Activation of human meningeal cells is modulated by lipopolysaccharide (LPS) and non-LPS components of Neisseria meningitidis and is independent of Toll-like receptor (TLR)4 and TLR2 signalling

Proteomic interrogation of the meninges reveals the molecular identities of structural components and regional distinctions along the CNS axis

The meninges surround the brain and spinal cord, affording physical protection while also serving as a niche of neuroimmune activity. Though possessing stromal qualities, its complex cellular and extracellular makeup has yet to be elaborated, and it remains unclear whether the meninges vary along the neuroaxis. Hence, studies were carried-out to elucidate the protein composition and structural organization of brain and spinal cord meninges in normal, adult Biozzi ABH mice. First, shotgun, bottom-up proteomics was carried-out. Prominent proteins at both brain and spinal levels included Type II collagen and Type II keratins, representing extracellular matrix (ECM) and cytoskeletal categories, respectively. While the vast majority of total proteins detected was shared between both meningeal locales, more were uniquely detected in brain than in spine. This pattern was also seen when total proteins were subdivided by cellular compartment, except in the case of the ECM category where brain and spinal meninges each had near equal number of unique proteins, and Type V and type III collagen registered exclusively in the spine. Quantitative analysis revealed differential expression of several collagens and cytoskeletal proteins between brain and spinal meninges. High-resolution immunofluorescence and immunogold-scanning electronmicroscopy on sections from whole brain and spinal cord - still encased within bone -identified major proteins detected by proteomics, and highlighted their association with cellular and extracellular elements of variously shaped arachnoid trabeculae. Western blotting aligned with the proteomic and immunohistological analyses, reinforcing differential appearance of proteins in brain vs spinal meninges. Results could reflect regional distinctions in meninges that govern protective and/or neuroimmune functions.

Development of a multicellular in vitro model of the meningeal blood-CSF barrier to study Neisseria meningitidis infection

Background

Bacterial meningitis is a life-threatening disease that occurs when pathogens such as Neisseria meningitidis cross the meningeal blood cerebrospinal fluid barrier (mBCSFB) and infect the meninges. Due to the human-specific nature of N. meningitidis, previous research investigating this complex host–pathogen interaction has mostly been done in vitro using immortalized brain endothelial cells (BECs) alone, which often do not retain relevant barrier properties in culture. Here, we developed physiologically relevant mBCSFB models using BECs in co-culture with leptomeningeal cells (LMCs) to examine N. meningitidis interaction.

Methods

We used BEC-like cells derived from induced pluripotent stem cells (iBECs) or hCMEC/D3 cells in co-culture with LMCs derived from tumor biopsies. We employed TEM and structured illumination microscopy to characterize the models as well as bacterial interaction. We measured TEER and sodium fluorescein (NaF) permeability to determine barrier tightness and integrity. We then analyzed bacterial adherence and penetration of the cell barrier and examined changes in host gene expression of tight junctions as well as chemokines and cytokines in response to infection.

Results

Both cell types remained distinct in co-culture and iBECs showed characteristic expression of BEC markers including tight junction proteins and endothelial markers. iBEC barrier function as determined by TEER and NaF permeability was improved by LMC co-culture and remained stable for seven days. BEC response to N. meningitidis infection was not affected by LMC co-culture. We detected considerable amounts of BEC-adherent meningococci and a relatively small number of intracellular bacteria. Interestingly, we discovered bacteria traversing the BEC-LMC barrier within the first 24 h post-infection, when barrier integrity was still high, suggesting a transcellular route for N. meningitidis into the CNS. Finally, we observed deterioration of barrier properties including loss of TEER and reduced expression of cell-junction components at late time points of infection.

Conclusions

Here, we report, for the first time, on co-culture of human iPSC derived BECs or hCMEC/D3 with meningioma derived LMCs and find that LMC co-culture improves barrier properties of iBECs. These novel models allow for a better understanding of N. meningitidis interaction at the mBCSFB in a physiologically relevant setting.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12987-022-00379-z.

Dietary Supplementation of Inulin Ameliorates Subclinical Mastitis via Regulation of Rumen Microbial Community and Metabolites in Dairy Cows

Subclinical mastitis (SCM) is one of the highly infectious diseases in dairy cows with the characteristics of high incidence and nonvisible clinical symptoms. The gastrointestinal microbiota is closely related to mastitis. Inulin is a prebiotic fiber with functions in improving intestinal microbial communities and enhancing the host’s immunity. However, the impact of dietary inulin on the rumen inner environment remains unknown. The current study investigated whether inulin could relieve SCM by affecting the profiles of ruminal bacterial and metabolites in dairy cows. Inulin inclusion rates were 0, 100, 200, 300, and 400 g/day per cow, respectively. Inulin increased milk yield, milk protein, and lactose and reduced the somatic cell counts (SCC) in milk. In serum, the concentration of proinflammatory cytokines, such as interleukin-6 (IL-6), IL-8, tumor necrosis factor α (TNF-α), and malondialdehyde (MDA) were decreased, and IL-4 and superoxide dismutase (SOD) were increased. Meanwhile, inulin increased the concentration of propionate, butyrate, and lactic acid (LA), while it decreased NH₃-N in rumen. The propionate- and butyrate-producing bacteria (e.g., Prevotella and Butyrivibrio) and several beneficial commensal bacteria (e.g., Muribaculaceae and Bifidobacterium) as well as metabolites related to energy and amino acid metabolism (e.g., melibiose and l-glutamate) were increased. However, several proinflammatory bacteria (e.g., Clostridia UCG-014, Streptococcus, and Escherichia-Shigella) were decreased, accompanied by the downregulation of lipid proinflammatory metabolites, for example, ceramide(d18:0/15:0) [Cer(d18:0/15:0)] and 17-phenyl-18,19,20-trinor-prostaglandin E₂. In the current study, the above indicators showed the best response in the 300 g/day inulin group. Overall, dietary supplementation of inulin could alleviate inflammatory responses in cows with SCM through improving the rumen inner environment.

