Erythrocyte Deformability As a Potential Biomarker for Chronic Fatigue Syndrome

Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is arguably the last major disease we know almost nothing about. It is a multi-systemic illness of unknown etiology affecting millions of individuals worldwide, with the capacity to persist for several years. ME/CFS is characterized by disabling fatigue of at least 6 months, accompanied serious fatigue and musculoskeletal pain, in addition to impaired short-term memory or concentration, and unrefreshing sleep or extended post-exertional. While the etiology of the disease is still debated, evidence suggest oxidative damage to immune and hematological systems as one of the pathophysiological mechanisms of the disease. Erythrocytes are potent scavengers of oxidative stress, and their shape changes appreciably in response to oxidative stress and certain inflammatory conditions including obesity and diabetes. The shape of erythrocytes change from biconcave discoid to an ellipsoid due shear flow in microcapillaries that provides a larger specific surface area-to-volume ratio for optimal microvascular perfusion and tissue oxygenation establishing the importance not only of total hematocrit but also of the capacity for large deformations in physiology. Clinically, ME/CFS patients show normal arterial oxygen saturation but nothing much is known about microvascular perfusion. In this work, we tested the hypothesis that the erythrocyte deformability in ME/CFS is adversely affected, using a combination of biophysical and biochemical techniques.

We tested the deformability of RBCs using a high-throughput microfluidic device which mimics blood flow through microcapillaries. We perfused RBCs (suspension in plasma) from ME/CFS patients and from age and sex matched healthy controls (n=9 pairs of donors) through a high-throughput microfluidic platform of 5µm width and 3-5 µm height. We recorded the movement of the cells at high speed (4000 fps), followed by image analysis to assess the following parameters: entry time (time required by the cells to completely enter the test channels), average transit velocity (velocity of the cells inside the test channels) and elongation index (ratio of the major diameter before and after deformation in the test channel). We observed that RBCs from ME/CFS patients had higher entry time (~12%, p<0.0001), lower average transit velocity (~17%, p<0.0001) and lower elongation index (~14%, p<0.0001) as compared to RBCs from healthy controls. Taken together, this data shows that RBCs from ME/CFS patients have reduced deformability. To corroborate our findings, we also measured the erythrocyte sedimentation rate (ESR) for these donors which show that the RBCs from ME/CFS patients had lower (~40%, p<0.01) sedimentation rates.

To understand the basis for differences in deformability, we investigated the changes in the fluidity of the membrane using a lateral diffusion assay using pyrenedecanoic acid (PDA), and observed that RBCs from ME/CFS patients have lower membrane fluidity (~30%, p<0.01). Apart from the fluidity, Zeta potential measurements showed that ME/CFS patients had lower net negative surface charge on the RBC plasma membrane (~18%, p<0.0001). Higher levels of reactive oxygen species (ROS) in RBCs from ME/CFS patients (~30%, p<0.008) were also observed, as compared to healthy controls. Using scanning electron microscopy (SEM), we also observed changes in RBC morphology between ME/CFS patients and healthy controls (presence of different morphological subclasses like biconcave disc, leptocyte, acanthocyte and burr cells; area and aspect ratio; levels of RBC aggregation). Despite these changes in RBC physiology, the hemoglobin levels remained comparable between healthy donors and ME/CFS patients. Finally, preliminary studies show that RBCs from recovering ME/CFS patients do not show such differences in cellular physiology, suggesting a connection between RBC deformability and disease severity.

Taken together, our data demonstrates that the significant decrease in deformability of RBCs from ME/CFS patients may have origins in oxidative stress, and suggests that altered microvascular perfusion can be a possible cause for ME/CFS symptoms. Our data also suggests that RBC deformability may serve as a potential biomarker for ME/CFS, albeit further studies are necessary for non-specific classification of the disease.