Ultrastructural localization of protein antigens I/II and III in Streptococcus mutans

Membrane proteomic analysis reveals overlapping and independent functions of Streptococcus mutans Ffh, YidC1, and YidC2

A comparative proteomic analysis was utilized to evaluate similarities and differences in membrane samples derived from the cariogenic bacterium Streptococcus mutans, including the wild-type strain and four mutants devoid of protein translocation machinery components, specifically ∆ffh, ∆yidC1, ∆yidC2, or ∆ffh/yidC1. The purpose of this work was to determine the extent to which the encoded proteins operate individually or in concert with one another and to identify the potential substrates of the respective pathways. Ffh is the principal protein component of the signal recognition particle (SRP), while yidC1 and yidC2 are dual paralogs encoding members of the YidC/Oxa/Alb family of membrane-localized chaperone insertases. Our results suggest that the co-translational SRP pathway works in concert with either YidC1 or YidC2 specifically, or with no preference for paralog, in the insertion of most membrane-localized substrates. A few instances were identified in which the SRP pathway alone, or one of the YidCs alone, appeared to be most relevant. These data shed light on underlying reasons for differing phenotypic consequences of ffh, yidC1 or yidC2 deletion. Our data further suggest that many membrane proteins present in a ∆yidC2 background may be non-functional, that ∆yidC1 is better able to adapt physiologically to the loss of this paralog, that shared phenotypic properties of ∆ffh and ∆yidC2 mutants can stem from impacts on different proteins, and that independent binding to ribosomal proteins is not a primary functional activity of YidC2. Lastly, genomic mutations accumulate in a ∆yidC2 background coincident with phenotypic reversion, including an apparent W138R suppressor mutation within yidC1.

Identification of a supramolecular functional architecture of Streptococcus mutans adhesin P1 on the bacterial cell surface

P1 (antigen I/II) is a sucrose-independent adhesin of Streptococcus mutans whose functional architecture on the cell surface is not fully understood. S. mutans cells subjected to mechanical extraction were significantly diminished in adherence to immobilized salivary agglutinin but remained immunoreactive and were readily aggregated by fluid-phase salivary agglutinin. Bacterial adherence was restored by incubation of postextracted cells with P1 fragments that contain each of the two known adhesive domains. In contrast to untreated cells, glutaraldehyde-treated bacteria gained reactivity with anti-C-terminal monoclonal antibodies (mAbs), whereas epitopes recognized by mAbs against other portions of the molecule were masked. Surface plasmon resonance experiments demonstrated the ability of apical and C-terminal fragments of P1 to interact. Binding of several different anti-P1 mAbs to unfixed cells triggered release of a C-terminal fragment from the bacterial surface, suggesting a novel mechanism of action of certain adherence-inhibiting antibodies. We also used atomic force microscopy-based single molecule force spectroscopy with tips bearing various mAbs to elucidate the spatial organization and orientation of P1 on living bacteria. The similar rupture lengths detected using mAbs against the head and C-terminal regions, which are widely separated in the tertiary structure, suggest a higher order architecture in which these domains are in close proximity on the cell surface. Taken together, our results suggest a supramolecular organization in which additional P1 polypeptides, including the C-terminal segment originally identified as antigen II, associate with covalently attached P1 to form the functional adhesive layer.

Functional characterization of cell-wall-associated protein WapA in Streptococcus mutans

Streptococcus mutans is known as a primary pathogen responsible for dental caries. One of the virulence factors of S. mutans in cariogenicity is its ability to attach to the tooth surface and form a biofilm. Several surface proteins have been shown to be involved in this process. A 29 kDa surface protein named wall-associated protein A (WapA, antigen A or antigen III), was previously used as a vaccine in animal studies for immunization against dental caries. However, the function of WapA in S. mutans is still not clear. This study characterized the function of WapA in cell surface structure and biofilm formation. Compared to the wild-type, the wapA mutant had much-reduced cell chain length, diminished cell-cell aggregation, altered cell surface ultrastructure, and unstructured biofilm architecture. Furthermore, in vivo force spectroscopy revealed that the cell surface of the wapA mutant was less sticky than that of the wild-type cells. More interestingly, these phenotypic differences diminished as sucrose concentration in the medium was increased to 0.5 %. Real-time RT-PCR analysis demonstrated that sucrose strongly repressed wapA gene expression in both planktonic and biofilm cells. These results suggest that the WapA protein plays an important structural role on the cell surface, which ultimately affects sucrose-independent cell-cell aggregation and biofilm architecture.

Salivary and serum antibody response to Streptococcus mutans antigens in humans

Humoral immunity against Streptococcus mutans infection was analyzed in caries-active and caries-free young adults by immunoblotting. All volunteers from both groups had detectable salivary immunoglobulin A (IgA) and serum IgG antibodies, with similar profiles. They could be classified on the basis of relative intensity of the immunoblot bands into categories of high or low responders. Common protein antigens with molecular weight ranging from approximately 45 to 190 kDa could be found either extracellularly or associated with the cell wall of S. mutans cultured in vitro. The predominant reactive antigens recognized by both IgA and IgG were of proteins around 63 and 60 kDa. Detection of IgA antibodies to the various antigens of S. mutans in individual saliva samples did not always correlate with serum IgG antibody profiles. In addition, distinct bands, which reacted preferentially with either IgA or IgG, could be detected by antibodies from specific subjects. Differential reactivities of salivary IgA and serum IgG antibodies to two, cell-wall associated protein antigens around 33 and 36 kDa were found in caries-active and caries-free young adults; 30.8% of caries-free subjects and 12% of caries-active subjects (P < 0.01) exhibited detectable antibody response to these antigens. This difference was not attributable to variations in antibody levels, since antibody response to these proteins were still detectable in some caries-free but not caries-active individuals whose levels of antibodies to other antigens were low. Thus, a new antibody profile which correlates with dental caries disease activity has been identified in a selected population. Differences in mucosal and systemic immune responses to S. mutans seem to be both antigen and host dependent.

