Biophysical characterization of the type III secretion tip proteins and the tip proteins attached to bacterium-like particles

Bacterium-like particles (BLPs), derived from Lactococcus lactis, offer a self-adjuvanting delivery vehicle for subunit protein vaccines. Proteins can be specifically loaded onto the BLPs via a peptidoglycan anchoring (PA) domain. In this study, the tip proteins IpaD, SipD, and LcrV belonging to type III secretion systems of Shigella flexneri, Salmonella enterica, and Yersinia enterocolitica, respectively, were fused to the PA and loaded onto the BLPs. Herein, we biophysically characterized these nine samples and condensed the spectroscopic results into three-index empirical phase diagrams (EPDs). The EPDs show distinctions between the IpaD/SipD and LcrV subfamilies of tip proteins, based on their physical stability, even upon addition of the PA. Upon attachment to the BLPs, the BLPs become defining moiety in the spectroscopic measurements, leaving the tip proteins to have a subtle yet modulating effect on the structural integrity of the tip proteins-BLPs binding. In summary, this work provides a comprehensive view of physical stability of the tip proteins and tip protein-BLPs and serves as a baseline for screening of excipients to increase the stability of the tip protein-BLPs for future vaccine formulation.

Intradermal delivery of Shigella IpaB and IpaD type III secretion proteins: kinetics of cell recruitment and antigen uptake, mucosal and systemic immunity, and protection across serotypes

Shigella is one of the leading pathogens contributing to the vast pediatric diarrheal disease burden in low-income countries. No licensed vaccine is available, and the existing candidates are only partially effective and serotype specific. Shigella type III secretion system proteins IpaB and IpaD, which are conserved across Shigella spp., are candidates for a broadly protective, subunit-based vaccine. In this study, we investigated the immunogenicity and protective efficacy of IpaB and IpaD administered intradermally (i.d.) with a double-mutant of the Escherichia coli heat-labile enterotoxin (dmLT) adjuvant using microneedles. Different dosage levels of IpaB and IpaD, with or without dmLT, were tested in mice. Vaccine delivery into the dermis, recruitment of neutrophils, macrophages, dendritic cells, and Langerhans cells, and colocalization of vaccine Ag within skin-activated APC were demonstrated through histology and immunofluorescence microscopy. Ag-loaded neutrophils, macrophages, dendritic cells, and Langerhans cells remained in the tissue at least 1 wk. IpaB, IpaD, and dmLT-specific serum IgG- and IgG-secreting cells were produced following i.d. immunization. The protective efficacy was 70% against Shigella flexneri and 50% against Shigella sonnei. Similar results were obtained when the vaccine was administered intranasally, with the i.d. route requiring 25-40 times lower doses. Distinctively, IgG was detected in mucosal secretions; secretory IgA, as well as mucosal and systemic IgA Ab-secreting cells, were seemingly absent. Vaccine-induced T cells produced IFN-γ, IL-2, TNF-α, IL-17, IL-4, IL-5, and IL-10. These results demonstrate the potential of i.d. vaccination with IpaB and IpaD to prevent Shigella infection and support further studies in humans.

N-terminus of IpaB provides a potential anchor to the Shigella type III secretion system tip complex protein IpaD

The type III secretion system (T3SS) is an essential virulence factor for Shigella flexneri , providing a conduit through which host-altering effectors are injected directly into a host cell to promote uptake. The type III secretion apparatus (T3SA) is composed of a basal body, external needle, and regulatory tip complex. The nascent needle is a polymer of MxiH capped by a pentamer of invasion plasmid antigen D (IpaD). Exposure to bile salts (e.g., deoxycholate) causes a conformational change in IpaD and promotes recruitment of IpaB to the needle tip. It has been proposed that IpaB senses contact with host cell membranes, recruiting IpaC and inducing full secretion of T3SS effectors. Although the steps of T3SA maturation and their external triggers have been identified, details of specific protein interactions and mechanisms have remained difficult to study because of the hydrophobic nature of the IpaB and IpaC translocator proteins. Here, we explored the ability for a series of soluble N-terminal IpaB peptides to interact with IpaD. We found that DOC is required for the interaction and that a region of IpaB between residues 11-27 is required for maximum binding, which was confirmed in vivo. Furthermore, intramolecular FRET measurements indicated that movement of the IpaD distal domain away from the protein core accompanied the binding of IpaB11-226. Together, these new findings provide important new insight into the interactions and potential mechanisms that define the maturation of the Shigella T3SA needle tip complex and provide a foundation for further studies probing T3SS activation.

