Streptococcus mutans and the problem of heart cross-reactivity

Lab-Test(®) 4: Dental caries and bacteriological analysis

Dental caries is one of the most common infectious ultifactorial diseases worldwide, characterized by the progressive demineralization of the tooth, following the action of bacterial acid metabolism. The main factors predisposing the onset of the carious process are: 1) the presence of bacterial species able to lower the pH until critical values of 5.5, 2) the absence of adequate oral hygiene, 3) an inefficient immune response anti-caries, 4) the type of alimentary diet and 5) the structure of the teeth. Among the 200 bacterial species isolated from dental plaque the most pathogenic for dental caries are: Streptococcus mutans, Streptococcus sobrinus, Lactobacillus acidophilus, Actinomices viscusus and Bifidobacterium dentium. Our laboratory (LAB^® s.r.l., Codigoro, Ferrara, Italy) has developed a test for absolute and relative quantification of the most common oral cariogenic bacteria. The test uses specific primers and probes for the amplification of bacteria genome sequences in Polymerase Chain Reaction Real Time. The results provide a profile of patient infection, helpful for improving the diagnosis and planning of preventive treatment to reduce the bacterial load.

Passive immunization against dental caries and periodontal disease: development of recombinant and human monoclonal antibodies

Indigenous micro-organisms in the oral cavity can cause two major diseases, dental caries and periodontal diseases. There is neither agreement nor consensus as to the actual mechanisms of pathogenesis of the specific virulence factors of these micro-organisms. The complexity of the bacterial community in dental plaque has made it difficult for the single bacterial agent of dental caries to be determined. However, there is considerable evidence that Streptococcus mutans is implicated as the primary causative organism of dental caries, and the cell-surface protein antigen (SA I/II) as well as glucosyltransferases (GTFs) produced by S. mutans appear to be major colonization factors. Various forms of periodontal diseases are closely associated with specific subgingival bacteria. Porphyromonas gingivalis has been implicated as an important etiological agent of adult periodontitis. Adherence of bacteria to host tissues is a prerequisite for colonization and one of the important steps in the disease process. Bacterial coaggregation factors and hemagglutinins likely play major roles in colonization in the subgingival area. Emerging evidence suggests that inhibition of these virulence factors may protect the host against caries and periodontal disease. Active and passive immunization approaches have been developed for immunotherapy of these diseases. Recent advances in mucosal immunology and the introduction of novel strategies for inducing mucosal immune responses now raise the possibility that effective and safe vaccines can be constructed. In this regard, some successful results have been reported in animal experimental models. Nevertheless, since the public at large might be skeptical about the seriousness of oral diseases, immunotherapy must be carried out with absolute safety. For this goal to be achieved, the development of safe antibodies for passive immunization is significant and important. In this review, salient advances in passive immunization against caries and periodontal diseases are summarized, and the biotechnological approaches for developing recombinant and human-type antibodies are introduced. Furthermore, our own attempts to construct single-chain variable fragments (ScFv) and human-type antibodies capable of neutralizing virulence factors are discussed.

Immunization against dental caries

Dental caries is one of the most common infectious diseases. Of the oral bacteria, mutans streptococci, such as Streptococcus mutans and S. sobrinus, are considered to be causative agents of dental caries in humans. There have been numerous studies of the immunology of mutans streptococci. To control dental caries, dental caries vaccines have been produced using various cell-surface antigens of these organisms. Progress in recombinant DNA technology and peptide synthesis has been applied to the development of recombinant and synthetic peptide vaccines to control dental caries. Significant protective effects against dental caries have been shown in experimental animals, such as mice, rats and monkeys, which have been subcutaneously, orally, or intranasally immunized with these antigens. Only a few studies, however, have examined the efficacy of dental caries vaccines in humans. Recently, local passive immunization using murine monoclonal antibodies, transgenic plant antibodies, egg-yolk antibodies, and bovine milk antibodies to antigens of mutans streptococci have been used to control the colonization of the organisms and the induction of dental caries in human. Such immunization procedures may be a safer approach for controlling human dental caries than active immunization.

