The effect of methotrexate and mycophenolic acid on monokine production in vitro

Methotrexate (MTX) and mycophenolic acid (MPA) are used clinically for their immunosuppressive properties. MTX is widely used for the treatment of RA. MPA is used to prevent graft rejection and is now experimentally used in SLE and RA. The precise mechanism of action is still debated. Both drugs, though in different ways, inhibit the de novo synthesis of DNA and RNA. We have analysed cytokine production in short cell cultures in whole blood and isolated cells by ELISA. We have shown before that both drugs inhibit the production of several cytokines after T-cell stimulation, and we concluded that MTX leads to irreversible elimination of activated T cells by apoptosis, whereas MPA reversibly prevents activation of resting T cells. We now show that when monocytes are stimulated by SAC or lipopolysaccharide, both MTX and MPA decrease TNF-α, IL-6 and IL-8 production. However the inhibition is not as profound as after T-cell stimulation. An exception is the effect on the production of the proinflammatory cytokine IL-1β. The production of IL-1β is not influenced by MTX after SAC or LPS stimulation, whereas the production is increased by MPA when cells are stimulated with LPS, but not with SAC. We have investigated the cause for this increase. The expression of IL-1β mRNA is not influenced by MPA. Immunoprecipitation and ELISA show that MPA leads to a decrease in the intracellular proform of IL-1β. We conclude that MPA leads to enhanced cleavage of pro-IL-1β.