Bone Abstracts

Within a living organism, the adaptive immune system, e.g. cytotoxic T lymphocytes (CTLs), induce apoptosis in tumor cells, and therefore limit tumor development. Bone marrow is a mysterious hidden place for different tumor cells and the anti-tumor immunity in the bone marrow is important because the specific microenvironment had been considered to contribute to escape of tumor cells from immune attack. However, the practical mechanism of anti-tumor immune elimination by CTLs in the bone marrow is still unclear. In this study, to elucidate how the CTLs response against tumor cells in bone marrow, we observed the interaction of leukemic cells and CTLs as well as CTL-induced apoptosis in the bone marrow using an intravital two-photon microscopy.

For visualizing the adaptive immune system, we used a fluorescent FRET-probe (SCAT3.1 FRET probe) that allows us to monitor caspase-3 activity and also used the ovalbumin-OT-I CTL system for inducing antigen specific T cell immunity.

First we visualized anti-tumor immune responses facilitated by CTLs in vitro. In results, leukemic cell death was dependent on the total cell number of CTLs. CTLs induced leukemic cell death by cell-cell contact; capturing leukemic cells, inducing caspase-3 activation and facilitating cell lysis. The CTLs took 3.5 hours on average to induce caspase-3 activation after capturing, and 2.4 hours on average to lysis the tumor cell after caspase-3 activation.

Secondly in mouse leukemia model, we observed that CTLs inducing apoptosis in leukemic cells upon directed cell-cell contact in the bone marrow by intravital 2-photon imaging. CTLs contact to leukemic tumor cells and induce caspase-3 activation, and remain in cell-cell contact to apoptotic leukemic cells until they undergo cell lysis.

Visualizing how tumor cells are killed by CTLs in vivo, especially in bone marrow, offers new perspectives for understanding anti-tumor immune elimination and the mechanism of escape from immune attack.