In murine Th1 cells, interferon gamma (IFN-γ) expression can be induced by two alternative pathways: a T-cell receptor (TCR)-dependent pathway and an IL-12/IL-18-dependent pathway. IL-12/IL-18-induced IFN-γ production might perpetuate inflammatory loops in autoimmunity leading to chronicity of inflammation. To evaluate a possible role for IL-12/IL-18 induced IFN-γ secretion in human autoimmune diseases we analyzed here the requirements for TCR-independent IFN-γ expression in human resting memory Th cells mimicking the inflammatory milieu found at the site of inflammation in rheumatoid arthiritis (RA).
Highly purified CD45RO+ memory/effector Th cells from healthy blood donors were cultured in the absence of antigen-presenting cells, but in the presence of different cytokine/chemokine combinations known to be overexpressed in the joints of RA patients. After different timepoints cells were analyzed on the single-cell level for the expression of IFN-γ and other cytokines. In addition, surface molecules such as activation markers and cytokine receptors were analyzed by FACS. To evaluate intracellular activation pathways, p38 MAP-kinase inhibitors or cyclosporinA were added in some experiments.
IFN-γ production could be induced, starting after 18 hours, peaking at 36 hours, in a subset of 2–10% of human memory/effector Th cells with a cocktail of inflammatory cytokines including IL-1β/IL-6/IL-7/IL-8/IL-12/IL-15/IL-18/MIP1α and tumour necrosis factor alpha. In contrast to mice, IL-12, IL-18 and an IL-2-receptor common γ-chain signalling cytokine (IL-2 or IL-7 or IL-15) were determined to be the minimum effective combination. Cytokine-stimulated IFN-γ+ Th cells did not co-produce IL-2/IL-4/IL-5/IL-10 or tumour necrosis factor alpha. TCR-dependent activation in cytokine-stimulated IFN-γ+ Th cells was excluded as cyclosporinA did not block IFN-γ production. However, cytokine-induced IFN-γ production was dependent on the p38 MAP-kinase pathway. In contrast to TCR-triggered IFN-γ+ Th cells, cytokine-stimulated IFN-γ+ Th cells did not upregulate 4-1BB (CD137).
In this study, we have characterized human cytokine-induced IFN-γ+ Th cells. Our results should help to clarify the role of inflammatory cytokine networks for the perpetuation of human Th-cell-driven autoimmune disorders such as RA.