Roles of follicular helper and regulatory T cells in allergic diseases and allergen immunotherapy.

Allergic diseases are characterized by overactive type 2 immune responses to allergens and immunoglobulin E (IgE)-mediated hypersensitivity. Emerging evidence suggests that follicular helper T (TFH ) cells, rather than type 2 T-helper (TH 2) cells, play a crucial role in controlling IgE production. However, follicular regulatory T (TFR ) cells, a specialized subset of regulatory T (TREG ) cells resident in B-cell follicles, restricts TFH cell-mediated help in extrafollicular antibody production, germinal center (GC) formation, immunoglobulin affinity maturation, and long-lived, high-affinity plasma and memory B-cell differentiation. In mouse models of allergic asthma and food allergy, CXCR5+ TFH cells, not CXCR5- conventional TH 2 cells, are needed to support IgE production, otherwise exacerbated by CXCR5+ TFR cell deletion. Upregulation of TFH cell activities, including a skewing toward type 2 TFH (TFH 2) and IL-13 producing TFH (TFH 13) phenotypes, and defects in TFR cells have been identified in patients with allergic diseases. Allergen immunotherapy (AIT) reinstates the balance between TFH and TFR cells in patients with allergic diseases, resulting in clinical benefits. Collectively, further understanding of TFH and TFR cells and their role in the immunopathogenesis of allergic diseases creates opportunities to develop novel therapeutic approaches.