BackgroundThe pathogenesis of sacroiliitis is unclear; therefore, we aimed to systematically study the immunopathology of sacroiliitis in patients with axial spondyloarthritis (axSpA), and explore the relationship between pannus formation, inflammation, and the structural damage caused by sacroiliitis.MethodsFine needle aspiration biopsy of the sacroiliac joint (SIJ) was performed in 193 patients with axSpA. Clinical, laboratory, and imaging data were collected at baseline and during the follow up. Immunohistochemistry analysis was performed to detect CD34+ microvessels, CD68+ osteoclasts/macrophages, vascular endothelial growth factor (VEGF), metalloproteinase-3 (MMP-3), tumor necrosis factor-α (TNF-α), and caspase-3. Autopsy subjects were used as controls.ResultsIn early sacroiliitis (grade 0–1) all pathological features could be observed, with the most common being subchondral pannus formation. Among the 193 patients, 98 were followed up for 1–13 years (mean 3.6 years); 63.3% had radiological progression at the endpoint. Multiple regression analysis showed that cartilage pannus invasion (OR 2.99, P = 0.010) and endochondral ossification (OR 3.97, P = 0.049) at baseline were risk factors for radiological structural damage. Compared to SIJ controls, the subchondral microvessel density, number of CD68+ multinuclear osteoclasts, and the levels of VEGF, caspase-3, MMP-3, and TNF-α expressed at the interface of the bone and cartilage were significantly higher in patients with sacroiliitis.ConclusionsSubchondral fibrovascular tissue formation is the most important pathological feature in early sacroiliitis. The existence of cartilage pannus invasion or endochondral ossification at baseline can predict radiological structural damage during the follow up.Electronic supplementary materialThe online version of this article (10.1186/s13075-018-1594-z) contains supplementary material, which is available to authorized users.

Background : Early diagnosis and treatment are the key to improve the prognosis of axial spondyloarthritis (axSpA), therefore, we aimed to evaluate the diagnostic value of magnetic resonance imaging (MRI) and pathological examination of the sacroiliac joint (SIJ) for non-radiographic axSpA (nr-axSpA). Methods : Fine needle aspiration biopsy of bilateral SIJs was performed in 107 patients with nr-axSpA after MRI examination. The active inflammatory manifestations and chronic structural changes in SIJs were evaluated using MRI. The pathological changes of SIJ specimens were examined using microscopy. Results : Bone marrow edema (BME) was present in 67/214 joints on MRI (31.3%). The proportion of pannus formation and inflammatory cell infiltration were up to 63.8% and 53.4% in patients without BME on MRI, which were not significantly lower than 75.0% and 58.3% in patients with BME on MRI. The inflammatory infiltrating cells were mainly CD3 + T lymphocytes and CD68 + macrophages. In the group with normal cartilage on MRI, the proportions of chondrocytes and cartilage matrix degeneration were 22.2% and 44.9%，respectively, which was not significantly lower than those in the group with abnormal cartilage on MRI (35.7% and 46.4%). However, a significantly higher proportion of pannus invasion was observed in the latter (26.6% vs 57.1%, P =0.001). In the group with a normal bone plate on MRI, the incidence of subchondral pannus formation and bone plate destruction was as high as 74.0% and 71.7%, with no significant difference compared with that in the group with bone erosion on MRI (88.9% and 88.3%). Conclusions: Pathological examination is more sensitive for detecting inflammatory and structural changes in early sacroiliitis than MRI examination. Key words : non-radiographic axial spondyloarthritis, sacroiliac joint, aspiration biopsy, magnetic resonance imaging.