Respiratory Adaptations of Immune Cells: Metabolic Links Between Exercise, Cardiorespiratory Fitness and Immunity

Background: This dissertation investigates how acute exercise, cardiorespiratory fitness (CRF), metabolic risk factors, inflammation, and aging influence mitochondrial respiration (MR) in human peripheral blood mononuclear cells (PBMCs) and CD4+ T cells. The aim was to examine whether MR reflects adaptive immunometabolic responses across different contexts. Methods: To examine modulators of MR in immune cells, three complementary studies were conducted. The first study investigated acute exercise-induced immunometabolic responses in healthy young adults (n = 15). After VO2max testing, participants performed two randomized treadmill sessions, consisting of 45-minutes of downhill running (12% decline) and level running (1% incline) at 70% of VO2max. Blood samples were collected before, immediately after, 3 hours after, and 24 hours after exercise. Freshly isolated CD4+ T cells were incubated in autologous serum obtained before and three hours after each exercise. MR was assessed by measuring oxygen consumption using high-resolution respirometry (HRR) with Oroboros O2k Oxygraph, complemented by transcriptomic, metabolomic, cytokine, and immune cell subset analyses. The second study examined MR of PBMCs in healthy older adults (n = 16, >55 years) under resting conditions. Following fasting blood sampling and CRF assessment via VO2max testing, PBMC MR was measured using HRR with Oroboros O2k Oxygraph. Associations with immune cell subsets, cytokines, metabolic markers, and lifestyle-related factors were analyzed. The third study comprised a narrative review of 20 studies employing HRR (Oroboros O2k Oxygraph or Seahorse XF Analyzer) to assess MR in PBMCs and their subtypes, as well as in platelets and neutrophils. The review examined the effects of acute exercise, training, and CRF across healthy and diseased populations to frame the current knowledge about MR in the field of exercise immunology. Results: The MR of PBMCs and CD4+ T cells is modulated through different factors. Acute endurance exercise induced a transient systemic inflammatory response but resulted in minor changes in MR of CD4+ T cells. In contrast, higher CRF was associated with enhanced maximal MR, accompanied by increased expression of genes involved in oxidative phosphorylation and a metabolite profile indicative of improved metabolic efficiency. In healthy older adults, higher basal MR was associated with lower pro-inflammatory cytokine levels such as TNF-α and IL-17. Higher maximal MR was linked to greater proportions of CD4+, CD8+, and naïve CD4+ T cells, and was further associated with STAT3, VEGF, and ICAM-1. The narrative review supported these findings, demonstrating that higher CRF and long-term training are frequently associated with improved MR in immune cells, whereas chronic diseases and systemic inflammation are characterized by impaired mitochondrial respiratory function. Conclusion: Collectively, this dissertation demonstrates that MR of immune cells represents a long-term marker of immunometabolic adaptation. MR of immune cells, particularly PBMCs and CD4+ T cells, appears to be modulated by multiple factors, including exercise, CRF, metabolic risk, inflammatory status, immune cell composition, and diseases. These findings place immune cell MR at the intersection of fitness, metabolic health, and inflammation. Future longitudinal studies should investigate whether long-term exercise interventions can enhance MR and thereby improve immunometabolic resilience, to examine causal relationships.