Background and aims: Interleukin-4 (IL-4) is an anti-inflammatory cytokine that can polarize macrophages from a pro-inflammatory M1 into an anti-inflammatory M2 phenotype, which is considered to exert the effects via IL-10.  This study aims to investigate the role of the endogenous opioid system in the analgesic actions of IL-4 and M2 macrophages.

Methods: Experiments were approved by the local ethics committee and performed following IASP guidelines. As a pain model, the sciatic nerve chronic constriction injury in male C57BL/6J mice was used. IL-4 was administered daily at the injury site on days 14-21, whereas opioid peptide antibodies and receptor antagonists were applied on days 22 and 26, and mechanical sensitivity was assessed by von Frey filaments. On day 22, immune cells from injured nerves were quantified by flow cytometry, F4/80+ macrophages were isolated using immunomagnetic separation, their M1/M2 state and opioid peptide precursor mRNA expression was assessed by qPCR and fluorescent in situ hybridization, and the opioid peptide content was measured by enzyme immunoassays. 
Results: IL-4 increased F4/80+ macrophage numbers and shifted them into an M2 phenotype. M2 macrophages synthetized and contained more opioids than M1 cells. IL-4 induced analgesia for several days after the discontinuation of IL-4 treatment. This analgesia was diminished by opioid peptide antibodies and receptor antagonists.

Conclusions: IL-4-induced M2 macrophages at injured nerves produced opioids, which activated peripheral opioid receptors to diminish pain. The endogenous opioid system is crucial to the analgesic actions of IL-4 and M2 macrophages.

Research support: Deutsche Forschungsgemeinschaft (MA 6432/2-1).

IL-4-INDUCED M2 MACROPHAGES PRODUCE LONG-LASTING ANALGESIA VIA ENDOGENOUS OPIOIDS