Activation of human neutrophils by phorbol ester decreases the cytoplasm compactness and the lactoferrin content of the granulocytes.

Human neutrophils isolated from the blood were incubated in vitro for 30 min with 25 ng/ml of phorbol 12-myristate 13-acetate (PMA), a protein kinase C activator, to study its effect on the fine structure of granulocytes and on the subcellular distribution of lactoferrin (Lf). Flow cytometry analysis of the human neutrophils showed that PMA induced a decrease in size and granularity of the cells. By electron microscopy, the PMA-treated cells showed numerous empty vesicles, smoothing of the cell surface, and a marked decrease in the compactness of the cytoplasmic ground substance. High-resolution immunogold ultracytochemistry showed that neutrophils lost most of their content in Lf after the PMA incubation. In thin sections, the cytoplasmic compartment of PMA-treated neutrophils contained an average of 1.03 +/- 0.3 Lf-positive vesicles per microns 2, whereas control granulocytes showed an average of 5.72 +/- 1.49 Lf vesicles per microns 2 of cytoplasm. Most of PMA-treated neutrophils kept some Lf-positive vesicles; a significant minority (10-15%) of the granulocytes showed cytological features of cell death. We conclude that the PMA-induced neutrophil activation is associated with microanatomical changes that include i) exocytosis of most, but not all, of the Lf-bearing vesicles; ii) rounding up of the cell outline; and iii) decrease in the compactness of the cytoplasmic ground substance.