PDCD1 and neoplasm: 15 Ahmadzadeh Ahmadzadeh M M Johnson Johnson LA LA Heemskerk Heemskerk B B Wunderlich Wunderlich JR JR Dudley Dudley ME ME White White DE DE Rosenburg Rosenburg SA SA Tumor antigen-specific CD8 T cells infiltrating the tumor express high levels of PD-1 and are functionally impaired Tumor antigen-specific CD8 T cells infiltrating the tumor express high levels of PD-1 and are functionally impaired Blood Blood 2009 2009 114 114 1537 1537 44 44 19423728 19423728