Metainflammation alters neutrophil function and migration in vivo in response to tissue injury.

Overnutrition and the consumption of Western-type diets lead to chronic low-grade systemic inflammation (i.e., metainflammation) and a dysfunctional immune response. Although neutrophils are affected by metainflammation, mechanistic evidence regarding the direct effects of dietary fat exposure on neutrophil function and migration in vivo, particularly in response to injury, remains limited. Here, we investigated how metainflammation induced by a high-cholesterol diet (HCD) influences neutrophil function and migration following tissue injury. We employed a tailfin transection model in juvenile zebrafish larvae with fluorescently tagged neutrophils fed an HCD and assessed neutrophil function and migration dynamics in vivo at the injury site and whole animal. We combined long-term, non-invasive intravital confocal microscopy with computational analysis to examine neutrophil behavior, and photoconversion techniques were used to track neutrophil mobilization across the larvae. Exposure to HCD resulted in a dysfunctional neutrophil response characterized by exacerbated recruitment, increased ROS production and NETosis, impaired apoptosis, and delayed inflammation resolution. Neutrophil forward and reverse migration were also significantly impacted at the injury site. Moreover, we identified diet-inflamed regions such as the liver and intestine as sources of activated neutrophils that reverse-migrate and respond to injuries at distant sites, contributing to inter-organ transmission of inflammation. Finally, ameliorating steatosis and systemic chronic inflammation rescued the exaggerated neutrophil recruitment to injury. Overall, our study highlights the crucial role of neutrophil dysregulation and reverse migration from diet-induced inflamed tissues in driving exaggerated and dysfunctional inflammatory responses to injury, providing insight into potential therapeutic strategies to alleviate these effects in metabolic disease.