Circulating immune complexes in mixed connective tissue disease.

Circulating immune complexes were measured by three assays (monoclonal rheumatoid factor radioimmunoassay, C1q-binding assay, Raji cell radioassay) in sera from patients with mixed connective tissue disease. Evidence for circulating immune complexes was found by at least one method in 94% of sera. The highest frequency of positive results was noted by the Raji cell radioassay (88%). However, a significant number of patients also had immune complexes measured by the C1q-binding assay (50%) and monoclonal rheumatoid factor radioimmunoassay (45%). Further, the levels of immune complexes are measured by the Raji cell radioassay, and to a lesser extent the C1q-binding assay, correlated with disease activity and represent a potentially useful clinical tool. The pattern of reactivity in mixed connective tissue disease sera differed from rheumatoid arthritis, scleroderma, and polymyositis. This suggests that a spectrum of immune complexes is found in these rheumatic diseases and supports the concept that mixed connective tissue disease is distinct from these diseases.