Three functional β-carbonic anhydrases in Pseudomonas aeruginosa PAO1: role in survival in ambient air

Bacterial β-class carbonic anhydrases (CAs) are zinc metalloenzymes catalysing reversible hydration of CO2. They maintain the intracellular balance of CO2/bicarbonate required for biosynthetic reactions and represent a new group of antimicrobial drug targets. Genome sequence analysis of Pseudomonas aeruginosa PAO1, an opportunistic human pathogen causing life threatening infections, identified three genes, PAO102, PA2053 and PA4676, encoding putative β-CAs that share 28-45 % amino acid sequence identity and belong to clades A and B. The genes are conserved among all sequenced pseudomonads. The CAs were cloned, heterologously expressed and purified. Metal and enzymic analyses confirmed that the proteins contain Zn(2+) and catalyse hydration of CO2 to bicarbonate. PAO102 (psCA1) was 19-26-fold more active, and together with PA2053 (psCA2) showed CA activity at both pH 7.5 and 8.3, whereas PA4676 (psCA3) was active only at pH 8.3. Circular dichroism spectroscopy suggested that psCA2 and psCA3 undergo pH-dependent structural changes. Taken together, the data suggest that psCA1 may belong to type I and psCA3 to type II β-CAs. Immunoblot analysis showed that all three CAs are expressed in PAO1 cells when grown in ambient air and at 5 % CO2; psCA1 appeared more abundant under both conditions. Growth studies of transposon mutants showed that the disruption of psCA1 impaired PAO1 growth in ambient air and caused a minor defect at high CO2. Thus, psCA1 contributes to the adaptation of P. aeruginosa to low CO2 conditions and will be further studied for its role in virulence and as a potential antimicrobial drug target in this organism.

Evaluation of immunogenicity and protective efficacy of orally delivered Shigella type III secretion system proteins IpaB and IpaD

Shigella spp. are food- and water-borne pathogens that cause shigellosis, a severe diarrheal and dysenteric disease that is associated with a high morbidity and mortality in resource-poor countries. No licensed vaccine is available to prevent shigellosis. We have recently demonstrated that Shigella invasion plasmid antigens (Ipas), IpaB and IpaD, which are components of the bacterial type III secretion system (TTSS), can prevent infection in a mouse model of intranasal immunization and lethal pulmonary challenge. Because they are conserved across Shigella spp. and highly immunogenic, these proteins are excellent candidates for a cross-protective vaccine. Ideally, such a vaccine could be administered to humans orally to induce mucosal and systemic immunity. In this study, we investigated the immunogenicity and protective efficacy of Shigella IpaB and IpaD administered orally with a double mutant of the Escherichia coli heat labile toxin (dmLT) as a mucosal adjuvant. We characterized the immune responses induced by oral vs. intranasal immunization and the protective efficacy using a mouse pulmonary infection model. Serum IgG and fecal IgA against IpaB were induced after oral immunization. These responses, however, were lower than those obtained after intranasal immunization despite a 100-fold dosage increase. The level of protection induced by oral immunization with IpaB and IpaD was 40%, while intranasal immunization resulted in 90% protective efficacy. IpaB- and IpaD-specific IgA antibody-secreting cells in the lungs and spleen and T-cell-derived IL-2, IL-5, IL-17 and IL-10 were associated with protection. These results demonstrate the immunogenicity of orally administered IpaB and IpaD and support further studies in humans.

