We have conducted a large screening of cytokine levels in CH patients and controls using serum and CSF samples in order to get an overview of the inflammatory state in the periphery and in the central nervous system (CNS). Nine cytokines were found to be higher in CSF from patients as compared to controls, particularly cytokines with chemoattractant properties on leukocytes. Interestingly, major differences in cytokine levels were detected between CSF and serum. In the discovery analysis two of the cytokines found at higher levels in CSF were lower in serum.
Previous studies on cytokines in CH have suggested an increase of IL-2 and IL-1β in blood of CH patients although data are conflicting for IL-1β[9, 10]. IL-2 gene expression and protein levels in blood was found to be increased in CH active bout compared to controls, while gene expression levels decreased to control levels during attacks[8, 11]. In our study, the assay for IL-1β did not pass quality control and we could not replicate the findings of an increase for IL-2 in our material. These differences could potentially be due to discrepancies between methodology, quantitative Real-time PCR (qPCR) and enzyme-linked immunosorbent assay (ELISA) in previous studies while we used PEA, an ELISA based array that increases the sensitivity coupling the detection to qPCR.
In this report we found evidence of a possible ongoing inflammation in the CNS of CH patients with elevated levels of several cytokines in the CSF of CH patients as compared to controls. To our knowledge, this is the first reported screening for cytokines in CSF from CH patients. There was very little difference between patients in remission phase and active bout for all the differentially expressed cytokines, which indicates that the neuroinflammatory state in CH patients is not exclusive to the active bout. The cytokines showing the largest increase in CSF from patients were HGF, MMP1, TNSF10 and 12 and several chemokines, all of which were found to have strong interactions in a network analysis. CCL8, CCL11, and CCL13 belong to the CC chemokine family and are primarily involved in attracting leukocytes to sites of inflammation as part of the innate immune system[14]. CCL8, along with other proinflammatory cytokines, have previously been found to be elevated in calvarial periosteum tissue of chronic migraine patients[15]. Experiments on rats have further shown that an inflammatory stimulation of the trigeminal ganglion afferent nerves innervating the calvarial periosteum results in periorbital hypersensitivity, a mechanism potentially relevant for CH, although we found increased CCL8 in CSF and not in serum[16]. Another chemokine, CCL2 has also been found to be elevated in CSF from patients with other primary headache disorders[17]. CXCL10 and CXCL11 are cytokines of the CXC chemokine family, CXC chemokines have chemoattractant properties on leukocytes like the CC chemokines described above, but mainly attract T lymphocytes[18]. Both CXCL10 and 11 bind to the CXCR3 receptor leading to activation of the phospholipase C-dependent pathway, which drives actin rearrangement and increase of intracellular calcium[19, 20]. CXCL10 further plays a role in activation of microglia and migration to sites of lesions or inflammation in the CNS[21, 22]. Considering the broad range of functions and substrates of these proteins, their potential role in CH remains to be clarified. Nociception, intracellular calcium signalling, and microglia activation are all potentially relevant for CH pathophysiology; the first line preventative CH treatment, being a calcium channel blocker, and the top GWAS loci identified for CH, MERTK, being primarily expressed in microglia and other glial cells[12, 23].
It is noteworthy that two of the cytokines that were higher in CSF from CH patients, CCL11 and CXCL11, inversely were lower in serum from CH patients compared to controls. A negative correlation between CXCL11 and galectin-3 has previously been found in serum from individuals with ulcerative colitis[24]. Similarly, we found increased concentrations of galectin-3 in serum from CH patients[25]. We also observed a trend for lower levels of CCL13 and CCL7 in serum in the post-hoc analysis, suggesting an overall downregulation of chemoattractant cytokines in serum from CH patients. The opposite direction of regulation of chemokines between CSF and serum is interesting and does not correlate with what is reported for example in multiple sclerosis[26].
