The CARD11-BCL10-MALT1 (CBM) complex drives NF-κB signaling and MALT1 protease activation after T cell receptor (TCR) stimulation, forming a central signaling hub in adaptive immunity. Both linear ubiquitin chain assembly complex (LUBAC), consisting of HOIP, HOIL-1 and SHARPIN, and TRAF6 interact with the CBM complex. Still, the coordinated activity of these E3 ligases in controlling CBM activity remains elusive. Here we demonstrate that LUBAC, unlike TRAF6, is largely dispensable for TCR-induced NF-κB activation in human CD4+ T cells. However, HOIP contributes to NF-κB target gene expression and, with TRAF6, modulates MALT1 substrate recognition, influencing T cell responses. Further, LUBAC-mediated conjugation of Met1-linked ubiquitin chains to BCL10 strictly depends on TRAF6, but putative Met1-ubiquitin acceptor lysines in BCL10 serve essential structural roles that limit accessibility within BCL10-MALT1 filaments. Thus, LUBAC acts downstream of TRAF6 to modulate MALT1 substrate recognition and to catalyze BCL10 ubiquitination, which is incompatible with BCL10-MALT1 filament formation.

Extracellular vesicles (EVs) are emerging as crucial players in cell communication and hold great promise as biomarkers and therapeutic tools. However, their diversity makes it challenging to detect, classify, and utilize them effectively, which limits their clinical applicability. A key challenge is the lack of reliable markers to identify EVs consistently. In this study, a novel high-affinity phosphatidylserine (PS)-binding reagent is introduced for EV analysis. PS is known as a marker of apoptotic cells and activated platelets, but its presence on EVs is debated due to variations in lipid composition. By comparing multiple PS-binding reagents, including MFG-E8 derivatives and Annexin V, it is demonstrated that ≈90% of EVs in human and mouse blood carry PS. Using the optimized reagent, the first in vivo insights into EV turnover are provided, showing that PS+ EVs in mouse blood are rapidly cleared (≈50% within 30 min) but persist on immune cells in the spleen. This discovery increases the potential of EVs as disease biomarkers and therapeutic targets by improving EV detection and isolation as well as opening the door for standardized quantification and diagnostic monitoring.

Breast cancer, one of the most common cancers in women, is classified by the expression of hormone receptors and the growth factor receptor HER2, which is important for personalised tumour treatment with HER2-targeted therapies. Tumour biopsies are required for histopathological diagnosis of HER2 expression by breast cancer cells but are subject to sampling error. In this study, we present a method for identifying and analysing cancer-derived EVs from plasma for the detection of HER2 expression in breast cancer without the need for additional processing steps. We detected nano-sized particles through an optimised flow cytometry approach that allows for the identification of HER2-expressing EVs and quantification of their HER2 expression levels. In a clinical study of 115 breast cancer patients, this optimised flow cytometric analysis detected a range of 1.3 to 50 × 103 HER2+EVs per µl of plasma. The number of HER2+EVs did not correlate directly with tumour size, grade, or metastasis. However, computational integration of data from the quantification of HER2pos EVs per µl/plasma and their HER2 expression levels on a single EV basis allowed for the reliable identification of HER2 expression levels in tumours. Our results reveal the potential for analysing cancer-derived EVs from plasma for the diagnosis and personalised therapy in breast cancer patients.

