A Multidisciplinary Structural Approach to the Identification of the Haemophilus influenzae Type b Capsular Polysaccharide Protective Epitope.

Glycoconjugate vaccines so far licensed are generally composed of a native or size-reduced capsular polysaccharide conjugated to carrier proteins. Detailed information on the structural requirements necessary for CPS recognition is becoming the key to accelerating the development of next-generation improved glycoconjugate vaccines. Structural glycobiology studies using oligosaccharides (OS) complexed with functional monoclonal antibodies represent a powerful tool for gaining information on CPS immunological determinants at the atomic level. Herein, the minimal structural epitope of Haemophilus influenzae type b (Hib) CPS recognized by a functional human monoclonal antibody (hmAb) is reported. Short and well-defined Hib oligosaccharides originating from the depolymerization of the native CPS have been used to elucidate saccharide-mAb interactions by using a multidisciplinary approach combining surface plasmon resonance (SPR), saturation transfer difference-nanomagnetic resonance (STD-NMR), and X-ray crystallography. Our study demonstrates that the minimal structural epitope of Hib is comprised within two repeating units (RUs) where ribose and ribitol are directly engaged in the hmAb interaction, and the binding pocket fully accommodates two RUs without any additional involvement of a third one. Understanding saccharide antigen structural characteristics can provide the basis for the design of innovative glycoconjugate vaccines based on alternative technologies, such as synthetic or enzymatic approaches.

Epitope Mapping of Human Polyclonal Antibodies to the fHbp Antigen of a Neisseria Meningitidis Vaccine by Hydrogen-Deuterium Exchange Mass Spectrometry (HDX-MS).

Antigen-antibody interactions play a key role in the immune response post vaccination and the mechanism of action of antibody-based biopharmaceuticals. 4CMenB is a multicomponent vaccine against Neisseria meningitidis serogroup B in which factor H binding protein (fHbp) is one of the key antigens. In this study, we use hydrogen/deuterium exchange mass spectrometry (HDX-MS) to identify epitopes in fHbp recognized by polyclonal antibodies (pAb) from two human donors (HDs) vaccinated with 4CMenB. Our HDX-MS data reveal several epitopes recognized by the complex mixture of human pAb. Furthermore, we show that the pAb from the two HDs recognize the same epitope regions. Epitope mapping of total pAb and purified fHbp-specific pAb from the same HD reveals that the two antibody samples recognize the same main epitopes, showing that HDX-MS based epitope mapping can, in this case at least, be performed directly using total IgG pAb samples that have not undergone Ab-selective purification. Two monoclonal antibodies (mAb) were previously produced from B-cell repertoire sequences from one of the HDs and used for epitope mapping of fHbp with HDX-MS. The epitopes identified for the pAb from the same HD in this study, overlap with the epitopes recognized by the two individual mAbs. Overall, HDX-MS epitope mapping appears highly suitable for simultaneous identification of epitopes recognized by pAb from human donors and to thus both guide vaccine development and study basic human immunity to pathogens, including viruses.

FHUSPA2/10 is a bactericidal monoclonal antibody targeting multiple repeated sequences of Moraxella catarrhalis UspA2.

Moraxella catarrhalis is an important and common respiratory pathogen that can cause Otitis Media, Community Acquired Pneumonia, and has been associated with an increased risk of exacerbations in chronic obstructive pulmonary disease in adults, leading to morbidity and mortality. Its ubiquitous surface protein A2 (UspA2) has been shown to interact with host structures and extracellular matrix proteins, suggesting a role at an early stage of infection and a contribution to bacterial serum resistance. The UspA proteins are homo-trimeric autotransporters that appear as a lollipop-shaped structure in electron micrographs. They are composed of an N-terminal head with adhesive properties, followed by a stalk, which ends by an amphipathic helix and a C-terminal membrane domain. The three family members UspA1, UspA2 and UspA2H, present different amino acid signatures both at the head and membrane-spanning regions. By combining electron microscopy, hydrogen deuterium exchange mass spectrometry and protein modeling, we identified a shared and repeated epitope recognized by FHUSPA2/10, a potent cross-bactericidal monoclonal antibody raised by UspA2 and deduced key amino acids involved in the binding. The finding strengthens the potential of UspA2 to be incorporated in a vaccine formulation against M. catarrhalis.

