Interactions between PD-1 and PD-L1 promote tolerance by blocking the TCR–induced stop signal
Brian T Fife, Kristen E Pauken, Todd N Eagar, Takashi Obu, Jenny Wu, Qizhi Tang, Miyuki Azuma, Matthew F Krummel & Jeffrey A Bluestone
Nature Immunology 10, 1185 - 1192 (2009)