Platelets are key players in cardiovascular disease, and platelet aggregation represents a central pharmacologic target, particularly in secondary prevention. However, inhibition of adenosine diphosphate and thromboxane signaling has low efficacy in preventing venous thromboembolism, necessitating the inhibition of the plasmatic coagulation cascade in this disease entity. Anticoagulation carries a significantly higher risk of bleeding complications, highlighting the need of alternative therapeutic approaches. We hypothesized that procoagulant activation (PA) of platelets promotes venous thrombus formation and that targeting PA could alleviate venous thrombosis. Here, we found elevated levels of procoagulant platelets in the circulation and in thrombi of patients with deep vein thrombosis (DVT) and pulmonary embolism, and in mice developing DVT following inferior vena cava stenosis. Furthermore, we detected PA of recruited platelets within murine venous thrombi and human pulmonary emboli. Mice with platelet-specific deficiency in central pathways of PA-cyclophilin D and transmembrane protein 16F-were more resistant toward low fl ow - induced venous thrombosis. Finally, we found that a clinically approved carbonic anhydrase inhibitor, methazolamide, reduced platelet procoagulant activity and alleviated murine thrombus formation without affecting trauma-associated hemostasis. These fi ndings identify an essential role of platelet procoagulant function in venous thrombosis and delineate novel pharmacologic strategies targeting platelets in the prevention of venous thromboembolism.

Phosphatidylserine (PS) is a well-established marker for apoptotic cells and activated platelets, and it is also found on extracellular vesicles (EVs). However, the frequency of PS+ EVs remains a topic of debate, probably due to the distinct lipid composition of EVs, which varies with the source material. This variation underscores the need for highly sensitive detection reagents to accurately differentiate between PS+ and PS- EVs. To address this issue, we compared several PS-binding reagents for the staining of blood EVs, including milk fat globule factor E8 (MFG-E8, lactadherin), a derivative of MFG-E8, and Annexin V, the most used compound in the literature. Here, we demonstrate the predominance of PS+ EVs in both murine and human blood and reveal significant differences in the affinities of the PS-binding proteins. With approximately 90% of the detected blood EVs being PS+, we utilized a derivative of MFG-E8 for in vivo monitoring of both circulating and cell-bound EVs, providing a novel tool to analyze endogenous EV kinetics. Our findings showed that murine PS+ plasma EVs were rapidly cleared from circulation (~50% after 30 minutes), yet they remained present on splenic B cells and monocytes/macrophages. In conclusion, MFG-E8-based reagents enable externalized PS to serve as a universal EV marker, greatly improving EV isolation, standardizing EV quantification, and facilitating diagnostic EV monitoring and disease biomarker discovery. ### Competing Interest Statement T.B. and J.K. declare competing interests due an exclusive licensing agreement with BioLegend, Inc.

Platelet homeostasis is essential for vascular integrity and immune defence1,2. Although the process of platelet formation by fragmenting megakaryocytes (MKs; thrombopoiesis) has been extensively studied, the cellular and molecular mechanisms required to constantly replenish the pool of MKs by their progenitor cells (megakaryopoiesis) remains unclear3,4. Here we use intravital imaging to track the cellular dynamics of megakaryopoiesis over days. We identify plasmacytoid dendritic cells (pDCs) as homeostatic sensors that monitor the bone marrow for apoptotic MKs and deliver IFNα to the MK niche triggering local on-demand proliferation and maturation of MK progenitors. This pDC-dependent feedback loop is crucial for MK and platelet homeostasis at steady state and under stress. pDCs are best known for their ability to function as vigilant detectors of viral infection5. We show that virus-induced activation of pDCs interferes with their function as homeostatic sensors of megakaryopoiesis. Consequently, activation of pDCs by SARS-CoV-2 leads to excessive megakaryopoiesis. Together, we identify a pDC-dependent homeostatic circuit that involves innate immune sensing and demand-adapted release of inflammatory mediators to maintain homeostasis of the megakaryocytic lineage.

