IgA antibodies against CD74 are associated with structural damage in the axial skeleton in patients with axial spondyloarthritis

To study the association between the presence of antibodies against CD74 and structural damage in the sacroiliac joints and spine in patients with axial spondyloarthritis (axSpA).Antibodies against CD74 were measured in the sera of patients with axSpA from 2 cohorts: 1. An observational cohort from Damp in Northern Germany and 2. from a clinical trial (ENRADAS), in which the modified Stoke Ankylosing Spondylitis Spine Score (mSASSS) had been evaluated by two readers blinded to the time point at baseline and two years later. The presence of antibodies against CD74 was correlated with the presence and grade of radiographic sacroiliitis in the observational cohort, and with baseline mSASSS in the ENRADAS cohort.The sensitivity of IgA anti-CD74 antibodies for axSpA was 50% in the Damp cohort and 42% in ENRADAS. The presence of IgA antibodies against CD74 was associated with a higher grade of sacroiliitis (observational cohort) and a higher baseline mSASSS in the ENRADAS cohort.IgA antibodies against CD74 are not only markers of AS, but are associated with structural damage development in the sacroiliac joints and in the spine.