Eosinophils in anti-neutrophil cytoplasmic antibody associated vasculitis.

Background: Anti-neutrophil cytoplasmic antibodies associated vasculitides (AAV) are characterized by autoimmune small vessel inflammation. Eosinophils are multifunctional cells with both pro-inflammatory and immunoregulatory properties. Tissue activated eosinophils secrete cyto- and chemokines and form extracellular traps (EETs), they release free granules and produce reactive oxygen species. The role of eosinophils is well established in eosinophilic granulomatosis with polyangiitis (EGPA) but very little is known about their role in granulomatosis with polyangiitis (GPA) and microscopic polyangiitis (MPA).

Methods: The expression of surface markers CD11c, CD11b, CD16, CD35, CD62L, CD64, CD88, Siglec-8 and CD193 and reactive oxygen species production by peripheral blood eosinophils were studied using flow cytometry. Fluorescence microscopy was used to visualize the release of eosinophil extracellular DNA traps (EETs). 98 GPA and MPA patients and 121 healthy controls were included in the study.

Results: Both GPA and MPA patients had decreased frequency of eosinophils in peripheral blood compared with healthy controls ( < 0.0001), which could not solely be explained by corticosteroid treatment. The patient's eosinophils showed increased surface expression of the Fc receptors CD16 ( < 0.0001) and CD64 ( = 0.0035) as well as CCR3 (CD193) ( = 0.0022). Decreased expression was found of the complement receptors CD35 ( = 0.0022), CD88 (p < 0,0001) as well as CD11c (p < 0,0001), CD11b ( = 0.0061) and Siglec-8 (p = 0,0015). Moreover, GPA and MPA eosinophils, showed decreased capacity to produce ROS ( < 0.0001). ANCA stimulation of eosinophils from GPA and MPA patients after C5a priming enhanced EETosis (p = 0,0088).

Conclusions: The percentage of eosinophils were decreased in peripheral blood in GPA and MPA patients and showed altered surface marker expression and function. The enhanced EETosis after ANCA stimulation, suggests that eosinophil can contribute to the autoantibody driven inflammatory process.