|ReceivedAug 12, 2022||RevisedAug 22, 2022||AcceptedAug 29, 2022||PublishedSep 16, 2022|
Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) remains an enigmatic highly disabling and complex long-term condition with a wide range of aetiologies and symptoms. A viral onset is commonly mentioned by patients and several bodily systems are ultimately disturbed. The parallel with long-covid is clear. However, immune dysregulation with impaired NK cell dysfunction and tendency to novel autoimmunity have been frequently reported. These may contribute to reactivation of previous acquired viruses/retrovirusesaccompanied by impaired endocrine regulation and mitochondrial energy generation. The unpredictable nature of seemingly unconnected and diverse symptoms that are poorly responsive to several allopathic and alternative therapies then contributes to an escalation of the illness with secondary dysfunction of multiple other systems. Treatment of established ME/CFS is therefore difficult and requires multi-specialty input addressing each of the areas affected by the illness.
Myalgicencephalomyelitis/chronic fatigue syndrome (ME/CFS), Immune system, Viruses, Retroviruses, Cytokines, NK cells, Mitochondria
Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is a long-term condition with a wide range of symptoms, but extreme fatigue affects all patients. There are no specific biomarkers, diagnostic tests or effective treatments for ME/CFS reflecting the inability to define the underlying cause(s) of the disorder. In June 2021 the InvestinME Research charity convened a meeting organized by The European ME Research Group (EMERG) that included ME/CFS researchers from across the world to discuss ME/CFS research both past and current that tests popular hypotheses that explain the aetiology of ME/CFS. These included: immunogenetics and dysfunctional immune responses; an abnormal autonomous nervous system and autoimmunity; mitochondrial metabolic dysfunction; and altered host-microbe (commensal and pathogen) interactions. This article summarizes discussions focusing on the immune system and addressing the hypothesis that immune dysfunction and viral factors may drive the development and progression of ME/CFS. Collectively, the variable patterns of immune dysfunction in ME/CFS may explain the specific combinations of symptoms experienced by patients and their propensity for improvement. Unravelling the details of these changes will not only help patients understand the nature of their illness and its multiple symptoms but also provide important clues as to how it is best treated. This understanding may ultimately help the increasing number of people suffering fatigue and cognitive dysfunction following SARS-CoV-2 infection as part of long-covid.
The clinical phenotype of ME/CFS reflects a combination of symptoms and pathologies that are either instigated by an initiating illness, or by factors associated with on-going neuro-immuno-endocrine disruption  and perpetuated and aggravated by the severity and multiplicity of never-ending inexplicable symptoms and extreme fatigue. This is illustrated in Figure 1, which shows the multiple pathways by which unfavorable interaction between the several bodily systems known to be impaired in CFS/ME can perpetuate and aggravate overall ill health.
Figure 1: Factors causing or perpetuating ME/CFS.
Here the central initiating factor contributes to worsening ill health by disturbance of one or more of the peripheral systems. If left unchecked the primary and secondary disruption leads to adverse activation or impairment of the other systems. This ultimately produces a clinical disorder with multiple, seemingly unrelated, symptoms and which defies a unifying known physical aetiology and which are now recognize to be CFS/ME.In respect to the factors which can initiate CFS/ME, persistent disabling fatigue is not unusual after viral and bacterial infections and is particularly common in rheumatological conditions [2-4]. In ME/CFS a significant immunological component is suggested by the nature of the symptoms (recurrent sore throats, lymph gland enlargement, flu-like sensations, arthralgia and myalgia) and by subtle abnormalities observed in the immune system . In the clinical setting, however, the majority of routine immune based blood tests are normal although low level anti-nuclear antibodies, mildly raised IgM and atypical lymphocytes can be seen in a small proportion of patients. However, despite mildly reduced levels of IgG3 and IgG4 and possible prevalence of reduced mannose binding lectin, an overt clinical immune deficiency and autoimmunity that can be demonstrated in routine blood tests is rarely evident in ME/CFS. Indeed, antibody deficiency warranting replacement therapy was evident in only one of over nine thousand CFS/ME patients seen in our clinic and a systemic connective tissue disorder in the form of Sjogrens syndrome was seen in only two people. Additionally, there many studies reporting inconsistent or contradictory findings from cytokine analyses. However, subtle dysregulation in Natural Killer , T cells  and B cells [9,10] associated with the generation of specific autoantibodies is a hallmark of disease in a significant proportion of CFS/ME sufferers  although there are conflicting reports for each cell type . These abnormalities can be accompanied by distinct transcription changes and epigenetic signatures involving genes that regulate cellular immunity, inflammation, mitochondrial energy generation and autoantibodies targeting autonomic system hormones and receptors [13-14]. However, it remains unclear whether these defects are the cause or the result of ME/CFS because the immune system is significantly influenced by stress, mood and sleep disturbance, all of which are affected by diverse aetiologies of ill health . Alongside this, there is evidence for reactivation of one or more previously-acquired conventional viruses and retroviruses which would normally be kept in check by an optimally functioning immune system[16-17]. Viral or retroviral reactivation caused by illness initiated immune paresis and contributing to inflammation and mitochondrial dysfunctionhas been considered important in precipitating and perpetuating ME/CFS symptoms .
Collectively, variable patterns in immune dysfunction may explain the specific combinations of symptoms experienced by patients and their likelihood of improvement. Unravelling the details of these changes will not only help patients understand the nature of their illness and its multiple symptoms, but also provide important clues as to how it is best treated.
Previous investigations of biochemical, microbiological and immunological abnormalities in subjects with ME/CFS have often considered this to be a single disorder with clinical and immune changes associated with particular stages of the disease and aetiological factors [19-21]. This may explain the absence of any consistent set of immune system related abnormalities in groups of patients for whom diagnosis is primarily achieved by exclusion as there are no specific diagnostic tests. Furthermore, the nature and timing of sample collection as well as the assays used for immune analysis may have an influence as differences in sample processing and cryopreservation can impact significantly on the expression of cellular markers and function .
