Background and Objectives
Nod2 polymorphisms increase the risk for developing Crohn’s disease, characterized by chronic intestinal inflammation. Bacterial peptidoglycan products chronically stimulate Nod2 in the intestine. Recent studies found that chronic Nod2 stimulation in human macrophages downregulates pro-inflammatory cytokines upon Nod2 or Toll-like receptor (TLR) restimulation. Therefore, an emerging hypothesis is that Nod2-mediated cytokine downregulation is required for intestinal homeostasis, but the mechanisms mediating this downregulation are incompletely understood.
Utilizing primary human macrophages, we examined secretory mediators as a mechanism of Nod2-mediated tolerance by inhibiting their function and assessing tolerance reversal through cytokine secretion. Signaling pathways contributing to secretory mediator induction and Nod2-mediated tolerance were identified through pathway inhibition.
We find that chronic Nod2 stimulation not only cross-tolerizes to TLRs, but also to the IL-1 receptor (IL-1R). Moreover, chronic IL-1β stimulation downregulates Nod2 responses. Accordingly, IL-1β blockade partially reverses Nod2-mediated tolerance. We find that an additional essential mechanism for Nod2-mediated tolerance is the early secretion of the anti-inflammatory mediators IL-10, TGF-β and IL-1Ra. Importantly, the mTOR pathway, involved in cell growth, differentiation and activation, significantly contributes to Nod2-induced anti- as opposed to pro-inflammatory cytokines and to Nod2-mediated tolerance.
Inflammatory responses through the IL-1R are downregulated upon chronic Nod2 stimulation, secretory mediators are a critical mechanism for Nod2-mediated cytokine downregulation, and the mTOR pathway is crucial for Nod2-mediated tolerance. These results further contribute to our understanding of the mechanisms through which Nod2, a protein critical to intestinal homeostasis, downregulates cytokine responses.