Anesthetic Pharmacology Preclinical Pharmacology Section Editor: Marcel E. Durieux Clinical Pharmacology Section Editor: Tony Gin The  Opioid Receptor Mediates Morphine-Induced Tumor Necrosis Factor and Interleukin-6 Inhibition in Toll-Like Receptor 2-Stimulated Monocytes Marie-Pierre Bonnet, MD* He ´le `ne Beloeil, MD, PhD* Dan Benhamou, MD* Jean-Xavier Mazoit, MD, PhD* Karim Asehnoune, MD, PhD† BACKGROUND: Morphine possesses immunomodulatory effects but its intrinsic mechanisms, especially in the toll-like receptor 2 (TLR2) signaling pathway, are only partially understood. In this study, we evaluated the effects of morphine on tumor necrosis factor (TNF), interleukin-6 (IL-6), and interleukin-10 (IL-10) pro- duction in TLR2-stimulated human monocytes and identified the involvement of the different opioid receptors, and of the lymphocyte-to-monocyte contact. METHODS: Peripheral blood mononuclear cells (PBMCs) were isolated from fresh blood by centrifugation on a density gradient. Monocytes were secondarily separated using a high-gradient magnetic cell sorting kit with specific anti-CD14 antibodies. Monocytes or PBMCs were pretreated with opioid receptors antago- nists before being cultured with morphine and peptidoglycan (PGN) from Staphy- lococcus aureus (specific TLR2 agonist). The amount of TNF, IL-6, and IL-10 was measured in the supernatant enzyme-linked immunosorbent assay. RESULTS: Proinflammatory cytokines: Morphine significantly inhibited the production of cytokines in a dose and concentration-dependent manner in PGN-stimulated monocytes.  Opioid receptor activation specifically mediated this morphine- induced TNF and IL-6 inhibition in monocytes. Morphine significantly inhibited the TNF, but not the IL-6 production, in PGN-stimulated PBMCs. The  opioid receptor was not involved in this morphine-induced TNF inhibition in PBMCs. Antiinflammatory cytokines: IL-10 was not a factor for the inhibition of TNF and IL-6 production after PGN stimulation in either monocytes or PBMCs cultures. CONCLUSIONS: The  opioid receptor mediates morphine-induced TNF and IL-6 inhibition in PGN-stimulated monocytes, but not in PBMCs. A direct monocyte- to-lymphocyte contact (PBMCs) alters the inhibitory effects of morphine observed on monocytes alone. IL-10 is not a factor for the inhibition of TNF or for IL-6 production. Interactions between TLR2 and  opioid intracellular pathways remain to be studied to delineate these morphine immunosuppressive effects. (Anesth Analg 2008;106:1142–9) Morphine and other opioids are potent immuno- modulators. 1 It has been suggested that chronic opioid users, for either therapeutic reasons or because of addiction, are more susceptible to bacterial and viral infections. 2,3 However, clinical evidence demonstrat- ing enhanced infectious susceptibility in opioid-using chronic pain patients is still missing. Opioid adminis- tration affects both innate and adaptative immunity, such as antibodies production, 4 natural killer activity, 5 cytotoxicity, cytokine production, 6,7 chimiotaxism, 8 and phagocytosis. 9 Monocytes play a central role in innate immunity. The presence of opioid receptors on monocytes has been demonstrated, 10 strengthening the link between immunity and opioid drugs, al- though this link has been questioned by others. 11 Morphine’s effects on innate immunity have been mainly studied in different cell populations after expo- sure of cells to lipopolysaccharide (LPS) from Gram- negative bacteria. However, infections by Gram-positive bacteria have been increasing during the last 20 yr 12 and are of poor prognosis. 13 Host-defense mechanisms against bacterial infections are under the control of toll-like receptors (TLRs). TLRs play a critical role in innate defense by sensing specific molecular patterns associated with microbial pathogens. Two TLRs are especially involved in specific identification of bacterial components: TLR4 recognizes the LPS from Gram- negative bacteria and TLR2 recognizes cocci Gram- positive components such as peptidoglycan (PGN). 14,15 From the *Ho ˆ pital Bice ˆtre, AP-HP, Le Kremlin-Bice ˆtre, cedex, France; and †Ho ˆ pital Hotel-Dieu, Nantes, France. Accepted for publication on November 28, 2007. Supported by the association Mises Au Point en Anesthe ´sie et Re ´animation, Le Kremlin-Bice ˆtre, France. Presented, in part, during the 47th Congress of the Socie ´te ´ Franc ¸aise d’Anesthe ´sie-Re ´animation, Paris, France, September 22, 2005. Address correspondence and reprint requests to Marie-Pierre Bonnet, MD, De ´partement d’Anesthe ´sie Re ´animation Chirurgicale, Groupement Hospitalier Universitaire Sud, Ho ˆ pital Bice ˆtre, 78, rue du Ge ´ne ´ral Leclerc, 94275 Le Kremlin-Bice ˆtre, cedex, France. Ad- dress e-mail to marie-pierre.bonnet@abc.aphp.fr. Copyright © 2008 International Anesthesia Research Society DOI: 10.1213/ane.0b013e318165de89 Vol. 106, No. 4, April 2008 1142