Bacteria-instructed B cells (BacB) cross-prime naïve CD8+T cells triggering effective cytotoxic responses

In addition to trigger humoral responses, conventional B cells have been described in vitro to cross-present exogenous antigens activating naïve CD8+ T cells. Nevertheless, the way B cells capture these exogenous antigens and the physiological roles of B cell-mediated cross-presentation remain poorly explored. Here, we show that B cells capture bacteria by trans-phagocytosis from previously infected dendritic cells (DC) during DC/B cells close contacts. Bacterial encounter 'instructs' the B cells that acquire antigen cross-presentation abilities, in a process that involves autophagy. Bacteria-instructed B cells, henceforth referred to as BacB cells, rapidly degrade up-taken bacteria, and process bacterial antigens, cross-priming naïve CD8+ T cells which differentiate into specific cytotoxic cells that efficiently control bacterial infections. Moreover, a proof of concept experiment shows that BacB cells that have captured bacteria expressing tumor antigens could be useful as novel cellular immunotherapies against cancer.