The mammalian actin-binding protein 1 (mAbp1, HIP-55, SH3P7) is phosphorylated by Syk, a tyrosine kinase that is important for recruitment of polymorphonuclear neutrophils (PMN) during acute inflammation.

Here we studied the role of mAbp1 for ß₂ integrin (CD11/CD18)-mediated PMN adhesion and migration using mAbp1^-/- and mAbp1^+/+ mice and neutrophil-like differentiated (dHL-60) cells. Upon adhesion on immobilized fibrinogen, mAbp1 was translocated to the leading edge of polarized dHL-60 cells and murine mAbp1^+/+ PMN. This process was impaired upon inhibition of Syk using piceatannol indicating a role of Syk for the translocation of mAbp1. Under static conditions, mAbp1 was not required for ß₂ integrin-mediated adhesion on fibrinogen or intercellular adhesion molecule 1 (ICAM-1). In addition, site-directed migration in a chemotactic gradient of 10 mM formyl-methionyl-leucyl-phenylalanine (fMLP) was not affected in mAbp1^-/- PMN under static conditions. In contrast, mAbp1 was essential for ß₂ integrin -mediated leukocyte adhesion in inflamed murine cremaster muscle venules upon stimulation with tumor necrosis factor a (TNFa). Similar results were obtained in a flow chamber coated with ICAM-1, P selectin glycoprotein ligand 1 (PSGL-1) and keratinocyte-derived chemokine (KC, CXCL1). Here, firm adhesion and migration of mAbp1^-/- PMN were significantly diminished compared to mAbp1^+/+ PMN. The observed adhesion defect in the absence of endothelial cells indicated that this defect was due to the indispensable role of mAbp1 in leukocytes. To elucidate the mechanism by which mAbp1 controlled firm arrest under flow conditions, we analyzed ß₂ integrin clustering by means of confocal microscopy. Upon down-regulation of mAbp1 using the RNAi technique, a decreased number of ß₂ integrin clusters in the high-affinity conformation was detectable under flow conditions compared to control cells. The fact that the high-affinity conformation of the ß₂ integrins is required to withstand shear forces under flow conditions during firm adhesion of PMN to the vascular wall identified mAbp1 as a novel molecular regulator of PMN recruitment during the inflammatory response in vivo.

This study was supported by DFG (Wa 1048/2-3).