Antigenic characterization of fimbria preparations from Streptococcus mutans isolates from caries-free and caries-susceptible subjects

The adhesion of pathogenic bacteria to the host surface is an essential step in the development of numerous infections, including dental caries. Attachment of Streptococcus mutans, the main etiological agent of human dental caries, to the tooth surface may be mediated by glucan synthesized by glucosyltransferase (GTF) and by cell surface proteins, such as P1, which bind to salivary receptors. Fimbriae on the surfaces of many microorganisms are known to function in bacterial adhesion. Previous studies in this laboratory have initially characterized the fibrillar surface of S. mutans. The purpose of this investigation was the comparison of the antigenic properties of fimbria preparations of S. mutans isolates from five caries-resistant (CR) and six caries-susceptible (CS) subjects. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of S. mutans fimbrial preparations revealed five major protein bands at 200, 175, 157, 86, and 66 kDa in preparations from CR and CS subjects. Immunoblot analysis indicated the presence of the same major bands recognized by anti-S. mutans fimbria antisera. Furthermore, the 175- and 157-kDa bands were recognized by antibodies to P1 and GTF, respectively. Immunoblot analysis with antisera to the fimbria preparation, to P1, or to GTF indicated that the levels of fimbria-reactive components and P1 and GTF antigens were higher in S. mutans fimbria preparations from CS subjects than in those from CR individuals. For example, four of six fimbria preparations from CS patients had demonstrable P1, and all had GTF. In contrast, only two of five CR fimbrial preparations exhibited P1 and GTF. Enzyme-linked immunosorbent assay demonstrated similar results for levels of GTF antigen in the fimbrial preparations from CR and CS subjects. The results suggest that differences between the compositions of S. mutans fimbriae in CR and CS individuals may play an important role in the virulence of this microorganism in dental caries.

Immunochemical analysis and localisation of a cell wall antigen of Streptococcus rattus with a specific monoclonal antibody

A monoclonal antibody specific to a cell wall antigen of Streptococcus rattus (S. rattus) was prepared after cell fusion of mouse myeloma cells to the spleen cells of mice immunized with whole cells of strain FA1. The monoclonal antibody reacted with a crude and purified polysaccharide antigen of S. rattus in precipitin reactions as well as in a solid phase enzyme assay (EIA). It also reacted in the EIA with whole cells of S. rattus strains, but did not react with any other species of the mutans group or of other oral and streptococcal species. Moreover, among many haptenic sugars and their derivatives tested, D-galactosamine and D-glucosamine strongly inhibited the reaction in a competitive enzyme immunoassay using the purified antigen and whole cells, indicating that the type-specific antigenic determinant of the organism has a structure similar to the amino group of the sugar molecules. Furthermore, immunoelectron microscopy revealed that the antigen epitopes formed an irregular fibrous structure over the entire surface of individual cells.

Identity of Streptococcus mutans surface protein antigen III and wall-associated protein antigen A

Preparations of Streptococcus mutans surface proteins AgIII and antigen A from different laboratories were compared with regard to amino acid composition, N-terminal amino acid sequence, electrophoretic mobility, and antigenic similarity. Despite previous observations of differences in physical properties, data indicate that these two preparations represent the same protein.

Localization on the cell surface of streptococcal protein antigen A

The cellular localization of the major surface protein SpaA of Streptococcus sobrinus 6715 was examined by immunoelectron microscopy with rabbit polyclonal antibodies directed against purified SpaA protein. Immunoelectron microscopic analysis of thin sections of S. sobrinus cells revealed that the SpaA protein is associated with the fibrillar fuzzy coat of S. sobrinus cells and appears to be distributed over the entire surface of S. sobrinus cells.

Inactivation of the Streptococcus mutans wall-associated protein A gene (wapA) results in a decrease in sucrose-dependent adherence and aggregation

A 0.8-kb HindIII-BamHI internal fragment of the Streptococcus mutans wall-associated protein A gene (wapA) was ligated to the 5.1-kb HindIII-BamHI fragment of the chimeric Streptococcus-Escherichia coli plasmid pVA891 (Emr Cmr). The resulting construct was used to transform S. mutans GS-5, and erythromycin-resistant mutants were isolated and analyzed. Directed mutagenesis of the wapA gene by plasmid insertion through homologous recombination was demonstrated by Southern blot hybridization with the wapA and pVA891 probes. Stable mutants were obtained, and the alteration of the wapA gene by insertional inactivation was associated with a significant decrease in S. mutans sucrose-dependent aggregation and binding to smooth surfaces. Thus, WapA may play an important role in the colonization of the tooth surface by S. mutans and in the buildup of dental plaque. These findings provided an explanation for previous studies which indicated that WapA was effective in the prevention of dental caries in animal models. Thus, the use of recombinant WapA in the preparation of a safe and effective human dental vaccine should be investigated further.

