T cell help is a limiting factor for rare anti-influenza memory B cells to reenter germinal centers and generate potent broadly neutralizing antibodies

Development of vaccines eliciting broadly neutralizing influenza antibodies (bnAbs) is an extraordinary challenge. One hypothetical proposal is that CD4+ T cell help to rare immuno-subdominant bnAb class memory B cells is one critical factor to cause these B cells to reenter secondary germinal centers (GCs) and generate potent bnAbs. In this regard, we previously showed that the prototypic hemagglutinin stem vaccine does not contain the dominant CD4+ T cell epitope. Here, to test the above hypothesis, we examined the effects of adding a single influenza T cell epitope to the stem vaccine in an influenza pre-infected booster mouse model. We found that this fused booster vaccine efficiently recruited anti-stem memory B cells with prior GC experience into the secondary GCs in draining lymph nodes. Furthermore, these secondary GC-experienced cells evolved, thereby contributing to generation of more potent neutralizing activity towards variant viruses. Thus, our results suggest the importance of T cell help in generating potent bnAbs by recruiting rare subdominant memory B cells into secondary GCs, and have implications for vaccine design.