Anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) constitute a group of vasculitides characterized by neutrophil-rich necrotizing inflammation of small vessels and the presence of ANCA in the circulation. Dying neutrophils surrounding the walls of small vessels are a histological hallmark of AAV. Traditionally it has been assumed that these neutrophils die by necrosis, but neutrophil extracellular traps (NETs) have recently been visualized at the sites of vasculitic lesions. NETs were first described to be involved in capture and elimination of pathogens but dysregulated production and/or clearance of NETs are thought to contribute to vessel inflammation in AAV; directly by damaging endothelial cells and indirectly by acting as a link between the innate and adaptive immune system through the generation of pathogenic PR3-ANCA and MPO-ANCA that can activate neutrophils. ANCA can, however, be found in all individuals and are therefore suggested to belong to the repertoire of natural antibodies produced by innate-like B cells, implying that not all ANCA are pathogenic. In paper I, we found neutrophils in patients to be more prone to undergo NETosis/necrosis spontaneously compared with neutrophils in healthy controls (HC), as well as that active patients possessed elevated levels of NETs in the circulation. Our results also suggest that ANCA during remission could contribute to the clearance of NETs as we observed an inverse relation between ANCA and NETs. In paper II, we observed neutrophils in patients to be more easily activated upon ANCA stimulation as they produced more ROS than neutrophils in HC. In paper III, we showed for the first time that cells of adaptive immunity (B and T cells) in addition to cells of innate immunity can release ET-like structures, in this case consisting of mitochondrial (mt) DNA. mtDNA can act as a damage-associated pattern molecule (DAMP) and promote inflammation, and increased levels of mtDNA has been observed in AAV. Our finding broadens our perspective of the possible roles of T and B cells in immunological responses, and should be further investigated in AAV. In paper IV, we observed reduced frequencies of MZ-like B cells, considered to be innate-like B cells that produce natural antibodies, and of the proposed regulatory B (Breg) cell populations CD24highCD27+ and CD25+CD27+ B cells in patients, particularly in those with active disease. We also observed the phenotypes of these different Breg cell populations to be different from the corresponding cells in HC. We hypothesize that the increased activation potential by neutrophils in AAV to produce ROS and undergo NETosis/necrosis contribute to the excessive inflammation as well as an increased antigen load of PR3 and MPO, and that this in combination with dysregulation of innate-like B cells and Breg cells could lead to break of tolerance to these antigens and production of pathogenic autoantibodies. ANCA can in turn activate neutrophils to release NETs, suggesting a vicious circle in disease development.
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