HOMOLOGY IN SYSTEMIC NEUTROPHIL RESPONSE INDUCED BY HUMAN EXPERIMENTAL ENDOTOXEMIA AND BY TRAUMA Tjaakje Visser,* Janesh Pillay, † Peter Pickkers, ‡ Luke P. H. Leenen,* and Leo Koenderman † *Departments of Surgery and † Respiratory Medicine, University Medical Center Utrecht, Utrecht; and ‡ Department of Intensive Care Medicine, University Medical Center Radboud, Nijmegen, the Netherlands Received 27 Jul 2011; first review completed 12 Aug 2011; accepted in final form 20 Oct 2011 ABSTRACT—The investigation of the trauma-induced innate immune responses is hampered by the wide variability in patients, type of trauma, and environmental factors. To circumvent this heterogeneity, we examined whether the systemic innate immune response toward human experimental endotoxemia is similar to the response during systemic inflammatory response syndrome after trauma. We tested the hypothesis that the innate immune response to pathogen-associated mole- cular pattern (e.g., lipopolysaccharides [LPSs]) and danger-associated molecular pattern (as induced by injury) leads to a comparable in vivo activation of human neutrophils. Escherichia coli LPS (2 ng/kg) was injected intravenously in nine healthy volunteers to induce a controlled systemic inflammatory response. Indices of systemic inflammation in this human inflam- mation model were compared with those of 12 trauma patients with a mean injury severity score of 19. Blood samples were withdrawn at 3 and 24 h after LPS-challenge or injury. Blood samples of nine healthy volunteers were used as control. Receptor expression was measured as readout for neutrophil activation by flow cytometry. Endotoxemia and injury resulted in a comparable activation phenotype of circulating neutrophils. This phenotype was characterized by downregulation of chemokine receptors CXCR1 and CXCR2 and of Fc+ receptors II and III. A significant difference between both conditions was seen in CD66b expression and for endotoxin resulted in an increased CD66b expression, whereas injury did not. Neutrophil activation was present 3 h after onset of inflammation, both during experimental endotoxemia as well as in trauma patients. Endotoxin and trauma appear to induce a similar neutrophil activation phenotype. KEYWORDS—DAMP, PAMP, neutrophil activation, systemic inflammation, injury, endotoxin INTRODUCTION Tissue injury results in activation of the innate immune system by danger-associated molecular patterns (DAMPs) (1). Danger-associated molecular patterns exist of endogeneous cyto- solic components such as high mobility group box 1, heat shock proteins, defensins, and annexins (2). After severe injury, activa- tion of the immune system can lead to a systemic inflammatory response syndrome with an increased risk of inflammatory com- plications such as acute respiratory distress syndrome and multi- ple organ dysfunction syndrome. A comparable systemic innate immune response is seen during severe infectious diseases such as sepsis and septic shock (3). Yet, during infection, the innate immune response is activated by microbial components in gene- ral, referred to as pathogen-associated molecular patterns (PAMPs) instead of by DAMPs (4, 5). Pathogen-associated molecular pat- terns are recognized by a limited number of germline-encoded pattern-recognition receptors, of which Toll-like receptors are most well known (4). Recently, there has been much discussion about the distinc- tion between PAMPs and DAMPs. It has been proposed that many microorganism components and endogenous alarm sig- nals belong to an ancient subfamily of universal DAMP (6, 7). In addition, several studies have shown that DAMP can also trigger the innate system via Toll-like receptors (8, 9). We hypo- thesized that activation by PAMP and DAMP results in a similar neutrophil response as part of the final common pathway of the innate immune response. Human experimental endotoxemia can be used to investigate cellular innate immune response to PAMP in a standardized manner. Human experimental endotoxemia was developed as model for the host response to infectious diseases and sepsis (10, 11). The model consists of an intravenous challenge of a human volunteer with endotoxin (lipopolysaccharide [LPS]) at low doses (1Y4 ng/kg) (12, 13). Earlier studies have already shown that the administration of LPS leads to a cytokine release, with increases of, e.g., tumor necrosis factor !, interleukin (IL) 1", IL-6, IL-8, and IL-10, comparable to that seen after trauma, albeit in a shorter time frame (10, 12, 13). It is unknown whether inflammation induced by DAMP re- sults in a similar cellular innate immune response as by PAMP- induced inflammation. A recent study has shown similarity in gene expression in circulating leukocytes from trauma patients and from subjects after LPS challenge (14). Therefore, the pre- sent study compared the early neutrophil response after LPS exposure with that of trauma patients, to see if the experimental endotoxemia could be used to investigate the acute systemic cellular response after trauma. Because the inflammatory stimu- lus is short lived after an LPS challenge, the endotoxemia model is, in particular, useful for investigating the kinetics of the early innate immune response (during the first couple of hours after Address reprint requests to Tjaakje Visser, MD, Department of Surgery/ Traumatology, University Medical Center Utrecht, PO Box 85500, 3508 GA, Utrecht, the Netherlands. E-mail: T.Visser@umcutrecht.nl. DOI: 10.1097/SHK.0b013e31823f14a4 Copyright Ó 2012 by the Shock Society 145 SHOCK, Vol. 37, No. 2, pp. 145Y151, 2012 Copyright © 2012 by the Shock Society. Unauthorized reproduction of this article is prohibited.