Monocyte proliferation induced by modified serum is associated with endogenous M-CSF production: evidence for involvement of a signalling pathway viascavenger receptors

It was found that human serum stored for 2 months at 4 degrees C (modified serum) induced monocyte proliferation and simultaneous macrophage colony st imulating factor (M-CSF) production by these cells in vitro. Cell number, e stimated by DNA content, doubled after 10 days in culture in the presence o f modified serum, while it decreased in culture with freshly thawed control serum. As the addition of more than 2.5 ng/ml of recombinant M-CSF signifi cantly supported monocyte survival/proliferation, cells were cultured for 1 0 days in medium supplemented with control serum, and endogenous M-CSF prod uction was investigated by enzyme-linked immunosorbent assay. M-CSF concent ration in the supernatants was 15-30 ng/ml after 10 day in culture with mod ified serum, a level that might be sufficient for monocyte proliferation. T he modified serum induced M-CSF from freshly isolated monocytes, while M-CS F was hardly detected in cultures supplemented with control serum. Assay fo r peroxidized lipid and agarose gel electrophoresis demonstrated that the m odified serum contained more oxidized low density lipoproteins (LDL) than t he control serum. Ligands of scavenger receptors, which are receptors for o xidized LDL, such as dextran sulphate, polyinosinic acid, heparin and acety lated LDL also significantly induced M-CSF production from human monocytes, although this was at levels below 2 ng/ml. These results indicate that ser um modified by oxidation stimulates monocytes to produce M-CSF resulting in their proliferation, and that signalling via scavenger receptors is one of the mechanisms responsible for this induction of M-CSF.