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Antiphospholipid antibodies and RA: presence of β2GP1 independent aCL
  1. C BONNET,
  2. P VERGNE,
  3. P BERTIN,
  4. R TREVES
  1. Department of Rheumatology
  2. University of Limoges, France
  3. Department of Immunology
  4. University of Limoges, France
  1. Dr C Bonnet, Service de Rhumatologie, CHU Dupuytren, 2 Avenue Martin Luther-King, 87042 Limoges- France
  1. M-O JAUBERTEAU
  1. Department of Rheumatology
  2. University of Limoges, France
  3. Department of Immunology
  4. University of Limoges, France
  1. Dr C Bonnet, Service de Rhumatologie, CHU Dupuytren, 2 Avenue Martin Luther-King, 87042 Limoges- France

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Anticardiolipin antibodies (aCL) are found in many conditions, such as lupus erythematosus, but also in other connective tissue diseases like rheumatoid arthritis (RA). To determine the prevalence and significance of aCL in RA, we evaluated the frequency of anticardiolipin and anti-β2 glycoprotein 1 (β2GP1) antibodies in patients with RA.

We studied serum samples from 50 consecutive patients (36 women, 14 men) with RA satisfying the 1987 American College of Rheumatology criteria for RA. Serum IgG and IgM aCL were characterised by enzyme linked immunosorbent assay (ELISA)1 using microtitration plates (Immunosorb, Nunc, Roksilde, Denmark) coated with cardiolipin purified from bovine heart (Sigma, St Louis, MO). Wells were saturated with 1% bovine serum albumin (BSA; Diamed, Cressier/Morat, Switzerland) in phosphate buffered saline solution (PBS). Serum samples diluted 1/100 in PBS-BSA were incubated for one hour at 37°C. The blocking and sample diluent buffer did not contain β2GP1 and differed from those using fetal calf serum, which are considered to add exogenous β2GP1. aCL levels were expressed in IgM and IgG units (U), calculated by including serum samples calibrated with Harris's standards on every plate.2 A search was made for IgG and IgM β2GP1 antibodies by an ELISA using human β2GP1 antigen coated on irradiated plates, according to Arvieux et al. 3 aCL and β2GP1 antibody levels were considered to be positive when greater or equal to 20 U. Rheumatoid factors (RF) (detected by nephelometry) and antinuclear antibodies (ANA) (detected by indirect immunofluorescence (IIF)) were determined for each patient. Additionally, antikeratin antibodies (detected by IIF on sections of rat oesophagus) and the presence of HLA-DR4 or HLA-DR1 were determined for 25 patients. The patients were assessed to determine the presence or absence of extra-articular manifestations of RA and sicca syndrome. A history of arterial or venous thrombosis, recurrent fetal loss, and current treatment—for example, steroid treatment, treatment with disease modifying antirheumatic drugs, and treatment for other diseases, were reviewed. Statistical analysis was performed with the χ2 test or Fisher's test, as appropriate.

Nine patients (18%) had low titre IgG isotype aCL, but no β2GP1 antibodies. There was no correlation with thrombosis or recurrent fetal loss. There was an increase in sicca syndrome and extra-articular manifestations of RA in the aCL+ group, but this was not statistically significant (table 1). No significant association was found between aCL and other autoantibodies (RF, ANA, antikeratin antibodies). No statistically significant association was found between any drug inducing aCL and the presence of aCL. In contrast with our patients, another study found IgG aCL in only 2% of healthy subjects.4

Table 1

Correlation of anticardiolipin antibodies (aCL) with serological and clinical findings in 50 patients with rheumatoid arthritis. Results are given as number (%) of patients

β2GP1 antibodies were found in 8% of patients with RA, belonging to the IgM class in 75% of these cases. The sera containing IgM β2GP1 antibodies also contained positive levels of RF. There was no correlation with any clinical manifestation. β2GP1 antibodies were not found in healthy subjects.4

The frequency of aCL, all of them β2GP1 independent in this study, was close to5 or lower than in other studies (39–49%).6 7 We found no association with clinical manifestations such as thrombotic events,8 or extra-articular manifestations,6 9 or with other autoantibodies (ANA).7 9 The relation between IgM β2GP1 antibodies and RA remains to be determined; it might be due to non-specific binding with RF.

References

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