Based on these observations, we hypothesized that the overabundance of functionally defective FoxP3+ T cells results from an expansion of effector-like cells that readily produce proinflammatory cytokines despite expression of FoxP3. Elucidating the functional and phenotypical characteristics of FoxP3+ T cells in GPA may form a next step in the search for novel monitoring tools and possible directed therapies. Here, FOXP3 is linked to granulomatosis with polyangiitis.