Blocking FSTL1 boosts NK immunity in treatment of osteosarcoma

Osteosarcoma (OS) is the most common primary bone malignancy. Many patients develop relapse and metastasis after treatments, and more effective treatments are needed for improving the clinical outcome. FSTL1 overexpression has been reported in murine and human OS, while the functional roles of FSTL1 remain unclear. Here, we elucidated tumor biological and immunological mechanisms underlying the refractory OS using mouse and human OS cell lines, mouse OS models, and clinical specimens. FSTL1 knockout in OS cells significantly suppressed cellular functions, including proliferation, invasion, sphere colony formation, and ALCAM expression. The FSTL1-ablated tumor cells were completely rejected due to generation of potent NK cells in the in vivo setting. Indeed, FSTL1 stimulation suppressed NK activity partly via apoptosis induction, but blocking FSTL1 or CD6, a receptor for ALCAM, significantly restored NK activity. Anti-FSTL1 therapy significantly suppressed tumor growth and metastasis in mouse OS models, and synergized with anti-CD6 therapy in providing significantly better prognosis. These suggest that blocking FSTL1 is a promising strategy for successfully treating OS. This study demonstrates a rationale of targeting the FSTL1-ALCAM axis in the treatment of OS in clinical settings. •FSTL1 expression is upregulated in osteosarcoma cell lines and patient's tissues.•FSTL1 knockout in osteosarcoma abrogates tumor progression.•FSTL1 knockout in osteosarcoma induces potent anti-tumor NK immunity.•FSTL1 and the related ALCAM collaboratively damage NK cells partly via cell death.•Anti-FSTL1 mAb therapy is significantly effective in mouse osteosarcoma models.