Platelets are first responders in thrombosis and hemostasis, but also central players in inflammation. Compared to platelets recruited to thrombi, immune-responsive platelets use distinct effector functions including Arp2/3-dependent migration along adhesive substrate gradients (haptotaxis), which prevents inflammatory bleeding and contributes to host defense. How platelet migration in this context is regulated on a cellular level is incompletely understood. Here, we use time-resolved morphodynamic profiling of individual platelets to show that migration, in contrast to clot retraction, requires anisotropic myosin IIa-activity at the platelet rear which is preceded by polarized actin polymerization at the front to initiate and maintain migration. Polarization of migrating platelets is coordinated by integrin GPIIb-dependent outside-in signaling via Gα13 to trigger tyrosine kinase c-Src/14-3-3ζ-dependent lamellipodium formation and functions independent of soluble agonists or chemotactic signals. Inhibitors of this signaling cascade, including the clinically employed ABL/c-Src inhibitor dasatinib, interfere predominantly with the migratory capacity of platelets, without major impairment of classical platelet functions. In murine inflammation models, this translates to reduced migration of platelets visualized by 4D intravital microscopy, resulting in increased inflammation-associated hemorrhage in acute lung injury. Finally, platelets isolated from dasatinib-treated leukemia patients prone to clinically relevant hemorrhage exhibit prominent migration defects, while other platelet functions are only partially affected. In summary, we define a distinct signaling pathway essential for migration, and provide novel mechanistic insights explaining dasatinib-related platelet dysfunction and bleeding.

CD8+ T cells are crucial for the clearance of viral infections. During the acute phase, proinflammatory conditions increase the amount of circulating phosphatidylserine+ (PS) extracellular vesicles (EVs). These EVs interact especially with CD8+ T cells; however, it remains unclear whether they can actively modulate CD8+ T cell responses. In this study, we have developed a method to analyze cell-bound PS+ EVs and their target cells in vivo. We show that EV+ cell abundance increases during viral infection and that EVs preferentially bind to activated, but not naive, CD8+ T cells. Superresolution imaging revealed that PS+ EVs attach to clusters of CD8 molecules on the T cell surface. Furthermore, EV-binding induces antigen (Ag)-specific TCR signaling and increased nuclear translocation of the transcription factor Nuclear factor of activated T-cells (NFATc1) in vivo. EV-decorated but not EV-free CD8+ T cells are enriched for gene signatures associated with T-cell receptor signaling, early effector differentiation, and proliferation. Our data thus demonstrate that PS+ EVs provide Ag-specific adjuvant effects to activated CD8+ T cells in vivo.

Phosphatidylserine (PS) is a phospholipid that is present in each cell membrane. In intact cells, PS is kept intracellularly, in dying cells, extracellular vesicles (EVs) and activated platelets, PS flips outside where it can be accessed by PS-binding reagents, such as Milk fat globule-EGF factor 8 (MFG-E8). In previous studies, we have established in vivouse of fluorescent MFG-E8 to label and characterize EVs. Here, we measured the antigen-specific immune responses elicited by targeting antigen (Ag) to PS +structures, presumably EVs via MFG-E8 as a novel vaccination strategy. In contrast to non-targeted conventional Ag, PS-targeting by fusing Ag to MFG-E8 caused accumulation in the marginal zone of lymphoid follicles soon after injection. Hours later, Ag-MFG-E8 was deposited on the follicular dendritic cell network without the need of previous primary immunization. Mice immunized with PS-targeted Ag/Alum induced long-lasting germinal centers (GCs) and significantly more GC B cells. Furthermore, it resulted in an approx. 13,000-fold increase in Ag-specific IgG-levels and strongly accelerated antibody class switch recombination as compared to immunization with conventional Ag/Alum. Ag targeted to PS also induced more efficient CD8 T cell activation and IFN-gamma production post immunization. Therefore, targeting Ag to EVs by harnessing the PS-binding properties of MFG-E8 might turn out to be of advantage for the induction of very rapid and long-lasting immune responses. This project was supported by the Deutsche Forschungsgemeinschaft (DFG, German Research foundation) – Project-ID 210592381 - SFB 1054 B03 to Thomas Brocker and Z02 to Jan Kranich.

