Although pain in rheumatoid arthritis (RA) is frequently thought to be inflammatory in nature, the association between measures of inflammation and pain intensity is low. This observation is likely due to the multifactorial nature of pain. In addition to pain from joint inflammation, RA patients may also have pain due to structural damage or central etiologies, such as aberrancies in the central nervous system (CNS) pain regulatory pathways. These CNS pathways include mechanisms that facilitate pain, as well as mechanisms that inhibit pain. Other factors, such as sleep disturbances, depression, anxiety, and catastrophizing, may also impact the perception of pain in RA patients. Since pain is frequently used as a proxy for inflammation in the assessment of RA disease activity, it is important that patients and physicians recognize that not all pain is inflammatory, and alternative management strategies, other than escalating disease-modifying antirheumatic drug treatment, may need to be considered.

Background Overexpression of IL-23 in adult mice by means of hydrodynamic tail vein injection of IL-23 minicircles has been reported to result in spondyloarthritis-like disease. The impact of genetic background and sex on the disease phenotype in this model has not been investigated. Methods We compared male B10.RIII mice with male C57BL/6 mice, and male with female B10.RIII mice after hydrodynamic injection of IL-23 enhanced episomal vector (EEV) at 8–12 weeks of age. We monitored clinical arthritis scores, paw swelling, and body weight. Animals were euthanized after two weeks and tissues were harvested for histology, flow cytometry and gene expression analysis. Serum cytokine levels were determined by ELISA. Findings Male B10.RIII mice developed arthritis in the forepaws and feet within 6 days after IL-23 EEV injection; they also exhibited psoriasis-like skin disease, colitis, weight loss, and osteopenia. In contrast to previous reports, we did not observe spondylitis or uveitis. Male C57BL/6 mice injected with IL-23 EEV had serum IL-23 levels comparable with B10.RIII mice and developed skin inflammation, colitis, weight loss, and osteopenia but failed to develop arthritis. Female B10.RIII mice had more severe arthritis than male B10.RIII mice but did not lose weight. Conclusions The phenotype of IL-23 induced disease in mice is controlled by genetic background and sex of the animals. The development of extra-articular manifestations but absence of arthritis in C57BL/6 mice suggests that organ-specificity of IL-23 driven inflammation is genetically determined. The mechanisms behind the strain-specific differences and the sexual dimorphism observed in this study may be relevant for human spondyloarthritis and warrant further exploration.

Background Overexpression of IL-23 in adult mice by means of hydrodynamic tail vein injection of IL-23 minicircles has been reported to result in spondyloarthritis-like disease. The impact of genetic background and sex on the disease phenotype in this model has not been investigated.  Methods We compared male B10.RIII mice with male C57BL/6 mice, and male with female B10.RIII mice after hydrodynamic injection of IL-23 enhanced episomal vector (EEV) at 8-12 weeks of age. We monitored clinical arthritis scores, paw swelling, and body weight. Animals were euthanized after two weeks and tissues were harvested for histology, flow cytometry and gene expression analysis. Serum cytokine levels were determined by ELISA.  Findings Male B10.RIII mice developed arthritis in the forepaws and feet within 6 days after IL-23 EEV injection; they also exhibited psoriasis-like skin disease, colitis, weight loss, and osteopenia. In contrast to previous reports, we did not observe spondylitis or uveitis. Male C57BL/6 mice injected with IL-23 EEV had serum IL-23 levels comparable with B10.RIII mice and developed skin inflammation, colitis, weight loss, and osteopenia but failed to develop arthritis. Female B10.RIII mice had more severe arthritis than male B10.RIII mice but did not lose weight.  Conclusions Systemic IL-23 overexpression results in spondyloarthritis-like disease in B10.RIII mice. The development of extra-articular manifestations but absence of arthritis in C57BL/6 mice suggests organ-specific genetic control mechanisms of IL-23 driven inflammation. Discrepancies regarding the phenotype of IL-23 induced disease in different labs and the sexual dimorphism observed in this study warrant further exploration.