Detection of CD26 by Fluorescence-activated cell sorting (FACS) analysis. Expression of CD26 was detected by FACS analysis. (a) Expression level of CD26 on unstimulated fibroblasts. Background staining with secondary antibody only (black histograms) was compared with specific CD26-staining (open histograms). Control staining with isotype antibodies resulted in a similar histogram as with secondary antibody alone. Confluent fibroblast monolayers were left untreated (Co) or were treated with IL-1β (100 U/ml), tumour necrosis factor alpha (TNF-α) (10 ng/ml), IFN-γ (200 ng/ml) or with combinations of these cytokines. (b) Regulation of CD26 expression as the percentage of the relative mean fluorescence intensity (MFI) for untreated fibroblasts (± standard error of the mean). The mean MFI of four experiments is shown (except for treatment with IFN-γ alone, for which n = 3). Statistical analysis was performed with the Mann–Whitney U test, *P < 0.05.