Documentation of galactosylation of human platelets. (A) Human apheresis platelets were galactosylated by incubation with UDP-galactose. Apheresis platelets from 12 collections were incubated with 1.2 mM UDP-galactose for 1 hour at 37°C and characterized by FACS with fluorescently labeled lectins RCA-1 (■) and sWGA (□) recognizing penultimate β-galactose and βGlcNAc, respectively. Incubation with UDP-galactose resulted in an increase in RCA-1 binding and a decrease in sWGA binding (right columns) compared with platelets incubated without UDP-galactose (left columns). Data are shown as relative MFI (mean fluorescence intensity) plus or minus 1 standard deviation (SD) with the value for the nonglycosylated platelets set at 1. (B) Galactosylation inhibits the phagocytosis of human platelets stored at 4°C for 48 hours by differentiated THP-1 cells expressing MAC-1 (α_Mβ₂ integrin receptor). The in vitro phagocytosis of fresh human platelets by THP-1 cells was set at 1. Comparisons were made between the in vitro phagocytosis of cold-stored (4°C) platelets with and without pretreatment with UDP-galactose (n = 3, ± 1 SD indicated by error bars).