Engagement of NOD2 has a dual effect on proIL-1 beta mRNA transcription and secretion of bioactive IL-1 beta

Synthesis and release of pro-inflammatory cytokines such as IL-1 beta play a crucial role in,, the intestinal inflammation that characterizes Crohn's disease. Mutations in the nucleotide oligomerization domain 2 (NOD2) gene are associated with an increased risk of Crohn's disease. Although it is known that NOD2 mediates cytokine responses to muramyl dipeptide (MDP), it is yet unclear whether NOD2 stimulation mediates only transcription of pro-IL-1 beta mRNA, or whether NOD2 is also involved in the activation of caspase-1 and release of active IL-1 beta. By investigating the response of MNC from Crohn's disease patients homozygous for the 3020insC NOD2 mutation, we were able to show that NOD2 signaling after stimulation with MDP has a dual effect by activating proIL- 1 beta mRNA transcription and inducing release of bioactive IL-1 beta. Because NOD2 engagement amplifies TLR stimulation, we investigated whether activation of caspase-1 by MDP is involved in the NOD2/TLR synergism. The synergy in IL-1 beta production between NOD2 and TLR is mediated at post-translational level in a caspase1-dependent manner, which indirectly suggests that NOD2 also induces caspase-1 activation. In contrast, the synergy in TNF-alpha production after stimulation with MDP and LPS is induced at transcriptional level. This demonstrates that both caspase-1-dependent and -independent mechanisms are involved in the synergy between NOD2 and TLR.