ANCA and Vasculitis Classification

Antineutrophil cytoplasmic antibodies (ANCA) are autoantibodies directed against constituents of neutrophil granules, principally proteinase 3 (PR3) and myeloperoxidase (MPO). They define the ANCA-associated vasculitides — granulomatosis with polyangiitis, microscopic polyangiitis, and eosinophilic granulomatosis with polyangiitis — and increasingly the field considers classifying these diseases by ANCA serotype (PR3-ANCA versus MPO-ANCA) alongside the traditional clinicopathologic syndromes.

Definition

ANCA are autoantibodies against neutrophil cytoplasmic antigens, chiefly proteinase 3 (PR3) and myeloperoxidase (MPO); their presence and specificity define and help classify the ANCA-associated vasculitides within contemporary nomenclature and classification criteria.

Scope

The entry covers what ANCA are, the two main antigen specificities and their immunofluorescence patterns (cytoplasmic c-ANCA, perinuclear p-ANCA), how ANCA enter the Chapel Hill nomenclature and the 2022 ACR/EULAR classification criteria, and the ongoing discussion of serotype-based versus syndrome-based classification. It is a conceptual reference topic, not clinical guidance, and is not itself a clinical entity.

Core questions

• What antigens do ANCA target, and how are PR3-ANCA and MPO-ANCA detected and distinguished?
• How do ANCA enter the definitions of the small vessel vasculitides?
• How do classification criteria incorporate ANCA serology and testing method?
• Should disease be classified by ANCA serotype or by clinicopathologic syndrome?

Key concepts

• Proteinase 3 (PR3-ANCA)
• Myeloperoxidase (MPO-ANCA)
• Cytoplasmic (c-ANCA) and perinuclear (p-ANCA) patterns
• Indirect immunofluorescence and antigen-specific immunoassay
• ANCA-associated vasculitis
• Serotype-based versus syndrome-based classification
• Classification criteria versus diagnostic testing

Mechanisms

ANCA are detected by indirect immunofluorescence, which yields a cytoplasmic (c-ANCA) or perinuclear (p-ANCA) staining pattern, and by antigen-specific immunoassays that identify PR3 or MPO reactivity; c-ANCA usually corresponds to PR3-ANCA and p-ANCA often to MPO-ANCA. Beyond serving as markers, ANCA are thought to participate in pathogenesis by binding antigens expressed on primed neutrophils, promoting their activation and injury to small-vessel endothelium. Because PR3-ANCA and MPO-ANCA disease differ in genetic associations, organ patterns, and relapse tendency, serotype is increasingly proposed as a classifying axis complementary to the named syndromes.

Clinical relevance

ANCA serology is central to how the small vessel vasculitides are classified and studied, and understanding antigen specificity and testing methods helps in appraising the vasculitis literature. This entry explains a classification and laboratory concept for reference; it characterises how disease is categorised and is not a basis for individual diagnostic or treatment decisions.

Epidemiology

Among the ANCA-associated vasculitides, PR3-ANCA and MPO-ANCA differ in distribution: PR3-ANCA disease is relatively more common in Northern European populations and MPO-ANCA disease in parts of Asia. These serotype patterns, rather than incidence of ANCA testing itself, are what the classification discussion turns on.

Evidence & guidelines

The 2012 Revised Chapel Hill Consensus Conference incorporated ANCA into the nomenclature of small vessel vasculitis, and the 2022 ACR/EULAR classification criteria for granulomatosis with polyangiitis and microscopic polyangiitis weight ANCA specificity heavily. EULAR management recommendations also frame their guidance around ANCA-associated disease. These documents define the classifying role of ANCA.

History

ANCA were first described in the 1980s in association with necrotising vasculitis and segmental glomerulonephritis, and the identification of PR3 and MPO as the main target antigens followed. This transformed vasculitis classification: the Chapel Hill nomenclature formalised the ANCA-associated category, and subsequent work raised the question of whether ANCA serotype should supplant or complement the traditional syndrome names.

Debates

Should ANCA-associated vasculitis be classified by serotype or by syndrome?

Evidence that PR3-ANCA and MPO-ANCA disease differ in genetics, organ involvement, and relapse risk has prompted proposals to classify by ANCA serotype rather than by the clinicopathologic syndromes alone, while criteria such as the 2022 ACR/EULAR system retain syndrome labels but weight ANCA specificity heavily.

Key figures

• J. Charles Jennette
• Ronald J. Falk
• Peter A. Merkel
• Joanna C. Robson

Seminal works

• jennette-2012
• robson-2022
• suppiah-2022

Frequently asked questions

What is the difference between PR3-ANCA and MPO-ANCA?

They are ANCA directed against different neutrophil antigens — proteinase 3 (PR3) and myeloperoxidase (MPO) — which tend to correspond to the cytoplasmic and perinuclear immunofluorescence patterns respectively and are associated with somewhat different clinical and genetic profiles.

Why is ANCA important for classifying vasculitis?

ANCA define the ANCA-associated category of small vessel vasculitis in the Chapel Hill nomenclature, and antibody specificity is heavily weighted in modern classification criteria, with growing interest in classifying disease by ANCA serotype.