Cross-reactivity of major outer membrane proteins of Enterobacteriaceae, studied by crossed immunoelectrophoresis

Comparative Outer Membrane Protein Analysis of High and Low-Invasive Strains of Cronobacter malonaticus

Cronobacter are an important group of foodborne pathogens that has been linked to life-threatening infections in both infants and adults. The major infections associated with Cronobacter species are neonatal meningitis, necrotizing enterocolitis, and septicaemia. There are seven species in the Cronobacter genus, of which only three are of clinical importance; Cronobacter sakazakii, Cronobacter malonaticus, and Cronobacter turicensis. To date most studies have focussed on C. sakazakii as it is the major species associated with neonatal infections. However, recently C. malonaticus, in particular sequence type 7 (ST7), has been noted as being prevalent in adult infections and therefore warranting further investigation. In this study, eight strains of C. malonaticus ST7, that had been isolated from a wide range of sources and varied in their in vitro virulence, were chosen for proteomic analysis of their outer membrane proteins (OMPs). One-dimensional gel analysis revealed a ~29 kDa size band that was only present in the highly invasive strains. Subsequent mass spectrometric analysis identified several peptides that matched the flagellin protein. The presence of flagellin protein was confirmed in 2D gel spot. Mass spectrometry analysis of total OMPs revealed that the four highly invasive C. malonaticus strains expressed the main flagellum proteins that were absent from the four low invasive strains. These were the flagellar hook protein FlgE, flagellar hook-associated protein 1, flagellar hook-associated protein, flagellin, and flagellar hook-filament junction protein FlgL. This data indicates that C. malonaticus flagellar proteins may have an important role in the organism's invasion properties.

Outer membrane protein A (OmpA): a new player in shigella flexneri protrusion formation and inter-cellular spreading

Outer membrane protein A (OmpA) is a multifaceted predominant outer membrane protein of Escherichia coli and other Enterobacteriaceae whose role in the pathogenesis of various bacterial infections has recently been recognized. Here, the role of OmpA on the virulence of Shigella flexneri has been investigated. An ompA mutant of wild-type S. flexneri 5a strain M90T was constructed (strain HND92) and it was shown to be severely impaired in cell-to-cell spreading since it failed to plaque on HeLa cell monolayers. The lack of OmpA significantly reduced the levels of IcsA while the levels of cell associated and released IcsP-cleaved 95 kDa amino-terminal portion of the mature protein were similar. Nevertheless, the ompA mutant displayed IcsA exposed across the entire bacterial surface. Surprisingly, the ompA mutant produced proper F-actin comet tails, indicating that the aberrant IcsA exposition at bacterial lateral surface did not affect proper activation of actin-nucleating proteins, suggesting that the absence of OmpA likely unmasks mature or cell associated IcsA at bacterial lateral surface. Moreover, the ompA mutant was able to invade and to multiply within HeLa cell monolayers, although internalized bacteria were found to be entrapped within the host cell cytoplasm. We found that the ompA mutant produced significantly less protrusions than the wild-type strain, indicating that this defect could be responsible of its inability to plaque. Although we could not definitely rule out that the ompA mutation might exert pleiotropic effects on other S. flexneri genes, complementation of the ompA mutation with a recombinant plasmid carrying the S. flexneri ompA gene clearly indicated that a functional OmpA protein is required and sufficient for proper IcsA exposition, plaque and protrusion formation. Moreover, an independent ompA mutant was generated. Since we found that both mutants displayed identical virulence profile, these results further supported the findings presented in this study.

Recombinant antibodies encoded by IGHV1-69 react with pUL32, a phosphoprotein of cytomegalovirus and B-cell superantigen

Leukemia cells from patients with chronic lymphocytic leukemia (CLL) express a highly restricted immunoglobulin heavy variable chain (IGHV) repertoire, suggesting that a limited set of antigens reacts with leukemic cells. Here, we evaluated the reactivity of a panel of different CLL recombinant antibodies (rAbs) encoded by the most commonly expressed IGHV genes with a panel of selected viral and bacterial pathogens. Six different CLL rAbs encoded by IGHV1-69 or IGHV3-21, but not a CLL rAb encoded by IGHV4-39 genes, reacted with a single protein of human cytomegalovirus (CMV). The CMV protein was identified as the large structural phosphoprotein pUL32. In contrast, none of the CLL rAbs bound to any other structure of CMV, adenovirus serotype 2, Salmonella enterica serovar Typhimurium, or of cells used for propagation of these microorganisms. Monoclonal antibodies or humanized rAbs of irrelevant specificity to pUL32 did not react with any of the proteins present in the different lysates. Still, rAbs encoded by a germ line IGHV1-69 51p1 allele from CMV-seropositive and -negative adults also reacted with pUL32. The observed reactivity of multiple different CLL rAbs and natural antibodies from CMV-seronegative adults with pUL32 is consistent with the properties of a superantigen.

