Ankylosing spondylitis and seronegative spondylarthropathies such as Reiter's syndrome and reactive arthritis are strongly associated with HLA-B27. However, the mechanisms by which HLA-B27 is involved in disease susceptibility and pathogenesis are unknown. If the disease association is a consequence of HLA-B27's physiological function in antigen presentation, the disease should be mediated by cytotoxic T lymphocytes (CTLs) that recognise bacterial or self peptides presented by HLA-B27. Proof of this arthritogenic peptide model requires isolation of B27-restricted CD8 T cells from arthritic joints of patients with spondylarthropathies. An important question is whether "arthritogenic" bacteria such as yersinia or salmonella can generate HLA-B27-restricted bacteria-specific CTLs. We describe such HLA-B27-restricted CTLs. We tested a panel of 354 αβ-TCR CD8 T lymphocyte clones (TLCs) that had been derived from the synovial fluid of 4 patients with reactive arthritis and 2 patients with ankylosing spondylitis. In 1 patient with yersinia-induced arthritis, 2 TLCs were identified that killed specifically yersinia-infected B27 target cells. In another patient with salmonella-induced arthritis, 1 B27-restricted CD8 TLC that recognised both salmonella and yersinia was identified. In 5 of the 6 patients autoreactive CTLs were found, 5 of which showed B27-restricted killing of uninfected cell lines. 827-restricted CTLs with specificity for arthritogenic bacteria or autoantigens provide a missing link in the pathogenesis of the HLA-B27-associated spondylarthropathies.