Potential Factors Affecting SARS-CoV Subunit Vaccines
A number of factors may affect the expression of proteins to be used as SARS subunit vaccines; apart from their immunogenicity and/or protective efficacy. Understanding of these factors is important to generate subunit vaccines with good quality, high immunogenicity, and excellent protection against SARS-CoV infection.
The expression of recombinant protein-based SARS subunit vaccines may be changed by the following factors. First, addition of an intron splicing enhancer to the truncated SARS-CoV S protein fragments results in better enhancement of protein expression in mammalian cells than the exon splicing enhancers, and different cells may result in different fold increase of protein expression (Chang et al., 2006). Second, inclusion of a post-transcriptional gene silencing suppressor p19 protein from tomato bushy stunt virus to a SARS-CoV N protein may significantly increase its transient expression in tobacco (Zheng et al., 2009).
The following factors may affect the immunogenicity and protective efficacy of protein-based SARS subunit vaccines, including same proteins expressed in different expression systems, and same proteins with various lengths, amino acid mutations, or deletions (He et al., 2006b; Du et al., 2009b). For example, RBD proteins containing different lengths (193-mer: RBD193-CHO or 219-mer: RBD219-CHO) elicited different immune responses and protective efficacy against SARS-CoV challenge (Du et al., 2009c, 2010). A recombinant SARS-CoV RBD (RBD-293T) protein expressed in mammalian cell system was able to induce stronger neutralizing antibody response than those expressed in insect cells (RBD-Sf9) and E. coli (RBD-Ec) (Du et al., 2009b), suggesting that RBD purified from mammalian cells has preference for further development due to its ability to maintain native conformation. Notably, a single mutation (R441A) in the RBD of SARS-CoV disrupted its major neutralizing epitopes and affinity to bind viral receptor ACE2, thus abolishing the vaccine’s immunogenicity, and hence, its ability to induce neutralizing antibodies in immunized animals (He et al., 2006b). Additionally, deletion of a particular amino acid by changing a glycosylation site in the SARS-CoV RBD (RBD219-N1) also resulted in the alteration of subunit vaccine’s immunogenicity (Chen et al., 2014).
Other factors that potentially affect the immunogenicity of SARS subunit vaccines include immunization routes and adjuvants (Zakhartchouk et al., 2007; Li et al., 2013). Significantly high-titer antibodies were induced by monomeric or trimeric SARS-CoV S and S1 proteins through the intramuscular (I.M.) route compared to the subcutaneous (S.C.) route (Li et al., 2013). Moreover, a SARS-CoV RBD subunit vaccine conjugated with Alum plus CpG adjuvants elicited a higher level of IgG2a antibody and interferon gamma (IFN-γ) secretion than the RBD with Alum alone (Zakhartchouk et al., 2007).