INTRACELLULAR SITES FOR STORAGE AND RECYCLING OF C3B RECEPTORS IN HUMAN NEUTROPHILS

Upon activation of human neutrophils, C3b receptors (type 1 complement receptors, CR1) are rapidly translocated from an intracellular pool to the surface, increasing plasma membrane expression 6- to 10-fold. This is followed by reinternalization and degradation of the receptors, even in the absence of ligand. Upregulation of surface CR1 may occur without exocytosis of primary or secondary granules, and intracellular CR1 in resting neutrophils does not sediment with these granules on density gradient fractionation. To directly localize the intracellular pools of CR1, we used immunoelectron microscopy of fixed, permeabilized cells. In resting neutrophils, CR1 is associated with the membranes of smooth-surfaced, empty-appearing vesicles whose irregular borders clearly distinguish them from primary and secondary granules. After activation by fMet-Leu-Phe, the intracellular pool is primarily found in large, conspicuous, multivesicular bodies. These bodies also incorporate colloidal gold from the extracellular fluid, suggesting that they are formed by endocytosis. Thus, the sites in which CR1 is stored in resting cells, and recycled in activated cells, are structurally unique and morphologically distinguishable.