Syk was required for efficient β₂ integrin–mediated phagocytosis of serum-opsonized E coli. Confocal microscopy images of (A) murine Syk^+/+ and Syk^−/− PMNs and (B-C) piceatannol-treated (PIC) and untreated (control) dHL-60 cells phagocytosing serum-opsonized Alexa 594–conjugated E coli (red). Syk was enriched at the site of particle binding (arrows) in murine Syk^+/+ PMNs (A) and in dHL-60 control cells (B). (B) In contrast, Syk was not translocated in cells after the inhibition of Syk by 30μM piceatannol (PIC, arrowheads). (C) Syk was activated at the site of particle binding, assessed by a specific anti–phospho-Syk antibody (P-Syk). (A,C) As expected no Syk or P-Syk staining was detected in Syk^−/− PMNs or piceatannol-treated dHL-60 cells, respectively (arrowheads). Results shown are representative for 3 independent experiments. Bar = 10 μm. Phagocytosis of serum-opsonized Alexa 594–conjugated E coli by dHL-60 cells (D) after pharmacologic inhibition of Syk with 30μM piceatannol (PIC) or (E) genetic down-regulation with the RNAi technique (siSyk) compared with control cells (100%; control or siControl, respectively). (F) Phagocytosis of murine Syk^+/+ or Syk^−/− PMNs. (D-F) Mean relative phagocytosis ± SD; n = 4; *P < .05.