The PD-1 / PD-L 1 complex resembles the antigen-binding Fv domains of antibodies and T cell receptors

*Structural Biology Section, Laboratory of Immunogenetics, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Twinbrook 2, 12441 Parklawn Drive, Rockville, MD 20852; Departments of ‡Immunology and Cell Biology and ††Immunology and Genomic Medicine and **21st Century Center of Excellence Formation, Graduate School of Medicine, Kyoto University, Yoshida-Konoe-Cho, Sakyo-ku, Kyoto 606-8501, Japan; §Precursory Research for Embryonic Science and Technology, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 190-0012, Japan; and Laboratory of Applied Structural Biology, Division of Applied Life Science, Graduate School of Agriculture, Kyoto University, Uji, Kyoto 611-0011, Japan