Qualitative Superiority, Not Magnitude, of Lung-Resident CD8+ T Cells Determines Protective Efficacy of Influenza NP Versus PB1 Vaccines.

T cell-based universal influenza vaccines aim to generate robust lung-resident memory CD8+ T cells (TRM), yet it remains unclear whether conserved Ags elicit equally protective TRM pools. We compared recombinant adenoviral vectors expressing the influenza A nucleoprotein (rAd/NP) or polymerase basic protein 1 (PB1). After intranasal immunization, both induced comparable magnitudes of respiratory Ag-specific CD8+ T cells, yet only rAd/NP provided 100% survival after lethal influenza challenge. This disparity reflected qualitative differences in the TRM pool; nucleoprotein (NP)-specific cells were predominantly CD103-CD49a+, a phenotype associated with superior cytotoxicity, whereas PB1-specific cells were mainly CD103+CD49a-. Furthermore, NP-specific CD8+ T cells showed 100-fold higher functional avidity and stronger lung-local CTL activity than PB1-specific cells. Our findings demonstrate that TRM quality-specifically phenotypic bias and functional avidity-rather than magnitude, are the primary determinants of vaccine-mediated protection. This study underscores the critical importance of Ag selection in optimizing T cell-based universal vaccine strategies.