TNFSF15 alleviates myeloid-derived suppressor cell-mediated cancer immunosuppression in mice

Myeloid-derived suppressor cells (MDSCs) are a category of immature myeloid cells that have an important function in suppressing immune responses in a variety of pathological settings. Thus, MDSCs are the subject of intensive studies regarding their recruitment, expulsion, deactivation, and maturation promotion. Tumor necrosis factor superfamily member 15 (TNFSF15) is produced largely by vascular endothelial cells in mature blood vessels with expression also observed in tumor-associated macrophages (TAMs) and dendritic cells (DCs) within the tumor stroma. In addition to inhibiting the proliferation of vascular endothelial cells and the differentiation of bone marrow-derived endothelial cell progenitors, TNFSF15 is able to promote the maturation of DC, as well as to modulate the polarization of naive M2-macrophages into M1-macrophages capable of eliminating cancer cells, and activate T-cell. In this study, we investigated whether a recombinant TNFSF15 results in a substantial reduction of MDSC accumulation in Lewis lung cancer (LLC) tumor-bearing mice. LLC allograft model mice were administered recombinant TNFSF15 (5 mg·kg^-1·d^-1, i.p.) for 7 consecutive days. The tumor, bone marrow and spleen were retrieved on Day 8 and analyzed using flow cytometry or immunofluorescence staining. We showed that TNFSF15 treatment significantly inhibited the tumor growth, and caused a substantial reduction of MDSC accumulation in the tumors. The proportions of MDSC in the bone marrows and the spleens were also reduced. The diminished MDSC was mainly the monocyte-like MDSC (M-MDSC) subtype. Additionally, the reduction in M-MDSC population was accompanied by an increase of the proportions of macrophages and DCs in the tumors. We demonstrated that TNFSF15 promoted M-MDSC differentiation by activating the JAK1/STAT3 signaling pathway. Moreover, the treatment gave rise to a markedly escalated accumulation of cytotoxic T cells in the tumors, attributing to tumor growth inhibition. Our results support the view that TNFSF15-driven differentiation of M-MDSC into DCs and macrophages, and the subsequent activation of T cells, may contribute partially to reinstitution of immunity in the tumor microenvironment.