A cohort of fibromyalgia (FM) patients (n =90) and healthy controls (HC) (n= 93) was recruited to investigate the associations between human IgG binding to satellite glia cells (SGC) from dorsal root ganglia (DRG) and pathophysiological mechanisms. The study is based on previously identified mechanisms resulting from injecting human IgG antibodies from FM patients, but not HC, in mice (Goebel et al. J Clin Invest. 2021;131(13):e144201). Subjects have been carefully phenotyped using validated questionnaires and quantitative sensory testing (QST) was applied to determine pain sensitivity. A blood sample was taken to quantify anti-SGC IgG, as well as proteins, lipids and metabolites. Skin biopsies were taken to analyze changes in skin innervation (IENFD) and immune cell activation. Magnetic resonance spectroscopy (MRS) and functional magnetic resonance imaging (fMRI) was performed (n=122) to investigate central nervous system pain related mechanisms. Insular glutamate levels, as well as the levels of other brain metabolites will be determined (MRS) and related to symptom severity and anti-SGC IgG levels. Resting state as well as pain related cerebral activation (BOLD) during standardized evoked pain stimuli will be characterized (fMRI) and related to the MRS findings and to anti-SGC IgG levels.

Visit 1 (duration approx. 2,5-4 hours)

Questionnaires: A check of inclusion and exclusion criteria was made, including the palpation of tender points (ACR 1990 FM criteria). Subjects completed a set of validated questionnaires regarding pain (pain intensity ratings on visual analogue scale (VAS), pain duration, pain drawing, Short form McGill), fibromyalgia severity (fibromyalgia impact questionnaire, (FIQ)), sleep (Pittsburgh Sleep Inventory Questionnaire, (PSQI)), fatigue (Multidimensional Fatigue Inventory (MFI-20)), depression/anxiety (Hospital anxiety depression scale (HADS), Becks depression inventory (BDI), State Trait Anxiety Questionnaire (STAI)), pain coping (Coping Strategy Questionnaire (PCS)), acceptance and flexibility (chronic pain acceptance questionnaire (CPAQ-8), acceptance and action questionnaire (SAAQ)), the Tampa Scale for Kinesiophobia (TSK) and health related quality of life (EQ-5D).

Quantitative sensory testing: Perception thresholds for non-painful and painful heat/cold were examined using a Thermotest. A thermode (2x2cm) was placed at the skin and the temperature was modulated by a computer (0-50 degrees C). The subject signaled the respective perception by pressing a button, which terminated the stimulation. Subjects were instructed to press the button at the sensation of the slightest warmth/cold and when the heat/cold become painful. Sensitivity to suprathreshold heat/cold pain stimuli were determined by asking the subject to press the button when they would rate a pain intensity corresponding to 4 and 7/10. Pain sensitivity to pressure was assessed by pressure algometry at 8 different sites in the body. The subjects were instructed to press a signal button when the pressure turned into the slightest sensation of pain (pressure pain threshold) and when the pain corresponded to 4 and 7/10, respectively.

Blood test: Venous blood tests were taken (maximal volume 50 ml/individual). The levels of human IgG binding to murine satellite glia cells (SGC) will be determined as % SGC binding human IgG. In addition, we will also assess the amount of IgG binding to human DRG sections and possibly also to human SGC (postmortem donors). The blood samples will also be analysed for algogenic and inflammatory substances, lipids and metabolites and immune cell activation, including fluorescence-activated cell sorting (FACS)(in a subgroup). As an extension of the study, IgG antibodies will be purified to evaluate the effect on cell cultures and in the rodent FM model.

Skin biopsies: Two punch skin biopsies (4 mm diameter) were performed at the thigh. The skin was sterilized, and anesthetized using a local anesthetic (10mg/ml Xylocain with adrenalin). If needed the was treated with spongostan/BloodStop and a special plaster (Steri-Strip) was always applied for 7-10 days. The skin will be analyzed regarding morphological changes of intradermal small fibres and IENFD and thick myelinated fibers will be determined. In addition, immune cell activation will be assessed using various methods.

Visit 2 (duration approx. 1,5 hours)

Imaging: The subjects were asked to make a new pain rating (VAS). They were familiarized with the evoked pain stimulus used in the scanner. Following the safety routines to exclude contraindications for MR scanning the subjects were placed in the scanner. The assessment started with structural scans (T1, T2), followed by resting state and MR spectroscopy (anterior and posterior insula) and finally an evoked pain protocol using a series of painful pressure stimuli generated by an automatic pressure applicator at the leg of the subjects, corresponding to 150 and 300kPa, respectively, (blood oxygenation level dependent, BOLD).

Aims not specified elsewhere:

1. To run assays aiming at identifying the antigen/antigens and if successful to quantify the specific antibodies and relate them to FM severity.
2. To assess if IgG antibodies form individuals with severe FM activate satellite cell cultures to release pro-inflammatory and algesic substances more efficiently than IgG antibodies from individuals with less severe FM.
3. To investigate if there are abnormalities in the number, characteristics, or activation of immune cells in the blood or skin of FM patients compared to HC.
4. To assess effects of FM IgG on animal models.

Inclusion Criteria:

• Female sex,
• age 20-65 years

For the FM arm:

• fulfilling the ACR 1990 fibromyalgia diagnostic criteria
• fulfilling the ACR 2016 fibromyalgia diagnostic criteria

Exclusion Criteria:

• autoimmune or inflammatory diseases (other than FM in the FMS cohort)
• other somatic diseases that could influence the study outcome (e.g. peripheral neuropathy etc)
• pain problems (other than FM in the FMS cohort)
• severe depression/anxiety that requires specific treatments
• medication with anticonvulsants, antidepressants or corticosteroids
• inability to refrain from NSAIDs, analgesics, sedatives or sleep medication 48 hours before examinations
• inability to communicate in Swedish or other factors that the investigator judges would interfere with the participation in the study
• smoking > 5 cigarettes/day
• pregnancy
• drug or alcohol abuse
• contraindications to skin biopsy (allergy to local anesthetics, hemophilia, medication with anticoagulants)
• contraindications to fMRI (metal implants, pacemaker)

Subjects will be recruited by advertising in daily press and social media. All subjects will be pre-screened by a telephone interview from our research nurse regarding inclusion and exclusion criteria before invited to the first visit. All subjects will have to sign informed consent before being included.