Objective: Antibodies to ß(2) glycoprotein I (IgG and IgM isotypes) have recently been added to the laboratory criteria of the revised antiphospholipid syndrome classification criteria. We investigated whether IgM anti-ß(2)-glycoprotein I (anti-ß(2)-GPI) is associated with clinical manifestations of systemic lupus erythematosus (SLE).
Methods: Anti-ß(2)-GPI was measured in 796 patients with SLE (93% women, 53% white, 38% African American, mean age 45 yrs). IgM anti-ß(2)-GPI (> 20 phospholipid units) was found in 16%. Associations were determined with clinical manifestations of SLE and with components of the Systemic Lupus International Collaborating Clinics/American College of Rheumatology Damage Index.
Results: As expected, IgM anti-ß(2)-GPI was highly associated with both the lupus anticoagulant and with anticardiolipin. It was associated with transient ischemic attack (OR 2.64, p = 0.04), but not significantly with venous or arterial thrombosis. IgM anti-ß(2)-GPI was protective against lupus nephritis (OR 0.54, p = 0.049), renal damage (p = 0.019), and hypertension (OR 0.58, p = 0.008). This protective effect remained after adjustment for ethnicity.
Conclusion: In SLE, IgM anti-ß(2)-GPI is not associated with thrombosis but is protective against lupus nephritis and renal damage. "Natural" autoantibodies of the IgM isotype may have a protective effect.