Humoral and CD4⁺ T helper (Th) cell responses to the hepatitis C virus non-structural 3 (NS3) protein: NS3 primes Th1-like responses more effectively as a DNA-based immunogene than as a recombinant protein

The non-structural 3 (NS3) protein is one of the most conserved proteins of hepatitis C virus, and T helper 1 (Th1)-like responses to NS3 in humans correlate with clearance of infection. Several studies have proposed that DNA-based immunizations are highly immunogenic and prime Th1-like responses, although few head-to-head comparisons with exogenous protein immunizations have been described. A full-length NS3/NS4A gene was cloned in eukaryotic vectors with expression directed to different subcellular compartments. Inbred mice were immunized twice in regenerating tibialis anterior (TA) muscles with either plasmid DNA or recombinant NS3 (rNS3). After two 100 μg DNA immunizations, specific antibody titres of up to 12960 were detected at week 5, dominated by IgG2a and IgG2b. NS3-specific CD4⁺T cell responses in DNA-immunized mice peaked at day 13, as measured by proliferation and IL-2 and IFN-γ production. Mice immunized with 1-10 μg rNS3 without adjuvant developed antibody titres comparable to those of the DNA-immunized mice, but dominated instead by IgG1. CD4⁺ T cell responses in these mice showed peaks of IL-2 response at day 3 and IL-6 and IFN-γ responses at day 6. With adjuvant, rNS3 was around 10-fold more immunogenic with respect to speed and magnitude of the immune responses. Thus, immunization with rNS3 in adjuvant is superior to DNA immunization with respect to kinetics and quantity in priming specific antibodies and CD4⁺ T cells. However, as a DNA immunogen, NS3 elicits stronger Th1-like immune responses, whereas rNS3 primes a mixed Th1/Th2-like response regardless of the route, dose or adjuvant.