Anti-neutrophil cytoplasmic antibodies (ANCAs) are a group of autoantibodies, mainly of the IgG type, against antigens in the cytoplasm of neutrophil granulocytes, and monocytes.
They are found in a number of autoimmune disorders, but are particularly associated with systemic vasculitis, so called ANCA-associated vasculitides (AAV).
ANCA-associated vasculitides (AAV) affects small and medium size blood vessels, and is characterized by sinonasal inflammation, pulmonary parenchymal involvement, pauci-immune glomerulonephritis, and the production of ANCA.
Immunofluorescence (IF) on ethanol-fixed neutrophils is used to detect ANCA.
Formalin-fixed neutrophils may be used to help differentiate ANCA patterns.
ANCA can be divided into four patterns when visualised by IF.
Cytoplasmic ANCA (c-ANCA),
C-ANCA (atypical),
perinuclear ANCA (p-ANCA)
Atypical ANCA (a-ANCA), also known as x-ANCA.
c-ANCA has cytoplasmic granular fluorescence with central interlobular accentuation.
C-ANCA (atypical) shows cytoplasmic staining that is usually uniform and has no interlobular accentuation
p-ANCA has three subtypes, classical p-ANCA, p-ANCA without nuclear extension and granulocyte specific-antinuclear antibody (GS-ANA).
Classical p-ANCA shows perinuclear staining with nuclear extension,
p-ANCA without nuclear extension has perinuclear staining without nuclear extension
Granulocyte specific-antinuclear antibody (GS-ANA). -ANA shows nuclear staining on granulocytes only.
a-ANCA often shows combinations of both cytoplasmic and perinuclear staining.
The c-ANCA antigen is specifically proteinase 3 (PR3).
p-ANCA antigens include myeloperoxidase and bacterial permeability increasing factor
protein (BPI).
Classical p-ANCA occurs with antibodies directed to myeloperoxidase.
p-ANCA without nuclear extension occurs with antibodies to permeability increasing factor protein, cathepsin G, elastase, lactoferrin and lysozyme.
Granulocyte specific-antinuclear antibody (GS-ANA) are antibodies directed to granulocyte specific nuclear antigens.
Atypical ANCA are thought to be antigens similar to that of the p-ANCAs, however may occur due to differences in neutrophil processing.
Enzyme-linked immunosorbent assay (ELISA) is used in diagnostic laboratories to detect ANCAs.
Immunofluorescence can be used to screen for many ANCAs.
ELISA is used to detect antibodies to individual antigens.
Microbial superantigens are molecules expressed by bacteria and other microorganisms that have the power to stimulate a strong immune response by activation of T-cells.
These molecules generally have regions that resemble self-antigens that promote a residual autoimmune response and this is the theory of molecular mimicry.
Staphylococcal and streptococcal superantigens have been characterized in autoimmune diseases, with similar proteins to body tissues.
The classical example in post group A streptococcal rheumatic heart disease, where there is similarity between M proteins of Streptococcus pyogenes to cardiac myosin and laminin.
Up to 70% of patients with granulomatosis with polyangiitis are chronic nasal carriers of Staphylococcus aureus, with carriers having an eight times increased risk of relapse: considered a type II hypersensitivity reaction.
ANCAs are associated with small vessel vasculitides including granulomatosis with polyangiitis, microscopic polyangiitis, primary pauci-immune necrotizing crescentic glomerulonephritis, eosinophilic granulomatosis with polyangiitis and drug induced vasculitides.
PR3 directed c-ANCA is present in 80-90% of granulomatosis with polyangiitis, 20-40% of microscopic polyangiitis, 20-40% of pauci-immune crescentic glomerulonephritis and 35% of eosinophilic granulomatosis with polyangiitis.
c-ANCA (atypical) is present in 80% of cystic fibrosis and also in inflammatory bowel disease, primary sclerosing cholangitis and rheumatoid arthritis
p-ANCA with myeloperoxidase specificity is found in 50% of microscopic polyangiitis, 50% of primary pauci-immune necrotizing crescentic glomerulonephritis and 35% of eosinophilic granulomatosis with polyangiitis.
p-ANCA with specificity to other antigens are associated with inflammatory bowel disease, rheumatoid arthritis, drug-induced vasculitis, autoimmune liver disease, drug induced syndromes and parasitic infections.
Atypical ANCA is associated with drug-induced systemic vasculitis, inflammatory bowel disease and rheumatoid arthritis.
The ANCA‐positive rate is much higher in patients with type 1 diabetes mellitus than in healthy individuals.
Levamisole, can cause an ANCA positive vasculitis.
The association of ANCA and disease activity is controversial: the reappearance of ANCA after treatment can indicate a relapse.
Pathogenesis is suggested by antibodies that have a direct role in the formation of small vessel vasculitides.
Myeloperoxidase (MPO) and proteinase 3 (PR3) specific ANCA can activate neutrophils and monocytes through their Fc and Fab’2 receptors, which can be enhanced by cytokines which cause neutrophils to display MPO and PR3 on their surface.
Activated neutrophils can then adhere to endothelial cells where degranulation occurs, releasing free oxygen radicals and lytic enzymes, resulting in damage to the endothelium via the induction of necrosis and apoptosis.
Neutrophils release chemoattractive signalling molecules that recruit more neutrophils to the endothelium, acting as a positive feedback loop.
ANCA titres have limited correlation with disease activity, except for kidney disease, and with risk of relapse, this is explained by differences in the epitopes and affinity of ANCAs.
ANCAs induce excess activation of neutrophils.
ANCA activation of neutrophils
result in the production of neutrophil extracellular traps (NETs), which cause damage to small blood vessels.
Treatment with an anti-CD20 antibody which removes circulating B-cells, clinical remission correlates more to the decreasing number of circulating B-cells than decrease in ANCA titer.
Clinical relapse in some patients is associated with the return of circulating B-cells.