IMPORTANCE The correlation between mastitis and the gastrointestinal microbiome in dairy cows has been demonstrated. Regulating the profile of rumen microorganisms may contribute to remission of subclinical mastitis (SCM). Supplementation of inulin in the diets of cows with SCM could increase the abundance of short-chain fatty acid (SCFA)-producing bacteria and beneficial commensal bacteria in rumen and meanwhile the levels of amino acids and energy metabolism. Conversely, the abundance of ruminal bacteria and metabolites with proinflammatory effects were decreased. Our study suggests that the improvement of the rumen internal environment by inulin supplementation could ameliorate inflammatory responses during SCM in dairy cows and thus improve lactation performance and milk quality. Our results provide a theoretical basis for regulation measures of SCM in dairy cows.

Interactions and Signal Transduction Pathways Involved during Central Nervous System Entry by Neisseria meningitidis across the Blood-Brain Barriers

The Gram-negative diplococcus Neisseria meningitidis, also called meningococcus, exclusively infects humans and can cause meningitis, a severe disease that can lead to the death of the afflicted individuals. To cause meningitis, the bacteria have to enter the central nervous system (CNS) by crossing one of the barriers protecting the CNS from entry by pathogens. These barriers are represented by the blood–brain barrier separating the blood from the brain parenchyma and the blood–cerebrospinal fluid (CSF) barriers at the choroid plexus and the meninges. During the course of meningococcal disease resulting in meningitis, the bacteria undergo several interactions with host cells, including the pharyngeal epithelium and the cells constituting the barriers between the blood and the CSF. These interactions are required to initiate signal transduction pathways that are involved during the crossing of the meningococci into the blood stream and CNS entry, as well as in the host cell response to infection. In this review we summarize the interactions and pathways involved in these processes, whose understanding could help to better understand the pathogenesis of meningococcal meningitis.

The meninges as barriers and facilitators for the movement of fluid, cells and pathogens related to the rodent and human CNS

Meninges that surround the CNS consist of an outer fibrous sheet of dura mater (pachymeninx) that is also the inner periosteum of the skull. Underlying the dura are the arachnoid and pia mater (leptomeninges) that form the boundaries of the subarachnoid space. In this review we (1) examine the development of leptomeninges and their role as barriers and facilitators in the foetal CNS. There are two separate CSF systems during early foetal life, inner CSF in the ventricles and outer CSF in the subarachnoid space. As the foramina of Magendi and Luschka develop, one continuous CSF system evolves. Due to the lack of arachnoid granulations during foetal life, it is most likely that CSF is eliminated by lymphatic drainage pathways passing through the cribriform plate and nasal submucosa. (2) We then review the fine structure of the adult human and rodent leptomeninges to establish their roles as barriers and facilitators for the movement of fluid, cells and pathogens. Leptomeningeal cells line CSF spaces, including arachnoid granulations and lymphatic drainage pathways, and separate elements of extracellular matrix from the CSF. The leptomeningeal lining facilitates the traffic of inflammatory cells within CSF but also allows attachment of bacteria such as Neisseria meningitidis and of tumour cells as CSF metastases. Single layers of leptomeningeal cells extend into the brain closely associated with the walls of arteries so that there are no perivascular spaces around arteries in the cerebral cortex. Perivascular spaces surrounding arteries in the white matter and basal ganglia relate to their two encompassing layers of leptomeninges. (3) Finally we examine the roles of ligands expressed by leptomeningeal cells for the attachment of inflammatory cells, bacteria and tumour cells as understanding these roles may aid the design of therapeutic strategies to manage developmental, autoimmune, infectious and neoplastic diseases relating to the CSF, the leptomeninges and the associated CNS.

