Efficient phagocytosis of iC3b‐opsonized bacteria requires CD11b/CD18‐mediated signaling via the non‐receptor tyrosine kinase Syk

Polymorphonuclear neutrophils (PMN) have an important function in innate host defense that involves phagocytosis and the killing of bacteria. Phagocytosis is facilitated upon opsonization of the bacteria by the complement factor iC3b which binds to the major neutrophil iC3b receptor CD11b/CD18. Here, we studied the functional impact of the non‐receptor tyrosine kinase Syk for efficient CD11b/CD18‐mediated phagocytosis of iC3b‐opsonized E. coli. By means of confocal microscopy, we found that CD18 colocalized with Syk during phagocytosis of iC3b‐opsonized E. coli. Here, Syk was found to be associated with its downstream target Vav, a guanine nucleotide exchange factor for the small GTPases Rac and Cdc42. The Syk specific inhibitor piceatannol (30 μM) abolished the colocalization of Syk and Vav with the iC3b‐opsonized bacteria. Inhibition of Syk using piceatannol (30 μM) or downregulation of Syk by RNAi technique significantly inhibited phagocytosis of iC3b‐opsonized E. coli within 10 min to 41.6% ± 2.5 % and 24.8% ± 18.7 % (p < 0.005) when compared to phagocytosis of untreated control cells at the same time point (100%). Similar results were obtained using Syk deficient murine PMN from bone marrow chimeric mice. Here, phagocytosis of Syk deficient PMN was significantly reduced within 10 min after the onset of the experiment to 31.2% ± 18.5% (p < 0.005) when compared to wild‐type PMN (100%) that were obtained from wildtype control chimeras. In conclusion, our data indicate that Syk is required for efficient CD11b/CD18‐mediated phagocytosis of iC3b‐opsonized bacteria. (Supported by DFG WA 1048/2‐1).