Cytokine dysregulation has been documented frequently in ME/CFS but the findings have often been contradictory [23-25]. Early studies suggested increased levels of pro-inflammatory cytokines despite normal levels of C-reactive protein  while others have found no difference  or only increases in specific cytokines such as TGF-β [28,21]. A systematic review showed that patients had significantly elevated TNF-α, IL-2, IL-4, TGF-β and CRP . Using a proteomics approach and aptamers able to detect 4797 human protein abundances , found the levels of IL18 binding protein and TNF-α receptor to be raised in 20 female patients with CFS/ME. The results suggested immune dysregulation with efforts to control inflammation. Notwithstanding, lipopolysaccharide (LPS)-induced IL10 secretion in whole blood cultures that was highly sensitive to suppression by dexamethasone, was significantly increased in patients with ME/CFS compared with controls . In a later study , noted significant glucocorticoid suppression of LPS induced IL10 production by PBMCs but this was no different to health controls. In addition to raised inflammatory cytokines, other factors have been reported, specifically: lowered overall plasma anti-oxidant activity; increased N-terminal pro-brain derived natriuretic factor; increased fibroblast growth factor 21 . Positive correlations between some of these variables has also been reported . Interestingly, serum levels of IL4, IFN and soluble CD23 in monozygotic (MZ) and dizygotic (DZ) twins discordant for chronic fatigue  were similar while persistent fatigue was more frequently concordant in the MZ versus the DZ twins. To reduce the variability of individual variations in cytokine levels, the use of cytokine signatures unique to ME/CFS have been reported and used to distinguish the early stages of ME/CFS  and were shown to be correlated with disease severity . More recently attention has been focused on correlating psychological variables and sleep disturbance with cytokine levels . This suggested an association between executive function and IL-1 and IL-6, interpersonal function and IL-6 and TNF-α and sleep with IL1, IL2, IL6 and TNFa . Further correlative studies have shown IL17 levels to be raised in CFS/ME patients in whom IL4, IL10, IL18 and IFNg were normal and there was no correlation with tryptophan metabolites .
The only immune abnormality consistently demonstrated in the majority of reports on ME/CFS is a reduction in the number [38-39] or function  or both of NK cells . However, a recent report based on data from large numbers of well characterized ME/CFS patients (some with severe disease) showed no significant differences in NK cell numbers or cytotoxicity  compared with healthy subjects and patients with Multiple Sclerosis. However, they did note an interesting reduction in terminally differentiated effector CD8 T cells and an increase in effector memory CD8 T cells and mucosal associated invariant T cells . Regardless,  confirmed reduced NK cell numbers in ME/CFS patients and increased numbers of adhesion (CD11b, CD11c and CD54) and activation (CD38) markers on those NK cells. However, others have found NK cells numbers to be higher in subjects with ME/CFS but that NK cell cytotoxicity was reduced compared with healthy controls . Functionally, NK cells recognize their targets by the absence of classical HLA class I proteins and by binding NK stimulatory and inhibitory receptors of the killer immunoglobulin-like receptor (KIR) superfamily. Findings have been variable with some studies showing no change in expression of CD94, numerous inhibitory killer immunoglobulin–like receptors (KIRs) and activating receptors (NKG2D)  while others in severely affected ME/CFS patients reported KIR3DL1 to be significantly increased and NK cytotoxicity reduced compared with healthy controls [45,46]. As further evidence of the disarray in NK results in CFS/ME, Rivas et al in 2018  reported a lower expression of NKG2C, an activating receptor that recognizes Human Leucocyte Antigen E (HLA-E), in contrast to an earlier study where no differences were observed and with the differences attributed to frozen PMBC used in one study and whole blood in the other . Importantly, altered expression of activating and inhibitory NK cell receptors are evident in chronic cytomegalovirus (CMV) infections and in co-infections with other viruses, notably Epstein-Barr virus (EBV) .In patients with severe ME/CFS, defects in protein kinase gene expression in NK cells has been reported . Thirty-seven genes showed increased expression while 55 showed significantly reduced expression, with a significant reduction in calcium-dependent protein kinases, lymphocyte-specific protein kinase (Lck) and Zeta-chain-associated protein kinase 70 (ZAP 70). The authors concluded that this dysregulation could contribute to impaired NK cell function as these kinases play critical roles in intracellular signaling and could disrupt NK responses to its environment. Defects in the mitogen activated protein kinases (MAPK) pathway were observed in ME/CFS patients, such as a reduction in ERK1/2 in cytolytic NK cells while the cytokine producing NK cells showed a significant increase in MEK 1/2 and p38 expression . ERK 1/2 has been identified as having an important role in cytotoxicity and furthermore, phosphorylation of ERK1/2 induces a conformational change that is required for NK cell cytotoxic activity. Thus, aberrant signaling through ERK 1/2 could interfere with the release of lytic proteins and thus explain the reduced NK cytotoxicity. More recently it has been suggested that the cause of NK cell dysfunction is due to a loss in function of Transient Receptor Potential Mela statin ion channel-3 (TRPM-3) activity which is critical for NK cytotoxicity  and could be improved by naltrexone, an opioid receptor antagonist mitigating its inhibitory function on TRPM3 . Interestingly, the latter used in low doses has been found helpful by patients with ME/CFS and also in fibromyalgia.
While a reduction in the numbers of CD19/IgM+ B cells has been observed , ME/CFS has never been linked to symptomatic antibody immunodeficiency with recurrent bacterial infections. Conversely, B cell depletion using a monoclonal anti-CD20 antibody, Rituximab, was initially found to markedly improve clinical symptoms in three patients with ME/CFS  and then in a double-blind placebo-controlled trial with 30 CFS patients . However, no significant benefit was reported by the same group in a more recent report using 12 months of Rituximab .
Recently, B cell subsets were examined and no differences found between ME/CFS patients and healthy controls for classical B cell markers, immunoglobulin D (IgD), CD27 and CD38 as well as CD5, CD21, CD23, B cell activating factor (BAFF) receptor and IgM . Intriguingly, a significant increase in CD24 expression on IgD positive B cells in ME/CFS was evident but there was no correlation with disease duration. Interestingly, CD24 is a glycoprotein expressed on most B cells and is a marker for transitional B cells that are at an intermediate stage in their development between immature bone marrow cells and peripheral mature B cells. These are highly pleomorphic with some polymorphisms associated with autoimmune disease  and possibly also with altered mitochondrial function .
