Opioid receptors found on the surface of immune cells may be useful biomarkers to diagnose people with fibromyalgia, a study found.
The study, “Identification of MOR-Positive B Cell as Possible Innovative Biomarker (Mu Lympho-Marker) for Chronic Pain Diagnosis in Patients with Fibromyalgia and Osteoarthritis Diseases,” was published in the International Journal of Molecular Sciences.
Fibromyalgia can be difficult to diagnose, as symptoms can be nonspecific and overlap with other medical conditions.
The identification of specific biomarkers to diagnose fibromyalgia and assess its severity and progression can help support the development of new, effective therapies.
Researchers have investigated the connection between immune cells and neurons that share common opioid receptors called Mu (MOR/Oprm1), delta (DOR/Oprd1), and kappa (KOR/Oprk1). While these receptors are known to play key roles in pain perception, little is known about their function on circulating immune cells during the development of a chronic pain disease.
Scientists in Italy set out to investigate whether the Mu opioid receptor found on the surface of immune cells could be used as a biomarker for fibromyalgia and osteoarthritis, a chronic pain condition caused by the breakdown of joint cartilage and bone.
They also looked at whether levels of Mu receptors could be influenced by psychological aspects relevant to chronic pain disorders, particularly fibromyalgia.
All patients enrolled in the study (ISRCTN24645566) were women, with an average age of 51. Fifty-nine had been diagnosed with fibromyalgia, 19 with osteoarthritis, and 24 were healthy, pain-free controls.
Pain intensity assessed using an 11-point numerical rating scale (NRS) revealed that nearly 85% of women with fibromyalgia reported severe pain (NRS between 7-10), while only about 37% of those with osteoarthritis reported feeling the same pain intensity.
Researchers collected blood samples, and participants were asked to complete a series of questionnaires, including the Fibromyalgia Impact Questionnaire (FIQ), the Osteoarthritis-FIQ-Revised (OA-FIQ-R), the Illness Perception Questionnaire-Revised (IPQ-R), the Coping Strategies Questionnaire (CSQ), and the Depression, Anxiety and Stress Scale (DASS-21).
Findings showed the percentage of T cells — immune cells responsible for destroying harmful microbes — containing the receptor was very low in all three groups. In contrast, the percentage of Mu-positive B cells — immune cells responsible for producing antibodies — was significantly lower in women with fibromyalgia and osteoarthritis compared with the pain-free controls.
While there were no differences in the percentage of Mu-positive B cells among women with fibromyalgia who reported moderate to severe pain, the number of Mu-positive B cells in this group was significantly lower compared with the pain-free control group.
Likewise, women with osteoarthritis reporting moderate to severe pain had a significantly lower percentage of Mu-positive B cells compared with controls.
Women who had fibromyalgia for more than five years had a significantly lower percentage of Mu-positive B cells than those who had the disease for less than five years.
The researchers found no link between the percentage of Mu-positive B cells and medications those with fibromyalgia were taking, or with any of the psychological aspects evaluated in questionnaires.
“We propose the percentage of Mu-positive B cells as a biological marker for an objective diagnosis of chronic pain suffering patients, also contributing to the legitimacy of FM [fibromyalgia] as a truly painful disease,” the researchers said.