Lipid A, the immunostimulatory principle of lipopolysaccharides?

Periodontitis and cardiometabolic disorders: The role of lipopolysaccharide and endotoxemia

Lipopolysaccharide is a virulence factor of gram-negative bacteria with a crucial importance to the bacterial surface integrity. From the host's perspective, lipopolysaccharide plays a role in both local and systemic inflammation, activates both innate and adaptive immunity, and can trigger inflammation either directly (as a microbe-associated molecular pattern) or indirectly (by inducing the generation of nonmicrobial, danger-associated molecular patterns). Translocation of lipopolysaccharide into the circulation causes endotoxemia, which is typically measured as the biological activity of lipopolysaccharide to induce coagulation of an aqueous extract of blood cells of the assay. Apparently healthy subjects have a low circulating lipopolysaccharide activity, since it is neutralized and cleared rapidly. However, chronic endotoxemia is involved in the pathogenesis of many inflammation-driven conditions, especially cardiometabolic disorders. These include atherosclerotic cardiovascular diseases, obesity, liver diseases, diabetes, and metabolic syndrome, where endotoxemia has been recognized as a risk factor. The main source of endotoxemia is thought to be the gut microbiota. However, the oral dysbiosis in periodontitis, which is typically enriched with gram-negative bacterial species, may also contribute to endotoxemia. As endotoxemia is associated with an increased risk of cardiometabolic disorders, lipopolysaccharide could be considered as a molecular link between periodontal microbiota and cardiometabolic diseases.

Whole-blood cultures from patients with chronic periodontitis respond differently to Porphyromonas gingivalis but not Escherichia coli lipopolysaccharide

BACKGROUND

Porphyromonas gingivalis lipid A heterogeneity modulates cytokine expression in human cells. This study investigates the effects of two lipid A isoforms of P. gingivalis, lipopolysaccharide (LPS)1435/1449 and LPS1690, on the secretion of proinflammatory and regulatory cytokines in total blood cultures from patients with and without chronic periodontitis (CP).

METHODS

A cross-sectional study was conducted in 38 systemically healthy individuals divided in two groups: 1) the CP group (n = 19), in which patients were diagnosed with CP; and 2) the no periodontitis (NP) group (n = 19), which included control patients without CP. Blood samples were collected from all patients, and whole-blood cell cultures (WBCCs) were stimulated for 48 hours with P. gingivalis LPS1435/1449 and LPS1690 and Escherichia coli LPS. Unstimulated WBCCs served as negative controls. The secretion of interferon-γ (IFN-γ), interleukin-10 (IL-10), and transforming growth factor-β (TGF-β) was detected in WBCC supernatants by enzyme-linked immunosorbent assays.

RESULTS

E. coli LPS significantly increased the expression of all cytokines in WBCCs from both the NP and CP groups when compared to non-stimulated cells (control treatment). P. gingivalis LPS preparations increased IFN-γ levels in the CP group but not in the NP group when compared with controls (P <0.05). P. gingivalis LPS preparations also increased IL-10 and TGF-β levels in both CP and NP groups, but P. gingivalis LPS1690 showed a three-fold increase on IL-10 production in the NP group (P <0.05) when compared to P. gingivalis LPS1435/144.

CONCLUSIONS

These data demonstrate that WBCC cell populations obtained from healthy individuals and patients with CP may differ in the cytokine response to P. gingivalis but not E. coli LPS. This is consistent with the notion that CP alters the systemic WBCC response and that this can be detected by the different P. gingivalis LPS structures.

Characterization of WRSs2 and WRSs3, new second-generation virG(icsA)-based Shigella sonnei vaccine candidates with the potential for reduced reactogenicity

