Introduction
Three highly pathogenic human coronaviruses (CoVs) have been identified so far, including Middle East respiratory syndrome coronavirus (MERS-CoV), severe acute respiratory syndrome (SARS) coronavirus (SARS-CoV), and a 2019 novel coronavirus (2019-nCoV), as previously termed by the World Health Organization (WHO).1–3 Among them, SARS-CoV was first reported in Guangdong, China in 2002.4 SARS-CoV caused human-to-human transmission and resulted in the 2003 outbreak with about 10% case fatality rate (CFR),1 while MERS-CoV was reported in Saudi Arabia in June 2012.5 Even though with its limited human-to-human transmission, MERS-CoV showed a CFR of about 34.4%.2 The 2019-nCoV was first reported in Wuhan, China in December 2019 from patients with pneumonia,6 and it has exceeded both SARS-CoV and MERS-CoV in its rate of transmission among humans.7 2019-nCoV was renamed SARS-CoV-2 by Coronaviridae Study Group (CSG) of the International Committee on Taxonomy of Viruses (ICTV),8 while it was renamed HCoV-19, as a common virus name, by a group of virologists in China.9 The disease and the virus causing it were named Coronavirus Disease 2019 (COVID-19) and the virus responsible for COVID-19 or the COVID-19 virus, respectively, by WHO.3 As of March 5, 2020, a total of 95,333 confirmed cases of COVID-19 were reported, including 3,282 deaths, in China and at least 85 other countries and/or territories.7 Currently, the intermediate host of SARS-CoV-2 is still unknown, and no effective prophylactics or therapeutics are available. This calls for the immediate development of vaccines and antiviral drugs for prevention and treatment of COVID-19.
A coronavirus contains four structural proteins, including spike (S), envelope (E), membrane (M), and nucleocapsid (N) proteins.2,10,11 Among them, S protein plays the most important roles in viral attachment, fusion and entry, and it serves as a target for development of antibodies, entry inhibitors and vaccines.1,12–17 The S protein mediates viral entry into host cells by first binding to a host receptor through the receptor-binding domain (RBD) in the S1 subunit and then fusing the viral and host membranes through the S2 subunit.16,18,19 SARS-CoV and MERS-CoV RBDs recognize different receptors. SARS-CoV recognizes angiotensin-converting enzyme 2 (ACE2) as its receptor, whereas MERS-CoV recognizes dipeptidyl peptidase 4 (DPP4) as its receptor.20,21 Similar to SARS-CoV, SARS-CoV-2 also recognizes ACE2 as its host receptor binding to viral S protein.22 Therefore, it is critical to define the RBD in SARS-CoV-2 S protein as the most likely target for the development of virus attachment inhibitors, neutralizing antibodies, and vaccines.
In this study, we identified the RBD fragment in SARS-CoV-2 S protein and found that the recombinant RBD protein bound strongly to human ACE2 (hACE2) and bat ACE2 (bACE2) receptors. In addition, it blocked the entry of SARS-CoV-2 and SARS-CoV into their respective hACE2-expressing cells, suggesting that it may serve as a viral attachment inhibitor against SARS-CoV-2 and SARS-CoV infection. Moreover, we demonstrated that SARS-CoV RBD-specific polyclonal antibodies cross-reacted with SARS-CoV-2 RBD protein and inhibited SARS-CoV-2 entry into hACE2-expressing cells. We have also shown that SARS-CoV RBD-specific polyclonal antibodies could cross-neutralize SARS-CoV-2 pseudovirus infection, suggesting the potential to develop SARS-CoV RBD-based vaccine for prevention of infection by SARS-CoV-2 and SARS-CoV.