T cells activated with PHA + IFN-γ or PHA + IL-12 exhibit an increase in IL-12–induced IFN-γ production. (A) PBMC activated with either PHA alone, PHA + IFN-γ, or PHA + IFN-γ + anti–IL-12 were plated at 3 × 10⁴ cells/well and cultured for an additional 72 hours with the indicated concentrations of IL-12. Supernatants were then harvested and the IFN-γ concentration measured by ELISA. The values shown were obtained after subtracting the amount of IFN-γ produced by the same T cells in response to medium alone for 72 hours. Results are representative of two separate experiments. (□) PHA; (▪) PHA + IFN-γ; (▩) PHA + IFN-γ + anti–IL-12. (B) PBMC activated for 4 days with either PHA alone or PHA + IL-12 + anti–IFN-γ were plated at 3 × 10⁴ cells/well and cultured for an additional 72 hours with the indicated concentrations of IL-12. Supernatants were then harvested and the IFN-γ concentration determined by ELISA as described in (A). Results are representative of two separate experiments. (□) PHA; (▪) PHA + 1 pmol/L IL-12 + anti–IFN-γ.