A mechanism of passive immunization with monoclonal antibodies to a 185,000 M(r) streptococcal antigen

The cell surface streptococcal antigen (SA) I/II of 185,000 M(r) is an immunodominant molecule that expresses one or more adhesion determinants. A series of 14 monoclonal antibodies (MAb) to defined parts of SA I/II were generated and some of these were used in passive immunization of macaques. Topical administration of selected MAb to the teeth of macaques prevented colonization of endogenous or implanted exogenous Streptococcus mutans for a period of 1 year. Significant reduction of both smooth surface and fissure caries was found in macaques who had MAb (Guy's 1) applied to their teeth, as compared with saline-treated animals. A series of in vivo passive immunization experiments was then carried out in 57 human subjects. Topical application of MAb to SA I/II prevented colonization of both artificially implanted exogenous strains of S. mutans, as well as natural recolonization by indigenous S. mutans. The properties of the protective MAb were then investigated and the epitope specificity within the SA I/II molecule was found to be essential but not the isotype specificity of the immunoglobulin (Ig). The requirement for complement activating and the phagocyte binding sites of the Fc fragment of MAb was not essential, as the F(ab')2 fragment of the MAb was as protective as the intact IgG, but the Fab fragment failed to prevent recolonization of S. mutans. Prevention of recolonization was specifically restricted to S. mutans, as the proportion of other organisms, such as S. sanguis, failed to show a significant change. The surprising feature of these experiments was that protection of re-colonization of S. mutans lasted up to 2 years, although MAb was applied for only 3 weeks and functional MAb was detected on the teeth only 3 days following application of the MAb. The long-term protection could therefore not be accounted for by a persistence of MAb on the teeth, but may be due to a shift in the microbial balance in which other bacteria occupy the ecological niche vacated by S. mutans, resulting in colonization resistance to S. mutans. Gene cloning and sequencing the SA from S. mutans, S. sobrinus and S. sanguis identified a conserved region (residues 955-1213) which on Southern hybridization and partial DNA sequence analysis was also found in 19 alpha-haemolytic oral streptococci. The results suggest that the SA molecule may constitute a family of adhesins in oral alpha haemolytic streptococci.(ABSTRACT TRUNCATED AT 400 WORDS)

Molecular, immunological and functional characterization of the major surface adhesin of Streptococcus mutans

In the 15 years since the last major NIH conference that dealt with anti-caries vaccines, we have learned much. Certainly, whole bacteria or bacterial fractions may not be proper immunogens due to the possibility of inducing tissue cross-reactivity. Our own experience (van de Rijn et al., 1976) illustrates that pitfall. But even in the era of genetically engineered vaccines, we first must understand the biological functions of our chosen immunogen before employing that pure protein in a vaccine. Our recent work (Brady et al., 1991c) indicates that antigen P1, a ubiquitous protein found on several oral streptococci, may possess different, but possibly overlapping, functional domains influencing reactions with fluid-phase salivary agglutinin (aggregation) versus fixed agglutinin (adherence). A proper vaccine would induce antibodies against the latter domain(s) thereby retarding colonization. An improper vaccine that induces antibodies against aggregation-related domains on P1 would lessen the host's ability to clear those bacteria from the oral cavity. After carefully identifying appropriate functional domains and obtaining sub-clones of the larger gene that yield truncated polypeptides typical of adherence-specific regions that are also immunogenic, we may be in a position to create the most effective vaccine. In studies employing the polymerase chain reaction (PCR) and standard cloning procedures, we have already begun to produce such polypeptides. Once a library of polypeptides is assembled, they may be tested for functional activity and for lack of induction of cross-reactivity with nonpathogenic streptococci (i.e., S. gordonii). Certain of these recombinant-specified polypeptides could serve as the basis for an anti-caries vaccine. Alternatively, peptides may be synthesized that resemble these sub-molecular regions for use in a vaccine or as competitive inhibitors of adherence but not aggregation. Clearly, a vaccine against dental caries remains a real possibility for the future.

Isolation of a human T cell line specific for a streptococcal cell surface antigen

A streptococcal cell surface antigen of Mr 185,000 (SAI/II) expressed by Streptococcus mutans has previously been well characterised. A T cell line specific for native SAI/II has been isolated from peripheral blood mononuclear cells (PBMC) of a naturally sensitised normal individual. This line has been maintained in culture for several months and was shown to be highly specific, not only for different preparations of native antigen but also for recombinant SAI/II protein. It did not respond to a homologous antigen SpaA (Mr 210,000), extracted from Strep. sobrinus. The phenotype of the line was CD3+ CD4+ CD8- TcR alpha beta +. HLA typing and inhibition studies showed that the response was restricted by both DR and DP encoded class II.

Immunological cross-reactivity between Streptococcus mutans and human heart tissue examined by cross-immunization experiments

Hyperimmunization of rabbits with Streptococcus mutans or other related cariogenic streptococci sometimes induces serum antibodies that react with human heart muscle. To determine whether antigen I/II (AgI/II), a major surface protein antigen present in most human isolates of these organisms, was responsible for inducing cross-reactive antibodies, we tested it for antigenic similarity to heart components, exploiting the ability of immune systems to mount anamnestic responses to antigens previously encountered. Mice immunized with a strain of Streptococcus pyogenes type M6, known to be heart cross-reactive, or with intact S. mutans cells developed antibodies that could be detected on a human heart sarcolemmal preparation. However, mice immunized with AgI/II and boosted with sarcolemma were unable to develop significant antisarcolemmal antibodies attributable to prior sensitization by AgI/II. Similarly, AgI/II was unable to recall antisarcolemmal responses in mice previously immunized with sarcolemma. Nevertheless, strong immunoglobulin G antibody responses to AgI/II were detected at the single-cell level in spleens and as circulating antibodies in all mice immunized with AgI/II or AgI/II-bearing S. mutans. We conclude that the ability of S. mutans to induce heart-reactive antibodies is not due to antigenic similarity between AgI/II and components of human heart but may be caused by other cross-reactive antigens in the bacterial cells or by nonspecific stimulation of the immune system.

