Vaccine development targeting lipopolysaccharide structure modification

The Brucella Cell Envelope

The cell envelope is a multilayered structure that insulates the interior of bacterial cells from an often chaotic outside world. Common features define the envelope across the bacterial kingdom, but the molecular mechanisms by which cells build and regulate this critical barrier are diverse and reflect the evolutionary histories of bacterial lineages. Intracellular pathogens of the genus Brucella exhibit marked differences in cell envelope structure, regulation, and biogenesis when compared to more commonly studied gram-negative bacteria and therefore provide an excellent comparative model for study of the gram-negative envelope. We review distinct features of the Brucella envelope, highlighting a conserved regulatory system that links cell cycle progression to envelope biogenesis and cell division. We further discuss recently discovered structural features of the Brucella envelope that ensure envelope integrity and that facilitate cell survival in the face of host immune stressors.

Bacterial lipopolysaccharide-induced endothelial activation and dysfunction: a new predictive and therapeutic paradigm for sepsis

BACKGROUND

Lipopolysaccharide, a highly potent endotoxin responsible for severe sepsis, is the major constituent of the outer membrane of gram-negative bacteria. Endothelial cells participate in both innate and adaptive immune responses as the first cell types to detect lipopolysaccharide or other foreign debris in the bloodstream. Endothelial cells are able to recognize the presence of LPS and recruit specific adaptor proteins to the membrane domains of TLR4, thereby initiating an intracellular signaling cascade. However, lipopolysaccharide binding to endothelial cells induces endothelial activation and even damage, manifested by the expression of proinflammatory cytokines and adhesion molecules that lead to sepsis.

MAIN FINDINGS

LPS is involved in both local and systemic inflammation, activating both innate and adaptive immunity. Translocation of lipopolysaccharide into the circulation causes endotoxemia. Endothelial dysfunction, including exaggerated inflammation, coagulopathy and vascular leakage, may play a central role in the dysregulated host response and pathogenesis of sepsis. By discussing the many strategies used to treat sepsis, this review attempts to provide an overview of how lipopolysaccharide induces the ever more complex syndrome of sepsis and the potential for the development of novel sepsis therapeutics.

CONCLUSIONS

To reduce patient morbidity and mortality, preservation of endothelial function would be central to the management of sepsis.

Brucella melitensis Wzm/Wzt System: Changes in the Bacterial Envelope Lead to Improved Rev1Δwzm Vaccine Properties

The lipopolysaccharide (LPS) O-polysaccharide (O-PS) is the main virulence factor in Brucella. After synthesis in the cytoplasmic membrane, O-PS is exported to the periplasm by the Wzm/Wzt system, where it is assembled into a LPS. This translocation also engages a bactoprenol carrier required for further biosynthesis pathways, such as cell wall biogenesis. Targeting O-PS export by blockage holds great potential for vaccine development, but little is known about the biological implications of each Wzm/Wzt moiety. To improve this knowledge and to elucidate its potential application as a vaccine, we constructed and studied wzm/wzt single- and double-deletion mutants, using the attenuated strain Brucella melitensis Rev1 as the parental strain. This allowed us to describe the composition of Brucella peptidoglycan for the first time. We observed that these mutants lack external O-PS yet trigger changes in genetic transcription and in phenotypic properties associated with the outer membrane and cell wall. The three mutants are highly attenuated; unexpectedly, Rev1Δwzm also excels as an immunogenic and effective vaccine against B. melitensis and Brucella ovis in mice, revealing that low persistence is not at odds with efficacy. Rev1Δwzm is attenuated in BeWo trophoblasts, does not infect mouse placentas, and is safe in pregnant ewes. Overall, these attributes and the minimal serological interference induced in sheep make Rev1Δwzm a highly promising vaccine candidate.

T6SS secretes an LPS-binding effector to recruit OMVs for exploitative competition and horizontal gene transfer

Outer membrane vesicles (OMVs) can function as nanoscale vectors that mediate bacterial interactions in microbial communities. How bacteria recognize and recruit OMVs inter-specifically remains largely unknown, thus limiting our understanding of the complex physiological and ecological roles of OMVs. Here, we report a ligand-receptor interaction-based OMV recruitment mechanism, consisting of a type VI secretion system (T6SS)-secreted lipopolysaccharide (LPS)-binding effector TeoL and the outer membrane receptors CubA and CstR. We demonstrated that Cupriavidus necator T6SS1 secretes TeoL to preferentially associate with OMVs in the extracellular milieu through interactions with LPS, one of the most abundant components of OMVs. TeoL associated with OMVs can further bind outer membrane receptors CubA and CstR, which tethers OMVs to the recipient cells and allows cargo to be delivered. The LPS-mediated mechanism enables bacterial cells to recruit OMVs derived from different species, and confers advantages to bacterial cells in iron acquisition, interbacterial competition, and horizontal gene transfer (HGT). Moreover, our findings provide multiple new perspectives on T6SS functionality in the context of bacterial competition and HGT, through the recruitment of OMVs.

