Background/Purpose: Joint pain is one of the main reasons for patients with rheumatoid arthritis (RA) to seek medical care. Collagen type II (CII) antibodies (Abs) are detectable in serum and synovial fluid of many RA patients and are thought to contribute to the inflammatory process in the joint. A common view is that factors produced during inflammation are the major cause of joint pain in RA. However, pain can appear prior to diagnosis and continue to be a significant problem even when the disease is under medical control. Earlier, we have shown that while injection of CII Abs in mice induces transient joint inflammation, pain-like behavior not only outlasts the inflammation but also precedes the onset of inflammation. Moreover, Fc gamma receptors (FcγRs), which are activated by immune complexes (ICs) on inflammatory cells, are also present on peripheral sensory neurons. The aim of this study is to examine if antibodies can directly activate sensory neurons via neuronally expressed FcγRs and thus act as pain-inducing molecules.

Methods: Male BALB/c mice (10-12 weeks) were used to prepare dorsal root ganglia (DRG) primary neuronal cell cultures. Neuronal expression of FcγRI was evaluated by immunocytochemistry. The effect of stimulation with CII mAb cocktail, control IgG2b and CII (all 1 μg/ml) and ICs formed by CII mAb cocktail and CII (CII-IC; 0.1-10 µg/ml) on neuronal excitability were examined by a) calcitonin-gene related peptide (CGRP) release in wild type (WT) and Fcγ-chain deficient mice b) electrophysiology and c) calcium imaging

Results: FcγRI expression was detected in DRG neurons and co-localized with PGP9.5, a neuronal marker. The levels of CGRP, a pain-associated neurotransmitter were increased in a bell shaped fashion in supernatants from WT DRG neurons stimulated with CII-IC, peaking at 1 µg/ml. Note that, 1 µg/ml concentration was used in subsequent studies. No increase in CGRP was detected after stimulation with CII mAb cocktail, CII or control IgG2b. CGRP release was induced by capsaicin (Cap; 50 nM), a ligand for transient receptor potential vanilloid 1 (TRPV1), was used as a positive control. In contrast, while DRG cultures from Fcγ-chain deficient mice responded to Cap, stimulation with CII-IC did not evoke CGRP release. Electrophysiological recordings in voltage clamp mode showed ionic current changes in the presence of CII-IC. Of 114 cells, 52 cells gave inward current response to Cap and 42% (22) of the Cap⁺ cells gave response to CII-IC. In control experiments, none of the Cap⁺ cells (18) responded to IgG2b. Calcium influx assessed by calcium imaging increased in DRG neurons stimulated with CII-IC and KCl (50 mM) (positive control to detect functional neurons) but not control IgG2b. Among the 1555 cells, 1119 cells gave response to KCl and 22.1% (247) of the KCl⁺ cells responded to CII-IC, but not to control IgG2b (<1%). 

Conclusion: The present study demonstrates for the first time that CII Abs in immune complex formation, presumably via activation of FcγRs expressed on DRG neurons directly contribute to nociception. These findings have the potential to define new roles for antibodies in pain transmission and open new avenues for treatment of pain in RA and other autoimmune diseases.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/a-novel-mechanism-of-arthritis-induced-pain-activation-of-sensory-neurons-by-autoantibodies/