Mechanisms Underlying Reduced Apoptosis in Neonatal Neutrophils

Abstract

Apoptosis, which leads to phagocytosis by mononuclear cells, represents the primary mechanism for removing neutrophils from inflamed tissues and minimizing injury. The present studies show that membrane phosphatidylserine turnover and permeability, as well as DNA fragmentation, were reduced in neutrophils from neonates when compared with adults. The activity of caspase 3 and expression of the proapoptotic proteins Bax, Bad, and Bak were also decreased in neonatal relative to adult neutrophils. These findings are consistent with impaired apoptosis in neonatal cells, which may contribute to prolonged inflammation in infants after oxidative stress or infection. Neutrophil apoptosis is induced by endogenous ligands such as Fas (FasL), which engage death receptors of the tumor necrosis factor/nerve growth factor superfamily, including Fas receptor (FasR). We found that expression of FasR was decreased in neonatal when compared with adult cells. Moreover, neonatal neutrophils did not undergo apoptosis in response to anti-FasR antibody and exhibited impaired chemotaxis to soluble FasL. However, in both adult and neonatal cells, p38 mitogen-activated protein kinase and phosphatidylinositol 3-kinase inhibitors blocked Fas-induced activity. These data suggest that prolonged survival of neonatal neutrophils at injured sites is due, in part, to reduced responsiveness to FasL. This may be related to decreased expression of both FasR and Bcl-2-family proteins that mediate neutrophil apoptosis. Human neutrophils are the primary effector cells in acute inflammation and are rapidly recruited from the bloodstream to injured sites. Although neutrophils from newborns exhibit defects in chemotaxis, phagocytosis, and oxidative metabolism (1-4), infants are at high risk for neutrophil-mediated tissue injury. Activated neutrophils are cleared from inflamed sites by the process of apoptosis, followed by macrophage phagocytosis. This promotes resolution rather than persistence of tissue injury (5,6). Attenuation of neutrophil apoptosis in neonates may contribute to severe and prolonged inflammatory responses. In premature infants, this may play a role in conditions such as bronchopulmonary dysplasia and necrotizing enterocolitis, which result in significant mortality and morbidity.Neutrophil longevity and functional activity are regulated by inflammatory mediators present in the microenvironment.Whereas proinflammatory cytokines, such as interferon-␥ (IFN-␥), granulocyte-monocyte colony-stimulating factor (GM-CSF), and bacterial-derived lipopolysaccharide (LPS), promote neutrophil activation and survival, anti-inflammatory mediators, such as IL-10 (IL-10) and Fas ligand (FasL), are proapoptotic (7-10). These mediators regulate the expression of pro-and antiapoptotic mitochondrial proteins, including Bak, Bad, Bax, and A1, as well as the proapoptotic effector protease caspase 3 (10 -14). Previous studies have demonstrated that the rate of apoptosis is reduced in peripheral blood neutrophils from neonates relative to adult cells (15,16). W...

Influence of Labor on Neonatal Neutrophil Apoptosis, and Inflammatory Activity

“…1). In both neonatal cell populations, apoptosis was significantly reduced when compared with adult cells, which is consistent with previous reports (1,2,14). In further studies, we analyzed potential mechanisms underlying the inhibitory effects of labor on neonatal neutrophil apoptosis.…”

“…The rate of apoptosis is related to the activity of these effector pathways, which induce cell and nuclear membrane permeability (13). Consistent with previous findings (2,14), we found that apoptosis is impaired in neonatal neutrophils relative to adult cells and that this is associated with reduced caspase-3 activity in neonatal cells. Interestingly, labor had no effect on caspase-3 activity or on expression of IAP-2, an intracellular protein induced by inflammatory stimuli and hypoxia that inhibits caspase activity (20).…”

“…This has been observed in patients with acute respiratory distress syndrome, as well as oxygen-induced lung injury, pathogenic states associated with markedly reduced neutrophil apoptosis (1). Neutrophil apoptosis is also impaired in neonates when compared with adults, which is consistent with increased incidence of severe inflammatory diseases in newborns (2). We have previously demonstrated that expression of proapoptotic proteins, including members of the Bcl-2 family, as well as FasR and caspase-3, is reduced in neonatal when compared with adult neutrophils, suggesting that specific developmental defects contribute to impaired apoptosis in neonates (2).…”

The Role of Neutr Ophils and Their Apoptosis in There Solution of Inflammation

“…Thus, compared to adults, the markers of apoptosis such as caspase 3 activity, histone-associated DNA fragments and strand breaks are reduced in neonatal neutrophils. The proapoptotic Bcl-2-family proteins are also decreased in the neutrophils of a newborn relative to adult cells [77]. These features mean that programmed cell death is impaired.…”

“…Thus, the delayed programmed death of these cells contributes to the pathogenesis of respiratory distress-syndrom, bronchopulmonary dysplasia and necrotizing enterocolitis, improving the resolution of an inflammatory response, prolonging tissue injury and causing the acute course of the disease [77,78].…”