Parenteral immunization with IpaB/IpaD protects mice against lethal pulmonary infection by Shigella

Shigellosis is an important diarrheal disease, especially among children in the developing world. About 90 million infections with Shigella spp are estimated to appear each year. We previously demonstrated that the type III secretion apparatus (T3SA) proteins IpaB and IpaD are protective antigens when administered intranasally using the mouse lethal pulmonary model. To simplify vaccine administration, we tested the parenteral route for IpaB and IpaD with several adjuvants and compared the immune response and protective efficacy via the intranasal route. We found that the intramuscular administration generated a response consisting of similar levels of serum IgG, a lack of IgA response and higher IL-17 secretion. Therefore, while parenteral administration yielded a unique pattern of immune responses, it retained the ability to protect mice in a lethal pulmonary challenge against S. flexneri when both proteins were used. Our results show the feasibility of generating protective parenteral vaccines against Shigella spp.

Studies of the conformational stability of invasion plasmid antigen B from Shigella

Shigella spp. are the causative agent of shigellosis, the second leading cause of diarrhea in children of ages 2-5. Despite many years of research, a protective vaccine has been elusive. We recently demonstrated that invasion plasmid antigens B and D (IpaB and IpaD) provide protection against S. flexneri and S. sonnei. These proteins, however, have very different properties which must be recognized and then managed during vaccine formulation. Herein, we employ spectroscopy to assess the stability of IpaB as well as IpgC (invasion protein gene), IpaB's cognate chaperone, and the IpaB/IpgC complex. The resulting data are mathematically summarized into a visual map illustrating the stability of the proteins and their complex as a function of pH and temperature. The IpaB/IpgC complex exhibits thermal stability at higher pH values but, though initially stable, quickly unfolds with increasing temperature when maintained at lower pH. In contrast, IpaB is a much more complex protein exhibiting increased stability at higher pH, but shows initial instability at lower pH values with pH 5 showing a distinct transition. IpgC precipitates at and below pH 5 and is stable above pH 7. Most strikingly, it is clear that complex formation results in stabilization of the two components. This work serves as a basis for the further development of IpaB as a vaccine candidate as well as extends our understanding of the structural stability of the Shigella type III secretion system.

Shigella antigen-specific B memory cells are associated with decreased disease severity in subjects challenged with wild-type Shigella flexneri 2a

The role of Shigella-specific B memory (BM) in protection has not been evaluated in human challenge studies. We utilized cryopreserved pre- and post-challenge peripheral blood mononuclear cells and sera from wild-type Shigella flexneri 2a (wt-2457T) challenges. Challenged volunteers were either naïve or subjects who had previously ingested wt-2457T or been immunized with hybrid Escherichia coli-Shigella live oral candidate vaccine (EcSf2a-2). BM and antibody titers were measured against lipopolysaccharide (LPS) and recombinant invasion plasmid antigen B (IpaB); results were correlated with disease severity following challenge. Pre-challenge IgA IpaB-BM and post-challenge IgA LPS-BM in the previously exposed subjects negatively correlated with disease severity upon challenge. Similar results were observed with pre-challenge IgG anti-LPS and anti-IpaB titers in vaccinated volunteers. Inverse correlations between magnitude of pre-challenge IgG antibodies to LPS and IpaB, as well as IgA IpaB-BM and post-challenge IgA LPS-BM with disease severity suggest a role for antigen-specific BM in protection.

