Levels of CD3+CD4+CD28- T cells in patients with ankylosing spondylitis and healthy control individuals. (a) Accumulation of CD3+CD4+CD28- cells in peripheral blood mononuclear cells of 95 patients with ankylosing spondylitis (AS; ●) and 65 age-matched healthy control individuals (3). The Mann–Whitney test was used to determine statistical difference. (b) Logarithmic transformation of percentages of CD3+CD4+CD28- T cells was performed to detect different populations of CD4+ T cells and to correct for data skewing. We found a bimodal distribution of frequencies of CD3+CD4+CD28- T cells (line for healthy control individuals, boxes for patients with AS). The cutoff value, which was determined at the intersection of the two bimodal distribution curves, was 1.7%. Using this value as cutoff, 70.3% of the patients had increased levels as compared with only 6.5% of the control group. (c, d) As a regression model, receiver operating curves were used to display sensitivity and specificity of CD28- T cell levels for AS disease and age. The area under the curve (AUC) was determined for both independent parameters.