Strategies for development of potential candidate Shigella vaccines

HIV/AIDS vaccines for Africa: scientific opportunities, challenges and strategies

More than decades have already elapsed since human immunodeficiency virus (HIV) was identified as the causative agent of acquired immunodeficiency syndrome (AIDS). The HIV has since spread to all parts of the world with devastating effects. In sub-saharan Africa, the HIV/AIDS epidemic has reached unprecedented proportions. Safe, effective and affordable HIV/AIDS vaccines for Africans are therefore urgently needed to contain this public health problem. Although, there are challenges, there are also scientific opportunities and strategies that can be exploited in the development of HIV/AIDS vaccines for Africa. The recent RV144 Phase III trial in Thailand has demonstrated that it is possible to develop a vaccine that can potentially elicit modest protective immunity against HIV infection. The main objective of this review is to outline the key scientific opportunities, challenges and strategies in HIV/AIDS vaccine development in Africa.

Cross-protection provided by live Shigella mutants lacking major antigens

The immune response elicited by Shigella infections is dominated by serotype-specific antibodies recognizing the LPS O-antigens. Although a marked antibody response to invasion plasmid antigens (Ipa-s) shared by all virulent strains is also induced, the varying level of immunity elicited by natural infections is serotype-restricted. Previous vaccines have tried to mimic and achieve this serotype-specific, infection-induced immunity. As, however, the four Shigella species can express 50 different types of O-antigens, current approaches with the aim to induce a broad coverage use a mixture of the most common O-antigens combined in single vaccines. In the current study we present data on an alternative approach to generate immunity protective against multiple serotypes. Mutants lacking both major immune-determinant structures (i.e. the Ipa and O-antigens) were not only highly attenuated, but, unlike their avirulent counterparts still expressing these antigens, elicited a protective immune response to heterologous serotypes in a murine model. Evidence is provided that protection was mediated by the enhanced immunogenic potential of minor conserved antigens. Furthermore, the rough, non-invasive double mutants triggered an immune response different from that induced by the smooth, invasive strains regarding the isotype of antibodies generated. These non-invasive, rough mutants may represent promising candidates for further development into live vaccines for the prophylaxis of bacillary dysentery in areas with multiple endemic serotypes.

Intra-abdominal and pelvic emergencies

The diversity in intra-abdominal/pelvic infections is more than any other organ system. Several clinical scenarios can end up in intra-abdominal infections. The common causes include penetrating abdominal trauma, abdominal surgery, diverticulitis, appendicitis, pancreatitis, biliary disease, perforated viscus, and primary peritonitis. Intra-abdominal infections can masquerade as fever of obscure origin or as dysfunction of neighboring organs, such as lower lobe pneumonia related to a subphrenic abscess or an abscess causing small bowel obstruction. An urgent surgical intervention is the mainstay of the management of serious intra-abdominal infections.

Protective role of lactobacilli in Shigella dysenteriae 1–induced diarrhea in rats

OBJECTIVE

Studies on lactic acid bacteria exemplify their use against various enteropathogens in vitro. Nevertheless, in vivo effects of Lactobacillus during Shigella infection have not been evaluated. The present study evaluated the effect of Lactobacillus rhamnosus and Lactobacillus acidophilus on neutrophil infiltration and lipid peroxidation during Shigella dysenteriae 1-induced diarrhea in rats.

METHODS

The rats were divided into eight groups (n = 6 in each group). Induced rats received single oral dose of S. dysenteriae (12 x 10(8) colony-forming units [cfu]/mL). Treated rats received L. rhamnosus (1 x 10(7) cfu/mL) or L. acidophilus (1 x 10(7) cfu/mL) orally for 4 d, alone or in combination, followed by Shigella administration. At the end of the experimental period, animals were sacrificed and the assay of the activity of alkaline phosphatase, myeloperoxidase, and antioxidants and the estimation of lipid peroxides were performed. Activity staining of superoxide dismutase and catalase was done in addition to gelatin zymography for matrix metalloproteinase (MMP; MMP-2 and MMP-9) activity. A portion of the intestinal tissue was fixed in 10% formalin for histologic studies.

