Project summary There is an urgent need to develop new vaccination strategies that induce antibodies that target relevant epitopes on complex microbes. Moreover, approaches that allow control of the number and fine specificity repertoire of responding naïve or memory B cells would fuel new vaccine designs for a variety of mutating viruses. This project centers on the hypothesis that co-engagement of the B cell receptor (BCR) for antigen with the Notch2 receptor will amplify responses by naïve and memory B cells. This hypothesis builds on preliminary data showing that BCR+Notch2 co-stimulation increases both number of B cells that respond and the number of proliferative events experienced by these cells. To test our hypothesis, we will generate unique mosaic nanoparticles based on immunogenic and sub-immunogenic variants of the HIV envelop (Env) antigen. We will answer the following questions: 1) Does BCR+Notch2 co-engagement prime naïve clones to target native-like Env variants?, and 2) Does BCR+Notch2 co-engagement on memory B cells increase their contribution to GC and PC pools? These studies will provide unique and needed insights into the potential exploitation of BCR signaling thresholds in emerging vaccine design.