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

Putative binding sites for prokallikrein, the endogenous zymogen of the vasoactive 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 laser-scanning microscopy on unstimulated human blood neutrophils. Our results describe, for the first time, the presence of tissue prokallikrein on the membrane of circulating neutrophils. Immunoreactive prokallikrein was associated into punctate clusters occupying the external surface of the neutrophil membrane and, after addition of exogenous zymogen, immunolabeling was enhanced four-fold. In contrast, only moderate immunoreactivity to prokallikrein was observed intracellularly. These results suggest that resting neutrophils provide a circulating platform for tissue prokallikrein whose surface density may be upregulated as part of the inflammatory process.