Characterization of surface proteins of Cronobacter muytjensii using monoclonal antibodies and MALDI-TOF Mass spectrometry

BACKGROUND

Cronobacter spp. is a newly emerging pathogen that causes meningitis in infants and other diseases in elderly and immunocompromised individuals. This study was undertaken to investigate surface antigenic determinants in Cronobacter spp. using monoclonal antibodies (MAbs) and MALDI-TOF Mass spectrometry.

RESULTS

Spleenocytes from mice that were immunized with heat-killed (20 min, 80°C) Cronobacter cells were fused with SP2 myeloma cells. Five desirable MAbs (A1, B5, 2C2, C5 and A4) were selected. MAbs A1, B5, 2C2 and C5 were of IgG2a isotype while A4 was an IgM. Specificity of the MAbs was determined by using immunoblotting with outer membrane protein preparations (OMPs) extracted from 12 Cronobacter and 6 non-Cronobacter bacteria. All MAbs recognized proteins with molecular weight ranging between 36 and 49 kDa except for one isolate (44) in which no OMPs were detected. In addition, MAbs recognized two bands (38-41 kDa) in four of the non-Cronobacter bacteria. Most of the proteins recognized by the MAbs were identified by MALDI-TOF peptide sequencing and appeared to be heterogeneous with the identities of some of them are still unknown. All MAbs recognized the same epitope as determined by an additive Index ELISA with their epitopes appeared to be conformational rather than sequential. Further, none of the MAbs recognized purified LPS from Cronobacter spp. Specificity of the MAbs toward OMPs was further confirmed by transmission electron microscopy.

CONCLUSIONS

Results obtained in this study highlight the immunological cross-reactivity among Cronobacter OMPs and their Enterobacteriaceae counterparts. Nevertheless, the identity of the identified proteins appeared to be different as inferred from the MALDI-TOF sequencing and identification.

"Gently rough": the vaccine potential of a Salmonella enterica regulatory lipopolysaccharide mutant

BACKGROUND

An alternative to multivalent vaccines could be to construct strains capable of conferring broad protection through shared antigens. Down-regulation of immunodominant major antigens has been proposed to enhance the immunogenicity of conserved antigens.

METHODS

The protection provided by an aroA as well as structural and regulatory lipopolysaccharide (LPS) mutants of Salmonella enterica serovar Typhimurium against homologous and heterologous challenges was assessed in the murine model of typhoid. The reactivity and cross-reactivity of the immune sera raised was tested by enzyme-linked immunospot assay and immunoblots. Conserved outer membrane proteins were identified by mass spectrometry.

RESULTS

Unlike any structural LPS mutants, the regulatory mutant lacking RfaH was finely balanced between safety and immunogenicity, and its vaccine potential was comparable to that of the well-characterized DeltaaroA mutant. Loss of the transcriptional antiterminator RfaH resulted in a heterogeneous length of LPS chains, designated here as the "gently rough" phenotype. Our study also provides evidence that the rough phenotype enhances the immunogenicity of minor antigens, which may improve cross-protection against heterologous bacteria. A panel of conserved antigens shared by members of the Enterobacteriaceae family was identified as abundant porins and lipoprotein antigens.

CONCLUSIONS

Fine-tuned down-regulation of immunodominant epitopes can create live vaccine strains that are not only desirably attenuated but that also exhibit an improved cross-protective potential.

Identification of Enterobacteriaceae bacteria by direct matrix-assisted laser desorptiom/ionization mass spectrometric analysis of whole cells

Several members of Enterobacteriaceas were analyzed by matrix-assisted laser desorption ionization (MALDI) time-of-flight mass spectrometry (TOFMS). Characteristic mass spectral peaks and patterns were observed in the mass range of 2 to 20 kDa. The mass peaks reported to be reproducibly observed by previous researchers, which were claimed to serve as species/strain-specific biomarkers, are consistently observed in our current study. Despite the high degree of similarity found in the MALDI mass spectra within the enteric bacteria, minor yet notable differences existed to allow their differentiation. Five spectral peaks at m/z 4364, 5380, 6384, 6856, and 9540, generated reproducibly for each genus studied here, are assigned as family-specific biomarkers for the Family Enterobacteriaceae. The mass peaks at m/z 7324, 7724, 9136, and 9253 are assigned as genus-specific biomarkers for Salmonella. Some unique biomarkers characterizing the species and strains of E. coli are also presented.

Potential virulence factors of Proteus bacilli

The object of this review is the genus Proteus, which contains bacteria considered now to belong to the opportunistic pathogens. Widely distributed in nature (in soil, water, and sewage), Proteus species play a significant ecological role. When present in the niches of higher macroorganisms, these species are able to evoke pathological events in different regions of the human body. The invaders (Proteus mirabilis, P. vulgaris, and P. penneri) have numerous factors including fimbriae, flagella, outer membrane proteins, lipopolysaccharide, capsule antigen, urease, immunoglobulin A proteases, hemolysins, amino acid deaminases, and, finally, the most characteristic attribute of Proteus, swarming growth, enabling them to colonize and survive in higher organisms. All these features and factors are described and commented on in detail. The questions important for future investigation of these facultatively pathogenic microorganisms are also discussed.