Oligomeric states of the Shigella translocator protein IpaB provide structural insights into formation of the type III secretion translocon

The Shigella flexneri Type III secretion system (T3SS) senses contact with human intestinal cells and injects effector proteins that promote pathogen entry as the first step in causing life threatening bacillary dysentery (shigellosis). The Shigella Type III secretion apparatus (T3SA) consists of an anchoring basal body, an exposed needle, and a temporally assembled tip complex. Exposure to environmental small molecules recruits IpaB, the first hydrophobic translocator protein, to the maturing tip complex. IpaB then senses contact with a host cell membrane, forming the translocon pore through which effectors are delivered to the host cytoplasm. Within the bacterium, IpaB exists as a heterodimer with its chaperone IpgC; however, IpaB's structural state following secretion is unknown due to difficulties isolating stable protein. We have overcome this by coexpressing the IpaB/IpgC heterodimer and isolating IpaB by incubating the complex in mild detergents. Interestingly, preparation of IpaB with n-octyl-oligo-oxyethylene (OPOE) results in the assembly of discrete oligomers while purification in N,N-dimethyldodecylamine N-oxide (LDAO) maintains IpaB as a monomer. In this study, we demonstrate that IpaB tetramers penetrate phospholipid membranes to allow a size-dependent release of small molecules, suggesting the formation of discrete pores. Monomeric IpaB also interacts with liposomes but fails to disrupt them. From these and additional findings, we propose that IpaB can exist as a tetramer having inherent flexibility, which allows it to cooperatively interact with and insert into host cell membranes. This event may then lay the foundation for formation of the Shigella T3SS translocon pore.

Identification of the bile salt binding site on IpaD from Shigella flexneri and the influence of ligand binding on IpaD structure

Type III secretion (TTS) is an essential virulence factor for Shigella flexneri, the causative agent of shigellosis. The Shigella TTS apparatus (TTSA) is an elegant nanomachine that is composed of a basal body, an external needle to deliver effectors into human cells, and a needle tip complex that controls secretion activation. IpaD is at the tip of the nascent TTSA needle where it controls the first step of TTS activation. The bile salt deoxycholate (DOC) binds to IpaD to induce recruitment of the translocator protein IpaB into the maturing tip complex. We recently used spectroscopic analyses to show that IpaD undergoes a structural rearrangement that accompanies binding to DOC. Here, we report a crystal structure of IpaD with DOC bound and test the importance of the residues that make up the DOC binding pocket on IpaD function. IpaD binds DOC at the interface between helices α3 and α7, with concomitant movement in the orientation of helix α7 relative to its position in unbound IpaD. When the IpaD residues involved in DOC binding are mutated, some are found to lead to altered invasion and secretion phenotypes. These findings suggest that adoption of a DOC bound structural state for IpaD primes the Shigella TTSA for contact with host cells. The data presented here and in the studies leading up to this work provide the foundation for developing a model of the first step in Shigella TTS activation.

Binding affects the tertiary and quaternary structures of the Shigella translocator protein IpaB and its chaperone IpgC

Shigella flexneri uses its type III secretion system (T3SS) to promote invasion of human intestinal epithelial cells as the first step in causing shigellosis, a life-threatening form of dysentery. The Shigella type III secretion apparatus (T3SA) consists of a basal body that spans the bacterial envelope and an exposed needle that injects effector proteins into target cells. The nascent Shigella T3SA needle is topped with a pentamer of the needle tip protein invasion plasmid antigen D (IpaD). Bile salts trigger recruitment of the first hydrophobic translocator protein, IpaB, to the tip complex where it senses contact with a host membrane. In the bacterial cytoplasm, IpaB exists in a complex with its chaperone IpgC. Several structures of IpgC have been determined, and we recently reported the 2.1 Å crystal structure of the N-terminal domain (IpaB(74.224)) of IpaB. Like IpgC, the IpaB N-terminal domain exists as a homodimer in solution. We now report that when the two are mixed, these homodimers dissociate and form heterodimers having a nanomolar dissociation constant. This is consistent with the equivalent complexes copurified after they had been co-expressed in Escherichia coli. Fluorescence data presented here also indicate that the N-terminal domain of IpaB possesses two regions that appear to contribute additively to chaperone binding. It is also likely that the N-terminus of IpaB adopts an alternative conformation as a result of chaperone binding. The importance of these findings within the functional context of these proteins is discussed.

