Threonine Phosphorylation and the Yin and Yang of STAT1: Phosphorylation-Dependent Spectrum of STAT1 Functionality in Inflammatory Contexts.

Threonine phosphorylation promotes inflammatory functions of STAT1 while restricting its interferon (IFN) signaling in innate immune responses. However, it remains unclear whether the restriction of STAT1-mediated IFN signaling conferred by threonine phosphorylation is a ubiquitous mechanism or one that is context-dependent. To address this, we utilized pristane-induced lupus, a prototype IFN-driven systemic autoimmune disease model characterized by the production of high-titer autoantibodies against nucleic acid-associated antigens. Through genetic and biochemical assays, we demonstrate that Thr748 phosphorylation is dispensable for STAT1 functionality in pristane-induced lupus. Genetically engineered mice expressing the phospho-deficient threonine 748-to-alanine (T748A) mutant STAT1 exhibited similar survival rates, high titers of anti-dsDNA IgG, and nephritis compared to their wild-type littermates. In sharp contrast, STAT1 deficiency protected mice against pristane-induced lupus, as evidenced by increased survival, low titers of anti-dsDNA IgG, and less severe nephritis in the STAT1 knockout mice compared to their T748A littermates. Our study suggests a phosphorylation-dependent modularity that governs the spectrum of STAT1 functionality in inflammatory contexts: IFN phospho-tyrosine-dependent and inflammatory phospho-threonine-dependent, with Thr748 phosphorylation driving selective inflammatory activities, particularly those not driven by the canonical JAK pathway. From a broader perspective, our findings provide deeper insights into how distinct phosphorylation events shape the combinatorial logic of signaling cassettes, thereby regulating context-dependent responses.