RESULTS

Administration of S. dysenteriae 1 alone resulted in increased levels of myeloperoxidase, lipid peroxidation, alkaline phosphatase, and the expression of MMP-2 and MMP-9 with concomitant decrease in the antioxidant levels. Pretreatment with the combination of L. rhamnosus (1 x 10(7) cfu/mL) and L. acidophilus (1 x 10(7) cfu/mL) significantly attenuated these changes when compared with the diseased group. Histologic observations were in correlation with biochemical parameters.

CONCLUSION

Lactobacillus rhamnosus plus L. acidophilus offered better protection when compared with individual treatment with these strains during Shigella infection.

Construction of a novel Shigella live-vector strain co-expressing CS3 and LTB/STm of enterotoxigenic E.coli

AIM: To construct and evaluate a polyvalent recombinant vaccine strain Shigella flexneri 2a T32 against enterotoxigenic E.coli (ETEC).

METHODS: By using a host-plasmid balanced lethal system based on asd gene, a polyvalent recombinant strain was constructed to highly express CS3 and regularly express fusion enterotoxin of LTB subunit and mutant ST (LTB/STm) in a vaccine strain Shigella flexneri 2a T32 with specific deletion of asd gene. Fimbria CS3 was observed by immunofluorescence and electron microscopy assay. The security of LTB/STm was examined by ileal loop assay and suckling mouse assay. To evaluate this new candidate vaccine, it was compared with a previous vaccine strain in plasmid and protein level, growth assay and immunogenicity in Balb/c mice.

RESULTS: The newly constructed vaccine expressed CS3 and grew better than the previously constructed vaccine except for the lower expression of LTB/STm. Serum IgG and mucosal IgA against CS3, LTB, ST, and host lipopolysaccharide (LPS) were produced after immunization of Balb/c mice by oral route with the new strain. The titers were not significantly different from the Balb/c mice with the previous strain.

CONCLUSION: This novel candidate diarrheal vaccine can effectively induce serum and mucosal antibody responses against ETEC and Shigella.

Vaccines against Infections Caused by Salmonella, Shigella, and Pathogenic Escherichia coli

Infectious diseases represent one of the most common causes of death worldwide, with the enteropathogenic bacteria Salmonella and Shigella and pathogenic Escherichia coli being among the most detrimental. Currently, vaccination represents the preferred method of preventing such infections. For stimulating the adaptive immune response, immunizations are frequently based on formulations which include inactivated whole-cell vaccines, live attenuated vaccines, or subunit vaccines. These can be administered via a parenteral or mucosal route, the latter having the advantage that it most closely mimics the actual course of infection. In addition to the type of vaccine and method of application, important consideration needs to be paid to safety, efficacy, and cost, which are often major bottlenecks in the successful implementation of vaccines. In this chapter we take a limited look at the history surrounding vaccinations involving Salmonella, Shigella, and pathogenic E. coli. Salmonella infections, which can lead to typhoid fever, are becoming increasing difficult to treat with antibiotics due to multi-drug-resistant strains. At present, the parenteral Vi-based subunit vaccines and the live attenuated oral vaccine Ty21a have proven to be the vaccines of choice, with high levels of protective efficacy and limited side effects. Shigella infections are responsible for the diarrheal disease shigellosis. Various live and nonliving mucosal and parenteral vaccines have been tested, with the most promising candidates evolving around those that stimulate the production of O-antigen-specific antibodies. Pathogenic Escherichia coli infections can lead to severe diseases due to the bacterium's production of several specific toxins. Vaccines against this bacterium target its toxins, as well as surface-exposed antigens, all of which have been found to be effective as immunogens.

Role of 57 kDa major antigenic component of Shigella dysenteriae outer membrane proteins in induction of major histocompatibility complex II-restricted T-cell response

BACKGROUND

In the past, many Shigella surface antigens were used to activate both T and B lymphocytes but failed to induce antigen-specific responses in Shigellosis. Our objective was to identify in vitro T-cell components using 57 kDa major antigenic fraction of Shigella dysenteriae 1 (IPC-31) outer membrane proteins (OMPs) in modulating specific T-cell subset responses against Shigellosis.

METHODS

Antigen-specific T- and B-cell activation was studied in immunized Balb/c mice against 57 kDa antigen by proliferative responses using [3H]-thymidine incorporation and avidin-biotin complex (ABC) peroxidase staining for CD4, CD8, CD3, CD22, and CD25 followed by IL-2 and IL-4 estimation. Macrophage functional assays for migration inhibition factors (MIF) and superoxide (O2-) anions were also performed against 57 kDa antigen, whole OMPs, and phytohemagglutinin (PHA) stimulation.