Live, attenuated Shigella vaccine candidates, such as Shigella sonnei strain WRSS1, Shigella flexneri 2a strain SC602, and Shigella dysenteriae 1 strain WRSd1, are attenuated principally by the loss of the VirG(IcsA) protein. These candidates have proven to be safe and immunogenic in volunteer trials and in one study, efficacious against shigellosis. One drawback of these candidate vaccines has been the reactogenic symptoms of fever and diarrhea experienced by the volunteers, that increased in a dose-dependent manner. New, second-generation virG(icsA)-based S. sonnei vaccine candidates, WRSs2 and WRSs3, are expected to be less reactogenic while retaining the ability to generate protective levels of immunogenicity seen with WRSS1. Besides the loss of VirG(IcsA), WRSs2 and WRSs3 also lack plasmid-encoded enterotoxin ShET2-1 and its paralog ShET2-2. WRSs3 further lacks MsbB2 that reduces the endotoxicity of the lipid A portion of the bacterial LPS. Studies in cell cultures and in gnotobiotic piglets demonstrate that WRSs2 and WRSs3 have the potential to cause less diarrhea due to loss of ShET2-1 and ShET2-2 as well as alleviate febrile symptoms by loss of MsbB2. In guinea pigs, WRSs2 and WRSs3 were as safe, immunogenic and efficacious as WRSS1.

Particle size distributions and concentrations of airborne endotoxin using novel collection methods in homes during the winter and summer seasons

A comparison study of novel collection methods for airborne bacteria and endotoxin was performed in an environmentally controlled chamber and in pilot-field studies. Airborne particulate matter was collected in swirling liquid impingers, air-monitoring filter cassettes, and with a micro-orifice uniform deposit impactor (MOUDI) to evaluate aerodynamic particle size distributions. Environmentally controlled chamber studies showed that impingers and MOUDI recovered significantly more airborne bacteria than filter cassettes, whereas collection methods for airborne endotoxin were not significantly different. In addition, total airborne bacteria and endotoxin concentrations were measured indoors and outdoors at three homes in Boulder, CO during winter and summer seasons. Indoor concentrations collected with the three different samplers were significantly different for airborne endotoxin, but not for airborne bacteria. Total airborne bacteria indoors and outdoors significantly varied with seasons. Outdoor airborne endotoxin significantly varied with season; no seasonal variation was seen for indoor airborne endotoxin. Indoor and outdoor levels were not significantly different for both airborne bacteria and endotoxin. The largest proportion of endotoxin was associated with airborne particulate matter <1 microm.

PRACTICAL IMPLICATIONS

This study compared sampling methods for airborne endotoxin, a potent and nonspecific immune system stimulant which can induce negative health responses. The data from this study showed that swirling liquid impingers and the micro-orifice uniform deposit impactor (MOUDI) recovered significantly more airborne endotoxin than the more widely adapted method of collecting airborne endotoxin on membrane filters, when collection methods were applied in realistic settings (homes). The MOUDI measured the particle size distribution of airborne endotoxin, which can be useful for determining endotoxin respiratory toxicity and its health effects.

Lipopolysaccharide heterogeneity: innate host responses to bacterial modification of lipid a structure

The innate host response system is composed of various mechanisms designed to detect and facilitate host responses to microbial components, such as lipopolysaccharides (LPS). To enable this to occur, innate systems contain multiple pattern recognition receptors (i.e., LBP, CD14, and TLRs), which identify certain features within bacterial LPS that are foreign to the host, as well as essential and uniquely specific for bacteria. Innate host identification of unique bacterial components or patterns, therefore, relies on the inability of bacteria to alter these essential or critical components dramatically. Historically, LPS have been viewed as essential outer-membrane molecules containing both a highly variable outer region (O-segment) as well as a relatively conserved inner region (lipid A). However, over the last decade, new evidence has emerged, revealing that increased natural diversity or heterogeneity within specific components of LPS, such as lipid A-resulting in minor to moderate changes in lipid A structure-can produce dramatic host responses. Therefore, examples of natural lipid A heterogeneity, and the mechanisms that control it, represent a novel approach in which bacteria modulate host responses and may thereby confer specific advantages to certain bacterial species under changing environmental host conditions.