Granulocyte Colony-Stimulating Factor Alleviates Bacterial-Induced Neuronal Apoptotic Damage in the Neonatal Rat Brain through Epigenetic Histone Modification

Bacterial meningitis during the perinatal period may cause long-term neurological deficits. The study investigated whether bacterial lipopolysaccharide (LPS) derived from E. coli. led to neuronal apoptosis with an impaired performance of long-term cognitive function involving the activation of histone modification in the TNF-α gene promoter. Further, we looked into the therapeutic efficacy of granulocyte colony-stimulating factor (G-CSF) in a neonatal brain suffering from perinatal bacterial meningitis. We applied the following research techniques: neurobehavioral tasks, confocal laser microscopy, chromatin immunoprecipitation, and Western blotting. At postnatal day 10, the animals were subjected to LPS and/or G-CSF. The target brain tissues were then collected at P17. Some animals (P45) were studied using neurobehavioral tasks. The LPS-injected group revealed significantly increased expression of NF-κB phosphorylation and trimethylated H3K4 in the TNFA gene promoter locus. Furthermore, the caspase-3, neuronal apoptosis expression, and an impaired performance in cognitive functions were also found in our study. Such deleterious outcomes described above were markedly alleviated by G-CSF therapy. This study suggests that selective therapeutic action sites of G-CSF through epigenetic regulation in the TNFA gene promoter locus may exert a potentially beneficial role for the neonatal brain suffering from perinatal bacterial-induced meningitis.

Host-pathogen interactions in bacterial meningitis

Bacterial meningitis is a devastating disease occurring worldwide with up to half of the survivors left with permanent neurological sequelae. Due to intrinsic properties of the meningeal pathogens and the host responses they induce, infection can cause relatively specific lesions and clinical syndromes that result from interference with the function of the affected nervous system tissue. Pathogenesis is based on complex host–pathogen interactions, some of which are specific for certain bacteria, whereas others are shared among different pathogens. In this review, we summarize the recent progress made in understanding the molecular and cellular events involved in these interactions. We focus on selected major pathogens, Streptococcus pneumonia, S. agalactiae (Group B Streptococcus), Neisseria meningitidis, and Escherichia coli K1, and also include a neglected zoonotic pathogen, Streptococcus suis. These neuroinvasive pathogens represent common themes of host–pathogen interactions, such as colonization and invasion of mucosal barriers, survival in the blood stream, entry into the central nervous system by translocation of the blood–brain and blood–cerebrospinal fluid barrier, and induction of meningeal inflammation, affecting pia mater, the arachnoid and subarachnoid spaces.

Interactions of Streptococcus suis serotype 2 with human meningeal cells and astrocytes

BACKGROUND

Streptococcus suis serotype 2 is an important porcine pathogen and emerging zoonotic agent responsible for meningitis, of which different sequence types predominate worldwide. Though bacterial meningitis is defined as an exacerbated inflammation of the meninges, the underlying astrocytes of the glia limitans superficialis may also be implicated. However, the interactions between this pathogen and human meningeal cells or astrocytes remain unknown. Furthermore, the roles of well-described virulence factors (capsular polysaccharide, suilysin and cell wall modifications) in these interactions have yet to be studied. Consequently, the interactions between S. suis serotype 2 and human meningeal cells or astrocytes were evaluated for the first time in order to better understand their involvement during meningitis in humans.

RESULTS

Streptococcus suis serotype 2 adhered to human meningeal cells and astrocytes; invasion of meningeal cells was rare however, whereas invasion of astrocytes was generally more frequent. Regardless of the interaction or cell type, differences were not observed between sequence types. Though the capsular polysaccharide modulated the adhesion to and invasion of meningeal cells and astrocytes, the suilysin and cell wall modifications only influenced astrocyte invasion. Surprising, S. suis serotype 2 induced little or no inflammatory response from both cell types, but this absence of inflammatory response was probably not due to S. suis-induced cell death.

CONCLUSIONS

Though S. suis serotype 2 interacted with human meningeal cells and astrocytes, there was no correlation between sequence type and interaction. Consequently, the adhesion to and invasion of human meningeal cells and astrocytes are strain-specific characteristics. As such, the meningeal cells of the leptomeninges and the astrocytes of the glia limitans superficialis may not be directly implicated in the inflammatory response observed during meningitis in humans.

Leptomeningeal cells transduce peripheral macrophages inflammatory signal to microglia in reponse to Porphyromonas gingivalis LPS

We report here that the leptomeningeal cells transduce inflammatory signals from peripheral macrophages to brain-resident microglia in response to Porphyromonas gingivalis (P.g.) LPS. The expression of Toll-like receptor 2 (TLR2), TLR4, TNF-α, and inducible NO synthase was mainly detected in the gingival macrophages of chronic periodontitis patients. In in vitro studies, P.g. LPS induced the secretion of TNF-α and IL-1β from THP-1 human monocyte-like cell line and RAW264.7 mouse macrophages. Surprisingly, the mean mRNA levels of TNF-α and IL-1β in leptomeningeal cells after treatment with the conditioned medium from P.g. LPS-stimulated RAW264.7 macrophages were significantly higher than those after treatment with P.g. LPS alone. Furthermore, the mean mRNA levels of TNF-α and IL-1β in microglia after treatment with the conditioned medium from P.g. LPS-stimulated leptomeningeal cells were significantly higher than those after P.g. LPS alone. These observations suggest that leptomeninges serve as an important route for transducing inflammatory signals from macrophages to microglia by secretion of proinflammatory mediators during chronic periodontitis. Moreover, propolis significantly reduced the P.g. LPS-induced TNF-α and IL-1 β production by leptomeningeal cells through inhibiting the nuclear factor-κB signaling pathway. Together with the inhibitory effect on microglial activation, propolis may be beneficial in preventing neuroinflammation during chronic periodontitis.

