Eosinophil cationic protein is stored in, but not produced by, peripheral blood neutrophils

Background Eosinophil cationic protein (ECP) is an eosinophil-derived protein, which has been shown to be present in circulating neutrophils. Objective To establish whether ECP is produced or internalized by peripheral blood neutrophils. Methods This was done using microscopy, flow cytometry, fractionation of cells and RT-PCR techniques. Results No ECP mRNA was detected after extensive cell purification to eliminate all traces of contaminating eosinophils. Examination of immunostained neutrophils by light, confocal, electron microscopy together with cell fraction experiments, established that ECP is present intracellularly and is mostly associated to cell granules. Uptake studies by flow cytometry and by using both cold and radiolabelled ECP showed that it is internalized by neutrophils and stored in some proportion in their primary granules. Upon stimulation with serum-treated Sephadex particles, the internalized ECP was partially released from cells. Conclusion ECP is not produced but can be internalized by circulating neutrophils, which take it from the environment and partially store it in their primary granules.