IgA is the predominant isotype of anti‐β2 glycoprotein I antibodies in rheumatoid arthritis

BACKGROUND: The aim of this study was to determine the frequency of anti‐cardiolipin antibodies (aCL) and anti‐β2 glycoprotein I antibodies (aβ2GPI) among Tunisian patients with rheumatoid arthritis (RA). METHODS: Ninety RA patients with positive anti‐cyclic citrullinated antibodies (anti‐CCP) and 90 healthy blood donors (HBD) were studied. aCL and aβ2GPI of isotype IgG, IgA and IgM were detected by ELISA. RESULT: The frequency of antiphopholipid antibodies (aPL) (aCL and/or aβ2GPI) was significantly higher in patients with RA than in HBD (35.5% vs 11.1%, P = .0001). The frequencies of aCL and aβ2GPI were significantly higher in patients than in healthy subjects (15.5% vs 5.5%, P = .04 and 32.2% vs 11.1%, P = .0005 respectively). aβ2GPI‐IgA were significantly more frequent in patients than in the control group (26.7% vs 7.8%, P = .0007). In patients, aβ2GPI‐IgA were significantly more frequent than aβ2GPI‐IgG (26.7% vs. 6.7%, P = .0003) and aβ2GPI‐IgM (26.7% vs 5.6%, P = .0001). In RA patients, the frequency of aβ2GPI was significantly higher than that of aCL (32.2% vs 15.5%, P = .008). aβ2GPI‐IgA was significantly more frequent than aCL‐IgA (26.7% vs 4.4%, P = .00005). The average titer of anti‐CCP in aPL positive patients was significantly higher than in aPL negative patients (170.6 ± 50 RU/mL vs 147.7 ± 51 RU/mL, P = .04). Significant correlation was found between aβ2GPI‐IgA and anti‐CCP (r = .235, P = .026). CONCLUSIONS: aPL and particularly aβ2GPI‐IgA are frequent in RA and are correlated with anti‐CCP.