HMGB1 down-regulation drives efferocytosis and inflammation suppression: A novel immune adaptation mechanism in Schizopygopsis younghusbandi responding to heat stress.

Schizopygopsis younghusbandi is a fish species endemic to the Yarlung Tsangpo River system on the Qinghai-Tibet Plateau and possesses unique adaptive capabilities to extreme environments; however, its thermoadaptive mechanism remains unclear. In this study, an acute heat stress model (16 °C, 20 °C, and 24 °C for 48 h) was established, and histopathological analysis, transcriptome sequencing, and in vitro cell experiments were integrated to systematically investigate the effects of heat stress on head kidney tissue structure, expression of immune-related genes, and cell death pathways. The results showed that acute heat stress induced only mild hemorrhage and congestion in the head kidney without causing severe structural damage. Transcriptomic analysis revealed that heat stress downregulated HMGB1 and multiple inflammation-related genes while promoting efferocytosis-related pathways. In vitro experiments confirmed that heat stress inhibited HMGB1 expression in CIK cells and upregulated apoptosis- and efferocytosis-related genes. Overexpression experiments demonstrated that extracellular HMGB1 (SP-HMGB1-GFP) significantly suppressed cell viability, activated inflammatory responses, and inhibited efferocytosis-related genes, suggesting that suppression of HMGB1 contributes to cell survival under high temperature. LPS exposure further validated the central role of HMGB1 in regulating inflammation and the protective effect of HMGB1 suppression under high-temperature conditions. In conclusion, S. younghusbandi may actively downregulate HMGB1 under acute heat stress to suppress excessive inflammation and activate efferocytosis, thereby avoiding the positive feedback loop of "efferocytosis inhibition - secondary necrosis - inflammation amplification" and maintaining immune homeostasis in the head kidney. This finding reveals a novel thermoadaptive mechanism in plateau fish and provides a theoretical basis for the conservation and aquaculture of endemic species under climate warming.