Demonstration of Circulating Immune Complexes in Sjögren's Syndrome

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Abstract
The sera of patients with Sjögren's syndrome have been examined for circulating immune complexes with the 125I-C1q binding assay and the Raji cell radioimmunoassay. Elevated 125I-C1q binding was found in the sera of 47 of 55 patients (85%) and a positive Raji cell assay was found in the sera of 17 of 20 patients (85%). The two assay showed concordance of 90% in the detection of circulating immune complexes. The amount of immune complexes as detected by the Raji cell assay was not correlated with the titer of IgM or IgG rheumatoid factor. Interestingly, however, a positive association was found between the C1q-binding activity and the titer of IgM rheumatoid factor (Spearman rank correlation coefficient [rho = 0.551], p < 0.0005) although there was no association between C1q-binding activity and the amount of IgG rheumatoid factor. In order to investigate further the positive association of C1q-binding activity and IgM rheumatoid factor, sera were treated with either 2-mercaptoethanol to dissociate 19S IgM RF or solid phase protein A to absorb IgG and were then analyzed for C1q-binding activity and IgM rheumatoid factor. Pretreatment of the sera with 2-mercaptoethanol resulted in complete elimination of IgM rheumatoid factor activity but only slight to moderate reductions in C1q-binding activity. Partially purified IgM rheumatoid factor when added to normal human serum did not result in increased C1q-binding activity. All C1q-binding activity could be removed from Sjögren's syndrome sera by absorption with solid phase protein A, which at the same time caused slight to moderate decreases in the IgM rheumatoid factor titer. These findings indicate that a high percentage of patients with Sjögren's syndrome have circulating immune complexes most of which are distinct from 19S IgM and IgG rheumatoid factor.