Cloning and expression in Escherichia coli of the Ibc protein genes of group B streptococci: binding of human immunoglobulin A to the beta antigen

Surface Structures of Group B Streptococcus Important in Human Immunity

The surface of the Gram-positive opportunistic pathogen Streptococcus agalactiae, or group B Streptococcus (GBS), harbors several carbohydrate and protein antigens with the potential to be effective vaccines. Capsular polysaccharides of all clinically-relevant GBS serotypes coupled to immunogenic proteins of both GBS and non-GBS origin have undergone extensive testing in animals that led to advanced clinical trials in healthy adult women. In addition, GBS proteins either alone or in combination have been tested in animals; a fusion protein construct has recently advanced to human clinical studies. Given our current understanding of the antigenicity and immunogenicity of the wide array of GBS surface antigens, formulations now exist for the generation of viable vaccines against diseases caused by GBS.

Streptococcus agalactiae CAMP factor/protein B does not bind to human IgG

CAMP factor is an extracellular cytolytic protein produced by Streptococcus agalactiae. CAMP factor has been reported to bind the Fc fragments of immunoglobulin G (IgG) and has therefore also been called protein B, in analogy to protein A of Staphylococcus aureus. We attempted to characterize the interaction of protein B with IgG in more detail. In contrast to protein A, CAMP factor does not inhibit the activation of complement by hemolysin antibodies bound to sheep red cell surfaces. IgG also failed to inhibit the co-hemolytic activity of CAMP factor, which is in disagreement with previous findings. After co-incubation, CAMP factor and IgG were cleanly separated by gel filtration, indicating that no binding had occurred.

A unique serine-rich repeat protein (Srr-2) and novel surface antigen (epsilon) associated with a virulent lineage of serotype III Streptococcus agalactiae

Group B streptococci (GBS) are pathogens of both neonates and adults, with serotype III strains in particular being associated with invasive disease and meningitis. In this study, a novel GBS surface antigen, epsilon, was found to be co-expressed with the previously reported delta antigen on an identical subset of serotype III GBS. Expression of delta/epsilon on the surface of serotype III GBS was shown to distinguish the restriction digest pattern (RDP) III-3 and multilocus sequence typing (ST)-17 lineage. epsilon-Specific antibodies were reactive with a unique, high-molecular-mass, serine-rich repeat protein (Srr-2) found exclusively in RDP III-3 strains. The gene encoding Srr-2 was located within a putative accessory secretory locus that included secY2 and secA2 homologues and had a genetic organization similar to that of the secY2/A2 locus of staphylococci. In contrast, serotype III delta/epsilon-negative strains and strains representative of serotypes Ia, Ib, Ic and II shared a common Srr-encoding gene, srr-1, and an organization of the secY2/A2 locus similar to that of previously reported serotype Ic, delta/epsilon-negative serotype III and serotype V GBS strains. Representative serotype III delta/epsilon-positive strains had LD(90) values 3-4 logs less than those of serotype III delta/epsilon-negative strains in a neonatal mouse model of infection. These results indicate that the RDP III-3/ST-17 lineage expresses Srr-2 and is highly virulent in an in vivo model of neonatal sepsis.

Bovine immunoglobulin A (IgA)-binding activities of the surface-expressed Mig protein of Streptococcus dysgalactiae

