Leukocyte PI3Kγ and PI3Kδ have temporally distinct roles for leukocyte recruitment in vivo

Phosphoinositide 3-kinases (PI3Ks) have been considered important in leukocyte motility. PI3K gamma, the class I-B PI3K, expressed prominently in leukocytes and also in endothelial cells, mediates leukocyte functional responses induced by chemoattractants. To reveal its role in leukocyte recruitment, we used intravital microscopy to directly visualize leukocyte rolling, adhesion, and emigration in postcapillary venules in PI3K gamma-deficient (PI3K gamma(-/-)) mice. We report here that PI3K gamma deficiency had no significant effects on leukocyte rolling flux or rolling velocity and minor effects on adhesion (30% to 35%) in response to CXC chemokine MIP-2 (CXCL2) or KC (CXCL1). However, leukocyte emigration was severely impaired in PI3K gamma(-/-) mice in an early (first 90 minutes) response to MIP-2 or KC. Chimeric mice receiving bone marrow transplants revealed that this early response was entirely dependent upon PI3K gamma in neutrophils but not parenchymal cells (endothelium and others). Identical responses were observed when endogenous chemokine production was induced by TNF alpha; leukocyte emigration was reduced in PI3K gamma(-/-) mice. More prolonged responses to MIP-2 (for 4 to 5 hours) or TNFo, (6 to 8 hours) were almost entirely PI3K gamma independent and largely dependent on PI3K gamma. Our results reveal that leukocyte emigration response to CXC chemokines is entirely dependent upon PI3K gamma or PI3K gamma, but these are nonoverlapping, temporally distinct events in inflamed tissues in vivo.