PDCD1 and viral encephalitis: To investigate whether PD-1 affects T cell function and differentiation during MuPyV encephalitis, we crossed PD-1fl/fl mice with Rosa26-CreERT2 (Rosa-Cre) mice to generate PD-1fl/fl Rosa-CreERT2 (PD-1fl/fl Rosa-Cre) mice, in which exon 2 of Pdcd1 is conditionally deleted after administration of tamoxifen.