Cellular and molecular mechanisms of inflammatory arthritis and fibromyalgia

Abstract

In Study I, we examined the impact of the hR100E-NGF mutation on inflammatory pain and bone erosion in both female and male mice. Our findings indicate that the hR100E-NGF mutation did not affect the development of the peripheral sensory nervous system at the lumbar DRG, sciatic nerve, ankle joint, or glabrous skin. Moreover, hR100E-NGF mice displayed sensory thresholds similar to those of the hWT-NGF mice in response to mechanical, heat, or cold stimulation under normal conditions. The hR100E-NGF and hWT-NGF mice developed comparable mechanical and heat sensitivity impairments after the intra-articular injection of complete Freund’s adjuvant. Notably, the hR100E-NGF mice were insensitive to nociceptive stimulation in the deeper tissues assessed by weight bearing and gait analysis. Furthermore, mRNA analysis from the inflamed joint showed a differential sex-dependent gene expression profile between hR100E-NGF female and male mice. Finally, the hR100E-NGF female but not the male mice were protected against the CFA-bone erosion. These data collectively demonstrate that the R100E NGF mutation effectively protects against joint pain-like behaviors in both male and female mice while providing bone protection exclusively to female mice in a monoarthritis model. We propose that manipulating the signaling of NGF and its receptors in a manner similar to the R100E mutation could be a promising approach to treating chronic pain and maintaining bone health, particularly in women.

Study II investigated the effects of injecting purified IgG from fibromyalgia (FM) patients and healthy controls (HC) in mice. We found that the injection of FM IgG but not IgG from healthy controls (HC) induces pressure, mechanical, and cold hypersensitivity in mice that were coupled to enhanced nociceptor responsiveness to mechanical and cold stimulation. The FM IgG-injected mice also developed impaired muscular strength and decreased locomotor activity. Moreover, FM IgG bound and stimulated satellite glial cells (SGCs) in vivo and in vitro. No FM or HC IgG accumulation was found in the brain or spinal cord of the injected mice. Our study also demonstrated that FM IgG can bind to satellite glial cells and neurons in the human DRG. In addition, we observed a significant reduction in the intraepidermal nerve fiber density in the mice 14 days after the FM IgG injection. Our results suggest that transferring FM IgG into mice can replicate some peripheral FM symptoms. This study can provide a valuable animal model for studying the peripheral physiology of FM. Our discovery could significantly advance the understanding and treatment of fibromyalgia and other related conditions. However, more research is needed to understand the cellular and molecular mechanisms involved in FM-IgG-mediated changes in mice.

Study III aimed to investigate the frequency of anti-satellite glial cell (SGC) antibodies and the antibody association with the disease severity in FM patients. We used serum (Karolinska Institutet, Sweden; n=30/group) and plasma (McGill University, Canada; n=35/group) samples collected from FM patients and HCs. Our results showed a higher binding intensity of the FM IgG to SGC in vitro. Furthermore, the frequency of SGC bound to FM IgG was significantly higher than HC IgG-treated cells. These findings correlated with pain intensity and fibromyalgia impact questionnaire scores (FIQ, questionnaire was only assessed in the Karolinska cohort). Further cluster analysis separated the FM group into severe and mild groups. Additionally, we found that serum from FM patients contains IgG that binds in greater proportion to SGC in the human DRG, measured by higher signal intensity. There were no differences in the binding intensity to neuronal cell bodies or axons between FM and HC serum samples. Finally, the previous results were confirmed using an FM serum sample with high levels of anti-SGC antibodies in 5 more human DRGs. To summarize, our report indicates that levels of anti-human SGC and anti-mouse SGC antibodies are elevated in patients with FM, which are linked to a more severe form of the disease. Patient stratification based on their profile of anti-SGC antibodies might benefit from therapies aiming to decrease circulating IgG or prevent IgG binding. Our results point to the possible involvement of anti-SGC antibodies and SGCs in the severity of FM; however, more in-depth studies are necessary to elucidate the antigen or antigens expressed in the SGC that bind to the circulating anti-SGC antibodies.

In Study IV, we aimed to explore the neuroimmune signature of the FM skin. We processed 16 FM and 16 HC sex-matched skin biopsies by immunohistochemistry. Using a pan-neuronal marker, we found lower intraepidermal nerve fiber density (IENFD) in the FM compared with HC skin. Moreover, the length and volume of dermal NF200+ nerve profiles were significantly elevated, but we found no changes in the length of dermal or epidermal Gap43+ nerve profiles in the FM group. Similarly, we found no changes in the total volume of CD31+ blood vessels between FM and HC skin. Our results showed that the density of non-nerve associated S100b+, CD68+, and CD163+ cells was significantly lower in the FM skin. Furthermore, the dermal CD117+FcERI+ mast cells in the dermis of FM patients were significantly increased compared with the HCs. Additionally, we found similar densities of CD207+, CD3+, or Neutrophil elastase+ cells between FM and HC skin biopsies. mRNA analysis of FM skin showed no changes in Cd68, Cd163, Cx3cr1, or FceR1 mRNA levels between FM and HC skin. In summary, this study reveals crucial dermal and epidermal changes in FM skin, particularly regarding nerve fibers and certain immune cell populations. These findings are highly relevant as they provide deeper insights into the complex interactions between the nervous and immune systems in FM. Understanding these changes could be key to developing more effective treatments for FM, focusing on both the neuropathic and immune components of the disease.