DEFINING ANTIBODY TARGETS IN STREPTOCOCCUS-ORALIS INFECTION

Immunoblotting of sera from 12 neutropenic patients with Streptococcus oralis septicemia and 18 patients with endocarditis due to viridans g roup streptococci revealed immunodominant S. oralis antigens at 85 and 180 kDa. The former cross-reacted with a mouse monoclonal antibody to hsp90. The latter was identified by sequencing positive clones obtain ed by screening a genomic expression library of S. oralis with pooled sera from patients who had been infected with S. oralis. Antibody elut ed from one of these clones reacted with the 180-kDa antigen of S. ora lis. Southern blotting confirmed the origin of the clone from S. orali s. The derived amino acid sequence showed 76.2% homology with the PAc protein precursor of Streptococcus mutans and 73.8% homology with the SpaA protein precursor of Streptococcus sobrinus. Epitope mapping of t he derived amino acid sequence with sera from patients with viridans g roup streptococcal endocarditis delineated nine epitopes. Peptides 1 ( TMYPNRQPGSCWDSS) and 2 (WYSLNGKIRAVDVPK), representing two of these ep itopes, and peptide 3 (YEVEKPLEPAPVAPS), representing the repeat proli ne region, were synthesized. These three peptides were used to screen a phage antibody display library derived from a patient who had recove red from S. oralis infection. Two of the human recombinant antibodies produced (SORAL 3 and SORAL 4 against peptide 3) and a human recombina nt antibody (B3.7) against the conserved epitope (LKVIRK) of hsp90 gav e statistically significant protection, compared with control groups, in a mouse model of lethal S. oralis infection.