Secretory vesicles are the intracellular reservoir of complement receptor 1 in human neutrophils

The subcellular localization of complement receptor 1 (CR1) was investigated in human neutrophils. CR1 was located exclusively in the light membrane fractions containing secretory vesicles and plasma membranes in Percoll density gradients of unperturbed neutrophils. Separation of plasma membranes from secretory vesicles by high-voltage free-flow electrophoresis of the light membranes from the Percoll gradient revealed that more than 80% of the CR1 was located intracellularly in secretory vesicles. After weak stimulation of neutrophils, the increase in CR1 surface expression closely paralleled both surface increase of known secretory vesicle membrane Ags and the release of matrix proteins from secretory vesicles. More potent stimulation of the neutrophils did not enhance CR1 surface expression further, in agreement with the lack of CR1 in granules as demonstrated on Percoll gradient. CR3 (Mac-1), which has been shown to be located both in secretory vesicles and in neutrophil granules, was up-regulated in parallel with CR1 after weak stimuli, whereas a profound increase of CR3 was observed after more potent stimuli in accordance with granule mobilization. These results identify secretory vesicles as the reservoir of CR1, which translocates to the plasma membrane after weak stimulation, and underscore the functional significance of this recently identified organelle.