The Mig protein of Streptococcus dysgalactiae is a type III immunoglobulin G (IgG)-binding protein, expressing IgG- and alpha2-macroglobulin (alpha2-M)-binding receptors. This study showed that the Mig protein also displays binding activities to bovine immunoglobulin A (B-IgA). Biotin-labelled bovine serum IgA bound immobilized recombinant Mig and alpha2-M receptors derived from Mig, as well as the native Mig extracted from the surface of S. dysgalactiae strain SDG8 and the alpha(2)-M receptor released from the isogenic mig mutant strain Mig8-Mt, as determined by Western blotting and ELISA. There was no B-IgA binding activity to the immobilized IgG receptor derived from Mig or the proteins in the culture supernatant from the mig mutant strain Mig7-Mt, in which expression of Mig or Mig-related peptides on the cell surface was completely abolished. In a reciprocal experiment, biotin-labelled Mig was found to bind immobilized bovine serum IgA but not human IgA (H-IgA). The binding of Mig to bovine serum IgA was competitively inhibited by unlabelled Mig, intact and truncated alpha(2)-M receptors, and bovine serum IgA, but not by the Mig-IgG receptor, H-IgA or B-IgG. The binding of Mig and partially purified bovine secretory IgA (B-sIgA) was also characterized by Western blotting. Membrane-immobilized B-sIgA did not react with the biotin-labelled Mig, whereas soluble B-sIgA showed binding activity to the immobilized alpha2-M receptor of Mig. It is therefore concluded that the 11 kDa N-terminal region of the alpha2-M receptor of the S. dysgalactiae Mig protein specifically binds soluble and immobilized bovine serum IgA, as well as soluble B-sIgA. This is believed to be the first report of a B-IgA-binding protein in S. dysgalactiae.

Investigation of the translation-initiation factor IF2 gene, infB, as a tool to study the population structure of Streptococcus agalactiae

The sequence of infB, encoding the prokaryotic translation-initiation factor 2 (IF2), was determined in eight strains of Streptococcus agalactiae (group B streptococcus) and an alignment revealed limited intraspecies diversity within S. agalactiae. The amino acid sequence of IF2 from S. agalactiae and from related species were aligned and revealed an interspecies conserved central and C-terminal part, and an N-terminal part that is highly variable in length and amino acid sequence. The diversity and relationships in a collection of 58 genetically distinct strains of S. agalactiae were evaluated by comparing a partial sequence of infB. A total of six alleles were detected for the region of infB analysed. The alleles correlated with the separation of the same strains of S. agalactiae into major evolutionary lineages, as shown in previous work. The partial sequences of infB were furthermore used in phylogenetic analyses of species closely related to S. agalactiae, yielding an evolutionary tree which had a topology similar to a tree constructed using 16S rRNA sequences from the same species.

An Escherichia coli-Enterococcus faecalis shuttle vector as a tool for the construction of a group B Streptococcus heterologous mutant expressing the beta antigen (Bac) of the C protein complex

Group B streptococci (GBS) represent a very important group of human pathogens. So far little is known about the mechanisms by which these bacteria can cause disease and the bacterial factors involved. One putative virulence factor is the beta antigen of the C protein complex (Bac), which can bind to the Fc region of human IgA. Its binding function might represent an important virulence mechanism. However, the genetic manipulation of this group of bacteria, necessary to prove involvement of bacterial factors in pathogenesis, is still in its infancy. We therefore tested the pAM401 vector system for its suitability in the construction of a heterologous expression mutant using the Bac protein as a model antigen. The bac gene, including its own promoter, was cloned into the Escherichia coli-Enterococcus faecalis shuttle vector pAM401 and was stably maintained extrachromosomally in the bac-deficient GBS strain 335. Expression of Bac was assessed by extracting the protein from transformed 335(pPJTU1) cells, negative controls (335 wild-type, 335(pAM401)) and other Bac-expressing GBS strains (A909, LA239). Blots of the extracted proteins probed with IgA, polyclonal sera and a monoclonal antibody raised against Bac clearly revealed expression of the 130-kDa protein in the transformed GBS 335(pPJTU1) cells. The correct processing and surface anchoring of the expressed Bac was demonstrated by binding of (125)I-labelled IgA to whole cells. Strain 335(pPJTU1) bound 12 times as much IgA compared to the parental strain LA239 and the GBS 335 negative controls, and a total of 25% compared to the high-level-expressing strain A909. Our studies show that the pAM401 shuttle vector can be used for stable heterologous expression of surface proteins in GBS. Our strategy is also of major importance for the complementation of deletion mutants in GBS and other Gram-positive human pathogens to fulfill Koch's postulates. The Bac mutant constructed in this study, 335(pPJTU1), can be used in animal models to assess the importance of Bac in GBS pathogenesis.

