The current standard for the diagnosis of COVID-19 is the nucleic acid test of SARS-CoV-2 RNA, however, virus antibody detection has the advantages of convenient sample collection, high throughout, and low cost. When combining detection with nucleic acid detection, antibody detection can effectively compensate for nucleic acid detection. Virus infection always induce high antibody titer against SARS-CoV-2 nucleocapsid protein (N protein), which can be used to detect COVID-19 at both infected and convalescent patients. In this study we reported the expression and purification of N protein in E.coli from inclusion bodies by a combination of two cation exchange chromatography, and the yield of N protein was around 50 mg/L fermentation broth with more than 90% purity. A corresponding colloidal gold detection kit prepared with our purified N protein was used to verify the efficiency and accuracy our N protein in antibody detection method. Of the 58 COVID-19 PCR positive patients' inactivated serum samples, 40 samples were IgM positive (69.0%), and 42 samples were IgG positive (72.4%), and all 95 COVID-19 negative patients’ inactivated serum samples were both IgM and IgG negative. Our results indicates that the refolded soluble N protein could be used for the preliminary detection of IgG and IgM antibodies against SARS-CoV- 2.

目前COVID-19的诊断标准是SARS-CoV-2 RNA核酸检测，而病毒抗体检测具有样本采集方便、通量高、成本低等优点。当检测与核酸检测相结合时，抗体检测可以有效弥补核酸检测的不足。病毒感染总是会诱发针对 SARS-CoV-2 核衣壳蛋白（N 蛋白）的高抗体滴度，可用于检测感染者和恢复期患者的 COVID-19。在这项研究中，我们报道了 N 蛋白在大肠杆菌中的表达和纯化采用两种阳离子交换层析相结合的方法从包涵体中分离得到N蛋白，产率在50 mg/L发酵液左右，纯度在90%以上。采用我们纯化的N蛋白制备相应的胶体金检测试剂盒，验证了我们的N蛋白抗体检测方法的有效性和准确性。58例COVID-19 PCR阳性患者灭活血清样本中，40例IgM阳性（69.0%），42例IgG阳性（72.4%），95例COVID-19阴性患者灭活血清样本均为IgM并且IgG阴性。我们的结果表明，重新折叠的可溶性 N 蛋白可用于初步检测针对 SARS-CoV-2 的 IgG 和 IgM 抗体。