Cytokines

A decade of cytokine research has revealed that these mediators play a pivotal role in the functioning of many organ systems. The immune system is not the exclusive producer of cytokines. These mediators are also produced in, e.g., the brain, gastrointestinal tract and liver. In view of the fact that the immune system can react to cytokines produced by cell types other than leukocytes, immunologist had to accept that the immune system is not functioning autonomously, but that it is constantly communicating with other organ systems in order to maintain its homeostasis.

cytokines are proteins any of a number of substances, such as interferon, interleukin, and growth factors, that are secreted by certain cells of the immune system and have an effect on other cells. Cytokines are a large group of proteins, peptides or glycoproteins that are secreted by specific cells of immune system. Cytokines are a category of signaling molecules that mediate and regulate immunity, inflammation and hematopoiesis.

With Focus on IL-1β, IL-6, TNF

from: <a href="https://link.springer.com/article/10.1007/s12035-018-0928-9#Sec9">A Molecular Neurobiological Approach to Understanding the Aetiology of Chronic Fatigue Syndrome (Myalgic Encephalomyelitis or Systemic Exertion Intolerance Disease) with Treatment Implications | SpringerLink</a>

Cytokines and Infections

The finding of increased CSF TNF-α in CFS has been mentioned above [<a title="View reference" href="https://link.springer.com/article/10.1007/s12035-018-0928-9#CR51">51</a>]. Several other studies have confirmed that CFS/ME/SEID is associated with increased levels of cytokines, particularly those which are pro-inflammatory. Four further recent examples are described here.

Using a high-throughput 51-multiplex array, Montoya and colleagues studied serum cytokines in 192 ME/CFS patients and 392 healthy controls in the important Stanford Myalgic Encephalomyelitis Initiative [<a title="View reference" href="https://link.springer.com/article/10.1007/s12035-018-0928-9#CR55">55</a>]. The following cytokines showed a significant elevation which correlated with disease severity: CCL11 (Eotaxin-1), CXCL1 (GROα), CXCL10 (IP-10), IFN-γ, IL-4, IL-5, IL-7, IL-12p70, IL-13, IL-17F, leptin, G-CSF, GM-CSF, LIF, NGF, SCF and TGF-α; of these, 13 are PICs and are likely to contribute to symptoms [<a title="View reference" href="https://link.springer.com/article/10.1007/s12035-018-0928-9#CR55">55</a>].

Using a 51-multiplex array, Hornig and colleagues studied CSF cytokines in 32 patients and 19 matched controls; their results pointed to a disturbance in IL-1 signalling [<a title="View reference" href="https://link.springer.com/article/10.1007/s12035-018-0928-9#CR56">56</a>]. In an earlier study, Horning and fellow workers also found evidence of cytokine activation and dissociation of inter-cytokine regulatory networks early in the course of CFS/ME, with illness duration being better correlated with cytokine changes than illness severity [<a title="View reference" href="https://link.springer.com/article/10.1007/s12035-018-0928-9#CR57">57</a>].

A study by Milrad and colleagues showed that poor sleep quality in CFS/ME patients is associated with higher levels of the PICs TNF-α, IL-1β and IL-6 [<a title="View reference" href="https://link.springer.com/article/10.1007/s12035-018-0928-9#CR58">58</a>].

These studies suggest that CFS/ME/SEID has a dynamic immunopathology with CNS immune activation and a shift to the Th2 pattern seen in autoimmunity [<a title="View reference" href="https://link.springer.com/article/10.1007/s12035-018-0928-9#CR56">56</a>, <a title="View reference" href="https://link.springer.com/article/10.1007/s12035-018-0928-9#CR57">57</a>]. Furthermore, neuroinflammation, together with O&NS and neural sensitivity, can be formulated into a robust model of the aetiopathology of CFS/ME/SEID involving sustained glial activation [<a title="View reference" href="https://link.springer.com/article/10.1007/s12035-018-0928-9#CR59">59</a>].

As mentioned earlier, increased PIC levels are associated with infections. It is therefore noteworthy that Underhill has suggested that CFS/ME is an infectious disease, on the basis of clinical, epidemiological and immunological evidence [<a title="View reference" href="https://link.springer.com/article/10.1007/s12035-018-0928-9#CR60">60</a>]. The following evidence particularly supports the role of Epstein–Barr virus (EBV) or human herpesvirus-4 (HHV-4), which can cause infectious mononucleosis, in this regard.

