By using immunofluorescent flow cytometry, we observed a profound up-regulation of CD45 on the plasma membrane of neutrophils exposed to low levels of a culture supernatant of the Gram-negative pathogen, Fusobacterium nucleatum (FN). Plasma membranes of neutrophils freshly prepared form human blood possessed little enzymatically active phosphotyrosine phosphatase. The activity of this enzyme was markedly potentiated in plasma membranes prepared from cells preexposed to the FN culture supernatant. This activity was vanadate sensitive and could be immunoprecipitated with anti-CD45 Ab. Cells preexposed to the FN culture supernatant were inhibited in their ability to release superoxide when challenged with the bacterial chemotactic factor, FMLP, but not PMA. The tyrosine kinase inhibitor, genistein, likewise inhibited FMLP but not PMA-induced superoxide release. Pretreatment of neutrophils with vanadate reversed FN-mediated inhibition of FMLP-triggered superoxide release but had no effect on genistein-mediated inhibition of FMLP-induced superoxide release. Of several proteins tyrosine phosphorylated in response to treatment of neutrophils with FMLP, Western analysis revealed one (m.w. approximately 93,000) that was lost when FMLP-stimulated cells were exposed to FN. This effect was inhibited when the cells were preexposed to vanadate. These results are consistent with the hypothesis that plasma membrane tyrosine phosphatase modulates FMLP-induced superoxide release by reversing the effects of tyrosine kinases activated in the initial phases of cell stimulation.

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