Isolation and characterization of a Shigella flexneri invasin complex subunit vaccine

The invasiveness and virulence of Shigella spp. are largely due to the expression of plasmid-encoded virulence factors, among which are the invasion plasmid antigens (Ipa proteins). After infection, the host immune response is directed primarily against lipopolysaccharide (LPS) and the virulence proteins (IpaB, IpaC, and IpaD). Recent observations have indicated that the Ipa proteins (IpaB, IpaC, and possibly IpaD) form a multiprotein complex capable of inducing the phagocytic event which internalizes the bacterium. We have isolated a complex of invasins and LPS from water-extractable antigens of virulent shigellae by ion-exchange chromatography. Western blot analysis of the complex indicates that all of the major virulence antigens of Shigella, including IpaB, IpaC, and IpaD, and LPS are components of this macromolecular complex. Mice or guinea pigs immunized intranasally with purified invasin complex (invaplex), without any additional adjuvant, mounted a significant immunoglobulin G (IgG) and IgA antibody response against the Shigella virulence antigens and LPS. The virulence-specific response was very similar to that previously noted in primates infected with shigellae. Guinea pigs (keratoconjunctivitis model) or mice (lethal lung model) immunized intranasally on days 0, 14, and 28 and challenged 3 weeks later with virulent shigellae were protected from disease (P<0.01 for both animal models).

Vaccination against shigellosis with attenuated Shigella flexneri 2a strain SC602

The Shigella flexneri 2a SC602 vaccine candidate carries deletions of the plasmid-borne virulence gene icsA (mediating intra- and intercellular spread) and the chromosomal locus iuc (encoding aerobactin) (S. Barzu, A. Fontaine, P. J. Sansonetti, and A. Phalipon, Infect. Immun. 64:1190-1196, 1996). Dose selection studies showed that SC602 causes shigellosis in a majority of volunteers when 3 x 10(8) or 2 x 10(6) CFU are ingested. In contrast, a dose of 10(4) CFU was associated with transient fever or mild diarrhea in 2 of 15 volunteers. All volunteers receiving single doses of >/=10(4) CFU excreted S. flexneri 2a, and this colonization induced significant antibody-secreting cell and enzyme-linked immunosorbent assay responses against S. flexneri 2a lipopolysaccharide in two-thirds of the vaccinees. Seven volunteers who had been vaccinated 8 weeks earlier with a single dose of 10(4) CFU and 7 control subjects were challenged with 2 x 10(3) CFU of virulent S. flexneri 2a organisms. Six of the control volunteers developed shigellosis with fever and severe diarrhea or dysentery, while none of the vaccinees had fever, dysentery, or severe symptoms (P = 0. 005). Three vaccinees experienced mild diarrhea, and these subjects had lower antibody titers than did the fully protected volunteers. Although the apparent window of safety is narrow, SC602 is the first example of an attenuated S. flexneri 2a candidate vaccine that provides protection against shigellosis in a stringent, human challenge model.

Identification of epitope and surface-exposed domains of Shigella flexneri invasion plasmid antigen D (IpaD)

Transport and surface expression of the invasion plasmid antigens (Ipa proteins) is an essential trait in the pathogenicity of Shigella spp. In addition to the type III protein secretion system encoded by the mxi/spa loci on the large virulence plasmid, transport of IpaB and IpaC into the surrounding medium is modulated by IpaD. To characterize the structural topography of IpaD, the Geysen epitope-mapping system was used to identify epitopes recognized by surface-reactive monoclonal and polyclonal antibodies produced against purified recombinant IpaD or synthetic IpaD peptides. Surface-exposed epitopes of IpaD were confined to the first 180 amino acid residues, whereas epitopes in the carboxyl-terminal half were not exposed on the Shigella surface. By using convalescent-phase sera from 10 Shigella flexneri-infected monkeys, numerous epitopes were mapped within a surface-exposed region of IpaD between amino acid residues 14 and 77. Epitopes were also identified in the carboxyl-terminal half of IpaD with a few convalescent-phase sera. Comparison of IpaD epitope sequences with Salmonella SipD sequences indicated that very similar epitopes may exist in the carboxyl-terminal region of each protein whereas the IpaD epitopes in the surface-exposed amino-terminal region were unique for the Shigella protein. Although the IpaD and SipD homologs may play similar roles in transport, the dominant serum antibody response to IpaD is against the unique region of this protein exposed on the surface of the pathogen.

Construction and characterization of virG (icsA)-deleted Escherichia coli K12-Shigella flexneri hybrid vaccine strains

Human challenge studies with EcSf2a-2, an aroD deletion-attenuated Escherichia coli K12-Shigella flexneri hybrid vaccine expressing S. flexneri 2a somatic antigen and the invasive phenotype indicated that, at doses of 2 x 10(9) bacteria, EcSf2a-2 was immunogenic but also reactogenic and therefore not sufficiently attenuated. Two factors that may contribute to the residual reactogenicity are the spontaneous appearance of plaque-positive variants in the E. coli K12 recipient and the presence of the arg locus encoding enterotoxin or cytotoxin, transferred from S. flexneri 2a into the E. coli recipient. EcSf2a-3 was derived from EcSf2a-2 by introducing a deletion in the virG gene, whose expression is required for plaque formation and keratoconjunctivitis in guinea pigs. EcSf2a-5 contains the same deletion in the E. coli-S. flexneri hybrid strain, 7921, but does not contain the arg locus. Lack of virG expression in these hybrid strains did not affect the immune response to LPS or the development of protective immunity in the guinea pig model.

