IL-6 Activity is Required for Maximal Catecholamine Release During High-Intensity Exercise in Men.

High-intensity exercise triggers a coordinated activation of metabolic, endocrine, and immune pathways, yet the mechanisms integrating these responses remain incompletely understood. Interleukin-6 (IL-6), released during exercise, has been proposed as a systemic signal linking skeletal muscle activity to whole-body stress responses. We tested whether exercise-induced IL-6 is required for full sympathoadrenal activation and immune cell mobilization during intense exercise in humans. Healthy young men received the IL-6 receptor (IL-6R) antibody tocilizumab prior to high-intensity interval exercise. IL-6R blockade reduced circulating epinephrine by ∼50%, lowered plasma glucose levels, and attenuated lactate accumulation, resulting in a smaller decline in blood pH. Immune cell mobilization was selectively impaired, with reduced recruitment of lymphocytes, CD8+ T cells, CD56ˆbright natural killer (NK) cells, monocytes, neutrophils, and dendritic cells, while CD4+ T cells, CD56ˆdim NK cells, and B cells were unaffected. Although upstream hypothalamic-pituitary-adrenal (HPA)-axis hormones corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP) were unchanged, adrenocorticotropic hormone (ACTH) was significantly reduced and associated with pH and catecholamine responses in the control condition. A lower lactate-to-pyruvate ratio during IL-6R blockade suggests enhanced pyruvate oxidation as a potential upstream mechanism. These findings position IL-6 as an integrative metabolic signal that mediates organ crosstalk and amplifies the HPA and sympathoadrenal response during high-intensity exercise. By linking skeletal muscle metabolic stress to endocrine activation, glucose regulation, and immune cell mobilization, IL-6 appears to coordinate the complex systemic fight-or-flight response to intense physical exertion.