Objective: To characterise the immunohistological features of sacroiliitis in ankylosing spondylitis (AS) at different disease stages.

Methods: Biopsy samples from sacroiliac joints (SIJs) of five patients with AS, two with early, three with advanced changes and samples from age matched controls from one necropsy SIJ and two iliac bone marrow (BM) biopsies were studied. Paraffin sections were immunostained in triplicate for T cells (CD3, CD8), macrophages (CD68), and the cytokines tumour necrosis factor α (TNFα), interferon γ, interleukin (IL) 1β, IL6, IL10, and transforming growth factor β₁ (TGFβ₁). Stained cells were counted over one entire high power field (×400) per section in BM, cartilage, and other connective tissue (CT). Results are the mean of three sections.

Results: CD3+ T cells were numerous in the BM of early AS, and in the CT of one patient with early and one with late AS, with variable proportions of CD8+ T cells. All patients with AS had more CD68+ macrophages than controls in BM and CT; in cartilage, one patient with early and one with late AS had increased CD68+ cells, some being osteoclasts. The patient with very early AS had large numbers of TNFα cells in the three tissular areas; for the other patient with early disease they were found only in CT and cartilage. IL6 was seen in 4/4 patients with AS in most areas. Patients with early disease had more T cells, TNFα, and IL6, and patients with advanced AS more TGFβ₁.

Conclusion: The immunohistological findings of a limited sample suggest a role for BM in sacroiliitis, for TNFα and IL6 in early, active lesions, and for TGFβ₁ at the time of secondary cartilage and bone proliferation.