In ME/CFS the function of both memory CD4 and CD8 T cells may be affected and associated with impaired mitochondrial function . T cell memory is complex and based partly on the strength of the initial antigenic stimulus with viral infections providing a very strong stimulus. For all T cells long-lived memory is maintained most significantly by continued antigen stimulation or cross-reactive antigen stimulation. This is evident in persistent viral infections such as those caused by EBV and HHV6 and both HHV6  and EBV can cause major alterations in T cell memory function. In the case of EBV, acute mononucleosis was accompanied by loss of IL-7R expression by all CD8+ T cells, including EBV epitope-specific populations . While expression was rapidly regained in total CD8+ cells it was only slowly and incompletely regained in EBV-specific memory cells  permitting bouts of viral reactivation. Therapeutically this could lead to treatments that would involve using agents active against EBV. T regulatory cells and Th17 cells have been studied in ME/CFS and the results showed that regulatory T cells are either increased in number in ME/CFS patients [62,63] or reduced . These findings need to be confirmed as do the associated increases in Th2 cells  which have central roles in immunity to viruses and the immune response and could be dysregulated in ME/CFS. Regardless, increased Treg function may hypothetically suppress CD4 and CD8 T cells having anti-EBV and HHV6 activity if the prevailing clones manifested virus induced cross reactive immunity to a self-antigen .
Several studies have sought to identify diagnostic biomarkers for ME/CFS, with varying results. Using whole blood gene expression in 29 adolescent ME/CFS patients  found that 176 genes were differentially expressed, including CD79a, which is involved in B cell differentiation, activation and signaling; Tumor necrosis factor receptor superfamily 13C, which is involved in B cell homeostasis; and FLT3 (fms-related tyrosine kinase 3) which has effects on numerous aspects of the immune system including B and pro B cell survival, proliferation, differentiation and responses to cytokines. Collectively, these findings suggest that abnormal B cell differentiation and survival is related to ME/CFS and linked to an altered hypothalamus–pituitary-adrenal (HPA) axis. These B cell abnormalities were also significantly linked to post exertional malaise. Interestingly they also found evidence of an up-regulated antiviral response. Presson  used weighted Gene Co-expression Network Analysis (WGCNA) to integrate gene expression and trait data for ME/CFS. They initially noted 299 genes correlating with chronic fatigue of which 20 candidate genes showed special prominence. Amongst the latter FOXN1, PRDX3 and SUCLA2 were noteworthy as they were involved in thymic T cell development and maturation, mitochondrial energy generation and apoptosis and mitochondrial function. Combining both mRNA expression and DNA methylation data of ME/CFS and healthy controls, performed recursive ensemble feature selection (REFS) on publicly available mRNA expression and DNA methylation data in peripheral blood mononuclear cells. They found a signature of 23 genes capable of distinguishing between ME/CFS patients and controls (AUC of 0.92), ten of which were indicative of a ‘derailed immune system in ME/CFS’ associated with downregulated IL2, CCR4 and MHC class II receptor function as well as proteoglycan 4, an immune cell receptor ligand, and intracellular signaling molecules important for anti-viral immunity.
Consistent evidence of specific viral infections causing ME/CFS remains elusive. However, several investigators have reported increased 2’5’OA synthetase (2-5A) activity in the mononuclear cells of patients with ME/CFS and with levels correlating with disease severity [68-70]. As 2-5A is induced by IFN- and IFN- and is important in combating viral proliferation , the raised levels appear to suggest chronic viral infection in ME/CFS. Intriguingly, increased 2-5A dimers compared to the normal oligomers that inhibit the proteolytic breakdown of RNase L, an important ribonuclease part of anti-viral innate immune defense,  may be responsible for elevated RNase L levels in ME/CFS and were significantly linked to exercise tolerance .
However, the detection of several herpes viruses (HHV), enteroviruses and Borna viruses in patients with ME/CFS by serology and PCR have provided conflicting results. Thus, suggested HHV-6 reactivation after detecting a raised frequency of anti-HHV-6 IgM and detection of HHV-6 antigen in short term PBMC cultures from their patients with ME/CFS. In marked contrast,  in their co-twin study detected no serological evidence of HHV-8, cytomegalovirus, herpes simplex virus 1 and 2 or hepatitis C virus in MZ twins discordant for ME/CFS.
Moreover, there was no PCR evidence of infection with HHV-6, HHV-7, HHV-8, CMV, EBV, herpes simplex virus, varicella zoster virus, JC virus, BK virus and parvovirus B19. This was also noted in an earlier study by in 1996  of 548 chronically fatigued patients using serological techniques for rubella, coxsackie B viruses and adenovirus. Similar negative results for increased viral prevalence in those with ME/CFS was presented by . In contrast, others found serological evidence of an increased frequency of previous EBV and Coxackie viruses B1 and B4 in patients with ME/CFS . IgM antibodies to non-structural genes in human CMV have also been detected in a subset of ME/CFS patients . This group also found IgM antibodies to EBV in a subset of ME/CFS patients indicating that a defect in the immune system could be permitting chronic infection by viruses. More recently salivary samples from patients with CFS/ME have shown increased DNA for HHV6 and HHV7 which in many patients correlated with fatigue and symptom severity .
Further dysregulation of the immune system is suggested by the detection of antibodies to mitochondrial components and also to serotonin, microtubule-associated protein 2 and muscarinic cholinergic receptor 1 . Similar endocrine receptor auto-antibodies were also subsequently reported by  and more recently against G-protein coupled receptors by . Furthermore, auto-antibodies against alpha 1 adrenergic and muscarinic M3 receptor was positively correlated with soluble CD23 in patients with an infective onset to their CFS/ME . In each case, infection stimulated autoimmunity has been proposed as an explanation for many of the symptoms in ME/CFS . In this respect, active EBV and HHV6, infection does not appear necessary for the development of autoimmunity [84,85] and several viral proteins can aid the persistence of immune cells with autoimmune tendency. In respect to CFS/ME, stress has been reported to reactivate EBV [16,86] and it is possible that the increased stress suffered by patients with ME/CFS may contribute to recurrent relapses in ME/CFS. For HHV6, however, reactivation may be necessary, and this has been shown to induce mitochondrial degeneration and impaired ATP generation which could theoretically at least contribute to fatigue symptoms .
Regarding other viruses, enterovirus particles in gastric biopsies of patients with confirmed ME/CFS have been reported  and supporting previous work suggesting enteroviral persistence in patients with ME/CFS . Previous negative associations between enteroviruses and ME/CFS have recently been attributed to sampling issues, methodological inadequacies using serological, tissue culture and molecular technologies and ‘incomprehensive’ detection panels for each method . Active lytic infection is also not necessary for clinical illness as many enteroviruses are capable of disrupting cellular homeostasis and mitochondrial energy production by several means .