Identification of an immunoglobulin A binding motif located in the beta-antigen of the c protein complex of group B streptococci

The beta-antigen of the c protein complex of group B streptococci contains two immunoglobulin A (IgA)-binding domains called A and B. A 73-amino-acid segment in domain A is responsible for most of the IgA-binding activity. To identify the IgA binding motif, the 73-amino-acid domain was divided into 60 14-amino-acid overlapping peptides spot synthesized onto a cellulose membrane. A 20-residue putative antigenic epitope was identified and expressed as a fusion protein. The fusion protein was purified by fast protein liquid chromatography and used to raise rabbit antiserum. By use of a membrane with spot-synthesized peptide amino acids of decreasing length (from 14 to 6 amino acids), the major antigenic epitope recognized by the anti-fusion protein antibodies was mapped to motif MLKKIE. Anti-fusion protein antibodies inhibited the binding of IgA to group B streptococci. This inhibition could be blocked by the peptide containing the motif MLKKIE. These results indicate that the motif MLKKIE is located in the IgA-binding site. The IgA-binding domain of beta-antigen from three group B streptococcal strains reacted with the anti-fusion protein antibodies, and their coding sequences gave positive signals in Southern hybridization. The sequences of beta-antigen from these strains were amplified by PCR, and sequence analysis showed them to be identical. The results indicate that the motif MLKKIE is required for IgA binding and is present in different group B streptococcal strains.

Protein B: an important human IgA-binding reagent

Protein B had a much higher affinity for human IgA than Jacalin, increasing the sensitivity and specificity of the measurement of total human IgA. Protein B, used as a capturing agent, greatly enhanced the measurement of antigen-specific IgA as compared to alpha chain-specific antibodies.

Electrotransformation of Streptococcus agalactiae with plasmid DNA

A protocol for efficient electrotransformation of Streptococcus agalactiae (group B streptococcus) Lancefield's strain O90R (NTCT 9993) (an unencapsulated derivative of type Ia strain O90) was developed. The Escherichia coli-Streptococcus shuttle vector pDP28 (7.8 kb) carrying the ermB gene for resistance to erythromycin was used as donor DNA. Frozen 'electrocompetent' cells were prepared by repeated washes in 10% glycerol. A 50-microliters aliquot containing about 5 x 10(9) colony forming units of bacteria was subjected to the electric pulse. Optimal conditions for electrotransformation were determined using different media, harvesting cells at different points of the growth curve, and using different field strengths. The dose-response curve for transformation of S. agalactiae with pDP28 showed one-hit kinetics as donor DNA varied between 0.01 and 3 micrograms. The efficiency of electrotransformation for this range of amounts of donor DNA was 1.2 x 10(4) cfu micrograms-1. The transformation frequencies obtained with this electroporation protocol are high enough to allow both subcloning and shotgun cloning of streptococcal DNA in S. agalactiae.

Prevalence of IgA receptors in clinical isolates of Streptococcus pyogenes and Streptococcus agalactiae: serologic distinction between the receptors by blocking antibodies

Group A and B streptococci (Streptococcus pyogenes and Streptococcus agalactiae) are the only known bacterial pathogens expressing IgA Fc-receptors. However, the IgA binding proteins of the two species have been found genetically unrelated. In the present investigation the binding of human IgA among clinical isolates of group A and group B streptococci was studied and the respective IgA-binding epitopes were compared serologically. Surface binding of radiolabelled, monoclonal human IgA1 occurred in 38% of 115 unselected group A streptococcal isolates. Comparing four predominant T-types, IgA-binding was found in 77% and 85%, respectively, of types T4 and T28 strains but only in 5% and 25%, respectively, of T1 and T12 strains. In group B streptococci, 70% of 58 type Ib strains but only 2% of 399 strains of other serotypes bound IgA. Using rabbit immune sera raised to the two streptococcal species it was found that strains exhibiting IgA Fc-receptors often induced antibodies blocking the binding of IgA to bacteria. Furthermore, the blocking shown by an individual serum was restricted to the streptococcal group used for immunization showing that also the IgA-binding eptiopes in group A and B streptococci are conformationally distinct. Though infections with serotypes often binding IgA, compared to other types, are not known to differ, it is assumed that the non-immune binding of IgA might favour mucosal colonization of the organisms.

