Impaired production of IL-12 in systemic lupus erythematosus. III: deficient IL-12 p40 gene expression and cross-regulation of IL-12, IL-10 and IFN-gamma gene expression.

Interleukin 12 (IL-12) is a heterodimer comprising p35 and p40 subunits which are encoded and regulated separately. The authors previously demonstrated deficient IL-12 production in SLE which correlates negatively with disease activity. The present study was designed to determine whether deficiency of IL-12 and excess production of IL-10 and IL-6 in systemic lupus erythematosus (SLE) are due to aberrant regulation at the gene level. Using semiquantitative RT-PCR assay, it was shown that constitutive expression of IL-12 p35 gene is somewhat impaired in SLE compared with controls and that IL-12 p40 mRNA, which was present at low levels in controls, was undetectable in unstimulated SLE peripheral blood mononuclear cells (PBMC). Gene expression of IL-12 p35 and p40 was significantly increased in response to SAC, with significantly lower SAC-induced expression of p40 in SLE patients than controls. SAC-stimulated IL-12 p35 and p40 mRNAs were significantly augmented by interferon gamma (IFN-gamma). Exogenous IL-12 or IFN-gamma significantly inhibited IL-10 gene expression, without affecting IL-6 mRNA or other proinflammatory cytokine mRNA levels. These observations were further confirmed by studies of protein production at the single cell level using ELISPOT assay. Downregulation of IL-12 p40 expression appears to be the cause of IL12 p70 deficiency in SLE. If this defect could be repaired, normalization of IL-12 and IFN-gamma production should reduce excessive IL-10 and prevent pathology.