Platelet
fibrinogen binding in whole blood has been measured in vitro by flow cytometry using a commercially available,
fluorescein isothiocyanate (
FITC)-conjugated polyclonal antifibrinogen antibody.
Fibrinogen-antifibrinogen
immune complexes were formed in experimental conditions approaching
antigen-antibody equivalence, but optimal reaction conditions in which their formation was prevented or minimized could be achieved.
Immune complex formation was associated with
fibrinogen binding to unstimulated platelets but did not significantly affect
ADP-induced
fibrinogen binding. Half-maximal
fibrinogen binding occurred at about 0.4 microM
ADP, and
ADP-induced
fibrinogen binding continued progressively during 20 min incubation with 10 microM
ADP.
Fibrinogen binding correlated closely with
platelet glycoprotein IIb-IIIa expression in members of a family with Glanzmann's
thrombasthenia, and, in double labelling experiments, with the binding of PAC1, a
monoclonal antibody that binds to
GP IIb-IIIa only after the exposure of
fibrinogen receptors. These studies show that platelet
fibrinogen binding can be reliably measured in whole blood by means of a polyclonal antifibrinogen antibody which does not discriminate between plasma and platelet-bound
fibrinogen, despite the presence of an approximately 100-fold excess of the former.