A non-excluding possibility to explain ME/CFS symptoms and their relationship with immune dysregulation, is as result of aberrant activation of human endogenous retrovirus (HERVs). HERVs are dormant viral sequences incorporated in our genome through evolution which actually represent about 8% of our genome (~four times the size of protein-coding sequences)) [91-93]. Acquired through ancient exogenous infections they provide essential functions and the main source for human evolution [94,95].
Although most elements appear permanently silenced in adult cells [96,97], environmental factors, including exogenous infections, may compromise their restriction [98-100]. Importantly, differential expression of HERVs in fibromyalgia and ME/CFS has been evidenced by several groups [101,102]. Furthermore, HERV activation has been found in lymphocytes of COVID patients correlating with inflammatory markers and pneumonia´s severity . Thus, the possibility that Long-COVID, a post-viral syndrome with symptoms that overlap those of ME/CFS [103,104], derives from the incapacity of some patients to re-silence HERVs seems to deserve interrogation. Symptomatically, HERV activation may translate into infection-like symptoms, such as pain, fatigue, immune and metabolic disturbances [105,106] which are common in ME/CFS.
In multiple sclerosis and amyotrophic lateral sclerosis HERV activation can lead to demyelination, inflammation and cognitive problems . HERVs detection and targeting opens up new diagnostic and therapeutic avenues [107,108].
Is the immune dysregulation hypothesis proven?
After over 20 year of attempting to identify specific immune defects and infectious agents that cause ME/CFS, clear pathways are still not evident. However, a complex interaction between the immune system and viruses/retroviruses is now emerging and manifesting subtle changes in cytokines, NK cells and T cells which directly or via autoimmunity may be responsible for the plethora of symptoms seen in ME/CFS. Combined with immune and pathogen-induced mitochondrial dysfunction now being dissected by several research groups it is quite possible that it will gain real insight into the cause of this enigmatic and highly disabling illness.
1. MurgaGandasegui I, AranburuLaka L, Gargiulo PÁ, Gómez Esteban JC, Lafuente Sánchez JV. Myalgic Encephalomyelitis/Chronic Fatigue Syndrome: A Neurological Entity?Medicina. 021;57(10):1030. PubMed | CrossRef
2. Galbraith S, Cameron B, Li H, Lau D, Vollmer-Conna U, Lloyd AR. Peripheral blood gene expression in postinfective fatigue syndrome following from three different triggering infections. J Infect Dis. 2011;204(10):1632-40. PubMed | CrossRef
3. Townsend L, Dyer AH, Jones K, Dunne J, Mooney A, Gaffney F, et al. Dowds J, Sugrue JA. Persistent fatigue following SARS-CoV-2 infection is common and independent of severity of initial infection. PLoS One. 2020;15(11):e0240784. PubMed | CrossRef
4. Iannuccelli C, Spinelli FR, Guzzo MP, Priori R, Conti F, Ceccarelli F, et al. Fatigue and widespread pain in systemic lupus erythematosus and Sjogren’s syndrome: symptoms of the inflammatory disease or associated fibromyalgia. Clin Exp Rheumatol. 2012;30:117-21.PubMed
5. Morris G, Berk M, Galecki P, Walder K, Maes M. The neuro-immune pathophysiology of central and peripheral fatigue in systemic immune-inflammatory and neuro-immune diseases. Mol Neurobiol. 2016;53(2):1195-219. PubMed | CrossRef
6. Nacul L, de Barros B, Kingdon CC, Cliff JM, Clark TG, Mudie K, et al. Evidence of clinical pathology abnormalities in people with myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) from an analytic cross-sectional study. Diagnostics. 2019;9(2):41. PubMed | CrossRef
7. Rivas JL, Palencia T, Fernández G, García M. Association of T and NK cell phenotype with the diagnosis of myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS). Front Immunol. 2018;9:1028. PubMed | CrossRef
8. Cliff JM, King EC, Lee JS, Sepúlveda N, Wolf AS, Kingdon C, et al.Cellular immune function in myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS). Front Immunol. 2019;10:796. PubMed | CrossRef
9. Mensah F, Bansal A, Berkovitz S, Sharma A, Reddy V, Leandro MJ, et al. Extended B cell phenotype in patients with myalgic encephalomyelitis/chronic fatigue syndrome: a cross-sectional study. Clin Exp Immunol. 2016;184(2):237-47. PubMed | CrossRef
10. Nguyen CB, Alsøe L, Lindvall JM, Sulheim D, Fagermoen E, Winger A, Kaarbø M,et al. Whole blood gene expression in adolescent chronic fatigue syndrome: an exploratory cross-sectional study suggesting altered B cell differentiation and survival. J Transl Med. 2017;15(1):1-21.PubMed | CrossRef
11. Freitag H, Szklarski M, Lorenz S, Sotzny F, Bauer S, Philippe A, et al. Autoantibodies to vasoregulative G-protein-coupled receptors correlate with symptom severity, autonomic dysfunction and disability in myalgic encephalomyelitis/chronic fatigue syndrome. J Clin Med. 2021;10(16):3675. PubMed | CrossRef
12. Bjørklund G, Dadar M, Pivina L, Doşa MD, Semenova Y, Maes M. Environmental, neuro-immune, and neuro-oxidative stress interactions in chronic fatigue syndrome. Mol Neurobiol. 2020;57(11):4598-607. PubMed | CrossRef
13. de Vega WC, Vernon SD, McGowan PO. DNA methylation modifications associated with chronic fatigue syndrome. PLoS One. 2014;9(8):e104757. PubMed | CrossRef
