title: Quantitative comparison of the efficiency of antibodies against S1 and S2 subunit of SARS coronavirus spike protein in virus neutralization and blocking of receptor binding: implications for the functional roles of S2 subunit.
abstract: Neutralizing effects of antibodies targeting the C-terminal stalk (S2) subunit of the spike protein of severe acute respiratory syndrome coronavirus have previously been reported, although its mechanism remained elusive. In this study, high titered mouse antisera against the N-terminal globular (S1) and S2 subunits of the S protein were generated and total immunoglobulin G (IgG) was purified from these antisera. The efficiency of these purified IgGs in virus neutralization and blocking of receptor binding were compared quantitatively using virus neutralization assay and a previously developed cell-based receptor binding assay, respectively. We demonstrated that anti-S1 IgG neutralizes the virus and binds to the membrane associated S protein more efficiently than anti-S2 IgG does. Moreover, both anti-S1 and anti-S2 IgGs were able to abolish the binding between S protein and its cellular receptor(s), although anti-S1 IgG showed a significantly higher blocking efficiency. The unexpected blocking ability of anti-S2 IgG towards the receptor binding implied a possible role of the S2 subunit in virus docking process and argues against the current hypothesis of viral entry. On the other hand, the functional roles of the previously reported neutralizing epitopes within S2 subunit were investigated using an antigen specific antibody depletion assay. Depletion of antibodies against these regions significantly diminished, though not completely abolished, the neutralizing effects of anti-S2 IgG. It suggests the absence of a major neutralizing domain on S2 protein. The possible ways of anti-S2 IgGs to abolish the receptor binding and the factors restricting anti-S2 IgGs to neutralize the virus are discussed.
authors.affiliation: Department of Zoology, Kadoorie Biological Science Building, The University of Hong Kong, Pokfulam Road, Hong Kong, China.
authors.forename: Fanya
authors.initials: F
authors.lastname: Zeng
authors.forename: Chung Chau
authors.initials: CC
authors.lastname: Hon
authors.forename: Chi Wai
authors.initials: CW
authors.lastname: Yip
authors.forename: Ka Man
authors.initials: KM
authors.lastname: Law
authors.forename: Yin Shan
authors.initials: YS
authors.lastname: Yeung
authors.forename: Kwok Hung
authors.initials: KH
authors.lastname: Chan
authors.forename: Joseph S
authors.initials: JS
authors.lastname: Malik Peiris
authors.forename: Frederick Chi Ching
authors.initials: FC
authors.lastname: Leung
mesh_terms: D000818:Animals; D000914:Antibodies, Viral / Q000276:immunology* / Q000494:pharmacology*; D002460:Cell Line; D002522:Chlorocebus aethiops; D006224:Cricetinae; D000939:Epitopes / Q000276:immunology; D008562:Membrane Glycoproteins / Q000276:immunology* / Q000378:metabolism*; D051379:Mice; D011485:Protein Binding; D021122:Protein Subunits / Q000276:immunology / Q000378:metabolism; D045473:Severe acute respiratory syndrome-related coronavirus / Q000187:drug effects* / Q000276:immunology*; D064370:Spike Glycoprotein, Coronavirus; D014759:Viral Envelope Proteins / Q000276:immunology* / Q000378:metabolism*
chemical_list: D000914:Antibodies, Viral; D000939:Epitopes; D008562:Membrane Glycoproteins; D021122:Protein Subunits; D064370:Spike Glycoprotein, Coronavirus; D014759:Viral Envelope Proteins; C578557:spike glycoprotein, SARS-CoV; C578553:spike protein, mouse hepatitis virus