Immunoglobulin class and subclass profile of the Ro/SS-A autoantibody response

We have developed an enzyme-linked immunosorbent assay (ELISA) for autoantibody to Ro/SS-A antigen (anti-Ro/SS-A) in order to more fully characterize the autoimmune response that occurs to this antigen in patients with subacute cutaneous lupus erythematosus (SCLE). The microtiter plate-immobilized, biochemically purified Ro/SS-A antigen reacted with anti-Ro/SS-A antibody, but not with other closely related specificities (anti-La/SS-B, anti-SM, anti-U1-RNP) or normal sera. The optimal pH of antigen-antibody reaction in this ELISA was 7.2. The binding of sera containing anti-Ro/SS-A was inhibited 80% by preincubation with the same amount of Ro/SS-A antigen used for coating the plate. Although 11 of the 14 (79%) SCLE sera studied had precipitating anti-Ro/SS-A antibody by immunodiffusion, 13 (93%) sera had abnormally elevated IgG, IgA, or IgM ELISA binding levels. A good correlation between IgG anti-Ro/SS-A ELISA binding levels and immunodiffusion titers was observed (r - 0.8588, p less than or equal to 0.001) suggesting that IgG is the major anti-Ro/SS-A antibody class detected by double immunodiffusion, Sera with a combination of high rheumatoid factor levels (latex 3+ or higher) and high anti-Ro/SS-A titers (1:8 or higher in immunodiffusion) tended to give an abnormally high IgM anti-Ro/SS-A ELISA binding levels. After rheumatoid factor activity was removed by absorption with heat-aggregated human IgG, a 50% decrease in IgM anti-Ro/SS-A ELISA binding was noted. On the other hand, absorption of rheumatoid factor-negative sera that contained high IgM anti-Ro/SS-A binding activity did not significantly decrease ELISA binding levels. Prednisone and 6-azathioprine reduced the level of IgG anti-Ro/SS-A autoantibody in sera of treated SCLE patients by 50%. The IgG subclass profile of anti-Ro/SS-A autoantibody was analyzed by using mouse monoclonal antibodies specific for the 4 human IgG subclasses. Of anti-Ro/SS-A positive SCLE sera, 91% had predominantly IgG1 subclass autoantibody. The coexistence of IgM and IgG anti-Ro/SS-A autoantibody and the predominance of the IgG1 subclass is compatible with the possibility that this autoantibody response is under T-cell control. The predominance of IgG1 in the autoimmune response to Ro/SS-A antigen in SCLE patients is consistent with the hypothesis that antibody dependent cell mediated cytotoxicity could be an important immunologic effector mechanism in this disorder.