Preservation of Platelet Function By Cold Storage for Perfusion

Platelets (thrombocytes) are enucleated blood cells formed by the fragmentation of megakaryocytes, mediating important physiological functions such as hemostatic response making them vital for survival. Platelets are very small cells, 2-3μm in diameter with a physiological concentration of 150,000-400,000 /μl of blood with a lifespan of 7-10 days. Thrombocytopenia, a medical condition where platelet counts can decline to as low as 10,000 per microliter, are often encountered with hematologic malignancies or blood loss during trauma. Platelet transfusions become imperative in such plights to decrease mortality; especially for cardiovascular surgeries, intensive cancer therapies, surgery or trauma to restore hemostasis. Thus, there is a very high demand of platelets transfusion and on an average 9 million platelet products are being used in the US for various therapeutic and prophylactic transfusions.

Currently, platelets for transfusion are stored at room temperature (20-24°C, RT) in gas-permeable bags with constant agitation for up to 5 days. This storage method is in practice for the past many decades for maximizing recovery and survival in vivo after transfusion.16 Ironically, this storage is far from ideal and more than a billion dollar is being wasted for medical expense annually because of expired shelf life or microbial contamination. Since platelets have no reliable substitute as of now, it is highly essential to devise more efficient platelet storage methods to save lives during life threatening traumas. It has been demonstrated by randomized clinical trials that platelets stored at 4°C for 72 h, either as whole blood or platelet concentrates, shortened the bleeding time and establish hemostasis in patients better than platelets stored at 22°C.17,18 In light of these and other similar studies, re-examination of platelet refrigeration is currently underway at several centers to determine whether decreased accumulation of deficits and preservation of immediate hemostatic function is associated with better outcome after traumatic hemorrhage compared to longer in vivo circulation times.

In this study, we characterized the consequences of cold storage on platelet adhesion to von Willebrand (VWF)-coated surfaces using a microfluidic assay, and platelet aggregation using a cone-and-plate rheometer at steady and complex patterns of physiological shear rates of 500 - 2,500 s-1. We observed that platelets stored at 4 °C for 2-5 days showed significantly more adhesion and aggregation compared to fresh platelets or platelets stored at RT. We also observed that platelets stored at 4 °C took significantly shorter time to clot upon addition of exogenous thrombin, and formed stronger clots as quantified by the rheological parameters including storage modulus and viscosity measured by small amplitude oscillatory and steady shear assays. Our data suggest that the cold storage yields platelets with superior functionality compared to room temperature storage and can be a potential candidate for long term storage.