Mst1 positively regulates B-cell receptor signaling via CD19 transcriptional levels

MST1/2: Important regulators of Hippo pathway in immune system associated diseases

The Hippo signaling pathway is first found in Drosophila and is highly conserved in evolution. Previous studies on this pathway in mammals have revealed its key role in cell proliferation and differentiation, organ size control, and carcinogenesis. Apart from these, recent findings indicate that mammalian Ste20-like kinases 1 and 2 (MST1/2) have significant effects on immune regulation. In this review, we summarize the updated understanding of how MST1/2 affect the regulation of the immune system and the specific mechanism. The effect of MST1/2 on immune cells and its role in the tumor immune microenvironment can alter the body's response to tumor cells. The relationship between MST1/2 and the immune system suggests new directions in the manipulation of immune responses for clinical immunotherapy, especially for tumor treatment.

LATS1 Promotes B-ALL Tumorigenesis by Regulating YAP1 Phosphorylation and Subcellular Localization

OBJECTIVE

YAP1 plays a dual role as an oncogene and tumor suppressor gene in several tumors; differentiating between these roles may depend on the YAP1 phosphorylation pattern. The specific function of YAP1 in B cell acute lymphoblastic leukemia (B-ALL), however, is currently unclear. Thus, in the present study, the role of YAP1 in B-ALL was investigated using relevant cell lines and patient datasets.

METHODS

The effects of shRNA-mediated knockdown on YAP1 and LATS1 levels in the NALM6 and MOLT-4 cell lines were examined using Western blotting, quantitative real-time polymerase chain reaction, flow cytometry, immunostaining, and nude mouse subcutaneous tumorigenesis experiments. Gene expression levels of Hippo pathway-related molecules before and after verteporfin (VP) treatment were compared using RNA-Seq to identify significant Hippo pathway-related genes in NALM6 cells.

RESULTS

Patients with ALL showing high YAP1 expression and low YAP1-Ser127 phosphorylation levels had worse prognoses than those with low YAP1 protein expression and high YAP1-Ser127 phosphorylation levels. YAP1-Ser127 phosphorylation levels were lower in NALM6 cells than in MOLT-4 and control cells; YAP1 was distributed in the nuclei in NALM6 cells. Knockdown of YAP1 inhibited MOLT-4 and NALM6 cell proliferation and arrested the NALM6 cell cycle in the G0/G1 phase. Before and after VP treatment, the expression of the upstream gene LATS1 was upregulated; its overexpression promoted YAP1-Ser127 phosphorylation. Further, YAP1 was distributed in the plasma.

CONCLUSION

LATS1 may downregulate YAP1-Ser127 phosphorylation and maintain B-ALL cell function; thus, VP, which targets this axis, may serve as a new therapeutic method for improving the outcomes for B-ALL patients.

MST1/2 in inflammation and immunity

The mammalian Sterile 20-like kinase 1/2 (MST1/2) belongs to the serine/threonine (GC) protein kinase superfamily. Collective studies confirm the vital role MST1/2 in inflammation and immunity. MST1/2 is closely related to the progress of inflammation. Generally, MST1/2 aggravates the inflammatory injury through MST1-JNK, MST1-mROS, MST1-Foxo3, and NF-κB pathways, as well as several regulatory factors such as tumor necrosis factor-α (TNF-α), mitochondrial extension factor 1 (MIEF1), and lipopolysaccharide (LPS). Moreover, MST1/2 is also involved in the regulation of immunity to balance immune activation and tolerance by regulating MST1/2-Rac, MST1-Akt1/c-myc, MST1-Foxos, MST1-STAT, Btk pathways, and lymphocyte function-related antigen 1 (LFA-1), which subsequently prevents immunodeficiency syndrome and autoimmune diseases. This article reviews the effects of MST1/2 on inflammation and immunity.

