CD8A and infection: To formally test this, 70–90 days after the last IAV infection, 1°M and 2°M CD8 T cell-bearing mice were IN challenged with 107 PFU Vac-NP, and half of them were injected IP with pertussis toxin (Ptx) before (D0) and 2 days after infection to block the access of circulating memory CD8 T cells into the infected lung (Figure 5A) (20).