14. Helliwell AM, Sweetman EC, Stockwell PA, Edgar CD, Chatterjee A, Tate WP. Changes in DNA methylation profiles of myalgic encephalomyelitis/chronic fatigue syndrome patients reflect systemic dysfunctions. Clin Epigenetics. 2020;12(1):1-20.PubMed | CrossRef
15. Zefferino R, Di Gioia S, Conese M. Molecular links between endocrine, nervous and immune system during chronic stress. Brain Behav. 2021;11(2):e01960. PubMed | CrossRef
16. Kerr JR. Epstein-Barr virus (EBV) reactivation and therapeutic inhibitors. J Clin Pathol. 2019;72(10):651-8. PubMed | CrossRef
17. Li L, Chi J, Zhou F, Guo D, Wang F, Liu G, et al. Human herpesvirus 6A induces apoptosis of HSB-2 cells via a mitochondrion-related caspase pathway. J Biomed Res. 2010;24(6):444-51. PubMed | CrossRef
18. Proal A, Marshall T. Myalgic encephalomyelitis/chronic fatigue syndrome in the era of the human microbiome: persistent pathogens drive chronic symptoms by interfering with host metabolism, gene expression, and immunity. Front Pediatr. 2018;6:373.PubMed | CrossRef
19. Anderson G, Maes M. Mitochondria and immunity in chronic fatigue syndrome. Prog Neuropsychopharmacol Biol Psychiatry. 2020;103:109976. PubMed | CrossRef
20. Chu L, Valencia IJ, Garvert DW, Montoya JG. Onset patterns and course of myalgic encephalomyelitis/chronic fatigue syndrome. Front Pediatr. 2019:5;7:12. PubMed | CrossRef
21. Montoya JG, Holmes TH, Anderson JN, Maecker HT, Rosenberg-Hasson Y, Valencia IJ, et al. Cytokine signature associated with disease severity in chronic fatigue syndrome patients. Proc Natl Acad Sci. 2017;114(34):E7150-8. PubMed | CrossRef
22. Gómez-Mora E, Carrillo J, Urrea V, Rigau J, Alegre J, Cabrera C, et al. Impact of long-term cryopreservation on blood immune cell markers in myalgic encephalomyelitis/chronic fatigue syndrome: implications for biomarker discovery. Front Immunol. 202017;11:582330. PubMed | CrossRef
23. AmelKashipaz MR, Swinden D, Todd I, Powell R. Normal production of inflammatory cytokines in chronic fatigue and fibromyalgia syndromes determined by intracellular cytokine staining in short-term cultured blood mononuclear cells. Clin Exp Immunol. 2003;132(2):360-5. PubMed | CrossRef
24. Fletcher MA, Zeng XR, Barnes Z, Levis S, Klimas NG. Plasma cytokines in women with chronic fatigue syndrome. J Transl Med. 2009;7(1):1-8. PubMed | CrossRef
25. terWolbeek M, van Doornen LJ, Kavelaars A, van de Putte EM, Schedlowski M, Heijnen CJ. Longitudinal analysis of pro-and anti-inflammatory cytokine production in severely fatigued adolescents. Brain Behav Immun. 2007;21(8):1063-74.PubMed | CrossRef
26. Cannon JG, Angel JB, Abad LW, Vannier E, Mileno MD, Fagioli L,et al. Interleukin-1β, interleukin-1 receptor antagonist, and soluble interleukin-1 receptor type II secretion in chronic fatigue syndrome. J Clin Immunol. 1997;17(3):253-61. PubMed | CrossRef
27. Nijs J, Van Oosterwijck J, Meeus M, Lambrecht L, Metzger K, Frémont M, et al. Unravelling the nature of postexertional malaise in myalgic encephalomyelitis/chronic fatigue syndrome: The role of elastase, complement C4a and interleukin‐1β. J Intern Med. 2010;267(4):418-35. PubMed | CrossRef
28. Bennett AL, Chao CC, Hu S, Buchwald D, Fagioli LR, Schur PH, et al. Elevation of bioactive transforming growth factor-β in serum from patients with chronic fatigue syndrome. J Clin Immunol. 1997;17(2):160-6. PubMed | CrossRef
29. Strawbridge R, Sartor ML, Scott F, Cleare AJ. Inflammatory proteins are altered in chronic fatigue syndrome-a systematic review and meta-analysis. NeurosciBiobehav Rev. 2019;107:69-83. PubMed | CrossRef
30. Germain A, Levine SM, Hanson MR. In-depth analysis of the plasma proteome in ME/CFS exposes disrupted ephrin-eph and immune system signaling. Proteomes. 2021;9(1):6. PubMed | CrossRef
31. Visser J, Graffelman W, Blauw B, Haspels I, Lentjes E, de Kloet ER, et al. LPS-induced IL-10 production in whole blood cultures from chronic fatigue syndrome patients is increased but supersensitive to inhibition by dexamethasone. J Neuroimmunol. 2001;119(2):343-9. PubMed | CrossRef
32. Lynn M, Maclachlan L, Finkelmeyer A, Clark J, Locke J, Todryk S, et al. Reduction of glucocorticoid receptor function in chronic fatigue syndrome. Mediators Inflamm. 2018;2018. PubMed | CrossRef
33. Domingo JC, Cordobilla B, Ferrer R, Giralt M, Alegre-Martín J, Castro-Marrero J. Are Circulating Fibroblast Growth Factor 21 and N-Terminal Prohormone of Brain Natriuretic Peptide Promising Novel Biomarkers in Myalgic Encephalomyelitis/Chronic Fatigue Syndrome? Antioxid Redox Signal. 2021;34(18):1420-27. PubMed | CrossRef
34. Lloyd AR, Hickie IB, Loblay RH. Illness or disease? The case of chronic fatigue syndrome. Med J Aust. 2000;172(10):471-2. PubMed | CrossRef
35. Hornig M, Montoya JG, Klimas NG, Levine S, Felsenstein D, Bateman L, et al. Distinct plasma immune signatures in ME/CFS are present early in the course of illness. Sci Adv. 2015;1(1):1400121. PubMed | CrossRef
36. Raanes EF, Stiles TC. Associations Between Psychological and Immunological Variables in Chronic Fatigue Syndrome/Myalgic Encephalomyelitis: A Systematic Review. Front Psychiatry. 2021;12:716320. PubMed | CrossRef
