HISTOCHEMICAL AND ELECTRON-MICROSCOPIC CHARACTERIZATION OF HEPATIC MACROPHAGE SUBFRACTIONS ISOLATED FROM NORMAL AND LIPOSOMAL MURAMYL DIPEPTIDE TREATED RATS

Subfractions of the hepatic macrophage population, differing in cell s ize, were isolated from normal rats and rats treated with liposomal mu ramyl dipeptide (lipMDP) and analyzed histochemically and by ultrastru ctural peroxidase cytochemistry. The majority of cells in all subfract ions of control rats displayed the ultrastructural endogenous peroxida se pattern of resident liver macrophages and showed positive staining with the general macrophage markers nonspecific esterase (NSE) and mon oclonal antibody ED1. Heterogeneity in intensity of NSE and EDI staini ng was observed among macrophages of different size. Generally, the in tensity of NSE and ED1 staining decreased with decreasing cell size. A fter injection of lipMDP, we, observed the appearance of a discrete su bpopulation of cells in the liver in addition to the resident macropha ges. These cells, containing a nucleus with a characteristic shape, we re predominantly recovered in the small-sized fractions and were chara cterized by an immature ultrastructural macrophage morphology (no or o nly a few lysosomes and phagosomes) and a lack of ED1 reactivity, NSE, ; and endogenous peroxidase. We suggest an important role for these ce lls in lipMDP induced antitumor capacity of the liver.