Summary
We investigated the interactions of vesicles containing human tissue factor (TF) with
platelets and evaluated responses induced by rFVIIa using standard aggregometry, ultrastructural
and flow-cytometry techniques. Washed platelets were exposed to a preparation of placental
human TF (pTF) or to a relipidated formulation of recombinant humanTF (rTF). Under
stirring conditions, pTF induced reversible aggregation with platelets returning to
their resting state after 5 minutes. This reversible response to pTF was partially
inhibited by antibodies against CD62-P, but not by antithrombin agents, and was not
observed with rTF. Sequential ultrastructural studies revealed uptake of both TF preparations
by platelets involving traffic of vesicles through channels of the open canalicular
system (OCS). Immunocytochemical studies on cryosections identified TF in the OCS,
and occasionally in the alpha-granules of the platelets. These processes were faster
with pTF than with rTF, but bothTF preparations accumulated in platelets at the end
of incubation periods. Flow cytometry studies revealed the presence of other cellular
antigens (CD62-P, CD14 and CD45) associated to the pTF. Addition of rFVIIa to washed
platelets exposed to pTF or rTF, caused a thrombin dependent irreversible platelet
aggregation. Our studies demonstrate that platelets possess mechanisms to capture
and incorporate TF-rich vesicles. These processes are accelerated by the presence
of other cellular antigens in the vesicles. Our findings may explain the hemostatic
action of rFVIIa in severely hemodiluted patients, but are also relevant for the understanding
of potential implications of TF-associated to platelets in the propagation of thrombus.
Keywords
Platelets - tissue factor - vesicles - traffic - rFVIIa