Anesthesiology 2006; 104:1274 – 82 © 2006 American Society of Anesthesiologists, Inc. Lippincott Williams & Wilkins, Inc. Blockade of the Complement C5a Receptor Reduces Incisional Allodynia, Edema, and Cytokine Expression J. David Clark, M.D., Ph.D.,* Yanli Qiao, M.D.,† Xiangqi Li, M.D.,† Xiaoyou Shi, M.D.,† Martin S. Angst, M.D.,‡ David C. Yeomans, Ph.D.‡ Background: Activation of the complement system is one component of the inflammatory response. Various components of the complement system participate in killing foreign organ- isms, recruiting immune cells, enhancing edema, and stimulat- ing cytokine formation. Complement-mediated enhancement of the inflammation surrounding surgical incisions may in- crease pain. Methods: In these studies, the authors used a murine hind paw incisional model to study the role of the complement C5a receptor in supporting incisional inflammation. At baseline and at various time points after incision, they measured the effects of a highly selective C5a receptor antagonist on nociceptive thresholds, edema formation, and cytokine production in the skin surrounding the incision. They also measured changes in C5a receptor expression near the incisions. Results: The once-daily injection of the C5a receptor antago- nist AcF-[OPdChaWR] reduced mechanical allodynia and edema in the incised hind paw. A multiplexed cytokine assay revealed that 8 of the 18 cytokines examined showed significant in- creases in skin tissue abundance after incision. Distinct time courses for the patterns of elevation were seen, though some degree of resolution occurred for all cytokines within 96 h. For 7 of these 8 cytokines, the C5a receptor antagonist reduced the enhancement of expression. In addition, the authors found that the C5a receptor messenger RNA level increased 15-fold in the skin surrounding the incisions within 24 h and then slowly declined. Conclusions: The tissue directly surrounding incisions in mouse hind paws undergoes large changes in the content of specific cytokines in addition to demonstrating edema and no- ciceptive sensitization. By blocking the receptor for one com- ponent of the complement system, C5a, all of these changes can be reduced. Complement receptor inhibitors may constitute a novel group of compounds useful in reducing the pain and swelling of surgical incisions. THE complement system has been shown to become activated in many acute, subacute and chronic injury states including sepsis, shock, blunt trauma, surgical trauma, burns, ischemia, arthritis, and others. Several excellent reviews tabulate the many demonstrated and proposed functions of the complement system. 1–3 Al- though generally helpful to the organism, in some in- stances, such as with autoimmune diseases or in sepsis, activation of the complement system can have deleteri- ous consequences. Many functions have been ascribed to the various components of the classic, alternative, and lectin complement pathways, but those most commonly invoked are enhancement of inflammation and direct attack of foreign organisms. Complement system activa- tion is commonly measured by the quantification of activated complement protein “split products.” In- creases of these complement split products have been demonstrated in the serum and in fluids from the wounds of surgical patients. 4 The participation of the complement system in wound inflammation suggests that inhibition of the complement cascade might reduce the liberation of inflammatory mediators which support nociception like hydrogen ion, biogenic amines, eico- sanoids, neuropeptides, and other molecules. Of special note, the skin surrounding incisions has been shown to contain increased amounts of several cytokines, includ- ing interleukin (IL)-1, IL-6, tumor necrosis factor (TNF)-, and others. 5,6 Each of these cytokines has been observed to support enhanced nociceptive sensitivity in various rodent models. 7–9 Although 18 cytokines were followed in these studies, our panel was not comprehen- sive of all known cytokines. Levels of these cytokines decrease as healing progresses in the 3–5 days after incisions. Recently, there has been a focus on methods to reduce complement anaphylatoxin activity to reduce inflamma- tion. These anaphylatoxins (C3a, C4a, and C5a) are ac- tive complement split products which have a number of biologic properties, including acting as chemoattractants and causing the degranulation of mast cells and polymor- phonuclear cells. 10 These substances act through G pro- tein– coupled cell-surface receptors to exert their bio- logic functions. 11,12 The existence of anaphylatoxin receptors, especially C5a receptors, on nonimmune cells including glia and central nervous system neurons has led to the hypothesis that these substances have func- tions in addition to ones strictly involving the immune system, including participation in neuroinflammation and neurodegenerative processes. 13–17 Therefore, efforts to develop C5a receptor antagonists have been intensive with the hope that these agents would demonstrate some utility in a number of different diseases character- ized by inflammation. 18,19 To reduce joint inflammation, Woodruff et al. 20 used a cyclic peptide C5a receptor antagonist in a rat model of inflammatory arthritis. The cyclic peptide compound AcF-[OPdChaWR] has been observed to block C5a-medi- ated effects for 24 h or more after single doses and is * Associate Professor, Department of Anesthesia, Stanford University School of Medicine, Stanford, California. Veterans Affairs Palo Alto Healthcare System, Palo Alto, California. † Life Sciences Research Associate, ‡ Associate Professor, Department of Anesthesia, Stanford University School of Medicine, Stanford, California. Received from the Veterans Affairs Palo Alto Healthcare System, Palo Alto, and Stanford University, Stanford, California. Submitted for publication October 3, 2005. Accepted for publication February 9, 2006. This work was supported by research grant No. GM061260 from the National Institute of General Medical Sciences, Bethesda, Maryland. Address correspondence to Dr. Clark: Department of Anesthesiology, 112A, Veterans Affairs Palo Alto Healthcare System, 3801 Miranda Avenue, Palo Alto, California 94304. djclark@stanford.edu. Individual article reprints may be pur- chased through the Journal Web site, www.anesthesiology.org. Anesthesiology, V 104, No 6, Jun 2006 1274