Characterization of a novel lipid-A from Rhizobium species Sin-1. A unique lipid-A structure that is devoid of phosphate and has a glycosyl backbone consisting of glucosamine and 2-aminogluconic acid

The structure of the lipid-A from Rhizobium species Sin-1, a nitrogen-fixing Gram-negative bacterial symbiont of Sesbania, was determined by composition, nuclear magnetic resonance spectroscopic, and mass spectrometric analyses. The lipid-A preparation consisted of a mixture of structures due to differences in fatty acylation and in the glycosyl backbone. There were two different disaccharide backbones. One disaccharide consisted of a distal glucosaminosyl residue beta-linked to position 6 of a proximal 2-aminoglucono-1,5-lactonosyl residue, and in the second disaccharide, the proximal residue was 2-amino-2,3-dideoxy-d-erythro-hex-2-enono-1,5-lactone. For both disaccharides, the distal glucosamine was acylated at C-2' primarily with beta-hydroxypalmitate (beta-OHC16:0) which, in turn, was O-acylated with 27-hydroxyoctacosanoic acid. For some of the lipid-A molecules, the distal glucosaminosyl residue was also acylated at C-3' with beta-hydroxymyristate (beta-OHC14:0), whereas other molecules were devoid of this acyl substituent. Both the 2-aminoglucono-1,5-lactonosyl and 2-amino-2,3-dideoxy-d-erythro-hex-2-enono-1,5-lactonosyl residues were acylated at C-2, primarily with beta-OHC16:0. Minor amounts of lipid-A molecules contained beta-OHC14:0 at C-3 and/or beta-hydroxystearate (beta-OHC18:0) or beta-hydroxyoctadecenoate (beta-OHC18:1) as the C-2 and C-2' N-acyl substituents.

Comparison of the effects of Salmonella minnesota Re595 lipopolysaccharide, lipid A and monophosphoryl lipid A on nitric oxide, TNF-alpha, and IL-6 induction from RAW 264.7 macrophages

Lipopolysaccharide (LPS) exhibits a wide variety of bioactivities. Although it was generally proposed that the lipid A component represented the active center responsible for most of the bioactivities of LPS, a variety of lipid A partial structures and analogues were reported to have different properties. Lipopolysaccharide of the Re595 mutant of Salmonella minnesota is lack of O and part of the core polysaccharide (2 keto-3-deoxyoctanate (KDO) left on lipid A). Re595 lipid A (LA) and Re595 monophosphoryl lipid A (MPLA) differ in structure from Re595 LPS by lacking KDO and KDO plus phosphoryl group respectively. Whether these lipid A-common Re595 LPS preparations differed in activities, we investigated their effects on nitric oxide (NO), TNF-alpha, IL-6, and IL-12 induction from murine macrophage cell line RAW 264.7. RAW 264.7 cells (2 x 10(5) cells ml(-1)) were stimulated with these LPS preparations at 1 microg ml(-1) for 48 h. Re595 LPS, Re595 LA and Re595 MPLA significantly induced NO, TNF-alpha and IL-6 production; NO, TNF-alpha and IL-6 inducing capacities were in the order of LPS = LA > MPLA, LPS = LA = MPLA, and LPS = LA > MPLA respectively. However, these preparations did not induce IL-12 production from RAW cells even when stimulated in combination with IFN-gamma (20 U ml(-1)). IFN-gamma itself also induced NO, TNF-alpha and IL-6 production from RAW 264.7 cells. When RAW 264.7 cells were stimulated with IFN-gamma plus any of these preparations, effects were additive and synergistic for NO and IL-6 responses respectively. But TNF-alpha responses of RAW cells against these preparations were almost equal when cultured alone or in combination with IFN-gamma. Pre-treatment of RAW cells either with LPS, LA or MPLA at low concentration (0.1 microg ml(-1)) for 60 min before pulsing with IFN-gamma (20 IU ml(-1)) plus LPS (1 microg ml(-1)) for an additional 48 h, significantly (P < 0.01) decreased NO response. Although to a lesser extent, TNF-alpha and IL-6 responses were also decreased. Complete inhibition of NO inducing effect of these LPS preparations was achieved with polymyxin B at 40 microg ml(-1). But the concentration of polymyxin B to get a significant (P < 0.05) inhibitory effect on LPS was four times higher than that for LA or MPLA. Unexpectedly, polymyxin B also inhibited INF-gamma-induced NO production from RAW cells in a dose-dependent fashion. These findings suggested that effect of LPS was dependent, at least in part, on both the LPS polysaccharide chain length and the hydrophilic portion of LPS. In addition, not only LPS but also LA and MPLA exert either enhancing or suppressive effects, depending on their concentrations and the timing of their addition with respect to co-stimulators.