Germinal centers: Dynamic signaling by T follicular helper cells during germinal center B cell selection

T follicular helper (T_FH) cells select high-affinity, antibody-producing B cells for clonal expansion in germinal centers (GCs), but the nature of their interaction is not well defined. Using intravital imaging, we found that selection is mediated by large but transient contacts between T_FHand GC B cells presenting the highest levels of cognate peptide bound to major histocompatibility complex II. These interactions elicited transient and sustained increases in T_FHintracellular free calcium (Ca²⁺) that were associated with T_FHcell coexpression of the cytokines interleukin-4 and-21. However, increased intracellular Ca²⁺did not arrest T_FHcell migration. Instead, T^FHcells remained motile and continually scanned the surface of many GC B cells, forming short-lived contacts that induced selection through further repeated transient elevations in intracellular Ca²⁺.