An investigation into the mechanism of protection by local passive immunization with monoclonal antibodies against Streptococcus mutans

Local oral passive immunization with Streptococcus mutans-specific monoclonal antibody (MAb) (Guy's 13) prevented recolonization by indigenous S. mutans in human volunteers who had first been treated with a conventional antibacterial agent (chlorhexidine). The F(ab')2 fragment of the MAb was as protective as the intact immunoglobulin G, but the Fab fragment of the molecule failed to prevent recolonization of S. mutans. In subjects receiving the MAb Fab fragment, S. mutans levels in dental plaque and saliva reappeared at a similar rate to that found in sham-immunized subjects who received either saline or a nonprotective MAb. In vitro, MAb had no bacteriostatic or bacteriocidal effect on S. mutans. However, S. mutans grown in the presence of either intact immunoglobulin G MAb or the F(ab')2 fragment formed very long chains, which resulted in clumping of the cells. S. mutans grown with either saline or the MAb Fab fragment formed significantly shorter chains, more characteristic of streptococcal growth in liquid media. The results suggest that the two binding sites of the MAb molecule may be an essential feature for preventing streptococcal colonization but that the ability to bind to phagocytes and activate complement which resides in the Fc fragment is not essential. Protection against colonization by S. mutans lasting up to 2 years was observed in immunized subjects, although MAb was applied over a period of only 3 weeks. Furthermore, functional MAb was detected up to 3 days following application of MAb to the teeth. The long-term protection could not be accounted for by a persistence of MAb on the tooth surface, and we have suggested that it may be due to a shift in the balance of the oral flora which discouraged recolonization by S. mutans. However, examination of the proportions of Streptococcus sanguis and veillonella species in the recolonization experiments failed to reveal a significant change in the proportions of either organism, which returned to approximately the preexperimental levels in both the immunized and control groups. These findings confirm the in vivo functional specificity of the MAb to S. mutans but are not consistent with the suggestion that S. sanguis or veillonella take over the niche vacated by S. mutans, unless the shift in the proportion of these organisms cannot be detected by the method used.

Regions of the Streptococcus sobrinus spaA gene encoding major determinants of antigen I

Surface protein antigen A (SpaA), also called antigen B, antigen I/II, or antigen P1, is an abundant cell envelope protein that is the major antigenic determinant of Streptococcus sobrinus and other members of the Streptococcus mutans group of cariogenic bacteria. This laboratory has previously reported the cloning and expression in Escherichia coli of a BamHI restriction fragment of S. sobrinus DNA containing most of the spaA gene (pYA726) and encoding antigen I. Regions of spaA encoding immunodeterminants of antigen I were analyzed by either deletion mapping or expressing selected restriction fragments from the trc promoter. SpaA proteins produced by mutants harboring nested deletions, constructed by BAL 31 exonuclease treatment at a unique SstI site located towards the 3' end of the gene, were examined by Western immunoblot with rabbit serum against SpaA from S. sobrinus. Only SpaA polypeptides larger than 56 kilodaltons reacted with anti-SpaA serum. Various restriction fragments of the region of spaA encoding the antigenic determinants were cloned into an expression vector. The immunoreactive properties of the polypeptides encoded by those fragments indicated that expression of the immunodominant determinant required topographically assembled residues specified by noncontiguous regions located within 0.48-kilobase PvuII-to-SstI and 1.2-kilobase SstI-to-HindIII fragments which were adjacent on the spaA map.

Streptococcus mutans and the problem of heart cross-reactivity

Investigations of immune responses to Streptococcus mutans have fostered consideration of vaccination as a possible preventive measure against dental caries. However, the finding that hyperimmune rabbit antisera to S. mutans sometimes give immunofluorescent reactions on human heart raised concerns over safety, especially as most individuals display circulating antibodies to this common oral organism. Recent progress in elucidating the molecular mechanisms of the well-established immunological cross-reactivity between group A streptococci and human heart tissue and the structure of S. mutans antigens permits a re-evaluation of this problem. This review examines the evidence for heart cross-reactivity induced by S. mutans in relation to studies on group A streptococci and current understanding of autoimmunity. Although the mechanisms involved in this phenomenon need further clarification, it now appears that it cannot be ascribed to antigenic similarity between heart tissue and a high-molecular-weight surface protein antigen of S. mutans.

Surface hydrophobicity, adherence, and aggregation of cell surface protein antigen mutants of Streptococcus mutans serotype c

The pac gene of the serotype c strain Streptococcus mutans MT8148 encodes a cell surface protein antigen (PAc) of approximate 190 kilodaltons. The serotype c strain S. mutans GS-5 does not produce the 190-kilodalton PAc but produces a lower-molecular-weight protein that reacts with anti-PAc serum. The SphI-BamHI fragment of the pac gene was ligated with the S. mutans-Escherichia coli shuttle vector pSA3. The chimeric shuttle vector was transformed into strain GS-5, and two transformants (TK15 and TK18) were isolated. These transformants produced a large amount of cell-free and cell-bound PAc of 190 kilodaltons. No plasmid was isolated from these transformants, and the EcoRI fragments of their chromosomal DNA hybridized with the erythromycin resistance gene in the shuttle vector DNA, indicating insertion of the chimeric shuttle vector DNA into the chromosomal DNA. The cell hydrophobicity of strains TK15 and TK18 as well as PAc-defective mutants constructed by inserting an erythromycin resistance gene into the pac gene of strain MT8148 was analyzed. Strains MT8148, TK15, and TK18 were hydrophobic. On the other hand, strain GS-5 and PAc-defective MT8148 transformants were hydrophilic. Resting cells of the hydrophobic strains attached in larger numbers to saliva-coated hydroxyapatite than did the hydrophilic strains. Human whole saliva induced the aggregation of cells of the hydrophobic strains but not that of cells of the hydrophilic strains. These results suggest that cell surface PAc of S. mutans serotype c participates in attachment of the streptococcal cell to experimental pellicles.

