REDUCED MITOGEN STIMULATION OF DNA-SYNTHESIS IN HUMAN-LYMPHOCYTES BY A HUMAN RECOMBINANT INTERLEUKIN-1 RECEPTOR ANTAGONIST

The monokine interleukin-1 is produced by monocytes/macrophages after antigen/LPS stimulation and is an important early signal for the activation of resting T cells to become antigen specific T cells. However, little is known about the regulation and inhibition of IL-1. Recently, a new monokine has been described, generated by human macrophages, called interleukin-1 receptor antagonist (IL-1ra). This new monokine adheres to IL-1 in solution and blocks IL-1 receptor binding. IL-1ra is a glycoprotein structurally similar to IL-1-beta but having no interleukin-1-like activity. Using as a model mitogen (PHA 20-mu-g/ml)-stimulated lymphocyte DNA synthesis, we found that hrIL-1ra (30 min lymphocyte pretreatment) inhibits [H-3]thymidine incorporation in a dose-dependent manner. This effect is most probably due to the inhibition of endogenous IL-1, which is a very important signal for T cell activation. The inhibition was maximum at the highest hrIL-1ra concentration used (250 ng/ml). However, when hrIL-1ra was added 2 h after PHA (20-mu-g/ml), a little, if any, inhibition of lymphocyte proliferation was found. The addition of hrIL-1ra simultaneously to the cell cultures with [H-3]thymidine ([H-3]TdR) 6 h before the end of culture incubation did not significantly modify the results compared to the cells treated with PHA alone, indicating no interference of hrIL-1ra on [H-3]TdR lymphocyte incorporation. We also found that the antibody anti-IL-1-beta inhibits mitogen stimulated lymphocyte DNA synthesis in dose-dependent concentrations. When hrIL-1ra was used in combination with anti-IL-1-beta, a synergistic inhibition of lymphocyte blastogenesis was found, whereas the inhibition was significantly less when the compounds were used alone. Similarly, when the two inhibitors were used without mitogen, no significant effect was found on resting lymphocytes. Our results also show that the down-regulation of DNA synthesis by hrIL-1ra can be partially restored by the addition of hrIL-2 plus mitogen at the beginning of the cell cultures. These results strongly suggest, for the first time, that hrIL-1ra is an inhibitor of lymphocyte DNA synthesis and that IL-1 plays a crucial role in lymphocyte activation and proliferation. The mechanism of action of hrIL-1ra is not yet determined, but it is possible that blocking the IL-1 receptors can lead to the down-regulation of IL-1 gene expression, thereby causing an inhibition of the orchestration of the immune system.