Antipeptide antisera define neutralizing epitopes on the adenovirus hexon

This study identified type-specific neutralizing epitopes on adenovirus type 2 (Ad2) hexon, the major capsid protein. Researchers synthesized two peptides corresponding to variable regions in loop 1 (L1, residues 281-292) and loop 2 (L2, residues 441-455) of the Ad2 hexon, which were predicted to be surface-exposed based on X-ray crystallography data. Antisera raised against these peptides bound both the synthetic peptides and native hexon protein, and effectively neutralized Ad2 infectivity in a type-specific manner, as neither antiserum neutralized Ad4 or Ad40. Mechanistic studies showed that neutralization did not prevent viral attachment or internalization; instead, antibody-treated virions were internalized normally but unable to establish infection, suggesting the antibodies block a post-entry step, likely an acid-induced conformational change required for endosomal escape. Importantly, antibodies against both L1 and L2 epitopes were detected in serum from a human previously infected with Ad2, demonstrating that these variable surface regions are naturally immunogenic during human infection. The work successfully mapped type-specific neutralizing determinants on the Ad2 hexon surface.

Key findings

• Two surface-exposed variable regions in adenovirus hexon loops 1 and 2 contain type-specific neutralizing epitopes
• Antipeptide antisera neutralize Ad2 infectivity in type-specific fashion without affecting viral attachment or internalization
• Neutralization occurs at a post-entry stage, likely by blocking acid-induced conformational changes in endosomes
• Antibodies against identified L1 and L2 epitopes are naturally present in serum from Ad2-infected humans
• Synthetic peptides specifically inhibit hexon-antibody binding and neutralizing activity, confirming epitope specificity