Follicular regulatory T cells (TFR cells) inhibit follicular helper T cell (TFH cell)-mediated antibody production. The mechanisms by which TFR cells exert their key immunoregulatory functions are largely unknown. Here we found that TFR cells induced a distinct suppressive state in TFH cells and B cells, in which effector transcriptional signatures were maintained but key effector molecules and metabolic pathways were suppressed. The suppression of B cell antibody production and metabolism by TFR cells was durable and persisted even in the absence of TFR cells. This durable suppression was due in part to epigenetic changes. The cytokine IL-21 was able to overcome TFR cell-mediated suppression and inhibited TFR cells and stimulated B cells. By determining mechanisms of TFR cell-mediated suppression, we have identified methods for modulating the function of TFR cells and antibody production.