Internalization of bacille Calmette-Guerin by bladder tumor cells

Mechanisms by which intravesical bacille Calmette-Guerin (BCG) treatment mediates antitumor activity are currently poorly understood. We have determined that both human bladder tumor (T-24) and mouse bladder tumor (MBT-2) cells are capable of internalizing the BCG and have characterized this process in vitro. The internalization of BCG by T-24 and MBT-2 cells was verified by histochemistry and electron microscopy. Time dependent internalization of BCG was observed with a maximum occurring at three hrs. Internalization was significantly inhibited by both incubation at 4C and cytochalasin B; conditions known to inhibit phagocytosis. Ultrastructural studies suggested that BCG were transported to membrane bound intracellular compartments and were degraded. The compartments containing the degraded mycobacteria labeled with the fluid phase marker HRP which is known to be transported to lysosomes in a variety of cell types. To determine if the internalization process occurred in vivo, we examined bladder washings of patients treated with intravesical BCG. The majority of the cells in the bladder washings were inflammatory cells which contained ingested BCG. In addition, internalized and degraded BCG were identified in urothelial cells. These data demonstrate that BCG are internalized by transitional epithelial cells both in vitro and in vivo. The internalization process is inhibitable by temperature and microfilament disruption. The potential therapeutic implications of this process and its relationship to antitumor activity of BCG therapy are currently being investigated.