The ontogeny of a 57-Kd cationic antimicrobial protein (CAP57) that has substantial similarities to bactericidal permeability increasing protein (BPI) has been determined immunocytochemically. CAP57 was detected in the granules of mature peripheral blood neutrophils. However, it was absent from other cells of the peripheral blood: eosinophils, red blood cells (RBCs), and mononuclear cells. In human bone marrow, CAP57 was confined to the neutrophilic series. The earliest stage of development of the myeloid cells at which CAP57 was demonstrated was the promyelocyte. Double immunofluorescent labeling showed that CAP57 was detected in cells positive for myeloperoxidase. The absence of lactoferrin in certain cells (promyelocytes) containing CAP57 indicated that CAP57 was synthesized and packaged in a population of granules prior to the development of granules that contain lactoferrin. CAP57 could not be demonstrated in HL60 cells either by enzyme-linked immunosorbent assay (ELISA) or by immunocytochemistry. However, the presence of another granule-associated cationic antimicrobial protein of molecular weight 37 Kd (CAP37) was readily detected in undifferentiated HL60 cells. Amino acid sequence analysis showed that CAP57 and BPI were identical. Further indication of the identity between CAP57 and BPI was that monoclonal anti-CAP57 antibodies cross reacted with BPI. Sucrose density-gradient centrifugations showed CAP57 was confined to a granule population that exhibited a buoyant density intermediate of the previously described light and heavy azurophil granules. Further resolution of the individual azurophil granule populations by Percoll density-gradient centrifugation revealed that CAP57 was most concentrated in the density range of 1.093 to 1.100 g/cc. These results strongly suggest the unique finding that CAP57 may be associated with a heretofore unreported granule type.

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