RESULTS

Greater increase of lymphocyte proliferation was observed after 57 kDa antigen stimulation than post-OMP and -PHA stimulation. Proportionately, CD4+ and CD25+ expression of total CD3+ T-cells was significantly dominant (p >0.05) over CD8+ T-cells. On day 7 of this stimulation, it was found to increase % MIF and O2- anions with decrease of IL-2 leading to activation of MHC-II antigens. Later, on day 28 of immunization, IL-2 levels were more increased than on days 7 and 14 but insignificant with non-immunized mice stimulated with 57 kDa. Levels of IL-2 were also noted with low degree of internalization to its IL-2R receptors rather than to IL-4 receptors. In parallel, expression of CD22 was also recorded higher in this stimulation than in PHA, indicating a T-cell-dependent humoral response.

CONCLUSIONS

Our results suggested that 57 kDa major antigenic OMP is immunogenic for MHC II-restricted T-cell response to acquire host defense against Shigella infection.

Identification of RanBMP interacting with Shigella flexneri IpaC invasin by two-hybrid system of yeast

AIM: Bacillary dysentery caused by Shigella flexneri is still a threat to human health. Of four invasion plasmid antigen proteins (IpaA, B, C and D), IpaC plays an important role in the pathogenicity of this pathogen. The purpose of this study was to investigate the proteins interacting with IpaC in the host cell during the pathogenic process of this disease.

METHODS: By applying two-hybrid system, the bait plasmid containing ipaC gene was constructed and designated pGBKT-ipaC. The bait plasmid was transformed AH109, and proved to express IpaC and then HeLa cDNA library plasmids were introduced into the above transformed AH109. The transformation mixture was plated on medium lacking Trp, Leu, and His in the initial screen, then restreaked on medium lacking Trp, Leu, His and Ade. Colonies growing on the selection medium were further assayed for β-galactosidase activity. BLAST was carried out in the database after sequencing the inserted cDNA of the positive library plasmid.

RESULTS: Among the 2 × 10⁶ transformants, 64 positive clones were obtained as determined by activation of His, Ade and LacZ reporter genes. Sequence analysis revealed that cDNA inserts of two colonies were highly homologous to a known human protein, RanBPM.

CONCLUSION: These results provide evidence that IpaC may be involved in the invasion process of S. flexneri by interacting with RanBPM, and RanBPM is most likely to be the downstream target of IpaC in the cascade events of S. flexneri infection.

Caspase-1 activation of IL-1beta and IL-18 are essential for Shigella flexneri-induced inflammation

Caspases are intracellular proteases that mediate mammalian cell apoptosis. Caspase-1 (Casp-1) is a unique caspase because it activates the proinflammatory cytokines interleukin (IL)-1beta and IL-18. Shigella flexneri, the etiological agent of bacillary dysentery, induces macrophage apoptosis, which requires Casp-1 and results in the release of mature IL-1beta and IL-18. Here we show that casp-1(-/-) mice infected with S. flexneri do not develop the acute inflammation characteristic of shigellosis and are unable to resolve the bacterial infection. Using casp-1(-/-) mice supplemented with recombinant cytokines and experiments with IL-1beta(-/-) and IL-18(-/-) mice, we show that IL-1beta and IL-18 are both required to mediate inflammation in S. flexneri infections. Together, these data demonstrate the importance of Casp-1 in acute inflammation and show the different roles of its substrates, IL-1beta and IL-18, in this response.

Identification of proteins required for the internalization of Campylobacter jejuni into cultured mammalian cells

