A new serological reaction in patients with polymyalgia rheumatica and/or giant cell (temporal) arteritis: deposition of complement C4 and C3 components on rat kidney structures detected by indirect immunofluorescence.

Using indirect immunofluorescence deposition of complement C4 and C3 components to rat kidney medullary structures was demonstrated. This serological reaction (C4/C3-IFT) is regularly obtained with fresh sera of patients suffering from active polymyalgia rheumatica (PMR) and/or giant cell (temporal) arteritis (GCA). Sera of normal controls and of steroid-treated PMR and/or GCA patients in clinical remission have a negative C4/C3-IFT reaction. A positive conversion of the test in steroid-treated patients indicates enhanced disease activity. Because of its technical simplicity, C4/C3-IFT can be routinely used first, as a reliable serological marker in the primary diagnosis of PMR and/or GCA and second, as a sensitive criterion of disease activity in these patients. C4/C3-IFT reactivity is, however, not specific for GCA and/or PMR. Various systemic inflammatory diseases may have a positive reaction as well (e.g., certain viral and bacterial infections, malignant tumors, vasculitides, inflammatory joint diseases of different etiology). Recent experimental findings suggest that C-reactive protein (CRP) in patients' sera mediates an activation of the classical complement pathway resulting in a deposition of C4 and C3 complement components to certain rat kidney structures.