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The human neutrophil FcRIII (CD16) on the plasma membrane can be replenished from an intracellular source
A L Kiss1, C R Jost, J A Fransen
1Laboratory I of Electron Microscopy, Semmelweis University of Medicine, Budapest, Hungary.
Abstract:
The internalization of peroxidase-antiperoxidase (PAP) immune complexes by human neutrophil granulocytes was studied at an ultrastructural level. PAP initially bound to the plasma membrane at 4 degrees C accumulated in endosomes within 5 min of internalization. By that time, all of the ligand bound to the plasma membrane had been removed from the cell surface and the cells were able to bind newly added PAP. Pre-embedding labelling of FcRIII on these cells showed that this receptor was present on the cell surface, indicating involvement of FcRIII in the rebinding of PAP in neutrophils. The origin of FcRIII present on the plasma membrane after Fc receptor-mediated internalization of PAP was investigated in another series of experiments. Incubation of the cells with pronase eliminated the epitope on the Fc receptor recognized by anti-FcRIII. After the pronase treatment hardly any Fc receptors were detected on the plasma membrane. However, incubation of the cells for only 5 min in a protease-free medium after the pronase treatment led to an abundance of FcRIII on the plasma membrane of the neutrophils. These findings support the hypothesis that FcRIII on the plasma membrane of human neutrophil granulocytes is replenished from an internal source of free Fc receptors and suggest that at least some of the receptors present on the cell surface after the binding and internalization of PAP originate from this source in the cytoplasm.
Insights
Human neutrophils internalize peroxidase-antiperoxidase (PAP) immune complexes. Fc receptor III (FcRIII) is involved in rebinding PAP, with surface receptors replenished from internal cytoplasmic sources.
Area of Science:
- Immunology
- Cell Biology
- Hematology
Background:
- Neutrophils play a critical role in the innate immune system.
- Immune complex uptake by neutrophils is essential for pathogen clearance.
- Fc receptors mediate the binding and internalization of immune complexes.
Purpose of the Study:
- To investigate the ultrastructural mechanism of peroxidase-antiperoxidase (PAP) immune complex internalization by human neutrophils.
- To determine the role of Fc receptor III (FcRIII) in PAP rebinding.
- To elucidate the origin of FcRIII on the neutrophil plasma membrane after immune complex internalization.
Main Methods:
- Ultrastructural analysis of human neutrophil granulocytes.
- Pre-embedding labeling of FcRIII.
- Pronase treatment to assess Fc receptor epitope accessibility.
- Incubation in protease-free medium to observe FcRIII re-expression.
Main Results:
- PAP immune complexes were internalized into endosomes within 5 minutes.
- Neutrophils regained the ability to bind new PAP after initial internalization.
- FcRIII was detected on the cell surface, indicating its involvement in PAP rebinding.
- Pronase treatment removed FcRIII from the plasma membrane, but receptors reappeared after short incubation in a protease-free medium.
Conclusions:
- FcRIII mediates the rebinding of PAP by human neutrophils.
- The plasma membrane FcRIII is replenished from an internal cytoplasmic pool.
- This replenishment mechanism ensures continued immune complex binding capacity of neutrophils.