Hydrogen-Deuterium Exchange Mass Spectrometry with Integrated Size-Exclusion Chromatography for Analysis of Complex Protein Samples.

The growing use of hydrogen-deuterium exchange mass spectrometry (HDX-MS) for studying membrane proteins, large protein assemblies, and highly disulfide-bonded species is often challenged by the presence in the sample of large amounts of lipids, protein ligands, and/or highly ionizable reducing agents. Here, we describe how a short size-exclusion chromatography (SEC) column can be integrated with a conventional temperature-controlled HDX-MS setup to achieve fast and online removal of unwanted species from the HDX sample prior to chromatographic separation and MS analysis. Dual-mode valves permit labeled proteins eluting after SEC to be directed to the proteolytic and chromatographic columns, while unwanted sample components are led to waste. The SEC-coupled HDX-MS method allows analyses to be completed with lower or similar back-exchange compared to conventional experiments. We demonstrate the suitability of the method for the analysis of challenging protein samples, achieving efficient online removal of lipid components from protein-lipid systems, depletion of an antibody from an antigen during epitope mapping, and elimination of MS interfering compounds such as tris(2-carboxyethyl)phosphine (TCEP) during HDX-MS analysis of a disulfide-bonded protein. The implementation of the short SEC column to the conventional HDX-MS setup is straightforward and could be of significant general utility during the HDX-MS analysis of complex protein states.

Epitope Mapping of Polyclonal Antibodies by Hydrogen-Deuterium Exchange Mass Spectrometry (HDX-MS).

Epitope mapping of antibodies (Abs) is crucial for understanding adaptive immunity, as well as studying the mode of action of therapeutic antibodies and vaccines. Especially insights into the binding of the entire polyclonal antibody population (pAb) raised upon vaccination would be of unique value to vaccine development. However, very few methods for epitope mapping can tolerate the complexity of a pAb sample. Here we show how hydrogen-deuterium exchange mass spectrometry (HDX-MS) can be used to map epitopes recognized by pAb samples. Our approach involves measuring the HDX of the antigen in absence or presence of varied amounts of pAbs, as well as dissociating additives. We apply the HDX-MS workflow to pAbs isolated from rabbit immunized with factor H-binding protein (fHbp), a Neisseria meningitidis vaccine antigen. We identify four immunogenic regions located on the N- and C-terminal region of fHbp and provide insights into the relative abundance and avidity of epitope binding Abs present in the sample. Overall, our results show that HDX-MS can provide a unique and relatively fast method for revealing the binding impact of the entire set of pAbs present in blood samples after vaccination. Such information provides a rare view into effective immunity and can guide the design of improved vaccines against viruses or bacteria.

Epitope and Paratope Mapping by HDX-MS Combined with SPR Elucidates the Difference in Bactericidal Activity of Two Anti-NadA Monoclonal Antibodies.

