Hyperosmotic Stress Induces Inflammation and Excessive Th17 Response to Blunt T-Cell Immunity in Tilapia

Abstract Despite the advances in study on osmotic physiology in bony fish, the mechanism by which the immune system, especially T-cell immunity, adapts and responds to osmotic stress remains unknown. In the current study, we investigated the response of T cells to hyperosmotic stress in the bony fish Nile tilapia (Oreochromis niloticus). As a euryhaline fish, tilapia was able to adapt to a wide range of salinities; however, hypertonic stress caused inflammation and excessive T-cell activation. Furthermore, hypertonic stress increased the expression of IL-17A in T cells, upregulated the transcription factor RORα, and activated STAT3 signaling, along with IL-6– and TGF-β1–mediated pathways, revealing an enhanced Th17 response in this early vertebrate. These hypertonic stress–induced events collectively resulted in an impaired antibacterial immune response in tilapia. Hypertonic stress elevated the intracellular ROS level, which in turn activated the p38–MK2 signaling pathway to promote IL-17A production by T cells. Both ROS elimination and the p38–MK2 axis blockade diminished the increased IL-17A production in T cells under hypertonic conditions. Moreover, the produced proinflammatory cytokines further amplified the hypertonic stress signaling via the MKK6–p38–MK2 axis–mediated positive feedback loop. To our knowledge, these findings represent the first description of the mechanism by which T-cell immunity responds to hypertonic stress in early vertebrates, thus providing a novel perspective for understanding the adaptive evolution of T cells under environmental stress.