Though most elevated cytokines in the CSF of CH patients had a proinflammatory profile, some, such as HGF, are considered as having anti-inflammatory effects[27]. HGF drives cell survival in various cell types and has been known to dampen proinflammatory cytokine release from macrophages[28]. A mendelian randomization study concluded HGF to be a potential causative factor for migraine onset[29]. The HGF-Met pathway has been known to drive differentiation of peptidergic neurons which are especially important in calcitonin gene-related peptide (CGRP) signalling[30]. The importance of CGRP in both migraine and CH pathophysiology suggests a similar mechanism may occur in CH. TNFSF10/tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) and TNFSF12/TNF-related weak inducer of apoptosis (TWEAK), two proapoptotic cytokines in the TNF ligand superfamily were also elevated in CSF in patients. Higher levels of the TNFSF10 receptor, TNFRSF10C, has been found in CH patients during an attack, supporting a role for TNF signalling in CH[5]. The link between TNFSF12 and CH is vague, experimental data supports a potential role for the TNFSF12 receptor, fibroblast growth factor-inducible-14 (Fn14), in neuropathic pain in rodents[31]. MMP1 is part of the matrix metalloproteinase family and are considered immune modulators. Their main function is to break down extracellular matrix, but they also play a role in cytokine release and generation of chemokine gradients[32]. Elevated levels of another member of the MMP family, MMP9, has previously been shown in migraine patients[33]. HGF and MMP1 among other cytokines were also found at higher levels in CH patients in active bout than controls in a posthoc analysis of serum samples. Several of the cytokines identified in the sub-threshold serum analysis are part of the IL-6 superfamily; IL-6, OSM and CSF3. VEGFA was slightly elevated in serum of CH patients and is an interesting candidate as it has repeatedly been shown to have a pro-nociceptive effect[34, 35]. Higher levels of VEGFA have also been found in migraine patients together with elevated CGRP and nitric oxide (NO).[36]
The differences observed between CSF and serum could point towards a greater importance of inflammation in the CNS in the pathophysiology of CH while the peripheral mechanisms have a weaker connection to the immune response. In particular, factors involved in the recruitment of immune cells were upregulated in CSF and we hypothesize that there is a chemoattractant gradient present in these patients, recruiting immune cells and concentrating the inflammation to the CNS. In concordance with our data, previous studies have shown that there is no systemic inflammation in CH patients[4, 37]. However, current data does not provide information on potential local inflammatory reactions that may occur in the peripheral nervous system during CH attacks such as in the trigeminal ganglion.
Initially our hypothesis stated that inflammatory reactions would be specifically upregulated during active bout and may even constitute a hallmark of the phenotypic switch occurring in CH patients when they transition between these two phases. Our results now show a completely different view, demonstrating very similar levels of cytokines between active bout and remission phase. CH patients are typically considered healthy in between bouts, but these data suggest that CH may be considered a chronic disorder, manifesting physiological changes also when patients are in remission, this is in line with several studies investigating biomarkers for CH not finding differences between active bout and remission[38–40]. It would be of interest to analyse if inflammatory markers normalize with time in elderly patients experiencing long-time/complete remission. The only cytokine specifically related to attacks, IL-13, displayed a trend for increased levels in serum during an attack. IL-13 is typically considered an anti-inflammatory cytokine and is commonly involved in asthma and allergies[41]. It is interesting to note that cultured primary microglia driven to an M1 phenotype have been found to increase their MERTK and galectin-3 gene expression in response to IL-13 stimulation, knowing that patients with CH have elevated MERTK and galectin-3 in peripheral blood[25, 42]. Visual inspection of the data revealed that serum cytokine levels during attacks were highly similar to those of controls, even when there was a difference in patients in active bout or during remission. This normalisation of cytokine levels during attacks has been described for IL-2 in an earlier study[8], and raises questions regarding the underlying mechanisms of the attacks, the cellular origin of the cytokines, and the activation and migration of immune cells in the different phases of the disease. One possibility may be that the CH attack somehow disturbs the chemokine gradient observed between serum and CSF. Unfortunately, we did not have access to CSF from patients during an attack to verify this hypothesis.
OLINK screening of cytokines gives the benefit of performing an unbiased search for immunological markers that may relate to the pathophysiology of CH. Another strength of the study is the large number of markers investigated maximizing the chances to reveal markers as compared to previous studies on CH investigating specific cytokines. Furthermore, we had the possibility to study cytokines both peripherally and in CSF, so our study provides a more complete view of the immune pathways involved than was previously available. This study also has limitations, including little information on the controls, which implies that factors that can influence the results such as age, sex and comorbidities may be overlooked. Also, immune cells have a variety of functions all of which are not completely covered by this OLINK array. In addition, the array does not include any indications of the underlying mechanisms or reveal which cells are responsible for the release of the detected cytokines. Future studies should comprise a broader panel of cytokines and preferably include a larger cohort to increase the power of detecting differences in cytokine levels.