37. Simonato M, Dall’Acqua S, Zilli C, Sut S, Tenconi R, Gallo N, et al. Tryptophan Metabolites, Cytokines, and Fatty Acid Binding Protein 2 in Myalgic Encephalomyelitis/Chronic Fatigue Syndrome. Biomedicines. 2021;9(11):1724. PubMed | CrossRef
38. Bansal AS, Bradley AS, Bishop KN, Kiani-Alikhan S, Ford B. Chronic fatigue syndrome, the immune system and viral infection. Brain Behav Immun. 2012;26(1):24-31. PubMed | CrossRef
39. Masuda A, Nozoe SI, Matsuyama T, Tanaka H. Psychobehavioral and immunological characteristics of adult people with chronic fatigue and patients with chronic fatigue syndrome. Psychosom Med. 1994;56(6):512-8. PubMed | CrossRef
40. Barker E, Fujimura SF, Fadem MB, Landay AL, Levy JA. Immunologic abnormalities associated with chronic fatigue syndrome. Clin Infect Dis. 1994;18:136-41. PubMed | CrossRef
41. Eaton Fitch N, du Preez S, Cabanas H, Staines D, Marshall Gradisnik S. A systematic review of natural killer cells profile and cytotoxic function in myalgic encephalomyelitis/chronic fatigue syndrome. Syst Rev. 2019;8(1):279. PubMed | CrossRef
42. Tirelli U, Marotta G, Improta S, Pinto A. Immunological abnormalities in patients with chronic fatigue syndrome. Scand J Immunol. 1994;40(6):601-8. PubMed | CrossRef
43. Klimas NG, Salvato FR, Morgan R, Fletcher MA. Immunologic abnormalities in chronic fatigue syndrome. J Clin Microbiol. 1990;28(6):1403-10. PubMed | CrossRef
44. Huth TK, Brenu EW, Ramos S, Nguyen T, Broadley S, Staines D,et al. Pilot study of natural killer cells in chronic fatigue syndrome/myalgic encephalomyelitis and multiple sclerosis. Scand J Immunol. 2016;83(1):44-51. PubMed | CrossRef
45. Brenu EW, Hardcastle SL, Atkinson GM, van Driel ML, Kreijkamp-Kaspers S, Ashton KJ, et al. Natural killer cells in patients with severe chronic fatigue syndrome. Auto Immun Highlights. 2013;4(3):69-80. PubMed | CrossRef
46. Ojo-Amaize EA, Conley EJ, Peter JB. Decreased natural killer cell activity is associated with severity of chronic fatigue immune dysfunction syndrome. Clin Infect Dis. 1994;18(Supplement_1):S157-9. PubMed | CrossRef
47. Theorell J, Bileviciute-Ljungar I, Tesi B, Schlums H, Johnsgaard MS, Asadi-Azarbaijani B,et al. Unperturbed cytotoxic lymphocyte phenotype and function in myalgic encephalomyelitis/chronic fatigue syndrome patients. Front Immunol. 2017;8:723.PubMed | CrossRef
48. Münz C. Natural Killer Cell Responses during Human γ-Herpesvirus Infections. Vaccines. 2021;9(6):655. CrossRef
49. Chacko A, Staines DR, Johnston SC, Marshall-Gradisnik SM. Dysregulation of protein kinase gene expression in NK cells from chronic fatigue syndrome/myalgic encephalomyelitis patients. Gene Regul Syst Bio. 2016;10:GRSB-S40036. PubMed | CrossRef
50. Huth TK, Staines D, Marshall-Gradisnik S. ERK1/2, MEK1/2 and p38 downstream signalling molecules impaired in CD56dimCD16+ and CD56brightCD16dim/− natural killer cells in Chronic Fatigue Syndrome/Myalgic Encephalomyelitis patients. J Transl Med. 2016;14(1):1-0. PubMed | CrossRef
51. Balinas C, Cabanas H, Staines D, Marshall-Gradisnik S. Transient receptor potential melastatin 2 channels are overexpressed in myalgic encephalomyelitis/chronic fatigue syndrome patients. J Transl Med. 2019;17(1):1-1. PubMed | CrossRef
52. Cabanas H, Muraki K, Staines D, Marshall-Gradisnik S. Naltrexone restores impaired transient receptor potential melastatin 3 ion channel function in natural killer cells from myalgic encephalomyelitis/chronic fatigue syndrome patients. Front Immunol. 2019;10:2545. PubMed | CrossRef
53. Lundell K, Qazi S, Eddy L, Uckun FM. Clinical activity of folinic acid in patients with chronic fatigue syndrome. Arzneimittelforschung. 2006;56(06):399-404. PubMed | CrossRef
54. Fluge Ø, Mella O. Clinical impact of B-cell depletion with the anti-CD20 antibody rituximab in chronic fatigue syndrome: a preliminary case series. BMC Neurol. 2009;9(1):1-7. PubMed | CrossRef
55. Fluge Ø, Bruland O, Risa K, Storstein A, Kristoffersen EK, Sapkota D, et al. Benefit from B-lymphocyte depletion using the anti-CD20 antibody rituximab in chronic fatigue syndrome. A double-blind and placebo-controlled study. PLoS One. 2011;6(10):e26358. PubMed | CrossRef
56. Fluge Ø, Rekeland IG, Lien K, Thürmer H, Borchgrevink PC, Schäfer C, et al. B-lymphocyte depletion in patients with myalgic encephalomyelitis/chronic fatigue syndrome: a randomized, double-blind, placebo-controlled trial. Ann Intern Med. 2019;170(9):585-93.PubMed | CrossRef
57. Mensah FF, Armstrong CW, Reddy V, Bansal AS, Berkovitz S, Leandro MJ, et al. CD24 expression and B cell maturation shows a novel link with energy metabolism: potential implications for patients with myalgic encephalomyelitis/chronic fatigue syndrome. Front Immunol. 2018;9:2421. PubMed | CrossRef
58. Lee YH, Bae SC. Association between functional CD24 polymorphisms and susceptibility to autoimmune diseases: A meta-analysis. Cell Mol Biol. 2015;61(8):97-104.PubMed
59. Mandarano AH, Maya J, Giloteaux L, Peterson DL, Maynard M, Gottschalk CG, et al.Myalgic encephalomyelitis/chronic fatigue syndrome patients exhibit altered T cell metabolism and cytokine associations. J Clin Invest. 2020;130(3):1491-505. PubMed | CrossRef
60. Gupta S, Agrawal S, Gollapudi S. Differential effect of human herpesvirus 6A on cell division and apoptosis among naive and central and effector memory CD4+ and CD8+ T-cell subsets. J Virol. 2009;83(11):5442-50. PubMed | CrossRef
