Interleukin-12-induced Interferon-γ Production by Human Peripheral Blood T Cells Is Regulated by Mammalian Target of Rapamycin (mTOR)

Abstract

Depending on the type of external signals, T cells can initiate multiple intracellular signaling pathways that can be broadly classified into two groups based on their sensitivity to the immunosuppressive drug cyclosporin A (CsA). Interleukin (IL)-12-mediated interferon (IFN)-␥ production by activated T cells has been shown to be CsA-insensitive. In this report, we demonstrate that the IL-12-induced CsA-resistant pathway of IFN-␥ production is sensitive to rapamycin. Rapamycin treatment resulted in the aberrant recruitment of Stat3, Stat4, and phospho-c-Jun to the genomic promoter region resulting in decreased IFN-␥ transcription. IL-12-induced phosphorylation of Stat3 on Ser-727 was affected by rapamycin, which may be due to the effect of rapamycin on the IL-12-induced interaction between mammalian target of rapamycin (mTOR) and Stat3. In accordance with this, reduction in the mTOR protein level by small interfering RNA resulted in suppression of Stat3 phosphorylation and decreased production of IFN-␥ after IL-12 stimulation. These results suggest that mTOR may play a major role in IL-12-induced IFN-␥ production by activated T cells.During an immune reaction in which antigen-presenting cells interact with T cells, both cell types get activated expressing both cell surface molecules and soluble factors that in turn are involved in further activating each cell type. The cell surface molecules CD40 ligand and IL-12-1 and IL-18 receptors, among others, are expressed on T cells, whereas B7 molecules IL-12 and IL-18 are expressed by antigen-presenting cells. One of the cytokines produced by activated T cells is IFN-␥, which is a potent activator of antigen-presenting cells. Activated antigen-presenting cells produce IL-12 and IL-18, which are potent activators of T cells, which in turn produce IFN-␥. This circular pathway of mutual stimulation is important in eliciting a complete cell-mediated immune response.IL-12 is an important cytokine involved in the production of IFN-␥ by T cells and NK cell, and plays an important role in the differentiation of T helper 1 (Th1) cell population (1-4). A unique aspect of IL-12 signaling is that it works together with IL-18 and the T cell co-receptor CD28 in inducing IFN-␥ production (1, 5, 6). Although IL-12-induced IFN-␥ expression has been shown to occur mostly at the level of transcription, the synergy between IL-12 and ␣CD28 or IL-12 and IL-2 has been shown to occur mostly at the post-transcriptional level, e.g. message stability and mobilization of nuclear mRNA (7-9).Upon binding to its receptor, IL-12 activates Janus family kinases Tyk2 and Jak2, which then phosphorylate the tyrosine residues of Stat3 and Stat4 (10). These tyrosine phosphorylations are responsible for the formation of Stat4/Stat4 homodimer and Stat3/Stat4 heterodimers (10 -12). These dimers then translocate to the nucleus and bind to the promoters of IL-12-responsive genes (11, 13). IL-12 stimulation also induces serine phosphorylation of Stat3 and Stat4, which are important for their transcriptional activ...

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