Polysaccharide-specific memory B cells generated by conjugate vaccines in humans conform to the CD27+IgG+ isotype-switched memory B Cell phenotype and require contact-dependent signals from bystander T cells activated by bacterial proteins to differentiate into plasma cells

The polysaccharides (PS) surrounding encapsulated bacteria are generally unable to activate T cells and hence do not induce B cell memory (BMEM). PS conjugate vaccines recruit CD4(+) T cells via a carrier protein, such as tetanus toxoid (TT), resulting in the induction of PS-specific BMEM. However, the requirement for T cells in the subsequent activation of the BMEM at the time of bacterial encounter is poorly understood, despite having critical implications for protection. We demonstrate that the PS-specific BMEM induced in humans by a meningococcal serogroup C PS (Men C)-TT conjugate vaccine conform to the isotype-switched (IgG(+)CD27(+)) rather than the IgM memory (IgM(+)CD27(+)) phenotype. Both Men C and TT-specific BMEM require CD4(+) T cells to differentiate into plasma cells. However, noncognate bystander T cells provide such signals to PS-specific BMEM with comparable effect to the cognate T cells available to TT-specific BMEM. The interaction between the two populations is contact-dependent and is mediated in part through CD40. Meningococci drive the differentiation of the Men C-specific BMEM through the activation of bystander T cells by bacterial proteins, although these signals are enhanced by T cell-independent innate signals. An effect of the TT-specific T cells activated by the vaccine on unrelated BMEM in vivo is also demonstrated. These data highlight that any protection conferred by PS-specific BMEM at the time of bacterial encounter will depend on the effectiveness with which bacterial proteins are able to activate bystander T cells. Priming for T cell memory against bacterial proteins through their inclusion in vaccine preparations must continue to be pursued.

Deciphering the contribution of human meningothelial cells to the inflammatory and antimicrobial response at the meninges

We have investigated the response of primary human meningothelial cells to Neisseria meningitidis. Through a transcriptome analysis, we provide a comprehensive examination of the response of meningothelial cells to bacterial infection. A wide range of chemokines are elicited which act to attract and activate the main players of innate and adaptive immunity. We showed that meningothelial cells expressed a high level of Toll-like receptor 4 (TLR4), and, using a gene silencing strategy, we demonstrated the contribution of this pathogen recognition receptor in meningothelial cell activation. Secretion of interleukin-6 (IL-6), CXCL10, and CCL5 was almost exclusively TLR4 dependent and relied on MyD88 and TRIF adaptor cooperation. In contrast, IL-8 induction was independent of the presence of TLR4, MyD88, and TRIF. Transcription factors NF-κB p65, p38 mitogen-activated protein kinase (MAPK), Jun N-terminal protein kinase (JNK1), IRF3, and IRF7 were activated after contact with bacteria. Interestingly, the protein kinase IRAK4 was found to play a minor role in the meningothelial cell response to Neisseria infection. Our work highlights the role of meningothelial cells in the development of an immune response and inflammation in the central nervous system (CNS) in response to meningococcal infection. It also sheds light on the complexity of intracellular signaling after TLR triggering.

The biology of Neisseria adhesins

Members of the genus Neisseria include pathogens causing important human diseases such as meningitis, septicaemia, gonorrhoea and pelvic inflammatory disease syndrome. Neisseriae are found on the exposed epithelia of the upper respiratory tract and the urogenital tract. Colonisation of these exposed epithelia is dependent on a repertoire of diverse bacterial molecules, extending not only from the surface of the bacteria but also found within the outer membrane. During invasive disease, pathogenic Neisseriae also interact with immune effector cells, vascular endothelia and the meninges. Neisseria adhesion involves the interplay of these multiple surface factors and in this review we discuss the structure and function of these important molecules and the nature of the host cell receptors and mechanisms involved in their recognition. We also describe the current status for recently identified Neisseria adhesins. Understanding the biology of Neisseria adhesins has an impact not only on the development of new vaccines but also in revealing fundamental knowledge about human biology.

Towards a meningitis free world--can we eliminate meningococcal meningitis?: contribution of the meningitis patient groups

Patient groups play a critical part in the fight against meningitis in all its forms. The UK has the world's three largest meningitis patient groups, which over the past 3 decades have worked tirelessly in the fight against meningitis. Within the UK, where the patient groups work to prevent or alleviate the suffering caused by meningitis and septicaemia, their work is in three areas: continued research; demonstrating burden; and awareness-raising and advocacy. The research relates to developing and improving vaccines, and to improving outcomes for forms of meningitis that are not vaccine preventable. Demonstrating burden - showing the real impact of meningitis from a human perspective - highlights the need for vaccines to prevent the disease. Lives are saved by raising awareness of signs and symptoms and of the need for fast action, whilst advocacy can bring about change to improve the quality of life of those affected by meningitis. Awareness raising and advocacy also have the wider benefit of creating a climate in which people recognise the need for vaccines to prevent this dreadful disease. In addition, the patient groups seek to influence the early introduction and uptake of vaccines as they are licensed and approved by the expert bodies, the UK body being the Joint Committee for Vaccination and Immunisation (JCVI). Each area of activity is explored, and examples given from each of the patient groups of work they have done or are doing in that area.

