E-034. Enhanced Epitope Exposure Explains Beneficial Immunomodulation by Monoclonal Antibodies against Streptococcus mutans P1

R. A. Robinette, M. W. Oli, W. P. McArthur, L. J. Brady;
Univ. of Florida Coll. of Dent., Gainesville, FL.

Certain non-adherence-inhibiting monoclonal antibodies (MAbs) (6-11A, 5-5D) against S. mutans adhesin P1 redirect the subsequent humoral immune response against this cariogenic oral pathogen towards a higher level of adherence inhibiting serum antibodies (Abs), as measured in vitro by BiaCore assay, when BALB/c mice are immunized with immune complexes (IC) comprised of whole bacteria and MAbs. Previous studies demonstrated that increased adherence inhibition correlated with increased IgG2a and/or IgG2b against a P1 polypeptide, NR21, recognized only by the adherence-inhibiting anti-P1 MAb 1-6F. Competition ELISA also demonstrated greater inhibition of binding of biotin-labeled MAb 1-6F to S. mutans by sera from IC compared to S. mutans only-immunized mice and the beneficial effects of MAbs 6-11A/5-5D were independent of complement, an intact Fc, and activating Fc receptors. The beneficial effects of MAbs 6-11A/5-5D appear to relate to an epitope requirement that depends on an interaction of the discontinuous alanine-rich (A) and proline-rich (P) regions of P1. Previous studies demonstrated that the A- and P-region-dependent anti-P1 MAb 4-10A was neutral in its immunomodulatory effects at the [MAb] tested; however, prozone-like effects are often observed with exogenously administered antibodies. To further investigate the contribution of MAb epitope specificity to beneficial immunomodulatory properties and the importance of [Ag/Ab], MAb 4-10 was re-evaluated over a broader range of [MAb] and the sera from S. mutans and IC-immunized mice were tested by BiaCore, anti-NR21 subclass ELISA and competition ELISA. Sera from mice immunized with IC containing lower [MAb 4-10A] exhibited increased adherence inhibition, higher levels of anti-NR21 IgG subclass Abs, and enhanced competition for S. mutans binding with MAb 1-6F. Additionally, binding of MAb 4-10A to P1 was shown by ELISA to significantly increase the binding of biotin-labeled MAb 1-6F to S. mutans suggesting that enhanced exposure of an epitope recognized by adherence-inhibiting antibodies represents a major mechanism of beneficial immunomodulation by anti-P1 MAbs.