TLRs play an integral role in host defense against a broad range of microorganisms1-3
, with TLR7 and TLR9 being particularly important components of the antiviral machinery4-7
. IRAK1 has been shown to be essential for TLR7 and TLR9 mediated IFN-α induction12-14
. However, the mechanisms underlying IRAK1 activation and regulation are not well understood. Our results demonstrate for the first time that Pin1 is a regulator of IRAK1, playing an essential role in TLR signaling and type I IFN-mediated innate and adaptive immunity. Given the major role of aberrant IRAK1 activation and type I IFN overproduction in various immune diseases10, 11
, these results suggest that Pin1 inhibitors, which are under active development21, 25
, may offer a useful therapeutic approach.
We found that Pin1 activity was increased during TLR signaling and that Pin1 genetic deletion modestly inhibited TLR7- and TLR9-dependent, proinflammatory cytokine production in mDCs, but completely abrogated type I IFN production from pDCs. Consistent with these phenotypes, Pin1 specifically acted on IRAK1 autophosphorylation sites in a TLR-dependent manner. Mechanistic evaluation of the consequences of mutations of the Pin1 binding and isomerizing sites in IRAK1 or genetic deletion of Pin1 demonstrated that the role of these phosphorylation sites is to promote a conformational change, leading to IRAK1 activation. Such Pin1-catalyzed conformational change facilitates the dissociation of IRAK1 from the receptor complex to activate downstream transcription factors for the induction of type I interferon. Specifically, Pin1 KO did not affect proximal IRAK1 signaling as its recruitment to the TLR complex and IRAK4 activation were normal. Furthermore, Pin1 KO did not affect other TLR activated kinases such as the MAPKs. However, Pin1 KO prevented IRAK1 activation and release from the receptor complex, leading to loss of recruitment of TRAF6 to the complex, resulting in failure of nuclear translocation of IRF7. As a result, Pin1-deficient cells and mice failed to mount a robust systemic type I IFN response following R-848 and CpG treatment or MCMV inoculation and became highly susceptible to viral infection. These results demonstrate an essential role for Pin1 in IFN-α mediated innate immunity.
In addition to their central role in innate immunity, TLRs have profound effects on the ensuing adaptive immune response. Type I IFNs not only limit viral replication, but also exert various immunoregulatory effects such as B and T cell activation39
as well as mDC maturation40
. Importantly, type I IFNs, following viral infection, also enable cross-priming of CD8+
T cells, thereby allowing the presentation of exogenous antigens in the context of major histocompatability complex (MHC) class I molecules38
KO mice were severely defective in triggering an antigen specific CD8+
T-cell response. These results demonstrate the critical role for Pin1 in regulating IRAK1, thereby representing an important new mechanism pivotally positioned at the interface of innate and adaptive immunity.
The tight regulation of IRAK1 is likely required to avoid inflammatory disease. For example, IRAK1 genetic changes are associated with pathological conditions such as SLE, leading to speculation that its increased expression or constitutive activation makes IRAK1 a disease susceptibility factor for IFN-driven autoimmune disorders41
. Similarly, elevated type I IFN production has been intimately linked to many autoimmune diseases10, 11, 42
In particular, aberrant type I IFN signaling now has a firmly demonstrated role in increased susceptibility to various viral infections, autoimmune disease and cancer. Important unifying themes of such diseases are the lack of treatments and the rather unusual prevalence of these diseases in females. For example, women progress to AIDS much faster than men who have the same viral load and this has been at least in part attributed to substantial sex differences in IFN-α secretion arising from HIV-1 stimulation of TLR7 in dendritic cells, which drives increased CD8 T cell activity43
. Furthermore, the TLR-IRAK1-IRF7-Type I IFN pathway as a key contributor in systemic lupus erythematosus (SLE) pathogenesis, a disease that is particularly prominent in females44, 45
. Specifically, in SLE patients increased concentrations of antibodies to self antigens such as nucleic acids activate TLRs to drive type I IFNs, which then perpetrate autoimmune tissue destruction and pathogenesis46
. Furthermore, recent genetic evidence has definitively associated components of the TLR-IRAK-IRF pathway with SLE47
. A single nucleotide polymorphism in IRAK1 has been associated with increased susceptibility to SLE and Irak1
KO effectively suppresses SLE in animal models41
. These examples highlight the important role of IRAK1 and type I IFN in a range of diseases.
One of the challenges arising from the recent wealth of knowledge on TLR signaling is how to develop a strategy to inhibit specific arms of TLR-mediated immune regulation while leaving other critical defensive nodes untouched, a question no doubt asked by many biologists but one rarely answered. The data here reveal that Pin1 inhibition completely abrogates activation of IRAK1 kinase, and fully suppresses type I IFN production, but with only a moderate effects on pro-inflammatory cytokine production. These results suggest that inhibiting either Pin1 and/or IRAK1 kinase activity might allow pharmacological discrimination, which would allow selective inhibition of the type I IFN response while leaving proinflammatory cytokine production unaffected. Such a pharmacological approach might have advantages over conventional immunosuppressing strategies.