Intracellular sites for storage and recycling of C3b receptors in human neutrophils
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M Berger1, E M Wetzler, E Welter
1Department of Pediatrics, Case Western Reserve University School of Medicine, Cleveland, OH 44106.
Insights
Human neutrophils rapidly move complement receptor 1 (CR1) from internal vesicles to their surface upon activation. These unique intracellular compartments store and recycle CR1, distinguishing them from typical granules.
Area of Science:
- Immunology
- Cell Biology
- Complement System
Background:
- Neutrophils play a key role in the innate immune system.
- Complement receptor 1 (CR1) is crucial for neutrophil function.
- The intracellular storage and trafficking of CR1 are not fully understood.
Purpose of the Study:
- To investigate the intracellular localization of CR1 in resting and activated human neutrophils.
- To characterize the unique cellular compartments involved in CR1 storage and recycling.
Main Methods:
- Immunoelectron microscopy was used to visualize CR1 within neutrophils.
- Cells were fixed and permeabilized for ultrastructural analysis.
- Neutrophil activation was induced using fMet-Leu-Phe.
Main Results:
- In resting neutrophils, CR1 is associated with smooth-surfaced vesicles distinct from granules.
- Upon activation, CR1 is found in large multivesicular bodies, suggesting endocytosis.
- These CR1-containing compartments are morphologically unique and distinguishable.
Conclusions:
- Human neutrophils store CR1 in unique intracellular vesicles, not in primary or secondary granules.
- Activated neutrophils utilize specific endocytic pathways involving multivesicular bodies for CR1 recycling.
- These findings reveal novel mechanisms of immune receptor trafficking in neutrophils.
Abstract:
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.