Abstract
Various cancer treatment approaches that inhibit the activity of the programmed death-1/programmed death-ligand 1 (PD-1/PD-L1) axis, a key player in tumor immune evasion, have been developed. We show that the immunomodulatory small tellurium complexes AS101 (ammonium trichloro(dioxoethylene-o,o')tellurate) and SAS (octa-O-bis(R,R)-tartarate ditellurane) suppress PD-L1 expression in a variety of human and mouse malignant cells via the modulation of α4β1 very late antigen-4 (VLA-4) integrin activity. Consequently, the expression of pAkt and its downstream effector pNFκB are inhibited. Additionally, SAS promotes the death of mouse malignant cells by activated syngeneic splenocytes or CD8(+) T cells, preventing the development of chemoresistance in malignant cells. Moreover, AS101 and SAS may increase, at least in part, chemosensitivity through inhibition of the VLA-4/IL-10/PD-L1 pathway. Additionally, AS101 or SAS treatment of B16/F10 melanoma-bearing mice decreased tumor cell PD-L1 expression, leading to increased CD8(+) T-cell infiltration into the tumors and tumor shrinkage. Combination treatment with an αPD-1 antibody and either tellurium compound significantly increased the antitumor efficacy of immunotherapy. Overall, VLA-4 integrin signaling is critical for tumor immune evasion and is a potential target for cancer treatment. Finally, AS101 or SAS, biologically active tellurium compounds, can effectively enhance the therapeutic efficacy of αPD-1-based cancer immunotherapy.