Coadministration of the cyanobacterial lipopolysaccharide antagonist CyP with antibiotic inhibits cytokine production by an in vitro meningitis model infected with Neisseria meningitidis

OBJECTIVES

In this study, the objective was to determine the anti-inflammatory properties of CyP, a cyanobacterial lipopolysaccharide (LPS) antagonist, used in combination with antibiotic chemotherapy during infection of an in vitro meningitis model infected with Neisseria meningitidis (meningococcus).

METHODS

Monocultures of human meningioma cells and meningioma-primary human macrophage co-cultures were infected with meningococci (10(2)-10(8) cfu/monolayer) or treated with isolated outer membranes or purified LPS (0.1-100 ng/monolayer) from N. meningitidis. CyP (1-20 μg/monolayer) was added at intervals from t = 0 to 4 h, with and without benzylpenicillin (1-20 μg/monolayer). The antagonistic effect of CyP and its adjunctive properties to benzylpenicillin administration was determined by measuring cytokine levels in culture supernatants after 24 h.

RESULTS

CyP significantly inhibited (P < 0.05) the secretion of interleukin (IL)-6, IL-8, monocyte chemoattractant protein (MCP)-1 and RANTES ('regulated upon activation, normal T cell expressed and secreted') (overall reduction levels from 50% to >95%) by meningioma cell lines and meningioma-macrophage co-cultures challenged with either live meningococci or bacterial components. Inhibition was effective when CyP was added within 2 h of challenge (P < 0.05) and was still pronounced by 4 h. In the co-culture model, CyP alone partially inhibited IL-1β secretion, but did not prevent tumour necrosis factor (TNF)-α secretion, whereas penicillin alone inhibited IL-1β and TNF-α but conversely did not reduce MCP-1 and RANTES secretion. However, coadministration of CyP and penicillin in both models had an additive effect and restored the overall inhibitory profile.

CONCLUSIONS

CyP inhibits cytokine production in an in vitro meningitis model and augments the anti-inflammatory response when combined with benzylpenicillin. Administration of an LPS antagonist with antibiotic merits consideration in the emergency treatment of patients presenting with meningococcal infection.

Visualising PAMP-PRR interactions using nanoscale imaging

The innate immune system utilises a set of receptors, called pattern recognition receptors (PRRs), in order to recognise specific molecular patterns or motifs called pathogen-associated molecular patterns (PAMPs) on invading pathogens. The toll-like receptor (TLR) family of proteins is an integral part of the mammalian innate immune system. We are now beginning to decipher which TLRs are involved in the recognition of particular microbial patterns, but questions remain as to the homo- and heterotypic associations that TLRs form and how these associations affect their activation. Technical advances in fluorescence microscopy has enabled us to investigate the functional associations of TLRs and other PPRs in living cells in response to different pathogens using non-invasive fluorescence imaging methods. In this chapter, we will describe some of the fluorescent imaging techniques, such as FRET and FRAP, that we employ in order to study PAMP-PRR associations.

Genome-wide expression profiling and mutagenesis studies reveal that lipopolysaccharide responsiveness appears to be absolutely dependent on TLR4 and MD-2 expression and is dependent upon intermolecular ionic interactions

Lipid A (a hexaacylated 1,4' bisphosphate) is a potent immune stimulant for TLR4/MD-2. Upon lipid A ligation, the TLR4/MD-2 complex dimerizes and initiates signal transduction. Historically, studies also suggested the existence of TLR4/MD-2-independent LPS signaling. In this article, we define the role of TLR4 and MD-2 in LPS signaling by using genome-wide expression profiling in TLR4- and MD-2-deficient macrophages after stimulation with peptidoglycan-free LPS and synthetic Escherichia coli lipid A. Of the 1396 genes significantly induced or repressed by any one of the treatments in the wild-type macrophages, none was present in the TLR4- or MD-2-deficient macrophages, confirming that the TLR4/MD-2 complex is the only receptor for endotoxin and that both are required for responses to LPS. Using a molecular genetics approach, we investigated the mechanism of TLR4/MD-2 activation by combining the known crystal structure of TLR4/MD-2 with computer modeling. According to our murine TLR4/MD-2-activation model, the two phosphates on lipid A were predicted to interact extensively with the two positively charged patches on mouse TLR4. When either positive patch was abolished by mutagenesis into Ala, the responses to LPS and lipid A were nearly abrogated. However, the MyD88-dependent and -independent pathways were impaired to the same extent, indicating that the adjuvant activity of monophosphorylated lipid A most likely arises from its decreased potential to induce an active receptor complex and not more downstream signaling events. Hence, we concluded that ionic interactions between lipid A and TLR4 are essential for optimal LPS receptor activation.

Location, location, location: is membrane partitioning everything when it comes to innate immune activation?

