MAC-1 (CD11b/CD18) mediates tyrosine signaling in human neutrophils

Tyrosine kinases play a prominent role in various intracellular signal transduction pathways. In this study, β₂-integrin (CD11/CD18)-mediated adhesive interactions of human neutrophils (PMN) were mimicked by antibody cross-linking of CD11/CD18. Within seconds, cross-linking of CD 18 induced transient tyrosine phosphorylation of several proteins with molecular masses of 120 kDa, 78 kDa, 72 kDa, 65 kDa, and 56 kDa, as shown by antiphosphotyrosine immunoblotting of whole cell lysates. Cross-linking of the α-subunits of the β₂-integrins demonstrated that only Mac-1 (CD1 lb/CD18) but not LF A-1 (CD 11 a/CD 18) or gp 150/95 (CD 11/CD 18) triggered tyrosine signaling. Comparison of Mac-1-mediated tyrosine signaling with that induced by chemoattractants gave similar results. Inhibition of tyrosine kinases by herbimycin A resulted in partial inhibition of phosphorylation of the 120 kDa and 65 kDa protein and in complete inhibition of phosphorylation the 78 kDa and 56 kDa protein. In contrast, herbimycin A had no inhibitory effect on phosphorylation of the 72 kDa protein. In unstimulated PMN, the tyrosine phosphatase inhibitor orthovanadate had no effect on phosphorylation of the 120 kDa protein, but induced phosphorylation of the 78 kDa, 72 kDa, 65 kDa, and 56 kDa proteins. These results show that (i) Mac-1 triggers complex intracellular signaling driven by different tyrosine phosphorylation and dephosphorylation events which (ii) may play a role in the regulation of PMN functions during inflammation. Supported by DFG (SFB 366/C3).