Background: Millions of platelet transfusions are given each year. Transfusion reactions occur in as many as 30% of patients receiving unmodified platelet transfusions. The cause of some transfusion reactions remains unclear. The current paradigm suggests that platelet concentrates (PC) contain proinflammatory mediators that are released by white blood cells during collection, processing and storage. CD154 (CD40 ligand, CD40L) is a potent inflammatory mediator, normally sequestered inside the resting platelet, that is known to translocate to the platelet membrane and be shed into plasma in response to agonist activation. We hypothesized that platelet-soluble CD154 (sCD154) is 'spontaneously' released by transfused platelets and plays a major role in transfusion reactions.
Objectives: To determine the time course and biological properties of CD154 translocation and release during collection and storage of platelets for transfusion.
Methods: We measured surface and sCD154 in platelets prepared by the platelet-rich plasma method or apheresis by fluorescence-activated cell sorting and enzyme-linked immunosorbent assay, respectively. The specific biological activity of platelet sCD154 was assayed by stimulation of the CD154/CD40 pathway in known CD40-positive cells with PC-derived supernatants.
Results and conclusions: We demonstrate that PCs prepared for transfusion have high levels of membrane-bound CD154 and sCD154, with maximum levels being seen 72 h after platelet collection. Importantly, we show that platelet-derived sCD154 potently stimulates CD40-positive cells. We propose that platelet-derived CD154 is a key 'cytokine' responsible for adverse reactions associated with platelet transfusions. Improved methods of platelet collection and/or storage, which limit CD154 expression, could reduce the risks of transfusion reaction.