Researchers find blocking neutralizing antibodies from recovered COVID-19 patients.
According to the latest report issued by the World Health Organization (WHO) on April 27th, 2020, the coronavirus disease 2019 (COVID-19, a novel coronavirus, namely severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has been confirmed in 3,145,932 cases and claimed 218,220 deaths worldwide.
At present, the development of prophylactic vaccines or therapeutic drugs for COVID-19 has been launched one after another. Blocking monoclonal antibodies (mAbs) are the best choice to neutralize virus infection because of their unique antigen specificity. Recently, a new report reported a very attractive method, that is to use B cell sorting method to identify and clone blocking mAbs from the memory B cell repertoire of recently recovered patients with COVID-19, so as to prevent COVID-19 virus from entering host cells.
The study, conducted by researchers from China, has been published in the journal Cellular & Molecular Immunology.
The article pointed out that SARS-CoV-2 is similar to SARS-CoV, using highly glycosylated homotrimeric spike (S) protein (composed of two subunits, S1 and S2) for receptor binding and virus entry. In order to bind to the host cell receptor human angiotensin-converting enzyme 2 (hACE2) shared by SARS-CoV and SARS-CoV-2, the S protein undergoes a significant conformational change to expose the RBD and key residues for receptor binding. The S protein is metastable, and the binding of RBD to the hACE2 receptor may cause the S1 protein to fall off from the S2 protein, thereby promoting S2-mediated virus-host membrane fusion and virus entry. In view of the key role of RBD in initiating SARS-CoV-2 invasion into host cells, it becomes a vulnerable target for neutralizing antibodies.
Chen and colleagues first confirmed by ELISA whether patients recovering from COVID-19 had anti-SARS-CoV-2 S1 protein IgG antibodies in their sera. It was found that most were able to produce high-titer SARS-CoV-2 S1 specific IgG antibodies, and all patients had SARS-CoV-2 RBD specific IgG antibodies in their sera.
Next, they established an ELISA-based inhibition test to check the blocking function of these antibodies. The results show that although all recovered COVID-19 patients can produce anti-S1 and anti-RBD antibodies, only a small proportion of these antibodies can block the binding of RBD to the hACE2 receptor. This observation can be explained by the transient and dynamic perfusion conformational state of the S protein, which provides a very limited window for the immunogenic epitopes of RBD exposure to specific B cells.
After ELISA assay and flow cytometry analysis, SARS-CoV-2 S pseudotyped lentiviral particles were used to determine the neutralizing effect of the mAb. The antibodies finally named 311mab-31B5 and 311mab-32D4 showed effective neutralization of the pseudovirus into host cells that ectopically expressed hACE2 (IC50 = 0.0338 and 0.0698 μg/ml, respectively).
In this report, the researchers successfully cloned two human blocking mAbs using SARS-CoV-2 RBD-specific memory B cells isolated from recovered COVID-19 patients. These two mAbs can specifically bind to SARS-CoV-2 RBD, block the interaction between SARS-CoV-2 RBD and hACE2 receptor, and lead to effective neutralization of SARS-CoV-2 S protein pseudotyped virus infection. This is the first report of such human anti-SARS-CoV-2 RBD-hACE2 blocking mAbs, which are expected to be used as specific preventive and therapeutic agent against the ongoing SARS-CoV-2 pandemic.
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