Molecular characterization of single-chain antibody variable fragments (scFv) specific to Pep27 from Streptococcus pneumoniae

Pep27 from Streptococcus pneumoniae is reported to initiate pneumococcal autolysis, thereby constituting a major virulence factor. Although a few antisera recognizing Pep27 have been reported, no monoclonal, well-characterized antibody for Pep27 has been developed. Here we screened two single-chain antibody variable fragments (scFv) using a phage display from a large human synthetic scFv library to select clones E2 and F9. Dissociation constants (Kd) of E2 and F9 were 1.1 μM and 0.50 μM, respectively. E2 and F9 did not cross-react with other pneumococcal and unrelated proteins. The epitopes of Pep27 were localized to residues 24, 26 and 27 by alanine scanning. Molecular docking analysis supported the experimentally investigated epitope. The E2 and F9 clones specifically detected Pep27 in an environment mimicking in vivo conditions, demonstrated in human serum. The scFv clones characterized here represent molecular tools for the detection of pneumococcal diseases with potential for further improvement in affinity.