Antibody profiles to the group B streptococcal beta antigen in maternal and infant paired sera

Antibody profiles to the purified beta antigen of the c protein of group B streptococci (GBS) were studied by ELISA and Western immunoblot (WB). The sera from 139 parturient women colonized with GBS, 35 non-colonized parturients and their newborn infants were studied by ELISA; WB was done on 76 maternal and 26 infant sera. Enzyme-labeled anti-IgA (alpha), -IgG (gamma), -IgM (mu), or -IgG (H&L) were used as secondary antibodies. A high prevalence of antibody to the beta antigen was observed by both ELISA and WB among parturient women and their newborns. IgG (H&L) ELISA titers > or = 200 were found in 84% and > or = 800 in 31% of the maternal sera. A significantly higher percentage of women colonized than those non-colonized with GBS had IgG (gamma) titers > or = 800. A significantly higher percentage of women colonized with c protein-positive than c-negative strains of GBS had IgG (H&L) titers > or = 3200. Twelve of 27 women with IgM antibody to the beta antigen also had IgG (gamma) titers > or = 800 and were, in addition, colonized with GBS. Multiple molecular forms of the antigen from 25 to 140 kDa were blotted by the maternal and infant sera. Concordance in the IgG but not in IgA or IgM antibody profiles of maternal and infant paired sera was observed in the overall blotting patterns and ELISA titers. The same titer as the mother was found in 55% of the infant sera and within one dilution in 97%. This suggests active transfer of IgG antibody to the beta antigen across the placenta from mother to baby.

Binding of human IgA to HCl-extracted c protein from group B streptococci (GBS)

The c beta protein of group B streptococci obtained by HCl extraction appears as a ladder-like pattern in SDS-PAGE when detected by a rabbit anti-c beta serum, and a similar picture is seen when the crude extract is incubated with human IgA and an anti-human IgA conjugate. Affinity-purified c beta antigen and IgA receptors from GBS gave identical pictures in Western blots using rabbit anti-c beta serum. Both the c beta antigen and the IgA receptor are exposed on the surface of GBS as demonstrated by immunofluorescence.

Large, identical, tandem repeating units in the C protein alpha antigen gene, bca, of group B streptococci

Group B Streptococcus (GBS) is the leading cause of neonatal sepsis and meningitis in the United States. The surface-associated C protein alpha antigen of GBS is thought to have a role in both virulence and immunity. We previously cloned the C protein alpha antigen structural gene (named bca for group B, C protein, alpha) into Escherichia coli. Western blots of both the native alpha antigen and the cloned gene product demonstrate a regularly laddered pattern of heterogeneous polypeptides. The nucleotide sequence of the bca locus reveals an open reading frame of 3060 nucleotides encoding a precursor protein of 108,705 Da. Cleavage of a putative signal sequence of 41 amino acids yields a mature protein of 104,106 Da. The 20,417-Da N-terminal region of the alpha antigen shows no homology to previously described protein sequences and is followed by a series of nine tandem repeating units that make up 74% of the mature protein. Each repeating unit is identical and consists of 82 amino acids with a molecular mass of 8665 Da, which is encoded by 246 nucleotides. The size of the repeating units corresponds to the observed size differences in the heterogeneous ladder of alpha C proteins expressed by GBS. The C-terminal region of the alpha antigen contains a membrane anchor domain motif that is shared by a number of Gram-positive surface proteins. The large region of identical repeating units in bca defines protective epitopes and may play a role in generating phenotypic and genotypic diversity of the alpha antigen.