Characterization of antigen-antibody interactions is crucial for understanding antibody-mediated protection against pathogens, biopharmaceutical development, as well as evaluation of the immune response post vaccination. Bexsero is a multicomponent vaccine against Neisseria meningitidis serogroup B in which one of the key vaccine antigens is Neisserial adhesin A (NadA), a trimeric coiled-coil protein. Two NadA-specific monoclonal antibodies (mAbs) isolated from Bexsero-vaccinated individuals have been shown to have similar binding affinity and appear to recognize a similar antigen region, yet only one of the mAbs is bactericidal. In this study, we use hydrogen/deuterium exchange mass spectrometry (HDX-MS) to perform an in-depth study of the interaction of the two mAbs with NadA antigen using a combined epitope and paratope mapping strategy. In addition, we use surface plasmon resonance (SPR) to investigate the stoichiometry of the binding of the two mAbs to NadA. While epitope mapping only identifies a clear binding impact of one of the mAbs on NadA, the paratope mapping analyses shows that both mAbs are binding to NadA through several complementarity determining regions spanning both heavy and light chains. Our results highlight the advantage of combined epitope and paratope mapping HDX-MS experiments and supporting biochemical experiments to characterize antigen-antibody interactions. Through this combined approach, we provide a rationale for how the binding stoichiometry of the two mAbs to the trimeric NadA antigen can explain the difference in bactericidal activity of the two mAbs.

Synergic complement-mediated bactericidal activity of monoclonal antibodies with distinct specificity.

The classical complement pathway is triggered when antigen-bound immunoglobulins bind to C1q through their Fc region. While C1q binds to a single Fc with low affinity, a higher avidity stable binding of two or more of C1q globular heads initiates the downstream reactions of the complement cascade ultimately resulting in bacteriolysis. Synergistic bactericidal activity has been demonstrated when monoclonal antibodies recognize nonoverlapping epitopes of the same antigen. The aim of the present work was to investigate the synergistic effect between antibodies directed toward different antigens. To this purpose, we investigated the bactericidal activity induced by combinations of monoclonal antibodies (mAbs) raised against factor H-binding protein (fHbp) and Neisserial Heparin-Binding Antigen (NHBA), two major antigens included in Bexsero, the vaccine against Meningococcus B, for prevention from this devastating disease in infants and adolescents. Collectively, our results show that mAbs recognizing different antigens can synergistically activate complement even when each single Mab is not bactericidal, reinforcing the evidence that cooperative immunity induced by antigen combinations can represent a remarkable added value of multicomponent vaccines. Our study also shows that the synergistic effect of antibodies is modulated by the nature of the respective epitopes, as well as by the antigen density on the bacterial cell surface.

The Streptococcus agalactiae complement interfering protein combines multiple complement-inhibitory mechanisms by interacting with both C4 and C3 ligands.

Group B Streptococcus (GBS) colonizes the human lower intestinal and genital tracts and constitutes a major threat to neonates from pregnant carrier mothers and to adults with underlying morbidity. The pathogen expresses cell-surface virulence factors that enable cell adhesion and penetration and that counteract innate and adaptive immune responses. Among these, the complement interfering protein (CIP) was recently described for its capacity to interact with the human C4b ligand and to interfere with the classical- and lectin-complement pathways. In the present study, we provide evidence that CIP can also interact with C3, C3b, and C3d. Immunoassay-based competition experiments showed that binding of CIP to C3d interferes with the interaction between C3d and the complement receptor 2/cluster of differentiation 21 (CR2/CD21) receptor on B cells. By B-cell intracellular signaling assays, CIP was confirmed to down-regulate CR2/CD21-dependent B-cell activation. The CIP domain involved in C3d binding was mapped via hydrogen deuterium exchange-mass spectrometry. The data obtained reveal a new role for this GBS polypeptide at the interface between the innate and adaptive immune responses, adding a new member to the growing list of virulence factors secreted by gram-positive pathogens that incorporate multiple immunomodulatory functions.-Giussani, S., Pietrocola, G., Donnarumma, D., Norais, N., Speziale, P., Fabbrini, M., Margarit, I. The Streptococcus agalactiae complement interfering protein combines multiple complement-inhibitory mechanisms by interacting with both C4 and C3 ligands.

Generation and characterization of a bivalent protein boost for future clinical trials: HIV-1 subtypes CR01_AE and B gp120 antigens with a potent adjuvant.

