Roles of T Follicular Helper Cells and T Follicular Regulatory Cells in Autoantibody Production in IL-2-Deficient Mice.

Autoantibodies can result from excessive T follicular helper (Tfh) cell activity, whereas T follicular regulatory (Tfr) cells negatively regulate autoantibody production. IL-2 knockout (KO) mice on the BALB/c background have elevated Tfh responses, produce autoantibodies, and develop lethal autoimmunity. We analyzed Tfh and Tfr cells in IL-2 KO mice on the C57BL/6 (B6) genetic background. In B6 IL-2 KO mice, the spontaneous formation of Tfh cells and germinal center B cells was greatly enhanced, along with production of anti-DNA autoantibodies. IL-2 has been reported to repress Tfr cell differentiation; however, Tfr cells were not increased over wild-type levels in the B6 IL-2 KO mice. To assess Tfh and Tfr cell regulation of autoantibody production in IL-2 KO mice, we generated IL-2 KO mice with a T cell-specific deletion of the master Tfh cell transcription factor Bcl6. In IL-2 KO Bcl6 conditional KO (2KO-Bcl6TC) mice, Tfh cells, Tfr cells, and germinal center B cells were ablated. In contrast to expectations, autoantibody IgG titers in 2KO-Bcl6TC mice were significantly elevated over autoantibody IgG titers in IL-2 KO mice. Specific deletion of Tfr cells with Foxp3-cre Bcl6-flox alleles in IL-2 KO mice led to early lethality, before high levels of autoantibodies could develop. We found IL-2+/+ Tfr cell-deficient mice produce significant levels of autoantibodies. Our overall findings provide evidence that Tfh cells are dispensable for high-level production of autoantibodies and also reveal a complex interplay between Tfh and Tfr cells in autoantibody production and autoimmune disease.

Xie MM ，Liu H ，Corn C ，Koh BH ，Kaplan MH ，Turner MJ ，Dent AL ... -  《-》

Interleukin-21 promotes germinal center reaction by skewing the follicular regulatory T cell to follicular helper T cell balance in autoimmune BXD2 mice.

Follicular regulatory T (Tfr) cells act as the regulatory counterpart of follicular helper T (Tfh) cells to suppress germinal center (GC) B cell differentiation. We recently showed that interleukin-21 (IL-21) promoted Tfh cell differentiation in autoimmune BXD2 mice that develop spontaneous GCs. This study was undertaken to determine the modulatory effects of IL-21 on Tfr cells and the Tfr cell to Tfh cell balance in BXD2 mice. The percentage and phenotype of Tfr cells were determined in BXD2 and BXD2-IL21(-/-) mice. The effects of IL-21 on Tfr cells and the Tfr cell:Tfh cell ratio were evaluated. Sorted Tfr cells from BXD2-IL21(-/-) mice were cocultured with Tfh cells and B cells, or transferred into BXD2 mice to determine their function. The percentages and numbers of GC B cells and Tfh cells were significantly reduced, but the percentage of Tfr cells was 2-fold higher in BXD2-IL21(-/-) mice than in wild-type BXD2 mice. Administration of AdIL-21 to BXD2-IL21(-/-) mice decreased the percentages and numbers of Tfr cells and the Tfr cell:Tfh cell ratio but increased the number of GC B cells in the spleen. Recombinant murine IL-21 suppressed FoxP3 and significantly reduced Tgfb1, Il2, and Gitr but enhanced Il21, Il6, Pd1, Cxcr5, and Icos expression in Tfr cells. IL-21 also counteracted Tfr cell-mediated inhibition of antibody secretion in the Tfh cell-B cell coculture system. Transfer of Tfr cells into young BXD2 mice reduced GC size and decreased the numbers of autoantibody-producing B cells. Our findings indicate that high levels of IL-21 selectively enhance Tfh cell differentiation but inhibit Tfr cell commitment and the suppressive function of Tfr cells on Tfh cells and B cells, suggesting that IL-21 skews the balance from Tfr cells to Tfh cells to promote autoreactive GC reactions in BXD2 mice.

Ding Y ，Li J ，Yang P ，Luo B ，Wu Q ，Zajac AJ ，Wildner O ，Hsu HC ，Mountz JD ... -  《-》

Follicular regulatory T cells repress cytokine production by follicular helper T cells and optimize IgG responses in mice.

