STK25 is an IRF5 kinase that promotes TLR7/8-mediated inflammation

Toll-like receptors (TLRs) represent a subset of pattern-recognition receptors (PRRs) employed by the innate immune system to detect pathogen-associated molecular patterns (PAMPs) and initiate the response to invading microbes. The transcription factor interferon regulatory factor 5 (IRF5) functions as an important mediator of the inflammatory response downstream of MyD88-dependent TLR activation. While the dysregulation of IRF5 activity has been implicated in the development of several autoimmune diseases including systemic lupus erythematosus (SLE) and rheumatoid arthritis, the factors that modulate TLR-induced IRF5 post-translational modifications (PTMs) are poorly understood. Therefore, the focus of this study was to identify and characterize the role(s) of novel kinases in the regulation of TLR7/8 signaling. We performed a kinome-wide siRNA screen in human THP-1 monocytic cells to identify mediators of TLR7/8-induced TNF- and IL-6 production. We identified serine/threonine protein kinase 25 (STK25) as a positive regulator of proinflammatory cytokine release in response to TLR7/8 activation in human primary myeloid cells. We determined that STK25 phosphorylates IRF5 in vitro via multiple biochemical assays. Phosphopeptide mapping by mass spectrometry revealed that STK25 phosphorylates IRF5 at a highly conserved residue, Thr265, that leads to the transcriptional activation of IRF5 in HEK293T cells. We determined that STK25 undergoes autophosphorylation in response to a variety of TLR triggers in multiple immune cell types. We demonstrated that R848-induced IRF5 nuclear translocation and proinflammatory cytokine production was significantly attenuated in immune cells from Stk25-deficient mice compared to wild-type. Finally, we determined that STK25 autophosphorylation is increased at steady-state in peripheral blood mononuclear cells (PBMCs) from SLE donors compared to healthy controls. Thus, our findings implicate STK25 as an important regulator of TLR7/8 signaling through the modulation of IRF5 activation. Significance StatementThe transcription factor IRF5 functions as a master regulator of innate and adaptive immunity. While the hyperactivation of IRF5 has been implicated in the pathogenesis of systemic lupus erythematosus (SLE), the mechanisms leading to the modulation of IRF5 activity are incompletely understood. Here, we conducted a screen of the human kinome to identify IRF5 kinases that function as positive regulators of TLR-induced inflammation. We demonstrate that STK25 directly phosphorylates IRF5 to drive proinflammatory cytokine responses downstream of TLR activation in both human and murine primary immune cells. Altogether, our findings implicate STK25 as a potential therapeutic target for the management of IRF5-mediated immunological disorders.