Cloned alpha and beta C-protein antigens of group B streptococci elicit protective immunity

Streptococcus agalactiae (group B streptococci [GBS]) is the leading cause of neonatal sepsis and meningitis in the United States. The surface-associated C proteins of GBS play a role in immunity, but their number, size, structure, function, and virulence properties have not been well characterized. A recombinant library of DNA fragments from GBS strain A909 (type Ia/C) was prepared in the plasmid pUX12, a specially constructed Escherichia coli expression vector. The library was screened with a rabbit antiserum shown to be protective for passive immunity to GBS infection in a mouse lethality model. Clones were divided into two distinct groups on the basis of DNA-DNA cross-hybridization, restriction enzyme analysis, and the expression of antigenic proteins in E. coli. A characteristic clone from each group was chosen for further study. Clone pJMS23 expresses gene products that biochemically and immunologically correspond to the trypsin-resistant, C-protein alpha antigen. Clone pJMS1 expresses a gene product that binds to immunoglobulin A and is similar to the trypsin-sensitive, C-protein beta antigen. Antisera raised in rabbits against E. coli containing each of the plasmid clones were able to elicit protective immunity in mice challenged by GBS strains carrying the C proteins but not by non-C-protein-bearing strains. Southern blot analysis shows no DNA homology between the clones, and there is no immunological cross-reactivity between the antigens they express. Therefore, pJMS23 and pJMS1 encode two different C proteins that define unique protective epitopes.

Molecular characterization of an IgA receptor from group B streptococci: sequence of the gene, identification of a proline-rich region with unique structure and isolation of N-terminal fragments with IgA-binding capacity

Certain strains of group B streptococci express a cell surface protein that binds IgA and acts as a virulence factor. This IgA receptor is referred to here as protein Bac. The gene for protein Bac was cloned and expressed in Escherichia coli, and the complete nucleotide sequence was determined. The deduced amino acid sequence of 1134 residues includes a signal sequence of 37 amino acids and a putative membrane anchor region at the C-terminal end. The processed form of the receptor, 1097 residues, has a calculated molecular weight of 123,786. There are no cysteines in protein Bac, suggesting a fibrillar structure. The C-terminal half of the protein includes a 90 residues long region with a novel type of periodic structure, the "XPZ motif", in which every third amino acid is proline. Unlike other bacterial immunoglobulin-binding proteins, there are no long repeats in protein Bac. Clones which express only part of the protein Bac gene were used to show that IgA-binding takes place in the N-terminal part of the molecule. Protein Bac was originally described as an antigen called beta, but N-terminal fragments that bind IgA do not react with a reference serum against the beta antigen. These and other data indicate that protein Bac can be divided into two regions with different functions: an N-terminal IgA-binding region and a C-terminal region corresponding to the beta antigen. The IgA-binding region of protein Bac does not show any homology to protein Arp, the IgA receptor from group A streptococci, although these receptors have similar binding properties. This indicates that convergent evolution has favored the appearance of these two structurally different streptococcal IgA receptors.

The IgA-binding beta antigen of the c protein complex of Group B streptococci: sequence determination of its gene and detection of two binding regions

The beta antigen of the lbc protein complex of Group B streptococci is a cell-surface receptor which binds the Fc region of human immunoglobulin A (IgA). Determination of the nucleotide sequence of the beta antigen gene shows that it encodes a preprotein having a molecular weight of 130,963 daltons and a polypeptide of 1164 amino acid residues that is typical of other Gram-positive cell-wall proteins. There is a long signal sequence of 37 amino acids at the N-terminus. Four of the five C-terminal amino acid residues are basic and are preceded by a hydrophobic stretch that appears to anchor the C-terminus in the cell membrane. To the N-terminal side of this hydrophobic stretch is a putative cell-wall-spanning region containing proline-rich repeated sequences. An unusual feature of these repeated sequences is a three-residue periodicity, whereby every first residue is a proline, the second residue is alternating positively or negatively charged, and the third residue is uncharged. The IgA-binding activity was approximately localized by expressing subfragments of the beta antigen as fusion proteins. Two distinct but adjacent DNA segments specified peptides that bound IgA, which indicates that the IgA-binding activity is located in two distinct regions of the protein.