Sequencing and characterization of the 185 kDa cell surface antigen of Streptococcus mutans

The gene spa P (formerly designated as spa P1) encoding the Mr 185,000 surface antigen I/II of Streptococcus mutans, serotype c (strain NG5) has been sequenced. The deduced amino acid sequence of antigen I/II (1561 residues) includes a putative signal peptide (residues 1-38), as well as a transmembrane region (residues 1537-1556). The N-terminal part of the protein (residues 39-550) is particularly rich in alanine and includes three tandem repeats of a sequence of 82 residues. This region is predicted to be alpha-helical, adopting a coiled-coil formation, and may account for the cell surface hydrophobicity associated with expression of antigen I/II. In contrast the C-terminal region (residues 800-1549) is proline-rich, favouring an extended conformation. Comparison with the sequence determined from Strep. mutans strain MT8148 showed that antigen I/II is highly conserved with the exception of a short central region (residues 750-805). N-terminal sequencing of purified antigens I and II components indicated that antigen I extends from the amino-terminus of the intact Mr 185,000 surface antigen while antigen II extends from residue 996.

Prevention of colonization of Streptococcus mutans by topical application of monoclonal antibodies in human subjects

Topical application of Streptococcus mutans-specific MAbs was examined as a means of preventing colonization in the mouth. The MAbs recognize the cell surface antigen (SA I/II) of Strep. mutans and also bind to the intact bacterial cell. In experiments with implantation of an exogenous Strep. mutans strain, specific MAb significantly reduced the levels of implantation when compared with that in controls given a non-specific MAb. In a second investigation, the effect of MAb on recolonization by indigenous Strep. mutans was studied in subjects after treatment with a topical antibacterial mouthwash (chlorhexidine), which had decreased Strep. mutans to undetectable levels. In control subjects, the indigenous Strep. mutans started to recolonize within days, but those receiving MAb remained free of Strep. mutans for up to 2 yr. By using different Strep. mutans-specific MAbs, it was found that the serotype and epitope specificities of the IgG MAb were essential, but that the IgG sub-class may not be important. No clinical side-effects were detected in any of the subjects receiving MAb and there was no evidence of serum, salivary or gingival fluid antibody responses against the antibody. Local passive immunization with MAbs is a safe method for preventing colonization of Strep. mutans in the mouth.

Identification of T- and B-cell epitopes in synthetic peptides derived from a Streptococcus mutans protein and characterization of their antigenicity and immunogenicity

Natural immunity to synthetic peptides (SP) derived from the sequences of a 3800 Mr Streptococcus mutans antigen was found in human subjects. Significant serum IgG antibodies were detected both to the native streptococcal antigen and to the SP17, containing essentially residues 1-15. A series of short peptides with deletions at the amino- and carboxy-termini were then tested to identify the B-cell epitopes. Residues 8-13 and 1-6 bound significant serum IgG antibodies but only the former consistently inhibited human antibodies, suggesting that residues 8-13 constitute a major B-cell epitope. The human CD4 subset of T-cells was then examined and this showed a significant uptake of [3H]-thymidine when stimulated with both the native streptococcal antigen and the SP17. The series of short peptides was then used to stimulate CD4 cells, in order to determine the T-cell epitope. The synthetic peptide with residues 6-15 was the shortest peptide that stimulated significant [3H]-thymidine uptake and this peptide was designated as a T-cell epitope. The immunogenicity and antigenicity of SP17 was also investigated in macaques. Immunization of monkeys with the free SP17 failed to elicit serum antibodies or T-cell responses. However, immunization with SP17 linked to tetanus toxoid as a carrier elicited serum antibodies and proliferative responses of lymphocytes, not only to the synthetic peptide but also to the native streptococcal antigen. As in the human studies a B-cell epitope was found in residues 8-13, whereas an overlapping T-cell epitope was located in residues 7-15.(ABSTRACT TRUNCATED AT 250 WORDS)

Sequence analysis of the cloned streptococcal cell surface antigen I/II

The gene spa P (formerly designated as spa P1) encoding the Mr 185,000 surface antigen (I/II) of Streptococcus mutans, serotype c (NG5), has been sequenced. The gene (4683 bp) encodes a protein of 1561 amino acid residues including putative signal peptide (residues 1-38) and transmembrane (residues 1537-1556) sequences. The N-terminal region (60-550) has alanine-rich repeats and is predicted to be alpha-helical. However, the C-terminal region (800-1540) is proline-rich and favours an extended structure. Except for a short central variable region the sequences appear to be highly conserved for S. mutans serotype c. N-Terminal sequencing of separated antigen I and antigen II polypeptides suggests that the former represents the N-terminal and the latter the C-terminal portions of the intact antigen.

Specificity of monoclonal antibodies in local passive immunization against Streptococcus mutans

