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Immunohistochemical and functional studies of glycoprotein 60 (gp60) in platelets
S A Mohamadein1, A E Ahmed, M Griffiths
1University Department of Pathology, Western Infirmary, Glasgow, UK.
Insights
Plasma glycoprotein 60 (gp60), an Fc gamma binding protein, is found in platelets and on their membranes. Fluid-phase gp60 may modulate immune complex interactions with platelets.
Area of Science:
- Immunology
- Hematology
- Biochemistry
Background:
- Plasma glycoprotein 60 (gp60) is an Fc gamma binding protein.
- gp60 inhibits complement-mediated prevention of immune precipitation.
Purpose of the Study:
- To investigate the presence and localization of gp60 in human platelets.
- To determine the role of platelet-associated gp60 in platelet activation and immune complex-mediated aggregation.
Main Methods:
- Immunofluorescence microscopy
- Immunoelectron microscopy
- Western blot analysis
- Platelet activation assays using thrombin, calcium ionophore, and immune complexes (IC)
Main Results:
- gp60 was detected in platelet cytoplasm, surface-connecting structures, and on platelet membranes.
- Platelet activation did not induce gp60 secretion.
- Fab anti-gp60 fragments did not inhibit IC-mediated platelet aggregation.
- Purified gp60 inhibited IC-mediated platelet aggregation in a dose-dependent manner.
Conclusions:
- Platelet-associated gp60 is not directly involved in IC-mediated platelet aggregation.
- Fluid-phase gp60 may play a modulatory role in the interaction between IC and platelets.
Abstract:
We showed by immunofluorescence, immunoelectron microscopy and Western blot analysis that the plasma glycoprotein (gp60), an Fc gamma binding protein which inhibits complement-mediated prevention of immune precipitation, is present in platelets. The gp60 content of platelets in normal individuals and patients with rheumatoid arthritis was similar (mean 0.028 and 0.024 fg/platelet respectively). Immunoelectron microscopic studies showed that gp60 was present in the cytoplasm and the surface connecting structures but not in the alpha granules, dense granules or lysosomes. Using this technique gp60 was also found on platelet membranes, an observation which was confirmed by immunofluorescence. Activation of platelets with thrombin, calcium ionophore, and immune complexes (IC) resulted in the release of the contents of the alpha granules (beta-thromboglobulin), dense granules (5-hydroxytryptamine) and lysosomes (beta-glucuronidase) but did not induce gp60 secretion. The inability of Fab anti-gp60 to inhibit IC-mediated platelet aggregation and of F(ab')2 anti-gp60 to produce platelet aggregation suggested that IC-mediated platelet aggregation did not occur as a result of the interaction of IC with platelet gp60. However, as the preincubation of IC with purified gp60 produced dose-dependent inhibition of the ability of IC to aggregate platelets it is possible that fluid-phase plasma gp60 modulates the interaction of IC with platelets.