Clinical and in vitro experimental data suggest that invasion of intestinal epithelial cells is an essential step in the pathogenesis of Campylobacter jejuni-mediated enteritis. However, the molecular mechanism of C. jejuni internalization remains poorly defined. The goal of this study was to identify a gene that encodes a protein required for the internalization of C. jejuni into host cells. A C. jejuni gene, designated ciaB, was identified upon immunoscreening C. jejuni genomic DNA-phage libraries with an antiserum generated against C. jejuni co-cultivated with INT 407 cells. The C. jejuni ciaB gene encodes a protein of 610 amino acids with a calculated molecular mass of 73,154 Da. The deduced amino acid sequence of the CiaB protein shares similarity with type III secreted proteins, associated with invasion of host cells, from other more extensively characterized bacterial pathogens. In vitro binding and internalization assays revealed that the binding of C. jejuni ciaB null mutants was indistinguishable from that of the parental isolate, whereas a significant reduction was noted in internalization. Immunoblot analysis using an anti-CiaB specific antibody revealed that CiaB is secreted into the supernatant fluids upon co-cultivation of C. jejuni with INT 407 cell conditioned medium. Metabolic labeling experiments revealed that at least eight C. jejuni proteins, ranging in size from 12.8 to 108 kDa, are secreted into the culture medium. C. jejuni ciaB null mutants were deficient in the secretion of all proteins, indicating that CiaB is required for the secretion process. Identification of the C. jejuni ciaB gene represents a significant advance in understanding the molecular mechanism of C. jejuni internalization.

Serotype-converting bacteriophages and O-antigen modification in Shigella flexneri

O-antigen modification (serotype conversion) in Shigella flexneri, which is an important virulence determinant, is conferred by temperate bacteriophages. Several serotype-converting phages have been isolated and preliminary characterization has identified the genes involved in O-antigen modification, and has also provided insight into the molecular biology of these phages.

Shigella-induced apoptosis is dependent on caspase-1 which binds to IpaB

We report here that the Shigella invasion plasmid antigen (Ipa)B, which is sufficient to induce apoptosis in macrophages, binds to caspase (Casp)-1, but not to Casp-2 or Casp-3. Casp-1 is activated and its specific substrate interleukin-1beta is cleaved shortly after Shigella infection. Macrophages isolated from Casp-1 knock-out mice are not susceptible to Shigella-induced apoptosis, although they respond normally to other apoptotic stimuli. Shigella kills macrophages from casp-3, casp-11, and p53 knock-out mice as well as macrophages overexpressing Bcl-2. We propose that Shigella induces apoptosis by directly activating Casp-1 through IpaB, bypassing signal transduction events and caspases upstream of Casp-1. Taken together these data indicate that Shigella-induced apoptosis is distinct from other forms of apoptosis and seems uniquely dependent on Casp-1.

Type III protein secretion systems in bacterial pathogens of animals and plants

Various gram-negative animal and plant pathogens use a novel, sec-independent protein secretion system as a basic virulence mechanism. It is becoming increasingly clear that these so-called type III secretion systems inject (translocate) proteins into the cytosol of eukaryotic cells, where the translocated proteins facilitate bacterial pathogenesis by specifically interfering with host cell signal transduction and other cellular processes. Accordingly, some type III secretion systems are activated by bacterial contact with host cell surfaces. Individual type III secretion systems direct the secretion and translocation of a variety of unrelated proteins, which account for species-specific pathogenesis phenotypes. In contrast to the secreted virulence factors, most of the 15 to 20 membrane-associated proteins which constitute the type III secretion apparatus are conserved among different pathogens. Most of the inner membrane components of the type III secretion apparatus show additional homologies to flagellar biosynthetic proteins, while a conserved outer membrane factor is similar to secretins from type II and other secretion pathways. Structurally conserved chaperones which specifically bind to individual secreted proteins play an important role in type III protein secretion, apparently by preventing premature interactions of the secreted factors with other proteins. The genes encoding type III secretion systems are clustered, and various pieces of evidence suggest that these systems have been acquired by horizontal genetic transfer during evolution. Expression of type III secretion systems is coordinately regulated in response to host environmental stimuli by networks of transcription factors. This review comprises a comparison of the structure, function, regulation, and impact on host cells of the type III secretion systems in the animal pathogens Yersinia spp., Pseudomonas aeruginosa, Shigella flexneri, Salmonella typhimurium, enteropathogenic Escherichia coli, and Chlamydia spp. and the plant pathogens Pseudomonas syringae, Erwinia spp., Ralstonia solanacearum, Xanthomonas campestris, and Rhizobium spp.

