Different proteolytic mechanisms involved in Fc gamma RIIIb shedding from human neutrophils

The Fc gamma receptor type IIIb (CD16) is highly expressed on human neutrop hils and is found in a soluble form in plasma and in other body fluids. Upo n activation of neutrophils in vitro, Fc gamma RIIIb is shed from the cell surface by proteolytic cleavage. We have now investigated the effect of met alloproteinase inhibitors and a serine proteinase inhibitor on the shedding of Fc gamma RIIIb induced by phorbol 12-myristate 13-acetate (PMA) or cyto chalasin B (cyto B) + N-formyl-methionyl-leucyl-phenylalanine (fMLP). Metal loproteinase inhibitors blocked to a large extent PMA-induced, but not cyto B + fMLP-induced shedding of Fc gamma RIIIb. Inhibition of members of the ADAM (a disintegrin and metalloproteinase) family appeared most efficient. In contrast, the serine protease inhibitor N-methoxysuccinyl-alanine-alanin e-proline-valine-chloromethylketone (MeOsuc-AAPV-CMK) largely blocked cyto B + fMLP-induced, but not PMA-induced shedding of Fc gamma RIIIb. Metallopr oteinase inhibitors in combination with the serine proteinase inhibitor res ulted in full inhibition of Fc gamma RIIIb shedding induced by either PMA o r cyto B + fMLP. The shedding of Fc gamma RIIIb that accompanied apoptosis was inhibited by 60% in the presence of inhibitors of metalloproteinases bu t was insensitive to inhibition of serine proteinases. These results show t hat distinct types of proteolytic enzyme are involved in the stimulus-induc ed shedding of Fc gamma RIIIb from human neutrophils and suggest that these proteinases may become differentially activated under various physiologica l or pathological conditions.