Mst1 regulates integrin-dependent thymocyte trafficking and antigen recognition in the thymus

Macrophage-Stimulating 1 Polymorphism rs3197999 in Pediatric Patients with Inflammatory Bowel Disease

Background and Objectives: Inflammatory bowel disease (IBD), which includes Crohn's disease (CD) and ulcerative colitis (UC), often necessitates long-term treatment and hospitalizations and also may require surgery. The macrophage-stimulating 1 (MST1) rs3197999 polymorphism is strongly associated with the risk of IBD but its exact clinical correlates remain under investigation. We aimed to characterize the relationships between the MST1 rs3197999 genotype and the clinical characteristics in children and adolescents with IBD within a multi-center cross-sectional study. Materials and Methods: Clinical data included serum C-reactive protein (CRP), albumin, activity indices (PUCAI, PCDAI), anthropometric data, pharmacotherapy details, surgery, and disease severity. Genotyping for rs3197999 was carried out using TaqMan hydrolysis probes. Results: The study included 367 pediatric patients, 197 with Crohn's disease (CD) (40.6% female; a median age of 15.2 years [interquartile range 13.2-17.0]) and 170 with ulcerative colitis (UC) (45.8% female; a median age of 15.1 years [11.6-16.8]). No significant relationships were found between MST1 genotypes and age upon first biologic use, time from diagnosis to biological therapy introduction, PUCAI, PCDAI, or hospitalizations for IBD flares. However, in IBD, the height Z-score at the worst flare was negatively associated with the CC genotype (p = 0.016; CC: -0.4 [-1.2-0.4], CT: -0.1 [-0.7-0.8], TT: 0.0 [-1.2-0.7)]). The TT genotype was associated with higher C-reactive protein upon diagnosis (p = 0.023; CC: 4.3 mg/dL [0.7-21.8], CT 5.3 mg/dL [1.3-17.9], TT 12.2 mg/dL [3.0-32.9]). Conclusions: This study identified links between MST1 rs3197999 and the clinical characteristics of pediatric IBD: height Z-score and CRP. Further studies of the associations between genetics and the course of IBD are still warranted, with a focus on more extensive phenotyping.

Thymocyte migration and emigration

Hematopoietic precursors (HPCs) entering into the thymus undergo a sequential process leading to the generation of a variety of T cell subsets. This developmental odyssey unfolds in distinct stages within the thymic cortex and medulla, shaping the landscape of T cell receptor (TCR) expression and guiding thymocytes through positive and negative selection. Initially, early thymic progenitors (ETPs) take residence in the thymic cortex, where thymocytes begin to express their TCR and undergo positive selection. Subsequently, thymocytes transition to the thymic medulla, where they undergo negative selection. Both murine and human thymocyte development can be broadly classified into distinct stages based on the expression of CD4 and CD8 coreceptors, resulting in categorizations as double negative (DN), double positive (DP) or single positive (SP) cells. Thymocyte migration to the appropriate thymic microenvironment at the right differentiation stage is pivotal for the development and the proper functioning of T cells, which is critical for adaptive immune responses. The journey of lymphoid progenitor cells into the T cell developmental pathway hinges on an ongoing dialogue between the differentiating cell and the signals emanating from the thymus niche. Herein, we review the contribution of the key factors mentioned above for the localization, migration and emigration of thymocytes.

Chemokine receptors in primary and secondary lymphoid tissues

Chemokine receptors are a complex superfamily of surface G protein-coupled receptors present mostly in leukocytes. In this chapter, we review the presence and functions of chemokine receptors in the immune cells of the primary and secondary lymphoid organs. Those include bone marrow, thymus, spleen, lymph nodes, and Peyer's patches as the main components of the gut-associated lymphoid tissue. There are general groups of chemokine receptors: conventional and atypical. We will mostly cover the role of conventional chemokine receptors, which are divided into four classes (CC, CXC, CX3C, and XC). Some relevant members are CXCR4, CXCR5, CCR4 and CCR7. For example, CXCR4 is a key chemokine receptor in the bone marrow controlling from the homing of progenitor cells into the bone marrow, the development of B cells, to the homing of long-lived plasma cells to this primary lymphoid organ. CCR7 and CCR4 are two of the main players in the thymus. CCR7 along with CCR9 control the traffic of thymic seed progenitors into the thymus, while CCR4 and CCR7 are critical for the entry of thymocytes into the medulla and as controllers of the central tolerance in the thymus. CXCR4 and CXCR5 have major roles in the spleen, guiding the maturation and class-switching of B cells in the different areas of the germinal center. In the T-cell zone, CCR7 guides the differentiation of naïve T cells. CCR7 also controls and directs the entry of T cells, B cells, and dendritic cells into secondary lymphoid tissues, including the spleen and lymph nodes. As new technologies emerge, techniques such as high dimensional spectral flow cytometry or single-cell sequencing allow a more comprehensive knowledge of the chemokine receptor network and their ligands, as well as the discovery of new interactions mediating unknown and overlooked mechanisms in health and disease.

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.

Protective effect of low-intensity pulsed ultrasound on immune checkpoint inhibitor-related myocarditis via fine-tuning CD4+ T-cell differentiation

PURPOSE

Immune checkpoint inhibitors (ICIs) have transformed traditional cancer treatments. Specifically, ICI-related myocarditis is an immune-related adverse event (irAE) with high mortality. ICIs activate CD4+ T-lymphocyte reprogramming, causing an imbalance between Th17 and Treg cell differentiation, ultimately leading to myocardial inflammatory damage. Low-intensity pulsed ultrasound (LIPUS) can limit inflammatory responses, with positive therapeutic effects across various cardiovascular inflammatory diseases; however, its role in the pathogenesis of ICI-related myocarditis and CD4+ T-cell dysfunction remains unclear. Accordingly, this study investigated whether LIPUS can alleviate ICI-related myocarditis inflammatory damage and, if so, aimed to elucidate the beneficial effects of LIPUS and its underlying molecular mechanisms.

METHODS

An in vivo model of ICI-related myocarditis was obtained by intraperitonially injecting male A/J mice with an InVivoPlus anti-mouse PD-1 inhibitor. LIPUS treatment was performed via an ultrasound-guided application to the heart via the chest wall. The echocardiographic parameters were observed and cardiac function was assessed using an in vivo imaging system. The expression of core components of the HIPPO pathway was analyzed via western blotting.

RESULTS

LIPUS treatment reduced cardiac immune responses and inflammatory cardiac injury. Further, LIPUS treatment alleviated the inflammatory response in mice with ICI-related myocarditis. Mechanistically, in the HIPPO pathway, the activation of Mst1-TAZ axis improved autoimmune inflammation by altering the interaction between the transcription factors FOXP3 and RORγt and regulating the differentiation of Treg and Th17 cells.

