The role of lactoferrin released by phagocytosing neutrophils in the regulation of colony-stimulating activity production by human mononuclear cells.

There remains a controversy about the alleged inhibitory effect of lactoferrin on production of colony-stimulating activity (C.S.A.) by mononuclear cells. We confirm the inhibitory action of both lactoferrin purified from human breast milk and that released from phagocytosing neutrophils. To show the inhibitory effect, it is necessary to plot the dose-response curve of medium conditioned by mononuclear cells with and without lactoferrin. Crowding cells to promote contact is essential for the fraction of C.S.A. production inhibitable by lactoferrin. Saturation of purified lactoferrin by addition of iron salts in vitro may introduce an artifact, as this lactoferrin retained its inhibitory activity at much greater dilutions than lactoferrin released from phagocytosing neutrophils.