The RV144 Phase III clinical trial with ALVAC-HIV prime and AIDSVAX B/E subtypes CRF01_AE (A244) and B (MN) gp120 boost vaccine regime in Thailand provided a foundation for the future development of improved vaccine strategies that may afford protection against the human immunodeficiency virus type 1 (HIV-1). Results from this trial showed that immune responses directed against specific regions V1V2 of the viral envelope (Env) glycoprotein gp120 of HIV-1, were inversely correlated to the risk of HIV-1 infection. Due to the low production of gp120 proteins in CHO cells (2-20 mg/L), cleavage sites in V1V2 loops (A244) and V3 loop (MN) causing heterogeneous antigen products, it was an urgent need to generate CHO cells harboring A244 gp120 with high production yields and an additional, homogenous and uncleaved subtype B gp120 protein to replace MN used in RV144 for the future clinical trials. Here we describe the generation of Chinese Hamster Ovary (CHO) cell lines stably expressing vaccine HIV-1 Env antigens for these purposes: one expressing an HIV-1 subtype CRF01_AE A244 Env gp120 protein (A244.AE) and one expressing an HIV-1 subtype B 6240 Env gp120 protein (6240.B) suitable for possible future manufacturing of Phase I clinical trial materials with cell culture expression levels of over 100 mg/L. The antigenic profiles of the molecules were elucidated by comprehensive approaches including analysis with a panel of well-characterized monoclonal antibodies recognizing critical epitopes using Biacore and ELISA, and glycosylation analysis by mass spectrometry, which confirmed previously identified glycosylation sites and revealed unknown sites of O-linked and N-linked glycosylations at non-consensus motifs. Overall, the vaccines given with MF59 adjuvant induced higher and more rapid antibody (Ab) responses as well as higher Ab avidity than groups given with aluminum hydroxide. Also, bivalent proteins (A244.AE and 6240.B) formulated with MF59 elicited distinct V2-specific Abs to the epitope previously shown to correlate with decreased risk of HIV-1 infection in the RV144 trial. All together, these results provide critical information allowing the consideration of these candidate gp120 proteins for future clinical evaluations in combination with a potent adjuvant.

Native State Organization of Outer Membrane Porins Unraveled by HDx-MS.

Hydrogen-deuterium exchange (HDx) associated with mass spectrometry (MS) is emerging as a powerful tool to provide conformational information about membrane proteins. Unfortunately, as for X-ray diffraction and NMR, HDx performed on reconstituted in vitro systems might not always reflect the in vivo environment. Outer-membrane vesicles naturally released by Escherichia coli were used to carry out analysis of native OmpF through HDx-MS. A new protocol compatible with HDx analysis that avoids hindrance from the lipid contents was setup. The extent of deuterium incorporation was in good agreement with the X-ray diffraction data of OmpF as the buried ß-barrels incorporated a low amount of deuterium, whereas the internal loop L3 and the external loops incorporated a higher amount of deuterium. Moreover, the kinetics of incorporation clearly highlights that peptides segregate well in two distinct groups based exclusively on a trimeric organization of OmpF in the membrane: peptides presenting fast kinetics of labeling are facing the complex surrounding environment, whereas those presenting slow kinetics are located in the buried core of the trimer. The data show that HDx-MS applied to a complex biological system is able to reveal solvent accessibility and spatial arrangement of an integral outer-membrane protein complex.

Outer Membrane Vesicles (OMV)-based and Proteomics-driven Antigen Selection Identifies Novel Factors Contributing to Bordetella pertussis Adhesion to Epithelial Cells.