A vaccine against dental caries: an overview

Dental caries continues to be a costly and prevalent oral disease. Research efforts towards developing a well tolerated and effective vaccine against dental caries were initiated following the demonstration of a specific bacterial aetiology for this disease. The cariogenic mutans streptococci are the principal bacteria causing this disease. Specific immune defence against these bacteria is provided mainly by secretory immunoglobulin (Ig) A antibodies present in saliva, which are generated by the common mucosal immune system. Progress in the development of a vaccine against dental caries has increased due to both advancements in molecular biology and our understanding of the mucosal immune system and mucosal vaccines. Advancements in molecular biology have facilitated the cloning and functional characterisation of virulence factors of the mutans streptococci, including the cell-surface fibrillar proteins, which mediate adherence to the tooth surface, and the glucosyltransferase enzymes, which synthesise adhesive glucans and allow microbial accumulation on the teeth. Current strategies for immunisation against dental caries are using these virulence factors as key antigens and incorporating them into novel mucosal vaccine systems and delivering them with or without adjuvants to mucosal IgA inductive sites. The most popular routes of mucosal immunisation are via the oral or nasal route. The mucosal immune system is functional in newborn infants, who develop salivary IgA antibodies as they become colonised by oral micro-organisms. Mucosal immunisation strategies result in the induction of salivary IgA antibody responses and pose fewer problems than parenteral injection of antigen. Therefore, mucosal immunisation of infants prior to the appearance of their first teeth may be a well tolerated and effective way to induce immunity against the colonisation of teeth by mutans streptococci and protection against subsequent dental caries. The purpose of this article is to provide an overview of the recent progress on the development of a vaccine against infection by Streptococcus mutans for the prevention of dental caries, with emphasis on the mucosal immune system and vaccine design.

Characterization of the mucosal immune response in breast milk after peroral immunization of chimpanzees (Pan troglodytes) with Streptococcus mutans

The characteristics of the mucosal immune response to Streptococcus mutans cells, antigen A, antigen B, glucosyltransferases and glucan-binding proteins were examined in four pregnant chimpanzees that had been immunized perorally with Strep. mutans. Six pregnant chimpanzees served as non-immunized controls. None of the chimpanzees harbored S. mutans. Samples of milk were collected from all animals throughout the experiment. Peroral immunization resulted in an overall 17-fold median increase in SIgA in milk. Although SIgA1 comprised almost two-thirds of milk SIgA, Strep. mutans whole-cell antibody activity was contained predominantly in the SIgA2 subclass. The difference between the specific activities of anti-Strep. mutans SIgA1 and SIgA2 antibodies compared over time reached the borderline of statistical significance (p = 0.08). The avidity of anti-Strep. mutans antibodies was low in three of four chimpanzees and there was no evidence of affinity maturation. SIgA antibodies from the milk of all four immunized chimpanzees recognized antigen A. In three animals these antibodies were restricted to the SIgA1 subclass and, in one animal, anti-A antibodies were confined to SIgA2. Antibodies from all of the immunized chimpanzees recognized degradation products of antigen B in both the SIgA1 and the SIgA2 subclasses. Only two of four immunized chimpanzees responded to glucosyltransferases and these antibodies were restricted to the SIgA1 subclass. None of the chimpanzees responded to the 74-kDa glucan-binding protein. However, three animals produced SIgA1 antibodies against the 59-kDa glucan-binding protein and two of these also produced SIgA2 antibodies against this protein.

