STK25 functions as an IRF5 kinase to promote TLR7/8-mediated inflammation

Abstract

Abstract Toll-like receptors (TLRs) represent a subset of pathogen-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 myeloid differentiation factor 88 (MyD88)-dependent TLR activation. While the dysregulation of IRF5 activity has been implicated in the development of several autoimmune diseases including systemic lupus erythematosus and rheumatoid arthritis, the factors that modulate TLR-induced IRF5 post-translational modifications (PTMs) are poorly understood. Therefore, the focus of this study is to identify protein kinases involved in the regulation of TLR7/8 signaling. We performed a kinome-based siRNA screen in human THP-1 monocytic cells and human primary myeloid cells to identify important mediators of TLR7/8-induced TNF-α and IL-6 production. We identified serine/threonine protein kinase 25 (STK25) as a positive regulator of proinflammatory cytokine production in response to TLR7/8 activation in human 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 threonine residue. Thus, our data implicate STK25 as a regulator of TLR7/8 signaling through the modulation of IRF5 activity.