NETs: Protection or Harm in Neonatal Inflammation or Infection

Many pediatric diseases including Systemic Inflammatory Response Syndrome (SIRS), sepsis, Acute Respiratory Distress Syndrome (ARDS) and neonatal Chronic Lung Disease (CLD) have been associated with dysregulation of the acute inflammatory response [1]. So it is with necrotizing enterocolitis (NEC). NEC, a disease of premature infants, afflicts 10% of very low birth weight infants. This disease is often fatal [2]. Less significant but nonetheless devastating sequelae include intestinal perforation, short gut syndrome, prolonged total parenteral nutrition with possible concomitant liver failure and a prolonged intensive care unit stay. The etiology remains unknown, although risk factors of prematurity, enteral feeds, infection and intestinal ischemia are associated with NEC [3].

The final common pathway for NEC appears, at least in part, to be mediated through the biologically active phospholipid platelet-activating factor (PAF). Each identified risk factor for NEC increases the serum levels of PAF in premature infants[4, 5]. Furthermore, serum levels of PAF-acetylhydrolase (PAF-AH), the enzyme responsible for catabolizing PAF, are lower in premature infants compared to term infants and lower in term infants compared to young children and adults [6, 7]. Although no clinical trials of PAF antagonists have been conducted in premature human infants, various PAF antagonists prevent NEC-like clinical disease in animal models [8-10].

Other data from animal models suggest a prominent role of the polymorphonuclear leukocyte (PMN) in the pathogenesis of NEC. Musemeche et al. induced NEC-like disease in rats by intra-aortic injection of PAF [11]. They used vinblastine, a chemotherapeutic agent with a side-effect profile significant for induction of neutropenia, to induce neutropenia in rats four days prior to intra-aortic injection of PAF. The vinblastine-induced neutropenia was protective for the clinical and pathologic manifestations of NEC-like disease. Other investigators have demonstrated an increase in PAF levels in the gastrointestinal tracts of rats subjected to gut ischemia/reperfusion. The elevated levels of intestinal PAF were then shown to chemo attract and prime PMNs [12].

The role of the human PMN in the acute inflammatory response is well documented. They play a fundamental role in the non-specific immune response and are rapidly recruited to areas of injury or inflammation where they participate in bacterial phagocytosis and killing. Disorders associated with a deficiency or impairment of PMNs (neutropenia, chronic granulomatous disease, leukocyte adhesion deficiency) predispose to infections with gram-negative and gram-positive bacteria [13]. However, regulation of this potent component of the acute inflammatory response is imperative. Disorders such as ARDS, ischemia/reperfusion injury and rheumatoid arthritis appear to result from the dysregulation of the PMNs' acute inflammatory response [1].

The molecular mechanisms regulating the PMNs' response in acute inflammation are not fully understood. In neutrophil priming, the activation of the NADPH oxidase enzyme via receptor-mediated stimulation with mediators such as fMLP or PAF is an increasingly complex process involving various cellular secondary messengers and the Rho family GTPase Rac2 [14, 15]. The mechanisms regulating PMN synthesis and release of pro-inflammatory cytokines such as IL-8 are less well understood. The mechanisms regulating PMN apoptosis are also not well understood. In vitro and in vivo studies indicate that the pro-inflammatory agents responsible for the priming of human PMNs also affect the longevity of those cells by delaying the PMNs in-built capacity to undergo apoptosis. In vitro studies on human PMNs show that pro-inflammatory mediators like Granulocyte/Macrophage - Colony Stimulating Factor (GM-CSF), Interleukin-8 (IL-8), Lipopolysaccharide (LPS), Complement 5a (C5a) and Interleukin-6 (IL-6) inhibit PMN apoptosis, while Tumor Necrosis Factor (TNF) and Fas-ligand (Fas-L) accelerate the rate of neutrophil apoptosis [16].