Antibody response of monkeys to invasion plasmid antigen D after infection with Shigella spp

The antigen preparation most often used for determining the levels of antibodies to virulence-associated proteins of Shigella spp. consists of a mixture of proteins (including IpaB, IpaC, IpaD, and VirG*) extracted from virulent shigellae with water (water extract). To overcome the lack of specificity for individual antigens in the water-extract enzyme-linked immunosorbent assay (ELISA), the ipaD gene from S. flexneri has been cloned, expressed to a high level, and purified for use in a new ELISA for the determination of the levels of antibody against IpaD in monkeys and humans challenged with shigellae. The IpaD ELISA for serum immunoglobulins G and A correlated well with the water-extract ELISA in that monkeys infected with S. flexneri or S. sonnei responded with high serum antibody titers in both assays. The IpaD assay required less antigen per well, had much lower background levels, and did not require correction with antigens from an avirulent organism. In conjunction with the water-extract ELISA, it was possible to identify infected animals that did not respond to IpaD but did produce antibodies that reacted in the water-extract ELISA. This indicates that even though IpaB, IpaC, and IpaD are essential for the invasiveness phenotype, the infected host does not always produce antibodies against all components of the invasiveness apparatus.

Recognition of three epitopic regions on invasion plasmid antigen C by immune sera of rhesus monkeys infected with Shigella flexneri 2a

The invasive ability of Shigella spp. is correlated with the expression of several plasmid-encoded proteins, including invasion plasmid antigen C (IpaC). By characterizing the antigenic structure of IpaC with monoclonal antibodies and convalescent-phase sera, it may be possible to determine the physical location of specific epitopes as well as the involvement of epitopes in a protective immune response or the host's susceptibility to disease. By using overlapping octameric synthetic peptides, which together represent the entire IpaC protein, the precise linear sequence of four surface-exposed epitopes was defined for four IpaC monoclonal antibodies. Furthermore, 17 unique peptide epitopes of IpaC were mapped by using 9-day-postinfection serum samples from 13 rhesus monkeys challenged with Shigella flexneri 2a. Each individual recognized a somewhat different array of IpaC peptide epitopes after infection with shigellae. However, the epitopes were clustered within three regions of the protein: region I (between amino acid residues 1 and 61), region II (between amino acid residues 177 and 258), and region III (between amino acid residues 298 and 307). Region II was recognized by 92% of S. flexneri-infected individuals and was considered to be a highly immunogenic region. Animals asymptomatic for shigellosis after challenge with S. flexneri recognized peptide epitopes within all three epitopic regions of IpaC, whereas symptomatic animals recognized peptides in only one or two of the epitopic regions. Antibody from monkeys challenged with S. sonnei recognized IpaC peptide epitopes which fell within and outside the three S. flexneri epitopic regions. While numerous potential epitopes exist on the IpaC protein, the identification of three regions in which epitopes are clustered suggests that these regions are significant with respect to the immune response and to subsequent pathogenesis postinfection.

Murine T-cell response to native and recombinant protein antigens of Rickettsia tsutsugamushi

A polyclonal T-cell line with TH1 characteristics was used to assess the murine cellular immune response to native and recombinant Rickettsia tsutsugamushi antigens. Proliferation of this T-cell line was observed in response to numerous native antigen fractions, which indicates that the murine T-helper-cell response is directed at multiple scrub typhus antigens with no apparent antigenic immunodominance. Subsequent analysis of recombinant R. tsutsugamushi antigens made it possible to identify a 47-kDa scrub typhus antigen (Sta47) that was stimulatory for the polyclonal T-cell line. Recombinant clones encoding 56-, 58-, and 110-kDa antigens (Sta56, Sta58, and Sta110, respectively) were unable to induce proliferation of this T-cell line. DNA sequence analysis of the cloned rickettsial insert encoding the Sta47 protein revealed the presence of four open reading frames potentially encoding proteins of 47, 30, 18, and 13 kDa. Analysis of sodium dodecyl sulfate-polyacrylamide gel electrophoresis-separated and eluted fractions of lysates from the recombinant HB101(pRTS47B4.3) demonstrated that the fractions containing the 47-kDa protein as well as those containing proteins less than 18 kDa were stimulatory. Selected synthetic amphipathic peptides derived from the Sta47 antigen sequence identified a 20-amino-acid peptide that gave a 10-fold increase in T-cell proliferation over a control malarial peptide of similar length. Recognition of the 47-kDa antigen by a T-cell line with TH1 characteristics implicates this protein as one of potential importance in protection studies and future vaccine development.

