Microbial-induced
cytokine regulation is critical to intestinal immune homeostasis. Acute stimulation of
nucleotide-binding oligomerization domain 2 (NOD2), the
Crohn's disease-associated sensor of bacterial
peptidoglycan, induces
cytokines. However,
cytokines are attenuated after chronic NOD2 and
pattern recognition receptor stimulation of macrophages; similar attenuation is observed in intestinal macrophages. The role of Tyro3, Axl, and Mer (TAM) receptors in regulating chronic
pattern recognition receptor stimulation and NOD2-induced outcomes has not been examined. Moreover, TAM receptors have been relatively less investigated in human macrophages. Whereas TAM receptors did not downregulate acute NOD2-induced
cytokines in primary human macrophages, they were essential for downregulating signaling and proinflammatory
cytokine secretion after chronic NOD2 and TLR4 stimulation. Axl and Mer were similarly required in mice for
cytokine downregulation after chronic NOD2 stimulation in vivo and in intestinal tissues. Consistently, TAM expression was increased in human intestinal myeloid-derived cells. Chronic NOD2 stimulation led to IL-10- and TGF-β-dependent TAM upregulation in human macrophages, which, in turn, upregulated suppressor of
cytokine signaling 3 expression. Restoring suppressor of
cytokine signaling 3 expression under TAM knockdown conditions restored chronic NOD2-mediated proinflammatory
cytokine downregulation. In contrast to the upregulated proinflammatory
cytokines, attenuated
IL-10 secretion was maintained in TAM-deficient macrophages upon chronic NOD2 stimulation. The level of MAPK activation in TAM-deficient macrophages after chronic NOD2 stimulation was insufficient to upregulate
IL-10 secretion; however, full restoration of MAPK activation under these conditions restored c-Fos, c-Jun, musculoaponeurotic
fibrosarcoma oncogene homolog K, and PU.1 binding to the
IL-10 promoter and
IL-10 secretion. Therefore, TAM receptors are critical for downregulating proinflammatory
cytokines under the chronic NOD2 stimulation conditions observed in the intestinal environment.