Ultrastructural analysis of IpaD at the tip of the nascent MxiH type III secretion apparatus of Shigella flexneri

Shigella flexneri is a Gram-negative enteric pathogen that is the predominant cause of bacillary dysentery. Shigella uses a type III secretion system to deliver effector proteins that alter normal target cell functions to promote pathogen invasion. The type III secretion apparatus (T3SA) consists of a basal body, an extracellular needle, and a tip complex that is responsible for delivering effectors into the host cell cytoplasm. IpaD [Ipa (invasion plasmid antigen)] is the first protein to localize to the T3SA needle tip, where it prevents premature effector secretion and serves as an environmental sensor for triggering recruitment of the translocator protein IpaB to the needle tip. Thus, IpaD would be expected to form a stable structure whose overall architecture supports its functions. It is not immediately obvious from the published IpaD crystal structure (Protein Data Bank ID 2j0o) how a multimer of IpaD would be incorporated at the tip of the first static T3SA intermediate, nor what its functional role would be in building a mature T3SA. Here, we produce three-dimensional reconstructions from transmission electron microscopy images of IpaD localized at the Shigella T3SA needle tip for comparison to needle tips from a Shigella ipaD-null mutant. The results demonstrate that IpaD resides as a homopentamer at the needle tip of the T3SA. Furthermore, comparison to tips assembled from the distal domain IpaD(Δ192-267) mutation shows that IpaD adopts an elongated conformation that facilitates its ability to control type III secretion and stepwise assembly of the T3SA needle tip complex.

The structures of coiled-coil domains from type III secretion system translocators reveal homology to pore-forming toxins

Many pathogenic Gram-negative bacteria utilize type III secretion systems (T3SSs) to alter the normal functions of target cells. Shigella flexneri uses its T3SS to invade human intestinal cells to cause bacillary dysentery (shigellosis) that is responsible for over one million deaths per year. The Shigella type III secretion apparatus is composed of a basal body spanning both bacterial membranes and an exposed oligomeric needle. Host altering effectors are secreted through this energized unidirectional conduit to promote bacterial invasion. The active needle tip complex of S. flexneri is composed of a tip protein, IpaD, and two pore-forming translocators, IpaB and IpaC. While the atomic structure of IpaD has been elucidated and studied, structural data on the hydrophobic translocators from the T3SS family remain elusive. We present here the crystal structures of a protease-stable fragment identified within the N-terminal regions of IpaB from S. flexneri and SipB from Salmonella enterica serovar Typhimurium determined at 2.1 Å and 2.8 Å limiting resolution, respectively. These newly identified domains are composed of extended-length (114 Å in IpaB and 71 Å in SipB) coiled-coil motifs that display a high degree of structural homology to one another despite the fact that they share only 21% sequence identity. Further structural comparisons also reveal substantial similarity to the coiled-coil regions of pore-forming proteins from other Gram-negative pathogens, notably, colicin Ia. This suggests that these mechanistically separate and functionally distinct membrane-targeting proteins may have diverged from a common ancestor during the course of pathogen-specific evolutionary events.

Antigen-specific IgA B memory cell responses to Shigella antigens elicited in volunteers immunized with live attenuated Shigella flexneri 2a oral vaccine candidates

We studied the induction of antigen-specific IgA memory B cells (B(M)) in volunteers who received live attenuated Shigella flexneri 2a vaccines. Subjects ingested a single oral dose of 10(7), 10(8) or 10(9) CFU of S. flexneri 2a with deletions in guaBA (CVD 1204) or in guaBA, set and sen (CVD 1208). Antigen-specific serum and stool antibody responses to LPS and Ipa B were measured on days 0, 7, 14, 28 and 42. IgA B(M) cells specific to LPS, Ipa B and total IgA were assessed on days 0 and 28. We show the induction of significant LPS-specific IgA B(M) cells in anti-LPS IgA seroresponders. Positive correlations were found between anti-LPS IgA B(M) cells and anti-LPS IgA in serum and stool; IgA B(M) cell responses to IpaB were also observed. These B(M) cell responses are likely play an important role in modulating the magnitude and longevity of the humoral response.