YAP1 expression is associated with survival and immunosuppression in small cell lung cancer

Immunotherapy is considered a major breakthrough in the treatment of small cell lung cancer (SCLC), although its anti-tumor efficacy is limited. With a high degree of malignancy and high heterogeneity, SCLC is difficult to treat in the clinic. A new combination strategy is urgently needed to further improve the efficacy of immunotherapy in patients with SCLC. By immunofluorescence, 100 SCLC patients in a local cohort were classified into the SCLC-A (high ASCL1 expression; n = 36), SCLC-N (high NEUROD1 expression; n = 32), SCLC-P (high POU2F3 expression; n = 14), and SCLC-Y (high YAP1 expression; n = 18) subtypes. Each SCLC molecular subtype represented different prognoses, tumor microenvironment traits, and immunotherapy sensitivities. Analysis of both the local and public cohorts suggested that the SCLC-Y subtype exhibited the worst clinical outcome (p < 0.05) when compared with other subtypes. SCLC with high YAP1 expression was characterized by high PD-L1 expression, high stromal score, T-cell functional impairment, and a close relationship with immune-related pathways. YAP1 upregulated PD-L1 expression and suppressed T cell activation, thus leading to immune evasion. In in vitro experiments, blockade of YAP1 promoted cancer cell apoptosis, immune cell proliferation, T-cell activation, and cytotoxic T-cell infiltration, thus further potentiating the efficacy of immunotherapy in patients with the SCLC-Y subtype.

Regulation of the tumor immune microenvironment by the Hippo Pathway: Implications for cancer immunotherapy

The tumor immune microenvironment (TIME) is a dynamic and complex ecosystem consisting of immune cells, stromal cells, and tumor cells. It plays a crucial role in shaping cancer progression and treatment outcomes. Notably, tumor-associated immune cells are key regulators within the TIME, influencing immune responses and therapeutic efficacy. The Hippo pathway is a critical signaling pathway involved in the TIME and cancer progression. In this review, we provide an overview of the Hippo pathway's role in the TIME, focusing on its interactions with immune cells and their implications in cancer biology and therapy. Specifically, we discuss the involvement of the Hippo pathway in regulating T-cell function, macrophage polarization, B-cell differentiation, MDSC activity, and dendritic cell-mediated immune responses. Furthermore, we explore its influence on PD-L1 expression in lymphocytes and its potential as a therapeutic target. While recent progress has been made in understanding the Hippo pathway's molecular mechanisms, challenges remain in deciphering its context-dependent effects in different cancers and identifying predictive biomarkers for targeted therapies. By elucidating the intricate crosstalk between the Hippo pathway and the TME, we aim to contribute to the development of innovative strategies for cancer treatment.

The hippo kinases MST1/2 in cardiovascular and metabolic diseases: A promising therapeutic target option for pharmacotherapy

Cardiovascular diseases (CVDs) and metabolic disorders are major components of noncommunicable diseases, causing an enormous health and economic burden worldwide. There are common risk factors and developmental mechanisms among them, indicating the far-reaching significance in exploring the corresponding therapeutic targets. MST1/2 kinases are well-established proapoptotic effectors that also bidirectionally regulate autophagic activity. Recent studies have demonstrated that MST1/2 influence the outcome of cardiovascular and metabolic diseases by regulating immune inflammation. In addition, drug development against them is in full swing. In this review, we mainly describe the roles and mechanisms of MST1/2 in apoptosis and autophagy in cardiovascular and metabolic events as well as emphasis on the existing evidence for their involvement in immune inflammation. Moreover, we summarize the latest progress of pharmacotherapy targeting MST1/2 and propose a new mode of drug combination therapy, which may be beneficial to seek more effective strategies to prevent and treat CVDs and metabolic disorders.

The Hippo signalling pathway and its implications in human health and diseases

As an evolutionarily conserved signalling network, the Hippo pathway plays a crucial role in the regulation of numerous biological processes. Thus, substantial efforts have been made to understand the upstream signals that influence the activity of the Hippo pathway, as well as its physiological functions, such as cell proliferation and differentiation, organ growth, embryogenesis, and tissue regeneration/wound healing. However, dysregulation of the Hippo pathway can cause a variety of diseases, including cancer, eye diseases, cardiac diseases, pulmonary diseases, renal diseases, hepatic diseases, and immune dysfunction. Therefore, therapeutic strategies that target dysregulated Hippo components might be promising approaches for the treatment of a wide spectrum of diseases. Here, we review the key components and upstream signals of the Hippo pathway, as well as the critical physiological functions controlled by the Hippo pathway. Additionally, diseases associated with alterations in the Hippo pathway and potential therapies targeting Hippo components will be discussed.

Dissection of multiple sclerosis genetics identifies B and CD4+ T cells as driver cell subsets

Background

Multiple sclerosis (MS) is an autoimmune condition of the central nervous system with a well-characterized genetic background. Prior analyses of MS genetics have identified broad enrichments across peripheral immune cells, yet the driver immune subsets are unclear.

