Activated human mononuclear phagocytes release a substance(s) that induces replication of quiescent human fibroblasts.

Human mononuclear phagocytes (HMP) treated in vitro with N-acetylmuramyl-L-alanyl-D-isoglutamine (MDP) released a soluble substance(s) that induced in vitro replication of subconfluent, quiescent, serum-free human fibroblasts. The response was enhanced if nonmitogenic plasma-derived serum (PDS) was added to the HMP-conditioned media after harvest. The capacity of serum-free, quiescent fibroblasts to respond to the HMP-released substance(s) diminished with time in culture. The diminished proliferative response was reversed by adding PDS to the MDP-HMP-conditioned media. Similar results were obtained in parallel experiments using platelet-derived growth factor (PDGF). These data suggest that the HMP-associated growth-promoting substances may, like PDGF, act co-ordinately with substances in PDS to stimulate the proliferation of cells in vitro.