Signal sequences modulate the immunogenic performance of human hepatitis C virus E2 gene

Envelope protein E2 of human hepatitis C virus (HCV) is an attractive component of a prototype HCV vaccine. Delivered by DNA immunogens, E2 evokes specific immune response of Th1-type, failing to induce either considerable antibody production, or T-helper cell proliferation. We aimed at modulating the immunogenic performance of E2 gene by changing the mode of protein expression in eukaryotic cells. Plasmids were constructed encoding full-length E2 and nonstructural protein 1 (p7) fused to either 13 or 38 C-terminal amino acids (aa) of HCV E1 that contain second hydrophobic segment of E1 stop-transfer signal, or a complete E1 stop-transfer signal with duplicated second hydrophobic segment. Injected into BALB/c mice, E2/p7 genes induced potent antibody and T-helper cell response targeted against hypervariable region 1, aa 472-586 of E2, and a novel epitope at aa 774-796 of p7. Profile of cytokines secreted by proliferating mouse splenocytes stimulated in vitro with E2- and p7-derived peptides, indicated mixed Th1/Th2 type of immune response. Thus, the full-length E2 and p7 genes supplied in one cassette were both immunogenic. E2/p7 containing a complete E1 stop-transfer signal with prolonged membrane spanning domain was superior to the shorter E2/p7 version in terms of both antibody and cellular immunogenicity. Optimal performance of HCV E2 could thus be achieved without the aid of external/heterologous signals by easing, through modification of the E2 signal sequence, the release of E2 from the rough ER while retaining full-length E2 and p7 sequences. This finding may help to improve the Th2 performance of HCV envelope genes as prototype vaccines.