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Fc-mediated endocytosis by human neutrophils. Ultrastructural studies
H B Fleit1, A P Lane, T J Watson
1Department of Pathology, State University of New York, Stony Brook.
Abstract:
Fc Receptors (FcR) mediate the binding and uptake by polymorphonuclear leukocytes (PMN) of antibody-coated particles and soluble immune complexes. We have studied Fc-mediated endocytosis by PMN ultrastructurally using a gold-conjugated monoclonal antibody (3G8) to block or to mark the location of FcR. Phagocytosis of antibody-coated erythrocytes (EIgG) was initiated rapidly after binding to discrete foci on the PMN plasma membrane. After the phagocytosis of EIgG, we examined the distribution of FcR remaining on the PMN plasma membrane. 3G8-Colloidal gold continued to bind to PMN after ingestion of up to three EIgG, demonstrating that all PMN FcR are not utilized during a brief phagocytic event. The endocytosis of soluble immune complexes was examined by labeling plasma membrane-bound rabbit immune complexes with goat anti-rabbit IgG conjugated to colloidal gold. Gold was found in clusters randomly distributed over the plasma membrane at 4 degrees C. When cells were warmed to 37 degrees C, numerous endocytic vesicles were observed as early as 2.5 minutes after warming. After 30 minutes at 37 degrees C, large vesicles, 1 micron in diameter, were found to contain 20 to 30 gold particles. The endocytosis of 3G8 was also examined using colloidal gold. After binding of 3G8-gold at 4 degrees C, clusters of large vesicles, up to 2 micron in diameter, were rapidly formed at 37 degrees C.
Insights
Polymorphonuclear leukocytes (PMN) utilize Fc receptors (FcR) for endocytosis. Studies show FcR are not fully depleted after phagocytosis and form large vesicles during immune complex uptake.
Area of Science:
- Immunology
- Cell Biology
Background:
- Fc Receptors (FcR) on polymorphonuclear leukocytes (PMN) facilitate the uptake of antibody-coated particles and immune complexes.
- Understanding FcR-mediated endocytosis is crucial for immune response mechanisms.
Purpose of the Study:
- To ultrastructurally investigate Fc-mediated endocytosis in PMN.
- To examine the distribution and utilization of FcR during phagocytosis and immune complex uptake.
Main Methods:
- Utilized gold-conjugated monoclonal antibody (3G8) to label FcR on PMN.
- Observed phagocytosis of antibody-coated erythrocytes (EIgG) and endocytosis of soluble immune complexes.
- Examined FcR distribution and vesicle formation using electron microscopy at different temperatures.
Main Results:
- FcR remained available on the PMN surface even after ingesting multiple antibody-coated erythrocytes.
- Soluble immune complexes triggered rapid formation of endocytic vesicles containing gold particles within minutes at 37°C.
- Uptake of the FcR-specific antibody (3G8-gold) also led to rapid formation of large vesicles.
Conclusions:
- Not all FcR are engaged during a single phagocytic event, indicating reserve capacity.
- PMN efficiently internalize soluble immune complexes and FcR via distinct endocytic pathways involving large vesicles.