Cross‐Linking of β2 Integrins Caused Diminished Responses of Neutrophils to Priming Agents Like Lipopolysaccharide or Tumor Necrosis Factor‐α: Possible Involvement of Tyrosine Kinase Syk

Neutrophils up‐regulate β2 integrins like CD11b/CD18 in response to lipopolysaccharide (LPS). Up‐regulation of β2 integrins causes neutrophils to adhere to surfaces, and to release superoxide anion (O2–). When neutrophils are exposed to LPS plus plasma under conditions not favorable for adherence (absence of Mg2+), the cells do not spontaneously release O2–, but instead they are primed for enhanced release of O2– after subsequent triggering by fMLP. In the presence of Mg2+, neutrophils adhere in response to LPS but fMLP‐triggered O2– release by LPS‐primed neutrophils is diminished. To understand why adherence interferes with the response of neutrophils to N‐formyl‐methionyl‐leucyl‐phenylalanine (fMLP), β2 integrins were cross‐linked by mouse monoclonal antibodies that had been immobilized by surface‐bound anti‐mouse antibody. When unprimed neutrophils were trapped on the surface by these cross‐linked monoclonal antibodies, O2– release was triggered, and priming by LPS for fMLP‐triggered O2– release was diminished, indicating that this cross‐linking of β2 integrins mimicked adherence. Alkaline phosphatase is up‐regulated by LPS or tumor necrosis factor‐α, and this response was also diminished by the cross‐linking antibodies. The diminished alkaline phosphatase up‐regulation was reversed by genistein, a general inhibitor of tyrosine kinases, and by piceatannol, an inhibitor for Syk kinase. Piceatannol also inhibited the phosphorylation of Syk caused by cross‐linking of β2 integrins. These results suggested that adherence‐induced triggering and Syk kinase activation might be responsible for the diminished response of LPS‐primed neutrophils to fMLP when neutrophils were adherent.