Serum protects against azurophil granule dependent down-regulation of complement receptor type 1 (CR1) on human neutrophils.

We have investigated the effect of gradual degranulation on the expression of functional receptors (CR1 and CR3) on human neutrophils. Incubation with increasing concentrations of fMLP (10(-10) - 10(-7) M) translocated CR1 and CR3 to the cell surface in a similar kinetic pattern. When reaching maximal expression of receptors (10(-7) M fMLP), 78 +/- 10% and 87 +/- 9% of the total pool of CR1 and CR3, respectively, were translocated to the cell surface. To drive the mobilization process further, cytochalasin B was introduced to increase the stimulatory effect of fMLP. No further increase in CR1 surface expression was obtained. However, we found a characteristic time course of surface appearance of CR1 and CR3 with a maximal surface expression within 1 minute, followed by a time-related down-regulation of CR1 but not CR3. In addition, the total pool of CR1 in cytochalasin B treated neutrophils was reduced after 15 minutes stimulation with fMLP measured by flow cytometry and immunoblotting, indicating degradation of CR1. The down-regulation of CR1 was concomitant with a translocation of azurophil granules, in terms of upregulation of CD63. Azurophil, but not specific nor secretory, granule fractions caused a down-regulation of CR1 on fMLP activated neutrophils. The presence of human sera and serine protease inhibitor protected CR1 from down-regulation. Together, these findings indicate that intracellular stored proteases, released in the late part of the sequential mobilization process, alters the expression of functional receptors mobilized in the early part of the mobilization process. The findings also focus on the importance of the microenvironment for the net outcome of neutrophil activation in terms of functional receptor expression.