Follicular helper T (Tfh) cells provide crucial help to germinal center B (GCB) cells for proper antibody production, and a specialized subset of regulatory T cells, follicular regulatory T (Tfr) cells, modulate this process. However, Tfr-cell function in the GC is not well understood. Here, we define Tfr cells as a CD4(+) Foxp3(+) CXCR5(hi) PD-1(hi) CD25(low) TIGIT(high) T-cell population. Furthermore, we have used a novel mouse model ("Bcl6FC") to delete the Bcl6 gene in Foxp3(+) T cells and thus specifically deplete Tfr cells. Following immunization, Bcl6FC mice develop normal Tfh- and GCB-cell populations. However, Bcl6FC mice produce altered antigen-specific antibody responses, with reduced titers of IgG and significantly increased IgA. Bcl6FC mice also developed IgG antibodies with significantly decreased avidity to antigen in an HIV-1 gp120 "prime-boost" vaccine model. In an autoimmune lupus model, we observed strongly elevated anti-DNA IgA titers in Bcl6FC mice. Additionally, Tfh cells from Bcl6FC mice consistently produce higher levels of Interferon-γ, IL-10 and IL-21. Loss of Tfr cells therefore leads to highly abnormal Tfh-cell and GCB-cell responses. Overall, our study has uncovered unique regulatory roles for Tfr cells in the GC response.

Wu H ，Chen Y ，Liu H ，Xu LL ，Teuscher P ，Wang S ，Lu S ，Dent AL ... -  《-》

Gut Commensal Segmented Filamentous Bacteria Fine-Tune T Follicular Regulatory Cells to Modify the Severity of Systemic Autoimmune Arthritis.

Autoantibodies play a major pathogenic role in rheumatoid arthritis. T follicular helper (Tfh) cells promote germinal center B cell and Ab responses. Excessive Tfh cell responses lead to autoimmunity, and therefore, counterregulation is crucial. T follicular regulatory (Tfr) cells, mainly differentiated from T regulatory cells, can negatively regulate Tfh and germinal center B cells. Dysbiosis is involved in rheumatoid arthritis's pathogenesis. We previously demonstrated that the gut microbiota, segmented filamentous bacteria (SFB), promote autoimmune arthritis by inducing Tfh cells. However, little is known regarding whether gut microbiota influence systemic (nongut) Tfr cells, impacting gut-distal autoimmunity. In this study, using SFB in autoimmune arthritic K/BxN mice, we demonstrated that SFB-induced arthritis is linked to the reduction of Tfr cells' CTLA-4, the key regulatory molecule of Tfr cells. This SFB-mediated CTLA-4 reduction is associated with increased Tfr glycolytic activity, and glycolytic inhibition increases Tfr cells' CTLA-4 levels and reduces arthritis. The surface expression of CTLA-4 is tied to TCR signaling strength, and we discovered that SFB-reduced CTLA-4 is associated with a reduction of Nur77, an indicator of TCR signaling strength. Nur77 is known for repressing glycolytic activity. Using a loss-of-function study, we demonstrated that Nur77+/- haplodeficiency increases glycolysis and reduces CTLA-4 on Tfr cells, which is associated with increased arthritis and anti-glucose-6-phosphate isomerase titers. Tfr-specific deletion (KRN.Foxp3CreBcl-6fl/fl) in autoimmune condition reveals that Tfr cells repress arthritis, Tfh cells, and autoantibody responses and that SFB can mitigate this repression. Overall, these findings demonstrated that gut microbiota distally impact systemic autoimmunity by fine-tuning Tfr cells.

Bates NA ，Li A ，Fan T ，Cutcliffe MP ，Dagenet CB ，Sleiman KC ，Ma H ，Tahsin S ，Garrett CS ，Altemus J ，Wu HJ ... -  《-》

Bcl6 promotes follicular helper T-cell differentiation and PD-1 expression in a Blimp1-independent manner in mice.

The transcription factors Bcl6 and Blimp1 have opposing roles in the development of the follicular helper T (TFH ) cells: Bcl6 promotes and Blimp1 inhibits TFH -cell differentiation. Similarly, Bcl6 activates, while Blimp1 represses, expression of the TFH -cell marker PD-1. However, Bcl6 and Blimp1 repress each other's expression, complicating the interpretation of the regulatory network. Here we sought to clarify the extent to which Bcl6 and Blimp1 independently control TFH -cell differentiation by generating mice with T-cell specific deletion of both Bcl6 and Blimp1 (double conditional KO [dcKO] mice). Our data indicate that Blimp1 does not control TFH -cell differentiation independently of Bcl6. However, a population of T follicular regulatory (TFR ) cells developed independently of Bcl6 in dcKO mice. We have also analyzed regulation of IL-10 and PD-1, two genes controlled by both Bcl6 and Blimp1, and observed that Bcl6 regulates both genes independently of Blimp1. We found that Bcl6 positively regulates PD-1 expression by inhibiting the ability of T-bet/Tbx21 to repress Pdcd1 transcription. Our data provide a novel mechanism for positive control of gene expression by Bcl6, and illuminate how Bcl6 and Blimp1 control TFH -cell differentiation.

Xie MM ，Koh BH ，Hollister K ，Wu H ，Sun J ，Kaplan MH ，Dent AL ... -  《-》