CONCLUSION

LIPUS therapy was shown to reduce ICI-related myocarditis inflammatory damage and improve cardiac function, representing an exciting finding for irAEs treatment.

The induction of SHP-1 degradation by TAOK3 ensures the responsiveness of T cells to TCR stimulation

Thousand-and-one-amino acid kinase 3 (TAOK3) is a serine and threonine kinase that belongs to the STE-20 family of kinases. Its absence reduces T cell receptor (TCR) signaling and increases the interaction of the tyrosine phosphatase SHP-1, a major negative regulator of proximal TCR signaling, with the kinase LCK, a component of the core TCR signaling complex. Here, we used mouse models and human cell lines to investigate the mechanism by which TAOK3 limits the interaction of SHP-1 with LCK. The loss of TAOK3 decreased the survival of naïve CD4+ T cells by dampening the transmission of tonic and ligand-dependent TCR signaling. In mouse T cells, Taok3 promoted the secretion of interleukin-2 (IL-2) in response to TCR activation in a manner that depended on Taok3 gene dosage and on Taok3 kinase activity. TCR desensitization in Taok3-/- T cells was caused by an increased abundance of Shp-1, and pharmacological inhibition of Shp-1 rescued the activation potential of these T cells. TAOK3 phosphorylated threonine-394 in the phosphatase domain of SHP-1, which promoted its ubiquitylation and proteasomal degradation. The loss of TAOK3 had no effect on the abundance of SHP-2, which lacks a residue corresponding to SHP-1 threonine-394. Modulation of SHP-1 abundance by TAOK3 thus serves as a rheostat for TCR signaling and determines the activation threshold of T lymphocytes.

Role of the Hippo pathway in autoimmune diseases

The immune system is an important defense against diseases, and it is essential to maintain the homeostasis of the body's internal environment. Under normal physiological conditions, the steady state of the immune system should be sustained to play normal immune response and immune function. Exploring the molecular mechanism of maintaining immune homeostasis under physiological and pathological conditions will provides understanding of the pathogenesis of autoimmune diseases, infections, metabolic disorders, and tumors, as well as new ideas and molecular targets for the prevention and treatment of these diseases. Hippo signaling pathway can not only regulate immune cells such as macrophages, T cells and dendritic cells, but also interact with immune-related signaling pathways such as NF-kB signaling pathway, TGF-β signaling pathway and Toll-like receptor signaling pathway, so as to resist the internal environment disorder caused by the invasion of exogenous pathogenic microorganisms and maintain the internal environment stability and physiological balance of the body. Hippo signaling pathway is also involved in the pathological process of immune system-related diseases such as rheumatoid arthritis, inflammatory bowel disease and psoriasis. Hippo pathway is closely related to organ development, stem cell biology, regeneration, and tumor biology. It affects cell differentiation by participating in extracellular and intracellular physiological signal reactions, sensing cell environment, and coordinating cell reactions. This pathway is crucial in maintaining immune homeostasis. This review summarizes the mechanism of Hippo pathway in different immune cells and some autoimmune diseases and the interaction between different immune signaling pathways and Hippo signaling pathway. It aims to explore the role of Hippo in autoimmune diseases and provide theoretical and practical basis for the treatment of autoimmune diseases through Hippo signaling pathway.

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.

Rap1 organizes lymphocyte front-back polarity via RhoA signaling and talin1

Lymphocyte trafficking requires fine-tuning of chemokine-mediated cell migration. This process depends on cytoskeletal dynamics and polarity, but its regulation remains elusive. We quantitatively measured cell polarity and revealed critical roles performed by integrin activator Rap1 in this process, independent of substrate adhesion. Rap1-deficient naive T cells exhibited impaired abilities to reorganize the actin cytoskeleton into pseudopods and actomyosin-rich uropods. Rap1-GTPase activating proteins (GAPs), Rasa3 and Sipa1, maintained an unpolarized shape; deletion of these GAPs spontaneously induced cell polarization, indicative of the polarizing effect of Rap1. Rap1 activation required F-actin scaffolds, and stimulated RhoA activation and actomyosin contractility at the rear. Furthermore, talin1 acted on Rap1 downstream effectors to promote actomyosin contractility in the uropod, which occurred independently of substrate adhesion and talin1 binding to integrins. These findings indicate that Rap1 signaling to RhoA and talin1 regulates chemokine-stimulated lymphocyte polarization and chemotaxis in a manner independent of adhesion.

Thymocyte Development of Humanized Mice Is Promoted by Interactions with Human-Derived Antigen Presenting Cells upon Immunization

Immune responses in humanized mice are generally inefficient without co-transplantation of human thymus or HLA transgenes. Previously, we generated humanized mice via the intra-bone marrow injection of CD133+ cord blood cells into irradiated adult immunodeficient mice (IBMI-huNSG mice), which could mount functional immune responses against HTLV-1, although the underlying mechanisms were still unknown. Here, we investigated thymocyte development in IBMI-huNSG mice, focusing on the roles of human and mouse MHC restriction. IBMI-huNSG mice had normal developmental profiles but aberrant thymic structures. Surprisingly, the thymic medulla-like regions expanded after immunization due to enhanced thymocyte expansion in association with the increase in HLA-DR+ cells, including CD205⁺ dendritic cells (DCs). The organ culture of thymus from immunized IBMI-huNSG mice with a neutralizing antibody to HLA-DR showed the HLA-DR-dependent expansion of CD4 single positive thymocytes. Mature peripheral T-cells exhibited alloreactive proliferation when co-cultured with human peripheral blood mononuclear cells. Live imaging of the thymus from immunized IBMI-huNSG mice revealed dynamic adhesive contacts of human-derived thymocytes and DCs accompanied by Rap1 activation. These findings demonstrate that an increase in HLA-DR+ cells by immunization promotes HLA-restricted thymocyte expansion in humanized mice, offering a unique opportunity to generate humanized mice with ease.

The mammalian Sterile 20-like kinase 4 (MST4) signaling in tumor progression: Implications for therapy

Cancer is a leading cause of death in humans, with a complex and dynamic nature that makes it challenging to fully comprehend and treat. The Mammalian Sterile 20-Like Kinase 4 (MST4 or STK26) is a serine/threonine-protein kinase that plays a crucial role in cell migration and polarity in both normal and tumor cells via activation of intracellular signaling molecules and pathways. MST4 is involved in tumor cell proliferation, migration and invasion, epithelial-mesenchymal transition (EMT), survival, and cancer metastasis through modulation of downstream signaling pathways including the extracellular signal-regulated kinase (ERK) and protein kinase B (AKT) pathways. Additionally, MST4 interacts with programmed cell death 10 (PDCD10) to promote tumor proliferation and migration. MST4 phosphorylates autophagy related 4B cysteine peptidase (ATG4B) to mediate autophagy signaling, promote tumor cell survival and proliferation, and contribute to treatment resistance. Taken together, MST4 functions as an oncogene and is a promising therapeutic target which deserves further exploration.