Dissecting Lipopolysaccharide Composition and Structure by GC-MS and MALDI Spectrometry

Lipopolysaccharides (LPSs) are the main components of the external leaflet of the outer membrane of Gram-negative bacteria. They exert multiple functions, starting from conferring stability to the bacterial membrane to mediating the interaction of the microbe with the external environment. The composition and the structure of LPSs present tremendous diversity even within bacteria of the same species, and for this reason, the determination of the structure of these molecules is crucial because it can provide information on the motifs key for the virulence of a pathogen or that are associated to a bacterium of the commensal or beneficial microbiota. In addition, structural data disclose the effects triggered from a mutation or from the use of an antibiotic, or they can be used as tools to check the quality of adjuvants and/or medications, as vaccines, that make use of LPS.The structural study of LPSs is complex, and it can be achieved with the right combination of different techniques. In this frame, this chapter focuses on the two MS-based approaches, the gas chromatography-mass spectrometry (GC-MS) and the matrix-assisted laser desorption/ionization (MALDI).

Development of attenuated live vaccine candidates against swine brucellosis in a non-zoonotic B. suis biovar 2 background

Brucella is a genus of gram-negative bacteria that cause brucellosis. B. abortus and B. melitensis infect domestic ruminants while B. suis (biovars 1-3) infect swine, and all these bacteria but B. suis biovar 2 are zoonotic. Live attenuated B. abortus S19 and B. melitensis Rev1 are effective vaccines in domestic ruminants, though both can infect humans. However, there is no swine brucellosis vaccine. Here, we investigated the potential use as vaccines of B. suis biovar 2 rough (R) lipopolysaccharide (LPS) mutants totally lacking O-chain (Bs2ΔwbkF) or only producing internal O-chain precursors (Bs2Δwzm) and mutants with a smooth (S) LPS defective in the core lateral branch (Bs2ΔwadB and Bs2ΔwadD). We also investigated mutants in the pyruvate phosphate dikinase (Bs2ΔppdK) and phosphoenolpyruvate carboxykinase (Bs2ΔpckA) genes encoding enzymes bridging phosphoenolpyruvate and the tricarboxylic acid cycle. When tested in the OIE mouse model at the recommended R or S vaccine doses (10⁸ and 10⁵ CFU, respectively), CFU/spleen of all LPS mutants were reduced with respect to the wild type and decreased faster for the R than for the S mutants. At those doses, protection against B. suis was similar for Bs2ΔwbkF, Bs2Δwzm, Bs2ΔwadB and the Rev1 control (10⁵ CFU). As described before for B. abortus, B. suis biovar 2 carried a disabled pckA so that a double mutant Bs2ΔppdKΔpckA had the same metabolic phenotype as Bs2ΔppdK and ppdK mutation was enough to generate attenuation. At 10⁵ CFU, Bs2ΔppdK also conferred the same protection as Rev1. As compared to other B. suis vaccine candidates described before, the mutants described here simultaneously carry irreversible deletions easy to identify as vaccine markers, lack antibiotic-resistance markers and were obtained in a non-zoonotic background. Since R vaccines should not elicit antibodies to the S-LPS and wzm mutants carry immunogenic O-chain precursors and did not improve Bs2ΔwbkF, the latter seems a better R vaccine candidate than Bs2Δwzm. However, taking into account that all R vaccines interfere in ELISA and other widely used assays, whether Bs2ΔwbkF is advantageous over Bs2ΔwadB or Bs2ΔppdK requires experiments in the natural host.

Top-Down Characterization of Lipooligosaccharides from Antibiotic-Resistant Bacteria

Modification of structures of lipooligosaccharides (LOS) represents one prevalent mechanism by which Gram-negative bacteria can become resistant to key antibiotics. Owing to the significant complexity of LOS, the structural characterization of these amphipathic lipids has largely focused on elucidation of the lipid A substructures. Analysis of intact LOS enables detection of core oligosaccharide modifications and gives insight into the heterogeneity that results from combinations of lipid A and oligosaccharide substructures. Top-down analysis of intact LOS also provides the opportunity to determine unknown oligosaccharide structures, which is particularly advantageous in the context of glycoconjugate vaccine development. Advances in mass spectrometry technologies, including the development of MSn capabilities and alternative ion activation techniques, have made top-down analysis an indispensable tool for structural characterization of complex biomolecules. Here we combine online chromatographic separations with MS3 utilizing ultraviolet photodissociation (UVPD) and higher-energy collisional dissociation (HCD). HCD generally provides information about the presence of labile modifications via neutral loss fragments in addition to the saccharide linkage arrangement, whereas UVPD gives more detailed insight about saccharide branching and the positions of nonstoichiometric modifications. This integrated approach was used to characterize LOS from Acinetobacter baumannii 1205 and 5075. Notably, MS3 analysis of A. baumannii 1205, an antibiotic-resistant strain, confirmed phosphoethanolamine and hexosamine modification of the lipid A substructure and further enabled derivation of a core oligosaccharide structure.

Microbes and Infection turns 20

The journal Microbes and Infection is celebrating its vigintennial anniversary and has reunited for this occasion two dozen reviews illustrating achievements of the past as well as future challenges in the field of infectious diseases. From top-notch vaccine development strategies, to high-throughput powered analysis of complex host-pathogen interactions, to innovative therapeutic designs, this issue covers the entire spectrum of pathogens and areas of their confrontation with the host.