Sequential activation of class IB and class IA PI3K is important for the primed respiratory burst of human but not murine neutrophils

It is well established that preexposure of human neutrophils to proinflammatory cytokines markedly augments the production of reactive oxygen species (ROS) to subsequent stimuli. This priming event is thought to be critical for localizing ROS to the vicinity of the inflammation, maximizing their role in the resolution of the inflammation, and minimizing the damage to surrounding tissue. We have used a new generation of isoform-selective phosphoinositide 3-kinase (PI3K) inhibitors to show that ROS production under these circumstances is regulated by temporal control of class I PI3K activity. Stimulation of tumor necrosis factor-alpha (TNF-alpha)-primed human neutrophils with N-formylmethionyl-leucyl-phenylalanine (fMLP) results in biphasic activation of PI3K; the first phase is largely dependent on PI3K gamma, and the second phase is largely dependent on PI3K delta. The second phase of PI3K activation requires the first phase; it is this second phase that is augmented by TNF-alpha priming and that regulates parallel activation of ROS production. Surprisingly, although TNF-alpha-primed mouse bone marrow-derived neutrophils exhibit superficially similar patterns of PI3K activation and ROS production in response to fMLP, these responses are substantially lower and largely dependent on PI3K gamma alone. These results start to define which PI3K isoforms are responsible for modulating neutrophil responsiveness to infection and inflammation.