IL-12 is required for the development of IFN-γ-dependent resistance to intracellular pathogens but is not thought to play a major role in its maintenance. To directly assess the requirement for continuous IL-12 signaling in long-term cell-mediated immunity, recombinant cytokine was transiently administered to IL-12 p40-deficient mice during the first 2 wk of infection with the intracellular pathogen Toxoplasma gondii. As expected, these animals survived the acute phase and established chronic infections. However, 4–6 wk after IL-12 withdrawal, the mice exhibited increased brain cyst burdens and succumbed to toxoplasmic encephalitis. Reactivation was associated with a loss of T-dependent IFN-γ production without a concomitant increase in Th2 cytokine expression. Importantly, parasite Ag-induced IFN-γ synthesis by purified T cells from these animals could be restored by in vitro exposure to IL-12. These results argue that endogenous IL-12 is required for the long-term maintenance of IFN-γ-dependent resistance against intracellular pathogens.

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