The current work presented two lines of novel data on the PGIA mouse model of RA. The first was the complex characterization of the different stages of the chronic PGIA model of the mouse using integrative methodologies. The PGIA model has been known as a T cell-dependent chronic model of RA, mimicking several molecular pathways of the human disease including autoantibody-production and the cytokine profile [
20]. It is also known that ankylosis of the affected joints occurs during the late phase, and both the joints and the spine demonstrate evidence of simultaneous bone erosion and neoformation [
17]. Although arthritic pain is a critical aspect of RA, nociceptive responses have not been studied in this model. Ours was the first study to show that PGIA induces a long-lasting mechanical hyperalgesia, and thus it can be suggested as an appropriate model of chronic arthritic pain. A limitation of our study is that the nocifensive behavior could not be investigated during the total experimental period due to the ankylosis of the ankle joint after the 3rd month. We also provided compelling evidence of the complex effects of PGIA on bone metabolism and architecture using novel non-invasive imaging methods. Arthritic mice demonstrate increased bone turnover in the hind paws, coupled with the development of periarticular osteophytes and erosions. The significant decrease in bone density in the control mice demonstrates the critical importance of using age-matched controls in studies investigating bone structural changes in RA models.
The other important novelty of our results is that we provide the first evidence about the pivotal role of the innervating capsaicin-sensitive sensory afferents in the T and B lymphocyte-dependent disease-mimicking model of RA. We showed that selective defunctionalization of these nerves leads to decreased arthritis severity, edema, and hyperalgesia. However, its effect on neutrophil ROS production follows an interesting biphasic pattern, demonstrated by MPO activity that is lower in the early, but higher in the late phase of arthritis. The functional implication of the late neutrophil activation is not clear as we did not see a corresponding worsening of functional parameters. On the other hand, desensitization had no protective effect on the arthritis-induced changes in bone turnover or bone morphometry.
Our present study demonstrated noteworthy differences when compared to our earlier experiments using the CFA-induced, K/BxN serum-transfer, and MCT models of arthritis. In both the CFA-induced and serum-transfer models, RTX-pretreatment aggravated arthritis severity but reduced inflammatory hyperalgesia [
2,
3]. However, in the MCT model, joint hyperemia, and spontaneous pain but not edema were diminished in desensitized animals [
23]. These essentially contradictory results of the earlier and the current experiments are at least partly due to distinct mechanisms and therefore, the limited translational value of the previously used models. For example, although the CFA-induced rat arthritis model was originally described as a RA disease model, it has been proven to rather model reactive arthritides with many features not found in human RA [
24,
34]. Arthritis in K/BxN mice is triggered by autoreactive T cells and maintained by the autoantibody production of B-cells, but serum-transfer arthritis is a transient T/B-cell-independent model mimicking primarily the innate immune response to joint-specific autoantibodies [
35,
36]. In a similar way, the MCT model evoked solely by a mast-cell-specific protease represents only one facet of the complexity of autoimmune arthritis [
37]. The PGIA model is the most translational RA model in term of adequately resembling all aspects of the autoinflammatory response. In summary, the observed differences highlight the importance of desensitization in a T/B cell-driven arthritis model. Of note, we cannot exclude that lymphocytes were directly affected by the RTX treatment. The extraneural expression of TRP receptors has recently became a topic of interest, and it has been proven that functional TRPV1 is expressed on T (but not B) cells, contributing to their pro-inflammatory response [
38,
39]. On the other hand, we have to mention that while the effect of desensitization on TRPV1 present on T cells has not been addressed, our own earlier studies involving other extraneural tissues, such as skin or oral mucosa, failed to demonstrate an effect of RTX-pretreatment on receptors present in such tissues [
40]. We have previously shown that TRPV1 and TRPA1 gene-deficient mice show diminished inflammation, nociception, and histological damage in CFA-induced model [
41,
42]. Moreover, in the K/BxN-serum transfer arthritis model reduced hyperalgesia but increased edema was detected in TRPV1 gene-deleted mice, while no differences were revealed in TRPA1 gene-deficient animals [
43]. These findings, taken together with the current results indicate that TRP channel activation aggravates local inflammation via the release of pro-inflammatory mediators, while it is also critical for inflammatory pain. Concerning a direct effect on the pain component, it has to be pointed out that the involvement of capsaicin-sensitive primary afferents in mechanonociception, mechanical hyperalgesia, and allodynia in models of inflammatory, neuropathic, or cancer pain had been extensively investigated. Several studies could not confirm a role for this subpopulation of sensory neurons in mechanical hyperalgesia in contrast with thermal hyperalgesia and spontaneous pain in these conditions [
44‐
49], which might be due to major differences in peripheral and central sensitization mechanisms in these chronic pathophysiological processes. However, in our long-lasting arthritis model with mixed inflammatory, neuropathic, and degenerative mechanisms, we found a markedly diminished mechanical hyperalgesia in desensitized arthritic animals, which points to the feasibility of targeting TRPV1 channels and TRPV1-expression sensory nerves for analgesia at the periphery.
The involvement of the axial skeleton is a unique feature of PGIA, characterized by a robust spondyloarthropathy, syndesmophyte formation, and intervertebral disc degeneration; hence it is also utilized as a model for ankylosing spondylitis [
51‐
53]. Our study demonstrated narrowing of the intervertebral space, an indirect sign of disc degeneration, is increased in desensitized arthritic mice, while other aspects of bone structural changes are similar to those of non-desensitized animals. This suggests that peptidergic afferents have an overall protective effect on PGIA-induced spondyloarthropathy.