Complement component C3 binds to activated normal platelets without preceding proteolytic activation and promotes...

Area of Science:

• Immunology
• Hematology
• Biochemistry

Background:

• Complement activation on activated platelets is reported, despite existing regulatory mechanisms.
• Understanding how activated platelets bind complement components is crucial for distinguishing physiological from pathological processes.

Purpose of the Study:

• To investigate the mechanisms of complement protein binding to activated platelets.
• To determine if complement activation occurs on platelet surfaces under physiological conditions.

Main Methods:

• Flow cytometry and Western blot analyses were used to detect complement proteins on nonactivated and activated platelets.
• Experiments involved inhibiting complement activation, diluting platelet-rich plasma, chelating divalent cations, and using monoclonal antibodies (mAbs) against C3 forms.
• Soluble complement receptor 1 was used to assess the form of bound C3.

Main Results:

• C1q, C4, C3, and C9 bound to activated platelets.
• Complement deposition was independent of complement activation, as inhibition at C1q or C3 levels did not block C3 binding.
• Bound C3 was identified as C3(H(2)O), a non-proteolytically activated form, which binds soluble complement receptor 1.

Conclusions:

• Complement does not activate on platelet surfaces under physiological conditions.
• The binding of C3(H(2)O) to activated platelets is independent of complement activation pathways.
• This finding contrasts with pathological conditions where complement dysregulation leads to thrombocytopenia and thrombotic disease.