Blood platelets are circulating stores for adhesive proteins, inflammatorymediators, and immunoglobulins - Role in nonhemolytic transfusion reactions

Objective: Blood platelets store within their alpha-granules a variety of s ubstances that are involved in blood clotting, inflammation, and repair pro cesses after a lesion of the vessel wall. Not all of these substances are s ynthesized within the megakaryocytes or platelets themselves, but some of t hem are taken up from the blood plasma by the circulating platelet. This ar ticle summarizes recent investigations on the internalization capabilities of platelets and the mechanism of endocytosis. Results: By means of immunoe lectron microscopy and by incubations with gold-labeled peptides the uptake of fibrinogen, IgG, IgE, and the chemokine RANTES by the platelet. can be- demonstrated. Two different mechanisms.:of internalization are discerned: i n resting platelets, the gold-labeled peptides are endocytosed and transfer red to a-granules, whereas in platelets stimulated by ADP or thrombin the g old particles appear within the open canalicular system and not within gran ules. Conclusion: Platelets store and release not only proinflammatory mediators that are typical for the megakaryocyte-platelet lineage,but also blood-born e substances from other cellular sources. Due to activation and disintegrat ion of platelets that are stored as concentrates far usage in transfusion m edicine, these substances can accumulate in the: platelet concentrate and m ight be responsible Tor transfusion-associated reactions. Further investiga tion in this field of platelet immunobiology is warranted.