Construction and characterization of genetically-marked bivalent anti-Shigella dysenteriae 1 and anti-Shigella flexneri Y live vaccine candidates

Bivalent vaccine candidates were developed against Shigella dysenteriae 1 and Shigella flexneri, which are among the most frequent causative agents of shigellosis in developing countries. The rfp and rfb gene clusters, which code for S. dysenteriae serotype 1 O-antigen biosynthesis, were inserted into an arsenite resistance minitransposon and randomly integrated into the attenuated S. flexneri aroD serotype Y strain SFL124. Nine recombinant clones that efficiently expressed both homologous and heterologous O-antigens were obtained. Southern blot analysis showed that in one clone the S. dysenteriae 1 genes had integrated into the chromosome, whereas in all the others they had integrated into the virulence plasmid. All recombinant clones exhibited normal growth characteristics, were able to invade and survive within eukaryotic cells to the same extent as the parental strain, and expressed efficiently the recombinant lipopolysaccharide within invaded cells. Immunization of mice with two of the recombinant clones resulted in the production of antibodies specific for both homologous and heterologous O-antigens. The recombinant clones constitute promising vaccine candidates which can readily be distinguished from endemic shigellae by their non-antibiotic resistance marker.

Construction and characterization of a live attenuated vaccine candidate against Shigella dysenteriae type 1

Vaccine candidates against Shigella dysenteriae type 1, which is associated with the most severe cases of bacillary dysentery, were constructed. The rfp and rfb gene clusters, which code for S. dysenteriae 1 O antigen biosynthesis, were randomly integrated into either the chromosome or the virulence plasmid of the rough attenuated Shigella flexneri aroD strain SFL124-27 with a minitransposon carrying an arsenite resistance selection marker. The recombinant clones efficiently expressed the recombinant O antigen, exhibited a normal growth pattern, were able to invade and survive within eukaryotic cells to the same extent as the parental strain, and expressed the recombinant antigen within invaded cells. A clone was selected as the vaccine candidate, which was demonstrated to be immunogenic and safe in animal models, leading to 47% full protection and 53% partial protection against challenge with the wild-type strain.

Apoptosis as a proinflammatory event: what can we learn from bacteria-induced cell death?

Infection of cells by some pathogenic bacteria triggers host cell apoptosis. Bacteria-induced apoptosis appears to promote an inflammatory response that causes tissue damage and further bacterial colonization. Shigella pathogenesis offers a paradigm for the role of apoptosis in bacterial infections.

Construction of a trivalent candidateShigella vaccine strain with host-vector balanced-lethal system

A trivalent liveShigella vaccine candidate FSD01 against S.flexneri 2a, S.sonnei and S.dysenteriae I was constructed. This candidate strain was based on the S.flexneri 2a vaccine T32. By homologous recombination exchange, the chromosomalasd gene of T32 was site-specifically inactivated, resulting in the strain unable to grow normally in LB broth, while anotherasd gene of S.mutans was employed to construct an Asd(+) complementary vector. This combination ofasd (-) host/Asd(+) vector formed a balanced-lethal expression system in T32 strain. By use of this system, two important protective antigen genes coding for S.sonnei Form I antigen and Shiga toxin B subunit were cloned and expressed in T32, which led to the construction of trivalent candidate vaccine FSD01. Experimental results showed that this strain was genetically stable, but its recombinant plasmid was non-resistant. Moreover, it was able to effectively express trivalent antigens in one host and induce protective responses in mice against the challenges of the above threeShigella strains.

IpaB, a Shigella flexneri invasin, colocalizes with interleukin-1 beta-converting enzyme in the cytoplasm of macrophages

Shigellae are the most prevalent etiological agents of dysentery. A crucial step in shigella pathogenesis is the induction of macrophage apoptosis. The invasion plasmid antigen B (IpaB) is necessary and sufficient to induce macrophage programmed cell death. IpaB activates apoptosis by binding to interleukin-1 beta (IL-1 beta)-converting enzyme (ICE) or a highly homologous protease. Here, we show that IpaB is disseminated throughout the cytoplasm of shigella-infected macrophages as detected by both immunofluorescence and immunoelectron microscopy. The cytoplasmic distribution of IpaB requires phagosome escape, and it is specific to IpaB, since lipopolysaccharide, used here as a bacterial marker, remains closely associated with the bacteria. In double-labeling experiments, we show that IpaB and ICE colocalize in the cytoplasm of the macrophage, suggesting that soon after secretion, IpaB binds to ICE to initiate apoptosis and to promote the cleavage of IL-1 beta.

