The structure of Neisseria meningitidis lipid A determines outcome in experimental meningococcal disease

The choroid plexus synergizes with immune cells during neuroinflammation

The choroid plexus (ChP) is a vital brain barrier and source of cerebrospinal fluid (CSF). Here, we use longitudinal two-photon imaging in awake mice and single-cell transcriptomics to elucidate the mechanisms of ChP regulation of brain inflammation. We used intracerebroventricular injections of lipopolysaccharides (LPS) to model meningitis in mice and observed that neutrophils and monocytes accumulated in the ChP stroma and surged across the epithelial barrier into the CSF. Bi-directional recruitment of monocytes from the periphery and, unexpectedly, macrophages from the CSF to the ChP helped eliminate neutrophils and repair the barrier. Transcriptomic analyses detailed the molecular steps accompanying this process and revealed that ChP epithelial cells transiently specialize to nurture immune cells, coordinating their recruitment, survival, and differentiation as well as regulation of the tight junctions that control the permeability of the ChP brain barrier. Collectively, we provide a mechanistic understanding and a comprehensive roadmap of neuroinflammation at the ChP brain barrier.

A collaboration between immune cells and the choroid plexus epithelium in brain inflammation

The choroid plexus (ChP) is a vital brain barrier and source of cerebrospinal fluid (CSF). Here, we use chronic two-photon imaging in awake mice and single-cell transcriptomics to demonstrate that in addition to these roles, the ChP is a complex immune organ that regulates brain inflammation. In a mouse meningitis model, neutrophils and monocytes accumulated in ChP stroma and surged across the epithelial barrier into the CSF. Bi-directional recruitment of monocytes from the periphery and, unexpectedly, macrophages from the CSF to the ChP helped eliminate neutrophils and repair the barrier. Transcriptomic analyses detailed the molecular steps accompanying this process, including the discovery of epithelial cells that transiently specialized to nurture immune cells, coordinate their recruitment, survival, and differentiation, and ultimately, control the opening/closing of the ChP brain barrier. Collectively, we provide a new conceptual understanding and comprehensive roadmap of neuroinflammation at the ChP brain barrier.

Lipid A structural diversity among members of the genus Leptospira

Lipid A is the hydrophobic component of bacterial lipopolysaccharide and an activator of the host immune system. Bacteria modify their lipid A structure to adapt to the surrounding environment and, in some cases, to evade recognition by host immune cells. In this study, lipid A structural diversity within the Leptospira genus was explored. The individual Leptospira species have dramatically different pathogenic potential that ranges from non-infectious to life-threatening disease (leptospirosis). Ten distinct lipid A profiles, denoted L1-L10, were discovered across 31 Leptospira reference species, laying a foundation for lipid A-based molecular typing. Tandem MS analysis revealed structural features of Leptospira membrane lipids that might alter recognition of its lipid A by the host innate immune receptors. Results of this study will aid development of strategies to improve diagnosis and surveillance of leptospirosis, as well as guide functional studies on Leptospira lipid A activity.

Outer Membrane Vesicle Induction and Isolation for Vaccine Development

Gram-negative bacteria release vesicular structures from their outer membrane, so called outer membrane vesicles (OMVs). OMVs have a variety of functions such as waste disposal, communication, and antigen or toxin delivery. These vesicles are the promising structures for vaccine development since OMVs carry many surface antigens that are identical to the bacterial surface. However, isolation is often difficult and results in low yields. Several methods to enhance OMV yield exist, but these do affect the resulting OMVs. In this review, our current knowledge about OMVs will be presented. Different methods to induce OMVs will be reviewed and their advantages and disadvantages will be discussed. The effects of the induction and isolation methods used in several immunological studies on OMVs will be compared. Finally, the challenges for OMV-based vaccine development will be examined and one example of a successful OMV-based vaccine will be presented.

Whole-blood incubation with the Neisseria meningitidis lpxL1 mutant induces less pro-inflammatory cytokines than the wild type, and IL-10 reduces the MyD88-dependent cytokines