Antibodies to Escherichia coli 06 porins cross-react with urinary pathogens

Antibodies to partially purified E. coli 06 35-40 KDa porin trimers recognized the reactive epitopes in the intact porin surface molecule present in various wild-type, heterologous, urinary pathogens. The presence of lipopolysaccharide in the membrane did not shield the antibody binding sites. The reactivity was shown to be specific for porins since LPS-absorbed porin antisera reacted with porins on immunoblots and showed no reactivity with LPS. Additionally, the cross-reactions were abolished by absorption of the porin antisera with E. coli 06 containing porin trimers. These data strengthen the rationale for exploring the enhancement of immunoprotection by monoclonal antibodies to specific immunoreactive antigens in the porin molecule.

Lipopolysaccharide-independent radioimmunoprecipitation and identification of structural and in vivo induced immunogenic surface proteins of Salmonella typhi in typhoid fever

The humoral response to Salmonella typhi is important for protective immunity against typhoid fever, as indicated by the protection obtained with killed cell vaccines and component vaccines (outer membrane proteins, Vi antigen) in animals and human beings. Nonetheless, analysis and interpretation of host humoral immune response to S. typhi surface antigens have been difficult because of the complex structure of the S. typhi envelope and the lack of purified reagents for detection of immune response to individual surface components. Normal and convalescent human sera from typhoid fever patients were absorbed with S. typhi lipopolysaccharide. These sera were used in radioimmunoprecipitation assays of whole S. typhi cells and S. typhi membranes labelled with either 125I or 35S-methionine. This strategy has permitted the unequivocal identification of a humoral immune response to structural and in vivo induced outer membrane proteins of S. typhi. In this manner, we have identified the porins, lipoprotein, the iron-starvation-induced proteins, and three proteins of 30, 18.5 and 15 kDa as surface-exposed immunogens of S. typhi in patients with typhoid fever. These studies suggest that further experimental work is needed to characterize the relevance of both anti-S. typhi outer membrane protein and antilipopolysaccharide antibodies in recovery from S. typhi infections and protective immunity.

Characterization of shared antigens of Fusobacterium nucleatum and Fusobacterium necrophorum

Fusobacterium nucleatum and Fusobacterium necrophorum are gram-negative, non-spore-forming anaerobic rods, frequently isolated from the normal flora and diseased lesions of the human oral cavity, gastrointestinal and genitourinary tracts. F. necrophorum is also known to be an animal pathogen. Studies were undertaken with rabbit anti-F. nucleatum sera and with human adult periodontitis (AP) sera that demonstrated the sharing of antigens between the two species. Immunodiffusion and immunoelectrophoresis studies of Fusobacterium species with rabbit anti-F. nucleatum sera demonstrated the presence of shared antigen(s) between F. nucleatum and F. necrophorum. Adsorption studies of AP sera in an enzyme-linked immunosorbent assay demonstrated the presence of antibodies reacting with the shared antigens of the two species. Immunoblot (IB) analysis of a soluble protein preparation of the two species of Fusobacterium, when allowed to react with rabbit anti-F. nucleatum 10197 serum, demonstrated 53 kDa and 30 kDa bands present in members of the two species. Further, IB analysis of protein preparations of the two species with AP sera indicated the presence of antibodies reacting with the shared 53 kDa band and in some cases the 30 kDa band. During serological testing with antisera or host immune studies with human sera to these species, the presence of shared antigens must be considered.

Efficacy of a Proteus mirabilis outer membrane protein vaccine in preventing experimental Proteus pyelonephritis in a BALB/c mouse model

A BALB/c mouse model of nonobstructive, ascending Proteus mirabilis pyelonephritis was characterized bacteriologically, histologically, and serologically from 3 to 28 days. Intravesicular administration of 2 X 10(8) P. mirabilis K7 resulted in the septic death of 9 (16%) of 57 mice by day 15. Among the survivors, K7 colonized the kidneys in great numbers until day 21. Histological examination of the kidneys revealed acute inflammation which was characterized by neutrophil infiltration by day 3, renal necrosis by day 7, and fibroblastic infiltration by day 14 which persisted at least until day 28. The immunoglobulin G response to the outer membrane proteins (OMP) was assessed by enzyme-linked immunosorbent assay and Western blotting (immunoblotting). Anti-OMP immunoglobulin G antibodies were detected as early as day 7, and the reciprocals of their titers rose progressively up to day 28 (i.e., greater than or equal to 500). This model was also used to assess the efficacy of OMP and lipopolysaccharide (LPS) immunization in preventing renal infection. K7 OMP or LPS (100 micrograms) preparations were administered intramuscularly in Freund's complete adjuvant. After 2 weeks, mice were intravesicularly challenged with 2 X 10(8) bacteria of the homologous K7 strain or one of four heterologous strains. Compared with the saline-immunized control group and K7 LPS-immunized mice, K7 OMP recipients were protected from death when challenged by homologous or heterologous strains. In addition, K7 OMP recipients were protected (P less than 0.003) from subsequent renal infection when challenged by the K7 strain and had more rapid bacterial renal clearance when challenged by three of four heterologous strains. OMP recipients produced antibodies which bound major OMP moieties (viz., 36- to 39-kDa cell wall constituents) as assessed by Western blotting. These results support the concept that immunization with selected bacterial protein surface coat constituents can prevent uromucosal infection by interfering with colonization or renal injury.

