Sepsis is a leading cause of morbidity and mortality in the intensive care unit. Improvements in the treatment of cancer have led to a growing population of immunocompromised patients with longer survival times and a propensity to develop sepsis and acute respiratory distress syndrome (ARDS) [1, 2]. It is important to study the role of neutrophils in sepsis in order to better understand the effect of neutropenia on the inflammatory response in sepsis. Different treatment modalities may be necessary in the neutropenic versus the non-neutropenic host.
Neutropenia is commonly induced by cyclophosphamide in cancer-stricken patients. In these patients, white cells are markedly diminished in number but not absent. It is known that cyclophosphamide has effects on other cells in the body [3–5]. This drug may decrease the activity of lymphoid cells and macrophages in the spleen [3, 4]. Cyclophosphamide may also modulate CD4+ T cells into a Th2 phenotype and cause a decrease IFN-gamma production as in patients with multiple sclerosis . However, in spite of the numerous cellular effects of cyclophosphamide, in a study of endotoxemia-associated acute lung injury, depletion of neutrophils by cyclophosphamide or anti-neutrophil antibodies showed no difference in lung injury as shown by lung edema or inflammation as demonstrated by similar amounts of the transcription factor, nuclear factor kappa B . Cyclophosphamide can be used to induce neutropenia because of its widespread use in treatment regimens and the fact that the effect is similar to anti-neutrophil antibodies when studying acute lung injury secondary to endotoxemia.
The immune response to sepsis has been widely studied in the immunocompetent host. Sepsis leads to early release of the cytokines; tumor necrosis factor (TNF)-α and interleukin (IL)-1β which are primarily produced by macrophages . Cytokines are low molecular weight proteins (<30 kd) that are responsible for intercellular signaling . After TNF-α and IL-1β are released, their signals are amplified many fold, leading to the activation of the inflammatory cascade and consequently, inflammation. The role of the neutrophil in producing or modulating this initial cytokine response to sepsis has not been well studied .
Recruitment of neutrophils in the lung for example, depends on communication between endothelial, stromal, parenchymal cells and the infiltrating neutrophils . These initial events are mediated by the early production of cytokines, specialized cytokines called chemokines and cell adhesion molecules . Chemokines belong to a superfamily of cytokines that promote chemotaxis of leukocytes to areas of infection, tissue injury or neoplasia . Four subfamilies of chemokines are categorized based on the spacing of the first two cysteine residues and are designated as C, C-C, C-X-C, C-X3-C. In general, the C-X-C group is responsible for neutrophil chemotaxis and activation [11, 12]. The most studied C-X-C chemokine in humans is IL-8. No murine homolog for IL-8 has been discovered but macrophage inflammatory protein (MIP)-2 is one of its functional homologs . In sepsis, the development of acute lung injury may be secondary to activation of the inflammatory response.
Sepsis syndrome is the single most common risk factor associated with the development of acute respiratory distress syndrome (ARDS) . Inflammatory mediators that may be produced as a result of sepsis are felt to play a central role in the development of ARDS. Migration of neutrophils to the lungs and disruption of the alveolar capillary membrane characterizes early ARDS [7, 15]. Mechanisms involved in recruiting neutrophils into the lung have not been well established but are probably dependent on chemotactic factors produced in the lung [7, 15].
The C-X-C chemokine, macrophage inflammatory protein (MIP-2), contributes to increasing neutrophil chemotaxis in murine acute lung injury [16–23]. Lipopolysaccharide, TNF-α and/or IL-1β have been reported to stimulate the production of these cytokines in the lung [19, 22]. In the neutropenic host, neutrophil adhesion and chemotaxis may be abnormal due to insufficient production of chemokines or disruption of at feedback mechanisms. Regulation of the inflammatory response by neutrophils is unknown and has not been well studied.
The first objective of this study is to determine if TNF-α, IL-1β and MIP-2 concentrations will be lower in the systemic circulation of neutropenic, endotoxemic rats compared to non-neutropenic, endotoxemic rats. The second objective is to ascertain if acute lung injury will be worse secondary to the increased production of TNF-α, IL-1β and MIP-2 in the lungs of the non-neutropenic, endotoxemic rats versus the neutropenic endotoxemic rats.