Summary
Inappropriate platelet activation is a feature of acute and chronic diseases such
as disseminated intravascular coagulation (DIC) and atherosclerosis. Since proinflammatory
microbial-derived agonists can be involved in the pathogenesis of these diseases,
we examined the potential role ofTLR4 (mediating responses to LPS) andTLR2 (which
responds to bacterial lipopeptides) in platelet activation. Our data suggested low-level
expression of TLR2 andTLR4 on platelets, determined by flow cytometry, and we also
observed expression of TLR4 on a megakaryocytic cell line by both flow cytometry and
immunohistochemistry. Stimulation of the platelets with the TLR4 agonist LPS, and
the synthetic TLR2 agonist Pam3CSK4, resulted in no platelet aggregation, no increase in CD62P surface expression and
no increase in the cytosolic concentration of Ca2+. The TLR agonists were also unable to directly activate platelets primed with epinephrine,
or pretreated with a low concentration ofADP or PAF. Pretreatment of platelets with
LPS or Pam3CSK4 also failed to modulate the platelet response to submaximal concentrations of the
classical platelet agonists ADP and PAF. We conclude that theTLR agonists LPS and
Pam3CSK4 have no direct effect on platelet activation and that platelet TLRs may be a remnant
from megakaryocytes. TLR2 and TLR4 agonists are thought to have a significant role
in diseases such as atherosclerosis and DIC, but our research suggests that this is
through a mechanism other than direct platelet activation or by modification of platelet
responses to other agonists.
Keywords
Platelets - TLR - LPS - atherosclerosis