out by lipoproteins, recent research has highlighted the crucial roles played by these proteins in the cell envelope of Gram-negative bacteria. Here, after discussing the wide range of activities carried out by lipoproteins in the model bacterium Escherichia coli, we review new insights into the essential mechanisms involved in lipoprotein maturation, sorting and targeting to their final destination. A special attention will also be given to the recent identification of lipoproteins on the surface of E. coli and of other bacteria. The renewed interest in lipoproteins is driven by the need to identify novel targets for antibiotic development.

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.

Sweet turning bitter: Carbohydrate sensing of complement in host defence and disease

The complement system plays a major role in threat recognition and in orchestrating responses to microbial intruders and accumulating debris. This immune surveillance is largely driven by lectins that sense carbohydrate signatures on foreign, diseased and healthy host cells and act as complement activators, regulators or receptors to shape appropriate immune responses. While carbohydrate sensing protects our bodies, misguided or impaired recognition can contribute to disease. Moreover, pathogenic microbes have evolved to evade complement by mimicking host signatures. While complement is recognized as a disease factor, we only slowly start to appreciate the role of carbohydrate interactions in the underlying processes. A better understanding of complement's sweet side will contribute to a better description of disease mechanisms and enhanced diagnostic and therapeutic options. This review introduces the key components in complement-mediated carbohydrate sensing, discusses their role in health and disease, and touches on the potential effects of carbohydrate-related disease intervention. LINKED ARTICLES: This article is part of a themed issue on Canonical and non-canonical functions of the complement system in health and disease. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.14/issuetoc.