Levels of bacterial LPS, pro-inflammatory cytokines and IL-10 are related to the severity of meningococcal septicaemia. Patients infected with a Neisseria meninigitidis lpxL1 mutant (Nm-mutant) with penta-acylated lipid A present with a milder meningococcal disease than those infected with hexa-acylated Nm wild type (Nm-wt). The aim was to compare the pro-inflammatory responses after ex vivo incubation with the heat-inactivated Nm-wt or the Nm-mutant in citrated whole blood, and the modulating effects of IL-10. Concomitantly, we measured intracellular IL-6, IL-8 and TNF-α to elucidate which cell types were responsible for the pro-inflammatory responses. Incubation with Nm-wt (10⁶/ml;10⁷/ml;10⁸/ml) resulted in a dose-dependent increase of the MyD88-dependent pro-inflammatory cytokines (IL-1β, IL-6, IL-8, TNF-α), which were mainly derived from monocytes. In comparison, only 10⁸/ml of the Nm-mutant significantly increased the concentration of these cytokines. The MyD88-independent cytokines (IP-10, RANTES) were evidently increased after incubation with the Nm-wt but were unaffected by the Nm-mutant. Co-incubation with IL-10 significantly reduced the concentrations of the MyD88-dependent cytokines induced by both the Nm-wt and the Nm-mutant, whereas the MyD88-independent cytokines were almost unaffected. In summary, the Nm-mutant is a weaker inducer of the MyD88-dependent/independent cytokines than the Nm-wt in whole blood, and IL-10 attenuates the Nm-stimulated increase in MyD88-dependent pro-inflammatory cytokines.

Innate immune recognition and inflammation in Neisseria meningitidis infection

Neisseria meningitidis (Nme) can cause meningitis and sepsis, diseases which are characterised by an overwhelming inflammatory response. Inflammation is triggered by host pattern recognition receptors (PRRs) which are activated by pathogen-associated molecular patterns (PAMPs). Nme contains multiple PAMPs including lipooligosaccharide, peptidoglycan, proteins and metabolites. Various classes of PRRs including Toll-like receptors, NOD-like receptors, C-type lectins, scavenger receptors, pentraxins and others are expressed by the host to respond to any given microbe. While Toll-like receptors and NOD-like receptors are pivotal in triggering inflammation, other PRRs act as modulators of inflammation or aid in functional antimicrobial responses such as phagocytosis or complement activation. This review aims to give an overview of the various Nme PAMPs reported to date, the PRRs they activate and their implications during the inflammatory response to infection.

A Meningococcal Outer Membrane Vesicle Vaccine Incorporating Genetically Attenuated Endotoxin Dissociates Inflammation from Immunogenicity

Background

Group B Neisseria meningitidis, an endotoxin-producing Gram-negative bacterium, causes the highest incidence of group B meningococcus (MenB) disease in the first year of life. The Bexsero vaccine is indicated in Europe from 8 weeks of age. Endotoxin components of outer membrane vesicles (OMVs) or soluble lipopolysaccharide (LPS) represent a potential source of inflammation and residual reactogenicity. The purpose of this study was to compare novel candidate MenB vaccine formulations with licensed vaccines, including Bexsero, using age-specific human in vitro culture systems.

Methods

OMVs from wild type- and inactivated lpxL1 gene mutant-N. meningitidis strains were characterized in human neonatal and adult in vitro whole blood assays and dendritic cell (DC) arrays. OMVs were benchmarked against licensed vaccines, including Bexsero and whole cell pertussis formulations, with respect to Th-polarizing cytokine and prostaglandin E2 production, as well as cell surface activation markers (HLA-DR, CD86, and CCR7). OMV immunogenicity was assessed in mice.

Results

ΔlpxLI native OMVs (nOMVs) demonstrated significantly less cytokine induction in human blood and DCs than Bexsero and most of the other pediatric vaccines (e.g., PedvaxHib, EasyFive, and bacillus Calmette–Guérin) tested. Despite a much lower inflammatory profile in vitro than Bexsero, ΔlpxLI nOMVs still had moderate DC maturing ability and induced robust anti-N. meningitidis antibody responses after murine immunization.

Conclusion

A meningococcal vaccine comprised of attenuated LPS-based OMVs with a limited inflammatory profile in vitro induces robust antigen-specific immunogenicity in vivo.

Biosynthetically engineered lipopolysaccharide as vaccine adjuvant

Lipopolysaccharide (LPS), a dominant component of the Gram-negative bacterial outer membrane, is a strong activator of the innate immune system, and thereby an important determinant in the adaptive immune response following bacterial infection. This adjuvant activity can be harnessed following immunization with bacteria-derived vaccines that naturally contain LPS, and when LPS or molecules derived from it are added to purified vaccine antigens. However, the downside of the strong biological activity of LPS is its ability to contribute to vaccine reactogenicity. Modification of the LPS structure allows triggering of a proper immune response needed in a vaccine against a particular pathogen while at the same time lowering its toxicity. Extensive modifications to the basic structure are possible by using our current knowledge of bacterial genes involved in LPS biosynthesis and modification. This review focuses on biosynthetic engineering of the structure of LPS and implications of these modifications for generation of safe adjuvants.

