Requirement for both IL-12 and IFN-γ signaling pathways in optimal IFN-γ production by human T cells

Phytohemagglutinin (PHA)-derived T lymphoblasts or T cell clones from patients genetically deficient in IL-12Rbeta1 (IL-12Rbeta1(-/-) or IFN-gammaR1 (IFN-gammaR1(-/-)) produced two- to threefold reduced IFN-gamma levels compared to the corresponding cells from healthy individuals after anti-CD3 and PMA stimulation. Moderate IFN-gamma production was observed in PHA-derived T lymphoblasts or T cell clones derived from healthy subjects in the presence of anti-IFN-gammaR1 or anti-IL-12 mAb, whereas it was negligible in the presence of both mAb. However, when anti-IFN-gammaR1 and/or anti-IL-12 mAb were added during restimulation, the cells produced normal levels of IFN-gamma, indicating that both IFN-gamma and IL-12 had an effect on the priming phase. Moderate production of IFN-gamma was partially enhanced only in IFN-gammaR1(-/-) T cell clones generated in the presence of IL-12, but was almost completely abolished when IL-12Rbeta1(-/-) and IFN-gammaR1(-/-) T cell clones were generated in the presence of anti-IFN-gammaR1 or anti-IL-12 mAb, respectively. IL-4 production was enhanced in T cell clones from IL-12Rbeta1(-/-), but not from IFN-gammaR1(-/-) patients, whereas IL-10 and IL-2 production did not differ significantly in polyclonal T cells or clones from healthy and deficient individuals. These results indicate that IL-12Rbeta1- and IFN-gammaR1-dependent signals co-ordinately regulate IFN-gamma, but not IL-2 and IL-10 production, whereas only IL-12 negatively controls IL-4 production by in vitro-generated T cell clones. Thus, although IL-12 and IFN-gamma signals are each sufficient for moderate production of IFN-gamma by human T cells, both are needed for optimal IFN-gamma production, and in the absence of both IFN-gamma production is completely abrogated.