Productive capacity of alveolar macrophages and pulmonary organ damage after femoral fracture and hemorrhage in IL-6 knockout mice

Abstract

Alveolar macrophages (AM) play an important role in the pathogenesis of posttraumatic pulmonary failure, and have been identified as major source of pulmonary cytokines. The effects of locally generated IL-6 as well as femoral fracture on the pulmonary inflammatory response and organ damage have not been fully elucidated. In the present study we evaluated the influence of femoral fracture, isolated or in combination with hemorrhage, on the immune function of AM and remote lung injury, and investigated the role of pulmonary IL-6 within this setting.

18 wild type (WT) and 18 IL-6 knockout mice (IL-6^−/−) underwent standardized femoral fracture, isolated or in combination with volume-controlled hemorrhage, followed by fluid resuscitation and splint fixation of the fracture. Animals were sacrificed 4 h after induction of fracture and hemorrhage. Animals were randomly assigned to three study groups (each consisting of six animals). Besides sham groups, experimental groups included animals with isolated femoral fracture or in combination with hemorrhagic shock. Cytokine release of AM was determined by flow cytometry. Pulmonary damage in terms of interstitial thickening and lung neutrophil infiltration was assessed by histology and immunohistology.

The productive capacity of AM for pro-inflammatory cytokines was increased after isolated femoral fracture in WT and IL-6^−/− mice. An additional hemorrhagic insult resulted in a further enhancement of pro-inflammatory cytokine release and an increased MCP-1 secretion in WT and IL-6^−/− animals. MCP-1 and pro-inflammatory cytokine production of AM was attenuated in IL-6^−/− mice compared to the respective WT groups. Interstitial thickening and lung neutrophil infiltration was only observed after femoral fracture combined with hemorrhagic shock with an attenuation of the pulmonary organ damage in IL-6^−/− compared to WT animals.

These results support the role of IL-6 as a therapeutic target for posttraumatic immune modulation. With an increased pro-inflammatory mediator release, already an isolated femoral fracture seems to influence the immune response of AM.

Introduction

After severe trauma and surgical interventions, the balance or imbalance of the pro- and anti-inflammatory immune response in part influences the clinical course after trauma. Whereas predominance of the pro-inflammatory response leads to the “Systemic Inflammatory Response Syndrome (SIRS)”, the anti-inflammatory reaction might result in immune suppression with an enhanced risk of infectious complications. SIRS as well as immune suppression play a decisive role in the development of sepsis and the “Multiple Organ Dysfunction Syndrome (MODS)” [1]. The lung is consistently the most susceptible organ in rodent models to sepsis and trauma as well as in clinical studies dealing with septic and trauma patients [2]. Therefore, pulmonary failure (Adult Respiratory Distress Syndrome, ARDS) remains one of the most common causes of trauma-related death [1], [2].

Alveolar macrophages are considered to be primary scavengers of particulate matter that reaches the alveoli. Alveolar macrophages have also been implicated in the pathogenesis of acute pulmonary injury [3]. In sepsis-induced lung injury, the pulmonary damage has been reported to be dependent on the presence of functional alveolar macrophages [4], [5]. Furthermore, experimental studies suggest a possible mechanistic role of mediators released by alveolar macrophages in the induction of alveolar type II cell apoptosis, thereby initiating or maintaining pulmonary injury [6], [7].

Alveolar macrophages have also been identified as the major source of pulmonary Interleukin (IL)-6 [8]. IL-6 affects a variety of biological functions, including immunoglobulin production, the acute phase response and inflammation. IL-6 is assumed to be a proximal mediator of pulmonary injury promoting lung neutrophil (polymorhonuclear leucocytes, PMN) infiltration. As reported by Solomkin locally instilled IL-6 resulted in a significant increase of PMN infiltration and lung injury [9]. This might support the hypothesis that locally generated IL-6 exerts pro-inflammatory effects that are central in pulmonary damage. However, the role of IL-6 in the pathophysiology of remote lung injury is still discussed controversially. After intraperitoneal administration of LPS a more severe pulmonary hemorrhage in IL-6^−/− mice than in WT animals has been observed, suggesting that IL-6 is protective against hemorrhagic organ damage related to LPS [2]. Thus, it appears that the role of IL-6 in acute inflammation depends on the stimulus and/or the model of inflammation used [10].

Therefore, further studies on the influence of trauma and hemorrhage on the immune function of alveolar macrophages are needed to clarify the importance of these cells in the development of complications after multiple trauma. Furthermore, the role of IL-6 within these pathophysiological changes has to be observed. In the present study we investigated the effects of femoral fracture with or without an additional hemorrhage on the immune function of alveolar macrophages and the remote pulmonary organ damage, and evaluated the role if pulmonary IL-6 under these conditions.