Immuno-electronmicroscopic demonstration of alpha and beta components of group B streptococcal protein antigen c

Affinity purified and absorbed monospecific antibodies against the protein type antigen c components c alpha and c beta were used in immuno-electronmicroscopic studies. As demonstrated with protein A gold particles the c alpha specific antibodies reacted with the c alpha antigen, the c beta specific antibodies with the c beta antigen. The respective antigens were distributed over the whole surface of the bacterial cells, mainly at the outermost layer of the cell wall.

A monoclonal antibody identifies a protective C-protein alpha-antigen epitope in group B streptococci

Group B streptococci (GBS) are the leading causes of neonatal sepsis and meningitis in the United States, with a high rate of fatality and serious morbidity despite appropriate therapy. The C-protein antigens of GBS appear to be important in immunity to experimental infection, yet these antigens remain incompletely characterized with respect to their number, structure, and function. None of these proteins has yet been purified to homogeneity. We have developed a novel method for extraction of surface proteins from the A909 (Ia/c) strain of GBS by using mutanolysin. Antibodies raised in rabbits against these partially purified proteins conferred passive protection to lethal GBS infection in mice challenged with a GBS strain expressing C proteins with a heterologous capsule type. In addition, mouse monoclonal antibodies were produced and identified by reactivity with the mutanolysin-extracted proteins. One of these monoclonal antibodies (4G8) identifies an epitope on the alpha-antigen of the GBS C proteins (identified by protease susceptibility and mouse protection). On sodium dodecyl sulfate-polyacrylamide gels, this epitope appears as a series of regularly spaced bands ranging in apparent molecular mass from 160,000 to 30,000 Da. The monoclonal antibody 4G8 induces opsonic killing of GBS and protects mice from lethal challenge with GBS. Thus, the 4G8 monoclonal antibody identifies a fully protective epitope on the C-protein alpha-antigen of GBS.

Opsonic effect of jacalin and human immunoglobulin A on type II group B streptococci

This study examined the effect of immunoglobulin A (IgA) and the IgA-binding lectin jacalin on the phagocytosis of type II group B streptococci (GBS). Strains possessing the trypsin-sensitive and trypsin-resistant components of the c protein (II/c) and type II GBS lacking the c protein (II) were examined by radiolabeled bacterial uptake, bactericidal assays, and electron microscopy. Type II/c GBS resisted phagocytosis by monocytes (4.9% +/- 0.8% uptake, mean +/- SE, n = 25) compared with type II GBS (8.5% +/- 1.4% uptake, n = 14, P = 0.03). Phagocytic killing by polymorphonuclear leukocytes was also less for the type II/c strain 78-471 than for the type II strain 79-176 (68% +/- 5% versus 86% +/- 4% reduction in CFU at 45 min, P = 0.03). IgA binding did not explain the resistance of type II/c GBS to phagocytosis. The uptake of type II/c GBS was not significantly different after opsonization in cord sera lacking endogenous IgA (5.93% +/- 1.4%) than in the same cord sera after addition of exogenous IgA (5.48% +/- 1.4%, P = 0.69, n = 9). Attempts to remove serum IgA with the IgA-binding lectin jacalin resulted in the binding of IgA-jacalin complexes to II/c GBS. This combination of nonspecific IgA and jacalin increased uptake of II/c GBS from 4.9% +/- 0.8% to 11.8% +/- 1.9% (P = 0.002). Jacalin also combined with specific, immune, monoclonal IgA bound to the surface of Haemophilus influenzae and promoted the uptake of these bacteria. Jacalin and IgA mediated phagocytosis of II/c GBS via receptors that were not dependent on divalent cations and that were not modulated by plating monocytes on antigen-antibody complexes.

