ANCA-positive vasculitides are a not uncommon cause of rapidly progressive glomerulonephritis. When we send off an ANCA test, what are we actually measuring?
ANCAs (anti-neutrophil cytoplasmic antibodies) are a type of IgG autoantibodies that are directed against antigens found within neutrophils. The test was developed by ethanol-fixing neutrophils and then adding patient serum to the cells. A secondary antibody with an immunofluorescent tag can be used detect the presence of ANCA, which can take one of two primary patterns: c-ANCA (demonstrating cytoplasmic staining) and p-ANCA (demonstrating peri-nuclear staining). It is important to note that these patterns are largely artifactual in the sense that the antigens detected by p-ANCA are also cytoplasmic, but with EtOH fixation the protein precipitates in the perinuclear pattern noted in the figures shown.
p-ANCA antibodies are usually directed against the antigen myeloperoxidase (MPO) although in some instances may be directed instead against lactoferrin, elastase, or cathepsin G, while c-ANCA antibodies are usually directly against the antigen PR3 (proteinase 3).
The most convincing evidence that ANCA antibodies play a pathogenic role in the development of disease is the finding that murine MPO knockout mice, when immunized with murine MPO, can generate anti-MPO antibodies which are able to induce a necrotizing, crescentic glomerulonephritis when injected into Rag2 knockout mice (which lack functioning T & B lymphocytes).