The antibody response to a previously defined B-cell epitope of Semliki 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 protein, was linked to an H-2d-restricted T-helper cell epitope of SFV located at positions 137 to 151 of the E2 protein. Colinearly synthesized peptides, of either T-B or B-T polarity, mixed with different adjuvants (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 titers were measured against either peptide. With Q VAC and W/O/W L 180.5 as adjuvants, the titers of SFV-reactive (nonneutralizing) antibodies were consistently much higher after immunization with the T-B peptide than with the B-T peptide, which was reflected in a higher vaccine efficacy. With these two adjuvants, the survival ratio in T-B peptide-immunized mice was 82%, compared with 8% in B-T peptide-immunized mice. Intermediate results were obtained with the adjuvant Montanide. L 180.5 alone was ineffective in this study. All immunoglobulin G (IgG) isotypes were induced with either adjuvant, but Q VAC was clearly the most effective 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 epitope. These nonneutralizing but SFV-reactive MAbs protected 40 to 80% of mice against a lethal challenge with SFV. Control mice all died. The availability of those antipeptide MAbs allowed competition binding assays with a previously characterized panel of E2-specific MAbs. Binding of enzyme-labeled antipeptide MAbs was very effectively inhibited by 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
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