Response to interferons and antibacterial innate immunity in the absence of tyrosine-phosphorylated STAT1

Signal transducer and activator of transcription 1 (STAT1) plays a pivotal role in the innate immune system by directing the transcriptional response to interferons (IFNs). STAT1 is activated by Janus kinase (JAK)-mediated phosphorylation of Y701. To determine whether STAT1 contributes to cellular responses without this phosphorylation event, we generated mice with Y701 mutated to a phenylalanine (Stat1 ^Y701F). We show that heterozygous mice do not exhibit a dominant-negative phenotype. Homozygous Stat1 ^Y701F mice show a profound reduction in Stat1 expression, highlighting an important role for basal IFN-dependent signaling. The rapid transcriptional response to type I IFN (IFN-I) and type II IFN (IFNγ) was absent in Stat1 ^Y701F cells. Intriguingly, STAT1Y701F suppresses the delayed expression of IFN-I-stimulated genes (ISG) observed in Stat1 ^-/- cells, mediated by the STAT2/IRF9 complex. Thus, Stat1 ^Y701F macrophages are more susceptible to Legionella pneumophila infection than Stat1 ^-/- macrophages. Listeria monocytogenes grew less robustly in Stat1 ^Y701F macrophages and mice compared to Stat1 ^-/- counterparts, but STAT1Y701F is not sufficient to rescue the animals. Our studies are consistent with a potential contribution of Y701-unphosphorylated STAT1 to innate antibacterial immunity. Synopsis This study shows that cells and mice expressing an unphosphorylated STAT1Y701F mutant are immunodeficient, but phenotypically distinguishable from Stat1 ^-/- animals. STAT1Y701F expression alters innate immunity to the bacterial pathogens Listeria monocytogenes and Legionella pneumophila. High levels of cellular STAT1 protein require STAT1 tyrosine phosphorylation-dependent tonic signaling (A). Heterozygosity of the Stat1 ^Y701F mutation reduces STAT1 protein expression and interferon-induced tyrosine phosphorylation of the WT allele. STAT1Y701F inhibits nuclear translocation of STAT2 and reduces interferon-induced transcription through STAT2/IRF9 complexes (B). This study shows that cells and mice expressing an unphosphorylated STAT1Y701F mutant are immunodeficient, but phenotypically distinguishable from Stat1 ^-/- animals.