COVID-19 diagnosis and study of serum SARS-CoV-2 specific IgA, IgM and IgG by a quantitative and sensitive immunoassay

Huan Ma; Weihong Zeng; Hongliang He; Dan Zhao; Yunru Yang; Dehua Jiang; Peigen Zhou; Yingjie Qi; Weihuang He; Changcheng Zhao; Ruting Yi; Xiaofang Wang; Bo Wang; Yuanhong Xu; Yun Yang; Arnaud John Kombe Kombe; Chengchao Ding; Jiajia Xie; Yong Gao; Linzhao Cheng; Yajuan Li; Xiaoling Ma; Tengchuan Jin

The RBD of the spike protein on viral surface is the ligand binding to the major host receptor ACE2; 309 therefore RBD could be a main target for neutralization antibodies. 32, 33 In this study, we explored the 310 possibility of using either NP or RBD as an immobilized antigen in for developing a clinical COVID-19 311 diagnostic kit. Our data (supplementary figure 2) showed RBD-based diagnostic kits were better 312 performed than that of NP in detecting all the three types of antibodies. A few previous studies reported 313 that RBD-based IgM and IgG detection is better than NP once a comparison was made 19, 23 , and the 314 measurement is agreeable with the titers measured by virus neutralization assays 23, 24 . We provided here 315 the evidence that RBD as an immobilized antigen is also better than NP in detection serum IgA from 316 COVID-19 patients. The exact mechanisms of difference between the use of two types of viral antigens 317 remain to be resolved. It could be that the NP as a highly basic protein interacts with acidic residues in 318 complementarity determining region in antibodies is less specific. It could also be due to the fact that the 319 NP antigen is expressed in bacteria as most investigators do, and the RBD protein we used is expressed 320 in a human cell line enabling critical glycosylation and high-affinity binding to antibodies raised in 321 COVID-19 patients. Nonetheless, we showed that our serological kits based on SARS-CoV-2 spike 322 protein RBD as an immobilized antigen provide a high sensitivity and specificity for detecting IgA, IgM, 323 and IgG in a quantitative manner.