Mapping of linear B cell epitopes on capsid proteins of bovine papillomavirus: identification of three external type-restricted epitopes.

Abstract

Immunodominant, conserved and type-restricted external epitopes of bovine papillomavirus (BPV) major (L1) capsid protein have been identified using BPV particles and synthetic peptides. Antisera to disrupted BPV-1 recognized BPV-1 and BPV-2 particles in immune electron microscopy (IEM) studies and inhibited BPV-2-induced focus formation of NIH/3T3 cells. Thus BPV-1/BPV-2 cross-reactive epitopes occur on the surface of virions. The L1 protein appeared to be immunodominant as the antisera reacted with three dominant BPV-1/BPV-2 conserved B cell epitopes (amino acids 111 to 125, 131 to 145 and 191 to 205) in Pepscan assays of BPV-1 L1, whereas no common epitopes and less frequent antibody binding to peptides were detected in Pepscans of the L2 protein of BPV-1. Four discrete variable regions were identified in the sequences of L1 proteins of BPV-1 and BPV-2. Antisera against synthetic peptides corresponding to three of the four variable regions (amino acids 42 to 56, 435 to 449 and 485 to 499) of BPV-2 L1 caused clumping of BPV-2, but not of BPV-1, particles as examined by IEM, and antisera to one peptide (amino acids 485 to 499) inhibited BPV-2-induced focus formation of NIH/3T3 cells. These data suggest that these regions are type-specific BPV-2 L1 epitopes and that they occur on the virion surface. Although conformation-dependent epitopes remain to be identified on papillomaviruses, the linear epitopes identified in this study may be worthy of further study as constituents of experimental prophylactic vaccines.