Current status of a mucosal vaccine against dental caries

The evidence of a specific bacterial cause of dental caries and of the function of the salivary glands as an effector site of the mucosal immune system has provided a scientific basis for the development of a vaccine against this highly prevalent and costly oral disease. Research efforts towards developing an effective and safe caries vaccine have been facilitated by progress in molecular biology, with the cloning and functional characterization of virulence factors from mutans streptococci, the principal causative agent of dental caries, and advancements in mucosal immunology, including the development of sophisticated antigen delivery systems and adjuvants that stimulate the induction of salivary immunoglobulin A antibody responses. Cell-surface fibrillar proteins, which mediate adherence to the salivary pellicle, and glucosyltransferase enzymes, which synthesize adhesive glucans and allow microbial accumulation, are virulence components of mutans streptococci, and primary candidates for a human caries vaccine. Infants, representing the primary target population for a caries vaccine, become mucosally immunocompetent and secrete salivary immunoglobulin A antibodies during the first weeks after birth, whereas mutans streptococci colonize the tooth surfaces at a discrete time period that extends around 26 months of life. Therefore, immunization when infants are about one year old may establish effective immunity against an ensuing colonization attempts by mutans streptococci. The present review critically evaluates recent progress in this field of dental research and attempts to stress the protective potential as well as limitations of caries immunization.

Control of specific plaque bacteria

The Specific Plaque Hypothesis posits that particular bacteria are of unique importance in the etiology of dental caries and periodontal diseases, and a logical conclusion is that these bacteria should be the targets for our 'magic bullets' in devising plaque-control methods. This paper considers the development of preventive measures based on understanding of the significance of particular bacterial species and the properties of those bacteria. Knowledge of the importance of specific organisms as mediators of disease and molecular studies on the properties of potential virulence factors may reveal potential targets for inhibition, blocking by synthetic analogues, or functional inactivation by antibodies.

Adherence of oral microorganisms to guided tissue membranes: an in vitro study

Microorganisms can adhere and colonize on an exposed guided tissue regeneration (GTR) membrane thus developing a nidus of infection. The purpose of this study was to compare early bacterial adhesion to three different GTR membranes. Expanded polytetrafluoroethylene, polyglactin 910, and collagen were used as the test membranes. In part I of this study 15 different oral microbes were used to compare their relative ability to adhere to the membranes. Six of the most strongly adherent bacteria (Actinomyces viscosus, Actinobacillus actinomycetemcomitans, Porphyromonas gingivalis, Streptococcus mutans, Fusobacterium nucleatum, and Selenomonas sputigena) were selected for part II of this study. The membranes were placed in tubes containing broth cultures containing identical concentrations (1 x 10(8) cells/ml) of these bacteria at 37 degrees C. Membranes were placed in tubes of media without bacteria as controls. At 4, 6, 12, and 24 hours, the bacterial cultures were decanted and the membranes in the tubes were agitated gently in reduced transfer fluid (RTF) 4 times to remove non-adherent bacteria. Each tube was then sonicated for 30 seconds in 10 ml RTF to detach adherent bacteria. The detached adherent bacteria were counted using a Petroff-Hausser chamber. Data were analyzed by using the SAS program. Analysis of variance was used to test for differences between multiple means. Results showed S. mutans had the strongest attachment to the collagen membrane at 4 and 6 hours. Selenomonas sputigena had the lowest adherence capability to all test membranes. Polyglactin 910 had significantly (P < 0.05) lower S. mutants adherence than either the ePTFE or the collagen membrane at 4 and 6 hours.(ABSTRACT TRUNCATED AT 250 WORDS)

Serum antibody responses to Streptococcus mutans antigens in humans systemically infected with oral streptococci

Sera from patients with subacute bacterial endocarditis (SBE) due to Streptococcus mutans or other oral streptococci and from normal subjects were assayed by enzyme-linked immunosorbent assay for antibodies to defined S. mutans antigens. Antibodies of IgG and IgA isotypes to Ag I/II and Ag III were greatly elevated in S. mutans-SBE sera, and the IgA antibodies in 3 sera included both polymeric and monomeric forms. Elevated IgM and IgG anti-lipoteichoic acid and IgG and IgA anti-serotype c polysaccharide antibodies were also found. The sera of 4 of 6 patients infected with other oral streptococci also displayed antibodies to S. mutans Ag I/II. Sera of 3 patients infected with Streptococcus mitis or Streptococcus oralis, but none of the S. mutans-infected cases, showed elevated antibodies to human heart sarcolemma, and all SBE sera had elevated rheumatoid factor. These results suggest that the known surface protein antigens of S. mutans are immunodominant in humans, and are not likely to be heart cross-reactive.

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.

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.