FIG. 4.

Alternative pathways for induction of T-cell-independent and T-cell-dependent IFN-γ synthesis. (A) The T-cell-independent pathway is driven largely by IL-12, as well as TNF-α, IL-1β, and IL-15. These proinflammatory cytokines induce NK cell production of IFN-γ, which can promote macrophage activation and acquisition of microbiostatic activity, as well as Th1-cell differentiation. (B) Presentation of parasite peptide by a professional Ag-presenting cell (APC) such as a macrophage or dendritic cell to a Thp cell. This results in Th1 differentiation in the presence of T-cell-derived IL-2 and IL-12 produced by the Ag-presenting cell. (C) A less well-characterized but potentially very important pathway of early IFN-γ production, in which T. gondii is able to directly activate T cells in a manner that does not require IL-12.