In the last twenty years, the general view of the plasma membrane has changed from a homogeneous arrangement of lipids to a mosaic of microdomains. It is currently thought that islands of highly ordered saturated lipids and cholesterol, which are laterally mobile, exist in the plane of the plasma membrane. Lipid rafts are thought to provide a means to explain the spatial segregation of certain signalling pathways emanating from the cell surface. They seem to provide the necessary microenvironment in order for certain specialised signalling events to take place, such as the innate immune recognition. The innate immune system seems to employ germ-lined encoded receptors, called pattern recognition receptors (PRRs), in order to detect pathogens. One family of such receptors are the Toll-like receptors (TLRs), which are the central “sensing” apparatus of the innate immune system. In recent years, it has become apparent that TLRs are recruited into membrane microdomains in response to ligands. These nanoscale assemblies of sphingolipid, cholesterol, and TLRs stabilize and coalesce, forming signalling platforms, which transduce signals that lead to innate immune activation. In the current paper, we will investigate all past and current literature concerning recruitment of extracellular and intracellular TLRs into lipid rafts and how this membrane organization modulates innate immune responses.

Biochemical investigation and gene expression analysis of the immunostimulatory functions of an edible Salacia extract in rat small intestine

Roots and bark from plants belonging to genus Salacia of the family Hippocrateaceae (Salacia reticulata, Salacia oblonga, etc.) have been used for traditional Ayurvedic medicine, particularly for the treatment of diabetes. In our study, we evaluated the gene expression profiles in the small intestinal epithelium of rats that were given a Salacia plant extract to gain insight into its effects on the small intestine. In detail, DNA microarray analysis was performed to evaluate the gene expression profiles in the rat ileal epithelium. The intestinal bacterial flora was also studied using T-RFLP (Nagashima method) in these rats. Expressions of many immune-related genes, especially Th1-related genes associated with cell-mediated immunity, were found to increase in the small intestinal epithelium and the intestinal bacterial flora became similar to those in the case with Salacia plant extract administration. Our study thus revealed that Salacia plant extract exerts bioregulatory functions by boosting intestinal immunity.

Neisseria meningitidis lipid A mutant LPSs function as LPS antagonists in humans by inhibiting TLR 4-dependent cytokine production

Lipopolysaccharide is a major constituent of the outer membrane of Gram-negative bacteria and important in the induction of pro-inflammatory responses. Recently, novel LPS species derived from Neisseria meningitidis H44/76 by insertional inactivation of the lpxL1 and lpxL2 genes have been created with a lipid A portion consisting of five (penta-acylated lpxL1) or four (tetra-acylated lpxL2) fatty acids connected to the glucosamine backbone instead of six fatty acids in the wild-type LPS. We show that these mutant LPS-types are poor inducers of cytokines (tumor-necrosis factor-α, IL-1β, IL-10, IL-RA) in human mononuclear cells. Both penta- and tetra-acylated meningococcal LPSs were able to inhibit cytokine production by wild-type Escherichia coli or meningococcal LPS. Binding of FITC-labelled E. coli LPS TLR4 transfected Chinese hamster ovary (CHO) cells was inhibited by both mutant LPS-types. Experiments with CHO fibroblasts transfected with human CD14 and TLR4 showed that the antagonizing effect was dependent on the expression of human TLR4. In contrast to the situation in humans, lpxL1 LPS has agonistic activity for cytokine production in peritoneal macrophages of DBA mice, and exacerbated arthritis in murine collagen induced arthritis model. N. meningitidis lipid A mutant LPSs lpxL1 and lpxL2 function as LPS antagonists in humans by inhibiting TLR4-dependent cytokine production but have agonistic activity in mice.

Membrane partitioning: is location everything when it comes to endotoxin recognition?

Lipid rafts are envisaged as islands of highly ordered saturated lipids and cholesterol that are laterally mobile in the plane of the plasma membrane. Lipid rafts are thought to provide a means to explain the spatial segregation of certain signalling pathways emanating from the cell surface. They seem to provide the necessary microenvironment in order for certain specialised signalling events to take place- such as the innate immune recognition. The innate immune system seems to employ germ-lined encoded receptors, called pattern recognition receptors (PRRs) in order to "sense" pathogens. One family of such receptors are the Toll like receptors (TLRs), which are the central "sensing" apparatus of the innate immune system. In recent years, it has become apparent that TLRs are recruited into membrane microdomains in response to ligands and these constitute signalling platforms, which transducer singals that lead to innate immune activation. In this chapter will review all past and current literature concerning recruitment of TLRs into lipid rafts and how this membrane compartmentalization is crucial for innate immune responses.

Interactions of Haemophilus parasuis and its LOS with porcine brain microvascular endothelial cells

Haemophilus parasuis is a swine pathogen that causes Glässer's disease, which is characterized by polyserositis and meningitis. The pathogenesis of the H. parasuis infection is poorly understood. To cause meningitis, H. parasuis has to cross the blood-brain barrier (BBB) to gain access to the central nervous system (CNS). We recently showed that H. parasuis adheres to and invades porcine brain microvascular endothelial cells (PBMEC). The aim of this study was to evaluate the role of H. parasuis lipooligosaccharide (LOS) in the adhesion to PBMEC and to determine if H. parasuis (and/or its LOS) is able to induce apoptosis and activation of PBMEC. Results showed that adhesion of H. parasuis to PBMEC was partially mediated by LOS. Moreover, H. parasuis induces caspase-3-mediated apoptosis of PBMEC in a time--and dose--dependent manner, but its LOS did not seem to be involved in such a process. Furthermore, H. parasuis and, to a lesser extent, its LOS, was able to induce the release of IL-8 and IL-6 by PBMEC. Field strains of H. parasuis serotypes 4 and 5 induced similar levels of these inflammatory mediators. Our data suggest that H. parasuis uses cellular adhesion, induction of apoptosis and up-regulation of inflammatory mediators as mechanisms to invade the CNS via the BBB, and that LOS would play a certain but limited role in such pathological process.

