Detection of dipeptidyl peptidase IV activity. (a) Soluble dipeptidyl peptidase IV (DPP IV) activity in serum-free conditioned medium from fibroblast cultures or (b)–(d) the activity of DPP IV associated with fibroblast membranes was evaluated by detecting the release of p-nitroanilide from GlyPro-p-nitroanilide (increase in UV absorption at 400 nm (OD400) compared with culture medium or untreated cells). Fibroblasts were either left untreated or were treated with the cytokines IFN-α (1000 U/ml), IFN-β (1000 U/ml), IFN-γ (200 ng/ml), IL-1β (10 U/ml) or tumour necrosis factor alpha (TNF-α) (10 ng/ml), or combinations thereof. A significantly increased membrane-bound DPP IV activity was detected on fibroblasts treated with IFN-γ (P < 0.0008), with IL-1β + IFN-γ (P < 0.0004) or with TNF-α + IFN-γ (P < 0.002). Statistical analysis for DPP IV activity on cytokine-treated versus untreated fibroblasts (n = 4 for cytokine combinations and n = 8 for individual cytokines) was performed with the Student t test with Bonferroni correction.