Interleukin-12 treatment during immunization elicits a T helper cell type 1-like immune response in mice challenged with respiratory syncytial virus and improves vaccine immunogenicity.

Adjuvant effects of exogenous interleukin (IL)-12 on induction of immune responses against respiratory syncytial virus (RSV) infection in mice were evaluated. Giving recombinant IL-12 at the time of immunization with a formalin-inactivated alum-precipitated RSV preparation resulted in significant reduction of virus replication in lungs 4 days after RSV challenge. Intraperitoneal or intramuscular IL-12 was effective when given at the time of immunization but not at the time of challenge. IL-12 treatment resulted in increased interferon-gamma mRNA in lungs, increased IgG2a RSV-specific antibody isotype utilization, and increased endogenous IL-12 p40 mRNA expression. IL-12 treatment did not significantly affect clinical outcome or cytotoxic T lymphocyte activity. These data demonstrate that IL-12 has potent adjuvant effects that may be due to induction of T helper cell type 1-like immune responses.