Surface expression and rapid internalization of macrosialin (mouse CD68) on elicited mouse peritoneal macrophages

Macrosialin, the mouse homolog of human CD68, is a heavily glycosylated transmembrane protein found almost exclusively in macrophages. Its function remains uncertain. If has a high affinity for oxidized low-density lipoprotein (LDL) in Ligand blots and antibodies against the human homolog, CD68, inhibit the binding of oxidized LDL to a human monocyte-derived cell line (THP-1), However, there is still controversy as to whether macrosialin, found predominantly in late endosomes, is expressed at all on the plasma membrane, The present studies, done in thioglycollate-elicited peritoneal macrophages, confirm that macrosialin is predominantly intracellular but show clearly that 10-15% of it is expressed on the cell surface. Exchange with intracellular pools occurs at an extremely high rate, The results are compatible with a surface function, including internalization of hound ligands or adhesion to surfaces.