Development of bilayered porous silk scaffolds for thymus bioengineering

The thymus coordinates the development and selection of T cells. It is structured into two main compartments: the cortex and the medulla. The replication of such complex 3D environment has been challenged by bioengineering approaches. Nevertheless, the effect of the scaffold microstructure on thymic epithelial cell (TEC) cultures has not been deeply investigated. Here, we developed bilayered porous silk fibroin scaffolds and tested their effect on TEC co-cultures. The small and large pore scaffolds presented a mean pore size of 84.33 ± 21.51 μm and 194.90 ± 61.38 μm, respectively. The highly porous bilayered scaffolds presented a high water absorption and water content (> 94 %), together with mechanical properties in the range of the native tissue. TEC (i.e., medullary (mTEC) and cortical (cTEC) cell lines) proliferation is increased in scaffolds with larger pores. The co-culture of both TEC lines in the bilayered porous silk scaffolds presents enhanced cell proliferation and metabolic activity when compared with mTEC in single culture. Also, when the co-culture occurred with cTEC in the small pores layer and mTEC in the large pores layer, a 9.2- and 18.9-fold increase in Foxn1 and Icam1 gene expression in cTEC is evident. These results suggest that scaffold microstructure and the co-culture influence TEC's behaviour. Bilayered silk scaffolds with adjusted microstructure are a valid alternative for TEC culture, having possible applications in advanced thymus bioengineering strategies.

Studying T Cell Development in Neonatal and Adult Thymic Slices

T cell development occurs in the thymus and is coordinated temporally and spatially within the highly complex thymic microenvironment. Therefore, T cell selection and maturation events cannot be fully recapitulated using traditional two-dimensional tissue culture in vitro. The thymic slice system provides a highly versatile system for studying T cell development ex vivo while preserving three-dimensional thymic architecture. Using the thymic slice system, T cell selection and maturation events can be visualized by live imaging and quantified by flow cytometry. Here we describe the process for generating slices from neonatal and adult mice.

Mammalian STE20-like kinase-1/2 are activated in human platelets stimulated by collagen or thrombin and play a vital role in collagen-activated platelets

INTRODUCTION

Mammalian ste20-like kinases-1/2 (MST1/2), the core kinases of the Hippo pathway, play critical roles in the biology of hematopoietic cells via noncanonical mechanisms and contributes to megakaryocyte differentiation, polyploidization, and maturation to produce platelets. However, the role of MST1/2 in platelet functions remains unclear.

MATERIALS AND METHODS

In this study, we investigated this topic by determining platelet aggregation and through flow cytometry, ATP release assay, clot retraction assay, and immunoblotting analysis.

RESULTS

We found that MST1/2 were rapidly phosphorylated and activated upon platelet stimulation by thrombin and collagen. XMU-MP-1, a specific inhibitor of MST1/2, blocks the activation of MST1/2 in platelets. Inhibitor-pretreated platelets showed impaired platelet aggregation and dense-granule secretion mediated by collagen, thrombin, and U46619, whereas ristocetin or ADP mediated platelet aggregation was unaffected by XMU-MP-1. Although platelet-mediated clot retraction was not affected by MST1/2 inhibitors, integrin αIIbβ3 activation was significantly attenuated in XMU-MP-1-treated platelets. Moreover, MST1/2 inhibition significantly attenuated the mobilization of platelet calcium ions and the secretion of α-granules induced by convulxin.

CONCLUSIONS

This study is the first to demonstrate that MST1/2 play vital roles in human platelets and contributes to collagen-induced platelet activation and aggregation.

A meta-analysis indicates that the regulation of cell motility is a non-intrinsic function of chemoattractant receptors that is governed independently of directional sensing

Chemoattraction, defined as the migration of a cell toward a source of a chemical gradient, is controlled by chemoattractant receptors. Chemoattraction involves two basic activities, namely, directional sensing, a molecular mechanism that detects the direction of a source of chemoattractant, and actin-based motility, which allows the migration of a cell towards it. Current models assume first, that chemoattractant receptors govern both directional sensing and motility (most commonly inducing an increase in the migratory speed of the cells, i.e. chemokinesis), and, second, that the signaling pathways controlling both activities are intertwined. We performed a meta-analysis to reassess these two points. From this study emerge two main findings. First, although many chemoattractant receptors govern directional sensing, there are also receptors that do not regulate cell motility, suggesting that is the ability to control directional sensing, not motility, that best defines a chemoattractant receptor. Second, multiple experimental data suggest that receptor-controlled directional sensing and motility can be controlled independently. We hypothesize that this independence may be based on the existence of separated signalling modules that selectively govern directional sensing and motility in chemotactic cells. Together, the information gathered can be useful to update current models representing the signalling from chemoattractant receptors. The new models may facilitate the development of strategies for a more effective pharmacological modulation of chemoattractant receptor-controlled chemoattraction in health and disease.

Current status of type 1 (IgG4-related) autoimmune pancreatitis

In 1995, Yoshida et al. proposed first the concept of "autoimmune pancreatitis" (AIP). Since then, AIP has been accepted as a new pancreatic inflammatory disease and is now divided two subtypes. Type 1 AIP affected immunoglobulin G4 (IgG4) and implicates the pancreatic manifestation of IgG4-related disease, while type 2 is characterized by neutrophil infiltration and granulocytic epithelial lesions (GEL). Recent research has clarified the clinical and pathophysiological aspects of type 1 AIP, which is more than type 2 among the Japanese population. However, many details remain unclear about the pathogenesis and progression of this disease. In this review, we discuss the current knowledge and recent advances relating to type 1 AIP.

A Stk4-Foxp3-NF-κB p65 transcriptional complex promotes Treg cell activation and homeostasis

The molecular programs involved in regulatory T (T_reg) cell activation and homeostasis remain incompletely understood. Here, we show that T cell receptor (TCR) signaling in T_reg cells induces the nuclear translocation of serine/threonine kinase 4 (Stk4), leading to the formation of an Stk4-NF-κB p65-Foxp3 complex that regulates Foxp3- and p65-dependent transcriptional programs. This complex was stabilized by Stk4-dependent phosphorylation of Foxp3 on serine-418. Stk4 deficiency in T_reg cells, either alone or in combination with its homolog Stk3, precipitated a fatal autoimmune lymphoproliferative disease in mice characterized by decreased T_reg cell p65 expression and nuclear translocation, impaired NF-κB p65-Foxp3 complex formation, and defective T_reg cell activation. In an adoptive immunotherapy model, overexpression of p65 or the phosphomimetic Foxp3^S418E in Stk3/4-deficient T_reg cells ameliorated their immune regulatory defects. Our studies identify Stk4 as an essential TCR-responsive regulator of p65-Foxp3-dependent transcription that promotes T_reg cell-mediated immune tolerance.

