Immunogenicity of amino acids 1–150 of Streptococcus GapC displayed on the surface of Escherichia coli

Streptococcus is one of the main pathogens that cause bovine mastitis. They includes into S.agalactiae, S.dysgalactiae, and S.uberis. The GapC protein is a virulence factor that is expressed on the surface of Streptococcus species. GapC is highly antigenic and immunization with GapC confers cross-protection against all three species. Our previous data showed that amino acids 1–150 of GapC (GapC1–150) of S. dysgalactiae conferred similar immunoprotection compared to full-length GapC. Thus, the present study aimed to construct a recombinant Escherichia coli XL1-Blue strain that displayed GapC1–150 on its surface, and to investigate the immunogenicity of the surface-localized GapC1–150. To do so, the ompA gene of the E. coli XL1-Blue strain was replaced with the lpp′-ompA-gapC11–150 or lpp′-ompA genes by λ Red recombination, the former of which fused GapC1–150 to an Lpp lipoprotein signal peptide and amino acids 1–159 of OmpA; the recombinant strains were named XL1-Blue/LOG76 and XL1-Blue/LO11, respectively. GapC1–150 was confirmed to localize to the surface of the XL1-Blue/LOG76 strain by an indirect enzyme-linked immunosorbent assay (ELISA), a fluorescence-activated cell sorter analysis, and laser-scanning confocal microscopy. Then, ICR mice were immunized intramuscularly with the XL1-Blue/LOG76 or XL1-Blue/LO11 strains, or recombinant GapC1–150. The sera of the immunized mice were collected and the anti-GapC1–150 antibody levels were detected by ELISA. Lymphocytes secreting interleukin (IL)-4 and interferon-γ were detected by an enzyme-linked ImmunoSpot assay, as was the level of IL-17A level in the supernatant of cultured splenic lymphocytes. The mice immunized with the XL1-Blue/LOG76 strain or GapC1–150 exhibited better cellular and humoral immunity. Lastly, the immunized mice were challenged with S. uberis, S. dysgalactiae, and S. agalactiae strains, and mice that were immunized with the XL1-Blue/LOG76 strain were better protected than those that were immunized with the XL1-Blue/LO11 strain. These results indicate that it is feasible to display GapC1–150 on the E. coli surface as a vaccine against Streptococcus species.