Follicular helper T (Tfh) cells are critical for the development of protective antibodies via germinal center (GC) B cell responses; however, uncontrolled Tfh cell expansion activates autoreactive B cells to produce antibodies that cause autoimmunity. The mechanisms that control Tfh cell homeostasis remain largely unknown. The aim of this study was to determine the contribution of BAFF to Tfh cell responses in autoimmunity.

We analyzed the properties of Tfh cells in lupus‐prone mice sufficient or deficient in BCMA. Adoptive transfer studies and mixed bone marrow chimeras were used to test BCMA signaling in T cells. We assessed BAFF stimulation of Tfh cells through in vitro cell cocultures and in vivo depletion studies using flow cytometry.

In Nba2 mice, Tfh cells expressed the BAFF receptors BCMA and B lymphocyte stimulator receptor 3 (BR‐3) and accumulated in the spleen when BCMA was absent. BCMA deficiency in T cells promoted the expansion of Tfh cells, GC formation, autoantibody production, and interferon‐γ (IFNγ) production by Tfh cells through BR‐3. IFNγ‐producing Tfh cells increased BAFF expression in dendritic cells. Blocking BAFF or IFNγ in vivo reduced Tfh cell accumulation and reduced autoimmunity in BCMA‐deficient animals. Moreover, circulating Tfh‐like cells that expressed BR‐3 (but not BCMA) were elevated in patients with systemic lupus erythematosus, and this correlated with serum BAFF and IFNγ levels.

In Nba2 mice, BCMA negatively regulates Tfh cell expansion, while BAFF signaling through BR‐3 promotes Tfh cell accumulation. Our findings suggest that the balance between BCMA and BR‐3 signaling in Tfh cells serves as a checkpoint of immune tolerance.