Monocytes, but not macrophages, produce the eosinophil cationic protein

The eosinophil cationic protein (ECP) is a cytotoxic protein with ribonucle ase activity, produced and stored in bone marrow eosinophil myelocytes. Mat ure circulating eosinophils contain about 10 pg ECP per cell. The aim of th is study was to investigate the possibility that monocytes produce and stor e ECP. By results from flow cytometry and specific protein measurement it i s shown that human monocytes contain ECP (monocytes about 10 fg ECP per cel l). RT-PCR analysis indicated the presence of mRNA coding for ECP in blood monocytes but not in alveolar macrophages. Furthermore, mRNA coding for ECP and low amounts of the protein were found in three myeloid cell lines repr esenting different stages of monocytic differentiation. Differentiation of U-937 cells to macrophages induced lowered transcription of the ECP gene an d reduced protein production. Immunohistochemical staining of lung tissue i ndicated that lung macrophages do not contain ECP It is concluded that ECP is produced to a low extent by human monocytes and that the production is s hut down during macrophage differentiation. This might indicate an alternat ive transcriptional regulation of the ECP gene in the monocytic lineage com pared to the eosinophil lineage.