Objectives. To gain insight into the immune pathogenesis of primary ANCA-associated vasculitides, the prevalence of circulating T lymphocytes expressing CD11b as a marker for activation was analysed in patients with WG or microscopic polyangiitis. Methods. Receptor expression and IFN synthesis were measured in T cells of patients with active disease by cytofluorometry and compared with expression in patients in remission and in healthy donors. Results. During active disease, a small but

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... ous population of CD8þCD28þCD11bþ was found which produced IFN. In healthy donors and in patients in remission or undergoing immunosuppressive therapy, CD11b was exclusively associated with CD8þCD28À cells, the latter being more frequent in patients with long-lasting or severe disease. In vitro experiments confirmed that CD11b is up-regulated when T cells are activated. After multiple rounds of restimulation, the CD11b expression persists whereas CD28 expression is lost, compatible with the notion that CD8þCD28þCD11bþ represents a transient phenotype in the course of T-cell activation. The IFN-producing T cells activated polymorphonuclear neutrophils (PMN) to express MHC class II, thus generating the same PMN phenotype as in patients with active ANCA-associated vasculitis. A similar PMN phenotype could be generated by cultivation with supernatants of activated T cells or by IFN alone, but not by antibodies to proteinase 3. Conclusions. In active primary vasculitis, a small population of CD8þ T cells, identified by the expression of CD11b, expands, producing IFN. These T cells could activate PMN, thus generating a long-living and potentially destructive PMN phenotype.