CD8A and infection: To this end, C57BL/6 mice were intranasally infected with influenza virus X-31(H3N2), or vaccinated intranasally with S-FLU(H3N2), and on day 30 after infection, the proportion of cross-reactive NP-specific CD8+ memory T cells in the lungs were measured by simultaneous staining with two H-2Db tetramers loaded with the NP366 peptide derived from either the parental/vaccine strain [ASNENMETM; NP(WT)] or a naturally occurring variant [ASNENMDAM; NP(68)], which differ in the sequence of the immunodominant NP CTL epitope at positions 7 and 8.