Shigella flexneri is trapped in polymorphonuclear leukocyte vacuoles and efficiently killed

We examined the bactericidal activity of polymorphonuclear leukocytes (PMN) against an invasive wild-type strain of Shigella flexneri (M90T) and a plasmid-cured noninvasive derivative (BS176). Both Shigella strains, as well as a rough strain of Escherichia coli, were killed with similar efficiencies by intact inflammatory PMN in room air and under N2 (i.e., killing was O2 independent). Bacterial killing by PMN extracts was substantially inhibited by antibodies to the bactericidal/permeability-increasing protein (BPI). Whereas wild-type Shigella escapes from the phagosome to the cytoplasm in epithelial cells and macrophages, wild-type Shigella was trapped in the phagolysosome of PMN as visualized by electron microscopy. The efficient killing of Shigella by PMN suggests that these inflammatory cells may not only contribute initially to the severe tissue damage characteristic of shigellosis but also ultimately participate in clearance and resolution of infection.

Development of Shigella sonnei live oral vaccines based on defined rfbInaba deletion mutants of Vibrio cholerae expressing the Shigella serotype D O polysaccharide

Previous experimentation has highlighted a number of difficulties in the development of carrier-based bivalent vaccines (J.-F. Viret and D. Favre, Biologicals 22:361-372, 1994) In an attempt to obviate these carrier strains. Toward this aim, a series of defined rfbInaba deletion (delta rfbInaba) mutants of the cholera vaccine strain V. cholerae CVD103-HgR (O1 Inaba serotype) and derivative bearing the chromosomally integrated locus encoding the S. sonnei O-PS were constructed and characterized. The various mutations disrupt genes thought to be involved in either the synthesis of perosamine, the synthesis of 3-deoxy-L-glycero tetronic acid, or the O-PS transport functions together with synthesis of the perosamine synthetase. Some deletions were obtained only in strains expressing the heterologous lipopolysaccharide (LPS). Viable delta rfbInaba deletions in CVD103-HgR profoundly altered some of its phenotypic properties. The same deletions present in CVD103-HgR derivatives expressing the heterologous LPS affected their phenotypes only to a lesser extent. Only in strains in which perosamine synthesis was specifically abolished could high amounts of core-bound S. sonnei O-PS be synthesized. Two such strains (CH21, which expresses both the R1 core and the S. sonnei O-PS, and CH22, which expresses only the latter antigenic determinant) were further analyzed and were found to be indistinguishable from CVD103-HgR with regard to lack of enterotoxin activity, choleragenoid production, mercury resistance, pilin production, and, for CH22, motility. Mice immunized with CH22 produced high titers of S. sonnei O-PS-specific antibodies.

Protection against local Shigella sonnei infection in mice by parenteral immunization with a nucleoprotein subcellular vaccine

Nucleoprotein subcellular (NPS) vaccine, consisting of ribosome-bound O polysaccharide, was prepared from avirulent Shigella sonnei. NPS vaccine was tested for safety and protective activity in the mouse intranasal challenge model of Shigella infection. The vaccine was nontoxic when injected in doses up to 10,000 micrograms, and a single subcutaneous injection of as little as 0.1 micrograms gave significant protection against a lethal intranasal challenge with S. sonnei. These data demonstrate the induction of local protective immunity by parenteral immunization, support the concept of the ribosome as a potent vaccine vector, and give additional evidence for the protective activity of the NPS vaccine against Shigella infection.

Construction and evaluation of a virG thyA double mutant of Shigella flexneri 2a as a candidate live-attenuated oral vaccine

A virG thyA double mutant of Shigella flexneri 2a was constructed as a candidate live-attenuated oral vaccine. In the keratoconjunctivitis model it did not provoke any adverse reaction by itself on guinea pigs' eyes and completely protected them from provoking keratoconjunctivitis. When (2.7-4.8) x 10(10) of the vaccine was inoculated intragastrically after 1 day fasting in cynomolgus monkeys three times at weekly intervals, a watery stool was observed at 40% as a side-effect. Upon intragastric challenge after 1 day fasting with 7.5 x 10(9) of the virulent strain four weeks after the last vaccination, a statistically significant difference was obtained in the mortality rate but not in the morbidity rate between the vaccine and the control group, although the clinical findings were less severe in the vaccine group than in the control group. These results together with the histopathological and immunological findings indicate that the vaccine deserve further detailed studies.

