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Objective. To characterize juvenile rheumatoid arthritis synovial fluid (SF) immune complexes and to examine their interaction with leukocytes. Methods. SF immunoglobulin‐containing fractions were prepared by sequential chromatography on protein A and Sephacryl 300. Fractions were subdivided according to molecular weight, characterized for immunoglobulin and complement content, and incubated with either promonocytic U937 cells or normal human peripheral blood mononuclear cells (PBMC). Results. High molecular weight SF immunoglobulin‐containing fractions stimulated the release of interleukin‐lβ (IL‐1β) from U937 cells. These same complexes stimulated tumor necrosis factor α (TNFα), IL‐lβ, IL‐6, IL‐8, and granulocytemacrophage colony‐stimulating factor (GM‐CSF) from PBMC. Lower molecular weight material was less efficient in inducing any of the cytokines. TNFα and IL‐1β were the earliest of the messenger RNAs examined to be induced by the high molecular weight complexes. However, the secretion of IL‐6, IL‐8, and GM‐CSF stimulated by the complexes was not completely dependent upon the secretion of IL‐1β. Addition of IL‐1 receptor antagonist to the cell cultures reduced GM‐CSF and IL‐6 production by 40% and IL‐8 production by 25% in PBMC. Conclusion. SF immunoglobulin fractions contain immune complexes that vary in size, composition, and phlogistic potential. High molecular weight complexes are capable of inducing a spectrum of proinflammatory cytokines, all of which have been implicated in the pathogenesis of rhematic disease.

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