Abstract
Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is a debilitating but poorly-understood disease. ME/CFS symptoms can range from mild to severe, and include immune system effects alongside incapacitating fatigue and post-exertional disease exacerbation. In this study, we examined immunological profiles of people living with ME/CFS by flow cytometry, focusing on cytotoxic cells, to determine whether people with mild/moderate (n= 43) or severe ME/CFS (n=53) expressed different immunological markers. We found that people with mild/moderate ME/CFS had increased expression of cytotoxic effector molecules alongside enhanced proportions of early-immunosenescence cells, determined by the CD28 (-) CD57 (-) phenotype, indicative of persistent viral infection. In contrast, people with severe ME/CFS had higher proportions of activated circulating lymphocytes, determined by CD69 (+) and CD38 (+) expression, and expressed more pro-inflammatory cytokines, including IFNγ, TNF and IL-17, following stimulation in vitro , indicative of prolonged non-specific inflammation. These changes were consistent across different cell types including CD8 (+) T cells, mucosal associated invariant T cells and Natural Killer cells, indicating generalised altered cytotoxic responses across the innate and adaptive immune system. These immunological differences likely reflect different disease pathogenesis mechanisms occurring in the two clinical groups, opening up opportunities for the development of prognostic markers and stratified treatments.