Characterization of the Streptococcus mutans P1 epitope recognized by immunomodulatory monoclonal antibody 6-11A

An intramolecular lock facilitates folding and stabilizes the tertiary structure of Streptococcus mutans adhesin P1

The cariogenic bacterium Streptococcus mutans uses adhesin P1 to adhere to tooth surfaces, extracellular matrix components, and other bacteria. A composite model of P1 based on partial crystal structures revealed an unusual complex architecture in which the protein forms an elongated hybrid alpha/polyproline type II helical stalk by folding back on itself to display a globular head at the apex and a globular C-terminal region at the base. The structure of P1's N terminus and the nature of its critical interaction with the C-terminal region remained unknown, however. We have cocrystallized a stable complex of recombinant N- and C-terminal fragments and here describe a previously unidentified topological fold in which these widely discontinuous domains are intimately associated. The structure reveals that the N terminus forms a stabilizing scaffold by wrapping behind the base of P1's elongated stalk and physically "locking" it into place. The structure is stabilized through a highly favorable ΔG(solvation) on complex formation, along with extensive hydrogen bonding. We confirm the functional relevance of this intramolecular interaction using differential scanning calorimetry and circular dichroism to show that disruption of the proper spacing of residues 989-1001 impedes folding and diminishes stability of the full-length molecule, including the stalk. Our findings clarify previously unexplained functional and antigenic properties of P1.

Alterations in immunodominance of Streptococcus mutans AgI/II: lessons learned from immunomodulatory antibodies

Streptococcus mutans antigen I/II (AgI/II) has been widely studied as a candidate vaccine antigen against human dental caries. In this report we follow up on prior studies that indicated that anti-AgI/II immunomodulatory monoclonal antibodies (MAbs) exerted their effects by destabilizing the native protein structure and exposing cryptic epitopes. We show here that similar results can be obtained by immunizing mice with truncated polypeptides out of the context of an intra-molecular interaction that occurs within the full-length molecule and that appears to dampen the functional response against at least two important target epitopes. Putative T cell epitopes that influenced antibody specificity were identified immediately upstream of the alanine-rich repeat domain. Adherence inhibiting antibodies could be induced against two discrete domains of the protein, one corresponding to the central portion of the molecule and the other corresponding to the C-terminus.

An intramolecular interaction involving the N terminus of a streptococcal adhesin affects its conformation and adhesive function

BACKGROUND

P1 is an adhesin on the surface of Streptococcus mutans.

RESULTS

Destroying the high affinity interaction between the N and C termini of S. mutans P1 creates a non-adherent phenotype.

CONCLUSION

The N terminus facilitates proper folding, function, and stability within recombinant P1.

SIGNIFICANCE

The relationship between folding, maturation, and cell surface assembly is critical to understanding the P1 mechanism of action. The adhesin P1 is localized on the surface of the oral pathogen Streptococcus mutans and facilitates an interaction with the glycoprotein complex salivary agglutinin that is comprised primarily of the scavenger receptor gp340. Recent crystal structures of P1 display an unusual structure in which the protein folds back upon itself to form an elongated hybrid helical stalk with a globular head at the apex and a globular C-terminal region at the base. The N terminus of P1 has not yet been characterized. In this report we describe the contribution of an interaction between the N-terminal and C-terminal portions of the protein that is required for proper function of P1 on the surface of S. mutans. Utilizing recombinant N-terminal and C-terminal fragments, we employed isothermal titration calorimetry and native gel electrophoresis to demonstrate that these fragments form a high affinity and stable complex in solution. Furthermore, circular dichroism and surface plasmon resonance measurements indicated that the N-terminal fragment contributes to the folding and increases the functionality of the C-terminal fragment in trans. Finally, we utilized circular dichroism, surface plasmon resonance, and differential scanning calorimetry to show that an N-terminal 106-amino acid segment within P1 contributes to the proper folding and function of the full-length recombinant molecule and increases the stability of its elongated hybrid helical stalk.

A therapeutic anti-Streptococcus mutans monoclonal antibody used in human passive protection trials influences the adaptive immune response

The adhesin known as Antigen I/II, P1 or PAc of the cariogenic dental pathogen Streptococcus mutans is a target of protective immunity and candidate vaccine antigen. Previously we demonstrated that immunization of mice with S. mutans complexed with anti-AgI/II monoclonal antibodies (MAbs) resulted in changes in the specificity, isotype and functionality of elicited anti-AgI/II antibodies in the serum of immunized mice compared to administration of bacteria alone. In the current study, an anti-AgI/II MAb reported in the literature to confer unexplained long term protection against S. mutans re-colonization following passive immunization in human clinical trials (MAb Guy's 13), and expressed in tobacco plants (MAb Guy's 13 plantibody), was evaluated for its potential immunomodulatory properties. Immunization of BALB/c mice with immune complexes of Guy's 13 plantibody bound to S. mutans whole cells resulted in a similar change in specificity, isotype, and functionality of elicited anti-AgI/II antibodies as had been observed for other immunomodulatory MAbs. This new information, coupled with the recently solved crystal structure of the adhesin, now provides a rational explanation and plausible mechanism of action of passively administered Guy's 13/Guy's 13 plantibody in human clinical trials, and how long-term prevention of S. mutans carriage well past the application period of the therapeutic antibody could have been achieved.