Antigen transfer and its effect on vaccine-induced immune amplification and tolerance

Antigen transfer refers to the process of intercellular information exchange, where antigenic components including nucleic acids, antigen proteins/peptides and peptide-major histocompatibility complexes (p-MHCs) are transmitted from donor cells to recipient cells at the thymus, secondary lymphoid organs (SLOs), intestine, allergic sites, allografts, pathological lesions and vaccine injection sites via trogocytosis, gap junctions, tunnel nanotubes (TNTs), or extracellular vesicles (EVs). In the context of vaccine inoculation, antigen transfer is manipulated by the vaccine type and administration route, which consequently influences, even alters the immunological outcome, i.e., immune amplification and tolerance. Mainly focused on dendritic cells (DCs)-based antigen receptors, this review systematically introduces the biological process, molecular basis and clinical manifestation of antigen transfer.

Involvement of MST1/mTORC1/STAT1 activity in the regulation of B-cell receptor signalling by chemokine receptor 2

BACKGROUND

CCR2 is involved in maintaining immune homeostasis and regulating immune function. This study aims to elucidate the mechanism by which CCR2 regulates B-cell signalling.

METHODS

In Ccr2-knockout mice, the development and differentiation of B cells, BCR proximal signals, actin movement and B-cell immune response were determined. Besides, the level of CCR2 in PBMC of SLE patients was analysed by bioinformatics.

RESULTS

CCR2 deficiency reduces the proportion and number of follicular B cells, upregulates BCR proximal signalling and enhances the oxidative phosphorylation of B cells. Meanwhile, increased actin filaments aggregation and its associated early-activation events of B cells are also induced by CCR2 deficiency. The MST1/mTORC1/STAT1 axis in B cells is responsible for the regulation of actin remodelling, metabolic activities and transcriptional signalling, specific MST1, mTORC1 or STAT1 inhibitor can rescue the upregulated BCR signalling. Glomerular IgG deposition is obvious in CCR2-deficient mice, accompanied by increased anti-dsDNA IgG level. Additionally, the CCR2 expression in peripheral B cells of SLE patients is decreased than that of healthy controls.

CONCLUSIONS

CCR2 can utilise MST1/mTORC1/STAT1 axis to regulate BCR signalling. The interaction between CCR2 and BCR may contribute to exploring the mechanism of autoimmune diseases.

Modulatory action of withaferin-A on oxidative damage through regulation of inflammatory mediators and apoptosis via PI3K/AKT signaling pathway in high cholesterol-induced atherosclerosis in experimental rats

Statins are widely used lipid-lowering drugs that cause many side effects. Withaferin-A (WA), popularly known as Ashwagandha, an ancient Indian medicinal herb, is extracted from Withania somnifera. Anti-atherosclerotic effect of WA has been reported. However, the mechanism remains unknown. Hence, we planned this study to investigate the WA mechanism in anti-atherosclerosis in a rat model. High cholesterol diet (HCD) was fed to induce atherosclerosis in Sprague-Dawley male rats. Five groups (N = 6 rats/group) were fed with normal diet, HCD, WA (10 mg/kg bw)+HCD, lovastatin (LS: 10 mg/kg bw)+HCD, WA (10 mg/kg bw) respectively for 90 days. Statistical analysis was done by GraphPad Prism (version 8.0.1) using one-way analysis of variance (ANOVA) followed by post hoc Duncan's test with a significance level (p < 0.05). The groups were compared for lipid profiles, oxidative stress, lipid peroxidation, inflammatory mediators, apoptotic markers, and histopathological changes in the liver and aorta. Treatment with HCD increased lipid profiles, inflammatory mediators, cytokines, and lipid peroxidation. WA as well as LS treatments significantly decreased these parameters restored the antioxidant status, and reduced lipid peroxidation (p < 0.05). Histopathological studies revealed that WA and LS reduced the hepatic fat and aortic plaque. WA reduced apoptosis via augmentation of phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt) pathway; increased B-cell lymphoma 2 and inhibited Bcl-2 associated X-protein proapoptotic proteins; TNF receptor superfamily member 6, Bim, caspase-3, and -9; demonstrated significant hypolipidemic and anti-inflammatory properties against HCD induced atherosclerosis in rats through regulation of inflammatory mediators and apoptosis via the PI3K/AKT signaling pathway.

Recent Progress on the Roles of Regulatory T Cells in IgG4-Related Disease

IgG4-related disease (RD) is a proposed concept of systemic inflammatory condition from Japanese researchers. Patients with IgG4-RD manifest several immunological and histological characterizations in the organs involved, including elevated levels of serum IgG4 and lympho-plasmacytic infiltration, storiform fibrosis, IgG4-positive plasma cells infiltration, and obstructive phlebitis. Nevertheless, the pathogenesis of IgG4-RD still remains unclear. It has been made clear that several immune cells with regulatory function play a vital part in several diseases. In particular, abnormalities in the function and proportion of regulatory T cells (Tregs) are implicated in several diseases, and their part in IgG4-RD has been investigated. This review offers an overview of the research in IgG4-RD related to Tregs. Herein, the basic information of Tregs, knowledge gained from animal models involving Tregs, and the role of IgG4-RD has been provided. We also included the immunological mechanisms of IgG4-RD based on the data accumulated so far in our hypothesis.

Molecular remission at T cell level in patients with rheumatoid arthritis

While numerous disease-modifying anti-rheumatic drugs (DMARDs) have brought about a dramatic paradigm shift in the management of rheumatoid arthritis (RA), unmet needs remain, such as the small proportion of patients who achieve drug-free status. The aim of this study was to explore key molecules for remission at the T cell level, which are known to be deeply involved in RA pathogenesis, and investigate the disease course of patients who achieved molecular remission (MR). We enrolled a total of 46 patients with RA and 10 healthy controls (HCs). We performed gene expression profiling and selected remission signature genes in CD4⁺ T cells and CD8⁺ T cells from patients with RA using machine learning methods. In addition, we investigated the benefits of achieving MR on disease control. We identified 9 and 23 genes that were associated with clinical remission in CD4⁺ and CD8⁺ T cells, respectively. Principal component analysis (PCA) demonstrated that their expression profiling was similar to those in HCs. For the remission signature genes in CD4⁺ T cells, the PCA result was reproduced using a validation cohort, indicating the robustness of these genes. A trend toward better disease control was observed during 12 months of follow-up in patients treated with tocilizumab in deep MR compared with those in non-deep MR, although the difference was not significant. The current study will promote our understanding of the molecular mechanisms necessary to achieve deep remission during the management of RA.

