PCIF1 Attenuates Type I Interferon Induction by Inhibiting IRF3 Activation in a Methyltransferase-Independent Manner

PCIF1 is primarily recognized as an RNA methyltransferase that mediates <i>N</i>⁶-methylation of cap-proximal adenosine (m⁶Am) and plays diverse roles in gene expression. In this study, we uncover a novel role for PCIF1 as a crucial negative regulator of type I interferon (IFN) induction, a pathway critical for antiviral immunity whose dysregulation leads to inflammatory and autoimmune diseases. We demonstrate that PCIF1 deficiency robustly enhances the poly(I:C)-induced type I IFN response, accompanied by augmented STAT1 activation and interferon-stimulated gene (ISG) expression. Mechanistically, PCIF1 suppresses <i>IFNB1</i> transcription by attenuating IRF3 phosphorylation and nuclear translocation, as shown by increased nascent <i>IFNB1</i> mRNA synthesis and promoter activity in PCIF1-deficient cells, without affecting the mRNA stability. Crucially, this suppressive function was independent of PCIF1's canonical RNA methyltransferase activity, as both wild-type PCIF1 and a methyltransferase-inactive mutant effectively attenuated type I IFN induction. Collectively, our findings establish PCIF1 as a novel methyltransferase-independent suppressor of type I IFN responses, revealing its previously unrecognized non-catalytic function. This discovery offers critical insights into the multifaceted regulation of innate immunity and highlights PCIF1's non-catalytic function as a promising therapeutic target for modulating antiviral responses and inflammatory diseases.