Epitope specificity shapes the CD4+ T-cell response to influenza virus infection in mice

CD4+ T helper (Th) cells play a central role in orchestrating protective immune responses during infection and vaccination. Activation of naïve CD4+ T cells is initiated by the recognition of antigenic peptides presented on MHC class II (MHC-II) molecules, leading to their differentiation into distinct effector subsets. In the context of influenza virus infection, the quality of CD4+ T-cell responses is a critical determinant of both cellular and humoral immunity. Given the limited breadth of protection conferred by current influenza vaccines, which primarily induce strain-specific neutralizing antibodies, there is growing interest in harnessing CD4+ T-cell responses targeting conserved viral epitopes to achieve broader and more durable protection. Identifying potent T-cell epitopes within viral antigens is therefore critical for designing effective vaccines that elicit robust, durable, and cross-protective Th-cell and antibody responses. In this study, using a mouse model of influenza virus infection, we identified CD4+ T-cell epitopes within the hemagglutinin (HA) and nucleoprotein (NP) of the H1N1 strain. We generated two NP peptide/MHC-II tetramers, NP264-274/I-Ab and NP418-428/I-Ab, which enabled in vivo tracking and characterization of epitope-specific CD4+ T cells during infection. These epitope-specific T cells exhibited distinct T-cell receptor (TCR) repertoires and differentiation patterns toward either Th1 or T follicular helper (Tfh) lineages, suggesting that epitope specificity shapes the quality of the T-cell response. Our findings provide new insights into the epitope-dependent functional diversity of Th-cell responses and offer valuable tools for the rational design of broadly protective influenza vaccines.