Platelet activation induces increased Fc gamma receptor expression

Area of Science:

• Immunology
• Hematology
• Cell Biology

Background:

• Platelets express Fc gamma RII, but its regulation remains poorly understood.
• Platelet activation by agonists can increase surface protein expression.

Purpose of the Study:

• To investigate the effect of platelet activation on Fc gamma RII expression.
• To quantify changes in Fc gamma RII number per platelet following activation.

Main Methods:

• Flow cytometry was used to measure the binding of fluorescently labeled IgG oligomers and anti-Fc gamma RII mAb (IV.3) to activated and non-activated platelets.
• Quantification of Fc gamma RII per platelet was achieved by correlating flow cytometry data with a standard filtration assay using radiolabeled IV.3.
• Platelets were activated by thrombin, PMA, ADP, or epinephrine at 25°C or 4°C, with or without cAMP-elevating agents.

Main Results:

• Platelet activation with thrombin or PMA at 25°C increased Fc gamma RII expression by 55% (mean 891 to 1382 receptors/platelet).
• ADP caused a smaller increase in Fc gamma RII expression, while epinephrine had no effect.
• The increase in Fc gamma RII expression was dependent on platelet activation (not observed at 4°C or with elevated cAMP) and did not require dense-granule secretion.

Conclusions:

• Platelet activation significantly upregulates Fc gamma RII surface expression.
• This upregulation may involve exposure of receptors from the open canalicular system or alpha-granule fusion.
• Increased Fc gamma RII expression could enhance immune complex clearance and contribute to immune complex-mediated thrombocytopenia.