Local oral passive immunization in human subjects with a monoclonal antibody (MoAb) raised against the 185-kD antigen I/II from S. mutans significantly reduced or prevented oral colonization of an exogenous strain of the organism. In subjects sham-immunized with either saline or an unrelated MoAb, however, significantly greater proportions of S. mutans persisted for a longer duration than in those immunized with the specific anti-streptococcal MoAb. Recolonization of indigenous S. mutans after this organism was reduced to undetectable levels by an antimicrobial agent has also been completely prevented with specific MoAb. Indeed, S. mutans was not detected for a period of over 1 year, as compared with recolonization within 10-82 days in the control subjects. The specificity of MoAb in preventing colonization of the streptococci was studied with four MoAb. This revealed that: (1) the sub-class of antibody is not an essential factor, as both MoAb Guy's 1 and 13 prevented colonization, although Guy's 1 is an IgG2a and Guy's 13 is an IgG1 class of antibody; (2) serotype specificity is important, as MoAb Guy's 9, which only recognizes S. sobrinus (serotypes d and g), does not prevent colonisation by S. mutans (serotype c); (3) neither protein nor carbohydrate nature of the putative adhesin was a determining factor, because MoAb Guy's 1 recognizes a carbohydrate and Guy's 13 a protein determinant and both MoAb prevented adherence of S. mutans; and (4) epitope specificity appears to be the most important factor in preventing adherence of S. mutans, as MoAb Guy's 11 and 13 share the same serotype specificity and both recognize a protein determinant, yet only Guy's 13 prevents colonisation. The long duration of protection from re-colonization by indigenous S. mutans, lasting about 1 year after application of the specific MoAb was stopped, cannot be accounted for by functional MoAb remaining on the teeth. We suggest that initially the MoAb prevents colonization by S. mutans and that the ecological niche vacated by this streptococcus is filled by other organisms from the oral flora, thereby discouraging re-colonization by S. mutans.

Characterisation of monoclonal antibodies to common protein epitopes on the cell surface of Streptococcus mutans and Streptococcus sobrinus

Three monoclonal antibodies (MAb) were prepared against a cell surface antigen which cross-react between Streptococcus mutans (serotypes c, e and f) and Streptococcus sobrinus (serotypes d and g). Two of the MAb also recognise a determinant on the surface of Streptococcus cricetus (serotype a). The common antigen shared between S. mutans and S. sobrinus was demonstrated by Western blotting to be about 200 kD in size. This antigen is shared not only by the cell surfaces of serotypes a, c, d, e, f and g, but also by the major cell surface antigen of S. mutans of 185 kD and another of 150 kD. These MAb identify all but one mutans type of streptococci and can be utilised as analytical reagents.

Interaction between surface protein antigens of Streptococcus mutans and human salivary components

The potential involvement of surface antigens (Ags) I/II and III of Streptococcus mutans in its adherence to salivary pellicle-coated tooth surfaces was investigated. The binding of radiolabelled Ag I/II to hydroxyapatite was increased by pretreating the mineral with human parotid saliva, and binding was maintained in the continuous presence of saliva. Binding of Ag III to hydroxyapatite was inhibited by pretreatment with, or in the presence of, saliva. Various aminohexoses, and also tris, inhibited the binding of Ag I/II. When Ags I/II and III were tested for their ability to bind to salivary components separated by SDS gel electrophoresis, several proteins capable of binding Ag I/II were identified, notably 2 proteins of apparent relative molecular mass 28,000 and 38,000. Analysis of these proteins, isolated by micro-preparative electrophoresis, indicated high proportions of proline, glycine, and glutamic acid, and overall compositions similar to basic proline-rich salivary proteins.

Proteolysis of the 185,000 MW streptococcal cell wall antigen generating 4000 and 6000 MW peptides with distinct antigenic determinants

A 185,000 MW glycoprotein antigen derived from Streptococcus mutans was digested with subtilisin. Purification by reversed phase high-powered liquid chromatography (HPLC) resulted in a homogeneous 20,000 MW protein which possesses the streptococcal antigen (SA) I and II determinants. This protein was immunogenic in mice both for the 20,000 MW and the native 185,000 MW SA. Further proteolysis with subtilisin generated four peptides of 18,000, 10,000, 6000 and 4000 MW. Whereas the 20,000, 18,000 and 10,000 MW peptides contained both SAI and SAII determinants, the 4000 MW peptide possessed only the SAI and the 6000 MW peptide the SAII determinant. The 4000-SAI peptide is of special significance, as the smallest SAI material separated in the past was 150,000 MW. This was difficult to purify and proved to be protective against dental caries on immunization of rhesus monkeys.

Isolation and characterization of monoclonal antibodies specific for antigen P1, a major surface protein of mutans streptococci

A panel of 15 murine monoclonal antibodies (MAbs; 14 immunoglobulin G1, 1 immunoglobulin G2a) directed against antigen P1, a major surface protein of mutans streptococci, was prepared. All of these MAbs reacted by the enzyme-linked immunosorbent assay with solubilized wall material from Streptococcus mutans Ingbritt 175 (a serotype c strain which retains significant amounts of P1 in its cell wall), culture supernatant fluid from Ingbritt 162 (a strain which excretes large amounts of P1 into the culture medium), and purified P1. By Western immunoblotting, these MAbs were observed to react with a high-molecular-weight polypeptide which comigrated with antigen P1. None of these MAbs cross-reacted with human heart tissue or with various eucaryotic proteins. When whole cells of various strains of mutans streptococci were screened against the panel of MAbs, the strongest reactivities were noted with strains of serotype c and e S. mutans, while a serotype f strain of S. mutans, along with S. sobrinus and S. cricetus strains, reacted somewhat more weakly. S. rattus strains were completely negative. Results obtained with bacterial culture supernatants were qualitatively similar. The surface localization of antigen P1 was confirmed by electron microscopy with an indirect immunogold technique. In sectioned S. mutans cells, labeling appeared to be associated with a fibrillar "fuzzy coat" layer, which was far more prominent on cells of Ingbritt 175 than on those of Ingbritt 162.

