Platelets do not express the oxidized or reduced forms of tissue factor.

Area of Science:

• Hematology
• Immunology
• Biochemistry

Background:

• The expression of tissue factor (TF) antigen and activity in platelets remains controversial, with results varying based on laboratory methods and reagents.
• Two forms of TF have been described: an oxidized functional form and a reduced nonfunctional form, interconvertible via disulfide bond formation.
• This study investigated whether differential expression of these two TF forms explains the discrepancies in reported platelet TF expression.

Purpose of the Study:

• To test the hypothesis that differential expression of oxidized (functional) and reduced (nonfunctional) tissue factor (TF) forms accounts for conflicting reports on platelet TF expression.
• To definitively determine if platelets express either form of TF.

Main Methods:

• Utilized specific reagents recognizing both oxidized and reduced TF for flow cytometry and western blotting of platelets.
• Employed TF-dependent activity assays, including Factor Xa (FXa) and Factor IXa (FIXa) generation, and thromboelastography, to confirm findings.
• Western blotting of placental TF was used to validate antibody specificity for both TF forms.

Main Results:

• A sheep anti-human TF polyclonal antibody recognized both reduced and oxidized TF forms, unlike an antibody specific to oxidized TF.
• Flow cytometry and Factor VIIa (FVIIa) binding assays showed no TF expression on the surface of unactivated or activated platelets.
• Western blotting and functional assays confirmed the absence of both TF forms and TF-related activity in platelets and whole blood.

Conclusions:

• Platelets from healthy donors do not express either the oxidized (functional) or reduced (nonfunctional) forms of tissue factor (TF).
• This study resolves the controversy surrounding platelet TF expression, indicating its absence in healthy individuals.