| The polymorphic neutrophil is an abundantly produced immune cell charged with protecting the host organism from potentially harmful prokaryotes and fungi. Neutrophils utilize an array of defenses to neutralize these microorganisms including reactive oxygen production and proteolytic enzymes which degrade an assortment of microbial proteins. Of particular importance to the effective clearance of bacteria is the enhanced survival of the normally short-lived neutrophil. The half-life of the neutrophil, which undergoes apoptosis rapidly both in vitro and in vivo, can be extended through treatment with several microbial metabolites including lipopolysaccharide. The stress hormone glucocorticoids (GC)s also prolong the life of the neutrophil which is especially surprising given the detrimental effects of these compounds on other immune cell types (e.g., lymphocytes). The induction of the stress response and a corresponding increase in neutrophil production at the expense of other immune cells likely represents a shift in strategy for dealing with microorganisms during periods of stress and is the subject of investigation presented herein.;We observed a rapid translocation of the Glucocorticoid Receptor (GR) to the nucleus of neutrophils within 30 minutes of GC exposure. Like other members of this family of proteins, ligand-activated GR migrates to the nucleus where it affects gene expression through direct DNA binding and indirect protein-protein interactions. Of the perhaps hundreds of genes induced by GCs, induction of glucocorticoid-induced leucine zipper (GILZ) has shown particular promise in playing a role in GC-mediated neutrophil survival as this protein specifically promotes the survival of T-lineage cells in response to GCs. Similarly, induction of both GILZ mRNA and protein was observed in GC-treated neutrophils, also lending to the suggestion that this protein participates in the survival of these cells. Since several genes related to apoptosis are likely to be altered by GCs, apoptosis-centric microarray analysis was performed on neutrophils treated with GCs. Using this broad approach, 25 genes were identified whose expression changed at least 2.0-fold in response to GCs. Among these gene candidates, a 2.0-fold decrease in Bid mRNA was detected in response to GCs. Further analysis of Bid revealed that GC treatment resulted in loss of both Bid mRNA as well as the apoptogenic t-Bid protein. Moreover, both upstream (FasL) and downstream (caspase 8) molecules involved in death receptor apoptosis signaling were similarly downregulated by GCs. Finally, treatment of neutrophils with a BH3-only cell-permeable peptide representing the apoptosis-inducing domain of Bid resulted in a significant increase in apoptosis of these cells. Thus, the loss of Bid likely represents an important mechanism through which GCs act to promote neutrophil survival. |