Identification of genes particularly sensitive to lipopolysaccharide (LPS) in human monocytes induced by wild-type versus LPS-deficient Neisseria meningitidis strains

Lipopolysaccharide (LPS) in the outer membrane of Neisseria meningitidis plays a dominant role as an inflammation-inducing molecule in meningococcal disease. We have used microarray analysis to study the global gene expression after exposure of human monocytes for 3 h to wild-type N. meningitidis (10(6)), LPS-deficient N. meningitidis (10(6) and 10(8)), and purified N. meningitidis LPS (1 ng [33 endotoxin units]/ml) to identify LPS-inducible genes. Wild-type N. meningitidis (10(6)) induced 4,689 differentially expressed genes, compared with 72 differentially expressed genes induced by 10(6) LPS-deficient N. meningitidis organisms. However, 10(8) LPS-deficient N. meningitidis organisms induced 3,905 genes, indicating a dose-response behavior of non-LPS cell wall molecules. A comparison of the gene expression patterns from 10(6) wild-type N. meningitidis and 10(8) LPS-deficient N. meningitidis organisms showed that 2,401 genes in human monocytes were not strictly LPS dependent. A list of "particularly LPS-sensitive" genes (2,288), differentially induced by 10(6) wild-type N. meningitidis but not by 10(8) LPS-deficient N. meningitidis organisms, showed an early expression of beta interferon (IFN-beta), most likely through the Toll-like receptor-MyD88-independent pathway. Subsequently, IFN-beta may activate the type I IFN signaling pathway, and an unknown number of IFN-beta-inducible genes, such as those for CXCL9, CXCL10, CXCL11, IFIT1, IFIT2, IFIT3, and IFIT5, are transcribed. Supporting this, human monocytes secreted significantly higher levels of CXCL10 and CXCL11 when stimulated by 10(6) wild-type N. meningitidis organisms than when stimulated by 10(8) LPS-deficient N. meningitidis organisms. Plasma CXCL10, but not CXCL11, was positively correlated (r = 0.67; P < 0.01) to LPS in patients (n = 24) with systemic meningococcal disease. Thus, new circulating biomarkers in meningococcal disease may be suggested through LPS-induced gene expression changes in human monocytes.

Non-lipooligosaccharide-mediated signalling via Toll-like receptor 4 causes fatal meningococcal sepsis in a mouse model

Meningococcal lipooligosaccharide (LOS) is a major inflammatory mediator of fulminant meningococcal sepsis and meningitis with disease severity correlating with circulating concentrations of LOS and proinflammatory cytokines. In this study we show that the proinflammatory response to live meningococci in a mouse model of sepsis involves TLR4, but can develop independently of the expression of LOS. This is supported by data showing that in vivo an isogenic LOS-deficient strain, lpxA, induced equivalent disease severity and similar proinflammatory responses as the serogroup C wild-type parent strain FAM20. This response was abolished in TLR4-/- mice, and neither the wild-type strain of meningococci nor the LOS-deficient mutant was able to cause fatal sepsis in these mice. Mouse survival correlated with low levels of cytokines and chemokines, the chemotactic complement factor C5a and neutrophil levels in blood at 24 h post infection. These data suggest that during meningococcal sepsis the recognition of one or more unidentified non-LOS component(s) by TLR4 is important in stimulating proinflammatory responses, and that fatality associated with meningococcal sepsis in mice is induced by the proinflammatory host response.

Comparison of the inflammatory responses of human meningeal cells following challenge with Neisseria lactamica and with Neisseria meningitidis

The rationale for the present study was to determine how different species of bacteria interact with cells of the human meninges in order to gain information that would have broad relevance to understanding aspects of the innate immune response in the brain. Neisseria lactamica is an occasional cause of meningitis in humans, and in this study we investigated the in vitro interactions between N. lactamica and cells derived from the leptomeninges in comparison with the closely related organism Neisseria meningitidis, a major cause of meningitis worldwide. N. lactamica adhered specifically to meningioma cells, but the levels of adherence were generally lower than those with N. meningitidis. Meningioma cells challenged with N. lactamica and N. meningitidis secreted significant amounts of the proinflammatory cytokine interleukin-6 (IL-6), the C-X-C chemokine IL-8, and the C-C chemokines monocyte chemoattractant protein 1 (MCP-1) and RANTES, but it secreted very low levels of the cytokine growth factor granulocyte-macrophage colony-stimulating factor (GM-CSF). Thus, meningeal cells are involved in the innate host response to Neisseria species that are capable of entering the cerebrospinal fluid. The levels of IL-8 and MCP-1 secretion induced by both bacteria were essentially similar. By contrast, N. lactamica induced significantly lower levels of IL-6 than N. meningitidis. Challenge with the highest concentration of N. lactamica (10(8) CFU) induced a small but significant down-regulation of RANTES secretion, which was not observed with lower concentrations of bacteria. N. meningitidis (10(6) to 10(8) CFU) also down-regulated RANTES secretion, but this effect was significantly greater than that observed with N. lactamica. Although both bacteria were unable to invade meningeal cells directly, host cells remained viable on prolonged challenge with N. lactamica, whereas N. meningitidis induced death; the mechanism was overwhelming necrosis with no significant apoptosis. It is likely that differential expression of modulins between N. lactamica and N. meningitidis contributes to these observed differences in pathogenic potential.

