Figure 2.

Immunogenicity of MERS-CoV spike MPSP vaccines. (A) Vaccination scheme for rabbit immunizations. Six groups of rabbits (5/group) were vaccinated in a prime/boost regimen with 15 μg of adjuvanted vaccine at 4-week interval and challenged with MERS-CoV (EMC strain; accession no. NC_019843) 3 weeks post-boost. Anti-MERS-CoV spike S2 (B) and S1 (D) IgG titres measured by ELISA in rabbits at different time points. Shown is the mean ± s.e.m. antibody titres from five rabbits per group. (C,E) MERS-CoV neutralizing antibody titres measured by a 90% reduction in a plaque reduction neutralization assay (PRNT₉₀). (B-E) Shown is the mean ± s.e.m. of five rabbits per group. (F,G) Vaccine-induced antibodies in nasal swabs of vaccinated rabbits. Anti MERS-CoV S2 (F) and S1 (G) antibody responses in the nasal swabs (tested at a 1:50 dilution) of vaccinated rabbits pre-challenge (three weeks post-boost). The difference in antibody responses between monomeric (RBD + LS) and multimeric (RBD-LS) RBD was tested for statistical significance using a student’s t-test, with asterisks indicating the level of significance. *P ≤ 0.05. Error bars indicate mean ± s.e.m. The dotted lines represent the limits of detection. HR2, heptad repeat 2; FP, fusion peptide; LS, lumazine synthase 60-meric particles; I3, I3-01 60-meric particles; RBD, receptor binding domain; RBD + LS, monomeric uncoupled RBD; RBD-LS, multimeric RBD coupled to LS through covalent SpyTag/SpyCatcher.