Bordetella pertussis naturally occurring isolates with altered lipooligosaccharide structure fail to fully mature human dendritic cells

Bordetella pertussis is a Gram-negative bacterium and the causative agent of whooping cough. Despite high vaccination coverage, outbreaks are being increasingly reported worldwide. Possible explanations include adaptation of this pathogen, which may interfere with recognition by the innate immune system. Here, we describe innate immune recognition and responses to different B. pertussis clinical isolates. By using HEK-Blue cells transfected with different pattern recognition receptors, we found that 3 out of 19 clinical isolates failed to activate Toll-like receptor 4 (TLR4). These findings were confirmed by using the monocytic MM6 cell line. Although incubation with high concentrations of these 3 strains resulted in significant activation of the MM6 cells, it was found to occur mainly through interaction with TLR2 and not through TLR4. When using live bacteria, these 3 strains also failed to activate TLR4 on HEK-Blue cells, and activation of MM6 cells or human monocyte-derived dendritic cells was significantly lower than activation induced by the other 16 strains. Mass spectrum analysis of the lipid A moieties from these 3 strains indicated an altered structure of this molecule. Gene sequence analysis revealed mutations in genes involved in lipid A synthesis. Findings from this study indicate that B. pertussis isolates that do not activate TLR4 occur naturally and that this phenotype may give this bacterium an advantage in tempering the innate immune response and establishing infection. Knowledge on the strategies used by this pathogen in evading the host immune response is essential for the improvement of current vaccines or for the development of new ones.

Mycobacterial membrane vesicles administered systemically in mice induce a protective immune response to surface compartments of Mycobacterium tuberculosis

Pathogenic and nonpathogenic species of bacteria and fungi release membrane vesicles (MV), containing proteins, polysaccharides, and lipids, into the extracellular milieu. Previously, we demonstrated that several mycobacterial species, including bacillus Calmette-Guerin (BCG) and Mycobacterium tuberculosis, release MV containing lipids and proteins that subvert host immune response in a Toll-like receptor 2 (TLR2)-dependent manner (R. Prados-Rosales et al., J. Clin. Invest. 121:1471–1483, 2011, doi:10.1172/JCI44261). In this work, we analyzed the vaccine potential of MV in a mouse model and compared the effects of immunization with MV to those of standard BCG vaccination. Immunization with MV from BCG or M. tuberculosis elicited a mixed humoral and cellular response directed to both membrane and cell wall components, such as lipoproteins. However, only vaccination with M. tuberculosis MV was able to protect as well as live BCG immunization. M. tuberculosis MV boosted BCG vaccine efficacy. In summary, MV are highly immunogenic without adjuvants and elicit immune responses comparable to those achieved with BCG in protection against M. tuberculosis.

This work offers a new vaccine approach against tuberculosis using mycobacterial MV. Mycobacterium MV are a naturally released product combining immunogenic antigens in the context of a lipid structure. The fact that MV do not need adjuvants and elicit protection comparable to that elicited by the BCG vaccine encourages vaccine approaches that combine protein antigens and lipids. Consequently, mycobacterium MV establish a new type of vaccine formulation.

Hexa-acylated lipid A is required for host inflammatory response to Neisseria gonorrhoeae in experimental gonorrhea

Neisseria gonorrhoeae causes gonorrhea, a sexually transmitted infection characterized by inflammation of the cervix or urethra. However, a significant subset of patients with N. gonorrhoeae remain asymptomatic, without evidence of localized inflammation. Inflammatory responses to N. gonorrhoeae are generated by host innate immune recognition of N. gonorrhoeae by several innate immune signaling pathways, including lipooligosaccharide (LOS) and other pathogen-derived molecules through activation of innate immune signaling systems, including toll-like receptor 4 (TLR4) and the interleukin-1β (IL-1β) processing complex known as the inflammasome. The lipooligosaccharide of N. gonorrhoeae has a hexa-acylated lipid A. N. gonorrhoeae strains that carry an inactivated msbB (also known as lpxL1) gene produce a penta-acylated lipid A and exhibit reduced biofilm formation, survival in epithelial cells, and induction of epithelial cell inflammatory signaling. We now show that msbB-deficient N. gonorrhoeae induces less inflammatory signaling in human monocytic cell lines and murine macrophages than the parent organism. The penta-acylated LOS exhibits reduced toll-like receptor 4 signaling but does not affect N. gonorrhoeae-mediated activation of the inflammasome. We demonstrate that N. gonorrhoeae msbB is dispensable for initiating and maintaining infection in a murine model of gonorrhea. Interestingly, infection with msbB-deficient N. gonorrhoeae is associated with less localized inflammation. Combined, these data suggest that TLR4-mediated recognition of N. gonorrhoeae LOS plays an important role in the pathogenesis of symptomatic gonorrhea infection and that alterations in lipid A biosynthesis may play a role in determining symptomatic and asymptomatic infections.

