LAUSR

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.