Plasmid DNA Expressing a Secreted or a Nonsecreted Form of Hepatitis C Virus Nucleocapsid: Comparative Studies of Antibody and T-Helper Responses Following Genetic Immunization

Abstract

In a murine model, we have compared humoral and T-helper (Th) responses induced following genetic immunization with two hepatitis C virus (HCV) plasmid-derived immunogens: a plasmid expressing the fulllength nucleocapsid (CAP) as a nonsecreted antigen (pCMVC2) and a plasmid expressing the amino-terminal part of CAP as a secreted antigen (pS2S.C2N). In BALB/c mice, intramuscular injection of either plasmid induced IgG_2a antibodies associated with a Thl-like profile characterized by the in vitro splenic production of interleukin-2 (IL-2) and interferon-γ (IFN-γ). The pS2S.C2N plasmid induced antibody titers three- to fivefold higher than those obtained with the pCMVC2 plasmid (maximal titers 1:1,500 versus 1:500). In control experiments, immunization using purified CAP antigen induced a predominant, but not exclusive, Th2-like profile as determined by the splenic production of IL-4 and IL-10. Six putative Th determinants were identified using a panoply of overlapping synthetic peptides in in vitro stimulation assays: amino acids 20-44, 39-63, 79-113, 89-113,118-142, and 138-152. For all CAP immunogens, MHC haplotype of immunized mice was found to influence seroconversion rates but not the type of cytokines produced in vitro. H-2^d mice were faster responders and displayed recall T-cell activation by a larger number of peptides than H-2^b mice, whereas H-2^S mice were overall very poor responders. Splenic stimulation by at least one determinant, amino acids 79-103, appeared to be highly specific of the H-2^b background and of DNA immunization only. These data indicate that DNA immunogens expressing different forms of HCV-CAP are not associated with different Th profiles but rather different seroconversion rates and antibody titers and that collaboration of distinct T-help epitopes can be restricted by the MHC background.