Results

We utilize chromatin accessibility data across hematopoietic cells to identify cell type-specific enrichments of MS genetic signals. We find that CD4 T and B cells are independently enriched for MS genetics and further refine the driver subsets to T_h17 and memory B cells, respectively. We replicate our findings in data from untreated and treated MS patients and find that immunomodulatory treatments suppress chromatin accessibility at driver cell types. Integration of statistical fine-mapping and chromatin interactions nominate numerous putative causal genes, illustrating complex interplay between shared and cell-specific genes.

Conclusions

Overall, our study finds that open chromatin regions in CD4 T cells and B cells independently drive MS genetic signals. Our study highlights how careful integration of genetics and epigenetics can provide fine-scale insights into causal cell types and nominate new genes and pathways for disease.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13059-022-02694-y.

YAP/Hippo Pathway and Cancer Immunity: It Takes Two to Tango

Hippo pathway with its main molecule YAP is a crucial pathway for development, tissue homeostasis, wound healing, tissue regeneration, and cancer. In this review, we discuss the multiple effects of the YAP/Hippo pathway in the immune system and cancer. We analyzed a series of effects: extracellular vesicles enhanced immunity through inhibition of LATS1/2, ways of modulation of the tumor microenvironment, YAP- and TAZ-mediated upregulation of PDL1, high expression of YAP and PDL1 in EGFR-TKI-resistant cells, enhanced YAP activity in inflammation, and the effect of the Hippo pathway on T cells, B cells, Tregs, macrophages, and myeloid-derived suppressor cells (MDSCs). These pleiotropic effects render the YAP and Hippo pathway a key pathway for exploitation in the future, in order to enhance our immunotherapy treatment strategies in oncology.

CCL2 regulation of MST1-mTOR-STAT1 signaling axis controls BCR signaling and B-cell differentiation

Chemokines are important regulators of the immune system, inducing specific cellular responses by binding to receptors on immune cells. In SLE patients, decreased expression of CCL2 on mesenchymal stem cells (MSC) prevents inhibition of B-cell proliferation, causing the characteristic autoimmune phenotype. Nevertheless, the intrinsic role of CCL2 on B-cell autoimmunity is unknown. In this study using Ccl2 KO mice, we found that CCL2 deficiency enhanced BCR signaling by upregulating the phosphorylation of the MST1-mTORC1-STAT1 axis, which led to reduced marginal zone (MZ) B cells and increased germinal center (GC) B cells. The abnormal differentiation of MZ and GC B cells were rescued by in vivo inhibition of mTORC1. Additionally, the inhibition of MST1-mTORC1-STAT1 with specific inhibitors in vitro also rescued the BCR signaling upon antigenic stimulation. The deficiency of CCL2 also enhanced the early activation of B cells including B-cell spreading, clustering and signalosome recruitment by upregulating the DOCK8-WASP-actin axis. Our study has revealed the intrinsic role and underlying molecular mechanism of CCL2 in BCR signaling, B-cell differentiation, and humoral response.

WASP and Mst1 coregulate B-cell development and B-cell receptor signaling

Mst1 is a serine/threonine kinase involved in cell survival, proliferation, apoptosis, and tumorigenesis. In mice, Mst1 regulates actin dynamics required for T-cell adhesion and migration, which correlate with thymic egress and entry into lymphatic tissue. The role of Mst1 in B cells and how it may control actin-dependent processes has not been well characterized. Wiskott-Aldrich syndrome protein (WASP) deficiency only moderately affects development and B-cell receptor (BCR) signaling, suggesting WASP likely associates with other molecules. We investigated whether Mst1 associates with WASP to regulate B-cell development and activation. Experimenting on Mst1/WASP double knockout (DKO) mice, we found a severe defect in the bone marrow B-cell development, and BCR signaling in the DKO mice was severely reduced. Even though WASP or Mst1 could influence the early B-cell activation, we found that the early activation events such as B-cell spreading, BCR clustering, and BCR signaling were much more impaired in the B cells from DKO mice. Furthermore, reciprocal regulation between Mst1 and WASP was observed in WASP and Mst1 KO mice, whereby the localization and function of phosphorylated WASP were affected in Mst1 KO mice. Most importantly, Mst1 inhibits the expression of WASP by decreasing the expression of WASP-interacting protein. Interestingly, we also found that WASP deficiency in patients and mice interferes with phosphorylated Mst1 localization and therefore function in B cells. Overall, our study provides a partner for WASP to regulate B-cell development and BCR signaling, as well as the reciprocal regulating molecular mechanism of one another.

