ANCA-Associated Vasculitis: How Neutrophils Attack the Vessel Wall

What It Is and Why It Matters Clinically

ANCA-associated vasculitis is small-vessel inflammation of arterioles, capillaries, and venules driven by antibodies against neutrophil granule enzymes. It is defined as pauci-immune because, unlike immune-complex vasculitides or lupus nephritis, biopsies show little or no immunoglobulin/complement deposition — the damage is cell-mediated, not deposit-mediated.

It matters because AAV is one of the great mimics and one of the true rheumatologic emergencies. Before effective immunosuppression, one-year mortality from generalized GPA approached 80%; today, with treatment, 5-year survival exceeds 75%, but only if it is recognized early.

• Multisystem and fast: a patient can progress from sinusitis and malaise to dialysis-dependent kidney failure or life-threatening lung hemorrhage in weeks.
• Common cause of RPGN: pauci-immune crescentic glomerulonephritis is the single most frequent cause of rapidly progressive glomerulonephritis in adults.
• Relapsing: it is a chronic, relapsing disease — PR3-ANCA/GPA relapse more often than MPO-ANCA/MPA.

The Mechanism: How Neutrophils Attack the Vessel Wall

AAV is a two-hit process centered on the neutrophil:

• Priming (first hit): infection or inflammation releases cytokines (TNF-α, C5a, IL-1) that prime neutrophils, translocating PR3 and MPO from cytoplasmic azurophilic granules onto the cell surface, where they become accessible to antibody.
• ANCA binding (second hit): circulating anti-PR3 or anti-MPO IgG engages these surface antigens by their Fab arms while the Fc tail cross-links neutrophil FcγRIIa/FcγRIIIb receptors — dual ligation that fully activates the cell.
• Respiratory burst & degranulation: activated neutrophils undergo an oxidative burst (NADPH oxidase → reactive oxygen species) and release proteases and MPO directly onto the endothelium.
• NETosis: they extrude neutrophil extracellular traps — decondensed chromatin studded with PR3/MPO — which are both cytotoxic and a source of fresh autoantigen, propagating the response.
• Complement amplification: the alternative complement pathway generates C5a, which primes still more neutrophils via the C5a receptor (C5aR/CD88) — the loop that avacopan blocks.

The net effect is fibrinoid necrosis of the vessel wall and, in the glomerulus, capillary rupture and crescent formation.

Clinical Presentation and Classic Signs

AAV usually opens with weeks of nonspecific constitutional symptoms — fever, weight loss, arthralgias, malaise — before organ-specific disease declares itself. The pulmonary–renal syndrome (alveolar hemorrhage plus glomerulonephritis) is the classic dangerous presentation.

• Kidney: hematuria, red-cell casts, proteinuria, and rising creatinine — rapidly progressive glomerulonephritis. Often asymptomatic until advanced.
• Upper airway (GPA): chronic sinusitis, nasal crusting/epistaxis, saddle-nose deformity from cartilage destruction, otitis, and subglottic stenosis.
• Lung: nodules or cavities (GPA), fleeting infiltrates, and diffuse alveolar hemorrhage — hemoptysis, dropping hemoglobin, hypoxemia.
• Nerve: mononeuritis multiplex (e.g., wrist/foot drop) — a hallmark, especially in EGPA.
• Eye/skin: scleritis, orbital pseudotumor, and palpable purpura.
• EGPA sequence: late-onset asthma → blood/tissue eosinophilia → vasculitis, with cardiomyopathy as a leading cause of death.

Diagnosis: Tests, Antibodies, and Biopsy

Diagnosis rests on the triad of serology, biopsy, and clinical pattern. The 2022 ACR/EULAR classification criteria (weighted scores; GPA/MPA ≥5, EGPA ≥6) are for research classification — you must first exclude mimics.