Use of monoclonal antibodies in local passive immunization to prevent colonization of human teeth by Streptococcus mutans

Local passive immunization with monoclonal antibodies (MAbs) raised against streptococcal antigen (SA) I/II protects monkeys against colonization of teeth by Streptococcus mutans and the subsequent development of dental caries. In this study we extended the preclinical experiments to human subjects. In the first study of eight healthy subjects, four had anti-SA I/II MAb (immunoglobulin G2a [IgG2a]) and four had saline applied to their teeth on three occasions. A streptomycin-resistant S. mutans strain (Guy K2 strain, serotype c) was then implanted onto the teeth, and the organism was cultured sequentially from dental plaque and saliva up to 100 days after the first treatment with MAb. Decreased colonization by S. mutans was found in the dental plaque collected from smooth surfaces and fissures and in saliva of subjects whose teeth were treated with the MAb, as compared with the saline-treated control subjects. The experiment was then repeated on seven new subjects, and the effect of anti-SA I/II MAb was compared with that of an unrelated MAb to Campylobacter jejuni. The results again showed a consistently lower level of colonization of teeth in the anti-SA I/II MAb-treated subjects as compared with those sham immunized with the unrelated MAb. There was little difference in serum IgG, IgM, or IgA, gingival fluid IgG, or salivary IgA anti-SA I/II antibodies between the immunized and sham-immunized subjects, before and after the investigation. No side effects were observed, and the gingival and plaque indices remained unchanged. A sensitive radioimmunoassay failed to detect changes in anti-MAb (IgG2a) antibodies in any of the three fluids examined. We suggest that local passive immunization by means of MAb might be an alternative approach in the prevention of colonization of teeth by S. mutans and the development of dental caries.

Analysis of heart-reactive antibodies induced in rabbits by immunization with Streptococcus mutans

The induction of antibodies that react with human heart muscle in rabbits immunized with Streptococcus mutans and related organisms has been investigated in order to re-examine the reported antigenic cross-reactivity between the major cell wall protein antigen I/II of S. mutans and heart tissue. Serum from 2 of 8 rabbits hyperimmunized with whole cells of S. mutans, S. sobrinus, S. rattus, or S. mitis, assayed by immunofluorescence and radioimmunoassay, revealed antibodies reactive with sarcolemma that could be attributed to immunization. Antisera raised against several crude and purified antigens of S. mutans did not reveal heart-reactive antibodies, that could be attributed to antigenic similarity between heart and these antigens. An assay designed to detect antibodies that could simultaneously bind to both heart and antigen I/II of S. mutans failed to reveal cross-reactivity due to this antigen, although antibodies separately expressing these binding activities were demonstrable. IgM rheumatoid factor was elevated in several antisera to S. mutans, and could have contributed to some previous observations ascribed to antigenic cross-reactivity between S. mutans and human heart muscle.

Detection and specificity of antibodies secreted by spleen cells in mice immunized with Streptococcus mutans

Immune responses of mice to Streptococcus mutans serotype c were analyzed by means of the enzyme-linked immunospot assay to determine the predominant specificities of the antibodies developed. In general, the numbers of splenic antibody-secreting cells correlated with serum antibody levels. A low dose (10(8) CFU) of killed whole cells injected twice intraperitoneally induced antibodies mainly against surface protein antigen I/II. A higher dose (10(9) CFU) given two to six times also resulted in a predominance of antigen I/II antibody-secreting cells and, in addition, antibody responses to surface protein antigen III and lipoteichoic acid occurred. Cells producing antibodies to serotype c polysaccharide were elicited only on repeated immunization. These results agreed with the development of antibodies in rabbits repeatedly immunized intravenously with killed whole cells of S. mutans, S. rattus, and S. sobrinus, which induced specific antibodies in accordance with the surface antigens that they express. Mice immunized twice with the same dose of purified antigens I/II and III developed greater numbers of antigen I/II splenic antibody-forming cells than antigen III splenic antibody-forming cells and higher serum antibody levels to antigen I/II than to antigen III. Furthermore, a single injection of antigen I/II but not of antigen III was sufficient to induce a strong specific-antibody response. Some evidence was also obtained for weak polyclonal stimulation of spleen cells by S. mutans cells and by antigen I/II, a result which could be relevant to the induction by S. mutans of antibodies reactive with mammalian tissues. It was concluded that for the antigens examined, S. mutans elicited the strongest antibody response against antigen I/II, which was also highly immunogenic in purified form.

Local active gingival immunization by a 3,800-molecular-weight streptococcal antigen in protection against dental caries

Local gingival immunization was attempted in an effort to confine the immune response to the oral cavity and bypass the systemic immune response. A low-molecular-weight (3.8K) streptococcal antigen (SA) I/II was applied 10 times over a period of 1 year to the gingival crevices of rhesus monkeys. The antigen was maintained in situ by means of silicone rubber appliances. Serial examinations over a period of 1 year showed that topical gingival immunization with the 3.8K SA results in a significantly lower incidence of dental caries and colonization of Streptococcus mutans compared with that of the sham-immunized controls. This was associated with an increase in gingival crevicular immunoglobulin G and salivary immunoglobulin A anti-SA I/II antibodies, whereas no change occurred in serum antibodies to SA I/II. The immune mechanism which prevents the colonization of S. mutans and the development of caries may involve antibodies that prevent the adherence of S. mutans to the teeth and facilitate phagocytosis and killing by the local neutrophils. This novel route of local immunization is noninvasive, does not cause side effects, and bypasses systemic immunization.

Salivary immunoglobulin A and serum antibodies to Streptococcus mutans ribosomal preparations in dental caries-free and caries-susceptible human subjects

Caries-free subjects or individuals with low caries susceptibility exhibited significantly higher (P less than 0.001) levels of naturally occurring salivary immunoglobulin A (IgA) and serum IgG, IgA, and IgM antibodies to a Streptococcus mutans ribosomal preparation than subjects with high caries susceptibility. Absorption of saliva and serum samples with S. mutans ribosomal preparations, but not with other S. mutans antigens or with Escherichia coli and Neisseria gonorrhoeae ribosomal preparations, removed the antibody activity. Absorption with Streptococcus sanguis ribosomes and NH4Cl-washed S. mutans ribosomes partially removed the anti-S. mutans ribosome antibody activity. These results provide evidence that naturally occurring salivary and serum antibodies to the S. mutans ribosomal preparation correlate with susceptibility to dental caries.