AKT2 deficiency impairs formation of the BCR signalosome

BACKGROUND

AKT2 is one of the key molecules that involves in the insulin-induced signaling and the development of cancer. In B cells, the function of AKT2 is unclear.

METHODS

In this study, we used AKT2 knockout mice model to study the role of AKT2 in BCR signaling and B cell differentiation.

RESULTS

AKT2 promotes the early activation of B cells by enhancing the BCR signaling and actin remodeling. B cells from AKT2 KO mice exhibited defective spreading and BCR clustering upon stimulation in vitro. Disruption of Btk-mediated signaling caused the impaired differentiation of germinal center B cells, and the serum levels of both sepecific IgM and IgG were decreased in the immunized AKT2 KO mice. In addition, the actin remodeling was affected due to the decreased level of the activation of WASP, the actin polymerization regulator, in AKT2 KO mice as well. As a crucial regulator of both BCR signaling and actin remodeling during early activation of B cells, the phosphorylation of CD19 was decreased in the AKT2 absent B cells, while the transcription level was normal.

CONCLUSIONS

AKT2 involves in the humoral responses, and promotes the BCR signaling and actin remodeling to enhance the activation of B cells via regulating CD19 phosphorylation. Video Abstract.

The Emerging Role of YAP/TAZ in Tumor Immunity

Yes-associated protein (YAP)/WW domain-containing transcription regulator 1 (TAZ) is an important transcriptional regulator and effector of the Hippo signaling pathway that has emerged as a critical determinant of malignancy in many human tumors. YAP/TAZ expression regulates the cross-talk between immune cells and tumor cells in the tumor microenvironment through its influence on T cells, myeloid-derived suppressor cells, and macrophages. However, the mechanisms underlying these effects are poorly understood. An improved understanding of the role of YAP/TAZ in tumor immunity is essential for exploring innovative tumor treatments and making further breakthroughs in antitumor immunotherapy. This review primarily focuses on the role of YAP/TAZ in immune cells, their interactions with tumor cells, and how this impacts on tumorigenesis, progression, and therapy resistance.

SCINA: A Semi-Supervised Subtyping Algorithm of Single Cells and Bulk Samples

Advances in single-cell RNA sequencing (scRNA-Seq) have allowed for comprehensive analyses of single cell data. However, current analyses of scRNA-Seq data usually start from unsupervised clustering or visualization. These methods ignore prior knowledge of transcriptomes and the probable structures of the data. Moreover, cell identification heavily relies on subjective and possibly inaccurate human inspection afterwards. To address these analytical challenges, we developed SCINA (Semi-supervised Category Identification and Assignment), a semi-supervised model that exploits previously established gene signatures using an expectation-maximization (EM) algorithm. SCINA is applicable to scRNA-Seq and flow cytometry/CyTOF data, as well as other data of similar format. We applied SCINA to a wide range of datasets, and showed its accuracy, stability and efficiency, which exceeded most popular unsupervised approaches. SCINA discovered an intermediate stage of oligodendrocytes from mouse brain scRNA-Seq data. SCINA also detected immune cell population changes in cytometry data in a genetically-engineered mouse model. Furthermore, SCINA performed well with bulk gene expression data. Specifically, we identified a new kidney tumor clade with similarity to FH-deficient tumors (FHD), which we refer to as FHD-like tumors (FHDL). Overall, SCINA provides both methodological advances and biological insights from perspectives different from traditional analytical methods.

Hippo Pathway in Mammalian Adaptive Immune System

The Hippo pathway was originally identified as an evolutionarily-conserved signaling mechanism that contributes to the control of organ size. It was then rapidly expanded as a key pathway in the regulation of tissue development, regeneration, and cancer pathogenesis. The increasing amount of evidence in recent years has also connected this pathway to the regulation of innate and adaptive immune responses. Notably, the Hippo pathway has been revealed to play a pivotal role in adaptive immune cell lineages, as represented by the patients with T- and B-cell lymphopenia exhibiting defective expressions of the pathway component. The complex regulatory mechanisms of and by the Hippo pathway have also been evident as alternative signal transductions are employed in some immune cell types. In this review article, we summarize the current understanding of the emerging roles of the Hippo pathway in adaptive immune cell development and differentiation. We also highlight the recent findings concerning the dual functions of the Hippo pathway in autoimmunity and anti-cancer immune responses and discuss the key open questions in the interplay between the Hippo pathway and the mammalian immune system.

