Secretory mediators regulate Nod2-induced tolerance in human macrophages.
OBJECTIVE
Nucleotide oligomerization domain 2 (Nod2) polymorphisms increase the risk of developing Crohn's disease, which is characterized by chronic intestinal inflammation. Bacterial peptidoglycan products chronically stimulate Nod2 in the intestine. Recent studies found that chronic Nod2 stimulation in human macrophages down-regulates proinflammatory cytokines on Nod2 or Toll-like receptor (TLR) restimulation. Therefore, an emerging hypothesis is that Nod2-mediated cytokine down-regulation is required for intestinal homeostasis, but the mechanisms mediating this down-regulation are incompletely understood.


METHODS
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.


RESULTS
We found that chronic Nod2 stimulation cross-tolerizes not only to TLRs but also to the interleukin (IL)-1 receptor. Moreover, chronic IL-1β stimulation down-regulates Nod2 responses. Accordingly, IL-1β blockade partially reverses Nod2-mediated tolerance. We found that an additional essential mechanism for Nod2-mediated tolerance is the early secretion of the anti-inflammatory mediators IL-10, transforming growth factor β, and IL-1Ra. Importantly, the mammalian target of rapamycin (mTOR) pathway, involved in cell growth, differentiation, and activation, significantly contributes to Nod2-induced anti-inflammatory as opposed to proinflammatory cytokines and to Nod2-mediated tolerance.


CONCLUSIONS
Inflammatory responses through the IL-1R are down-regulated upon chronic Nod2 stimulation, secretory mediators are a critical mechanism for Nod2-mediated cytokine down-regulation, 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, down-regulates cytokine responses.
Hedl
Matija
M
Department of Medicine, Section of Digestive Diseases, Yale University, New Haven, Connecticut, USA.
Abraham
Clara
C
D002478:Cells, Cultured; D015536:Down-Regulation; D006801:Humans; D007108:Immune Tolerance; D018836:Inflammation Mediators / Q000276:immunology*; D016753:Interleukin-10 / Q000276:immunology; D053583:Interleukin-1beta / Q000276:immunology; D008264:Macrophages / Q000276:immunology*; D053473:Nod2 Signaling Adaptor Protein / Q000276:immunology*; D017472:Receptors, Interleukin-1 / Q000276:immunology; D058570:TOR Serine-Threonine Kinases / Q000276:immunology; D051193:Toll-Like Receptors / Q000276:immunology; D016212:Transforming Growth Factor beta / Q000378:metabolism
D018836:Inflammation Mediators; D053583:Interleukin-1beta; C429018:NOD2 protein, human; D053473:Nod2 Signaling Adaptor Protein; D017472:Receptors, Interleukin-1; D051193:Toll-Like Receptors; D016212:Transforming Growth Factor beta; D016753:Interleukin-10; C546842:MTOR protein, human; D058570:TOR Serine-Threonine Kinases