Autoantibodies in Connective Tissue Diseases — ANA Patterns and Antibody-Disease Pairs (SLE, MCTD, Scleroderma, Myositis, Sjögren)

Antinuclear antibodies (ANAs) bind to molecules inside the cell nucleus — nucleic acids and nuclear proteins. They are one of the few specific markers for systemic autoimmune connective tissue disease, so an ANA is the usual first step. The most common method is indirect immunofluorescence (IIF) on fixed HEp-2 cells; the American College of Rheumatology affirmed HEp-2 IIF as the gold standard in 2010. ELISA and multiplex assays are used to confirm specific antibodies.

The test is sensitive but not specific. ANA is positive in more than 97% of SLE but its specificity is only about 20%, because it is also found in other autoimmune diseases, infections, malignancy and healthy people — more than 20% of healthy people (especially women and relatives of patients) can be positive, although titres above 1:320 are uncommon. A negative ANA is expected in some inflammatory conditions such as ankylosing spondylitis. Interpret it with the clinical picture, the titre and the pattern.

On HEp-2 IIF the pattern of nuclear fluorescence points toward the target antigen. The pattern is a clue, not a diagnosis.

More than 150 autoantibodies have been described in SLE, but only a few are used clinically. ANA is the screening test; the specific antibodies refine diagnosis and predict organ involvement. Immunological criteria for SLE include abnormal ANA titre (in the absence of drugs) and the presence of anti-dsDNA or anti-Sm antibodies.

• Anti-dsDNA assays: the Farr radioimmunoassay is the gold standard (rarely used); ELISA carries a high false-positive risk; the *Crithidia luciliae* immunofluorescence test confirms. Anti-ssDNA is non-specific and testing is not recommended.
• The Smith antigen is a protein within small nuclear ribonucleoprotein (snRNP) particles; the U1-RNP complex converts pre-messenger RNA to mature RNA.
• Before pregnancy, women with SLE should have antiphospholipid and anti-SSA/SSB antibodies checked, because positive results change management.

Mixed connective tissue disease (MCTD) is a rare autoimmune disease defined by a high titre of anti-U1-ribonucleoprotein (anti-U1 RNP) antibodies plus Raynaud phenomenon and overlapping features of at least two of: SLE, systemic sclerosis, polymyositis/dermatomyositis and rheumatoid arthritis. It was first described by Sharp in 1972. The 70 kDa component of the U1-RNP complex (with the A and C proteins) is the main antibody target.

ANA is positive in more than 90% of systemic sclerosis, and 60–70% also have a more specific antibody. These specific antibodies are mutually exclusive, may precede clinical disease by months to years, and predict the phenotype. Localised scleroderma (morphea) lacks them.

• Limited cutaneous systemic sclerosis (formerly CREST: calcinosis, Raynaud, oesophageal dysmotility, sclerodactyly, telangiectasia) has skin thickening distal to the elbows and knees; diffuse disease extends proximally and to the trunk.
• Raynaud phenomenon is present in more than 95% of patients; scleroderma renal crisis occurs in about 10%, mainly in diffuse disease, and ACE inhibitors changed its outcome.
• In the classification criteria quoted by StatPearls, positive anti-centromere, anti-Scl-70 or anti-RNA polymerase III earns points toward the diagnosis.

ANA is positive in most patients with dermatomyositis but does not support the diagnosis. Myositis-specific autoantibodies (MSAs) are present in about 60–80% of dermatomyositis and predict the pattern of organ involvement, prognosis and cancer risk. The overall relative risk of malignancy in dermatomyositis is about 4.7, highest in the first year after diagnosis.

Sjögren disease is a systemic autoimmune disease with sicca symptoms (dry eyes, dry mouth) from immune-mediated inflammation of the lacrimal and salivary glands. B cells produce rheumatoid factor, anti-SSA/Ro and anti-SSB/La, and their expansion explains the raised risk of MALT B-cell lymphoma. Anti-Ro and anti-La are present in up to about 90% of cases and are weighted at 3 points in the classification criteria; the diagnosis also uses ocular staining scores and a lip biopsy with a focus score. Anti-Ro/La also occur in SLE.

• ANA is the screening test (IIF on HEp-2 is gold standard); high sensitivity (>97% in SLE), low specificity (~20%).
• Anti-dsDNA (>95% specific, 40–80% sensitive) and anti-Sm (~99% specific, 10–55% sensitive) are SLE-specific; anti-dsDNA reflects activity and nephritis.
• Anti-U1 RNP = MCTD (high titre, speckled ANA, Raynaud, pulmonary hypertension).
• Anticentromere = limited scleroderma; Scl-70 = diffuse scleroderma with ILD; RNA polymerase III = renal crisis.
• Jo-1 = antisynthetase syndrome; Mi-2 = classic dermatomyositis with a good prognosis; TIF1-γ/NXP2 = cancer.
• Anti-Ro/SSA, anti-La/SSB = Sjögren; neonatal lupus and congenital heart block in an SLE or Sjögren mother.
• Anti-ribosomal P = lupus psychosis (neuropsychiatric lupus), but present in fewer than 5%.