Regulating germinal center contractionGerminal centers (GCs) in secondary lymphoid organs are where mature B cells expand and differentiate. Although GC formation is well studied, the control of GC duration and contraction is less well understood. Using intravital imaging of mouse GCs and single-cell RNA sequencing, Jacobsenet al.report that T follicular helper (T_FH) cells are a critical player in this process. They found that some late-GC T_FHcells upregulate the transcription factor FOXP3 and acquire a regulatory T cell–like phenotype. These cells are distinct from T follicular regulatory (T_FR) cells and, unlike T_FRcells, are needed to shut down the GC reaction. Tweaking this process may be key to extending GC lifetimes and enhancing antibody responses in the context of vaccination.Science, abe5146, this issue p.eabe5146