Despite the superior efficacy of high-dose influenza vaccines, over one-third of older adults fail to respond. Yet, the mechanisms underlying this impaired vaccine responsiveness remain poorly understood. Here, we performed longitudinal profiling of older adults (n=60) receiving high-dose influenza vaccination to identify immune programs associated with vaccine responsiveness. Strong responders exhibited a primed baseline immune state characterized by elevated plasma cytokines and chemokines, followed by a reduction in circulating cDC2 and moDC frequencies at Day 1 after vaccination, consistent with migration to lymphoid tissues. These subsets showed increased expression of interferon-stimulated genes (ISGs) and higher CD40 expression, both of which were associated with stronger antibody responses. At the epigenetic level, cDC2s from responders showed increased chromatin accessibility at interferon-related loci such as GBP5 and IRF1, along with higher IRF/JUN motif activity. These Day 1 transcriptional changes in cDC2 and moDC correlated with IgG1⁺ plasmablast expansion at Day 7, linking dendritic cell activation to downstream humoral responses.
By Day 7, CD4⁺ T-cell trajectories diverged: strong responders preferentially expanded influenza-specific activated cTfh1 (CXCR5⁺CXCR3⁺ICOS⁺CD38⁺) and influenza-specific Th10 (CXCR5⁻CXCR3⁺PD1⁺IL10⁺) cells, whereas weak responders expanded regulatory cTfr (CXCR5⁺FOXP3⁺) cells. Th10 expansion correlated with plasmablast and antibody responses and was independently validated in a larger influenza vaccination cohort, including younger adults. Functionally, Th10 cells promoted memory B-cell differentiation into plasmablasts and production of influenza-specific IgGs. TCR analyses revealed minimal clonal overlap between Th10 and cTfh1 cells. Together, these findings link early dendritic cell transcriptional and epigenetic remodeling to divergent helper and regulatory CD4⁺ T cell programs associated with vaccine responsiveness, and establish Th10 cells as a previously unrecognized component of vaccine-induced humoral immunity.