To explore the regulatory defects underlying the overexpression of CD40 ligand (CD40L, CD154) in human lupus we studied the effects of cyclosporin-A (CsA), which blocks Ca2+/calcineurin-dependent CD40L gene expression, on peripheral blood-derived T cells and monocytes. In contrast to control subjects, CsA failed to inhibit the prolonged CD40L expression observed in vitro on anti-CD3-activated lupus T cells. Resistance to CsA was not restricted to CD4+ or CD8+ T cell subsets and was disease activity-independent. Experiments assessing the effects of dexamethasone on CD40L expression, as well as of CsA on the early activation marker CD69 expression and on surface CD40L cleavage, confirmed the unique regulation of CD40L in lupus T cells. On the other hand, co-culture with anti-CD3-activated T cells caused surface CD40L expression on monocytes, which was not an Fc receptor-mediated event. Lupus monocytes clearly overexpressed CD40L comparing to healthy and disease-control monocytes, and, similarly to lupus T cells, displayed a prominent resistance to CsA inhibitory effects. These findings indicate that, besides Ca2+/calcineurin-dependent mechanisms, other pathways are involved in the dysregulation of CD40L in SLE immune cells, dissection of which may have important therapeutic implications.