Surface presentation of Shigella flexneri invasion plasmid antigens requires the products of the spa locus

An avirulent, invasion plasmid insertion mutant of Shigella flexneri 5 (pHS1059) was restored to the virulence phenotype by transformation with a partial HindIII library of the wild-type invasion plasmid constructed in pBR322. Western immunoblot analysis of pHS1059 whole-cell lysates revealed that the synthesis of the invasion plasmid antigens VirG, IpaA, IpaB, IpaC, and IpaD was similar to that seen in the corresponding isogenic S. flexneri 5 virulent strain, M90T. IpaB and IpaC, however, were not present on the surface of pHS1059 as was found in M90T, suggesting that the transport or presentation of the IpaB and IpaC proteins onto the bacterial surface was defective in the mutant. pHS1059 was complemented by pWR266, which carried contiguous 1.2- and 4.1-kb HindIII fragments of the invasion plasmid. pHS1059(pWR266) cells were positive in the HeLa cell invasion assay as well as colony immunoblot and enzyme-linked immunosorbent assays, using monoclonal antibodies to IpaB and IpaC. These studies established that the antigens were expressed on the surface of the transformed bacteria. In addition, water extraction of pHS1059 and pHS1059(pWR266) whole cells, which can be used to remove IpaB and IpaC antigens from the surface of wild-type M90T bacteria, yielded significant amounts of these antigens from pHS1059(pWR266) but not from pHS1059. Minicell and DNA sequence analysis indicated that several proteins were encoded by pWR266, comprising the spa loci, which were mapped to a region approximately 18 kb upstream of the ipaBCDAR gene cluster. Subcloning and deletion analysis revealed that more than one protein was involved in complementing the Spa- phenotype in pHS1059. One of these proteins, Spa47, showed striking homology to ORF4 of the Bacillus subtilis flaA locus and the fliI gene sequence of Salmonella typhimurium, both of which bear strong resemblance to the alpha and beta subunits of bacterial, mitochondrial, and chloroplast proton-translocating F0F1 ATPases.

Myosin-cross-reactive epitope of Shigella flexneri invasion plasmid antigen B

IpaB, invasion plasmid antigen B, of Shigella flexneri is a 62-kDa protein required for invasion of intestinal epithelial cells. IpaB is also one of several major protein antigens recognized by the humoral immune systems of most humans and monkeys after infection with shigellae. Computer analysis of the deduced IpaB amino acid sequence indicates that an alpha-helical structure is likely through much of the molecule. Homology searches with protein data banks show that one alpha-helical domain between amino acid residues 95 and 181 has a moderate level of identity with myosin and streptococcal M protein. By using a monoclonal antibody (2F1) which recognizes an epitope in the amino-terminal third of the IpaB protein, it was possible to demonstrate a cross-reactive epitope(s) on skeletal muscle myosin. Epitope mapping localized the 2F1 epitope to three noncontiguous regions of the IpaB protein within the alpha-helical domain that contains homology with myosin. Antibodies produced in rabbits immunized with synthetic peptides from one of the 2F1 epitope regions (residues 99 to 110) of IpaB were capable of reacting with IpaB as well as myosin. Furthermore, sera from several monkeys previously infected with S. flexneri 2a contained antibodies to IpaB pep 101-116 (IpaB peptide 101-116) and also myosin. Sera from animals with antibodies against other IpaB peptides did not contain antibodies against myosin.

Small-animal model to measure efficacy and immunogenicity of Shigella vaccine strains

The development of a small-animal model to test the protective efficacy and immunogenicity of a vaccine strain against shigellosis would greatly facilitate the evaluation of potential vaccine candidates. In guinea pigs, the ability of shigellae to invade and multiply within the corneal epithelium, causing keratoconjunctivitis, closely mimics the invasion process in the intestinal epithelium (B. Sereny, Acta Microbiol. Acad. Sci. Hung. 4:367-376, 1957). The serum response of animals recovering from a Shigella keratoconjunctival infection was determined and found to be consistent with that shown by convalescent humans and primates. This model was used to test the efficacy of two vaccine candidates, and the immune response of the guinea pigs to the vaccine strains was examined. Both vaccine strains demonstrated significant protection against challenge by homologous virulent Shigella strains, and the results were comparable with results obtained in trials with monkeys. The guinea pig model also provides a rapid and inexpensive means of evaluating different immunization regimens as well as of testing other variables such as length of protection against disease.

Effect of prior infection with virulent Shigella flexneri 2a on the resistance of monkeys to subsequent infection with Shigella sonnei

All virulent shigellae have large plasmids. Plasmid-associated genes encode the expression of membrane-associated proteins (MAP), some of which correlate with the ability to invade susceptible epithelial cells. These MAP are serologically related in all of the shigella serotypes and evoke an antibody response after infection. To determine whether the MAP have a significant role in protection, 24 monkeys were infected with virulent Shigella flexneri 2a. After recovery, one group (with controls) was rechallenged with S. flexneri 2a; another group (with controls) was fed Shigella sonnei. The animals that were rechallenged with S. flexneri 2a were protected, while those that were fed S. sonnei experienced the same incidence of disease as controls. No differences in serum immune response to MAP after primary infection with S. flexneri were detected in immunoblots using lysates of S. flexneri or S. sonnei or in ELISA using water extracts of these strains.