Purification and immunochemical properties of a protein antigen from serotype g Streptococcus mutans

A proteinaceous antigen (PAg) was purified from the culture supernatant of Streptococcus mutans 6715 (serotype g) by ultrafiltration, ammonium sulfate precipitation, DEAE-Sephacel ion-exchange chromatography, Phenyl-Sepharose CL-4B hydrophobic chromatography, and subsequent Sephacryl S-300 gel filtration. A yield of 0.1 mg of PAg was obtained from a liter of culture supernatant. The isoelectric point and molecular weight of PAg were pH 4.6 and 210,000, respectively. It contained 35% sugar, which was identified as glucose by gas-liquid chromatography. Amino acid analysis revealed that PAg contains 28% acidic and 11% basic amino acid residues. PAg retained its antigenicity after heating at 80 C for 10 min in deionized water, or after treatment with 0.1 M HC1 or 0.1 M NaOH at 37 C for 1 hr. Immunodiffusion and immunoelectrophoresis analyses revealed that PAg is serologically distinct from other cell-surface antigens such as serotype-specific polysaccharide and lipoteichoic acid. A cross-reaction between PAg and a protein antigen similarly prepared from serotype c S. mutans was observed in immunodiffusion tests.

Local passive immunization by monoclonal antibodies against streptococcal antigen I/II in the prevention of dental caries

Local passive immunization with monoclonal antibodies (Mc Ab) to Streptococcus mutans was attempted as an alternative approach to active systemic immunization. We prepared an immunoglobulin G class Mc Ab to the cell surface protein determinant of streptococcal antigen I/II and applied it repeatedly to the teeth of rhesus monkeys. This resulted in decreased colonization by S. mutans in fissures and smooth surfaces of teeth and no dental caries, unlike the results in control animals, which developed caries and showed a high proportion of S. mutans on their teeth. There was no significant difference in serum, salivary, or gingival fluid antibodies to S. mutans between the two groups of animals. Any objections raised over systemic immunization inducing cross-reactive antibodies are therefore overcome by local passive immunization. The mechanism of prevention of colonization has not been established, but we postulate that the Mc Ab which is directed against an important cell surface antigenic determinant of S. mutans (streptococcal antigen I/II) prevents adherence of S. mutans to the acquired pellicle on the tooth surface. S. mutans reacts with the Mc Ab and becomes opsonized, phagocytosed, and killed by the local gingival traffic of neutrophils.

Characterization of monoclonal antibodies to Streptococcus mutans antigenic determinants I/II, I, II, and III and their serotype specificities

Monoclonal antibodies (McAb) were developed to four protein components of Streptococcus mutans serotype c, some of which are significant in the protection against dental caries. The six McAb used in this investigation support the identities of streptococcal antigens (SA) I/II, I, II, and III. The specificities of these antigenic determinants were established both by direct binding and inhibition with the pure SA with a solid-phase radioassay. Whereas conventional antisera to S. mutans serotype c cross-react with serotypes c, e, and f (and g), McAb to serotype c-derived SA I/II react predominantly with serotype c and show some low-titer reactivity with serotype f. The slight cross-reactivity between S. mutants cells of serotypes c and f could be further differentiated by absorption of any of the three McAb to SA I/II with cells of serotype c. Parallel studies of McAb with cells of S. mutans and their ammonium sulfate-precipitated culture supernatants suggest that some SA determinants are retained predominantly on the cell surface, but others are readily shed into the culture medium, so that they are detected both on the cell surface and culture medium. Unlike polyclonal antibodies, McAb are capable of discriminating single antigenic determinants and can be applied to the study of shedding of antigens from microorganisms into the environment, such as the gut or gingival sulcus.

Cell surface proteins of oral streptococci

Whole cells of representative strains of oral streptococci (Streptococcus sanguis, S. mitis, and S. salivarius) were radiolabeled by the lactoperoxidase method of radioiodination. The labeled polypeptides obtained by extraction of whole cells with boiling sodium dodecyl sulfate were analyzed by polyacrylamide gel electrophoresis and autoradiography. Of the total radioactivity, ca. 70% was released by treating whole cells with trypsin, suggesting that the labeling was confined to proteins located on the cell surface. Most S. sanguis strains studied gave a characteristic banding pattern consisting of a high-molecular-weight (120,000 [120K] to 63K) group of six proteins. Three low-molecular-weight (12K, 16K, and 18K) proteins were also detected in many strains.

Biochemical and immunological differences between hydrophobic and hydrophilic strains of Streptococcus mutans

Hydrophobic strains of Streptococcus mutans were compared with paired variants showing reduced hydrophobicity. Extracts of hydrophobic cells contained a number of high-molecular-weight proteins which were not present on cells with decreased hydrophobicity. The proteins were found in purified cell walls, suggesting that they are located on the bacterial surface. Trypsin treatment of whole cells destroyed the proteins and reduced the hydrophobicity. Chemical analysis did not reveal any marked differences in the proportion of cell wall constituents. The amino acid compositions and lipoteichoic acid contents of hydrophobic and hydrophilic cell walls were similar. Culture supernatants from the hydrophilic variants contained high-molecular-weight proteins similar to those extracted from the cell walls of the hydrophobic parent strains, indicating that the variants were impaired in their ability to incorporate the hydrophobicity-associated proteins into the cell wall. The dominant protein had a molecular weight of 190,000, similar to that of antigen I/II (B) of S. mutans.