DOCK8 deficiency diminishes thymic T-regulatory cell development but not thymic deletion

Objective

To define the effect of DOCK8 deficiency on thymic tolerance in mice.

Methods

Thymocytes from wild‐type (Dock8^+/+) and DOCK8‐deficient (Dock8^pri/pri) mice were examined by flow cytometry. Some mice had transgenic expression of the BCL2 anti‐apoptotic protein in haemopoietic cells. Some mice expressed the transgenic 3A9 T‐cell receptor (TCR), which triggers thymocyte deletion in mice also expressing hen egg lysozyme under the insulin promoter.

Results

In Dock8^pr/pri mice, the proportion of thymocytes induced to acquire tolerance at the immature CCR7⁻ stage was normal. Deletion of strongly self‐reactive CD4⁺ thymocytes occurred efficiently in Dock8^pri/pri mice in a TCR‐transgenic model that requires self‐antigen transfer from epithelial cells to bone marrow (BM)‐derived antigen‐presenting cells. Thymic Foxp3⁺ T‐regulatory cells (T_REG) and Helios⁺ Foxp3⁻ T_REG precursors were decreased in Dock8^pri/pri mice, including when apoptosis was inhibited by BCL2 transgene expression. Dock8^pri/pri thymic T_REG expressed CD25 and CTLA‐4 at normal levels. The results suggest that DOCK8 deficiency does not affect the function of BM‐derived antigen‐presenting cells in the thymus, the TCR self‐reactivity threshold that activates tolerance mechanisms in thymocytes or the apoptotic deletion of these thymocytes. However, DOCK8 is required to prevent a subset of developing T_REG cells from undergoing cell death via a mechanism that is distinct from apoptosis.

Conclusion

DOCK8 deficiency diminishes T_REG development in the thymus without compromising thymocyte deletion.

Factors that influence the thymic selection of CD8αα intraepithelial lymphocytes

Thymocytes bearing αβ T cell receptors (TCRαβ) with high affinity for self-peptide-MHC complexes undergo negative selection or are diverted to alternate T cell lineages, a process termed agonist selection. Among thymocytes bearing TCRs restricted to MHC class I, agonist selection can lead to the development of precursors that can home to the gut and give rise to CD8αα-expressing intraepithelial lymphocytes (CD8αα IELs). The factors that influence the choice between negative selection versus CD8αα IEL development remain largely unknown. Using a synchronized thymic tissue slice model that supports both negative selection and CD8αα IEL development, we show that the affinity threshold for CD8αα IEL development is higher than for negative selection. We also investigate the impact of peptide presenting cells and cytokines, and the migration patterns associated with these alternative cell fates. Our data highlight the roles of TCR affinity and the thymic microenvironments on T cell fate.

Hippo Signal Transduction Mechanisms in T Cell Immunity

Hippo signaling pathways are evolutionarily conserved signal transduction mechanisms mainly involved in organ size control, tissue regeneration, and tumor suppression. However, in mammals, the primary role of Hippo signaling seems to be regulation of immunity. As such, humans with null mutations in STK4 (mammalian homologue of Drosophila Hippo; also known as MST1) suffer from recurrent infections and autoimmune symptoms. Although dysregulated T cell homeostasis and functions have been identified in MST1-deficient human patients and mouse models, detailed cellular and molecular bases of the immune dysfunction remain to be elucidated. Although the canonical Hippo signaling pathway involves transcriptional co-activator Yes-associated protein (YAP) or transcriptional coactivator with PDZ motif (TAZ), the major Hippo downstream signaling pathways in T cells are YAP/TAZ-independent and they widely differ between T cell subsets. Here we will review Hippo signaling mechanisms in T cell immunity and describe their implications for immune defects found in MST1-deficient patients and animals. Further, we propose that mutual inhibition of Mst and Akt kinases and their opposing roles on the stability and function of forkhead box O and β-catenin may explain various immune defects discovered in mutant mice lacking Hippo signaling components. Understanding these diverse Hippo signaling pathways and their interplay with other evolutionarily-conserved signaling components in T cells may uncover molecular targets relevant to vaccination, autoimmune diseases, and cancer immunotherapies.

Phosphatidic acid-dependent localization and basal de-phosphorylation of RA-GEFs regulate lymphocyte trafficking

Background

Lymphocytes circulate between peripheral lymphoid tissues via blood and lymphatic systems, and chemokine-induced migration is important in trafficking lymphocytes to distant sites. The small GTPase Rap1 is important in mediating lymphocyte motility, and Rap1-GEFs are involved in chemokine-mediated Rap1 activation. Here, we describe the roles and mechanisms of Rap1-GEFs in lymphocyte trafficking.

Results

In this study, we show that RA-GEF-1 and 2 (also known as Rapgef2 and 6) are key guanine nucleotide exchange factors (GEF) for Rap1 in lymphocyte trafficking. Mice harboring T cell-specific knockouts of Rapgef2/6 demonstrate defective homing and egress of T cells. Sphingosine-1-phosphate (S1P) as well as chemokines activates Rap1 in a RA-GEF-1/2-dependent manner, and their deficiency in T cells impairs Mst1 phosphorylation, cell polarization, and chemotaxis toward S1P gradient. On the other hand, B cell-specific knockouts of Rapgef2/6 impair chemokine-dependent retention of B cells in the bone marrow and passively facilitate egress. Phospholipase D2-dependent production of phosphatidic acid by these chemotactic factors determines spatial distribution of Rap1-GTP subsequent to membrane localization of RA-GEFs and induces the development of front membrane. On the other hand, basal de-phosphorylation of RA-GEFs is necessary for chemotactic factor-dependent increase in GEF activity for Rap1.

Conclusions

We demonstrate here that subcellular distribution and activation of RA-GEFs are key factors for a directional movement of lymphocytes and that phosphatidic acid is critical for membrane translocation of RA-GEFs with chemokine stimulation.