The complex entanglement of Hippo-Yap/Taz signaling in tumor immunity

The Hippo-Yap/Taz pathway, originally identified as a central developmental regulator of organ size, has been found perturbed in many types of human tumors, and linked to tumor growth, survival, evasion, metastasis, stemness, and drug resistance. Beside these tumor-cell-intrinsic functions, Hippo signaling also plays important immune-regulatory roles. In this review, we will summarize and discuss recent breakthroughs in our understanding of how various components of the Hippo-Yap/Taz pathway influence the tumor immune microenvironment, including their effects on the tumor secretome and immune infiltrates, their roles in regulating crosstalk between tumor cells and T cells, and finally their intrinsic functions in various types of innate and adaptive immune cells. While further research is needed to integrate and reconcile existing findings and to discern the overall effects of Hippo signaling on tumor immunity, it is clear that Hippo signaling functions as a key bridge connecting tumor cells with both the adaptive and innate immune systems. Thus, all future therapeutic development against the Hippo-Yap/Taz pathway should take into account their multi-faceted roles in regulating tumor immunity in addition to their growth-regulatory functions. Given that immune therapies have become the mainstay of cancer treatment, it is also important to pursue how to manipulate Hippo signaling to boost response or overcome resistance to existing immune therapies.

The Coordination Between B Cell Receptor Signaling and the Actin Cytoskeleton During B Cell Activation

B-cell activation plays a crucial part in the immune system and is initiated via interaction between the B cell receptor (BCR) and specific antigens. In recent years with the help of modern imaging techniques, it was found that the cortical actin cytoskeleton changes dramatically during B-cell activation. In this review, we discuss how actin-cytoskeleton reorganization regulates BCR signaling in different stages of B-cell activation, specifically when stimulated by antigens, and also how this reorganization is mediated by BCR signaling molecules. Abnormal BCR signaling is associated with the progression of lymphoma and immunological diseases including autoimmune disorders, and recent studies have proved that impaired actin cytoskeleton can devastate the normal activation of B cells. Therefore, to figure out the coordination between the actin cytoskeleton and BCR signaling may reveal an underlying mechanism of B-cell activation, which has potential for new treatments for B-cell associated diseases.

The Mst1 Kinase Is Required for Follicular B Cell Homing and B-1 B Cell Development

The Mst1 and 2 cytosolic serine/threonine protein kinases are the mammalian orthologs of the Drosophila Hippo protein. Mst1 has been shown previously to participate in T-cell and B-cell trafficking and the migration of lymphocytes into secondary lymphoid organs in a cell intrinsic manner. We show here that the absence of Mst1 alone only modestly impacts B cell homing to lymph nodes. The absence of both Mst1 and 2 in hematopoietic cells results in relatively normal B cell development in the bone marrow and does not impact migration of immature B cells to the spleen. However, follicular B cells lacking both Mst1 and Mst2 mature in the splenic white pulp but are unable to recirculate to lymph nodes or to the bone marrow. These cells also cannot traffic efficiently to the splenic red pulp. The inability of late transitional and follicular B cells lacking Mst 1 and 2 to migrate to the red pulp explains their failure to differentiate into marginal zone B cell precursors and marginal zone B cells. Mst1 and Mst2 are therefore required for follicular B cells to acquire the ability to recirculate and also to migrate to the splenic red pulp in order to generate marginal zone B cells. In addition B-1 a B cell development is defective in the absence of Mst1.

Sulfur dioxide exposure reduces the quantity of CD19+ cells and causes nasal epithelial injury in rats

Background

Reactive airway dysfunction syndrome (RADS), also called irritant-induced asthma, is a type of occupational asthma that can occur within a very short period of latency. The study sought to investigate the influence of sulfur dioxide (SO₂) exposure on CD19+ cells and nasal epithelial injury.

Methods

We investigated the effects of SO₂ on CD19 expression and morphological changes of nasal epithelia in rats. In the study, 20 rats were randomly divided into the SO₂ exposure group that were exposed to 600 ppm SO₂, 2 h/day for consecutive 7 days, and the control group that were exposed to filtered air).

