Response: Are PDMVs the biologically active source of CD154 in ATR?

In vivo animal data reporting biologic platelet CD154 function,^1-4  coupled with reports of direct biologic activity on isolated primary human neutrophils and B cells,^5-7  indicate platelet CD154 could have clinically significant function. Indeed, recent reports indicate that acute transfusion reactions (ATRs) may result from soluble CD154 (sCD154) released from platelets during storage.^5,8-10  By demonstrating that only platelet concentrates (PCs) that resulted in clinical ATR can stimulate CD154-specific B cell IL-6 production, Cognasse and colleagues make a compelling case for investigation into improved platelet storage that minimizes platelet release of CD154.

Upon activation, platelets can release microparticles and exosomes¹¹  (collectively referred to as platelet-derived membrane vesicles [PDMVs]) that can deliver platelet-derived signals in vitro ^12-14  or in vivo.³  This becomes an important consideration when examining what is thought to be “soluble” CD154. Some studies use centrifugation at g forces too low to separate PDMV from sCD154, while others are unclear about centrifugation speeds. Transwell experiments have also been used, but the small size of PDMVs (0.04-1.0 μm) would have permitted their free diffusion along with sCD154. Separating PDMV CD154 from sCD154 is of significance because we have reported that in the absence of whole murine platelets, the ability to deliver a CD154 signal in vitro or in vivo resides almost exclusively with PDMVs and only minimally with truly soluble CD154.³  This is in spite of the fact that most platelet CD154 is in the soluble fraction and very little remains on PDMVs. We have also made the same observation with human platelets in vitro (manuscript in preparation). It will be important to identify if PDMVs are the biologically active source of CD154 in ATR when considering methods of collection and storage of PCs designed to inhibit release of CD154.