According to the consensus introduced at the Chapel-Hill-Consensus conference and the generally renowned classification system, granulomatosis with polyangiitis (GPA, formerly: Wegener’s granulomatosis, WG), microscopic polyangiitis (MPA), eosinophilic granulomatosis with polyangiitis (EGPA, formerly: Churg-Strauss syndrome (CSS)) are classed as the group of ANCA-associated vasculitides (AAV). ANCA (antineutrophil cytoplasm antibodies) are autoantibodies directed against antigens found in cytoplasmic granules of neutrophils and monocytes. They are important serological markers for the diagnosis of AAV.
Moreover, an association with ANCA has been described for some of the immune complex vasculitides. Patients with anti-GBM glomerulonephritides (serological marker: antibodies against the glomerular basement membrane; anti-GBM) are often ANCA positive (>35%). A positive result can indicate rapid-progressive glomerulonephritis or GPA. In patients with AAV with renal involvement, the parallel analysis of ANCA and anti-GBM antibodies is thus recommended.
AAV diagnostics are primarily based on IIFT. Standard for IIFT is a BIOCHIP mosaic of ethanol (EOH)- and formaldehyde (HCHO)-fixed human granulocytes. Further EUROIMMUN-exclusive BIOCHIPs, e.g. HEp-2 cells with sedimented granulocytes, further increase the diagnostic certainty. The EUROPLUS technique allows the combination of conventional cell culture substrates with defined single antigens (PR3, MPO, GBM) on one test field. This considerably simplifies the interpretation of the immunofluorescence patterns.
IIFT allows the differentiation of two ANCA types: the cytoplasmic type (cANCA), which mostly is associated with GPA and is almost always directed against proteinase 3 (PR3), and the perinuclear type (pANCA), which indicates a spectrum of various diseases. The main target antigen of pANCA in MPA and EGP is myeloperoxidase (MPO), but antibodies against granulocyte elastase, lactoferrin, lysozyme, cathepsin G, beta-glucoronidase, azurocidin, h-lamp-2 and alpha-enolase are also found in connection with pANCA.
Positive IIFT results should always be confirmed with a monospecific anti-PR3 and anti-MPO test (e.g. ChLIA and ELISA) (International Consensus Statement, Savige et al., Am J Clin Pathol, 1999 & 2003). Since not all cANCA and pANCA are positive in the ELISA, the highest sensitivity and specificity for ANCA detection can only be achieved with parallel performance of IIFT and ChLIA/ ELISA.
The sCD163 ELISA is therefore especially suited for targeted monitoring of patients with diagnosed AAV with renal involvement. As opposed to systemic inflammation markers, the sCD163 level is a specific indicator for renal inflammation and reacts promptly under treatment or when the inflammatory activity deviates.
pANCA are also of great relevance in the differentiation of chronic inflammatory bowel diseases (67% ulcerative colitis, 7% Crohn’s disease). DNA-bound lactoferrin has been identified as the main target antigen (Teegen et al., Ann N Y Acad Sci, 2009).