Toll-like receptor-mediated induction of type I interferon in plasmacytoid dendritic cells requires the rapamycin-sensitive PI ( 3 ) K-mTOR-p 70 S 6 K pathway

Robust production of type I interferon (IFN-α/β) in plasmacytoid dendritic cells (pDCs) is crucial for antiviral immunity. Here we show involvement of the mammalian target of rapamycin (mTOR) pathway in regulating interferon production by pDCs. Inhibition of mTOR or its ‘downstream’ mediators, the p70 ribosomal S6 protein kinases p70S6K1 and p70S6K2, during pDC activation by Toll-like receptor 9 (TLR9) blocked the interaction of TLR9 with the adaptor MyD88 and subsequent activation of the interferon-regulatory factor IRF7, which resulted in impaired IFN-α/β production. Microarray analysis confirmed that inhibition of mTOR by the immunosuppressive drug rapamycin suppressed antiviral and anti-inflammatory gene expression. Consistent with this, targeting rapamycin-encapsulated microparticles to antigen-presenting cells in vivo resulted in less IFN-α/β production in response to CpG DNA or the yellow fever vaccine virus strain 17D. Thus, mTOR signaling is crucial in TLR-mediated IFN-α/β responses by pDCs. Plasmacytoid dendritic cells (pDCs) are specialized immune cells able to rapidly produce large amounts of type I interferon for antiviral innate immunity1–3. Induction of the antiviral innate immune response depends on recognition of viral components by Toll-like receptors (TLRs)4. The pDCs ‘preferentially’ express virus-recognizing TLR7 and TLR9, which sense viral single-stranded RNA and unmethylated CpG-rich DNA, respectively5. ‘Downstream’ of TLR7 and TLR9 engagement, interferon production by pDCs depends absolutely on the adaptor protein MyD88–interferon-regulatory protein IRF7 signaling pathway6,7. Data have shown that phosphatidylinositol-3-OH kinase (PI(3)K)–Akt kinase activity influences TLRinduced cytokine production in myeloid DCs8. The kinase mTOR (mammalian target of rapamycin) is a crucial ‘downstream’ mediator of PI(3)K-Akt signaling and a key regulator © 2008 Nature Publishing Group Correspondence should be addressed to B.P. (bpulend@rmy.emory.edu). AUTHOR CONTRIBUTIONS W.C. and B.P. designed the experiments; W.C. did the experiments; S.M. did the siRNA transfection; H.T. did the in vivo experiments; S.P.K. and N.M. prepared the microparticles; A.P. analyzed the microarray data; and W.C. and B.P. wrote the manuscript. Note: Supplementary information is available on the Nature Immunology website. Reprints and permissions information is available online at http://npg.nature. com/reprintsandpermissions/ NIH Public Access Author Manuscript Nat Immunol. Author manuscript; available in PMC 2013 August 03. Published in final edited form as: Nat Immunol. 2008 October ; 9(10): 1157–1164. doi:10.1038/ni.1645. N IH PA Athor M anscript N IH PA Athor M anscript N IH PA Athor M anscript of cell growth and proliferation9–11. Increasing evidence suggests that mTOR integrates signals from nutrients, energy status and growth factors to regulate many processes, including autophagy, ribosome biogenesis and metabolism9–11. It has been found that mTOR also is important in the survival and maturation of myeloid DCs12, which suggests there is crosstalk between TLR signaling and the PI(3)K-Akt-mTOR pathway. However, it is unclear whether mTOR influences the function of pDCs. Here we show that the rapamycin-sensitive PI(3)K-mTOR–p70 ribosomal S6 protein kinase (p70S6K) pathway is crucial for TLR-mediated induction of type I interferon in pDCs. Inhibition of mTOR or p70S6K by various approaches resulted in much less TLR-mediated production of interferon-α (IFN-α) by pDCs. The mechanism of this inhibition seemed to involve disruption of the TLR9-MyD88 complex and subsequent impairment of phosphorylation and nuclear translocation of IRF7. A physiological function for the mTOR pathway in regulating IFN-α production was emphasized by the finding that mTOR inhibition in vivo resulted in less IFN-α in serum and in pDCs in response to stimulation with a TLR ligand or viral vaccine, which ultimately led to impaired immune responses.