MST1/2 Balance Immune Activation and Tolerance by Orchestrating Adhesion, Transcription, and Organelle Dynamics in Lymphocytes

The STE20-like serine/threonine kinases MST1 and MST2 (MST1/2) are mammalian homologs of Hippo in flies. MST1/2 regulate organ size by suppressing the transcription factor YAP, which promotes proliferation. MST1 is predominantly expressed in immune cells, where it plays distinct roles. Here, we review the functions of MST1/2 in immune cells, uncovered by a series of recent studies, and discuss the connection between MST1/2 function and immune responses. MST1/2 regulate lymphocyte development, trafficking, survival, and antigen recognition by naive T cells. MST1/2 also regulate the function of regulatory T cells and effector T cell differentiation, thus acting to balance immune activation and tolerance. Interestingly, MST1/2 elicit these functions not by the “canonical” Hippo pathway, but by the non-canonical Hippo pathway or alternative pathways. In these pathways, MST1/2 regulates cellular processes relating to immune response, such as chemotaxis, cell adhesion, immunological synapse, gene transcriptions. Recent advances in our understanding of the molecular mechanisms of these processes have revealed important roles of MST1/2 in regulating cytoskeleton remodeling, integrin activation, and vesicular transport in lymphocytes. We discuss the significance of the MST1/2 signaling in lymphocytes in the regulation of organelle dynamics.

From stem cell to immune effector: how adhesion, migration, and polarity shape T-cell and natural killer cell lymphocyte development in vitro and in vivo

Lymphocyte development is a complex and coordinated pathway originating from pluripotent stem cells during embryogenesis and continuing even as matured lymphocytes are primed and educated in adult tissue. Hematopoietic stem cells develop in a specialized niche that includes extracellular matrix and supporting stromal and endothelial cells that both maintain stem cell pluripotency and enable the generation of differentiated cells. Cues for lymphocyte development include changes in integrin-dependent cell motility and adhesion which ultimately help to determine cell fate. The capacity of lymphocytes to adhere and migrate is important for modulating these developmental signals both by regulating the cues that the cell receives from the local microenvironment as well as facilitating the localization of precursors to tissue niches throughout the body. Here we consider how changing migratory and adhesive phenotypes contribute to human natural killer (NK)- and T-cell development as they undergo development from precursors to mature, circulating cells and how our understanding of this process is informed by in vitro models of T- and NK cell generation.

Phosphatidic acid regulates subcellular distribution of RA-GEFs critical for chemokine-dependent migration

Integrin activation by Rap1-GTP is pivotal for lymphocyte trafficking. In this study, we show the phosphatidic acid (PA)-dependent membrane distribution of RA-GEF-1 and -2 (also known as Rapgef2 and 6), which are guanine nucleotide exchange factors for Rap1, plays important roles in lymphocyte migration. RA-GEF-1 associates with PA through 919-967 aa within CDC25 homology domain, and the deletion of this region of RA-GEF-1 inhibits chemokine-dependent migration. Chemokine stimulation induces temporal production of PA on the plasma membrane, which is not necessary for Rap1 activation, but the translocation of RA-GEFs. Thus, chemokine-dependent generation of PA is critical for lymphocyte migration through membrane localization of RA-GEFs.

Testing the Efficiency and Kinetics of Negative Selection Using Thymic Slices

Central tolerance is an efficient barrier to autoimmunity and negative selection of self-reactive thymocytes is one of its major manifestations. Because of its importance, negative selection has been studied extensively through numerous in vitro and in vivo approaches that have tremendously increased our understanding of the process. Recently, in situ experimental systems using thymus slices have been developed that combine some of the advantages of in vitro assays such as ease of manipulation and high throughput with the existence of three dimensional mature thymus microenvironment. These approaches offer unprecedented opportunity to study negative selection. Here, we describe how thymic slices can be used to measure the kinetics and magnitude of negative selection. Taking the OT-1/Ova system as an example, we provide detailed guidance on cutting thymic slices, labeling and overlaying thymocytes on them and reading out the extent of negative selection by flow cytometry. The system can easily be adapted to evaluate the effects of various mutations or treatments on negative selection or to study the behavior of different cells in the thymus through time-lapse imaging.

Yap suppresses T-cell function and infiltration in the tumor microenvironment

A major challenge for cancer immunotherapy is sustaining T-cell activation and recruitment in immunosuppressive solid tumors. Here, we report that the levels of the Hippo pathway effector Yes-associated protein (Yap) are sharply induced upon the activation of cluster of differentiation 4 (CD4)-positive and cluster of differentiation 8 (CD8)-positive T cells and that Yap functions as an immunosuppressive factor and inhibitor of effector differentiation. Loss of Yap in T cells results in enhanced T-cell activation, differentiation, and function, which translates in vivo to an improved ability for T cells to infiltrate and repress tumors. Gene expression analyses of tumor-infiltrating T cells following Yap deletion implicates Yap as a mediator of global T-cell responses in the tumor microenvironment and as a negative regulator of T-cell tumor infiltration and patient survival in diverse human cancers. Collectively, our results indicate that Yap plays critical roles in T-cell biology and suggest that Yap inhibition improves T-cell responses in cancer.

The Hippo Pathway: Biology and Pathophysiology

The Hippo pathway was initially discovered in Drosophila melanogaster as a key regulator of tissue growth. It is an evolutionarily conserved signaling cascade regulating numerous biological processes, including cell growth and fate decision, organ size control, and regeneration. The core of the Hippo pathway in mammals consists of a kinase cascade, MST1/2 and LATS1/2, as well as downstream effectors, transcriptional coactivators YAP and TAZ. These core components of the Hippo pathway control transcriptional programs involved in cell proliferation, survival, mobility, stemness, and differentiation. The Hippo pathway is tightly regulated by both intrinsic and extrinsic signals, such as mechanical force, cell-cell contact, polarity, energy status, stress, and many diffusible hormonal factors, the majority of which act through G protein-coupled receptors. Here, we review the current understanding of molecular mechanisms by which signals regulate the Hippo pathway with an emphasis on mechanotransduction and the effects of this pathway on basic biology and human diseases.

Hippo kinase loss contributes to del(20q) hematologic malignancies through chronic innate immune activation

Heterozygous deletions within chromosome 20q, or del(20q), are frequent cytogenetic abnormalities detected in hematologic malignancies. To date, identification of genes in the del(20q) common deleted region that contribute to disease development have remained elusive. Through assessment of patient gene expression, we have identified STK4 (encoding Hippo kinase MST1) as a 20q gene that is downregulated below haploinsufficient amounts in myelodysplastic syndrome (MDS) and myeloproliferative neoplasm (MPN). Hematopoietic-specific gene inactivation in mice revealed Hippo kinase loss to induce splenomegaly, thrombocytopenia, megakaryocytic dysplasia, and a propensity for chronic granulocytosis; phenotypes that closely resemble those observed in patients harboring del(20q). In a JAK2-V617F model, heterozygous Hippo kinase inactivation led to accelerated development of lethal myelofibrosis, recapitulating adverse MPN disease progression and revealing a novel genetic interaction between these 2 molecular events. Quantitative serum protein profiling showed that myelofibrotic transformation in mice was associated with cooperative effects of JAK2-V617F and Hippo kinase inactivation on innate immune-associated proinflammatory cytokine production, including IL-1β and IL-6. Mechanistically, MST1 interacted with IRAK1, and shRNA-mediated knockdown was sufficient to increase IRAK1-dependent innate immune activation of NF-κB in human myeloid cells. Consistent with this, treatment with a small molecule IRAK1/4 inhibitor rescued the aberrantly elevated IL-1β production in the JAK2-V617F MPN model. This study identified Hippo kinase MST1 (STK4) as having a central role in the biology of del(20q)-associated hematologic malignancies and revealed a novel molecular basis of adverse MPN progression that may be therapeutically exploitable via IRAK1 inhibition.

