Regulatory T-cells and IL-5 mediate pain outcomes in a preclinical model of chronic muscle pain

Pregabalin produces analgesia in males but not females in an animal model of chronic widespread muscle pain

Introduction

Pregabalin, which acts on the α2δ-1 subunit of voltage-gated calcium channels, relieves ≥50% of pain in a third of individuals with fibromyalgia. Thus far, preclinical studies of pregabalin have predominantly used male animals.

Objectives

The purpose of our study was to investigate potential sex differences in the analgesic efficacy of pregabalin that may contribute to disparities in human outcomes.

Methods

We used a mouse model of chronic widespread muscle pain (CWP) to test the effects of pregabalin on muscle hyperalgesia, nonreflexive pain, and motor behaviors. The CWP pain model combines 2 pH 4.0 saline injections, spaced 5 days apart, into the gastrocnemius muscle and produces bilateral muscle hyperalgesia. Furthermore, we explored sex differences in the mRNA and protein expression of the α2δ-1 subunit of voltage-gated calcium channels in the dorsal horn of the spinal cord and dorsal root ganglia after development of CWP.

Results

Pregabalin fully attenuated muscle hyperalgesia bilaterally in male but not female mice with equal motor deficits produced in both sexes. In addition, using the conditioned place preference test, mice of both sexes with CWP spent significantly more time in the pregabalin-paired chamber compared with baseline, but not significantly greater than pain-free controls. Chronic widespread muscle pain produced no changes in α2δ-1 subunit mRNA or protein expression in the dorsal horn of the spinal cord or dorsal root ganglia in either sex.

Conclusion

Overall, these findings indicate pregabalin may be more effective in treating CWP in males, but the factors leading to these differences are not fully understood.

The influence of sex on neuroimmune communication, pain, and physiology

With the National Institutes of Health's mandate to consider sex as a biological variable (SABV), there has been a significant increase of studies utilizing both sexes. Historically, we have known that biological sex and hormones influence immunological processes and now studies focusing on interactions between the immune, endocrine, and nervous systems are revealing sex differences that influence pain behavior and various molecular and biochemical processes. Neuroendocrine-immune interactions represent a key integrative discipline that will reveal critical processes in each field as it pertains to novel mechanisms in sex differences and necessary therapeutics. Here we appraise preclinical and clinical literature to discuss these interactions and key pathways that drive cell- and sex-specific differences in immunity, pain, and physiology.

B-cell and plasma cell activation in a mouse model of chronic muscle pain

Fibromyalgia (FM) is a complex chronic musculoskeletal pain disorder with an elusive pathogenesis, with a strong implication of immune interactions. We recently found that IL-5 and the adaptive immune system mediates pain outcomes in fibromyalgia (FM) patients and preclinical models of FM-like chronic widespread pain (CWP). However, there is an active debate if FM/CWP has an autoimmune etiology. Preclinical models of CWP utilize a repeated insult paradigm, which resembles a primary, then secondary response similarly observed in the antibody response, in which the subsequent event causes a potentiated pain response. Recent translational studies have implicated immunoglobulins (Ig) and B-cells in FM/CWP pathophysiology. To understand if these are involved in preclinical models of CWP, we performed comprehensive B-cell phenotyping in the bone marrow, circulation, and popliteal (draining) lymph nodes in the two-hit acidic saline model of CWP. We found increased MHC class II-expressing B-cells in peripheral blood, increased activated plasma cells in peripheral blood, and increased memory B-cells in the bone marrow. Interestingly, acidic pH (4.0) injected mice have reduced levels of IgG1, independent of treatment with IL-5. We have demonstrated that the acidic saline model of CWP induces T-cell mediated activation of B-cells, increased active plasma cells, and increased memory B-cells in female mice.