• ANCA serology: screen by indirect immunofluorescence, confirm with antigen-specific ELISA. c-ANCA → PR3-ANCA (mostly GPA); p-ANCA → MPO-ANCA (mostly MPA). PR3-ANCA ELISA specificity for GPA exceeds 90%. Note EGPA is ANCA-positive in only ~40%.
• Renal biopsy (gold standard): focal necrotizing, pauci-immune crescentic glomerulonephritis — crescents with scant immunofluorescence staining.
• Labs: elevated creatinine, active urinary sediment (dysmorphic RBCs, RBC casts), high ESR/CRP, normocytic anemia; eosinophilia >10% or >1500/µL points to EGPA.
• Imaging: chest CT for nodules, cavities, or ground-glass hemorrhage; sinus imaging for GPA.

Do-not-miss: a positive p-ANCA is nonspecific (seen in IBD, drug-induced disease, endocarditis) — always confirm the antigen target.

Management at a Mechanism Level

Treatment is split into remission induction and maintenance, each targeting a step in the neutrophil cascade.

• Rituximab (anti-CD20 monoclonal) depletes B cells, cutting off production of ANCA-generating plasmablasts. In the RAVE trial it was non-inferior to cyclophosphamide and superior for relapsing PR3-ANCA disease; it is now a preferred first-line induction agent.
• Cyclophosphamide — an alkylating agent that broadly suppresses lymphocyte proliferation — remains an alternative for severe or refractory disease.
• Glucocorticoids blunt neutrophil priming and cytokine signaling. Per the PEXIVAS trial, a reduced-dose steroid taper is non-inferior for death/ESKD and causes fewer serious infections.
• Avacopan is an oral C5a-receptor (C5aR/CD88) antagonist that severs the complement-driven neutrophil amplification loop; the ADVOCATE trial showed it sustains remission while allowing near-total steroid withdrawal, and it is now in EULAR-2022/KDIGO-2024 guidelines.
• Plasma exchange removes circulating ANCA; per PEXIVAS it does not reduce death/ESKD overall but may aid early kidney recovery in severe renal disease or alveolar hemorrhage.
• Maintenance: low-dose rituximab, azathioprine, or methotrexate — with PJP prophylaxis.

Distinctions from Mimics, Pitfalls, and Significance

AAV must be separated from its dangerous look-alikes because the treatments diverge sharply.

• Anti-GBM (Goodpasture) disease: also pulmonary–renal, but biopsy shows linear IgG along the GBM (not pauci-immune); ~20–30% are double-positive for anti-GBM and MPO-ANCA — check both.
• Immune-complex vasculitis / lupus nephritis: bright granular immunofluorescence and low complement (C3/C4) — opposite of pauci-immune AAV.
• Infective endocarditis: can produce low-titer ANCA and glomerulonephritis — blood cultures and echo before immunosuppressing.
• Drug-induced ANCA: hydralazine, propylthiouracil, levamisole-adulterated cocaine, and minocycline cause high-titer MPO-ANCA; withdrawal of the drug is central.
• Cocaine-induced midline destruction: mimics GPA with a saddle nose but shows anti-human neutrophil elastase (HNE) antibodies.

Significance: AAV illustrates how autoantibodies against intracellular enzymes can weaponize the innate immune system — and it is now a template for mechanism-targeted therapy (complement and B-cell blockade replacing blunt cytotoxics).

The three ANCA-associated vasculitides: distinguishing features

Feature	GPA (Granulomatosis w/ Polyangiitis)	MPA (Microscopic Polyangiitis)	EGPA (Eosinophilic GPA)
Dominant ANCA	PR3-ANCA (c-ANCA), ~80–90%	MPO-ANCA (p-ANCA), ~60–70%	MPO-ANCA in ~40% (ANCA-negative common)
Signature pathology	Necrotizing granulomas + vasculitis	Necrotizing vasculitis, NO granulomas	Eosinophil-rich granulomas + vasculitis
Classic organ pattern	Upper airway (saddle nose, sinusitis), lung nodules, RPGN	Kidney (RPGN) + alveolar hemorrhage; no ENT granuloma	Asthma, eosinophilia >10%, sinusitis, neuropathy, cardiac
Distinctive clue	Subglottic stenosis, orbital pseudotumor	Pauci-immune GN with minimal ENT disease	Late-onset asthma; mononeuritis multiplex; peripheral eosinophilia
ACR/EULAR 2022 score threshold	≥5	≥5	≥6