61. Sauce D, Larsen M, Curnow SJ, Leese AM, Moss PA, Hislop AD,et al. EBV-associated mononucleosis leads to long-term global deficit in T-cell responsiveness to IL-15. Blood. 2006;108(1):11-8. PubMed | CrossRef
62. Liu JQ, Liu Z, Zhang X, Shi Y, Talebian F, Carl JW Jr,et al. Increased Th17 and regulatory T cell responses in EBV-induced gene 3-deficient mice lead to marginally enhanced development of autoimmune encephalomyelitis. J Immunol. 2012;188(7):3099-106. PubMed | CrossRef
63. Ramos S, Brenu E, Broadley S, Kwiatek R, Ng J, Nguyen T, et al. Regulatory T, natural killer T and γδ T cells in multiple sclerosis and chronic fatigue syndrome/myalgic encephalomyelitis: a comparison. Asian Pac J Allergy Immunol. 2016;34(4):300-305. PubMed | CrossRef
64. Skowera A, Cleare A, Blair D, Bevis L, Wessely SC, Peakman M. High levels of type 2 cytokine-producing cells in chronic fatigue syndrome. Clin Exp Immunol. 2004;135(2):294-302. PubMed | CrossRef
65. Sepúlveda N, Carneiro J, Lacerda E, Nacul L. Myalgic Encephalomyelitis/Chronic Fatigue Syndrome as a Hyper-Regulated Immune System Driven by an Interplay Between Regulatory T Cells and Chronic Human Herpesvirus Infections. Front Immunol. 2019;10:2684. PubMed | CrossRef
66. Presson AP, Sobel EM, Papp JC, Suarez CJ, Whistler T, Rajeevan MS, et al. Integrated weighted gene co-expression network analysis with an application to chronic fatigue syndrome. BMC Syst Biol. 2008;2:95. PubMed | CrossRef
67. Metselaar PI, Mendoza-Maldonado L, Li Yim AYF, Abarkan I, Henneman P, Te Velde AA, et al. Recursive ensemble feature selection provides a robust mRNA expression signature for myalgic encephalomyelitis/chronic fatigue syndrome. Sci Rep. 2021;11(1):4541. PubMed | CrossRef
68. Ikuta K, Yamada T, Shimomura T, Kuratsune H, Kawahara R, Ikawa S, et al. Diagnostic evaluation of 2', 5'-oligoadenylate synthetase activities and antibodies against Epstein-Barr virus and Coxiella burnetii in patients with chronic fatigue syndrome in Japan. Microbes Infect. 2003;5(12):1096-102. PubMed | CrossRef
69. Meeus M, Nijs J, McGregor N, Meeusen R, De Schutter G, Truijen S, et al. Unravelling intracellular immune dysfunctions in chronic fatigue syndrome: interactions between protein kinase R activity, RNase L cleavage and elastase activity, and their clinical relevance. In Vivo. 2008;22(1):115-21. PubMed
70. Vojdani A, Lapp CW. Interferon-induced proteins are elevated in blood samples of patients with chemically or virally induced chronic fatigue syndrome. ImmunopharmacolImmunotoxicol. 1999;21(2):175-202. PubMed | CrossRef
71. Maes M. Inflammatory and oxidative and nitrosative stress cascades as new drug targets in myalgic encephalomyelitis and chronic fatigue syndrome. Mod Trends Pharmacopsychiatry. 2013;28:162-74. PubMed | CrossRef
72. Frémont M, El Bakkouri K, Vaeyens F, Herst CV, De Meirleir K, Englebienne P. 2',5'-Oligoadenylate size is critical to protect RNase L against proteolytic cleavage in chronic fatigue syndrome. Exp Mol Pathol. 2005;78(3):239-46. PubMed | CrossRef
73. Snell CR, Vanness JM, Strayer DR, Stevens SR. Physical performance and prediction of 2-5A synthetase/RNase L antiviral pathway activity in patients with chronic fatigue syndrome. In Vivo. 2002;16(2):107-9. PubMed
74. Ablashi DV, Eastman HB, Owen CB, Roman MM, Friedman J, Zabriskie JB, et al. Frequent HHV-6 reactivation in multiple sclerosis (MS) and chronic fatigue syndrome (CFS) patients. J Clin Virol. 2000;16(3):179-91. PubMed | CrossRef
75. Koelle DM, Barcy S, Huang ML, Ashley RL, Corey L, Zeh J, et al. Markers of viral infection in monozygotic twins discordant for chronic fatigue syndrome. Clin Infect Dis. 2002;35(5):518-25. PubMed | CrossRef
76. Buchwald D, Ashley RL, Pearlman T, Kith P, Komaroff AL. Viral serologies in patients with chronic fatigue and chronic fatigue syndrome. J Med Virol. 1996;50(1):25-30. PubMed | CrossRef
77. Manian FA. Simultaneous measurement of antibodies to Epstein-Barr virus, human herpesvirus 6, herpes simplex virus types 1 and 2, and 14 enteroviruses in chronic fatigue syndrome: is there evidence of activation of a nonspecific polyclonal immune response? Clin Infect Dis. 1994;19(3):448-53. PubMed |
78. Lerner AM, Beqaj SH, Deeter RG, Fitzgerald JT. IgM serum antibodies to human cytomegalovirus nonstructural gene products p52 and CM2(UL44 and UL57) are uniquely present in a subset of patients with chronic fatigue syndrome. In Vivo. 2002;16(3):153-9. PubMed | CrossRef
79. Lee JS, Lacerda EM, Nacul L, Kingdon CC, Norris J, O'Boyle S, et al. Salivary DNA Loads for Human Herpesviruses 6 and 7 Are Correlated with Disease Phenotype in Myalgic Encephalomyelitis/Chronic Fatigue Syndrome. Front Med. 2021; 8:656692. PubMed
80. Bassi N, Amital D, Amital H, Doria A, Shoenfeld Y. Chronic fatigue syndrome: characteristics and possible causes for its pathogenesis. Isr Med Assoc J. 2008;10(1):79. PubMed
81. Loebel M, Grabowski P, Heidecke H, Bauer S, Hanitsch LG, Wittke K, et al. Antibodies to β adrenergic and muscarinic cholinergic receptors in patients with Chronic Fatigue Syndrome. Brain Behav Immun. 2016;52:32-9.PubMed | CrossRef
