Effect of the combination of the proinflammatory cytokines IL-1, tumor necrosis factor-α and IL-17

The list of cytokines that may contribute to joint destruction in rheumatoid arthritis is growing extensively. At the same time, inhibition of a single cytokine such as tumor necrosis factor (TNF)-α or IL-1 was able to control disease in a significant proportion of patients. Accordingly, the mode of interactions between these cytokines has to be clarified in order to understand how a single cytokine inhibition can still be effective. These interactions were studied with mesenchymal cells, which are targets of cytokines (synoviocytes, osteoblasts, myoblasts), as well as with the respective organ samples (synovium, bone, muscle) in order to keep intact interactions as seen in vivo. Cells and explants were exposed to cytokines used at low concentrations and high concentrations in order to look at their effect alone and a possible additive or synergistic effect when combined. Their respective soluble receptors were used as specific inhibitors alone and in combination.

Effects on AP-1, NF-κB and Egr-1 activation were explored by RT-PCR and immunocytochemistry. IL-1 and TNF-α induced most of these transcription factors while IL-17 had a weak effect on the different mesenchymal cells. More importantly, when these cytokines were used at low concentrations with no effect alone, their combinations showed a synergistic effect on transcription and nuclear translocation of AP-1 members, Egr-1 and NF-κB. Moreover, cytokine combinations allowed an enhanced recruitment of factors not expressed by cytokines used alone. Conversely, combination of specific inhibitors (p75 TNF and type II IL-1 soluble receptors) was needed to completely abrogate NF-κB nuclear translocation in myoblasts stimulated with both IL-1 and TNF-α. Low concentrations of cytokines can have a significant biological effect through their interactions mediated by synergistic mechanisms.