MyD88-dependent signaling affects the development of meningococcal sepsis by nonlipooligosaccharide ligands

The Toll-like receptors (TLRs) and the adaptor myeloid differentiation factor 88 (MyD88) are important in the innate immune defenses of the host to microbial infections. Meningococcal ligands signaling via TLRs control inflammatory responses, and stimulation can result in fulminant meningococcal sepsis. In this study, we show that the responses to nonlipooligosaccharide (non-LOS) ligands of meningococci are MyD88 dependent. An isogenic LOS-deficient mutant of the serogroup C meningococcal strain FAM20 caused fatal disease in wild type C57BL/6 mice that was not observed in MyD88-/- mice. Fatality correlated with high proinflammatory cytokine and C5a levels in serum, high neutrophil numbers in blood, and increased bacteremia at 24 h postinfection in the wild-type mice. Infection with the parent strain FAM20 resulted in fatality in 100% of the wild-type mice and 50% of the MyD88-/- mice. We conclude that both LOS and another neisserial ligand cause meningococcal sepsis in an in vivo mouse model and confirm that meningococcal LOS can act via both the MyD88- dependent and -independent pathways, while the non-LOS meningococcal ligand(s) acts only via the MyD88-dependent pathway.

Protein secretion and secreted proteins in pathogenicNeisseriaceae

Secreted proteins of pathogenic bacteria are often essential virulence factors. They are involved, for example, in the adherence of the bacteria to host cells or required to suppress the host's defence mechanisms. Until recently, only IgA1 protease had been studied in detail in the NeisseriaceaeNeisseria meningitidis and Neisseria gonorrhoeae. The availability of their genome sequences, however, has boosted research in this area. Here, we present a survey of the secretome of the pathogenic Neisseriaceae, based on the available genome sequences, and the current knowledge of the functions and structures of the secreted proteins. Of the six protein-secretion pathways that are widely disseminated among Gram-negative bacteria, three pathways appear to be present among the Neisseriaceae, i.e. the autotransporter-, the two-partner- and the type I-secretion mechanisms. Comparison of the predicted secretomes reveals a considerable flexibility. As compared with N. meningitidis and the nonpathogen N. lactamica, N. gonorrhoeae appears to have a considerably degenerated secretome, which may reflect its altered niche occupancy. The flexibility of the secretome may be enhanced by the presence of ORFs in the genomes potentially encoding fragments of secreted proteins. We hypothesize that these ORFs may substitute for the corresponding fragments in the full-length genes through genetic recombination, thereby changing the host-cell receptor specificity of the secreted protein.

Lipooligosaccharide-independent alteration of cellular homeostasis in Neisseria meningitidis-infected epithelial cells

Neisseria meningitidis (MC) is an important cause of meningitis and septic shock. Primary loose attachment of MC to host epithelial cells is mediated by type IV pili. Lipooligosaccharide (LOS), opacity (Opa) proteins and glycolipid adhesins facilitate subsequent tight attachment. MC infection causes numerous changes in host epithelial cell homeostasis. These include cortical plaque formation, increased expression of proinflammatory cytokines and alterations in host iron homeostasis. Using both biochemical and genetic approaches, we examined the role of LOS in mediating these events. We first examined specific cellular iron homeostasis changes that occur following addition of purified MC LOS to epithelial cells. Using an MC mutant that completely lacks LOS (MC lps tbp), we examined pili-mediated attachment and cortical plaque formation in human endocervical epithelial cells (A431). We also tested whether the lack of LOS alters cellular homeostasis, including changes in the levels of host stress response factors and proinflammatory cytokines. MC lps tbp elicited the formation of cortical plaques in A431 cells. However, the plaques were less pronounced than those formed by the MC parent. Surprisingly, the proinflammatory cytokine TNF(alpha) was upregulated during infection in MC lps tbp-infected cells. Furthermore, alterations in iron homeostasis, including lower transferrin receptor 1 (TfR-1) levels, altered TfR-1 trafficking, an 'iron-starvation' gene expression profile and low iron regulatory protein (IRP) binding activity are independent of LOS. Our results demonstrate that LOS is partially involved in both the attachment to host cells and formation of cortical plaques. However, TNFalpha induction and changes in iron homeostasis observed in MC-infected epithelial cells are independent of LOS.