Despite high vaccination coverage world-wide, whooping cough, a highly contagious disease caused by Bordetella pertussis, is recently increasing in occurrence suggesting that novel vaccine formulations targeted at the prevention of colonization and transmission should be investigated. To identify new candidates for inclusion in the acellular formulation, we used spontaneously released outer membrane vesicles (OMV)1 as a potential source of key adhesins. The enrichment of Bvg+ OMV with adhesins and the ability of anti-OMV serum to inhibit the adhesion of B. pertussis to lung epithelial cells in vitro were demonstrated. We employed a proteomic approach to identify the differentially expressed proteins in OMV purified from bacteria in the Bvg+ and Bvg- virulence phases, thus comparing the outer membrane protein pattern of this pathogen in its virulent or avirulent state. Six of the most abundant outer membrane proteins were selected as candidates to be evaluated for their adhesive properties and vaccine potential. We generated E. coli strains singularly expressing the selected proteins and assessed their ability to adhere to lung epithelial cells in vitro Four out of the selected proteins conferred adhesive ability to E. coli Three of the candidates were specifically detected by anti-OMV mouse serum suggesting that these proteins are immunogenic antigens able to elicit an antibody response when displayed on the OMV. Anti-OMV serum was able to inhibit only BrkA-expressing E. coli adhesion to lung epithelial cells. Finally, stand-alone immunization of mice with recombinant BrkA resulted in significant protection against infection of the lower respiratory tract after challenge with B. pertussis Taken together, these data support the inclusion of BrkA and possibly further adhesins to the current acellular pertussis vaccines to improve the impact of vaccination on the bacterial clearance.

Structural basis for potent antibody-mediated neutralization of human cytomegalovirus.

Human cytomegalovirus (HCMV) is the leading viral cause of birth defects and organ transplant rejection. The HCMV gH/gL/UL128/UL130/UL131A complex (Pentamer) is the main target of humoral responses and thus a key vaccine candidate. We report two structures of Pentamer bound to human neutralizing antibodies, 8I21 and 9I6, at 3.0 and 5.9 Å resolution, respectively. The HCMV gH/gL architecture is similar to that of Epstein-Barr virus (EBV) except for amino-terminal extensions on both subunits. The extension of gL forms a subdomain composed of a three-helix bundle and a ß hairpin that acts as a docking site for UL128/UL130/UL131A. Structural analysis reveals that Pentamer is a flexible molecule, and suggests sites for engineering stabilizing mutations. We also identify immunogenic surfaces important for cellular interactions by epitope mapping and functional assays. These results can guide the development of effective vaccines and immunotherapeutics against HCMV.

Surface and Exoproteomes of Gram-Positive Pathogens for Vaccine Discovery.

Reverse vaccinology has been very successful in the discovery of vaccine candidates against many pathogenic bacteria by integrating genome and proteome mining. This great achievement was facilitated by the complementarity of the in silico prediction of antigens and the empirical data on protein localization, expression, and immunogenicity obtained through different techniques based on electrophoresis, immunoblotting and mass spectrometry. An iterative process between information provided by DNA sequence analysis and proteomic data has been established leading to precise antigen identification. In this review, we report how the identification of surface and exoproteomes of Gram-positive pathogens have contributed to the selection of vaccine candidates. Moreover, we show how quantitative mass spectrometry is now paving the way for identifying protective antigens that play key roles during infection and represent the most promising vaccine targets.

Epitope Mapping of a Monoclonal Antibody Directed against Neisserial Heparin Binding Antigen Using Next Generation Sequencing of Antigen-Specific Libraries.

We explore here the potential of a newly described technology, which is named PROFILER and is based on next generation sequencing of gene-specific lambda phage-displayed libraries, to rapidly and accurately map monoclonal antibody (mAb) epitopes. For this purpose, we used a novel mAb (designated 31E10/E7) directed against Neisserial Heparin-Binding Antigen (NHBA), a component of the anti-group B meningococcus Bexsero® vaccine. An NHBA phage-displayed library was affinity-selected with mAb 31E10/E7, followed by massive sequencing of the inserts present in antibody-selected phage pools. Insert analysis identified an amino acid stretch (D91-A128) in the N-terminal domain, which was shared by all of the mAb-enriched fragments. Moreover, a recombinant fragment encompassing this sequence could recapitulate the immunoreactivity of the entire NHBA molecule against mAb 31E10/E7. These results were confirmed using a panel of overlapping recombinant fragments derived from the NHBA vaccine variant and a set of chemically synthetized peptides covering the 10 most frequent antigenic variants. Furthermore, hydrogen-deuterium exchange mass-spectrometry analysis of the NHBA-mAb 31E10/E7 complex was also compatible with mapping of the epitope to the D91-A128 region. Collectively, these results indicate that the PROFILER technology can reliably identify epitope-containing antigenic fragments and requires considerably less work, time and reagents than other epitope mapping methods.

