SARS-CoV Subunit Vaccines Based on Full-Length S Protein
Subunit vaccines based on SARS-CoV S protein, including full-length or trimeric S protein, are immunogenic with protection against SARS-CoV infection (He et al., 2006a; Kam et al., 2007; Li et al., 2013). Either insect cell-expressed full-length (FL-S) or extracellular domain (EC-S) SARS-CoV S protein developed high-titer S-specific antibodies with neutralizing activity against pseudotyped SARS-CoV expressing S protein of representative SARS-CoV human and palm civet strains (Tor2, GD03, and SZ3) isolated during the 2002 and 2003 or 2003 and 2004 outbreaks (He et al., 2006a). In addition, full-length S-ectodomain proteins fused with or without a foldon trimeric motif (S or S-foldon) could elicit specific antibody responses and neutralizing antibodies, protecting immunized mice against SARS-CoV challenge with undetectable virus titers in the lungs (Li et al., 2013). Moreover, a subunit vaccine (triSpike) based on a full-length S protein trimer induced specific serum and mucosal antibody responses and efficient neutralizing antibodies against SARS-CoV infection (Kam et al., 2007). Nevertheless, this vaccine also resulted in Fcγ receptor II (FcγRII)-dependent and ACE2-independent ADE, particularly in human monocytic or lymphoblastic cell lines infected with pseudotyped SARS-CoV expressing viral S protein, or in Raji B cells (B-cell lymphoma line) infected with live SARS-CoV (Kam et al., 2007; Jaume et al., 2012), raising significant concerns over the use of full-length S protein as a SARS vaccine target.