In ME/CFS patients, we found that (1) the changes in the M-wave amplitude post-exercise, the alterations of the redox status induced by muscle exercise, and the CD26-expression level are correlated; (2) the LHS score was correlated to the alterations of the redox status induced by exercise and CD26-expression; (3) the pain component of MOS SF-36 was correlated to the CD26-expression level. These variables were mainly altered in patients with a history of infectious disease.

The oxidative stress refers to an imbalance in the pro- and anti-oxidant status in favour of the former. In healthy subjects, this situation is common in skeletal muscle following exercise because the muscle anti-oxidant defences are weak [9, 36]. In ME/CFS patients, the present work and others reported increased levels of blood markers of oxidative stress (here, TBARS) and a decreased antioxidant defense (here, RAA), a situation that promotes an oxidative stress, and eventually affects the muscle membrane excitability [10–16]. The correlation found here between the level of oxidative stress and the M-wave alteration is consistent with these data.

Our work also reports the correlation of the redox markers with the LHS score of quality-of- life. This relationship is in agreement with studies that examined the impact of an altered oxidant-antioxidant status in ME/CFS and found that the resting blood levels of malondialdehyde [27] and TBARS [28] are associated with variations in cognitive symptoms and sleep disturbances whereas the total symptom score, joint pain, and postexertional malaise correlate with the isoprostane levels [29].

Regarding CD26, we found that its expression was decreased in ME/CFS patients. This observation supports a previous report [18] but contrasts with a study on ME/CFS patients examined after Giardia infection [40]. The CD26 protease activity controls the circulating level of several mediators of pain and mood [21–24]. Because mood changes and myalgia often occur in ME/CFS [1, 2], concentrations of CD26 peptide substrates were addressed in patients, and discrepant conclusions were reached regarding Neuropeptide Y and Substance-P [41, 42]. Here, we found that the scores reflecting the quality-of-life are correlated with CD26-expression. This result is in line with the present observations that myalgia was often reported by our patients (24/36 patients), impacting their quality-of-life scores. We also observed that the CD26 expression level decreased, which may elevate the circulating level of pro-nociceptive peptides and promote myalgia.

Recently, two studies have investigated the CD26-expression level in a context of redox alterations. In acute lymphoblastic leukemia, the CD26 level is correlated with the myeloperoxidase activity, glutathione-s-transferase activity and xanthine oxidase activity [25]. Moreover, a long term loss of CD26-expression was shown to increase the capability to protect against the oxidative stress [26]. The low CD26-expression level here reported in ME/CFS may constitute therefore an adaptive response to the chronic redox imbalance found in ME/CFS patients [10–14 and the present study], this hypothesis being supported by the correlation we found between the CD26 activity and the redox status.

Finally, when patients were sorted according to the presence of infection/intense physical activity/emotional stress in their medical history, we observed that the subjects who reported episode(s) of severe infection had levels of biomarkers and scores of quality-of-life that were more altered compared with those measured in patients who did not report severe infections. These results further support the role of infectious stressors in ME/CFS [7, 8].

We believe that this study has three main limitations: (1) we did not properly quantify the emotional stress because only the presence of a major stressful life event was considered; (2) in order to address the effect of CD26 activity on its peptide substrates, we undertook to measure the circulating levels of neuropeptide Y and substance-P in patients but we did not detect correlation with the CD26-expression level, which may be explained by the fact that only local concentrations of pain and mood mediators act in vivo,their sampling in situ being impossible; (3) as in previous ME/CFS studies [8, 10–14], the finding of an altered redox balance in our subjects was only based on measurements of TBARS (which quantifies the level of membrane lipoperoxydes resulting from an excessive ROS production) and of the reduced ascorbic acid (a major plasma antioxidant). Thus, the effective enhancement of intracellular ROS production in ME/CFS cannot be evaluated first because it would have required muscle biopsies and second because it is counterbalanced by the intracellular antioxidants that are not explored in the present study.