82. Szklarski M, Freitag H, Lorenz S, Becker SC, Sotzny F, Bauer S, et al. Delineating the association between soluble CD26 and autoantibodies against G-protein coupled receptors, immunological and cardiovascular parameters identifies distinct patterns in post-infectious vs. non-infection-triggered Myalgic Encephalomyelitis/Chronic Fatigue Syndrome. Front Immunol. 2021;12:644548. PubMed | CrossRef
83. Blomberg J, Gottfries CG, Elfaitouri A, Rizwan M, Rosén A. Infection elicited autoimmunity and myalgic encephalomyelitis/chronic fatigue syndrome: an explanatory model. Front Immunol. 2018:229. PubMed | CrossRef
84. Ariza ME. Myalgic encephalomyelitis/chronic fatigue syndrome: the human herpesviruses are back! Biomolecules. 2021;11(2):185. PubMed | CrossRef
85. Nagata K, Hayashi K. Epstein–Barr Virus Reactivation-Induced Immunoglobulin Production: Significance on Autoimmunity. Microorganisms. 2020;8(12):1875. PubMed | CrossRef
86. Zhang N, Zuo Y, Jiang L, Peng Y, Huang X, Zuo L. Epstein-Barr virus and neurological diseases. Front Mol Biosci. 2021:1310. PubMed | CrossRef
87. Schreiner P, Harrer T, Scheibenbogen C, Lamer S, Schlosser A, Naviaux RK, et al. Human herpesvirus-6 reactivation, mitochondrial fragmentation, and the coordination of antiviral and metabolic phenotypes in myalgic encephalomyelitis/chronic fatigue syndrome. Immunohorizons. 2020;4(4):201-15. PubMed | CrossRef
88. Chia JK, Chia AY. Chronic fatigue syndrome is associated with chronic enterovirus infection of the stomach. J Clin Pathol. 2008;61(1):43-8.PubMed | CrossRef
89. Galbraith DN, Nairn C, Clements GB. Phylogenetic analysis of short enteroviral sequences from patients with chronic fatigue syndrome. J Gen Virol. 1995;76(7):1701-7. PubMed | CrossRef
90. O'Neal AJ, Hanson MR. The enterovirus theory of disease etiology in myalgic encephalomyelitis/chronic fatigue syndrome: a critical review. Front Med. 2021:18;8:688486.PubMed | CrossRef
91. Griffiths DJ. Endogenous retroviruses in the human genome sequence. Genome Biol. 2001;2(6):1-5. PubMed | CrossRef
92. Lander ES, Linton LM, Birren B, Nusbaum C, Zody MC, Baldwin J, et al. International Human Genome Sequencing Consortium. Initial sequencing and analysis of the human genome. Nature. 2001;409(6822):860-921. Erratum in: Nature. 2001;412(6846):565. Erratum in: Nature. 2001;411(6838):720. Szustakowki, J [corrected to Szustakowski, J].
93. Tang W, Mun S, Joshi A, Han K, Liang P. Mobile elements contribute to the uniqueness of human genome with 15,000 human-specific insertions and 14 Mbp sequence increase. DNA Res. 2018;25(5):521-33. PubMed | CrossRef
94. Ferrari R, Grandi N, Tramontano E, Dieci G. Retrotransposons as drivers of mammalian brain evolution. Life. 2021;11(5):376. PubMed | CrossRef
95. Kazazian Jr HH. Mobile elements: drivers of genome evolution. Science. 2004;303(5664):1626-32.PubMed | CrossRef
96. Geis FK, Goff SP. Silencing and transcriptional regulation of endogenous retroviruses: an overview. Viruses. 2020;12(8):884. PubMed | CrossRef
97. Hurst TP, Magiorkinis G. Epigenetic control of human endogenous retrovirus expression: focus on regulation of long-terminal repeats (LTRs). Viruses. 2017;9(6):130. PubMed | CrossRef
98. Balestrieri E, Minutolo A, Petrone V, Fanelli M, Iannetta M, Malagnino V,et al. Evidence of the pathogenic HERV-W envelope expression in T lymphocytes in association with the respiratory outcome of COVID-19 patients. EBioMedicine. 2021;66:103341. PubMed | CrossRef
99. Macchietto MG, Langlois RA, Shen SS. Virus-induced transposable element expression up-regulation in human and mouse host cells. Life Sci Alliance. 2020;3(2). PubMed | CrossRef
100. Pappalardo AM, Ferrito V, Biscotti MA, Canapa A, Capriglione T. Transposable elements and stress in vertebrates: An overview. Int J Mol Sci. 2021;22(4):1970. PubMed | CrossRef
101. Ovejero T, Sadones O, Sánchez-Fito T, Almenar-Pérez E, Espejo JA, Martín-Martínez E, et al. Activation of transposable elements in immune cells of fibromyalgia patients. Int J Mol Sci. 2020;21(4):1366. PubMed | CrossRef
102. Rodrigues LS, da Silva Nali LH, Leal CO, Sabino EC, Lacerda EM, Kingdon CC, et al. HERV-K and HERV-W transcriptional activity in myalgic encephalomyelitis/chronic fatigue syndrome. Auto Immun Highlights. 2019;10(1):1-5. PubMed | CrossRef
103. Komaroff AL, Bateman L. Will COVID-19 lead to myalgic encephalomyelitis/chronic fatigue syndrome? Front Med. 2021:1132. PubMed | CrossRef
104. González-Hermosillo JA, Martínez-López JP, Carrillo-Lampón SA, Ruiz-Ojeda D, Herrera-Ramírez S, Amezcua-Guerra LM, et al. Post-acute COVID-19 symptoms, a potential link with myalgic encephalomyelitis/chronic fatigue syndrome: a 6-month survey in a Mexican cohort. Brain Sci. 2021;11(6):760. PubMed | CrossRef
105. Gröger V, Emmer A, Staege MS, Cynis H. Endogenous retroviruses in nervous system disorders. Pharmaceuticals. 2021;14(1):70. PubMed | CrossRef
106. Mu X, Ahmad S, Hur S. Endogenous retroelements and the host innate immune sensors. AdvImmunol. 2016;132:47-69. PubMed | CrossRef
107. Giménez-Orenga K, Oltra E. Human endogenous retrovirus as therapeutic targets in neurologic disease. Pharmaceuticals. 2021;14(6):495. PubMed | CrossRef
108. Goerner-Potvin P, Bourque G. Computational tools to unmask transposable elements. Nat Rev Genet. 2018;19(11):688-704. PubMed | CrossRef