Evolution of the ferric enterobactin receptor in gram-negative bacteria

Using sodium dodecyl sulfate-polyacrylamide gel electrophoresis of iron-deficient and replete cell envelopes, 59Fe-siderophore uptake studies, and Western immunoblots and cytofluorimetric analyses with monoclonal antibodies (MAbs), we surveyed a panel of gram-negative bacteria to identify outer membrane proteins that are structurally related to the Escherichia coli K-12 ferric enterobactin receptor, FepA. Antibodies within the panel identified FepA epitopes that are conserved among the majority of the bacteria tested, as well as epitopes present in only a few of the strains. In general, epitopes of FepA that are buried in the outer membrane bilayer were more conserved among gram-negative bacteria than epitopes that are exposed on the bacterial cell surface. The surface topology and tertiary structure of FepA are quite similar in E. coli and Shigella flexneri but differ in Salmonella typhimurium. Of the 18 different genera tested, 94% of the bacteria transported ferric enterobactin, including members of the previously unrecognized genera Citrobacter, Edwardsiella, Enterobacter, Haemophilus, Hafnia, Morganella, Neisseria, Proteus, Providencia, Serratia, and Yersinia. The ferric enterobactin receptor contains at least one buried epitope, recognized by MAb 2 (C. K. Murphy, V. I. Kalve, and P. E. Klebba, J. Bacteriol. 172:2736-2746, 1990), that is conserved within the structure of an iron-regulated cell envelope protein in all the bacteria that we have surveyed. With MAb 2, we identified and determined the Mr of cell envelope antigens that are immunologically related to E. coli FepA in all the gram-negative bacteria tested. Collectively, the library of anti-FepA MAbs showed unique patterns of reactivity with the different bacteria, allowing identification and discrimination of species within the following gram-negative genera: Aeromonas, Citrobacter, Edwardsiella, Enterobacter, Escherichia, Haemophilus, Hafnia, Klebsiella, Morganella, Neisseria, Proteus, Providencia, Pseudomonas, Salmonella, Serratia, Shigella, Vibrio, and Yersinia.

The ompH gene of Yersinia enterocolitica: cloning, sequencing, expression, and comparison with known enterobacterial ompH sequences

We have recently described a previously uncharacterized outer membrane protein of Salmonella typhimurium and Escherichia coli and cloned and sequenced the corresponding gene, the ompH gene, of S. typhimurium (P. Koski, M. Rhen, J. Kantele, and M. Vaara, J. Biol. Chem. 264:18973-18980, 1989). We report here the cloning, sequencing, and expression of the corresponding gene of Yersinia enterocolitica. It is significantly homologous to the ompH genes of E. coli and S. typhimurium (homology percentages, 65 and 64%, respectively), has a promoter region strongly homologous to the E. coli 17-bp class consensus promoter, and encodes a protein consisting of 165 amino acids (22 of which form the signal sequence). The plasmid-borne Y. enterocolitica ompH was found to be expressed both in the E. coli host and in minicells. The isolated outer membrane of Y. enterocolitica was shown to contain OmpH. The homology of the Y. enterocolitica OmpH protein is 66% with E. coli OmpH and 64% with S. typhimurium OmpH. All OmpH proteins have almost identical hydrophobic profiles, charge distributions, and predicted secondary structures. Because yersiniae are considered rather distant relatives of E. coli and S. typhimurium in the Enterobacteriaceae family, these results might indicate that most or all strains of the family Enterobacteriaceae have OmpH proteins remarkably homologous to those now sequenced.

A conserved domain on enterobacterial porin protein analysed by monoclonal antibody

Broadly cross-reactive monoclonal antibodies (MAbs) against enterobacterial outer membrane (OM) porin (Po) protein were isolated after immunization of BALB/c mice with whole cells of E. coli 055:B5. MAbs (n = 6) of the IgG class but of four different isotypes were studied. Based on a competition ELISA, all of the MAbs were directed against one and the same Po protein domain (Po I). The MAbs cross-reacted with 72 of 74 strains from 10 different genera of the Enterobacteriaceae. One Morganella and one Salmonella strain showed no cross-reactivity. Also, nine strains of various Neisseria spp. cross-reacted while 21 strains of various other nonenteric Gram-negative bacteria showed no cross-reactivity. The Po I sites were inaccessible in intact homologous bacteria but partially accessible in the OM. Digestion of OM with lysozyme or lysostaphin affected the accessibility of the Po I sites in OMs of various enterobacteria. Lysostaphin strongly enhanced the immunoaccessibility, whereas lysozyme had lesser effects. The enzymes also affected the binding by Neisseria OMs of the anti-Po I MAb. The Po I site was immunogenic both in humans and rabbits. The data indicate that Po I is an important Po protein domain, and that the effects of peptidoglycan-degrading enzymes must be considered in studies of Po protein domains.

