Application of phage display technology for the production of antibodies againstStreptococcus suis serotype 2

Streptococcus suis (S. suis) serotype 2 infection is problematic in the swine industry and responsible for most cases of human infection worldwide. Since current multiplex PCR cannot differentiate between serotypes 2 and 1/2, then serotype-specific antibodies (Abs) are required for serotype identification to confirm S. suis serotype 2 infection. This study aimed to generate Abs specific to S. suis serotype 2 by phage display from a human heavy chain variable domain (VH) antibody library. For biopanning, whole cells of S. suis serotype 2 were used as the target antigen. With increasing selection stringency, we could select the VH Abs that is specifically bound to S. suis serotype 2 surface antigen, which was identified as the capsular polysaccharide (CPS). From ELISA analysis, the specific phage clone 47B3 VH with the highest binding activity to S. suis serotype 2 was selected and shown to have no cross-reactivity with S. suis serotypes 1/2, 1, and 14 that share a common epitope with serotype 2 and occasionally cause infections in human. Moreover, 47B3 VH shown no cross-reactivity with other septic bacteria colonizing blood specimens. Then, 47B3 VH was successfully expressed as soluble 47B3 VH in SHuffle® T7 E. coli. The soluble 47B3 VH was tested for the binding ability in a dose-dependent ELISA assay. The results indicated that the activity of phage clone 47B3 was still retained even in the soluble form. The specificity test of the soluble 47B3 VH was further determined. The results showed that the soluble 47B3 VH was also specifically bound with serotype 2 human clinical isolate and had no cross-reactivity with other virulent S. suis serotypes in human cases. The quellung reaction demonstrated that the soluble 47B3 VH could differentiate between S. suis serotypes 2 and 1/2 in terms of bioactivity. Thus, this VH Ab is beneficial in the diagnosis of disease or the discrimination of S. suis serotypes. Furthermore, the results of this study here could warrant the phage display VH platform to produce serotyping Abs.