Normal peritoneal lymphocytes: A population with increased capacity for endocytosis.

The existence of lymphocyte to macrophage transformation among normal peritoneal lymphocytes (NPLs) was considered. We described the conditions under which pinocytosis of the enzyme tracer horseradish peroxidase (HRP) and phagocytosis of latex particles by NPLs occurred and compared these data with results obtained with splenic lymphocytes (SpLs), a better characterized source of lymphocytes. In the presence of relatively large concentrations (200 mug/ml to 1 mg/ml) of HRP, at least three times as many NPLs took up the protein by pinocytosis as did corresponding SpLs. Also, on the average, NPLs took up about twice as much HRP per cell than did SpLs. The uptake was rapid, and pinocytotic vesicles containing HRP were seen as early as 15 minutes after exposure to the tracer in NPLs but not in SpLs. In contrast to peritoneal macrophages, NPLs retained HRP in a variety of cytoplasmic vesicles and vacuoles in relatively undegraded form for at least 24 hours. Finally, if NPLs, which were nonphagocytic when freshly isolated, were cultured for 24 hours, both erythrophagocytosis and phagocytosis of latex spheres were observed. The cells which had taken up the latex were nonadherent to plastic, resembled lymphocytes ultrastructurally and were peroxidase negative. We concluded that under these culture conditions, NPLs had not transformed into morphologic macrophages, but phagocytic lymphocytes.