Safety and immunogenicity study of the auxotrophic Shigella flexneri 2a vaccine SFL1070 with a deleted aroD gene in adult Swedish volunteers

The live auxotrophic Shigella flexneri 2a vaccine strain SFL1070 with a deleted aroD gene was given orally to 37 adult Swedish volunteers who received three doses within 5 days. Each dose comprised 1 x 10(5) (n = 9), 1 x 10(7) (n = 10), 1 x 10(8) (n = 9) or 1 x 10(9) (n = 9) c.f.u. S. flexneri SFL1070. One volunteer vaccinated with 1 x 10(7) and three vaccinated with 1 x 10(8) c.f.u. reported mild gastrointestinal symptoms after the first dose. Vaccination with 1 x 10(9) c.f.u. caused abdominal pain and watery diarrhoea in four volunteers who all recovered spontaneously within 72 h. S. flexneri SFL1070 was not recovered from volunteers given 1 x 10(5) c.f.u., but was shed in faeces by six volunteers vaccinated with 1 x 10(7), by all nine vaccinated with 1 x 10(8), and by seven volunteers vaccinated with 1 x 10(9) c.f.u. The mean excretion time was 2.6 (range 0-4) days in the 1 x 10(8) and the 1 x 10(9) groups. Serum antibody responses against either S. flexneri 2a and Y lipopolysaccharides (LPSs) or Shigella invasion plasmid antigens (Ipa) were seen in eight volunteers vaccinated with 1 x 10(9) (p < 0.01 to p < 0.05 for mean relative titres of IgA and IgG against S. flexneri 2a and Y LPSs), in four vaccinated with 1 x 10(8), and in two and one volunteers each vaccinated with 1 x 10(7) and 1 x 10(5) c.f.u. of S. flexneri SFL1070. Intestinal sIgA responses to the same antigens were elicited in all volunteers in the 1 x 10(9) and the 1 x 10(8) groups, and in six and one volunteers vaccinated with 1 x 10(7) and 1 x 10(5) c.f.u., respectively. The sIgA responses against S. flexneri 2a and Y LPSs were significant in all but the 1 x 10(5) group (p < 0.01 to p < 0.05). Significant antibody-secreting cell (ASC) responses specific to S. flexneri 2a LPS were seen in peripheral blood from eight volunteers each in the 1 x 10(9) and 1 x 10(8) groups and from five volunteers vaccinated with 1 x 10(7) c.f.u. (p < 0.01 to p < 0.05). The number of volunteers showing anti-Shigella Ipa ASC responses in these groups were five (p < 0.01 to p < 0.05), three and one, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Purification, pore-forming ability, and antigenic relatedness of the major outer membrane protein of Shigella dysenteriae type 1

The major outer membrane protein (MOMP), the most abundant outer membrane protein, was purified to homogeneity from Shigella dysenteriae type 1. The purification method involved selective extraction of MOMP with sodium dodecyl sulfate in the presence of 0.4 M sodium chloride followed by size exclusion chromatography with Sephacryl S-200 HR. MOMP was found to form hydrophilic diffusion pores by incorporation into artificial liposome vesicles composed of egg yolk phosphatidylcholine and dicetylphosphate, indicating that MOMP of S. dysenteriae type 1 exhibited significant porin activity. However, the liposomes containing heat-denatured MOMP were barely active. The molecular weight of MOMP found by size exclusion chromatography was 130,000, and in sodium dodecyl sulfate-10% polyacrylamide gel it moved as an oligomer of 78,000 molecular weight. Upon boiling, fully dissociated monomers of 38,000 molecular weight were seen for S. dysenteriae type 1. However, among the four Shigella spp., the monomeric MOMP generated upon boiling ranged from 38,000 to 35,000 in molecular weight. Antibody raised in BALB/c mice immunized with MOMP of S. dysenteriae type 1 reacted strongly with purified MOMP of S. dysenteriae type 1 in an enzyme-linked immunosorbent assay (ELISA). The antibody reacted with whole-cell preparations of S. dysenteriae type 1 in an ELISA, suggesting that MOMP possessed surface components. Moreover, MOMP could be visualized on the bacterial surface by immunoelectron microscopy with anti-MOMP antibody. S. dysenteriae type 1 MOMP-specific immunoglobulin eluted from MOMP bound to a nitrocellulose membrane was found to cross-react with MOMP preparations of S. flexneri, S. boydii, and S. sonnei, indicating that MOMPs were antigenically related among Shigella species. The strong immunogenicity, surface exposure, and antigenic relatedness make MOMP of Shigella species an immunologically significant macromolecule for study.

