Different proteolytic mechanisms involved in FcγRIIIB shedding from human neutrophils

The Fcγ receptor type IIIb (CD16) is highly expressed on human neutrophils and is found in a soluble form in plasma and in other body fluids. Upon activation of neutrophils in vitro, FcγRIIIb is shed from the cell surface by proteolytic cleavage. We have now investigated the effect of metalloproteinase inhibitors and a serine proteinase inhibitor on the shedding of FcγRIIIb induced by phorbol 12-myristate 13-acetate (PMA) or cytochalasin B (cyto B) + N-formyl-methionyl-leucyl-phenylalanine (fMLP). Metalloproteinase inhibitors blocked to a large extent PMA-induced, but not cyto B + fMLP-induced shedding of FcγRIIIb. Inhibition of members of the ADAM (a disintegrin and metalloproteinase) family appeared most efficient. In contrast, the serine protease inhibitor N-methoxysuccinyl-alanine-alanine-proline-valine-chloromethylketone (MeOsuc-AAPV-CMK) largely blocked cyto B + fMLP-induced, but not PMA-induced shedding of FcγRIIIb. Metalloproteinase inhibitors in combination with the serine proteinase inhibitor resulted in full inhibition of FcγRIIIb shedding induced by either PMA or cyto B + fMLP. The shedding of FcγRIIIb that accompanied apoptosis was inhibited by 60% in the presence of inhibitors of metalloproteinases but was insensitive to inhibition of serine proteinases. These results show that distinct types of proteolytic enzyme are involved in the stimulus-induced shedding of FcγRIIIb from human neutrophils and suggest that these proteinases may become differentially activated under various physiological or pathological conditions.