Intracellular production of perforin and interferon-γ in CD8+ CD28- and CD28+ T cells. Peripheral blood mononuclear cells of patients with ankylosing spondylitis were stimulated with phorbol 12-myristate 13-acetate and ionomycin in the presence of brefeldin A. Cells were stained with fluorescence-marked monoclonal antibodies (mAbs) directed against CD8, CD28 and either perforin or interferon-γ and counted by flow cytometry. Whiskers boxblots show the results of 5 independent experiments for perforin (a) and 6 experiments for interferon-γ (b) with 50 % of cases within the boxes and 80 % between the endpoints of the whiskers (lines). The Mann–Whitney test was used to determine the statistical difference between the percentage of perforin-positive or interferon-γ-positive CD28+ and CD28- cells out of the CD8+ T cell compartment (each with P < 0.001).