Molecular cloning and sequence analysis of a Rickettsia tsutsugamushi 22 kDa antigen containing B- and T-cell epitopes

The identification of Rickettsia tsutsugamushi T-cell epitopes is necessary for the characterization of the protective immune response (of which the T-cell response is essential) against scrub typhus rickettsiae. A T-helper cell line derived from R. tsutsugamushi (Karp strain) immune mice reacted with rickettsial protein antigens eluted from the 18-35 kDa region of polyacrylamide gels. Within this region is a 22 kDa protein which is reactive with immune serum. The gene encoding the 22 kDa scrub typhus antigen (sta22) was cloned and expressed in Escherichia coli. Nucleotide sequence analysis of the sta22 gene revealed a potential open reading frame (ORF) in the sta22 sequence encoding a 22 kDa protein. A recombinant 22 kDa protein synthesized in E. coli maxicells was reactive with anti-rickettsial antibodies. The codon usage of the adenine and thymine rich sta22 sequence was similar to other previously sequenced R. tsutsugamushi genes. Computer analysis of the deduced amino acid sequence suggested that the Sta22 protein has several amphipathic regions which may be potential T-cell epitopes. The recombinant Sta22 protein eluted from polyacrylamide gels induced a strong proliferative response from the scrub typhus rickettsiae reactive T-cell line. Recognition of the R. tsutsugamushi Sta22 polypeptide by both cellular and humoral immune mechanisms implicates this antigen as one of potential importance in vaccine development.

The 56-kilodalton major protein antigen of Rickettsia tsutsugamushi: molecular cloning and sequence analysis of the sta56 gene and precise identification of a strain-specific epitope

Lasting immunity against Rickettsia tsutsugamushi, the causative agent of scrub typhus fever, has been demonstrated to be strain specific. Two protein antigens of 110 and 56 kilodaltons (kDa) have been shown to exhibit strain-specific epitopes. The 56-kDa scrub typhus antigen (Sta56) is an abundant outer membrane protein of R. tsutsugamushi and is an antigen often recognized by humans infected with this obligate intracellular bacterium. In this study the complete gene encoding Sta56 (strain Karp) was cloned into pBR322 on a 2.3-kilobase genomic HindIII DNA fragment and the complete 56-kDa polypeptide was expressed in Escherichia coli. DNA sequence analysis of the 2.3-kilobase HindIII fragment revealed an open reading frame large enough to encode a 56-kDa polypeptide. A putative signal sequence was identified at the deduced amino terminus of the Sta56 polypeptide, and pulse-chase analysis of maxicells labeled with [35S]methionine demonstrated that a higher-molecular-weight precursor matures into the 56-kDa polypeptide. Epitope scanning analysis with synthetic peptides derived from the deduced amino acid sequence identified an octapeptide (located from amino acid residues 117 to 124) that was reactive with a Karp strain-specific monoclonal antibody (K13F88A). Other epitopes recognized by different monoclonal antibodies, including another Karp strain-specific monoclone (K1E106), were localized to different regions of the protein based on their reactivities with lambda gt11 recombinants expressing various portions of the sta56 gene.

Detection of Rickettsia tsutsugamushi by gene amplification using polymerase chain reaction techniques

Scrub typhus is commonly undiagnosed in endemic areas due, in part, to dependence on retrospective serodiagnosis. Since the etiologic agent, R. tsutsugamushi, will not grow in cell-free systems, a rapid direct-agent detection system such as provided by polymerase chain reaction (PCR) methodology is needed. Genes coding for the variable 56-kDa antigen of R. tsutsugamushi were amplified through 35 cycles using 20-mer oligonucleotide primers and Taq polymerase. Amplification of 1-ng samples of DNA extracted from purified prototype R. tsutsugamushi Karp, Gilliam, and Kato strains was detected by direct visual inspection of the electrophoresed, ethidium bromide-stained, specific bands. Specificity of the PCR was shown when PCR amplification of various non-scrub typhus rickettsial DNAs was unsuccessful. R. tsutsugamushi DNA extracted from the blood of infected mice could be PCR amplified and the 1477-base pair product detected by either direct visualization or by specific hybridization with amplified non-radioactive digoxigenin-11-dUTP-labeled Karp 56-kDa DNA probe.

