Correlation of interleukin‐6 production and disease activity in polymyalgia rheumatica and giant cell arteritis

Objective. To explore the role of proinflammatory cytokines in giant cell arteritis (GCA) and polymyalgia rheumatica (PMR), two clinically related syndromes characterized by an intense acute‐phase reaction. In particular, to determine plasma concentrations of interleukin‐6 (IL‐6) and tumor necrosis factor α (TNFα) and to correlate changes in plasma IL‐6 levels with clinical symptoms during corticosteroid therapy. Methods. IL‐6 and TNFα concentrations were determined in plasma samples from patients with untreated PMR or GCA, and plasma IL‐6 levels were monitored in patients receiving long‐term therapy (14 months) with corticosteroids. To identify IL‐6–producing cells, the polymerase chain reaction was used to detect IL‐6 messenger RNA. In vitro production of IL‐6 and IL‐2 by peripheral blood mononuclear cells (PBMC) from treated and untreated patients was quantified using IL‐6– and IL‐2–specific bioassay systems. Results. IL‐6 concentrations were increased in PMR and GCA patients, whereas TNFα concentrations were similar to those in normal donors. Administration of corticosteroids rapidly reduced the levels of circulating IL‐6 but did not correct the underlying mechanism inducing the increased IL‐6 production. In individual patients, changes in plasma IL‐6 levels and clinical manifestations during prolonged therapy were closely correlated. Short‐term withdrawal of corticosteroids, even after several months of treatment, was followed by an immediate increase in plasma IL‐6 concentrations. To identify the cellular source of plasma IL‐6, PBMC from treated and untreated patients with PMR or GCA were analyzed for their ability to secrete IL‐6 and the T cell–specific cytokine IL‐2. Polyclonal T cell stimulation caused a rapid release of IL‐6, which was shown to be derived exclusively from CD14+ cells. Conclusion. Increased production of IL‐6, but not TNFα, is a characteristic finding in patients with PMR or GCA. Corticosteroids rapidly suppress IL‐6 production but do not correct the underlying mechanism inducing the increased IL‐6 production. The close correlation of plasma IL‐6 concentrations with clinical symptoms suggests a direct contribution of this cytokine to the disease manifestations and presents the possibility that monitoring IL‐6 levels would be useful in making decisions on adjustment of corticosteroid dosage in individual patients.