(Study type: Blood collected in a red-top tube; related body system: Circulatory, digestive, immune, respiratory, urinary systems.)

Antineutrophil cytoplasmic autoantibodies (ANCA) have long been associated with vasculitis (inflammation of the blood vessels) and glomerulonephritis. Although progress has been made in understanding this association, the exact etiology of ANCA-associated vasculitis is unclear. Whatever the actual mechanism, there is a connection between ANCAs, activated neutrophils, the formation of neutrophil extracellular traps (NETs), and instigation of NETosis, a type of neutrophil cell death. The immune function of NETs is to entrap pathogens and other “foreign” targets. Neutrophil activation is involved in other diseases and is triggered by exposure to things other than ANCAs. NET research is an active area of study with demonstrated connections to the development of other conditions such as cancer, coagulation disorders, diabetes, rheumatoid arthritis, and systemic lupus erythematosus.

The most thoroughly studied theory suggests that ANCAs may activate neutrophils and also target cell surface proteins (myeoloperoxidase [MPO] and proteinase 3 [PR3]). Activated neutrophils produce NETs, fiberlike structures that are pushed out into circulation from activated neutrophils. The NETs become stuck to the walls of capillaries and other small vessels, a process that subsequently initiates the process of NETosis. When NETosis occurs, inflammatory fragments of nuclear material (e.g., DNA) and enzymes are released. The dying neutrophils break apart and release cytoplasmic components that include the granular inclusion proteins MPO and PR3. The proteins, which are associated with the neutrophils’ cell surfaces and cytoplasmic granules, become circulating targets, and the autoimmune response from ANCAs deepens. Other than the ANCA and specific PR3 blood studies, tissue biopsy is the definitive test with presence of granuloma being the positive finding.

ANCAs react with enzymes in the cytoplasmic granules of neutrophils and can be identified by their cellular staining characteristics when present. The most common types of cytoplasmic granules include PR3, MPO, cathepsin G, elastase, and lactoferrin. PR3-ANCA and MPO-ANCA are most associated with ANCA-associated vasculitides.

c-ANCA (cytoplasmic) is most often associated with the PR3 proteinase in neutrophils and monocytes and is found in the sera of patients with GPA, an autoimmune vasculitis that primarily affects the kidneys and lungs. Autoantibodies to PR3 develop in GPA. Antibodies to PR3-ANCA produce a characteristic pattern of granular cytoplasmic fluorescence called the cytoplasmic ANCA pattern. GPA includes granulomatous inflammation of the upper and lower respiratory tract and vasculitis.

p-ANCA (perinuclear) is most often associated with myeloperoxidase MPO and produces a perinuclear cytoplasmic staining pattern. Antibodies to MPO-ANCA occur predominately in patients with microscopic polyangiitis (MPA); p-ANCA is also present in the sera of patients with pauci-immune necrotizing glomerulonephritis.