Conformational changes in IpaD from Shigella flexneri upon binding bile salts provide insight into the second step of type III secretion

Shigella flexneri uses its type III secretion apparatus (TTSA) to inject host-altering proteins into targeted eukaryotic cells. The TTSA is composed of a basal body and an exposed needle with invasion plasmid antigen D (IpaD) forming a tip complex that controls secretion. The bile salt deoxycholate (DOC) stimulates recruitment of the translocator protein IpaB into the maturing TTSA needle tip complex. This process appears to be triggered by a direct interaction between DOC and IpaD. Fluorescence spectroscopy and NMR spectroscopy are used here to confirm the DOC-IpaD interaction and to reveal that IpaD conformational changes upon DOC binding trigger the appearance of IpaB at the needle tip. Förster resonance energy transfer between specific sites on IpaD was used here to identify changes in distances between IpaD domains as a result of DOC binding. To further explore the effects of DOC binding on IpaD structure, NMR chemical shift mapping was employed. The environments of residues within the proposed DOC binding site and additional residues within the "distal" globular domain were perturbed upon DOC binding, further indicating that conformational changes occur within IpaD upon DOC binding. These events are proposed to be responsible for the recruitment of IpaB at the TTSA needle tip. Mutation analyses combined with additional spectroscopic analyses confirm that conformational changes in IpaD induced by DOC binding contribute to the recruitment of IpaB to the S. flexneri TTSA needle tip. These findings lay the foundation for determining how environmental factors promote TTSA needle tip maturation prior to host cell contact.

Formulation and immunogenicity studies of type III secretion system needle antigens as vaccine candidates

Bacterial infections caused by Shigella flexneri, Salmonella typhimurium, and Burkholderia pseudomallei are currently difficult to prevent due to the lack of a licensed vaccine. Here we present formulation and immunogenicity studies for the three type III secretion system (TTSS) needle proteins MxiH(Δ5), PrgI(Δ5), and BsaL(Δ5) (each truncated by five residues at its C terminus) as potential candidates for vaccine development. These antigens are found to be thermally stabilized by the presence of carbohydrates and polyols. Additionally, all adsorb readily to aluminum hydroxide apparently through a combination of hydrogen bonds and/or Van der Waals forces. The interaction of these proteins with the aluminum-based adjuvant changes with time resulting in varying degrees of irreversible binding. Peptide maps of desorbed protein, however, suggest that chemical changes are not responsible for this irreversible association. The ability of MxiH(Δ5) and PrgI(Δ5) to elicit strong humoral immune responses was tested in a murine model. When administered intramuscularly as monomers, the needle components exhibited dose dependent immunogenic behavior. The polymerized version of MxiH was exceptionally immunogenic even at low doses. The responses of both monomeric and polymerized forms were boosted by adsorption to an aluminum salt adjuvant.

Evidence for alternative quaternary structure in a bacterial Type III secretion system chaperone

BACKGROUND

Type III secretion systems are a common virulence mechanism in many Gram-negative bacterial pathogens. These systems use a nanomachine resembling a molecular needle and syringe to provide an energized conduit for the translocation of effector proteins from the bacterial cytoplasm to the host cell cytoplasm for the benefit of the pathogen. Prior to translocation specialized chaperones maintain proper effector protein conformation. The class II chaperone, Invasion plasmid gene (Ipg) C, stabilizes two pore forming translocator proteins. IpgC exists as a functional dimer to facilitate the mutually exclusive binding of both translocators.

RESULTS

In this study, we present the 3.3 A crystal structure of an amino-terminally truncated form (residues 10-155, denoted IpgC10-155) of the class II chaperone IpgC from Shigella flexneri. Our structure demonstrates an alternative quaternary arrangement to that previously described for a carboxy-terminally truncated variant of IpgC (IpgC1-151). Specifically, we observe a rotationally-symmetric "head-to- head" dimerization interface that is far more similar to that previously described for SycD from Yersinia enterocolitica than to IpgC1-151. The IpgC structure presented here displays major differences in the amino terminal region, where extended coil-like structures are seen, as opposed to the short, ordered alpha helices and asymmetric dimerization interface seen within IpgC1-151. Despite these differences, however, both modes of dimerization support chaperone activity, as judged by a copurification assay with a recombinant form of the translocator protein, IpaB.

