Antibodies with specificities against a dispase-produced 15-kilodalton hexon fragment neutralize adenovirus type 2 infectivity.

During the entrance of adenovirus type 2 into cells, it has been suggested that the virion undergoes a conformational change. In this investigation, we have further characterized the hypothetical conformational change, which the structural protein hexon undergoes in response to low pH. From pH 5.0 to pH 6.0, the proteolytic enzyme dispase cleaved the hexon into a few distinct fragments with a dominating low-molecular-weight fragment with a molecular weight of 15,000 (15K peptide), whereas between pH 6.5 and pH 8.0, the cleavage of the hexon was negligible. The degradation of the hexon with dispase at low pH was not due to an increased activity or alteration of the active site of dispase at low pH. The 15K fragment was identified as a segment of the N-terminal part of the hexon polypeptide beginning at amino acid residue 5. An immune serum produced in response to acid-treated and glutaraldehyde-fixed hexons contained a small amount of antibodies directed towards the 15K fragment, as judged by Western immunoblotting. An anti-15K antibody fraction was isolated by affinity chromatography by removing antibodies recognizing the hexon in the alkaline configuration. Such antibodies displayed a higher relative titer at pH 5.0 than at pH 7.5 in an enzyme-linked immunosorbent assay. The isolated antibodies showed a specific neutralizing capacity five times higher than that of the corresponding unfractionated polyclonal anti-hexon serum; however, the neutralizing ability was independent of pH. The neutralization of adenovirus type 2 infection by the isolated anti-15K antibodies implies that the N-terminal end of the hexon may play a critical role in the early steps of the virion-cell interaction.