STAT3 signaling in B cells controls germinal center zone organization and recycling

Germinal centers (GCs), sites of antibody affinity maturation, are organized into dark (DZ) and light (LZ) zones. Here, we show a B cell-intrinsic role for signal transducer and activator of transcription 3 (STAT3) in GC DZ and LZ organization. Altered zonal organization of STAT3-deficient GCs dampens development of long-lived plasma cells (LL-PCs) but increases memory B cells (MBCs). In an abundant antigenic environment, achieved here by prime-boost immunization, STAT3 is not required for GC initiation, maintenance, or proliferation but is important for sustaining GC zonal organization by regulating GC B cell recycling. Th cell-derived signals drive STAT3 tyrosine 705 and serine 727 phosphorylation in LZ B cells, regulating their recycling into the DZ. RNA sequencing (RNA-seq) and chromatin immunoprecipitation sequencing (ChIP-seq) analyses identified STAT3 regulated genes that are critical for LZ cell recycling and transiting through DZ proliferation and differentiation phases. Thus, STAT3 signaling in B cells controls GC zone organization and recycling, and GC egress of PCs, but negatively regulates MBC output.

Context-Dependent miR-21 Regulation of TLR7-Mediated Autoimmune and Foreign Antigen-Driven Antibody-Forming Cell and Germinal Center Responses

MicroRNAs (miRNAs) are involved in healthy B cell responses and the loss of tolerance in systemic lupus erythematosus (SLE), although the role of many miRNAs remains poorly understood. Dampening miR-21 activity was previously shown to reduce splenomegaly and blood urea nitrogen levels in SLE-prone mice, but the detailed cellular responses and mechanism of action remains unexplored. In this study, using the TLR7 agonist, imiquimod-induced SLE model, we observed that loss of miR-21 in Sle1b mice prevented the formation of plasma cells and autoantibody-producing Ab-forming cells (AFCs) without a significant effect on the magnitude of the germinal center (GC) response. We further observed reduced dendritic cell and monocyte numbers in the spleens of miR-21-deficient Sle1b mice that were associated with reduced IFN, proinflammatory cytokines, and effector CD4⁺ T cell responses. RNA sequencing analysis on B cells from miR-21-deficient Sle1b mice revealed reduced activation and response to IFN, and cytokine and target array analysis revealed modulation of numerous miR-21 target genes in response to TLR7 activation and type I IFN stimulation. Our findings in the B6.Sle1bYaa (Sle1b ^Yaa) spontaneous model recapitulated the miR-21 role in TLR7-induced responses with an additional role in autoimmune GC and T follicular helper responses. Finally, immunization with T-dependent Ag revealed a role for miR-21 in foreign Ag-driven GC and Ab, but not AFC, responses. Our data suggest a potential multifaceted, context-dependent role for miR-21 in autoimmune and foreign Ag-driven AFC and GC responses. Further study is warranted to delineate the cell-intrinsic requirements and mechanisms of miR-21 during infection and SLE development.

STAT4 Is Largely Dispensable for Systemic Lupus Erythematosus-like Autoimmune- and Foreign Antigen-Driven Antibody-Forming Cell, Germinal Center, and Follicular Th Cell Responses

Genome-wide association studies identified variants in the transcription factor STAT4 gene and several other genes in the STAT4 signaling pathway, such as IL12A, IL12B, JAK2, and TYK2, which are associated with an increased risk of developing systemic lupus erythematosus (SLE) and other autoimmune diseases. Consistent with the genome-wide association studies data, STAT4 was shown to play an important role in autoimmune responses and autoimmunity development in SLE mouse models. Despite such important role for STAT4 in SLE development in mice and humans, little is known whether and how STAT4 may regulate extrafollicular Ab-forming cell (AFC) and follicular germinal center (GC) responses, two major pathways of autoreactive B cell development and autoantibody production. To our surprise, we found STAT4 to be largely dispensable for promoting autoimmune AFC and GC responses in various autoimmune- and SLE-prone mouse models, which strongly correlated with autoantibody production, and immune complex deposition and immune cell infiltration in the kidney. We further observed that STAT4 deficiency had no effects on AFC, GC, and Ag-specific Ab responses during protein Ag immunization or influenza virus infection. Additionally, CD4+ effector and follicular Th cell responses in autoimmune- and SLE-prone mice and protein Ag-immunized and influenza virus-infected mice were intact in the absence of STAT4. Together, our data demonstrate a largely dispensable role for STAT4 in AFC, GC, and Ab responses in SLE mouse models and in certain foreign Ag-driven responses.