At 6 months, 1 year and 2 years after infection, of 301 adolescents, aged between 12 and 18 years, with infectious mononucleosis, 13, 7 and 4%, respectively, met CFS criteria in a study by Katz and colleagues [<a title="View reference" href="https://link.springer.com/article/10.1007/s12035-018-0928-9#CR61">61</a>]. While CFS/ME/SEID patients and controls have been found to have similar IgG antibody response patterns to EBV, patients show increased IgG reactivity to an EBNA-6 repeat sequence and also to EBNA-6 protein; it may be that homologous sequences of human proteins containing this sequence could give rise to antigenic mimicry [<a title="View reference" href="https://link.springer.com/article/10.1007/s12035-018-0928-9#CR62">62</a>].

It is also interesting to note that there is evidence that CFS/ME/SEID patients suffer from sensitised fatigue pathways [<a title="View reference" href="https://link.springer.com/article/10.1007/s12035-018-0928-9#CR63">63</a>], as this may be related to the spread of reactivated EBV to ectopic lymphoid aggregates [<a title="View reference" href="https://link.springer.com/article/10.1007/s12035-018-0928-9#CR64">64</a>].

Besides EBV, Montoya and colleagues have suggested that HHV-6 may also be an infectious trigger for CFS/ME [<a title="View reference" href="https://link.springer.com/article/10.1007/s12035-018-0928-9#CR65">65</a>]. Importantly, Pantry and colleagues have shown that chromosomal integrated HHV-6 can be associated with persistent infection with exogenous HHV-6, which in turn may be associated with neurological symptomology [<a title="View reference" href="https://link.springer.com/article/10.1007/s12035-018-0928-9#CR66">66</a>].

</div>

from: <a href="http://www.pnas.org/content/114/34/E7150">Cytokine signature associated with disease severity in chronic fatigue syndrome patients | PNAS</a>

Although only two cytokines were found to be different (TGF-β higher and resistin lower) in ME/CFS patients compared with controls, 17 cytokines correlated with ME/CFS severity. Thirteen of these cytokines are proinflammatory and may contribute to many of the symptoms these patients experience for several years. Only CXCL9 (MIG) inversely correlated with fatigue duration.

The average pMFI of two cytokines was found to be significantly different in ME/CFS patients and in healthy controls. TGF-β (P = 0.0052) was elevated in ME/CFS patients, and resistin (P = 0.0052) was lower (<a id="xref-table-wrap-2-1" class="xref-table" href="http://www.pnas.org/content/114/34/E7150#T2">Table 2</a>). In addition, three cytokines were significantly different in cases and controls when stratified by severity, as shown in <a id="xref-fig-1-1" class="xref-fig" href="http://www.pnas.org/content/114/34/E7150#F1">Fig. 1</a> (red brackets). The average pMFI of IL-13 was significantly higher in the severe group (P = 0.0250) than in controls; leptin was significantly lower in the mild group (P = 0.0495), and resistin was significantly lower in the mild (P = 0.0370) and severe (P = 0.0208) groups (<a id="xref-table-wrap-3-1" class="xref-table" href="http://www.pnas.org/content/114/34/E7150#T3">Table 3</a>).

Overall, 17 cytokines were found to have a statistically significant upward linear trend across the sequence of mild, moderate, and severe ME/CFS severity: CCL11 (P = 0.0069), CXCL1 (P = 0.0266), CXCL10 (P = 0.0100), G-CSF (P = 0.0110), GM-CSF (P = 0.0063), IFN-γ (P = 0.0101), IL-4 (P = 0.0103), IL-5 (P = 0.0073), IL-7 (P = 0.0063), IL-12p70 (P = 0.0069), IL-13 (P = 0.0069), IL-17F (P = 0.0103), leptin (P = 0.0100), leukemia inhibitory factor (LIF) (P = 0.0100), nerve growth factor (NGF) (P = 0.0069), stem cell factor (SCF) (P= 0.0145), and TGF-α (P = 0.0367) (<a id="xref-fig-2-1" class="xref-fig" href="http://www.pnas.org/content/114/34/E7150#F2">Fig. 2</a> and <a id="xref-table-wrap-4-2" class="xref-table" href="http://www.pnas.org/content/114/34/E7150#T4">Table 4</a>). Intercellular adhesion molecule 1 (ICAM1) and resistin exhibited a statistically significant, nonlinear, inverted trend (P = 0.0334 for each) (<a id="xref-fig-1-2" class="xref-fig" href="http://www.pnas.org/content/114/34/E7150#F1">Fig. 1</a>).

