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Foamy cells associated with platelet phagocytosis.

In order to gain an insight into the mechanism for the formation of foamy cells (macrophages with foamy cytoplasm) frequently seen in spleens affected by idiopathic thrombocytopenic purpura (ITP), these cells were experimentally induced in mice by subcutaneous injection of platelets with or without accompanied administration of corticosteroid. The light- and electron-microscopic features of experimentally reproduced foamy cells were essentially similar to those seen in the spleens of ITP patients. Corticosteroid had no significant effect on the formation of foamy cells. Most macrophages with foamy cytoplasm contained various amounts of phospholipids, which were derived from platelet membranes. By electron microscopy, myelinlike materials were frequently demonstrated in the cytoplasm of foamy cells. Although lysosomal enzyme activity was revealed in the macrophages that contained morphologically recognizable platelets, there was no demonstrable activity in the cells that contained myelinlike materials. From these results, the following conclusion has been suggested as the mechanism for the formation of foamy cells. Under the state of accelerated phagocytosis of platelets by the macrophages, such as in ITP, the amount of ingested platelet membranes is beyond the capacity of lysosomal digestion. Thus, the incompletely degraded membrane constituents, especially membrane-derived phospholipids, remain in the macrophages, and they are most responsible for the foamy appearance of these macrophages.