A strong antibody response to the periplasmic C-terminal domain of the OmpA protein of Escherichia coli is produced by immunization with purified OmpA or with whole E. coli or Salmonella typhimurium bacteria

We produced in Bacillus subtilis the complete, as well as the N-terminal two-thirds, OmpA protein of Escherichia coli (called here Bac-OmpA and Bac-OmpA-dN, respectively). These Bac-OmpA proteins were used to examine the immunological properties of different parts of OmpA, free of lipopolysaccharide and other components of the outer membrane. The full-length Bac-OmpA was indistinguishable from the authentic protein isolated from E. coli (Coli-OmpA) both as immunogen and as antigen in enzyme immunoassay (EIA). The N-terminal Bac-OmpA-dN was a poor immunogen which gave rise to significantly lower titers of anti-OmpA antibody than did the full-length OmpA preparations. When used as an antigen in EIA, the Bac-OmpA-dN detected anti-OmpA antibody in serum samples from animals immunized with the full-length OmpA much less efficiently than did either Bac-OmpA or Coli-OmpA. The periplasmic C-terminal domain therefore appears to be an immunodominant epitope of the purified OmpA protein. Also, when rabbits and mice were immunized with intact, live or dead E. coli, the antibody response detected by EIA with the full-length protein, Bac-OmpA, was much stronger than that detected with the N-terminal two-thirds, Bac-OmpA-dN. Similar results were obtained with the OmpA of Salmonella typhimurium. Because the ompA gene of enterobacteria is highly conserved, the Bac-OmpA might be useful as a group-specific EIA antigen to diagnose diseases caused by members of the family Enterobacteriaceae.

Outer membrane proteins of E. coli in the host-pathogen interaction in urinary tract infection

Outer membrane protein patterns (Omp) of Escherichia coli obtained directly from the urine of bacteriuric patients without passage on artificial culture media (ACM) were studied by polyacrylamide gel electrophoresis (SDS-PAGE) in an effort to determine whether in vivo conditions of growth affected the expression of these bacterial surface structures. Seventeen strains studied showed two distinct Omp patterns: one protein band appeared at the level of porin proteins (40 kDa) in both patterns, but Omp A protein was at the level of 36 kDa in the first pattern and a new protein was observed at 21.5 kDa in the second pattern suggesting that it is a fragment of Omp A. High molecular weight proteins were also observed in most of the strains and this finding was related to lack of free iron when the same strains were grown under iron restricted conditions in vitro. The same strains grown in pooled urine from normal females showed the first pattern mentioned above. Comparative growth on ACM of urinary strains and E. coli strains isolated from blood, feces and wounds showed an increase in the number of porins expressed (from 1 to 2 or 3, with some variability observed between strains). Differences in osmolality between pooled urine and ACM used, plus in vitro studies varying the osmolality of culture media, showed that osmolality accounted for differences in the number of porins expressed: porin expression decreased in urine the ACM of high osmolality, suggesting that the same phenomena occurred in vivo. It is concluded that host factors including low availability of iron and high osmolality present in the urinary tract influence the expression of several E. coli surface proteins. These proteins may relate to the ability of E. coli to colonize and invade the urinary tract by regulating the physiologic and/or metabolic state of the bacterial cell favoring survival of the organism in a hostile environment. Specific immune responses directed against porins could influence the outcome of this host-parasite interaction.

Antibody response to defined domains on enterobacterial outer membrane proteins in healthy persons and patients with bacteraemia

Monoclonal antibody (MAb)-based competitive enzyme immunoassays (cELISAs) were elaborated to measure antibodies against MAb-defined domains on three different enterobacterial outer membrane (OM) proteins in sera from healthy individuals (n = 30) and in paired serum samples from patients (n = 45) with bacteraemia caused by enterobacteria or by various nonenteric bacteria (n = 15). The MAb-defined domains were Hm I and Hm II on the heat-modifiable (Hm) protein, PALp I and PALp II on the peptidoglycan-associated lipoprotein (PALp), and BLp I on Braun's lipoprotein (BLp). All MAbs have shown broad cross-reactivity with and specificity for enterobacteria. Sera from healthy individuals and from patients with infections caused by nonenteric bacteria contained low levels of MAb-blocking antibodies. Bacteraemia caused by enterobacteria resulted in generation of antibodies against the MAb-defined domains in many of the patients. Thus, 40% and 69% showed a positive BLp I cELISA with the first and second serum samples, respectively. Of the second serum samples, 20-38% showed positive Hm and PALp cELISAs. The BLp I cELISA showed higher diagnostic sensitivity than the previously described indirect ELISA for IgG antibodies against E. coli 055 OM protein antigens. Assays using the MAbs as competitors showed that the patients bacteraemic with enterobacteria, also generated antibodies against other domains on the OM proteins. The cELISAs may be useful in the diagnosis and management of patients with serious infections caused by enterobacteria. In this regard, the BLp I cELISA showed the most promising results.

