T FH cells in bystander and cognate interactions with B cells.

Follicular T-helper (TFH ) cells play a crucial role in three aspects of the germinal center (GC) response. They promote GC formation, arbitrate competition among GC B cells to determine the outcome of affinity maturation, and regulate GC output of memory and plasma cells to shape the long-lived humoral immune memory. Of fundamental importance are dynamic physical interactions between TFH and B cells, which are the main platform for TFH cells to deliver "help" factors to B cells and also for reciprocal signaling from B cells to maintain the helper state of TFH cells. Recent work has significantly expanded our understanding of how T-B interactions are spatiotemporally regulated and molecularly orchestrated to fulfill those TFH functions. In this review, we elaborate two modes of T-B interactions, the antigen-specific or cognate mode in which TFH cells engage individual antigen-presenting B cells and the antigen nonspecific bystander mode in which TFH cells are engaged with the ensemble of follicular B cells. We discuss findings that indicate how short-lived cognate T-B contacts coupled with an intercellular positive feedback drive affinity-based selection and how bystander interactions between T and B cells regulate follicular T-cell recruitment and maintenance of an appropriate helper state. We argue that this combination of bystander and cognate interactions with B cells constantly shapes the internal state of TFH cells and provides the platform to execute their helper functions.