CONCLUSIONS

From primary to quaternary structure, these results presented here suggest that a symmetric dimerization interface is conserved across bacterial class II chaperones. In light of previous data which have described the structure and function of asymmetric dimerization, our results raise the possibility that class II chaperones may transition between asymmetric and symmetric dimers in response to changes in either biochemical modifications (e.g. proteolytic cleavage) or other biological cues. Such transitions may contribute to the broad range of protein-protein interactions and functions attributed to class II chaperones.

A repulsive electrostatic mechanism for protein export through the type III secretion apparatus

Many Gram-negative bacteria initiate infections by injecting effector proteins into host cells through the type III secretion apparatus, which is comprised of a basal body, a needle, and a tip. The needle channel is formed by the assembly of a single needle protein. To explore the export mechanisms of MxiH needle protein through the needle of Shigella flexneri, an essential step during needle assembly, we have performed steered molecular dynamics simulations in implicit solvent. The trajectories reveal a screwlike rotation motion during the export of nativelike helix-turn-helix conformations. Interestingly, the channel interior with excessive electronegative potential creates an energy barrier for MxiH to enter the channel, whereas the same may facilitate the ejection of the effectors into host cells. Structurally known basal regions and ATPase underneath the basal region also have electronegative interiors. Effector proteins also have considerable electronegative potential patches on their surfaces. From these observations, we propose a repulsive electrostatic mechanism for protein translocation through the type III secretion apparatus. Based on this mechanism, the ATPase activity and/or proton motive force could be used to energize the protein translocation through these nanomachines. A similar mechanism may be applicable to macromolecular channels in other secretion systems or viruses through which proteins or nucleic acids are transported.

Neonatal mucosal immunization with a non-living, non-genetically modified Lactococcus lactis vaccine carrier induces systemic and local Th1-type immunity and protects against lethal bacterial infection

Safe and effective immunization of newborns and infants can significantly reduce childhood mortality, yet conventional vaccines have been largely unsuccessful in stimulating the neonatal immune system. We explored the capacity of a novel mucosal antigen delivery system consisting of non-living, non-genetically modified Lactococcus lactis particles, designated as Gram-positive enhancer matrix (GEM), to induce immune responses in the neonatal setting. Yersinia pestis LcrV, used as model protective antigen, was displayed on the GEM particles. Newborn mice immunized intranasally with GEM-LcrV developed LcrV-specific antibodies, Th1-type cell-mediated immunity, and were protected against lethal Y. pestis (plague) infection. The GEM particles activated and enhanced the maturation of neonatal dendritic cells (DCs) both in vivo and in vitro. These DCs showed increased capacities for secretion of proinflammatory and Th1-cell polarizing cytokines, antigen presentation and stimulation of CD4(+) and CD8(+) T cells. These data show that mucosal immunization with L. lactis GEM particles carrying vaccine antigens represents a promising approach to prevent infectious diseases early in life.

The response of type three secretion system needle proteins MxiHDelta5, BsaLDelta5, and PrgIDelta5 to temperature and pH

