Mechanisms and implications of phosphoinositide 3-kinase delta in promoting neutrophil trafficking into inflamed tissue.

The phosphoinositide 3-kinase (PI3K) catalytic subunit p110 delta is expressed in neutrophils and is thought to play a role in their accumulation at sites of inflammation by contributing to chemoattractant-directed migration. We report here that p110 delta is present in endothelial cells and participates in neutrophil trafficking by modulating the proadhesive state of these cells in response to tumor necrosis factor alpha (TNF alpha). Specifically, administration of the selective inhibitor of PI3K delta, IC87114, to animals reduced neutrophil tethering to and increased rolling velocities on cytokine-activated microvessels in a manner similar to that observed in mice deficient in p110 delta. These results were confirmed in vitro as inhibition of this isoform in endothelium, but not neutrophils, diminished cell attachment in flow. A role for PI3K delta in TNF alpha-induced signaling is demonstrated by a reduction in Akt-phosphorylation and phosphatidylinositol-dependent kinase 1 (PDK1) enzyme activity upon treatment of this cell type with IC87114. p110 delta expressed in neutrophils also contributes to trafficking as demonstrated by the impaired movement of these cells across inflamed venules in animals in which this catalytic subunit was blocked or genetically deleted, results corroborated in transwell migration assays. Thus, PI3K delta may be a reasonable therapeutic target in specific inflammatory conditions as blockade of its activity reduces neutrophil influx into tissues by diminishing their attachment to and migration across vascular endothelium.

Selective role of PI3K delta in neutrophil inflammatory responses.

Although members of the class I phosphoinositide 3-kinases (PI3Ks) have been implicated in neutrophil inflammatory responses, the contribution of the individual PI3K isoforms in neutrophil activation has not been tractable with the non-selective inhibitors, LY294002 and wortmannin. We have developed a novel series of PI3K inhibitors that is selective for PI3K delta, an isoform expressed predominantly in hematopoietic cells. In addition to being selective between members of class I PI3Ks, representatives of these inhibitors such as IC980033 and IC87114 did not inhibit any protein kinases tested. Utilizing these inhibitors we report here a novel role for PI3K delta in neutrophil activation. Inhibition of PI3K delta with IC980033 and IC87114 blocked both fMLP- and TNF1 alpha-induced neutrophil superoxide generation and elastase exocytosis. The PI3K delta inhibitor IC87114 also blocked TNF1 alpha-stimulated elastase exocytosis from neutrophils in a mouse model of inflammation. To our knowledge, this is the first in vivo efficacy demonstration of a PI3K delta inhibitor in an animal model. Inhibition of PI3K delta, however, had no effect on in vitro neutrophil bactericidal activity and Fc gamma R-stimulated superoxide generation. Thus, PI3K delta plays an essential role in certain signaling pathways of neutrophil activation and appears to be an attractive target for the development of an anti-inflammatory therapeutic.