One of the most consistent findings in ME/CFS subjects is poor NK cell function. Using K562 cells as target cells, 16 of 17 studies reviewed found poor function in subjects compared with healthy controls. However, this finding should be interpreted with caution as even the strongest of these studies are subject to methodological limitations discussed at the beginning of
Chapter 4. Furthermore, it is unclear from the description of the methodology of some of the studies whether multiple studies included the same subjects. The largest study compared 176 ME/CFS subjects with 230 healthy controls and found a significant group effect of poorer NK cell function in the ME/CFS cohort (Fletcher et al., 2010). Curriu and colleagues (2013) showed that there were differences in mean cytotoxicity between ME/CFS subjects and healthy controls, but the range was the same. Brenu and colleagues (2012b) studied 65 ME/CFS patients and 21 matched controls in a longitudinal study of three time points over 12 months and found significant deficits in NK cytotoxic activity in the patient group at each time point using peripheral blood mononuclear cells (PBMCs) and a flow cytometric measure of killing. Caligiuri and colleagues (1987) demonstrated reduced cytotoxic activity of ME/CFS NK cells to K562 targets. On the other hand, one study with 26 ME/CFS patients and 50 controls failed to demonstrate impaired NK cell function in the ME/CFS patients using a K562 chromium (Cr) release assay of peripheral blood lymphocytes (PBLs) (Mawle et al., 1997). The authors of this study do not report NK cell counts or CD3-CD56+, but as described, NK numbers generally are not low in ME/CFS.
Low NK cytotoxicity is not specific to ME/CFS. It is also reported to be present in patients with rheumatoid arthritis, cancer, and endometriosis (Meeus et al., 2009; Oosterlynck et al., 1991; Richter et al., 2010). It is present as well in healthy individuals who are older, smokers, psychologically stressed, depressed, physically deconditioned, or sleep deprived (Fondell et al., 2011; Whiteside and Friberg, 1998; Zeidel et al., 2002).
A few studies found a correlation between the severity of NK cell functional impairment and the severity of disease in ME/CFS patients (Lutgendorf et al., 1995; Ojo-Amaize et al., 1994; Siegel et al., 2006). Others looked at mechanisms of cellular dysfunction in ME/CFS and identified abnormalities in early activation markers (Mihaylova et al., 2007) and perforin and granzyme concentration (Maher et al., 2005), as well as in the genes that regulate these cellular functions (Brenu et al., 2011, 2012a). However, no replication studies have been published.