CD4 and diphtheria: Expansion of the IFNγ− population in the spleen upon boosting with H1Y98F-VLP but not the H1WT-VLP is reminiscent of the CD4+ T cell responses to protein vaccines such as hepatitis B, diphtheria and tetanus, that are dominated by Thpp cells and elicit robust and durable antibody responses30,31,60.