Challenges for development of meningococcal vaccines in infants and children

Structural insight into the dual function of LbpB in mediating Neisserial pathogenesis

Lactoferrin-binding protein B (LbpB) is a lipoprotein present on the surface of Neisseria that has been postulated to serve dual functions during pathogenesis in both iron acquisition from lactoferrin (Lf), and in providing protection against the cationic antimicrobial peptide lactoferricin (Lfcn). While previous studies support a dual role for LbpB, exactly how these ligands interact with LbpB has remained unknown. Here, we present the structures of LbpB from N. meningitidis and N. gonorrhoeae in complex with human holo-Lf, forming a 1:1 complex and confirmed by size-exclusion chromatography small-angle X-ray scattering. LbpB consists of N- and C-lobes with the N-lobe interacting extensively with the C-lobe of Lf. Our structures provide insight into LbpB’s preference towards holo-Lf, and our mutagenesis and binding studies show that Lf and Lfcn bind independently. Our studies provide the molecular details for how LbpB serves to capture and preserve Lf in an iron-bound state for delivery to the membrane transporter LbpA for iron piracy, and as an antimicrobial peptide sink to evade host immune defenses.

Structural rearrangement of Neisseria meningitidis transferrin binding protein A (TbpA) prior to human transferrin protein (hTf) binding

Gram-negative bacterium Neisseria meningitidis, responsible for human infectious disease meningitis, acquires the iron (Fe³⁺) ion needed for its survival from human transferrin protein (hTf). For this transport, transferrin binding proteins TbpA and TbpB are facilitated by the bacterium. The transfer cannot occur without TbpA, while the absence of TbpB only slows down the transfer. Thus, understanding the TbpA-hTf binding at the atomic level is crucial for the fight against bacterial meningitis infections. In this study, atomistic level of mechanism for TbpA-hTf binding is elucidated through 100 ns long all-atom classical MD simulations on free (uncomplexed) TbpA. TbpA protein underwent conformational change from ‘open’ state to ‘closed’ state, where two loop domains, loops 5 and 8, were very close to each other. This state clearly cannot accommodate hTf in the cleft between these two loops. Moreover, the helix finger domain, which might play a critical role in Fe³⁺ ion uptake, also shifted downwards leading to unfavorable Tbp-hTf binding. Results of this study indicated that TbpA must switch between ‘closed’ state to ‘open’ state, where loops 5 and 8 are far from each other creating a cleft for hTf binding. The atomistic level of understanding to conformational switch is crucial for TbpA-hTf complex inhibition strategies. Drug candidates can be designed to prevent this conformational switch, keeping TbpA locked in ‘closed’ state.

Invasive meningococcal disease epidemiology and characterization of Neisseria meningitidis serogroups, sequence types, and clones; implication for use of meningococcal vaccines

BACKGROUND AND AIMS

Neisseria meningitidis (N. meningitidis) is a Gram-negative bacterium that can cause life-threatening invasive infections referred to as invasive meningococcal disease (IMD). In the last decade the incidence of IMD in Israel is about 1/100,000 population annually. We aimed to describe the epidemiology of IMD in Israel combining epidemiological data and characterization of N. meningitidis isolates.

METHODS

Invasive infection caused by N. meningitidis is a notifiable disease in Israel. Data were collected by epidemiological investigations and control measures were employed. Laboratory work-up included serogrouping, N. meningitides molecular characterization and whole-genome sequencing.

RESULTS

During 1998-2017, 1349 cases of IMD were notified in Israel (mean annual incidence rate 0.94/100,000). The peak incidence rates were observed in infants under 1 year of age (10.9/100,000). Case fatality rate was 9.7%. The majority of the N. meningitidis isolates were of serogroup B (67.9%). During 2007-2017, three clonal complexes (CC) 32, 41/44 and 23 (hyper-invasive clonal complexes) were the leading CC (61%). CC32 was the leading CC causing meningococcemia and mortality. In 2017, 35 isolates were tested for 4CMenB antigens variants; of the serogroup B isolates tested 46.7% showed a match to one or more antigens (fHbp or PorA:VR1), most were ST32 (CC32).

