PDCD1 and infection: We also found similar normalized bond numbers of CD8+ T cells from P14 mice infected with LCMV strain Armstrong (5 days post infection) or Clone 13 (5 or 8 days post infection) to RBCs bearing PD-L1 (Supplementary Fig. 2a) or gp33:H2-Db (Supplementary Fig. 2b), yet fewer bonds to dual-ligand RBCs than the sum of bonds to single-ligand RBCs for all conditions (Supplementary Fig. 2c), suggesting PD-1-mediated negative cooperativity for in vivo activated T cells during responses to antigen.