Results

Inhalation of high concentration of SO₂significantly reduced CD19 expression at both the mRNA transcript and protein levels, and reduced the percentages of CD19⁺ cells and CD19⁺/CD23⁺ cells in the nasal septum. However, inhalation of high concentration of SO₂ did not affect immunoglobulin (Ig) G, IgA and IgE levels in the serum and nasal septum. More importantly, SO₂ exposure also caused mild structural changes of the nasal septum.

Conclusion

Our results reveal that inhalation of a high concentration of SO₂ reduces CD19 expression and causes structural change of the nasal septum in rats.

The Hippo Pathway: Immunity and Cancer

Since its discovery, the Hippo pathway has emerged as a central signaling network in mammalian cells. Canonical signaling through the Hippo pathway core components (MST1/2, LATS1/2, YAP and TAZ) is important for development and tissue homeostasis while aberrant signaling through the Hippo pathway has been implicated in multiple pathologies, including cancer. Recent studies have uncovered new roles for the Hippo pathway in immunology. In this review, we summarize the mechanisms by which Hippo signaling in pathogen-infected or neoplastic cells affects the activities of immune cells that respond to these threats. We further discuss how Hippo signaling functions as part of an immune response. Finally, we review how immune cell-intrinsic Hippo signaling modulates the development/function of leukocytes and propose directions for future work.

Role of Hippo signaling in regulating immunity

The Hippo signaling pathway has been established as a key regulator of organ size control, tumor suppression, and tissue regeneration in multiple organisms. Recently, emerging evidence has indicated that Hippo signaling might play an important role in regulating the immune system in both Drosophila and mammals. In particular, patients bearing a loss-of-function mutation of MST1 are reported to have an autosomal recessive primary immunodeficiency syndrome. MST1/2 kinases, the mammalian orthologs of Drosophila Hippo, may activate the non-canonical Hippo signaling pathway via MOB1A/B and/or NDR1/2 or cross-talk with other essential signaling pathways to regulate both innate and adaptive immunity. In this review, we present and discuss recent findings of cellular mechanisms/functions of Hippo signaling in the innate immunity in Drosophila and in mammals, T cell immunity, as well as the implications of Hippo signaling for tumor immunity.

Mammalian sterile 20 kinase 1 and 2 are important regulators of hematopoietic stem cells in stress condition

The mammalian Hippo signaling pathway has been implicated in the self-renewal and differentiation of stem and progenitor cells. MST1 and MST2 (MST1/2) are core serine-threonine kinases in the Hippo signaling pathway, one of which, MST1, has been extensively investigated for its role in T cell and myeloid cell function. These studies have identified MST1 as a promising therapeutic target in immunological disease. However, the roles of MST1/2 in hematopoietic stem cell (HSC) function in vivo are not fully understood. Here, we report that mice with a conditional deletion of Mst1/2 exhibit impaired hematopoietic stem and progenitor cell (HSPC) function under stress condition. Furthermore, Mst1/2 deletion markedly altered mature cell output. Therefore, MST1/2 are indispensable for maintenance as well as function of stem and progenitor cells under steady state conditions and with transplantation stress.

MST1 deficiency promotes B cell responses by CD4+ T cell-derived IL-4, resulting in hypergammaglobulinemia

MST1 deficiency causes T and B cell lymphopenia, resulting in combined immunodeficiency. However, MST1-deficient patients also exhibit autoimmune-like symptoms such as hypergammaglobulinemia and autoantibody production. Recent studies have shown that the autoimmune responses observed in MST1-deficient patients were most likely attributable to defective regulatory T (Treg) cells instead of intrinsic signals in MST1-lacking B cells. Nevertheless, it is not determined how MST1 deficiency in T cells breaks B cell tolerance and causes systemic autoimmune-like phenotypes. In this study, we confirmed that Mst1^-/- mice developed hypergammaglobulinemia associated with increased levels of IgG, IgA, and IgE. We also showed that uncontrolled B cell responses were resulted from the IL-4-rich environment created by CD4⁺ T cells. Defective MST1-FOXO1 signaling down-regulated Treg cells, resulting in the collapse of immune tolerance where the populations of Th2 and T follicular helper cells expanded. In conclusion, we suggest that MST1 acts as a molecular brake to maintain immune tolerance by regulating T cell-mediated B cell activation.