Phage display revisited: Epitope mapping of a monoclonal antibody directed against Neisseria meningitidis adhesin A using the PROFILER technology.

There is a strong need for rapid and reliable epitope mapping methods that can keep pace with the isolation of increasingly larger numbers of mAbs. We describe here the identification of a conformational epitope using Phage-based Representation OF ImmunoLigand Epitope Repertoire (PROFILER), a recently developed high-throughput method based on deep sequencing of antigen-specific lambda phage-displayed libraries. A novel bactericidal monoclonal antibody (mAb 9F11) raised against Neisseria meningitidis adhesin A (NadA), an important component of the Bexsero(®) anti-meningococcal vaccine, was used to evaluate the technique in comparison with other epitope mapping methods. The PROFILER technology readily identified NadA fragments that were capable of fully recapitulating the reactivity of the entire antigen against mAb 9F11. Further analysis of these fragments using mutagenesis and hydrogen-deuterium exchange mass-spectrometry allowed us to identify the binding site of mAb 9F11 (A250-D274) and an adjoining sequence (V275-H312) that was also required for the full functional reconstitution of the epitope. These data suggest that, by virtue of its ability to detect a great variety of immunoreactive antigen fragments in phage-displayed libraries, the PROFILER technology can rapidly and reliably identify epitope-containing regions and provide, in addition, useful clues for the functional characterization of conformational mAb epitopes.

Expression of factor H binding protein in meningococcal strains can vary at least 15-fold and is genetically determined.

Factor H binding protein (fHbp) is a lipoprotein of Neisseria meningitidis important for the survival of the bacterium in human blood and a component of two recently licensed vaccines against serogroup B meningococcus (MenB). Based on 866 different amino acid sequences this protein is divided into three variants or two families. Quantification of the protein is done by immunoassays such as ELISA or FACS that are susceptible to the sequence variation and expression level of the protein. Here, selected reaction monitoring mass spectrometry was used for the absolute quantification of fHbp in a large panel of strains representative of the population diversity of MenB. The analysis revealed that the level of fHbp expression can vary at least 15-fold and that variant 1 strains express significantly more protein than variant 2 or variant 3 strains. The susceptibility to complement-mediated killing correlated with the amount of protein expressed by the different meningococcal strains and this could be predicted from the nucleotide sequence of the promoter region. Finally, the absolute quantification allowed the calculation of the number of fHbp molecules per cell and to propose a mechanistic model of the engagement of C1q, the recognition component of the complement cascade.

Exploiting chimeric human antibodies to characterize a protective epitope of Neisseria adhesin A, one of the Bexsero vaccine components.

Neisseria adhesin A (NadA) is one of the antigens of Bexsero, the recently licensed multicomponent vaccine against serogroup B Neisseria meningitidis (MenB). NadA belongs to the class of oligomeric coiled-coil adhesins and is able to mediate adhesion and invasion of human epithelial cells. As a vaccine antigen, NadA has been shown to induce high levels of bactericidal antibodies; however, the domains important for protective response are still unknown. In order to further investigate its immunogenic properties, we have characterized the murine IgG1 mAb (6E3) that was able to recognize the 2 main antigenic variants of NadA on the surface of MenB strains. The epitope targeted by mAb 6E3 was mapped by hydrogen-deuterium exchange mass spectrometry and shown to be located on the coiled-coil stalk region of NadA (aa 206-249). Although no serum bactericidal activity was observed for murine IgG1 mAb 6E3, functional activity was restored when using chimeric antibodies in which the variable regions of the murine mAb 6E3 were fused to human IgG3 constant regions, thus confirming the protective nature of the mAb 6E3 epitope. The use of chimeric antibody molecules will enable future investigations of complement-mediated antibody functionality independently of the Fc-mediated differences in complement activation.