Monoclonal antibodies against three different enterobacterial outer membrane proteins. Characterization, cross-reactivity, and binding to bacteria

BALB/c mice were immunized with whole-cells of Escherichia coli 055:B5 or Proteus mirabilis NCTC 60 to produce broadly cross-reacting monoclonal antibodies (MAbs) against outer membrane (OM) proteins. A total of 10 anti-OM MAbs of the IgG class were selected. These included 5 MAbs against the heat-modifiable (Hm) protein, 3 against the peptidoglycan-associated lipoprotein (PALp), and 2 against Braun's lipoprotein (BLp). Based on competition ELISA, the MAbs defined 2 Hm protein binding sites (Hm I and Hm II), 2 PALp sites (PALp I and PALp II), and one BLp site (BLp I). The MAbs showed broad cross-reactivity against 74 strains of 10 different genera of the Enterobacteriaceae. Non-cross-reacting enteric bacilli occurred only among bacteria of the genera Salmonella, Proteus, and Providentia. The results revealed that Proteus and Providentia strains differed from other enteric bacilli with regard to BLp synthesis or specificity. A panel of 30 non-enteric Gram-negative bacteria did not cross-react. Testing of MAb binding to bacteria showed that a part of the BLp I, PALp I, and PALp II sites was immunoaccessible in intact homologous bacteria, and that the Hm I and Hm II epitopes were inaccessible. The MAbs should facilitate studies of structure and immunobiological function of enterobacterial OM proteins and should have a potential as immunodiagnostic reagents.

Effect of lipopolysaccharide structure on reactivity of antiporin monoclonal antibodies with the bacterial cell surface

We studied the reactivity of 66 anti-Escherichia coli B/r porin monoclonal antibodies (MAbs) with several E. coli and Salmonella typhimurium strains. Western immunoblots showed complete immunological cross-reactivity between E. coli B/r and K-12; among 34 MAbs which recognized porin in immunoblots of denatured outer membranes of E. coli B/r, all reacted with OmpF in denatured outer membranes of E. coli K-12. Extensive reactivity, although less than that for strain B/r (31 of 34 MAbs), occurred for porin from a wild-type isolate, E. coli O8:K27. Only one of the MAbs reacted with porin in denatured outer membranes of S. typhimurium. Even with immunochemical amplification of the Western immunoblot technique, only six MAbs recognized S. typhimurium porin (OmpD), demonstrating that there is significant immunological divergence between the porins of these species. Antibody binding to the bacterial surface, which was analyzed by cytofluorimetry, was strongly influenced by lipopolysaccharide (LPS) structure. An intact O antigen, as in E. coli O8:K27, blocked adsorption of all 20 MAbs in the test panel. rfa+ E. coli K-12, without an O antigen but with an intact LPS core, bound seven MAbs. When assayed against a series of rfa E. coli K-12 mutants, the number of MAbs that recognized porin surface epitopes increased sequentially as the LPS core became shorter. A total of 17 MAbs bound porin in a deep rough rfaD strain. Similar results were obtained with S. typhimurium. None of the anti-E. coli B/r porin MAbs adsorbed to a smooth strain, but three antibodies recognized porin on deep rough (rfaF, rfaE) mutants. These data define six distinct porin surface epitopes that are shielded by LPS from reaction with antibodies.

Serum antibodies to outer membrane proteins of Escherichia coli in healthy persons and patients with bacteremia

