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Objective: TNF receptor-associated periodic syndrome (TRAPS) is a systemic autoinflammatory disorder caused by mutations in the type 1 TNF receptor (TNFRSF1A) gene. Because the pathomechanism of TRAPS may involve aberrant TNF-mediated intracellular signalling, we examined phosphorylation levels of nuclear factor kappaB (NF-kappaB) and p38 in response to TNF in 10 patients with three different TNFRSF1A mutations (C73R, C88Y and F112I)., Methods: Phosphorylation levels of NF-kappaB p65 and p38 were determined in fresh leucocytes stimulated with TNF (0-100 ng/ml) for 2.5-20 min and permeabilized for phospho-specific antibodies in a whole blood flow cytometry assay. As control agonists, we used bacterial lipopolysaccharide (LPS) and IFN-gamma, the latter mediating phosphorylation of the signal transducer and activator of transcription 1. Areas under curve values for dose-response and time course of NF-kappaB and p38 phosphorylation were calculated for the comparison of patients and reference subjects., Results: NF-kappaB and p38 phosphorylation levels of monocytes, lymphocytes and neutrophils stimulated with TNF were significantly lower in TRAPS patients than in reference subjects. Phosphorylation levels induced by LPS, or by IFN-gamma, in patient and reference samples were comparable, indicating that the defect was confined to TNF-mediated signalling., Conclusions: In the three families studied, TRAPS was associated with low TNF-mediated signalling in leucocytes. This deficiency of the innate immune system may result in the activation of as yet unidentified compensatory regulatory mechanisms yielding the hyperinflammatory phenotype of TRAPS.