Induction of neutralizing antibodies in mice immunized with an amino-terminal polypeptide of Streptococcus mutans P1 protein produced by a recombinant Bacillus subtilis strain

The oral pathogen Streptococcus mutans expresses a surface protein, P1, which interacts with the salivary pellicle on the tooth surface or with fluid-phase saliva, resulting in bacterial adhesion or aggregation, respectively. P1 is a target of protective immunity. Its N-terminal region has been associated with adhesion and aggregation functions and contains epitopes recognized by efficacious antibodies. In this study, we used Bacillus subtilis, a gram-positive expression host, to produce a recombinant N-terminal polypeptide of P1 (P1(39-512)) derived from the S. mutans strain UA159. Purified P1(39-512) reacted with an anti-full-length P1 antiserum as well as one raised against intact S. mutans cells, indicating preserved antigenicity. Immunization of mice with soluble and heat-denatured P1(39-512) induced antibodies that reacted specifically with native P1 on the surface of S. mutans cells. The anti-P1(39-512) antiserum was as effective at blocking saliva-mediated aggregation of S. mutans cells and better at blocking bacterial adhesion to saliva-coated plastic surfaces compared with the anti-full-length P1 antiserum. In addition, adsorption of the anti-P1 antiserum with P1(39-512) eliminated its ability to block the adhesion of S. mutans cells to abiotic surfaces. The present results indicate that P1(39-512), expressed and purified from a recombinant B. subtilis strain, maintains important immunological features of the native protein and represents an additional tool for the development of anticaries vaccines.

Beneficial immunomodulation by Streptococcus mutans anti-P1 monoclonal antibodies is Fc independent and correlates with increased exposure of a relevant target epitope

We showed previously that deliberate immunization of BALB/c mice with immune complexes (IC) of the cariogenic bacterium Streptococcus mutans and mAbs against its surface adhesin P1 results in changes in the specificity and isotype of elicited anti-P1 Abs. Depending on the mAb, changes were beneficial, neutral, or detrimental, as measured by the ability of the serum from immunized mice to inhibit bacterial adherence to human salivary agglutinin by a BIAcore surface plasmon resonance assay. The current study further defined changes in the host response that result from immunization with IC containing beneficial mAbs, and evaluated mechanisms by which beneficial immunomodulation could occur in this system. Immunomodulatory effects varied depending upon genetic background, with differing results in C57BL/6 and BALB/c mice. Desirable effects following IC immunization were observed in the absence of activating FcRs in BALB/c Fcer1g transgenic mice. mAb F(ab')(2) mediated desirable changes similar to those observed using intact IgG. Sera from IC-immunized BALB/c mice that were better able to inhibit bacterial adherence demonstrated an increase in Abs able to compete with an adherence-inhibiting anti-P1 mAb, and binding of a beneficial immumomodulatory mAb to S. mutans increased exposure of that epitope. Consistent with a mechanism involving a mAb-mediated structural alteration of P1 on the cell surface, immunization with truncated P1 derivatives lacking segments that contribute to recognition by beneficial immunomodulatory mAbs resulted in an improvement in the ability of elicited serum Abs to inhibit bacterial adherence compared with immunization with the full-length protein.

Requirements for surface expression and function of adhesin P1 from Streptococcus mutans

In this report, we define requirements for the successful translocation and functional maturation of the adhesin P1 of Streptococcus mutans. Conformational epitopes recognized by anti-P1 monoclonal antibodies (MAbs) were further characterized, thus facilitating the use of particular MAbs as tools to monitor the locations of various forms of the protein. We show that correct localization of P1 is dependent on structural features of the molecule itself, including a requisite A region-P region intramolecular interaction that occurs within the cell prior to secretion. P1 also was shown to be affected by several members of the protein-folding-secretion-turnover apparatus. It does not achieve a fully functional form in the absence of the trigger factor PPIase homolog RopA, and its translocation is delayed when DnaK levels are limited. In addition, dnaK message levels are differentially altered in the presence of P1 lacking the alanine-rich compared to the proline-rich repeat domains. Lastly, nonsecreted P1 lacking the P region accumulates within the cell in the absence of htrA, implying an intracellular HtrA protease function in the degradation and turnover of this particular internal-deletion polypeptide. However, the opposite effect is seen for full-length P1, suggesting a sensing mechanism and substrate-dependent alteration in HtrA's function and effect that is consistent with its known ability to switch between chaperone and protease, depending on environmental perturbations.

