Addition of DCs to the cultures significantly increased IFN production by TFH cells from both and mice at a high DC:TFH cell ratio (1:12), with the IFN levels induced by DCs on TFH cells from mice substantially higher. cell cocultures and depletion studies using flow cytometry. Results In Nba2 mice, TFH cells expressed the BAFF receptors BCMA and BR3, and accumulated in the spleen when BCMA was absent. BCMA deficiency in T cells promoted the growth of TFH cells, GC formation, autoantibody production, and IFN production by TFH cells through BR3. IFN-producing TFH cells increased BAFF expression in dendritic cells. Blocking BAFF or IFN reduced TFH cell accumulation and improved autoimmunity in BCMA-deficient animals. Moreover, circulating TFH-like cells that expressed BR3 (but not BCMA) were elevated in SLE patients, which correlated with serum BAFF and IFN titers. Conclusion In Nba2 mice, BCMA negatively regulates TFH cell growth whereas BAFF signaling through BR3 promotes TFH cell accumulation. Our work suggests the balance between BCMA and BR3 signaling in TFH cells serves as a checkpoint of immune tolerance. Peripheral B-cell responses to foreign antigen is usually a tightly regulated process with multiple checkpoints that generate protective antibodies and prevent the development of autoantibodies (1). The coordinated interplay between antigen-specific B-cells and TFH cells is crucial in this process by establishing GCs that facilitate the selection and differentiation of memory B-cells and plasma cells (PC) that produce high-affinity antibodies (2, 3). It has been shown in mouse models of SLE that accumulation of TFH cells is usually a significant catalyst of autoantibody production and inhibiting TFH cell formation reduces disease (4). Therefore, mechanisms must exist that maintain TFH cell homeostasis under normal circumstances to avoid unchecked TFH activity, 10074-G5 inhibiting the production of pathogenic autoantibodies that promotes autoimmunity. Family members belonging to the BAFF cytokine-receptor network (BCMA, BR3, TACI) have been closely linked to B-cell homeostasis and tolerance (5). Multiple innate immune cell types including dendritic cells produce BAFF (6). BR3 (but not BCMA) is usually expressed on mature B-cells, while PCs express BCMA 10074-G5 and reduced levels of BR3. BAFF signaling through BR3 on mature B-cells is critical for their survival (7). In contrast, BCMA is usually critically required for survival of bone marrow PCs but dispensable for maintaining peripheral B-cell and PC numbers (8, 9). Increased levels of BAFF have been linked to loss of B-cell tolerance in both autoimmune mice and humans (10C13). Given that extra BAFF promotes survival and differentiation of autoreactive B-cells that arise in the GC reaction, we initially reasoned that a deficiency in BCMA of lupus-prone mice would deprive autoantibody-producing PCs of a key survival factor and therefore reduce autoantibody production. Paradoxically, we found that BCMA deficiency exacerbates the formation of autoantibody-secreting PCs in spleens of autoimmune-prone mice and the reasons for this effect is not comprehended (14). Despite evidence that BR3 is usually expressed on a subset of T cells (15C17), our knowledge of the physiologic importance of BAFF function in T cells is usually minimal. Studies in BAFF transgenic mice and arthritic mice exhibited a role for BAFF in mediating proinflammatory CD4+ T cell responses (18, 19). However, the potential role for BAFF in TFH cell homeostasis is not known. MATERIALS AND METHODS Mice inbred C57BL/6 (B6) mice were previously described (14, 20). CD45.1, CD45.2, and IFNR1?/? B6 mice were obtained from The Jackson Laboratory. Taconic provided T cell-deficient CD3e?/? B6 mice. All mice were Vasp maintained at the University of Virginia and experiments used female mice. For chimera studies, CD45.1 B6 mice were lethally irradiated with 1200 Rad 10074-G5 and reconstituted with 4106 bone marrow cells from the following CD45.2 donors, 10074-G5 isolated as previously described (21): 100% WT, 100% and mice plus anti-CD3 anti-BR3 blocking antibody for 48 hours. To evaluate BAFF expression in DCs, purified DCs from WT and IFNR1?/? mice were cultured.