Nonimmune binding of human immunoglobulin A to type II group B streptococci

The binding of 125I-labeled human myeloma immunoglobulin A (IgA) to four type II strains and one nontypable strain of group B streptococci was measured after streptococcal chains were broken by brief sonication. Some IgA binding was observed with all strains, but specific binding (binding that was inhibited by excess unlabeled IgA, was dose dependent, and was saturable) occurred only with those strains possessing the trypsin-sensitive beta component of the c protein. Similar amounts of binding were observed with myeloma IgA and IgA1 purified from normal serum. Specific binding was more rapid at 25 degrees C than at 0 or 37 degrees C and reached a plateau in 6 to 8 h. Binding was drastically reduced (85 to 90%) when streptococci had been heated (90 degrees C for 1 h). Most radioactivity bound to group B streptococci could be displaced (greater than 60% in 3 days) by the addition of excess unlabeled IgA. The binding capacity of one strain (10(8) streptococci in 1 ml of buffer) was saturated by approximately 24 micrograms of IgA. When transformed for Scatchard analysis, these data indicated that there was a specific binding capacity of 16,000 molecules of monomeric serum IgA per single streptococcal cell. The dissociation constant for IgA binding was 19.3 nM. Since enzyme-linked immunosorbent assay studies showed that the myeloma IgA used for the studies described above was IgA1, our quantitative data apply only to the binding of this subclass to group B streptococci. However, an enzyme-linked immunosorbent-filtration assay showed that both IgA1 and IgA2 bound to a type II group B streptococcus bearing the c protein.

Colloidal gold immunolabeling of immunoglobulin-binding sites and beta antigen in group B streptococci

We have characterized the immunoglobulin A (IgA)-Fc-binding properties and beta-antigen expression of several strains of group B streptococci by using ultrastructural immunocytochemistry. Colloidal gold-labeled tracers were used with intact and sectioned bacteria in order to gain information regarding the location and distribution of cell surface and cytoplasmic IgA-Fc-binding molecules and beta antigen. Colloidal gold (5- or 15-nm particles) was conjugated to IgA to characterize IgA-binding properties and to IgG to test for IgG binding. Rabbit anti-beta antiserum was reacted with the bacteria and then with protein G labeled with 15-nm colloidal gold particles. A double-labeling technique was used for simultaneous localization of IgA-Fc- and anti-beta-antibody-binding properties on sectioned bacteria. The data corroborated previous results which indicated that (i) IgA-Fc-binding and IgA-Fc-nonbinding forms of beta antigen can be secreted by strains which do not express beta antigen on the cell surfaces (HG806, VC75); (ii) differences in levels of expression of beta antigen and/or IgA-Fc-binding proteins can be detected among various group B isolates; (iii) group B streptococci do not express human IgG-Fc-binding proteins; and (iv) not all forms of beta antigen are capable of binding human IgA.

Identification of non-immunoglobulin A-Fc-binding forms and low-molecular-weight secreted forms of the group B streptococcal beta antigen

The beta antigen expressed on the surfaces of certain strains of group B streptococci has been reported to bind to the Fc region of human immunoglobulin A (IgA). In this study, we screened 100 isolates of group B streptococci for expression of both beta antigen and IgA-Fc-binding activity. We identified two isolates which expressed the beta antigen but could not bind human IgA Fc fragments and also observed variability in IgA-Fc-binding activity among other beta-antigen-expressing strains. Novel low-molecular-weight forms of beta antigen were secreted by four beta-antigen surface-negative isolates and included IgA-Fc-binding (Mrs, 55,000 and 53,000) and non-IgA-Fc-binding (Mr, 38,000) molecules. These results suggest that the IgA-Fc-binding site represents a unique domain of the beta antigen. The 55,000- and 53,000-Mr forms of secreted beta antigen were functionally and antigenically representative of the size-heterogeneous (Mr, up to 145,000) beta-antigen molecules expressed by surface-positive strains. The cell surface-localized IgA-Fc-binding molecules could bind only human serum IgA efficiently; however, once solubilized, these molecules could bind both human serum and secretory IgAs.