Increased spontaneous but decreased mitogen-stimulated expression and excretion of interleukin 18 by mononuclear cells in patients with active systemic lupus erythematosus.

Objective: To measure serum concentration and analyze the expression of interleukin 18 (IL-18) mRNA in mononuclear cells of patients with systemic lupus erythematosus (SLE).

Methods: IL-18 concentrations in sera and culture supernatants of peripheral blood mononuclear cells (PBMC) from healthy controls and patients with active SLE were measured by ELISA. PBMC and polymorphonuclear leukocytes (PMN) purified from patients with active SLE were stimulated with phytohemagglutinin (PHA), pokeweed mitogen (PWM), and lipopolysaccharide (LPS). Expression of IL-18 mRNA in cells was analyzed by RT-PCR.

Results: Serum IL-18 levels were significantly higher in SLE patients than in controls, and correlated with disease activity in SLE patients (r(2) = 0.602). Two patients receiving intravenous methylprednisolone therapy (1.0 g/day for 3 days) showed profound decreases in serum IL-18 levels after therapy. The quiescent PBMC from SLE patients (30/30) expressed IL-18 transcript more frequently than control PBMC (20/30). PBMC from SLE patients produced more IL-18 than control PBMC after 72 hours of incubation, by RT-PCR. PHA and PWM inhibited the production of IL-18 in PBMC from both SLE patients and controls. Inhibition by PWM was more pronounced than that by PHA, especially in SLE-PBMC. Control and SLE-PMN with or without LPS stimulation produced negligible IL-18.

Conclusion: IL-18 is involved in the autoimmune derangement of leukocyte function in patients with active SLE.