New Research Highlights Potential Marker for Chronic Pain in Fibromyalgia

Mu-Lympho-Marker (MLM) has demonstrated promise as a marker for chronic pain (CP) in people with fibromyalgia (FM), according to new findings from an observational study.1

“The absence of a dependable diagnostic test represents a significant obstacle in advancing the comprehension of CP and in this specific contest of FM pain condition. Adding to the complexity, the lack of specific biomarkers for CP further hampers the FM treatment and often causes tensions between patients and physicians. This can lead patients to believe their clinical condition is being misunderstood or dismissed as emotional issues, resulting in worsened disease severity and quality of life. Consequently, patients may engage in “doctor-shopping,” which increases health care costs. Consequently, the syndrome typically follows a persistent course without remission,” lead investigator Valentina Malafoglia, PhD, Laboratory of Physiology and Pharmacology of Pain, IRCCS San Raffaele Roma, Rome, Italy, and colleagues wrote.1

Malafoglia and colleagues conducted an observational, longitudinal study following up a study prior to the pandemic that explored the MLM biomarker and found that on B cells, in painful patients with FM and osteoarthritis, MLM was statistically lower than in pain-free subjects. This reduction in Mu-positive B-cell expression was not associated with the investigated psychological characteristics and investigators hypothesized the “Mu-opioid receptor theory,” proposing that a reduction in the reserve of Mu-opioid receptors on immune cells could lead to a disruption of endogenous analgesic activity, thereby contributing to the onset and progression of pain.2

This study called back participants with FM enrolled in the first study for follow-up sampling. Participants reported using the Numerical Rating Scale (NRS). Investigators analyzed Mu+ B cells percentage of expression by flow cytometry and explored the cellular localization of Mu-opioid receptor with immunofluorescence analyses. The study used pain-free participants as a control group. All the participants also filled out self-reported psychological tests.1

Of the 59 patients with FM enrolled in the initial study, 21 participated in the follow-up study. The control group consisted of 43 pain-free participants. All participants were adult and female and had a mean age of 54 years (standard deviation [SD], 8.8).

At follow-up sampling (T1), all participants with FM reported moderate (n = 6; 28.5%) to severe (n = 15; 71.5%) pain on the NRS scale with a median score of 8.0 (IQR: 7.0–9.0), unchanged from baseline before the pandemic (T0; median, 8.0; IQR: 8.0–9.0; Wilcoxon Z = 1.635, P = .102). Control participants reported no chronic pain (NRS = 0) for at least 3 months prior to enrollment. No SARS-CoV-2 infections were reported during the study.1

Flow cytometry analysis showed significantly lower MLM expression on B cells in FM patients compared to controls at T1 (FM, 12.76; SD,1.9 vs. Ctrl(-), 42.71; SD, 2.7; mean difference, 32.0; SD, 4.4; t = 7.330; 95% CI, 23.2–40.9; P <.0001), consistent with findings at T0. No significant difference in MLM expression was observed over time within FM patients (T0, 18.4; SD, 2.5 vs. T1, 12.76; SD, 1.9; paired t test, NS), indicating stability.1

“The stability of MLM over an extended period, characterized by significant bioimmune and psychological stress, such as the Covid-19 pandemic period, leads us to hypothesize its suitability as a putative biomarker of CP in patients with FM. Extending this discussion beyond FM, objective pain assessment through MLM analysis could be of significant value in the assessment of CP in cases where traditional self-reporting is challenging. This includes people with cognitive/communicative impairments or psychiatric disorders for whom it is particularly difficult to obtain a reliable and objective assessment of pain,” Malafoglia and colleagues wrote.1

Confocal microscopy of PBMCs from 4 FM and 4 control participants revealed internalization of the Mu-opioid receptor in FM patients' B cells, with cytoplasmic localization not observed in controls. No significant differences in psychological questionnaire scores were found between T0 and T1 (paired t test, NS), nor were any significant correlations observed between questionnaire results and MLM expression.

“Further studies are necessary to understand the biological and genetic bases underlying Mu-opioid receptor underexpression on immune cells and to define its sensitivity and predictive reliability as a diagnostic marker, in different CP syndromes. Thus, we aim to achieve an even more precise understanding of MLM's role in clinical practice, pain management, and rehabilitation strategies,” Malafoglia and colleagues concluded.1

REFERENCES

1. Malafoglia V, Raffaeli W, Ilari S, et al. Mu-opioid receptor expression on B cells as a potential biomarker for chronic pain: a follow-up study with patients with fibromyalgia. Pain Rep. 2025;10(4):e1283. Published 2025 May 27. doi:10.1097/PR9.0000000000001283

2. Raffaeli W, Malafoglia V, Bonci A, Tenti M, Ilari S, Gremigni P, Iannuccelli C, Gioia C, Di Franco M, Mollace V, Vitiello L, Tomino C, Muscoli C. Identification of MOR-positive B cell as possible innovative biomarker (Mu Lympho-Marker) for chronic pain diagnosis in patients with fibromyalgia and osteoarthritis diseases. Int J Mol Sci. 2020;21:1499. Doi: 10.3390/ijms21041499