In the last decade, immunotherapy has significantly changed the landscape of cancer treatment . As representatives, antibodies targeting immune checkpoints, such as programmed cell death-1 (PD-1) inhibitors and programmed cell death-ligand 1 (PD-L1) inhibitors, have won great clinical success in treating various cancer indications . However, due to the complicated immune resistance mechanisms, these immune checkpoint inhibitors can only benefit a subset of patients . Thus, there is an urgent need to search for other immune negative regulators that can serve as complementary or alternative drug targets .