PDCD1 and infection: As expected, PD-1 density of p11CM+ CD8 T cells negatively correlated with Ki67 expression over time post infection (from 14 to 112 dpi), except for the colorectal samples (Figure 1H, I and J) suggesting that PD-1 expressing virus-specific CD8 T cells become functionally impaired in their ability to proliferate with progressive infection.