HLA-B27/Chlamydia peptide tetramer staining and intracellular cytokine staining in patient no. 6. When synovial T cells where stained with HLA-B27 Chlamydia peptide tetramers, only HLA-B27/Chlamydia peptide 133 was positive for CD8+ T cells (0.02%). All other HLA-B27 tetramers with Chlamydia peptides did not bind synovial T cells in this patient (not shown). The specificity of this staining could be confirmed by an increased quantity of tetramer-positive T cells after peptide-specific T cell stimulation (0.22%) after 1 week of peptide-specific stimulation loaded on synovial mononuclear cells (a) and Chlamydia peptide 133-specific induction of IFN-γ production in intracellular cytokine staining (b).