Molecular and cellular mechanisms of tissue invasion by Shigella flexneri

Shigella flexneri, a member of the family of enterobacteriaceae, causes bacillary dysentery by invading the human colonic mucosa and provoking a very intense inflammation. Recent in vitro data allow us to integrate different phenomena into a model of the infectious process during shigellosis. In vivo, bacteria appear to enter the submucosa via the M cells, specialized cells that cover the follicular structures of the intestinal mucosa. Once inside the submucosa, shigellae encounter resident tissue macrophages, which are infected, and apoptosis is rapidly induced. During programmed cell death the inflammatory cytokine interleukin-1 (IL-1) is released. Interleukin-1 triggers an inflammatory reaction characterized by extravasation of polymorphonuclear (PMN) cells. The inflammation is probably potentiated by the production of other cytokines by epithelial, endothelial, and PMN cells. Polymorphonuclear cells migrate through the epithelium into the lumen of the colon, destabilizing the integrity of the epithelial barrier. The damaged epithelium allows massive entry of bacteria into the submucosa. Further colonization of the epithelium aggravates inflammation, which in turn causes extensive tissue destruction. Both the in vitro and in vivo results that support this model are discussed.

Morphologic evaluation of the pathogenesis of bacterial enteric infections

Current advances in the understanding of the pathogenicity of the agents of diarrheal infections, Vibrio cholerae, diarrheagenic E. coli, Shigella, Salmonella, and enteropathogenic Yersinia, have, to a great extent, become possible due to morphological studies of host-pathogen interactions in natural and experimental infections. Despite a multigenic nature and a diversity of pathogenic features in the bacterial species and even in serogroups of the same species, it is now possible to delineate four major patterns of interaction of enteric pathogens with their cellular targets, the enterocytes, and with the immune apparatus of the gut. These patterns, epicellular cytotonic, epicellular restructuring cytotonic, invasive intraepithelial cytotonic and cytotoxic, and invasive transcellular cytotonic and cytotoxic bacteremic, underlie early pathogenesis and clinical manifestations in the respective diarrheal diseases. In this review, the results of the morphological analyses of these patterns over the last 3 decades as well as some methodological problems encountered in the interpretation of morphological observations are discussed.

Diarrhoeal disease: current concepts and future challenges. Diarrhoeal disease and vaccine development

Vaccination against diarrhoeal disease offers many opportunities to reduce significantly disease burden and childhood mortality from preventable disease world-wide. Regrettably, vaccine development has become an issue more of the development of the ultimate vaccine rather than the provision of a useful public health tool. The delay in implementing the delivery of vaccines with only 50% protective efficacy while awaiting the development of single dose vaccines that will provide lifelong immunity in more than 90% of recipients has resulted in no vaccine becoming available to those people who need it. Any alleviation of the disease burden would be of considerable benefit in an endemic region, to both the people and the governments, while researchers continue pursuing the ideal vaccine. The issues are discussed in this paper.

Vaccines to prevent bacterial enteric infections in children

Considerable progress has been made in the last decade in developing vaccines against the most important bacterial enteric infections. Two new vaccines against typhoid fever (oral Ty21a and parenteral Vi polysaccharide) have been licensed in many countries. Newer generations of more sophisticated typhoid vaccines are undergoing clinical testing including recombinant attenuated S typhi strains and Vi polysaccharide-carrier protein conjugate vaccines. Two inactivated oral cholera vaccines, consisting of inactivated V cholerae 01 bacteria alone or in combination with the B subunit of cholera toxin, each conferred 50% to 53% protection over 3 years in a field trial in Bangladesh where subjects were immunized with a three-dose regimen. An engineered live oral cholera vaccine, strain CVD 103-HgR, has been shown in extensive clinical trials to be well tolerated by children and adults in developing countries and highly immunogenic following administration of just a single oral dose; a large-scale field trial of the efficacy of this vaccine is underway. Several candidate vaccines against Shigella and enterotoxigenic E coli are in clinical trials.