Different cytokine production and Toll-like receptor expression induced by heat-killed invasive and carrier strains of Neisseria meningitidis

Neisseria meningitidis may cause severe invasive disease. The carriage state of the pathogen is common, and the reasons underlying why the infection becomes invasive are not fully understood. The aim of this study was to compare the differences between invasive and carrier strains in the activation of innate immunity. The monocyte expression of TLR2, TLR4, CD14, and HLA-DR, cytokine production, and the granulocyte oxidative burst were analyzed after in vitro stimulation by heat-killed invasive (n = 14) and carrier (n = 9) strains of N. meningitidis. The expression of the cell surface markers in monocytes, the oxidative burst, and cytokine concentrations were measured using flow cytometry. Carrier strains stimulated a higher production of inflammatory cytokines and oxidative burst in granulocytes than invasive strains (all p < 0.001), whereas invasive strains significantly up-regulated TLR2, TLR4 (p < 0.001), and CD14 (p < 0.01) expression on monocytes. Conversely, the monocyte expression of HLA-DR was higher after the stimulation by carrier strains (p < 0.05) in comparison to invasive strains. The LPS inhibitor polymyxin B abolished the differences between the strains. Our findings indicate different immunostimulatory potencies of invasive strains of N. meningitidis compared with carrier strains.

Lack of lipid A pyrophosphorylation and functional lptA reduces inflammation by Neisseria commensals

The interaction of the immune system with Neisseria commensals remains poorly understood. We have previously shown that phosphoethanolamine on the lipid A portion of lipooligosaccharide (LOS) plays an important role in Toll-like receptor 4 (TLR4) signaling. For pathogenic Neisseria, phosphoethanolamine is added to lipid A by the phosphoethanolamine transferase specific for lipid A, which is encoded by lptA. Here, we report that Southern hybridizations and bioinformatics analyses of genomic sequences from all eight commensal Neisseria species confirmed that lptA was absent in 15 of 17 strains examined but was present in N. lactamica. Mass spectrometry of lipid A and intact LOS revealed the lack of both pyrophosphorylation and phosphoethanolaminylation in lipid A of commensal species lacking lptA. Inflammatory signaling in human THP-1 monocytic cells was much greater with pathogenic than with commensal Neisseria strains that lacked lptA, and greater sensitivity to polymyxin B was consistent with the absence of phosphoethanolamine. Unlike the other commensals, whole bacteria of two N. lactamica commensal strains had low inflammatory potential, whereas their lipid A had high-level pyrophosphorylation and phosphoethanolaminylation and induced high-level inflammatory signaling, supporting previous studies indicating that this species uses mechanisms other than altering lipid A to support commensalism. A meningococcal lptA deletion mutant had reduced inflammatory potential, further illustrating the importance of lipid A pyrophosphorylation and phosphoethanolaminylation in the bioactivity of LOS. Overall, our results indicate that lack of pyrophosphorylation and phosphoethanolaminylation of lipid A contributes to the immune privilege of most commensal Neisseria strains by reducing the inflammatory potential of LOS.

Invasive meningococcal capsular group Y disease, England and Wales, 2007-2009

Increases may result from mutations that allow the organism to evade the immune system.

Enhanced national surveillance for invasive meningococcal disease in England and Wales identified an increase in laboratory-confirmed capsular group Y (MenY) disease from 34 cases in 2007 to 44 in 2008 and 65 in 2009. For cases diagnosed in 2009, patient median age at disease onset was 60 years; 39% of patients had underlying medical conditions, and 19% died. MenY isolates causing invasive disease during 2007–2009 belonged mainly to 1 of 4 clonal complexes (cc), cc23 (56% of isolates), cc174 (21%), cc167 (11%), and cc22 (8%). The 2009 increase resulted primarily from sequence type 1655 (cc23) (22 cases in 2009, compared with 4 cases each in 2007 and 2008). cc23 was associated with lpxL1 mutations and meningitis in younger age groups (<25 years); cc174 was associated with nonmeningitis, particularly pneumonia, in older age groups (>65 years). The increase in MenY disease requires careful epidemiologic and molecular monitoring.

The changing and dynamic epidemiology of meningococcal disease

The epidemiology of invasive meningococcal disease continues to change rapidly, even in the three years since the first Meningococcal Exchange Meeting in 2008. Control of disease caused by serogroup C has been achieved in countries that have implemented meningococcal C or quadrivalent meningococcal ACWY conjugate vaccines. Initiation of mass immunization programs with meningococcal A conjugate vaccines across the meningitis belt of Africa may lead to the interruption of cyclical meningococcal epidemics. A meningococcal B vaccination program in New Zealand has led to a decreased incidence of high rates of endemic serogroup B disease. Increases in serogroup Y disease have been observed in certain Nordic countries which, if they persist, may require consideration of use of a multiple serogroup vaccine. The imminent availability of recombinant broadly protective serogroup B vaccines may provide the tools for further control of invasive meningococcal disease in areas where serogroup B disease predominates. Continued surveillance of meningococcal disease is essential; ongoing global efforts to improve the completeness of reporting are required.