Anti-Neutrophil Cytoplasmic Antibodies (ANCA) are autoantibodies that target proteins within the cytoplasm of neutrophils. They are traditionally identified using Indirect Immunofluorescence (IFA) testing, which reveals distinct staining patterns, primarily cytoplasmic ANCA (cANCA) and perinuclear ANCA (pANCA). Atypical pANCA is a variant of the perinuclear staining pattern that does not match the profile of classic pANCA. This variant presents a diagnostic challenge and is linked to various non-vasculitic inflammatory conditions.
The Immune Basis of Atypical pANCA
Classic pANCA targets the enzyme Myeloperoxidase (MPO) and is associated with vasculitis, an inflammatory disease of the blood vessels. Atypical pANCA produces the same perinuclear staining pattern on IFA but does not recognize MPO. Instead, these antibodies target various non-MPO antigens stored in neutrophil granules.
Non-MPO Antigens
These targets include proteins such as Lactoferrin, Cathepsin G, and Elastase. The autoantibodies may also be directed against Chromatin or Bactericidal/Permeability-Increasing protein (BPI).
The IFA Artifact
The perinuclear pattern seen on IFA is an artifact of the laboratory process. ANCA testing uses ethanol fixation, which causes the contents of neutrophil granules to migrate toward the nucleus, resulting in rim-like fluorescence. Non-MPO antigens targeted by atypical pANCA are often more soluble or susceptible to denaturation by ethanol. This allows them to leak out of the granules and concentrate near the nucleus during fixation. This relocation generates fluorescence visually identical to MPO-ANCA, despite binding to different proteins. The difference in target antigen dictates the clinical association.
Identifying Atypical pANCA in the Laboratory
Identifying Atypical pANCA requires a two-step approach combining visual screening and antigen-specific confirmation. The initial step is Indirect Immunofluorescence (IFA), which identifies the pANCA staining pattern on neutrophils. The second step involves antigen-specific immunoassays, typically Enzyme-Linked Immunosorbent Assays (ELISA), to confirm antibodies against the classic targets: Myeloperoxidase (MPO) and Proteinase 3 (PR3).
A result is classified as Atypical pANCA when the IFA shows a pANCA pattern, but the confirmatory ELISA tests for both MPO and PR3 are negative. This discrepancy indicates the autoantibody is binding to a non-MPO/non-PR3 antigen. Relying solely on the IFA pattern would inaccurately suggest classic pANCA and potentially mislead clinical management toward vasculitis. The negative MPO/PR3 result is the definitive laboratory criterion that reclassifies the finding as atypical. Laboratories may conduct further testing for specific atypical antigens like Lactoferrin or Cathepsin G.
Clinical Significance and Associated Conditions
Atypical pANCA rarely associates with the small-vessel vasculitis linked to classic MPO-ANCA. Instead, this antibody profile serves as a serological marker for chronic inflammatory and autoimmune disorders, especially those affecting the gastrointestinal tract and liver. Its detection prompts clinical investigations focused on non-vasculitic diseases.
Atypical pANCA is strongly associated with Inflammatory Bowel Disease (IBD), particularly Ulcerative Colitis (UC). Studies show that 37% to 83% of UC patients may test positive for Atypical pANCA. This high frequency suggests the antibody plays a role in the disease process or is a reaction to chronic gut inflammation.
The finding helps differentiate UC from Crohn’s disease (CD), the other major form of IBD. Atypical pANCA is common in UC but found in only 10% to 25% of CD patients. While not a definitive diagnostic tool, a positive Atypical pANCA result supports a diagnosis of Ulcerative Colitis over Crohn’s disease.
Atypical pANCA is also frequently observed in patients with Primary Sclerosing Cholangitis (PSC), a chronic liver disease. Prevalence in PSC patients can be as high as 65% to 95%. This strong association suggests a shared immunological pathway between the gut and liver, known as the gut-liver axis. A substantial percentage of patients with Autoimmune Hepatitis (AIH) also detect Atypical pANCA. The presence of this antibody in UC, PSC, and AIH underscores its utility as a marker for chronic, non-vasculitic systemic inflammatory disorders.