Immunoglobulin A anti‐phospholipid antibodies in Swedish cases of systemic lupus erythematosus: associations with disease phenotypes, vascular events and damage accrual

Summary Immunoglobulin (Ig) G‐ and IgM‐class anti‐cardiolipin antibodies (aCL) and lupus anti‐coagulant (LA) are included in the 1997 update of the American College of Rheumatology (ACR‐97) systemic lupus erythematosus (SLE) criteria. Despite limited evidence, IgA‐aCL and IgA anti‐β2‐glycoprotein‐I (anti‐β2GPI) were included in the 2012 Systemic Lupus International Collaborating Clinics criteria. The present study aimed to evaluate IgG‐/IgA‐/IgM‐aCL and anti‐β2GPI occurrence in relation to disease phenotype, smoking habits, pharmacotherapy, anti‐phospholipid syndrome (APS) and organ damage among 526 Swedish SLE patients meeting ACR‐97. Patients with rheumatoid arthritis (n = 100), primary Sjögren’s syndrome (n = 50) and blood donors (n = 507) served as controls. Anti‐phospholipid antibodies (aPL) were analysed by fluoroenzyme‐immunoassays detecting aCL/anti‐β2GPI. Seventy‐six (14%) SLE cases fulfilled the Sydney APS‐criteria, and ≥ 1 aCL/anti‐β2GPI isotype (IgG/IgA/IgM) occurred in 138 SLE patients (26%). Forty‐five (9%) of the SLE cases had IgA‐aCL, 20 of whom (4%) lacked IgG‐/IgM‐aCL. Seventy‐four (14%) tested positive for IgA anti‐β2GPI, 34 (6%) being seronegative regarding IgG/IgM anti‐β2GPI. Six (1%) had APS manifestations but were seropositive regarding IgA‐aCL and/or IgA anti‐β2GPI in the absence of IgG/IgM‐aPL and LA. Positive LA and IgG‐aPL tests were associated with most APS‐related events and organ damage. Exclusive IgA anti‐β2GPI occurrence associated inversely with Caucasian ethnicity [odds ratio (OR) = 0·21, 95% confidence interval (CI) = 0·06–0·72) and photosensitivity (OR = 0·19, 95% CI = 0·05–0·72). Nephritis, smoking, LA‐positivity and statin/corticosteroid‐medication associated strongly with organ damage, whereas hydroxychloroquine‐medication was protective. In conclusion, IgA‐aPL is not rare in SLE (16%) and IgA‐aPL analysis may have additional value among SLE cases with suspected APS testing negative for other isotypes of aPL and LA. •IgA aPL is not rare in SLE, and may occur simultaneously or in the absence of other aPL isotypes. •IgA anti‐β2GPI was associated with non‐Caucasian ethnicity, and inversely with skin lupus. •IgA aPL analysis may be of value among SLE‐cases with suspected APS, testing negative for other aPLs/LA.