Platelets sialylate specific surface glycoproteins in neighboring cells. (A) Sialylation of GPIbα in ST3GalIV^−/− mouse platelets by activated human platelets. Human platelets were identified and separated from mouse platelets (G₁) by flow cytometry using PE-conjugated anti–human CD61. Flow cytometric analysis of terminal β-galactose using ECL-FITC-labeled lectin on mouse ST3GalIV^−/− platelets (G1) incubated with TRAP-activated human platelets (TRAP) or resting platelets (Rest) in the absence of CMP-SA. Relative binding of ECL-FITC to ST3GalIV^+/+ (WT) and ST3GalIV^−/− (KO) platelets, and to ST3GalIV^−/− platelets incubated with medium from resting (Rest) or TRAP-activated human platelets (TRAP) with (+SA) and without CMP-SA (-SA). (B) Histograms report the mean ± SEM. SA indicates CMP-SA (n = 3). (C) Incorporation of sialic acid in GPIbα. ST3GalIV^+/+ (WT) and ST3GalIV^−/− (KO) mouse platelets were incubated with supernatants from resting (Rest) or TRAP (TRAP)–activated human platelets with (+CMP-SA) or without (-CMP-SA) CMP-SA. Lysates were subjected to SDS-PAGE and immunoblotted with sWGA, RCA-I, or Abs to GPIbα and actin (n = 3). (D) Survival of ST3GalIV^+/+ (WT) or ST3GalIV^−/− mouse platelets incubated with resting (Rest) or TRAP-activated human platelet supernatants (TRAP) in the presence (+CMP-SA) or absence (-CMP-SA) of CMP-SA. The y-axis reflects the platelet recovery defined by the number of platelets circulating at the first sample point after injection, expressed as the percentage of circulating nonactivated WT platelets. Shown are means ± SEM for 6 recipients. **P < .01; ***P < .001.