<h3>Analysis of ME/CFS Cases by Fatigue Duration.</h3>

In Spearman correlation analyses, before controlling for potentially confounding variables, several cytokines were found to correlate inversely with fatigue duration: Growth related oncogene-α (GRO-α or CXCL1), IFN-β, IL-1α, IL-1RA, IL-2, IL-8, IL-15, TGF-α, and TGF-β (<a class="in-nw" href="http://www.pnas.org/lookup/suppl/doi:10.1073/pnas.1710519114/-/DCSupplemental/pnas.1710519114.sapp.pdf">SI Appendix, SI Materials and Methods and Table S1</a>). Following correction for age, nonspecific binding, race, and sex, only one cytokine, CXCL9 (monokine induced by interferon-γ, MIG), was found to correlate inversely with fatigue duration (unadjusted for multiple comparisons, P = 0.0123) (<a class="in-nw" href="http://www.pnas.org/lookup/suppl/doi:10.1073/pnas.1710519114/-/DCSupplemental/pnas.1710519114.sapp.pdf">SI Appendix, Table S1</a>). Although the inclusion of fatigue duration as an additional covariate was expected to result in a loss of statistical power, regression analysis by both disease severity and fatigue duration revealed that the upward linear trend across disease severity remained statistically significant for CCL11, CXCL10, G-CSF, GM-CSF, IFN-γ, IL-4, IL-5, IL-7, IL-12p70, IL-13, IL-17F, leptin, LIF, NGF, ICAM1, and resistin. The findings by severity for CXCL1, SCF, and TGF-α shifted slightly above the threshold for statistical significance (CXCL1, adjusted P = 0.0536; SCF, adjusted P = 0.0531; TGF-α, adjusted P = 0.0797), possibly because of variance inflation (<a id="xref-ref-18-1" class="xref-bibr" href="http://www.pnas.org/content/114/34/E7150#ref-18">18</a>) by the independent covariate of fatigue duration. Only CXCL9 and IL-1α inversely correlated with fatigue duration but lost statistical significance after correction for multiple comparisons (<a class="in-nw" href="http://www.pnas.org/lookup/suppl/doi:10.1073/pnas.1710519114/-/DCSupplemental/pnas.1710519114.sapp.pdf">SI Appendix, Table S2</a>). We also compared mean cytokine levels in cases with ≤3-y fatigue duration (n = 30) and those with >3-y fatigue duration (n = 156), as per Hornig et al. (<a id="xref-ref-17-2" class="xref-bibr" href="http://www.pnas.org/content/114/34/E7150#ref-17">17</a>). We did not find any cytokine to be significantly different between these two groups (<a class="in-nw" href="http://www.pnas.org/lookup/suppl/doi:10.1073/pnas.1710519114/-/DCSupplemental/pnas.1710519114.sapp.pdf">SI Appendix, Table S3</a>).

From: <a id="null" class="null" href="http://jpet.aspetjournals.org/content/jpet/early/2018/08/03/jpet.118.250845.full.pdf">http://jpet.aspetjournals.org/content/jpet/early/2018/08/03/jpet.118.250845.full.pdf</a>

However, even though TNF is elevated in the serum of patients with FMS, (Theoharides, et al., 2010c) it was not consistently elevated in ME/CFS (Brenu, et al., 2011), but was apparently associated only with increased IL-4 (Hanson, et al., 2001). There was also no significant difference in serum cytokine levels across the night (Nakamura, et al., 2010) or post exercise (Nakamura, et al., 2013). There is some evidence of stronger correlation of cytokines alterations early in the course of illness rather than severity (Hornig, et al., 2015). It has been proposed that “cytokine co-expression networks” may be more predictive of ME/CFS phenotype (Klimas, et al., 2012), (Hornig, et al., 2016), but looking for such biomarkers in the periphery would not reflect inflammation in the brain. One study reported that of 27 cytokines studied in CSF from ME/CFS patients, only IL-10 was significantly reduced {26107}. Another paper using network analysis of CSF cytokine levels reported an inverse relationship with interleukin 1 receptor antagonist only in classical, but not in atypical ME/CFS (Hornig, et al., 2017).