β₂ integrins mediate protein tyrosine phosphorylation 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. Cross-linking of CD18 induced tyrosine phosphorylation of several proteins with apparent molecular masses of ~120, 78, 72, 65, and 56 kDa, respectively, as shown by anti-phosphotyrosine immunoblotting of whole cell lysates. Cross-linking of the α-subunits of the β₂ integrins demonstrated that only cross-linking of Mac-1 but not LFA-1 or gp150/95 triggered tyrosine signaling. The tyrosine phosphorylations showed a rapid and transient time course. Comparison of the CD18-mediated tyrosine phosphorylation patterns with those induced by chemoattractants gave similar results. The observed tyrosine phosphorylation was specific, since binding and non-binding irrelevant primary antibodies had no effect. Furthermore, F(ab')₂ fragments of the anti-CD18 antibody IB4 and addition of F(ab')₂ fragments of secondary antibody were sufficient to induce tyrosine phosphorylation. Inhibition of tyrosine kinases by herbimycin A resulted in partial inhibition of the CD18-mediated tyrosine phosphorylation of the 120- and 65-kDa proteins and in complete inhibition of tyrosine phosphorylation of the 78- and 56-kDa proteins. In unstimulated PMN, the tyrosine phosphatase inhibitor sodium orthovanadate had no effect on tyrosine phosphorylation of the 120-kDa protein, but induced phosphorylation of the 78-, 72-, 65-, and 56-kDa proteins. These results indicate that the β₂ integrin-mediated intracellular signaling cascade involves different tyrosine phosphorylation (and dephosphorylation) events, which may play a role in the regulation of PMN functions during inflammation.