Molecular cloning and sequence analysis of the Sta58 major antigen gene of Rickettsia tsutsugamushi: sequence homology and antigenic comparison of Sta58 to the 60-kilodalton family of stress proteins

The scrub typhus 58-kilodalton (kDa) antigen (Sta58) of Rickettsia tsutsugamushi is a major protein antigen often recognized by humans infected with scrub typhus rickettsiae. A 2.9-kilobase HindIII fragment containing a complete sta58 gene was cloned in Escherichia coli and found to express the entire Sta58 antigen and a smaller protein with an apparent molecular mass of 11 kDa (Stp11). DNA sequence analysis of the 2.9-kilobase HindIII fragment revealed two adjacent open reading frames encoding proteins of 11 (Stp11) and 60 (Sta58) kDa. Comparisons of deduced amino acid sequences disclosed a high degree of homology between the R. tsutsugamushi proteins Stp11 and Sta58 and the E. coli proteins GroES and GroEL, respectively, and the family of primordial heat shock proteins designated Hsp10 Hsp60. Although the sequence homology between the Sta58 antigen and the Hsp60 protein family is striking, the Sta58 protein appeared to be antigenically distinct among a sample of other bacterial Hsp60 homologs, including the typhus group of rickettsiae. The antigenic uniqueness of the Sta58 antigen indicates that this protein may be a potentially protective antigen and a useful diagnostic reagent for scrub typhus fever.

Sequence and molecular characterization of a multicopy invasion plasmid antigen gene, ipaH, of Shigella flexneri

A lambda gt11 expression library of Tn5-tagged invasion plasmid pWR110 (from Shigella flexneri serotype 5, strain M90T-W) contained a set of recombinants encoding a 60-kilodalton protein (designated IpaH) recognized by rabbit antisera raised against S. flexneri invasion plasmid antigens (J. M. Buysse, C. K. Stover, E. V. Oaks, M. M. Venkatesan, and D. J. Kopecko, J. Bacteriol. 169:2561-2569, 1987). Southern blot analysis of wild-type S. flexneri serotype 5 invasion plasmid DNA (pWR100) digested with various combinations of five restriction enzymes and hybridized with defined ipaH probes showed complex hybridization patterns resulting from multiple copies of the ipaH gene on pWR100. DNA sequence analysis of a 2.9-kilobase (kb) EcoRI fragment directing IpaH antigen synthesis in plasmid recombinant pWR390 revealed an open reading frame coding for a 532-amino-acid protein (60.8 kilodaltons); this size matched well with the estimated size of IpaH determined by Western blot analysis of M90T-W cells and maxicell analysis of Escherichia coli HB101(pWR390) transformants. Examination of the amino acid sequence of IpaH revealed a hydrophilic protein with six evenly spaced 14-residue (L-X2-L-P-X-L-P-X2-L-X2-L) repeat motifs in the amino-terminal end of the molecule. Southern blot analysis of HindIII-digested pWR100 DNA probed with defined segments of the pWR390 2.9-kb insert demonstrated that the multiple band hybridization pattern resulted from repeats of a significant portion of the ipaH structural gene in five distinct HindIII fragments (9.8, 7.8, 4.5, 2.5, and 1.4 kb). Affinity-purified IpaH antibody, used to monitor the expression of the antigen in M90T-W cells grown at 30 and 37 degrees C, showed that IpaH synthesis was not regulated by growth temperature.

Molecular characterization of a trans-acting, positive effector (ipaR) of invasion plasmid antigen synthesis in Shigella flexneri serotype 5

A trans-acting, positive effector of invasion plasmid antigen (Ipa) synthesis has been identified and mapped on the pWR100 invasion plasmid of Shigella flexneri serotype 5 (strain M90T-W). Recombinant plasmids carrying this regulatory gene, designated ipaR, were found to restore full virulence to a non-invasive ipaR::Tn5 insertion mutant [M90T-W(pHS1042)] that had lost the ability to synthesize four Ipa antigens (IpaA, 70 kDa; IpaB, 62 kDa; IpaC, 42 kDa; and IpaD, 37 kDa). Genetic mapping of the ipaR gene positioned the locus on a 2.6 kb PstI-AccI fragment contained within a larger 8.0 kb EcoRI molecule that also encoded IpaD, IpaA, and two small proteins (27 kDa and 28 kDa). The trans regulatory effect of the ipaR product on ipaB, ipaC, ipaD, and ipaA expression was demonstrated by transforming compatible ipaBC, ipaDA, ipaR and ipaDAR plasmid recombinants, in various combinations, into M90T-A3, an isogenic invasion plasmid mutant of M90T-W that contained a deletion of the pWR100 ipaBCDA and ipaR loci; such transformants produced wild type levels of the IpaB, IpaC, IpaD and IpaA antigens only in the presence of IpaR+ plasmids. DNA sequence analysis of the ipaR region established that the intiation codon for ipaR is 459 bp from the 3'-end of the ipaA gene and that ipaR encodes a 309 amino acid residue protein. An interesting feature of the IpaR polypeptide was its strong sequence homology with the bacteriophage P1 partition protein ParB, consisting of a 42.8% amino acid identity over a 278 residue section of the aligned proteins.

