Stimulation of nucleotide-binding oligomerization domain-containing (Nod)2 and other pattern recognition receptors (PRR) in human monocyte-derived macrophages induces interleukin (IL)-1, which increases mitogen-activated protein kinase (MAPK) activation and cytokine secretion. Activation of MAPK by PRR has varied effects on inflammatory cytokine secretion. We investigated whether different levels of autocrine IL-1 mediate these varied effects.

Macrophage responses to PRR ligands were analyzed by enzyme-linked immunosorbent assay and flow cytometry. We overexpressed or reduced MAPK levels (using small inhibitory RNA).

Nod2 and other PRR activated signaling via extracellular signal-related kinase (ERK) and p38 that inhibited inflammatory cytokine production by human monocyte-derived macrophages; autocrine IL-1 production prevented this inhibition. ERK and p38 inhibited inflammatory cytokine production by human macrophages that produce low levels of IL-1 (such as M2, endotoxin-tolerant, and intestinal macrophages); adding exogenous IL-1 caused ERK and p38 to stimulate production of inflammatory cytokines in these cells. In mouse macrophages, which do not produce IL-1 in response to PRR stimulation alone, addition of exogenous IL-1 reversed the ERK-mediated inhibition of IL-12p40. Increasing activation of c-Jun N-terminal kinase in Nod2-stimulated human monocyte-derived macrophages, in the absence of autocrine IL-1 signaling, caused ERK and p38 to stimulate inflammatory cytokines secretion. Importantly, infection of human intestinal macrophages with pathogens that induce IL-1 production reversed the inhibition of inflammatory cytokine production by ERK and p38.

In response to PRR stimulation of macrophages, the level of MAPK signaling is regulated by autocrine IL-1 and determines whether production of inflammatory cytokines is inhibited or stimulated. This mechanism could account for reported differences in MAPK regulation of inflammatory cytokines and propagate the inflammatory response to pathogens.