Objectives Gut inflammation commonly occurs in axial spondyloarthritis (axSpA), and is linked to disease activity and outcome. Given the role of IgA in mucosal immunity, we explored the association between anti-CD74 IgA antibodies, gut inflammation and axSpA. Methods Anti-CD74 IgA was measured by ELISA in serum samples of axSpA patients (fulfilling the 2009 ASAS classification criteria). A group of fibromyalgia (FM) and rheumatoid arthritis (RA) patients served as non-inflammatory and inflammatory controls. Newly diagnosed axSpA patients underwent ileocolonoscopy; mucosal biopsies were histopathologically assessed as normal, acute or chronically inflamed. Optimal anti-CD74 IgA cut-off values were determined with a receiver operating characteristics (ROC) curve. Results AxSpA patients (n = 281) showed higher anti-CD74 IgA levels (mean±SD 18.8 ± 12.4 U/ml) compared with 100 FM patients (10.9 ± 5.0 U/ml, p< 0.001) and 34 RA patients (13.7 ± 9.6 U/ml, p= 0.02). The area under the ROC curve (AUC) for diagnosis (axSpA vs FM) was 0.70, providing a sensitivity of 60% and specificity of 87% (cut-off 15 U/ml). Antibody concentrations were not significantly different between axSpA patients with (n = 40) and without (n = 69) gut inflammation (p= 0.83), yielding an AUC of 0.51. Anti-CD74 IgA levels were not associated with degree of bone marrow oedema on MRI of the sacroiliac joints, CRP, or any other disease-specific feature such as the use of NSAIDs or biological treatment. Conclusion Serum anti-CD74 IgA is a potentially useful diagnostic biomarker for axSpA. However, antibody levels do not correlate with any phenotypical feature, including microscopic gut inflammation, suggesting this to be a disease-specific rather than an inflammatory marker.

中文翻译：

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抗 CD74 IgA 抗体显示出中轴型脊柱关节炎的诊断潜力，但与微观肠道炎症无关

目的 肠道炎症通常发生在中轴型脊柱关节炎 (axSpA) 中，并且与疾病活动度和结果相关。鉴于 IgA 在粘膜免疫中的作用，我们探索了抗 CD74 IgA 抗体、肠道炎症和 axSpA 之间的关联。方法 在 axSpA 患者（满足 2009 ASAS 分类标准）的血清样本中，通过 ELISA 测量抗 CD74 IgA。一组纤维肌痛 (FM) 和类风湿性关节炎 (RA) 患者充当非炎症和炎症对照。初诊axSpA患者行回结肠镜检查；粘膜活检组织病理学评估为正常、急性或慢性炎症。最佳抗 CD74 IgA 截止值由接受者操作特征 (ROC) 曲线确定。结果 AxSpA 患者 (n = 281) 显示出较高的抗 CD74 IgA 水平（平均值±标准差 18.8 ± 12. 4 U/ml) 与 100 名 FM 患者 (10.9 ± 5.0 U/ml, p<0.001) 和 34 名 RA 患者 (13.7 ± 9.6 U/ml, p= 0.02) 相比。用于诊断（axSpA 与 FM）的 ROC 曲线下面积 (AUC) 为 0.70，灵敏度为 60%，特异性为 87%（截止值 15 U/ml）。抗体浓度在有 (n = 40) 和没有 (n = 69) 肠道炎症 (p = 0.83) 的 axSpA 患者之间没有显着差异，产生的 AUC 为 0.51。抗 CD74 IgA 水平与骶髂关节 MRI 上的骨髓水肿程度、CRP 或任何其他疾病特异性特征（例如使用非甾体抗炎药或生物治疗）无关。结论 血清抗 CD74 IgA 是一种潜在有用的 axSpA 诊断生物标志物。然而，抗体水平与任何表型特征无关，包括微观肠道炎症、