TLR7 Negatively Regulates B10 Cells Predominantly in an IFNγ Signaling Dependent Manner

IL-10 producing B cells (B10 cells) play an important immunoregulatory role in various autoimmune and infection conditions. However, the factors that regulate their development and maintenance are incompletely understood. Recently, we and others have established a requirement for TLR7 in promoting autoimmune antibody forming cell (AFC) and germinal center (GC) responses. Here we report an important additional role of TLR7 in the negative regulation of B10 cell development. TLR7 overexpression or overstimulation promoted the reduction of B10 cells whereas TLR7 deficiency rescued these cells in both non-autoimmune and autoimmune-prone mice. TLR7 expression was further inversely correlated with B cell-dependent IL-10 production and its inhibition of CD4 T cell proliferation and IFNγ production in an in vitro B cell and T cell co-culture system. Further, B10 cells displayed elevated TLR7, IFNγR, and STAT1 expression compared to non-B10 cells. Interestingly, deficiency of IFNγR in TLR7 overexpressing lupus-prone mice rescued B10 cells from TLR7-mediated reduction. Finally, B cell intrinsic deletion of IFNγR was sufficient to restore B10 cells in the spleens of TLR7-promoted autoimmune mouse model. In conclusion, our findings demonstrate a novel role for the IFNγR-STAT1 pathway in TLR7-mediated negative regulation of B10 cell development.

Serine Phosphorylation of the STAT1 Transactivation Domain Promotes Autoreactive B Cell and Systemic Autoimmunity Development

Although STAT1 tyrosine-701 phosphorylation (designated STAT1-pY701) is indispensable for STAT1 function, the requirement for STAT1 serine-727 phosphorylation (designated STAT1-pS727) during systemic autoimmune and antipathogen responses remains unclear. Using autoimmune-prone B6.Sle1b mice expressing a STAT1-S727A mutant in which serine is replaced by alanine, we report in this study that STAT1-pS727 promotes autoimmune Ab-forming cell (AFC) and germinal center (GC) responses, driving autoantibody production and systemic lupus erythematosus (SLE) development. In contrast, STAT1-pS727 is not required for GC, T follicular helper cell (Tfh), and Ab responses to various foreign Ags, including pathogens. STAT1-pS727 is also not required for gut microbiota and dietary Ag-driven GC and Tfh responses in B6.Sle1b mice. By generating B cell-specific bone marrow chimeras, we demonstrate that STAT1-pS727 plays an important B cell-intrinsic role in promoting autoimmune AFC, GC, and Tfh responses, leading to SLE-associated autoantibody production. Our analysis of the TLR7-accelerated B6.Sle1b.Yaa SLE disease model expressing a STAT1-S727A mutant reveals STAT1-pS727-mediated regulation of autoimmune AFC and GC responses and lupus nephritis development. Together, we identify previously unrecognized differential regulation of systemic autoimmune and antipathogen responses by STAT1-pS727. Our data implicate STAT1-pS727 as a therapeutic target for SLE without overtly affecting STAT1-mediated protection against pathogenic infections.

B cell-intrinsic STAT3 regulates the germinal center dark zone

Germinal centers (GCs) are sites for antibody diversification, somatic hypermutation (SHM) and antigen specific B cell selection. GCs are anatomically divided into two distinct areas known as dark zone (DZ) and light zone (LZ) where SHM and clonal selection occurs, respectively. Although previous studies have suggested a B cell-intrinsic role of STAT3 in GC maintenance and antibody responses, how STAT3 signaling in B cells may maintain GCs for B cell selection is not clear. Using immunization and infection models, we show that STAT3 in B cells regulates the GC DZ, resulting in decreased high affinity antibody production. STAT3 deficient DZ B cells demonstrate a decreased cell proliferation and increased apoptotic activity. Further, STAT3 deletion in B cells led to defects in plasma cell formation and B cell memory response. Using GC B cell specific STAT3 conditional knockout mice and GC B cell culture system, we show that STAT3 deficient GC B cells display low amounts of FOXO1, a critical transcription factor required for GC DZ program. Transcriptomic analysis of GC B cells from STAT3 deficient and sufficient mice reveals STAT3-mediated gene regulation of multiple cellular pathways known to be involved in GC DZ maintenance. In conclusion, STAT3 signaling in B cells controls the GC DZ, which, in turn, helps maintain GCs for high-affinity antibody production. Our findings identify STAT3 as a novel target for enhancing humoral immunity as well as for the treatment of diseases involving abnormal GC activity.

