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The Ultrastructure of Target L-Cells and Immune Macrophages during Their Interaction In Vivo¹

Department of Microbiology, University of Washington, Seattle, Washington 98105

C57BL mice that had recently rejected Sarcoma I ascitic tumors responded to the i.p. injection of genetically related L-cells by the immediate and strong adherence of immune macrophages to the target cells. The interaction of attached cells was often marked by interdigitation of their surface membranes and by the uptake of portions of the living target cells by the macrophages. The term "piecemeal cytophagocytosis" is proposed to designate the ingestion of portions of cells as distinct from the ingestion of whole cells. The ultrastructural studies presented show that piecemeal cytophagocytosis apparently begins with the macrophage membrane spreading over a discrete area of the target cell membrane and adhering to it. The adhering portion of the target cell then appears to be pulled into the macrophage while the encroaching edge of the macrophage cytoplasm indents the surface of the target cell until the involved portion of target cell is nearly surrounded by the macrophage. At this stage, only a narrow neck of target cell cytoplasm communicates with the partially surrounded portion of the target cell. Final separation of the surrounded portion of cytoplasm is accomplished by a pinching-off process involving fusion of the contacting areas of the membrane of each cell.

¹ Supported in part by USPHS Research Grant CA-05698 from the National Cancer Institute.

Received 5/26/71. Accepted 7/21/71.