Signal regulatory protein α negatively regulates both TLR3 and cytoplasmic pathways in type I interferon induction

Recognition of double-stranded RNA (dsRNA) activates interferon-regulatory factor 3 (IRF3)-dependent expression of anti-viral factors. The innate immune system recognizes viral dsRNA through two distinct pathways. First, the Toll-like receptor 3 (TLR3) detects dsRNA phagocytosed in endosomes. In addition, the helicases retinoic acid induced protein I (RIG-I)/melanoma differentiation associated gene 5 (MDA5) binds cytoplasmic dsRNA generated during viral replication. Both RIG-I/MDA5 and TLR3 can bind polyriboinosinic:polyribocytidylic acid (poly(I:C)), the synthetic analog of viral dsRNA, and mediate type I IFN production. Here we show that signal regulatory protein (SIRP) α negatively regulates both TLR3- and RIG-1/MDA5-dependent anti-viral pathways. Suppression of SIRPα expression by RNA interference results in enhanced activation of IRF3 and MAPK pathways after poly(I:C) treatment, coupled with the up-regulation of IFN-β and IFN-β-inducible gene transcriptional activation. The requirement of phosphoinositide 3-kinase (PI3K) activity for the induction of IFN-β and IFN-β-inducible genes by dsRNA is supported by the observation that a PI3K inhibitor failed to activate IFN-β and IFN-β-inducible gene expression. PI3K, whose activity is essential for activation of IRF3, is recruited to the phosphorylated tyrosine residues of SIRPα upon poly(I:C) stimulation, which lead to a reduction in the activity of the downstream kinase AKT. Thus SIRPα may accomplish its inhibitory function in type I IFN induction, in part, through its association and sequestration of the signal transducer PI3K.