Serine phosphorylation of the STAT1 transactivation domain promotes autoreactive B cell and systemic autoimmunity development

Although STAT1 tyrosine-701 phosphorylation (STAT1-pY701) is indispensable for STAT1 function, the requirement for STAT1 serine-727 phosphorylation (STAT1-pS727) during autoimmune and anti-pathogen responses remains unclear. Here we report that STAT1-pS727 promotes autoimmune antibody-forming cell (AFC) and germinal center (GC) responses, driving systemic lupus erythematosus (SLE) development. STAT1-pS727, however, is not required for GC and antibody responses to foreign-antigens including pathogens or gut microbiota. STAT1-pS727 plays an important B cell-intrinsic role in driving autoimmunity. Transcriptomic analysis of B cells from TLR7-accelerated SLE-prone mice reveals STAT1-pS727-mediated gene regulation of cellular pathways known to be involved in autoimmune GC and AFC responses. Mechanistically, TLR7 activation in B cells induces STAT1-pS727 and during autoimmune responses TLR7 signaling converges with IFNγ-STAT1 signaling in B cells by recruiting STAT1 into the MyD88 signaling complex. Together, we identify previously unappreciated differential regulation of autoimmune and anti-pathogen responses by STAT1-pS727, and implicate STAT1-pS727 as a therapeutic target for SLE.

Type II but Not Type I IFN Signaling Is Indispensable for TLR7-Promoted Development of Autoreactive B Cells and Systemic Autoimmunity

TLR7 is associated with development of systemic lupus erythematosus (SLE), but the underlying mechanisms are incompletely understood. Although TLRs are known to activate type I IFN (T1IFN) signaling, the role of T1IFN and IFN-γ signaling in differential regulation of TLR7-mediated Ab-forming cell (AFC) and germinal center (GC) responses, and SLE development has never been directly investigated. Using TLR7-induced and TLR7 overexpression models of SLE, we report in this study a previously unrecognized indispensable role of TLR7-induced IFN-γ signaling in promoting AFC and GC responses, leading to autoreactive B cell and SLE development. T1IFN signaling in contrast, only modestly contributed to autoimmune responses and the disease process in these mice. TLR7 ligand imiquimod treated IFN-γ reporter mice show that CD4⁺ effector T cells including follicular helper T (Tfh) cells are the major producers of TLR7-induced IFN-γ. Transcriptomic analysis of splenic tissues from imiquimod-treated autoimmune-prone B6.Sle1b mice sufficient and deficient for IFN-γR indicates that TLR7-induced IFN-γ activates multiple signaling pathways to regulate TLR7-promoted SLE. Conditional deletion of Ifngr1 gene in peripheral B cells further demonstrates that TLR7-driven autoimmune AFC, GC and Tfh responses and SLE development are dependent on IFN-γ signaling in B cells. Finally, we show crucial B cell-intrinsic roles of STAT1 and T-bet in TLR7-driven GC, Tfh and plasma cell differentiation. Altogether, we uncover a nonredundant role for IFN-γ and its downstream signaling molecules STAT1 and T-bet in B cells in promoting TLR7-driven AFC, GC, and SLE development whereas T1IFN signaling moderately contributes to these processes.

Strain-Dependent Contribution of MAVS to Spontaneous Germinal Center Responses

Germinal centers (GCs) are essential for the production of somatically hypermutated, class-switched Abs that are protective against infection, but they also form in the absence of purposeful immunization or infection, and are termed spontaneous GCs (Spt-GCs). Although Spt-GCs can arise in nonautoimmune-prone mice, aberrant regulation of Spt-GCs in autoimmune-prone mice is strongly associated with the development of autoimmune diseases like systemic lupus erythematosus. The formation of Spt-GCs is crucially driven by TLR7-mediated RNA sensing. However, the impact of MAVS-dependent, Rig-like receptor-mediated RNA sensing on the Spt-GC response remains unknown. In this study, we assessed the Spt-GC response and splenic B cell development in two MAVS-deficient mice with distinct genetic backgrounds. Importantly, we found that MAVS differentially controls Spt-GC responses and B cell development, depending on genetic background. B6/129 mixed background MAVSKO mice had nearly absent Spt-GC responses in the spleen and cervical lymph nodes, which were associated with impaired splenic B cell development, in addition to impaired B cell activation and TLR7 expression. Interestingly, treatment of mice with TLR7 agonist could partially rescue GC responses by overcoming follicular B cell activation deficits. Contrastingly, the absence of MAVS on a B6 background resulted in normal B cell development and Spt-GC formation. Our results highlight important differences in the contribution of MAVS to B cell development and Spt-GC function, depending on the genetic background, warranting greater regard for the impact of genetic background in further studies using these mice for the study of autoimmunity.