We further demonstrated that Pdcd1−/− aged mice did not develop a cytotoxic CD8+ TRM population in the lung 40 d p.i. These results indicate a temporal role of PD-1 signaling in both the initial antiviral response to improve granzyme B production and during the resolution phase of infection, when the memory CD8+ T cell compartment is formed. The gene discussed is CD8A; the disease is infection.