Antigenic and genetic relatedness of eight Rickettsia tsutsugamushi antigens

The genetic and antigenic relatedness of eight antigens in three strains of Rickettsia tsutsugamushi has been studied by using recombinant organisms expressing epitopes of the 150-, 110-, 72-, 58-, 56-, 49-, 47-, and 20-kilodalton (kDa) polypeptide antigens of the Karp strain. Southern blot analysis of Karp, Kato, and Gilliam strain genomic DNA by using probes specific for each antigen class indicated that while strong homology exists between each of the corresponding antigen genes in these three strains, some restriction fragment length polymorphism exists. Antibodies affinity purified against each recombinant antigen class reacted with a comparably sized polypeptide in the Karp, Kato, and Gilliam strains in Western blots (immunoblots). Against more recent human isolates of R. tsutsugamushi, the affinity-purified antibodies against the 58-kDa recombinant antigen (anti-58-kDa) reacted with all nine isolates, anti-56-kDa reacted with eight of nine isolates, anti-47-kDa reacted with eight of nine isolates, anti-72-kDa reacted with eight of nine isolates, and anti-110-kDa reacted with four of nine isolates. Additional analysis indicated that the 110-kDa antigen may contain strain-specific epitopes similar to those previously reported for the 56-kDa polypeptide. Evidently, the strain heterogeneity among scrub typhus rickettsiae is a result of multiple components that exhibit variability in a background of strong homology.

Shigella flexneri invasion plasmid antigens B and C: epitope location and characterization with monoclonal antibodies

Invasion plasmid antigens B (IpaB) and C (IpaC) are associated with the ability of shigellae to invade cultured mammalian cells. Monoclonal antibodies against IpaB and IpaC polypeptides were produced and used in a whole-cell enzyme-linked immunosorbent assay to show that both IpaB and IpaC polypeptides were exposed on the surface of virulent shigellae. Moreover, these surface epitopes were shown to be highly conserved among different serotypes of Shigella spp. and enteroinvasive Escherichia coli. Cross-reactive epitopes were not found on noninvasive Shigella strains or on other enteric bacteria including Salmonella, Yersinia, Campylobacter, Vibrio, and Aeromonas spp. and various pathogenic strains of E. coli. The monoclonal antibodies were used in competitive binding assays to define three unique epitopes of the IpaB polypeptide and four unique epitopes of the IpaC polypeptide. Epitope locations and their corresponding DNA-encoding regions were defined by examining the IpaB and IpaC products expressed by lambda gt11 recombinants and by constructing a genetic map of the insert DNAs of these recombinants. Three IpaB epitopes (2F1, 1H4, 4C8) were found to be encoded on three contiguous DNA regions comprising a 700-base-pair (bp) segment that corresponded to the amino-terminal end of the IpaB polypeptide. Similarly, a 640-bp DNA segment that corresponded to the amino-terminal end of the IpaC polypeptide was found to encode three clustered IpaC epitopes (5H1, 9B6, 5B1). Approximately 50 bp downstream from this region a fourth IpaC epitope-encoding region (2G2) was found. The effect of the monoclonal antibodies on plaque formation by virulent Shigella flexneri on a monolayer of cultured mammalian cells (a sensitive measure of invasiveness) was determined. Only the IpaB-specific monoclonal antibody 2F1 was able to reduce the plaque-forming capacity by greater than 50%, suggesting that this epitope of the IpaB polypeptide is involved in the invasion process.

Molecular cloning of invasion plasmid antigen (ipa) genes from Shigella flexneri: analysis of ipa gene products and genetic mapping

Tn5-tagged invasion plasmid DNA (pWR110) from Shigella flexneri serotype 5 (strain M90T) was cloned into the expression vector lambda gt11. Recombinant phage (lambda gt11Sfl) expressing pWR110-encoded polypeptide antigens were identified by using rabbit antisera directed against S. flexneri M90T invasion plasmid antigens. Antigens encoded by lambda gt11Sfl recombinant phage were characterized by reacting affinity-purified antibodies, eluted from nitrocellulose-bound plaques of lambda gt11Sfl recombinants, with virulent, wild-type S. flexneri M90T polypeptides in Western blot analyses. lambda gt11Sfl clones directing the synthesis of complete, truncated, and beta-galactosidase fusion versions of three previously identified outer membrane polypeptides (57-, 43-, and 39-kilodalton [kDa] antigens) were isolated. A fourth polypeptide, similar in size to the 57-kDa antigen (ca. 58 kDa) but unrelated as determined by DNA homology and serological measurements, was also identified. Southern blot analysis of S. flexneri M90T invasion plasmid DNA hybridized with lambda gt11Sfl insert DNA probes was used to construct a map of invasion plasmid antigen genes (ipa) corresponding to the 57-kDa (ipaB), 43-kDa (ipaC), and 39-kDa (ipaD) polypeptides. Genes ipaB, ipaC and ipaD mapped to contiguous 4.6-kilobase (kb) and 1.0-kb HindIII fragments contained within a larger (23-kb) BamHI fragment. The ipaH gene, which encodes the synthesis of the 58-kDa polypeptide, did not map in or near the ipaBCD gene cluster, suggesting a distinct location of ipaH on the invasion plasmid.