CONCLUSIONS

Preliminary analysis based on limited number of samples suggests that the 4CMenB coverage would be about half the strains; further research is necessary. Integration of clinical, epidemiological and laboratory data is essential to support decision-making on the introduction of the novel MENB vaccines in Israel.

Cellular Immune Responses in Humans Induced by Two Serogroup B Meningococcal Outer Membrane Vesicle Vaccines Given Separately and in Combination

MenBvac and MeNZB are safe and efficacious outer membrane vesicle (OMV) vaccines against serogroup B meningococcal disease. Antibody responses have previously been investigated in a clinical trial with these two OMV vaccines given separately (25 μg/dose) or in combination (12.5 and 12.5 μg/dose) in three doses administered at 6-week intervals. Here, we report the results from analyzing cellular immune responses against MenBvac and MeNZB OMVs in terms of antigen-specific CD4(+)T cell proliferation and secretion of cytokines. The proliferative CD4(+)T cell responses to the combined vaccine were of the same magnitude as the homologous responses observed for each individual vaccine. The results also showed cross-reactivity in the sense that both vaccine groups receiving separate vaccines responded to both homologous and heterologous OMV antigen when assayed for antigen-specific cellular proliferation. In addition, a multiplex bead array assay was used to analyze the presence of Th1 and Th2 cytokines in cell culture supernatants. The results showed that gamma interferon, interleukin-4 (IL-4), and IL-10 responses could be detected as a result of vaccination with both the MenBvac and the MeNZB vaccines given separately, as well as when given in combination. With respect to cross-reactivity, the cytokine results paralleled the observations made for proliferation. In conclusion, the results demonstrate that cross-reactive cellular immune responses involving both Th1 and Th2 cytokines can be induced to the same extent by different tailor-made OMV vaccines given either separately or in combination with half the dose of each vaccine.

Structural insight into the lactoferrin receptors from pathogenic Neisseria

Neisseria are pathogenic bacteria that cause gonorrhea, septicemia, and meningitis. Like other pathogenic bacteria, Neisseria must acquire iron for survival from their local environment within the human host. Instead of secreting siderophores to scavenge iron, Neisseria steal iron from human iron binding proteins such as hemoglobin, transferrin and lactoferrin for survival. Recently we reported the crystal structures of the Neisseria meningitidis transferrin receptors TbpA and TbpB, as well as the structures of apo and holo human transferrin. We also analyzed these proteins using small angle X-ray scattering and electron microscopy to provide the molecular details explaining how Neisseria are able to interact with and extract iron from transferrin. Here, we utilize the structural reports, as well as the recently reported structure of the N-lobe of LbpB from Moraxella bovis, to assemble improved 3D homology models for the neisserial lactoferrin import receptors LbpA and LbpB, both of which are important vaccine targets against N. meningitidis. We then analyzed these models to gain structural insights into the lactoferrin-iron import system and form a mechanistic model fashioned in parallel to the homologous transferrin-iron import system.

Structural basis for iron piracy by pathogenic Neisseria

Neisseria are obligate human pathogens causing bacterial meningitis, septicaemia and gonorrhoea. Neisseria require iron for survival and can extract it directly from human transferrin for transport across the outer membrane. The transport system consists of TbpA, an integral outer membrane protein, and TbpB, a co-receptor attached to the cell surface; both proteins are potentially important vaccine and therapeutic targets. Two key questions driving Neisseria research are how human transferrin is specifically targeted, and how the bacteria liberate iron from transferrin at neutral pH. To address these questions, we solved crystal structures of the TbpA-transferrin complex and of the corresponding co-receptor TbpB. We characterized the TbpB-transferrin complex by small-angle X-ray scattering and the TbpA-TbpB-transferrin complex by electron microscopy. Our studies provide a rational basis for the specificity of TbpA for human transferrin, show how TbpA promotes iron release from transferrin, and elucidate how TbpB facilitates this process.