In Evolution's Unending Race: Ancestral STING Sensors in Salmo salar Mediate Intracellular Bacterial Detection and Programmed Cell Death Through Evolutionarily Conserved Pathways

Alejandro J Yañez^1,2Jorge F. Beltrán³Claudia Barrientos⁴Genaro Soto-Rauch⁴Marcelo Aguilar^2,4Adolfo Isla^2,4,5*Sandra Flores⁴Yassef Yuivar⁶Adriana Ojeda⁶Felipe Almendras¹Patricio Bustos Salgado⁶Marcos Mancilla⁶

• ¹Greenvolution Spa, Puerto Varas, Chile
• ²Interdisciplinary Center for Aquaculture Research, University of Concepcion, Concepción, VIII Biobío Region, Chile
• ³Department of Chemical Engineering, Faculty of Engineering and Sciences, University of La Frontera, Temucco, Chile
• ⁴Laboratorio de Dignostico y Terapia, Facultad de Ciencias, Universidad Austral de Chile, Valdivia, Chile
• ⁵Departamento de Ciencias Básicas, Facultad de Ciencias, Universidad Santo Tomás,, Valdivia, Chile
• ⁶ADL Diagnostic Chile Ltda, Puerto Montt, X Los Lagos Region, Chile

“In evolution’s unending race survival demands continuous adaptation to stop is to fall behind.” The Stimulator of Interferon Genes (STING) pathway embodies this principle, conserved from Drosophila to salmon and humans as a master regulator of cytosolic DNA detection. While its antiviral roles in mammals are extensively studied, structural features and regulatory functions in teleosts during intracellular bacterial stress remain poorly defined. Here, we structurally characterize the ancestral STING ortholog from Atlantic salmon (Salmo salar), revealing conserved essential motifs, including cyclic dinucleotide (CDN)-binding cleft and phosphorylation regulatory sites, through AlphaFold-guided modeling, supporting cross-species functional orthology. In silico docking demonstrates that a validated human STING agonist binds salmonid STING with high affinity, highlighting translational potential via conserved CDN-binding clefts. Transcriptomics reveal strong sting1 expression in immune tissues, rapidly induced by intracellular pathogen Piscirickettsia salmonis. In infected SHK-1 macrophage-like cells, sting1 peaks at 4 hours post-infection (hpi), alongside Interferon-alpha (IFN-α), tumor necrosis factor-alpha (TNF-α), and interleukin-1 beta (IL-1β). However, this inflammatory burst collapses by 5 days post-infection (dpi), despite sustained sting1 expression, indicating functional uncoupling through bacterial immune evasion. In vivo, sustained activation of DEAD-box Helicase 41 (DDX41)–STING signaling correlates with reduced pyroptosis, necroptosis and inflammatory responses, highlighting bacterial immune suppression strategies. This study positions S. salar as a high-resolution model for STING biology, introducing the Evolutionary Molecular Immunity Race (EMIR) framework, where STING orchestrates immune fate over 400 million years of host-pathogen coevolution.