The role of structural proteomics in vaccine development: recent advances and future prospects.

Vaccines are the most effective way to fight infectious diseases saving countless lives since their introduction. Their evolution during the last century made use of the best technologies available to continuously increase their efficacy and safety. Mass spectrometry (MS) and proteomics are already playing a central role in the identification and characterization of novel antigens. Over the last years, we have been witnessing the emergence of structural proteomics in vaccinology, as a major tool for vaccine candidate discovery, antigen design and life cycle management of existing products. In this review, we describe the MS techniques associated to structural proteomics and we illustrate the contribution of structural proteomics to vaccinology discussing potential applications.

Antigenic Characterization of the HCMV gH/gL/gO and Pentamer Cell Entry Complexes Reveals Binding Sites for Potently Neutralizing Human Antibodies.

Human Cytomegalovirus (HCMV) is a major cause of morbidity and mortality in transplant patients and in fetuses following congenital infection. The glycoprotein complexes gH/gL/gO and gH/gL/UL128/UL130/UL131A (Pentamer) are required for HCMV entry in fibroblasts and endothelial/epithelial cells, respectively, and are targeted by potently neutralizing antibodies in the infected host. Using purified soluble forms of gH/gL/gO and Pentamer as well as a panel of naturally elicited human monoclonal antibodies, we determined the location of key neutralizing epitopes on the gH/gL/gO and Pentamer surfaces. Mass Spectrometry (MS) coupled to Chemical Crosslinking or to Hydrogen Deuterium Exchange was used to define residues that are either in proximity or part of neutralizing epitopes on the glycoprotein complexes. We also determined the molecular architecture of the gH/gL/gO- and Pentamer-antibody complexes by Electron Microscopy (EM) and 3D reconstructions. The EM analysis revealed that the Pentamer specific neutralizing antibodies bind to two opposite surfaces of the complex, suggesting that they may neutralize infection by different mechanisms. Together, our data identify the location of neutralizing antibodies binding sites on the gH/gL/gO and Pentamer complexes and provide a framework for the development of antibodies and vaccines against HCMV.

The Human Pathogen Streptococcus pyogenes Releases Lipoproteins as Lipoprotein-rich Membrane Vesicles.

Bacterial lipoproteins are attractive vaccine candidates because they represent a major class of cell surface-exposed proteins in many bacteria and are considered as potential pathogen-associated molecular patterns sensed by Toll-like receptors with built-in adjuvanticity. Although Gram-negative lipoproteins have been extensively characterized, little is known about Gram-positive lipoproteins. We isolated from Streptococcus pyogenes a large amount of lipoproteins organized in vesicles. These vesicles were obtained by weakening the bacterial cell wall with a sublethal concentration of penicillin. Lipid and proteomic analysis of the vesicles revealed that they were enriched in phosphatidylglycerol and almost exclusively composed of lipoproteins. In association with lipoproteins, a few hypothetical proteins, penicillin-binding proteins, and several members of the ExPortal, a membrane microdomain responsible for the maturation of secreted proteins, were identified. The typical lipidic moiety was apparently not necessary for lipoprotein insertion in the vesicle bilayer because they were also recovered from the isogenic diacylglyceryl transferase deletion mutant. The vesicles were not able to activate specific Toll-like receptor 2, indicating that lipoproteins organized in these vesicular structures do not act as pathogen-associated molecular patterns. In light of these findings, we propose to name these new structures Lipoprotein-rich Membrane Vesicles.