Plasmid DNA vaccines are effective in the absence of IFN gamma

Intramuscular injection of bacterially derived plasmid DNA results in the d evelopment of both humoral and cellular immune responses against plasmid-en coded antigens. Immunostimulatory CpG sequences within bacterial DNA are th ought to enhance this process by stimulating the secretion of proinflammato ry cytokines such as interferon gamma (IFN gamma) by cells of the innate im mune system. Although IFN gamma induction by CpG elements within plasmid DN A has been documented in vitro and more recently in vivo, and coimmunizatio n with plasmids expressing IFN gamma has been shown to enhance DNA-immuniza tion-induced immune responses, it is unclear if IFN gamma is necessary for successful DNA immunization. To address this issue, we compared humoral and cellular immune responses in wild-type and IFN gamma-deficient mice vaccin ated with a plasmid (pCMVNP) expressing the nucleoprotein gene from the are navirus lymphocytic choriomeningitis virus (LCMV). IFN gamma-positive (BALB /c) and IFN gamma-negative (GKO) mice responded to DNA vaccination by the d evelopment of antigen-specific CD8(+) T cells, which were detectable direct ly ex vivo by intracellular cytokine staining and comprised 0.7-2.5% of all CD8(+) T cells in the vaccinee. DNA vaccines also induced virus-specific c ytotoxic T lymphocytes (CTL), even in the absence of IFN gamma. DNA vaccina tion of both mouse strains also was associated with a significant reduction in viral titers after LCMV challenge, indicating that, at least in the pre sence of other immune effector mechanisms, IFN gamma is not required for in duction of protective anti-viral immunity by DNA immunization. No quantitat ive differences were observed in antiviral IgG levels among GKO and BALB/c vacciness, although GKO mice did exhibit a significant reduction of the IgG 2a:IgG1 ratio, in agreement with the previously documented requirement for IFN gamma in isotype switching to IgG2a. Immunized BALB/c mice produced sim ilar levels of both IgG1 and IgG2a, indicating a mixed Th1/Th2 response to intramuscular immunization with pCMVNP. These results show that IFN gamma i nduction by bacterially derived plasmid DNA does not contribute to the magn itude of the antibody response and is not required for the induction or sho rt-term maintenance of DNA-induced CTL. However, IFN gamma is necessary for the development of IgG2a antibodies that may be crucial for protection aga inst some pathogens. (C) 1999 Academic Press.