Antibodies to Escherichia coli outer membrane proteins in sera from healthy persons and from patients bacteremic with various enteric or nonenteric bacteria were measured by an enzyme-linked immunosorbent assay (ELISA). Outer membranes were prepared from E. coli O55. Serum was absorbed with E. coli O55 lipopolysaccharide and diluted 1:100 for immunoglobulin A (IgA) or IgM and 1:1,000 for IgG antibodies. Paired serum specimens were obtained from the 56 patients included in the study (the first specimen on the day of positive blood culture and the second specimen 8 to 12 days later) and compared with sera from blood donors (n = 50) as controls. On an average, the patients bacteremic with enterobacteria (n = 40) showed increased levels of antibodies of all three immunoglobulin classes in the first serum specimens and significantly higher levels in the second specimens compared with the controls, although with considerable case-to-case variation. Increased levels of IgG antibodies showed the best combination of diagnostic specificity (100%) and sensitivity (53%) for bacteremia caused by enteric bacilli. Mostly, the antibody response was directed against the major E. coli O55 outer membrane proteins at 81,000, 38,500, 33,500, and 7,500 molecular weights as shown by Western blot (immunoblot) analysis. Some of the patients bacteremic with nonenteric bacteria showed increased levels of IgA antibodies, but not of IgG or IgM antibodies. Cross-reactivity of the nonenteric blood culture isolates with the E. coli outer membrane preparation was not demonstrated. The cross-reactivity of the E. coli O55 outer membrane proteins with those of enteric bacilli of other genera was examined by absorption experiments. Western blots with serum absorbed with live E. coli O55 provided evidence that the epitopes of the outer membrane protein at 7,500 molecular weight were available for antibody binding at the bacterial surface, and that at least some of the epitopes of the 38,500- and 33,500-molecular -weight proteins were accessible to antibodies. The results suggest that an ELISA for the measurement of antibodies against cross-reactive outer membrane proteins from enteric bacilli may be useful in the diagnosis of serious infections caused by members of the family Enterobacteriaceae, and that antibodies to the major outer membrane proteins may have an immunobiological function.

The porin protein of the outer membrane of Escherichia coli: reactivity in immunoblotting, antibody-binding by the native protein, and cross-reactivity with other enteric bacteria

The experimental conditions for antibody-binding by the 38.5 kD porin protein of an E. coli 055 strain in immunoblotting were investigated. A non-ionic detergent in the buffer which contained the primary antibody was required for antibody-binding by electroblots of the SDS-denatured protein. Immunoblotting, using antiserum absorbed with bacteria or the outer membrane (OM) of the E. coli 055 strain, showed results concordant with inaccessibility to antibodies of the 38.5 kD porin protein in its native configuration in the bacterial cells, but immunoreactivity when contained in the OM. OM from strains of different genera of the Enterobacteriaceae and antisera against these strains when used in immunoblot analyses showed that the E. coli 055 porin protein harboured antigenic determinants which are common to the various genera of the enteric bacilli. Cross-reactivity with non-enteric Gram-negative bacteria was not observed.

Evidence for the importance of O antigen specific antibodies in mouse-protective Salmonella outer membrane protein (porin) antisera

The antibodies responsible for the protective effect of sera obtained by immunizing rabbits with the major outer membrane protein porin of Salmonella typhimurium complexed with LPS (Kuusi et al., Infect. Immun., 1981; 34:328-332) were found to be directed to the O antigen. They were effective in very small concentration which probably accounted for our failure to identify them before or now by in vitro methods. Although both the porin and the LPS in the complexes used for immunization were isolated from rough (R) form organisms devoid of O antigen by all usually applied criteria, the antigen contained a small amount of smooth form LPS, most probably derived from a slight leakiness of the rfa mutation responsible for the R character. The small amount of contaminating O antigen was apparently rendered highly immunogenic by complexation with the outer membrane protein.

Antibody responses to Escherichia coli J5 lipopolysaccharide and to Salmonella porin in patients with bacteremia

This study was undertaken in order to evaluate whether patients with bacteremia respond with antibodies directed towards two outer membrane components of Gram-negative bacteria. The antibody responses to the core of the lipopolysaccharide molecule (LPS) of the rough E. coli J5 mutant and to a purified outer membrane protein (porin) from Salmonella were studied in bacteremic patients. Two or three serum samples were consecutively collected from 77 patients having 82 episodes of bacteremia altogether, of these 50 were caused by bacteria of the genus Enterobacteriaceae. As controls, sera from 82 age and sex matched patients and 100 healthy blood donors were analysed. The antibody titers were assessed by enzyme-linked immunosorbent assays (ELISA). None of the patients with bacteremia responded with an increase in antibody level to the E. coli J5 rough LPS. This finding indicates that the core portion of the E. coli J5 LPS contains no antigenic epitopes immunologically cross-reactive with Gram-negative bacteria causing bacteremia. By contrast, 13 patients showed significant titer increases to the porin preparation derived from Salmonella. Twelve of these patients had bacteremia caused by Gram-negative organisms belonging to Enterobacteriaceae. One patient had bacteremia with Bacteroides fragilis but also suffered from a severe peritonitis with growth of both E. coli and Klebsiella. It is suggested that the measurement of antibody response to porin may be of value for differential serological diagnosis in patients with bacteremia, to distinguish between Enterobacteriaceae and other organisms.

