Chronic Fatigue Syndrome: Exercise Performance Related to Immune Dysfunction. Journal: Med Sci Sports Exerc. 2005 Oct;37(10):1647-1654. Authors: Nijs J [1,2], Meeus M [1,2], McGregor NR , Meeusen R , DE Schutter G , VAN Hoof E , DE Meirleir K  Affiliations:  Department of Human Physiology, Faculty of Physical Education and Physiotherapy, Vrije Universiteit Brussel, Brussels, BELGIUM;  Division of Musculoskeletal Physiotherapy, Department of Health Sciences, Hogeschool Antwerpen, Antwerp, BELGIUM; and  Bio21, Institute of Biomedical Research, University of Melbourne, Parksville, Victoria, AUSTRALIA. NLM Citation: PMID: 16260962 PURPOSE: To date, the exact cause of abnormal exercise response in chronic fatigue syndrome (CFS) remains to be revealed, but evidence addressing intracellular immune deregulation in CFS is growing. Therefore, the aim of this cross-sectional study was to examine the interactions between several intracellular immune variables and exercise performance in CFS patients. METHODS: After venous blood sampling, subjects (16 CFS patients) performed a maximal exercise stress test on a bicycle ergometer with continuous monitoring of cardiorespiratory variables. The following immune variables were assessed: the ratio of 37 kDa Ribonuclease (RNase) L to the 83 kDa native RNase L (using a radiolabeled ligand/receptor assay), RNase L enzymatic activity (enzymatic assay), protein kinase R activity assay (comparison Western blot), elastase activity (enzymatic-colorimetric assay), the percent of monocytes, and nitric oxide determination (for monocytes and lymphocytes; flow cytometry, live cell assay). RESULTS: Forward stepwise multiple regression analysis revealed 1) that elastase activity was the only factor related to the reduction in oxygen uptake at a respiratory exchange ratio (RER) of 1.0 (regression model: R = 0.53, F (1,14) = 15.5, P < 0.002; elastase activity P < 0.002); 2) that the protein kinase R activity was the principle factor related to the reduction in workload at RER = 1.0; and 3) that elastase activity was the principle factor related to the reduction in percent of target heart rate achieved. CONCLUSION: These data provide evidence for an association between intracellular immune deregulation and exercise performance in patients with CFS. To establish a causal relationship, further study of these interactions using a prospective longitudinal design is required.