#$%^&*AU2020201069A120200305.pdf#####Abstract Provided are antibodies that specifically bind to Programmed Death-1 (PDI, or CD279) and inhibit PDI-mediated cellular signaling and activities in immune cells, antibodies binding to a set of amino acid residues required for its ligand binding, and uses of these antibodies to treat or diagnose cancer, infectious diseases or other pathological disorders modulated by PDI-mediated functions.