DNA vaccination with plasmids encoding the intracellular (HBcAg) or secreted (HBeAg) form of the core protein of hepatitis B virus primes T cell responses to two overlapping Kb- and Kd-restricted epitopes
Plasmid DNA encoding either the intracellular form HBcAg or the secreted form HBeAg of the core protein of hepatitis B virus (HBV) was injected into the muscle of H-2b, H-2d or F1b x d mice. Serum antibody responses and class I-restricted cytotoxic T lymphocyte (CTL) responses to HBcAg/ HBeAg were detected in all mice tested. Stable murine H-2b and H-2d transfectants that express either intracellular HBcAg were secreted HBeAg were constructed. With these cell lines we restimulated in vitro T cells primed in vivo and detected their specific cytolytic reactivity against naturally processed peptides. CD8+ CTL responses elicited by DNA vaccination with plasmids encoding HBcAg or HBeAg were specific for the (previously described) Kd-binding HBcAg93-100 peptide MGLKFRQL in H-2b mice or the (newly defined) Kd-binding HBcAg87-96 peptide SYVNTNMGL in H-2d mice. The overlapping epitopes span residues 87-100 of HBcAg, and are present on HBcAg and HBeAg. CTL responses were equally well elicited in vivo by injecting HBcAg- or HBeAg-expressing plasmid DNA, and CTL efficiently recognize in vitro HBcAg- and HBeAg-expressing transfectants. DNA vaccination of F1b x d mice with HBcAg- or HBeAg-expressing plasmid DNA primed CTL populations that recognized the Kb- or the Kd-restricted epitope. Both Kb- and Kd-binding peptides are thus generated from cytoplasmic/nuclear HBcAg and secreted HBeAg. These data make it unlikely that the appearance of HBeAg-negative variants during chronic HBV infection results from CTL-driven selection. DNA vaccination is an efficient technique to prime CTL responses against overlapping epitopes present on intracellular or secreted viral protein antigens.