Bone
metastasis greatly deteriorates the quality of life in patients with
cancer. Although mechanisms have been widely investigated, the relationship between
cancer bone
metastasis and antitumor immunity in the host has been much less studied. Here, we report a novel mechanism of bone
metastasis mediated by FSTL1, a
follistatin-like
glycoprotein secreted by Snail(+)
tumor cells, which metastasize frequently to bone. We found that FSTL1 plays a dual role in bone
metastasis-in one way by mediating
tumor cell invasion and bone tropism but also in a second way by expanding a population of pluripotent mesenchymal stem-like CD45(-)
ALCAM(+) cells derived from bone marrow. CD45(-)
ALCAM(+) cells induced bone
metastasis de novo, but they also generated CD8(low) T cells with weak CTL activity in the periphery, which also promoted bone
metastasis in an indirect manner. RNA interference-mediated attenuation of FSTL1 in
tumor cells prevented bone
metastasis along with the parallel increase in
ALCAM(+) cells and CD8(low) T cells. These effects were accompanied by heightened antitumor immune responses in vitro and in vivo. In clinical specimens of advanced
breast cancer,
ALCAM(+) cells increased with FSTL1 positivity in
tumor tissues, but not in adjacent normal tissues, consistent with a causal connection between these molecules. Our findings define FSTL1 as an attractive candidate therapeutic target to prevent or treat bone
metastasis, which remains a major challenge in patients with
cancer.