Immunoadsorbent-purified antibodies in the study of antigenic relatedness of outer membrane proteins of enteric bacilli

Immunoadsorbent chromatography was used for purification of antibodies to E. coli 055 outer membrane proteins. Antibodies to the 33.5 kD and 7.5 kD proteins were eluted when rabbit antisera were applied to an epoxy-activated Sepharose 6B column to which the outer membrane was coupled in the presence of dioxane. ELISA coats prepared with sonicated bacteria showed binding of the eluted antibodies with strains of all of seven different species of the enteric bacilli, but not with other Gram-negative bacilli or cocci, or with Gram-positive cocci; immunoblot analysis of transblots of SDS-PAGE-separated bacteria showed that antibodies to both of the 33.5 kD and 7.5 kD E. coli outer membrane proteins cross-reacted with the enteric bacilli of different species. Both of the anti-33.5 kD and -7.5 kD antibodies were bound by intact E. coli 055 cells, but more efficiently by sonically disrupted or heat-treated bacteria. The results show that affinity-purified anti-OM antibodies were useful for the study of the antigenic relatedness of E. coli OM proteins with proteins of other bacteria.

A set of two monoclonal antibodies specific for the cell surface-exposed 39K major outer membrane protein of Haemophilus influenzae type b defines all strains of this pathogen

Six murine plasma cell hybridomas producing monoclonal antibodies (mabs) directed against the 39,000-molecular-weight (39K) major outer membrane protein of Haemophilus influenzae type b were employed in the antigenic analysis of the 39K protein. The initial characterization of the mabs by radioimmunoprecipitation analysis showed that four of these mabs reacted with antigenic determinants of the 39K protein that are exposed on the bacterial cell surface and accessible to antibody. The other two mabs reacted with antigenic determinants of the 39K protein that are either not exposed on the H. influenzae type b cell surface or not accessible to antibody (internal determinants). A total of 126 clinical isolates of H. influenzae type b obtained from pediatric research centers throughout the United States were examined for reactivity with the six mabs by using a solid-phase radioimmunoassay in which bacterial colony growth from agar plates was placed on filter paper and used as antigen. The reactivities of these strains with two of the mabs recognizing cell surface-exposed antigenic determinants of the 39K protein were used to divide the 126 strains into four different groups. Group 1 strains reacted with mab 12D9, group 2 strains reacted with mab 4C4, group 3 strains reacted with both mabs 12D9 and 4C4, and group 4 strains (only one was found) did not react with either mab. The reactivities of two other mabs recognizing cell surface-exposed antigenic determinants of the 39K protein were used to further divide the four groups into eight subgroups. A single mab recognizing an internal antigenic determinant of the 39K protein reacted with every H. influenzae type b strain examined in this study. These data indicate that only limited antigenic heterogeneity exists among the cell surface-exposed antigenic determinants of the 39K outer membrane proteins among H. influenzae type b strains and that at least one internal antigenic determinant of the 39K protein is universally present in all H. influenzae type b strains. Radioimmunoprecipitation analysis also demonstrated that H. influenzae type b strains which lacked a 39K major outer membrane protein possessed a 38K major outer membrane protein which reacted with the anti-39K mabs, indicating that the 38K and 39K outer membrane proteins of different H. influenzae type b strains are antigenically related.

Human antibody response to individual outer membrane proteins of Haemophilus influenzae type b

To evaluate the potential of outer membrane proteins of Haemophilus influenzae as a vaccine, sera from 11 healthy persons and from 23 patients convalescing from disease caused by Haemophilus influenzae type b were assayed for antibodies to individual outer membrane proteins of a single type b isolate, strain Eag, by a gel radioimmunoassay. All 23 patients, ranging in age from 2 months to 62 years, with 17 patients being 24 months or less, had antibodies to some of these proteins in their sera (range, antibodies to 4 to 17 proteins per patient). Although the intensity and spectrum of the response varied, all patients had antibodies to one particular outer membrane protein and 19 patients had antibodies to another, with those patients 5 years and older having antibodies to more proteins than did infants (<==24 months). In the two cases examined, convalescent sera had greater amounts and broader spectra of antibodies than did acute sera. In addition, 10 of 11 healthy subjects not known to have had systemic H. influenzae disease also had antibodies to individual outer membrane proteins, with older children having greater amounts than did their younger siblings and with children showing a different spectrum of response than that for adults. Thus, antibodies to outer membrane proteins are commonly found in humans. Also, these results and those demonstrating that hyperimmune rabbit antisera to strain Eag reacted with each of five type b substrains possessing some different outer membrane proteins indicate considerable cross-reactivity among these proteins. These results encourage continued consideration of outer membrane proteins in a vaccine.

Immunization with major outer membrane protein (porin) preparations in experimental murine salmonellosis: effect of lipopolysaccharide

A crude major outer membrane (porin) preparation, obtained from a rough strain of Salmonella typhimurium earlier shown to be protective both in active and passive immunization of mice against challenge with smooth S. typhimurium, was further purified. Removal of the main impurities, lipopolysaccharide (LPS) and lipoprotein, was accompanied by loss of protective capacity in passive immunization experiments. Reconstitution with rough LPS restored the protective capacity. Protection was, however, concluded not to be due to anti-LPS, because a large fraction of the anti-LPS antibodies could be removed from the protective rabbit antiserum with an LPS immunosorbent without loss of protection.