Exercise, mitochondrial stress, and trained immunity: metabolic adaptation of innate immunity

Traditionally recognized as "cellular powerhouses", mitochondria have gained relevance as pivotal nodes in the integration of metabolism, stress signaling, and innate immunity. In this context, the present work seeks to answer the following question: Does the continuous, exercise-induced mitochondrial stress contribute towards training of innate immune cells by promoting the generation of DAMPs such as mtDNA and succinate? Exercise can be considered as a form of controllable mitochondrial stressor. Mechanistically, the temporary release of mtDAMPs through exercise results in activation of pattern recognition receptors (NLRP3, TLR9, cGAS-STING). Subsequently, there is a metabolic reprogramming event favoring switch from oxidative phosphorylation to aerobic glycolysis along with epigenetic changes (H3K4me3, H3K27ac) priming pro-inflammatory genes for enhanced secondary response. Moderate-intensity exercise develops an immune homeostatic condition with reduced low-grade inflammation and increased reactivity, while sedentary behavior fosters chronic low-grade inflammation, and excessive high-volume exercise can temporarily reduce immune competency. Herein, we present an integrative model where exercise-induced mitochondrial stress as a physiological "training vaccine" to enhance immune surveillance via trained immunity principles. The current model helps differentiate the immune status of elite athletes from sedentary subjects and paves the way for understanding the immunological benefit of exercise prescription in infection prevention, metabolic health, and cancer immunotherapy.