MOB1 regulates thymocyte egress and T-cell survival in mice in a YAP1-independent manner

Mammalian STE20-like protein kinase 1/2 (MST1/2) and nuclear Dbf2-related kinase 1/2 (NDR1/2) are core components of Hippo signaling that are also known to be important regulators of lymphocyte trafficking. However, little is understood about the roles of other Hippo pathway molecules in these cells. Here, we present the first analysis of the function of Mps one binder kinase activator-1 (MOB1) in T lymphocytes in vivo. T-cell-specific double knockout (DKO) of MOB1A/B in mice [tMob1 DKO mice] reduces the number of naïve T cells in both the circulation and secondary lymphoid organs, but leads to an accumulation of CD4⁺ CD8^- and CD4^- CD8⁺ single-positive (SP) cells in the thymus. In vitro, naïve MOB1A/B-deficient T cells show increased apoptosis and display impaired trafficking capacity in response to the chemokine CCL19. These defects are linked to suppression of the activation of MST and NDR kinases, but are independent of the downstream transcriptional co-activator Yes-associated protein 1 (YAP1). Thus, MOB1 proteins play an important role in thymic egress and T-cell survival that is mediated by a pathway other than conventional Hippo-YAP1 signaling.

Live-cell imaging reveals the relative contributions of antigen-presenting cell subsets to thymic central tolerance

Both medullary thymic epithelial cells (mTEC) and dendritic cells (DC) present tissue-restricted antigens (TRA) to thymocytes to induce central tolerance, but the relative contributions of these antigen-presenting cell (APC) subsets remain unresolved. Here we developed a two-photon microscopy approach to observe thymocytes interacting with intact APCs presenting TRAs. We find that mTECs and DCs cooperate extensively to induce tolerance, with their relative contributions regulated by the cellular form of the TRA and the class of major histocompatibility complex (MHC) on which antigen is presented. Even when TRA expression is restricted to mTECs, DCs still present self-antigens at least as frequently as mTECs. Notably, the DC subset cDC2 efficiently acquires secreted mTEC-derived TRAs for cross-presentation on MHC-I. By directly imaging interactions between thymocytes and APCs, while monitoring intracellular signaling, this study reveals that distinct DC subsets and AIRE+ mTECs contribute substantially to presentation of diverse self-antigens for establishing central tolerance.

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.

Scutellarin exerts protective effects against atherosclerosis in rats by regulating the Hippo-FOXO3A and PI3K/AKT signaling pathways

Atherosclerosis (AS), a progressive disorder, is one of the tough challenges in the clinic. Scutellarin, an extract from Herba Erigerontis, is found to have oxygen-free radicals scavenging effects and antioxidant effects. In this study, we aimed to investigate the anti-AS effects of scutellarin is related to controlling the Hippo-FOXO3A and PI3K/AKT signal pathway. To establish an AS model, the rats in the scutellarin and model groups were intraperitoneally injected with vitamin D ₃ and then fed a high-fat diet for 12 weeks. In addition, in vitro angiotensin II-induced apoptosis of human aortic endothelial cells (HAECs) were used to establish models. Scutellarin significantly reduced blood lipid levels and increased antioxidase levels in both models. Additionally, scutellarin inhibited reactive oxygen species generation and apoptosis in HAECs. The impaired vascular barrier function was restored by using scutellarin in AS rats and in HAECs cells characterized by inhibiting mammalian sterile-20-like kinases 1 (Mst1) phosphorylation, Yes-associated protein (YAP) phosphorylation, forkhead box O3A (FOXO3A) phosphorylation at serine 207, nuclear translocation of FOXO3A, and upregulating protein expression of AKT and FOXO3A phosphorylation at serine 253. Scutellarin significantly reduced Bcl-2 interacting mediator of cell death (Bim), caspase-3, APO-1, CD95 (Fas), and Bax: Bcl-2-associated X (Bax) levels and activated Bcl-2: B-cell lymphoma-2 (Bcl-2). Scutellarin also significantly inhibited the expression of Mst1, YAP, FOXO3A at the messenger RNA level. When Mst1 was overexpressed or phosphoinositide 3-kinases suppressed, the effects of scutellarin were significantly blocked. In conclusion, the results of the present study suggest that scutellarin exerts protective effects against AS by inhibiting endothelial cell injury and apoptosis by regulating the Hippo-FOXO3A and PI3K/AKT signal pathways.

Macrophage achieves self-protection against oxidative stress-induced ageing through the Mst-Nrf2 axis

Reactive oxygen species (ROS) production in phagocytes is a major defense mechanism against pathogens. However, the cellular self-protective mechanism against such potential damage from oxidative stress remains unclear. Here we show that the kinases Mst1 and Mst2 (Mst1/2) sense ROS and maintain cellular redox balance by modulating the stability of antioxidant transcription factor Nrf2. Site-specific ROS release recruits Mst1/2 from the cytosol to the phagosomal or mitochondrial membrane, with ROS subsequently activating Mst1/2 to phosphorylate kelch like ECH associated protein 1 (Keap1) and prevent Keap1 polymerization, thereby blocking Nrf2 ubiquitination and degradation to protect cells against oxidative damage. Treatment with the antioxidant N-acetylcysteine disrupts ROS-induced interaction of Mst1/2 with phagosomes or mitochondria, and thereby diminishes the Mst-Nrf2 signal. Consistently, loss of Mst1/2 results in increased oxidative injury, phagocyte ageing and death. Thus, our results identify the Mst-Nrf2 axis as an important ROS-sensing and antioxidant mechanism during an antimicrobial response.

Immune cells produce reactive oxygen species (ROS) to eliminate pathogens, but cell-spontaneous death and ageing may also be induced. Here the authors show that, upon sensing ROS, Mst1/2 kinases modulate the activity of Nrf2 transcription factor and downstream genetic programs to protect mouse macrophages from death and ageing.

