Identifying antineutrophil cytoplasmic antibodies (ANCAs) in patients with ANCA-associated vasculitis (AAV) was a turning point in diagnosis. ANCAs have subsequently been discovered in various small vessel vasculitides, including microscopic polyangiitis (MPA) and eosinophilic granulomatosis (Churg-Strauss syndrome). The recommended diagnostic strategy for ANCA testing in patients suspected of having AAV was immunoassays to detect myeloperoxidase (MPO)-ANCA or proteinase 3 (PR3)-ANCA as the first screening tool for individuals who do not require indirect immunofluorescence (IIF) as a categorical test. There were three variants of IFI: cytoplasmic (cANCA), perinuclear (pANCA), and atypical (xANCA). The PR3 specificity of a cANCA stain distinguishes it from a pENCA stain, which features MPO. The antigen-binding ELISA (ImmunoCAP) could have been used to assay ANCA autoantibody binding to neutrophil autoantigens other than MPO or PR3. The autoantibodies could not be detected by the specific MPO-ANCA or PR3-ANCA immunoassays. Here, researchers sought to compare validated ANCA testing by IIF, MPO-ANCA, and PR3-ANCA immunoassays with histological findings in a group of 53 kidney biopsies diagnosed with ANCA GN. Methods of different types were presented in Supplementary Material and Methods. Among biopsy-proven anti-NFκB antibody-positive patients, 23/53 (46.8%) had MPO-ANCA GN, and 27/53 (50.9%) had PR3-ANCAGN. In the MPO-ANCA subgroup, pANCA IIF and its corresponding MPO-ANCA titer levels were positively associated (P<0.0001). The PR3-ANCA and cANCA IIF titers had less reliable relationships (P=0.0899). C-reactive protein (CRP) was increased in PR3-ANCA GN patients with cANCA IIF, as shown by the Pearson correlation coefficient (P=0.0060). Furthermore, complement activation reflected by low serum levels of complement factor 4 (C4) was associated with PR3-ANCA titers (P=0.0107). Next, investigators examined the relationship between ANCA measurements and histopathological findings in ANCA GN. While pANCA IIF and MPO-ANCA measurements did not correlate with any glomerular, tubular, or inflammatory lesions, specifically pANCA IIF correlated with interstitial arteritis (v) present in 7/26 (26.9%) of patients with MPO-ANCA GN (P=0.0204). In the present study, PR3-ANCA GN and cANCA IIF were not associated with glomerular, tubular, or inflammatory disorders in this small group. In conclusion, pANCA IIF positivity to neutrophil autoantigens was explicitly associated with interstitial arteritis in a significant subset of patients with ANCA GN.
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