Use of conversion adaptors to clone antigen genes in lambda gt11

A strategy has been devised and tested to employ EcoRI conversion adaptors for cloning 5' cohesive-ended restriction fragments into the unique EcoRI site of the lambda gt11 expression vector. Five lambda gt11 chromosomal libraries were constructed with Rickettsia tsutsugamushi genomic DNA digested with restriction enzymes generating five different 5' cohesive ends. Recombinant phage yields as high as 10(7) plaque forming units were achieved without amplification of the five libraries. Sequences encoding epitopes of all eight R. tsutsugamushi polypeptide antigens, previously identified by Western blot analysis, were obtained in the five lambda gt11 expression libraries. Recombinant antigen expression was dependent on lambda gt11 lac promoter induction in 39% of the recombinants assayed. This method significantly improves the efficiency of genomic lambda gt11 library construction by eliminating blunt-ended ligation and simplifying the removal of unligated EcoRI-ended oligonucleotides.

Molecular cloning and expression of Rickettsia tsutsugamushi genes for two major protein antigens in Escherichia coli

Several polypeptide antigens of Rickettsia tsutsugamushi are recognized by human or primate convalescent sera and may be important protective immunogens. Molecular cloning and expression of the genes encoding the 110K (110 kilodalton) and 56K polypeptide antigens of R. tsutsugamushi Karp were accomplished in the lambda gt11 expression vector system. Southern blot analysis with the cloned fragments for the 56K polypeptide antigen (0.7 kilobases) and the 110K polypeptide antigen (5.4 kilobases) confirmed that the insert DNA was rickettsial and not host cell in origin. Expression of a complete 110K polypeptide was shown to be independent of isopropyl-beta-D-thiogalactopyranoside induction, suggesting that an intact rickettsial promoter was operational. Epitopes of the 56K polypeptide were expressed as lac promoter-dependent beta-galactosidase fusion proteins. Polyclonal antibody, affinity purified against the recombinant 110K and 56K polypeptides, reacted with polypeptides of similar size in the Kato and Gilliam strains of R. tsutsugamushi. Group-reactive, but not strain-specific, monoclonal antibodies against the 56K polypeptide reacted with the cloned portion of the 56K polypeptide. Western blot analysis demonstrated that the cloned 56K Karp antigen gene product is recognized by human convalescent serum.

Serum immune response to Shigella protein antigens in rhesus monkeys and humans infected with Shigella spp

The serum antibody response to proteins encoded by the virulence-associated plasmid of Shigella flexneri was determined in monkeys challenged with virulent S. flexneri serotype 2a. With water-extractable antigen in an enzyme-linked immunosorbent assay, a significant increase in antibody titer against proteins from a plasmid-carrying, virulent strain of S. flexneri serotype 5 could be demonstrated in convalescent sera. There were minimal antibody titers against proteins from an avirulent (plasmid-free) organism. Previously identified plasmid-coded polypeptides a, b, c, and d were predominant antigens recognized by a majority of the convalescent sera in immunoblots. An additional 140-megadalton plasmid-coded polypeptide was also recognized by half of the sera. Convalescent serum from an infected monkey recognized antigens on the bacterial surface in several different plasmid-containing Shigella species and in an enteroinvasive Escherichia coli strain. A survey of sera obtained from children 5 to 10 years of age who had been infected with S. flexneri or S. sonnei revealed high enzyme-linked immunosorbent assay titers in both acute and convalescent sera against a water extract from a virulent Shigella strain. In contrast, children under 3 years of age had no antibody titer in either acute or convalescent sera against the virulence-associated shigella proteins, while 3- to 4-year-old children mounted an immune response against these proteins only in convalescence.

Identification and antigenic characterization of virulence-associated, plasmid-coded proteins of Shigella spp. and enteroinvasive Escherichia coli

Seven plasmid-coded polypeptides, designated a through g, were identified by two-dimensional nonequilibrium pH gradient electrophoresis of radiolabeled extracts from minicells of virulent Shigella flexneri serotypes 2a and 5 and enteroinvasive Escherichia coli O143. These polypeptides were deemed to be products of 140-megadalton (MDa) virulence-associated plasmids because they were not synthesized in minicells which were not harboring a 140-MDa plasmid or in minicells which were carrying an F lac plasmid of the same incompatibility group. Synthesis of these polypeptides was repressed in minicells incubated at 30 degrees C and in minicells isolated from a noninvasive opaque colonial variant, even though these strains harbored a 140-MDa plasmid. Enriched fractions of polypeptides b, c, and d were obtained from S. flexneri serotype 5 by preparative isoelectric focusing, and polyclonal rabbit antisera recognizing each polypeptide were raised. These antisera were able to detect cross-reacting plasmid-coded polypeptide antigens in S. flexneri serotype 3, Shigella sonnei, and enteroinvasive E. coli O143. In addition, Western blots of minicell extracts from S. flexneri serotype 5 or E. coli O143 indicated that plasmid-coded polypeptides a through d were recognized by convalescent antiserum from a monkey infected with S. flexneri serotype 2a.

