Escape of phagocytosis-activating function from platelets into plasma during storage

Stimulation of the phagocytic activity of leukocytes by two glycoproteins, the macromolecular activators of phagocytosis 1-MAPP and s-MAPP, involved in platelet release products (PRPr), was not observed when platelet concentrate (PC), stored longer than 48 h, was used as a platelet source. The supernatant of the PC (Sup) acquired this activity during storage. Salting out with ammonium sulfate (AS) revealed that the phagocytosis-enhancing activity was recovered in the supernatant of Sup from PC with 60% saturated AS, and the reverse activity, phagocytosis suppression, was observed in the precipitate with 30% AS. Suppressing activity was observed in PRPr prepared from both fresh and 48-h-old PC. The phagocytosis-enhancing activity observed in the Sup from the 48-hour PC was characterized by Sephadex G200 gel filtration and consisted of two peaks (300 and 150 kD), matching these of l-MAPP and s-MAPP in the PRPr prepared from fresh platelets. These results indicate that l-MAPP and s-MAPP escape from platelets into the plasma during storage, whereas the suppressing activity is retained in platelets for at least 48 h.