Methotrexate (MTX) is a cornerstone in the treatment of rheumatoid arthritis (RA). Among its anti-proliferative activity, the anti-inflammatory mechanisms of MTX seem to play a major role in the treatment of RA. MTX reduces the production of pro-inflammatory cytokines such as interleukin (IL)-1, IL-2, IL-6 and interferon (INF)-γ, while the gene expression of anti-inflammatory Th2 cytokines like IL-4 and IL-10 is increased - altogether resulting in the anti-inflammatory effect. As little is known about the impact of MTX on other cytokines involved in the pathogenesis of RA, the present trial investigated the effect of MTX on IL-12A and IL-18 gene expression by peripheral blood mononuclear cells (PBMCs). For comparison, the effect on IL-6 and tumour necrosis factor (TNF) was analysed.
Using real-time PCR, mRNA concentrations of pro-inflammatory cytokines were determined in PBMCs from 17 patients before and during MTX therapy. Furthermore, gene expression was correlated with clinical and pharmacokinetic parameters such as methotrexate polyglutamate concentrations (Spearman's correlation coefficient). To eliminate concomitant corticosteroids as confounding factor, a subgroup analysis for methotrexate without corticosteroids was performed in 6 patients.
MTX statistically significantly reduced the mRNA expression of IL-12A by PBMCs in rheumatoid arthritis patients (Wilcoxon-test for paired samples, p<0.046). Consistent with other reports, IL-6 was reduced under MTX treatment. Although the combination of MTX and corticosteroids significantly reduced the gene expression of IL-18, this key molecule was unaffected by MTX without corticosteroids. Our results were further supported by a negative correlation of methotrexate polyglutamate concentrations and the mRNA expression of the pro-inflammatory cytokines IL-6 and IL-12A.
We describe a novel effect of MTX reducing the gene expression of IL-12A independently of corticosteroid application in patients. This impact was further enhanced by a reduction of IL-12A-producing lymphocytes and neutrophils under MTX treatment. These results expand the understanding of the mechanism of action of the most widely used drug in RA.