Localized exocytosis of primary (lysosomal) granules during phagocytosis: role of Ca2+-dependent tyrosine phosphorylation and microtubules

The uptake and killing of bacteria by human neutrophils are dependent on the fusion of secretory granules with forming phagosomes. The earliest component of exocytosis was found to precede phagosome closure, so that granular membrane constituents were detectable on the plasmalemma. We show that during phagocytosis of IgG-opsonized particles, this early secretory response is highly polarized in the case of primary granules, but less so for specific granules. The vectorial discharge of primary granules was dependent on calcium, but no evidence was found that calcium is involved in determining the polarity of exocytosis. In particular, a redistribution of endomembrane calcium stores toward forming phagosomes could not be detected. Polarized granule exocytosis was accompanied by focal tyrosine phosphorylation and actin polymerization, although the latter was not required for the response. Instead, microtubules seemed to contribute to the vectorial nature of the response. During particle ingestion, the microtubule-organizing center relocated toward forming phagosomes, and colchicine treatment altered the pattern of exocytosis, reducing its directionality. We hypothesize that the focal activation of tyrosine kinases generates localized signals that induce exocytosis in a calcium-dependent manner, and that reorientation of microtubules facilitates preferential delivery of granules toward the forming phagosome.