Abstract
A unique subpopulation of macrophages (MΦ) was identified among the spleen and bone marrow MΦ 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 MΦ (FF‐MΦ) appeared to be highly activated. Uncoated sheep erythrocytes (E) were ingested by FF‐MΦ indicating that opsonization was not a prerequisite for phagocytosis. However, IgM‐coated E (EIgM) were more readily phagocytosed by FF‐MΦ than were E suggesting that IgM is recognized as an effective opsonin by these cells. EIgM and E coated with IgM and complement(C) (ElgMC) were ingested by approximately the same percentage of FF‐ MΦ; thus, if these cells possess complement receptors in addition to structures which bind EIgM, the C receptors do not enhance the ability of FF‐MΦ to ingest opsonized particles.
The non‐focus‐forming MΦ, e.g. individual MΦ (I‐MΦ), in the spleen and bone marrow can, themselves, be divided into various subpopulations distinguished by their ability to bind and ingest E, EIgM and ElgMC. These may represent various subpopulations of MΦ or MΦ at various stages of activation or differentiation.
While spleen and bone marrow MΦ contained FF‐MΦ and I‐MΦ which vary in their ability to ingest E, EIgM and ElgMC, the MΦ of the peritoneum and blood of normal mice were far more homogeneous. Peritoneal and blood MΦ did not form foci, and did not ingest E or EIgM in significant amounts although a small percentage were able to ingest ElgMC. These data suggest that the population of MΦ in the spleen and bone marrow are far more heterogeneous 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.