Bone marrow is a mysterious hidden place for different tumors,including leukemic cells, which are to be eradicated by anti-tumor immunity by cytotoxic T lymphocytes (CTLs). However, the practical mechanism of anti-tumor immune response by CTLs in the bone marrow is still unclear. Especially it needs to be elucidated how specific local environments in bone marrowaffect the mode of immune reactions in situ.

　To elucidate the actual mode of immune reactions by CTLs against leukemic cells in bone marrow, we directly observed the interaction of leukemic cells and CTLs and resultant apoptosis of leukemic cells in the mouse bone marrow by using intravital two-photon microscopy technique that we have originally developed. To visualize antigen-specific CTL responses, we used ovalbumin-specific TCR transgenic OT-I CD8 T cells, and FRET based-fluorescent probe for monitoring caspase-3 activity (SCAT 3.1) was utilized for detecting apoptosis in vivo.

　In results, we succeeded in visualizing the anti-leukemia immune responses mediated by OT-I CTLs in the bone marrow. Time-lapse images of the bone marrow showed there were at least two patterns in the apoptosis process induced by OT-I CTLs; (1) the ‘touch & stay’ pattern, where a single OT-I CTL keeps stable cell-cell contact with leukemic cells more than 6 hours until the leukemia cell undergoes apoptosis. (2)the ‘touch & go’ pattern, where several (normally 2-7) CTLs alternately have a short contact (~10 minutes for each) with a single leukemic cell in the whole process.

Further analyses identified that initial speed of targeted leukemia cells tended to be lower in the touch & go pattern than in the touch & stay pattern. These results suggest that the mode of immune response may be variable depending on the initial dynamic status of targeted tumor cells, which should be considered for promoting anti-tumor immune therapies in future.