Cellular visualization of tissue prokallikrein in human neutrophils and myelocytes

The vasoactive peptides bradykinin and kallidin (lysyl-bradykinin) have been implicated in diapedesis, a cellular process by which neutrophils migrate through endothelial cell gap junctions. The kinin peptides are released from their precursor moiety, kininogen, by the specific action of endoproteinases, the kallikreins. Kininogens have been demonstrated on the surface of neutrophils, and the presence of a competent processing enzyme such as tissue prokallikrein in neutrophils has been postulated, but firm evidence for this is still lacking. We have raised antibodies to a synthetic peptide that is a sequence copy of the activation segment of human TK and demonstrated that the anti-peptide antibodies specifically recognized the zymogen but not the active form of kallikrein. Using these anti-peptide antibodies, we showed by Western blotting, immunocytochemistry and electron microscopy that the tissue prokallikrein antigen was localized in neutrophils and their precursor cells, the myelocytes. We further demonstrated by in situ hybridization the presence of tissue kallikrein mRNA in the mature neutrophils and myelocytes. Our findings lend credence to the hypothesis that upon release and activation, neutrophil-borne TK acts on cell-associated kininogens to trigger the release of kinins, which may open endothelial gates for neutrophil diapedesis.