Hippo Pathway Kinase Mst1 Is Required for Long-Lived Humoral Immunity

The protein kinase Mst1 is a key component of the evolutionarily conserved Hippo pathway that regulates cell survival, proliferation, differentiation, and migration. In humans, Mst1 deficiency causes primary immunodeficiency. Patients with MST1-null mutations show progressive loss of naive T cells but, paradoxically, mildly elevated serum Ab titers. Nonetheless, the role of Mst1 in humoral immunity remains poorly understood. In this study, we found that early T cell-dependent IgG1 responses in young adult Mst1-deficient mice were largely intact with signs of impaired affinity maturation. However, the established Ag-specific IgG1 titers in Mst1-deficient mice decayed more readily because of a loss of Ag-specific but not the overall bone marrow plasma cells. Despite the impaired affinity and longevity of Ag-specific Abs, Mst1-deficient mice produced plasma cells displaying apparently normal maturation markers with intact migratory and secretory capacities. Intriguingly, in immunized Mst1-deficient mice, T follicular helper cells were hyperactive, expressing higher levels of IL-21, IL-4, and surface CD40L. Accordingly, germinal center B cells progressed more rapidly into the plasma cell lineage, presumably forgoing rigorous affinity maturation processes. Importantly, Mst1-deficient mice had elevated levels of CD138⁺Blimp1⁺ splenic plasma cell populations, yet the size of the bone marrow plasma cell population remained normal. Thus, overproduced low-affinity plasma cells from dysregulated germinal centers seem to underlie humoral immune defects in Mst1-deficiency. Our findings imply that vaccination of Mst1-deficient human patients, even at the early stage of life, may fail to establish long-lived high-affinity humoral immunity and that prophylactic Ab replacement therapy can be beneficial to the patients.

Live-Cell FRET Imaging Reveals a Role of Extracellular Signal-Regulated Kinase Activity Dynamics in Thymocyte Motility

Extracellular signal-regulated kinase (ERK) plays critical roles in T cell development in the thymus. Nevertheless, the dynamics of ERK activity and the role of ERK in regulating thymocyte motility remain largely unknown due to technical limitations. To visualize ERK activity in thymocytes, we here developed knockin reporter mice expressing a Förster/fluorescence resonance energy transfer (FRET)-based biosensor for ERK from the ROSA26 locus. Live imaging of thymocytes isolated from the reporter mice revealed that ERK regulates thymocyte motility in a subtype-specific manner. Negative correlation between ERK activity and motility was observed in CD4/CD8 double-positive thymocytes and CD8 single-positive thymocytes, but not in CD4 single-positive thymocytes. Interestingly, however, the temporal deviations of ERK activity from the average correlate with the motility of CD4 single-positive thymocytes. Thus, live-cell FRET imaging will open a window to understanding the dynamic nature and the diverse functions of ERK signaling in T cell biology.

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Mice expressing EKAREV from ROSA26 locus enable ERK activity monitoring in T cells

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ERK activity negatively regulates the motility of thymocytes in the thymus

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Temporal dynamics of ERK activity regulates cell motility of CD4-SP in the medulla

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TCR signal from intercellular association induces ERK activity dynamics in CD4-SP

Hippo Kinases Mst1 and Mst2 Sense and Amplify IL-2R-STAT5 Signaling in Regulatory T Cells to Establish Stable Regulatory Activity

Interleukin-2 (IL-2) and downstream transcription factor STAT5 are important for maintaining regulatory T (Treg) cell homeostasis and function. Treg cells can respond to low IL-2 levels, but the mechanisms of STAT5 activation during partial IL-2 deficiency remain uncertain. We identified the serine-threonine kinase Mst1 as a signal-dependent amplifier of IL-2-STAT5 activity in Treg cells. High Mst1 and Mst2 (Mst1-Mst2) activity in Treg cells was crucial to prevent tumor resistance and autoimmunity. Mechanistically, Mst1-Mst2 sensed IL-2 signals to promote the STAT5 activation necessary for Treg cell homeostasis and lineage stability and to maintain the highly suppressive phosphorylated-STAT5⁺ Treg cell subpopulation. Unbiased quantitative proteomics revealed association of Mst1 with the cytoskeletal DOCK8-LRCHs module. Mst1 deficiency limited Treg cell migration and access to IL-2 and activity of the small GTPase Rac, which mediated downstream STAT5 activation. Collectively, IL-2-STAT5 signaling depends upon Mst1-Mst2 functions to maintain a stable Treg cell pool and immune tolerance.

An in vitro method for studying subcellular rearrangements during cell polarization in Drosophila melanogaster hemocytes

Thanks to the power of Drosophila genetics, this animal model has been a precious tool for scientists to uncover key processes associated to innate immunity. The fly immune system relies on a population of macrophage-like cells, also referred to as hemocytes, which are highly migratory and phagocytic, and can easily be followed in vivo. These cells have shown to play important roles in fly development, both at the embryonic and pupal stages. However, there is no robust assay for the study of hemocyte migration in vitro, which limits our understanding of the molecular mechanisms involved. Here, we contribute to fill this gap by showing that hemocytes adopt a polarized morphology upon ecdysone stimulation, allowing the study of the cytoskeleton rearrangements and organelle reorganization that take place during the first step of cell locomotion.

Noncanonical Hippo Signalling in the Regulation of Leukocyte Function

The mammalian sterile 20-like (MST) kinases are central constituents of the evolutionary ancient canonical Hippo pathway regulating cell proliferation and survival. However, perhaps surprisingly, MST1 deficiency in human patients leads to a severe combined immunodeficiency syndrome with features of autoimmune disease. In line with this, Mst1-deficient mice exhibit severe defects in lymphocyte and neutrophil functions as well as disturbed intracellular vesicle transport. These findings spurred research on the noncanonical functions of MST1 in leukocytes. Here, we summarise the latest findings on this topic and discuss MST1 as a critical regulator of various leukocyte functions.

CCR2 Signal Facilitates Thymic Egress by Priming Thymocyte Responses to Sphingosine-1-Phosphate

The signal mediated by sphingosine-1-phosphate receptor 1 (S1P1) is essential but seemingly insufficient for thymic export of newly generated T cells. Here, we reported the identification of CCR2 as an additional regulator of this process. CCR2 showed a markedly increased expression in the most mature subset of single-positive (SP) thymocytes. Its deficiency led to a reduction of recent thymic emigrants in the periphery and a simultaneous accumulation of mature SP cells in the thymus. The CCR2 signaling promoted thymic emigration primarily through modulating the chemotactic responses to S1P1 engagement. On the one hand, the chemokinesis induced by CCR2 activation endowed thymocytes with enhanced capacity to respond to S1P-induced migration. On the other hand, CCR2 signaling through Stat3 augmented forkhead box O1 activity, leading to increased expression of S1P1. Taken together, the present study highlights a unique and novel function of CCR2 signaling in the regulation of thymic egress.

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.