CbpM and CbpG of Streptococcus Pneumoniae Elicit a High Protection in Mice Challenged with a Serotype 19F Pneumococcus

Abstract

Among many pneumococcal antigens, choline-binding proteins (CPBs) display a high immunogenicity in animal models. This study aims to determine the immunogenicity of CbpM, CbpG and CbpL proteins of Streptococcus pneumoniae in a mice model. The genes were cloned into pET21a expression vector and the recombinant proteins were produced. Mice were immunized with the purified recombinant proteins. Subsequently, the mice were challenged with S. pneumoniae ATCC 49619 (2×10⁶ CFU) and their survival and bacterial clearances were followed 24 hours after infection. The antibody responses of the mice were determined by ELISA assay. The opsonophagocytosis assay was performed using rabbit’s sera. Passive immunization was carried out using two doses of anti-CbPs antibodies. Finally, these mice were  experimentally infected with virulent bacteria and the protective effects of two doses of 10 and 100 µg/mL by monitoring the survival rate and bacterial clearance were determined at 2, 3 and 7 days after bacterial challenge. The mice actively immunized with CbpM, CbpG and CbpL recombinant proteins showed survival rate of 100%, 85% and 75%, respectively. The survival rates among passively immunized mice groups which received 100 µg/mL dose of anti-CbpM, anti-CbpG and anti- CbpL were 50%, 50% and 25%, respectively. The rates of opsonization with rabbit’s antibodies against CbpM, CbpG, and CbpL  at 100 µg/mL doses was 45.6%, 14.7% and 82.3%, and at 10 µg/mL was 12.9%, 12.2% and 9.35%, respectively. Our findings suggest that the recombinant proteins particularly CbpM and CbpG can protect the mice against pneumococcus19F serotype and effectively induce a protective antibody response. Thus, CbpG and CbpM proteins might be used as suitable vaccine candidate in pneumococcal vaccine formulations.