Objective. To investigate whether systemic lupus erythematosus (SLE) is accompanied by increased serum nitrite levels, whether active compared with inactive disease is associated with greater nitric oxide (NO) production, and whether endothelial cells or keratinocytes serve as cellular sources of NO by virtue of their increased expression of either constitutive nitric oxide synthase (cNOS) or inducible NOS (iNOS).

Methods. Fifty-one serum samples (46 from patients with SLE) were analyzed for NO production by measuring nitrite levels in a calorimetric assay. Skin biopsy samples from 21 SLE patients and 11 healthy volunteers were evaluated immunohistochemically, using monoclonal antibodies, for endothelial cell and keratinocyte cNOS and iNOS expression.

Results. Serum nitrite levels were significantly elevated in the 46 patients with SLE (mean ± SEM 37 ± 6 μM/liter) compared with controls (15 ± 7 μM/liter; P < 0.01), and were elevated in patients with active SLE compared with those with inactive disease (46 ± 7 μM/liter versus 30 ± 7 μM/liter; P < 0.01). Serum nitrite levels correlated with disease activity (r = 0.47, P = 0.04) and with levels of antibodies to doublestranded DNA (r = 0.35, P = 0.02). Endothelial cell expression of iNOS in SLE patients (mean ± SEM score 1.5 ± 0.2) was significantly greater compared with controls (0.6 ± 0.2; P < 0.01), and higher in patients with active disease compared with those with inactive SLE (1.7 ± 0.2 versus 1.2 ± 0.2; P < 0.01). Keratinocyte expression of iNOS was also significantly elevated in SLE patients (0.9 ± 0.1) compared with controls (0.4 ± 0.1; P < 0.001). With regard to expression of cNOS, there were no differences between patients with active SLE, those with inactive SLE, and normal controls in either the vascular endothelium or the keratinocytes.

Conclusion. NO production is increased in patients with SLE, and 2 potential sources of excessive NO are activated endothelial cells and keratinocytes via up-regulated iNOS.