Basis of beneficial immunomodulation by monoclonal antibodies against Streptococcus mutans adhesin P1

We previously identified five monoclonal antibodies (MAbs) against Streptococcus mutans adhesin P1 that modulate the humoral response when bound to whole bacteria and immune complexes (ICs) are administered to BALB/c mice. The two MAbs that redirected the response towards increased efficacy recognize discontinuous epitopes involving pre-alanine-rich domain sequence; therefore, to evaluate whether epitope specificity contributes to a desirable outcome a further MAb with this characteristic was tested. A beneficial immune response was promoted. None of the three MAbs that promoted a beneficial response was opsonic, suggesting that increased uptake of ICs by phagocytes does not mediate the improvement of the IC-elicited antibodies to inhibit bacterial adherence. Finally, two of the six anti-P1 MAbs activated complement but did not partition according to desirable vs. nondesirable effects.

Characterization of epitopes recognized by anti-Streptococcus mutans P1 monoclonal antibodies

Sequences contributing to epitopes recognized by a panel of monoclonal antibodies (mAbs) against the Streptococcus mutans surface protein P1 were delineated by Western blot and enzyme-linked immunosorbent assay using a battery of deletion constructs and recombinant polypeptides. mAbs that recognize complex discontinuous epitopes reconstituted by combining the alanine-rich and proline-rich repeat domains and varying degrees of flanking sequence were identified as well as mAbs that bound epitopes contained within contiguous segments of P1. Cross-reactivity with SspA and SspB from Streptococcus gordonii is also reported. This information enables insight into the structure and function of a streptococcal adhesin and its correlates of protection and furthers our understanding of the immunomodulatory and bacterial-adherence inhibition activities of anti-P1 mAbs.

The role of peptide loops of the Bordetella pertussis protein P.69 pertactin in antibody recognition

Bordetella pertussis, the etiological agent of whooping cough, is re-emerging in several countries with a traditionally high vaccine uptake. In these B. pertussis strains, polymorphisms were found in several proteins, including P.69 pertactin (P.69 Prn). P.69 Prn, an adhesin, contains two variable regions which are composed of repeats, one of which flanks the receptor binding site. Antibody titers against P.69 Prn correlate with protection and P.69 Prn is one of the components of acellular pertussis vaccines. Nevertheless, little is known about the structure and location of P.69 Prn epitopes. We used a three pronged approach to identify discontinuous epitopes that are recognized by mouse monoclonal antibodies, i.e. site-directed mutagenesis, deletion mapping and competition assays. Site-directed mutagenesis was focused on regions of P.69 Prn predicted to form loops according to the crystal structure. In this report we describe the location of several discontinuous epitopes that are also recognized by human antibodies. Our results reveal an important role of the N-terminus in immune recognition. We provide data for an indirect role of loops in immune evasion by masking of epitopes. We propose that the repeat regions have evolved to allow rapid antigenic variation to deflect the immune response from the functional domain of P.69 Prn. The results presented here provide a better understanding of the structure and function of variable loops and their role in the persistence of pathogens in immunologically primed populations.

Redirecting the humoral immune response against Streptococcus mutans antigen P1 with monoclonal antibodies

The adhesin P1 of Streptococcus mutans has been studied as an anticaries vaccine antigen. An anti-P1 monoclonal antibody (MAb) bound to S. mutans prior to mucosal immunization of mice was shown previously to alter the amount, specificity, isotype, and biological activity of anti-P1 antibodies. The present study was undertaken to screen this and four additional anti-P1 MAbs for immunomodulatory activity when complexed with S. mutans and administered by a systemic route and to evaluate sera from immunized mice for the ability to inhibit adherence of S. mutans to immobilized human salivary agglutinin. All five MAbs tested influenced murine anti-P1 serum antibody responses in terms of subclass distribution and/or specificity. The effects varied depending on which MAb was used and its coating concentration. Two MAbs promoted a more effective, and two others a less effective, adherence inhibition response. An inverse relationship was observed between the ability of the MAbs themselves to inhibit adherence and the ability of antibodies elicited following immunization with immune complexes to inhibit adherence. Statistically significant correlations were demonstrated between the levels of anti-P1 serum immunoglobulin G2a (IgG2a) and IgG2b, but not of IgG1 or IgG3, and the ability of sera from immunized animals to inhibit bacterial adherence. These results indicate that multiple anti-P1 MAbs can mediate changes in the immune response and that certain alterations are potentially more biologically relevant than others. Immunomodulation by anti-P1 MAbs represents a useful strategy to improve the beneficial immune response against S. mutans.