Plaque formation by virulent Shigella flexneri

An in vitro tissue culture plaque assay was developed to investigate the intracellular replication and intercellular spread of virulent shigellae. Shigella plaques were formed in HeLa cell monolayers in the presence of an agarose overlay containing tissue culture medium and gentamicin, which eliminated extracellular bacterial growth. Microscopically, the plaques were characterized by a central area of dead host cells surrounded by cells infected with shigellae. Cells further away from the plaque center were uninfected. Inclusion of chloramphenicol or nalidixic acid in the overlay completely abolished plaque formation. Plaque formation was completely inhibited when infected monolayers were shifted from 37 to 30 degrees C. Shifting infected monolayers from 30 degrees C, where plaques do not form, to 37 degrees C resulted in the formation of plaques. Cultures of Shigella boydii, Shigella sonnei (form I), and all six serotypes of Shigella flexneri produced plaques. Shigellae isolated from plaques were Sereny test positive, contained a 140-megadalton plasmid, and were gentamicin sensitive. Noninvasive shigellae did not form plaques.

Impairment of folic acid absorption by oral pancreatic extracts

Higher serum folate levels were found among newly diagnosed, untreated patients with pancreatic insufficiency than among treated patients despite greater fat malabsorption in the former group. In vivo folate absorption tests using Tritium-labeled pteroylmonoglutamatic acid showed folate absorption to be enhanced in pancreatic insufficiency patients as compared to control subjects (P less than 0.01). Moreover, pancreatic extract significantly inhibited folate absorption in both normal subjects (P less than 0.05) and pancreatic insufficient patients (P less than 0.001). In vitro testing showed pancreatic extract to form insoluble complexes with folate. Such complex formation may diminish absorption of dietary folate and lead to folate deficiency. Since both pancreatic extract and bicarbonate are used in the treatment of pancreatic insufficiency and both are known to impair folate absorption, folate status should be monitored in patients being treated for pancreatic insufficiency; supplementation may be indicated.

Taxonomic relationship of vole agent to Rochalimaea quintana

We investigated the recent claim that the vole agent, a rickettsia-like microorganism isolated from wild voles by Baker in 1946, is actually a strain of Rochalimaea quintana, the etiological agent of trench fever. The two organisms were compared on the basis of percent guanine-plus-cytosine content, genome size, deoxyribonucleic acid-deoxyribonucleic acid hybridization, polypeptide composition, and serological relationships. Although the two organisms do have identical or nearly identical deoxyribonucleic acid base ratios and show some serological cross-reactions, they differ substantially by all of the other criteria employed. They are clearly different, although possibly related, organisms. It remains to be determined whether they can be regarded as two species of the same genus. On the other hand, an Old World strain and a New World strain of R. quintana were indistinguishable from one another by the same criteria.

Vitamin-A reversal of abnormal dark adaptation in cirrhosis. Study of effects on the plasma retinol transport system

Of 26 patients hospitalized with mild to moderate alcohol-associated cirrhosis, 14 had dark-adaptation abnormalities consistent with marginal vitamin-A status. The response of dark adaptation and the plasma retinol transport proteins, retinol-binding protein and prealbumin, was studied in 12 of these patients after daily oral vitamin-A supplements of 3300 microgram. Vitamin-A supplementation was associated with significant (p less than 0.05-0.005) improvement in dark adaptation and increased plasma concentrations of retinyl esters, retinol, and retinol-binding protein. Thus in patients with cirrhosis and marginal vitamin-A status, supplemental vitamin-A therapy appears to stimulate retinol-binding protein release from the liver. This enhancement of plasma retinol transport and delivery of retinol to peripheral tissues such as the retina is one of several factors that may serve to optimize vitamin-A nutritional status in patients with cirrhosis.

Zinc deficiency: a cause of abnormal dark adaptation in cirrhotics

Six stable alcoholic cirrhotics with serum zinc less than 70 microgram/100 ml had abnormal dark adaptation responses (mean dark adapted final threshold 3.2 +/- 0.6 versus 2.1 +/- 0.2 log lux in 21 age matched controls, P less than 0.01). Serum vitamin A ranged from 15 to 37 microgram/100 ml. Zinc sulfate (220 mg/day) was fed to three patients for 1 to 2 weeks and dark adapted final thresholds fell 0.9, 0.4, and 1.2 log lux without concurrent rises in serum vitamin A. Two patients were treated initially with oral vitamin A (10,000 IU/day) for 2 to 4 weeks, but their final thresholds fell to normal (2.1, 2.2 log lux) only after the addition of zinc for 1 to 2 weeks. The sixth patient, treated with vitamin A and zinc together, attained a normal final threshold in 2 weeks. The improvement in dark adaptation by zinc may be due to enhanced activity of previously depressed retinol dehydrogenase.