Identification of immunoreactive tissue prokallikrein on the surface membrane of human neutrophils

Putative binding sites for prokallikrein, the endogenous zymogen of the vas oactive and pro-inflammatory tissue kallikrein-kinin system, were recently demonstrated on human neutrophils. However, the occurrence and distribution of neutrophil-bound prokallikrein itself have so far not been examined. In this study, a specific anti-peptide antibody directed against the propart of the zymogen was used to localize the kallikrein precursor by confocal la ser-scanning microscopy on unstimulated human blood neutrophils. Our result s describe, for the first time, the presence of tissue prokallikrein on the membrane of circulating neutrophils. Immunoreactive prokallikrein was asso ciated into punctate clusters occupying the external surface of the neutrop hil membrane and, after addition of exogenous zymogen, immunolabeling was e nhanced four-fold. In contrast, only moderate immunoreactivity to prokallik rein was observed intracellularly. These results suggest that resting neutr ophils provide a circulating platform for tissue prokallikrein whose surfac e density may be upregulated as part of the inflammatory process.