INFLUENCE OF EPITOPE POLARITY AND ADJUVANTS ON THE IMMUNOGENICITY ANDEFFICACY OF A SYNTHETIC PEPTIDE VACCINE AGAINST SEMLIKI FOREST VIRUS

The antibody response to a previously defined B-cell epitope of Semlik i Forest virus (SFV) was investigated in male BALB/c (H-2d) mice. The B-cell epitope, located at amino acid positions 240 to 255 of the E2 p rotein, was linked to an H-2d-restricted T-helper cell epitope of SFV located at positions 137 to 151 of the E2 protein. Colinearly synthesi zed peptides, of either T-B or B-T polarity, mixed with different adju vants (the nonionic block copolymer L 180.5, a water-oil-water [W/O/W] emulsion of L 180.5, Montanide, and Q VAC) were used for immunization . Generally, after one booster immunization, high serum antibody titer s were measured against either peptide. With Q VAC and W/O/W L 180.5 a s adjuvants, the titers of SFV-reactive (nonneutralizing) antibodies w ere consistently much higher after immunization with the T-B peptide t han with the B-T peptide, which was reflected in a higher vaccine effi cacy. With these two adjuvants, the survival ratio in T-B peptide-immu nized mice was 82%, compared with 8% in B-T peptide-immunized mice. In termediate results were obtained with the adjuvant Montanide. L 180.5 alone was ineffective in this study. All immunoglobulin G (IgG) isotyp es were induced with either adjuvant, but Q VAC was clearly the most e ffective in inducing IgG2a and IgG2b isotypes with the T-B peptide as the antigen. Subsequently, monoclonal antibodies (MAbs) of IgM, IgG1, IgG2a, IgG2b, and IgG3 subclasses were prepared against the B-cell epi tope. These nonneutralizing but SFV-reactive MAbs protected 40 to 80% of mice against a lethal challenge with SFV. Control mice all died. Th e availability of those antipeptide MAbs allowed competition binding a ssays with a previously characterized panel of E2-specific MAbs. Bindi ng of enzyme-labeled antipeptide MAbs was very effectively inhibited b y two strongly SFV-neutralizing mutually competitive MAbs, suggesting that the linear B-cell epitope (amino acids 240 to 255) is associated with a major neutralization site of SFV.