The release of prion protein from platelets during storage of apheresis platelets

BACKGROUND: Recent studies using a time-resolved fluoroimmunoassay method ( dissociation-enhanced lanthanide fluoroimmunoassay) showed that platelets a nd plasma are the main reservoir of the normal isoform of cell-associated p rion protein (PrPc) in human blood. The aims of the present study were to m onitor PrPc levels in various fractions of apheresis platelets during stora ge by using the DELFIA method and to assess the association of this release with alpha-granule protein P-thromboglobulin and cytoplasmic LDH. STUDY DESIGN AND METHODS: Units of apheresis platelets (n = 6) were obtaine d from volunteer donors by the use of a cell separator and stored up to 10 days. Samples (7-9 mt) were aseptically collected from each unit on storage Days 1, 2, 3, 4, 5, 8, and 10. Platelet-poor plasma and apheresis platelet s were prepared and the former split into two fractions, one centrifuged at 40,000 x g for 2 hours at 4 degrees to remove microparticles. The spun mic roparticles, apheresis platelets and platelet samples, platelet-poor plasma , and high-spun plasma fractions were stored in a frozen state until they w ere tested. RESULTS: The results showed that the mean overall levels of PrPc throughout storage remained within 15 percent of Day 1 levels. In contrast, the mean cellular levels in platelets significantly decreased to 46 percent of Day 1 levels by Day 10 of storage (p<0.01), while the corresponding levels in pl asma significantly rose as much as 329 percent (p<0.01). Moreover, although microparticle-bound PrPc was released during storage, it was increasingly superseded by soluble protein. PrPc and P-thromboglobulin release exhibited very similar patterns (p<0.01). In contrast, LDH showed a significant incr ease in high-spun plasma only toward the end of the storage period (p<0.01) . CONCLUSION: These results indicate that PrPC is released from platelets dur ing the storage of apheresis platelets and that this release is probably du e mainly to platelet activation and alpha-granule release in the first few days of storage. Moreover, the released PrPC is increasingly composed of so luble proteins, as the storage period exceeds 5 days.