Background: the role of the T helper 17 (Th17) cell subset in anti-neutrophil cytoplasm antibodies (ANCA) associated vasculitis (AAV) is controversial. We hypothesized that a specific Th17 response to myeloperoxidase (MPO) or proteinase 3 (PR3) is detectable in AAV patients and is different among the disease phases. Methods: we analyzed 43 AAV patients with renal involvement (21 acute and 22 remission patients), and 12 healthy controls. Peripheral blood mononuclear cells (PBMCs) were cultured with PR3/MPO over 48 h. Thereafter, frequencies of MPO/PR3-specific Th17 cells were assessed using an enzyme-linked immunosorbent spot (ELISpot) assay. Supernatant IL-17 concentration was quantified using ELISA. Finally, specific Th17 response after depletion of T regulatory lymphocytes (T-regs) in some remission patients was compared to the non T-reg-depleted response. Results: specific Th17 cell number was higher in acute patients compared to remission (p = 0.004). Specific Th17 cell number performed well in the disease activity detection (ROC curve area under the curve (AUC) = 0.87; p = 0.0001) with an optimal cut-off of 6 spots/million. Patients above this cut-off showed higher serum creatinine (p = 0.004), C-reactive protein (CRP) (p = 0.001) and ANCA titer (p = 0.032). Supernatant IL-17 concentration was higher in acute patients compared to remission (p = 0.035) and did not normalize to healthy control levels (p = 0.01). Conclusions: a specific Th17 cell response is present in AAV patients. This response is more pronounced in the acute phase, but persists in remission