Inflammation, Autoimmunity, and Infection in Fibromyalgia: A Narrative Review

Fibromyalgia (FM) is a chronic disease characterized by widespread musculoskeletal pain of unknown etiology. The condition is commonly associated with other symptoms, including fatigue, sleep disturbances, cognitive impairment, and depression. For this reason, FM is also referred to as FM syndrome. The nature of the pain is defined as nociplastic according to the latest international classification and is characterized by altered nervous sensitization both centrally and peripherally. Psychosocial conditions have traditionally been considered critical in the genesis of FM. However, recent studies in animal models and humans have provided new evidence in favor of an inflammatory and/or autoimmune pathogenesis. In support of this hypothesis are epidemiological data of an increased female prevalence, similar to that of autoimmune diseases, and the frequent association with immune-mediated inflammatory disorders. In addition, the observation of an increased incidence of this condition during long COVID revived the hypothesis of an infectious pathogenesis. This narrative review will, therefore, discuss the evidence supporting the immune-mediated pathogenesis of FM in light of the most current data available in the literature.

T cells at the interface of neuroimmune communication

The immune system protects the host from infection and works to heal damaged tissue after infection or injury. There is increasing evidence that the immune system and the nervous system work in concert to achieve these goals. The sensory nervous system senses injury, infection, and inflammation, which results in a direct pain signal. Direct activation of peripheral sensory nerves can drive an inflammatory response in the skin. Immune cells express receptors for numerous transmitters released from sensory and autonomic nerves, which allows the nervous system to communicate directly with the immune system. This communication is bidirectional because immune cells can also produce neurotransmitters. Both innate and adaptive immune cells respond to neuronal signaling, but T cells appear to be at the helm of neuroimmune communication.

Origins and functions of eosinophils in two non-mucosal tissues

Eosinophils are a type of granulocyte named after the presence of their eosin-stained granules. Traditionally, eosinophils have been best known to play prominent roles in anti-parasitic responses and mediating allergic reactions. Knowledge of their behaviour has expanded with time, and they are now recognized to play integral parts in the homeostasis of gastrointestinal, respiratory, skeletal muscle, adipose, and connective tissue systems. As such, they are implicated in a myriad of pathologies, and have been the target of several medical therapies. This review focuses on the lifespan of eosinophils, from their origins in the bone marrow, to their tissue-resident role. In particular, we wish to highlight the functions of eosinophils in non-mucosal tissues with skeletal muscle and the adipose tissues as examples, and to discuss the current understanding of their participation in diseased states in these tissues.

Differential Effects of Regulatory T Cells in the Meninges and Spinal Cord of Male and Female Mice with Neuropathic Pain

Immune cells play a critical role in promoting neuroinflammation and the development of neuropathic pain. However, some subsets of immune cells are essential for pain resolution. Among them are regulatory T cells (Tregs), a specialised subpopulation of T cells that limit excessive immune responses and preserve immune homeostasis. In this study, we utilised intrathecal adoptive transfer of activated Tregs in male and female mice after peripheral nerve injury to investigate Treg migration and whether Treg-mediated suppression of pain behaviours is associated with changes in peripheral immune cell populations in lymphoid and meningeal tissues and spinal microglial and astrocyte reactivity and phenotypes. Treatment with Tregs suppressed mechanical pain hypersensitivity and improved changes in exploratory behaviours after chronic constriction injury (CCI) of the sciatic nerve in both male and female mice. The injected Treg cells were detected in the choroid plexus and the pia mater and in peripheral lymphoid organs in both male and female recipient mice. Nonetheless, Treg treatment resulted in differential changes in meningeal and lymph node immune cell profiles in male and female mice. Moreover, in male mice, adoptive transfer of Tregs ameliorated the CCI-induced increase in microglia reactivity and inflammatory phenotypic shift, increasing M2-like phenotypic markers and attenuating astrocyte reactivity and neurotoxic astrocytes. Contrastingly, in CCI female mice, Treg injection increased astrocyte reactivity and neuroprotective astrocytes. These findings show that the adoptive transfer of Tregs modulates meningeal and peripheral immunity, as well as spinal glial populations, and alleviates neuropathic pain, potentially through different mechanisms in males and females.