Immuno-electron microscopic tracing of lactoferrin, a regulator of myelopoiesis, into a subpopulation of human peripheral blood monocytes.

Lactoferrin (LF), an iron binding glycoprotein present in the specific granules of mature granulocytes, is capable of suppressing the production of granulocyte-macrophage colony stimulatory factors (GM-CSF) by monocytes and macrophages in vitro and rebound myelopoiesis in vivo. By an immuno-electron microscopic technique (peroxidase-antiperoxidase procedure) in combination with a three-step fixation and detergent treatment of mononuclear blood cells pulsed with LF, intracellular localization of LF in a sub-population of human monocytes with a well-preserved subcellular morphology was demonstrated. LF was found in the monocytes after a 30 min pulse with concentrations of LF as low as 10(-14) M. Under the electron microscope, deposits of approximately 110 nm diameter appeared scattered in the area of the endoplasmic reticulum. Large numbers of smaller deposits (20-70 nm) were found in the euchromatin. The heterochromatin and other cytoplasmic organelles were free of LF. These results suggest that the euchromatin might be the functional site for LF inhibition of the production of GM-CSF by a subpopulation of human monocytes.