Monocytes/Macrophages control resolution of transient inflammatory pain

Insights into mechanisms governing resolution of inflammatory pain are of great importance for many chronic pain-associated diseases. Here we investigate the role of macrophages/monocytes and the anti-inflammatory cytokine interleukin-10 (IL-10) in the resolution of transient inflammatory pain. Depletion of mice from peripheral monocytes/macrophages delayed resolution of intraplantar IL-1β- and carrageenan-induced inflammatory hyperalgesia from 1 to 3 days to >1 week. Intrathecal administration of a neutralizing IL-10 antibody also markedly delayed resolution of IL-1β- and carrageenan-induced inflammatory hyperalgesia. Recently, we showed that IL-1β- and carrageenan-induced hyperalgesia is significantly prolonged in LysM-GRK2(+/-) mice, which have reduced levels of G-protein-coupled receptor kinase 2 (GRK2) in LysM(+) myeloid cells. Here we show that adoptive transfer of wild-type, but not of GRK2(+/-), bone marrow-derived monocytes normalizes the resolution of IL-1β-induced hyperalgesia in LysM-GRK2(+/-) mice. Adoptive transfer of IL-10(-/-) bone marrow-derived monocytes failed to normalize the duration of IL-1β-induced hyperalgesia in LysM-GRK2(+/-) mice. Mechanistically, we show that GRK2(+/-) macrophages produce less IL-10 in vitro. In addition, intrathecal IL-10 administration attenuated IL-1β-induced hyperalgesia in LysM-GRK2(+/-) mice, whereas it had no effect in wild-type mice. Our data uncover a key role for monocytes/macrophages in promoting resolution of inflammatory hyperalgesia via a mechanism dependent on IL-10 signaling in dorsal root ganglia.

We show that IL-10-producing monocytes/macrophages promote resolution of transient inflammatory hyperalgesia. Additionally, we show that reduced monocyte/macrophage GRK2 impairs resolution of hyperalgesia and reduces IL-10 production. We propose that low GRK2 expression and/or impaired IL-10 production by monocytes/macrophages represent peripheral biomarkers for the risk of developing chronic pain after inflammation.