The cellular immune responses elicited by an investigational
vaccine against an emergent variant of
influenza (H3N2v) are not fully understood. Twenty-five subjects, enrolled in an investigational
influenza A/H3N2v
vaccine study, who received two doses of
vaccine 21 days apart, were included in a sub-study of cellular immune responses. H3N2v-specific plasmablasts were determined by ELISpot 8 days after each
vaccine dose and H3N2v specific CD4+ T cells were quantified by intracellular
cytokine and CD154 (
CD40 ligand) staining before vaccination, 8 and 21 days after each
vaccine dose. Results: 95% (19/20) and 96% (24/25) subjects had pre-existing H3N2v specific memory B, and T cell responses, respectively. Plasmablast responses at Day 8 after the first
vaccine administration were detected against contemporary H3N2 strains and correlated with hemagglutination inhibition HAI (
IgG: p = 0.018;
IgA: p < 0.001) and Neut (
IgG: p = 0.038;
IgA: p = 0.021) titers and with memory B cell frequency at baseline (
IgA: r = 0.76, p < 0.001;
IgG: r = 0.74, p = 0.0001). The CD4+ T cells at Days 8 and 21 expanded after prime vaccination and this expansion correlated strongly with early post-vaccination HAI and Neut titers (p ≤ 0.002). In an adult population, the rapid serological response observed after initial H3N2v vaccination correlates with post-vaccination plasmablasts and CD4+ T cell responses.