Immune efficacy evaluation of recombinant surface protein rSeseC-02147 of Streptococcus equi subsp. zooepidimicus

【Objective】 The aim of the study was to prepare the recombinant surface protein rSeseC-02147 of Streptococcus equi subsp. zooepidimicus（SEZ） using a prokaryotic expression system, and to systematically evaluate its immune efficacy against SEZ infections, providing a candidate antigen for the development of SEZ subunit vaccines. 【Method】 The prokaryotic expression vector pCold I-SeseC-02147 was constructed and transformed into Escherichia coli BL21（DE3） competent cells, followed by isopropyl thiogalacto side（IPTG） induction. The recombinant protein rSeseC-02147 was purified and recovered to immunize BALB/c mice, with the inactivated SEZ vaccine as the positive control and PBS as the negative control. Serum was collected from the submandibular vein of mice on the 14th day after the secondary immunization. The reactogenicity of the recombinant protein rSeseC-02147, as well as the antibody titers and antibody subtypes of the mouse serum were detected using Western blotting and enzyme-linked immunosorbent assay（ELISA）, respectively. In addition, the immunoprotective efficacy of the recombinant protein rSeseC-02147 in mice was systematically evaluated by mouse immune protection tests, bacterial load detection in the peritoneal lavage fluid and histopathological observation of organs. 【Result】 The constructed prokaryotic expression vector pCold I-SeseC-02147, successfully expressed the recombinant protein rSeseC-02147 after being induced by IPTG, achieving a purified concentration of 2.86 mg/mL. The recombinant protein rSeseC-02147 specifically reacted with the serum of mice in the immunized group and the positive control group, but did not react specifically with the serum of mice in the negative control group, indicating that rSeseC-02147 had good reactogenicity and specificity. The serum antibody titer produced by immunized mice with recombinant protein rSeseC-02147 was much higher than that from the inactivated SEZ vaccine, and the IgG1subtype was extremely significantly higher than the IgG2a subtype（P＜0.01, the same below）. The recombinant protein rSeseC-02147 provided mice with immune protection against SEZ infection, with a survival rate of 60% within 14 d post challenge. Additionally, the bacterial load in the peritoneal lavage fluid of the mice immunized with the recombinant protein rSeseC-02147 was extremely significantly lower than that in the negative control group, and these mice exhibited milder pathological damage to tissues such as lungs, kidneys, and spleens. 【Conclusion】 The recombinant protein rSeseC-02147, prepared using the prokaryotic expression system, exhibits good reactogenicity and specificity. It provides immune protection by inhibiting the SEZ proliferation and attenuating the damage of SEZ to bodily organs. Therefore, the recombinant protein rSeseC-02147 has the potential to be developed into an SEZ subunit vaccine.