The type III secretion system (TTSS) is a specialized supramolecular injectisome composed of 25 or more proteins which form basal and extracellular domains and share gross architectural similarities with bacterial flagella. The extracellular component of the "needle complex" is primarily composed of a single monomeric subunit organized in a helical array surrounding a hollow pore and protrudes from the bacterial membrane. It is through this surface appendage that virulence factors are translocated to the host cell cytoplasm and thereby subvert normal host cell functions. We present here a comprehensive biophysical analysis of the dynamic conformational behavior of the truncated monomeric needle subunit proteins MxiH(Delta5) (Shigella flexneri), BsaL(Delta5) (Burkholderia pseudomallei), and PrgI(Delta5) (Salmonella typhimurium) as well as their thermal stability over a pH range of 3-8. Circular dichroism spectroscopy indicates the secondary structure is largely alpha helical in all three proteins, and surprisingly thermally labile with transition midpoints in the range of 35-50 degrees C over the pH range of 3-8. Additionally, at the concentrations examined, the very broad thermal transitions were >90% reversible. Second derivative UV absorbance spectroscopy data indicates some disruption of the protein's tertiary structure occurs at temperatures in the range of 29-46 degrees C. The difference in the pH of maximal stability for each of the proteins and the variation for each protein with respect to both secondary and tertiary structural elements is striking. It appears, that at physiological temperatures all three proteins experience intermediate non-native molten globule like states in which they display significant secondary structure in the absence of extensive tertiary interactions. Because of the size difference between the inner pore of the needle and the fully folded needle proteins, it seems clear that the needle subunits must be secreted in a partially or completely unfolded state to reach the distal tip of the needle for assembly. It is proposed that the formation of these intermediate states in the physiological temperature range may play a role in passage through the pore and needle assembly.

Antigen-specific B memory cell responses to lipopolysaccharide (LPS) and invasion plasmid antigen (Ipa) B elicited in volunteers vaccinated with live-attenuated Shigella flexneri 2a vaccine candidates

We evaluated B memory responses in healthy adult volunteers who received one oral dose of live-attenuated Shigella flexneri 2a vaccine. LPS-specific B(M) cells increased from a median of 0 at baseline to 20 spot forming cells (SFC)/10(6) expanded cells following vaccination (p=0.008). A strong correlation was found between post-vaccination anti-LPS B(M) cell counts and peak serum anti-LPS IgG titers (rs=0.95, p=0.0003). Increases in B(M) specific for IpaB approaching significance were also observed. In sum, oral vaccination with live-attenuated S. flexneri 2a elicits B(M) cells to LPS and IpaB, suggesting that B(M) responses to Shigella antigens should be further studied as a suitable surrogate of protection in shigellosis.

Structural dissection of the extracellular moieties of the type III secretion apparatus

Many Gram-negative bacterial pathogens use type III secretion systems (TTSSs) for subverting the normal cellular functions of their target eukaryotic cells. The type III secretion apparatus (TTSA) functions like a syringe to inject proteins through an external needle and into a target cell's membrane and cytosol. The TTSA basal body spans the bacterial inner and outer membranes, and the external needle is topped with a tip complex that controls the secretion and delivery of translocator and effector proteins. Recently solved structures of TTSA proteins have greatly advanced our understanding of shared themes in apparatus assembly and function. In this highlight, the structure-function of TTSA needle and tip complex proteins are described and common themes discussed.

The C-terminus of IpaC is required for effector activities related to Shigella invasion of host cells

Invasion plasmid antigen C (IpaC) is secreted by the Shigella flexneri type III secretion system (TTSS) as an essential trigger of epithelial cell invasion. At the molecular level, IpaC possesses a distinct functional organization. The IpaC C-terminal region between amino acids 319 and 345 is predicted to form a coiled-coil structure. Such alpha-helical motifs appear to be a recurring structural theme among TTSS components. Together with IpaB, this IpaC region is also required for the formation of translocon pores in target cell membranes. In contrast, mutations within the C-terminal tail of IpaC (defined by residues 345-363) have no effect on contact hemolysis (a putative measure of translocon pore formation), but they can contribute significantly to IpaC's ability to trigger S. flexneri entry into cultured cells. Here we describe the molecular dissection of the IpaC C-terminus and how changes in this region affect selected virulence-related activities. IpaC invasion function requires its immediate C-terminus and this general region may be involved in its ability to trigger actin nucleation. In contrast, IpaC could not be shown to interact directly with Cdc42, a host GTPase closely tied to Shigella invasion.