Markers of macrophage heterogeneity. I. Studies of macrophages from various organs of normal mice.

A unique subpopulation of macrophages (M phi) was identified among the spleen and bone marrow M phi of normal mice. After 24 h of culture, approximately 2.5% of the adherent cells cluster into "foci" of 10-30 cells. On the basis of their phagocytic and morphologic characteristics, these focus-forming M phi (FF-M phi) appeared to be highly activated. Uncoated sheep erythrocytes (E) were ingested by FF-m phi indicating that opsonization was not a prerequisite for phagocytosis. However, IgM-coated E (EIgM) were more readily phagocytosed by FF-M phi than were E suggesting that IgM is recognized as an effective opsonin by these cells. EIgM and E coated with IgM and complement (C) (EIgMC) were ingested by approximately the same percentage of FF-M phi; thus, if these cells possess complement receptors in addition to structures which bind EIgM, the C receptors do not enhance the ability of FF-M phi to ingest opsonized particles. The non-focus-forming M phi, e.g. individual M phi (I-M phi), in the spleen and bone marrow can, themselves, be divided into various subpopulations distinguished by their ability to bind and ingest E, EIgM ana EIgMC. These may represent various subpopulations of M phi or M phi at various stages of activation or differentiation. While spleen and bone marrow M phi contained FF-M phi and I-M phi which vary in their ability to ingest E, EIgM and EIgMC, theM phi of the peritoneum and blood of normal mice were far more homogeneous. Peritoneal and blood M phi did not form foci, ad did not ingest E or EIgM in significant amounts although a small percentage were able to ingest EIgMC. These data suggest that the population of M phi in the spleen and bone marrow are far more heterogenous than those found in the peritoneum or blood and that binding and phagocytosis of various coated and uncoated erythrocytes can be studied to elucidate this heterogeneity.