Supplementary table 1: Supplementary Table 1: Gene expression measured in QIAGEN Apoptosis Array; Supplementary Table 2: Antibodies

BackgroundTreg cells have been shown to be an important part of immune-homeostasis and IL-2 which is produced upon T cell receptor (TCR)-dependent activation of T lymphocytes has been demonstrated to critically participate in Treg development.ObjectiveTo evaluate small molecule inhibitors (SMI) for the identification of novel IL-2/Treg enhancing compounds.Materials and methodsWe used TCR-dependent and allergen-specific cytokine secretion of human and mouse T cells, next generation messenger ribonucleic acid sequencing (RNA-Seq) and two different models of allergic airway inflammation to examine lead SMI-compounds.ResultsWe show here that the reported 3-phosphoinositide dependent kinase-1 (PDK1) SMI BX-795 increased IL-2 in culture supernatants of Jurkat E6-1 T cells, human peripheral blood mononuclear cells (hPBMC) and allergen-specific mouse T cells upon TCR-dependent and allergen-specific stimulation while concomitantly inhibiting Th2 cytokine secretion. RNA-Seq revealed that the presence of BX-795 during allergen-specific activation of T cells induces a bona fide Treg cell type highly similar to iTreg but lacking Foxp3 expression. When applied in mugwort pollen and house dust mite extract-based models of airway inflammation, BX-795 significantly inhibited Th2 inflammation including expression of Th2 signature transcription factors and cytokines and influx into the lungs of type 2-associated inflammatory cells such as eosinophils.ConclusionsBX-795 potently uncouples IL-2 production from Th2 inflammation and induces Th-IL-2 cells, which highly resemble induced (i)Tregs. Thus, BX-795 may be a useful new compound for the treatment of allergic diseases.

Platelet homeostasis is essential for vascular integrity and immune defense. While the process of platelet formation by fragmenting megakaryocytes (thrombopoiesis) has been extensively studied, the cellular and molecular mechanisms required to constantly replenish the pool of megakaryocytes by their progenitor cells (megakaryopoiesis) remains unclear. Here we use intravital 2 photon microscopy to track individual megakaryopoiesis over days. We identify plasmacytoid dendritic cells (pDCs) as crucial bone marrow niche cells that regulate megakaryopoiesis. pDCs monitor the bone marrow for platelet-producing megakaryocytes and deliver IFN-α to the megakaryocytic niche to trigger local on-demand proliferation of megakaryocyte progenitors. This fine-tuned coordination between thrombopoiesis and megakaryopoiesis is crucial for megakaryocyte and platelet homeostasis in steady state and stress. However, uncontrolled pDC function within the megakaryocytic niche is detrimental. Accordingly, we show that pDCs activated by SARS-CoV2 drive inappropriate megakaryopoiesis associated with thrombotic complications. Together, we uncover a hitherto unknown megakaryocytic bone marrow niche maintained by the constitutive delivery of pDC-derived IFN-α.

Impairment of vascular integrity is a hallmark of inflammatory diseases. We recently reported that single immune-responsive platelets migrate and reposition themselves to sites of vascular injury to prevent bleeding. However, it remains unclear how single platelets preserve vascular integrity once encountering endothelial breaches. Here we demonstrate by intravital microscopy combined with genetic mouse models that procoagulant activation (PA) of single platelets and subsequent recruitment of the coagulation cascade are crucial for the prevention of inflammatory bleeding. Using a novel lactadherin-based compound, we detect phosphatidylserine (PS)-positive procoagulant platelets in the inflamed vasculature. We identify exposed collagen as the central trigger arresting platelets and initiating subsequent PA in a CypD- and TMEM16F-dependent manner both in vivo and in vitro. Platelet PA promotes binding of the prothrombinase complex to the platelet membrane, greatly enhancing thrombin activity and resulting in fibrin formation. PA of migrating platelets is initiated by costimulation via integrin alpha IIb beta 3 (GPIIBIIIA)/G alpha(13)-mediated outside-in signaling and glycoprotein VI signaling, leading to an above-threshold intracellular calcium release. This effectively targets the coagulation cascade to breaches of vascular integrity identified by patrolling platelets. Platelet-specific genetic loss of either CypD or TMEM16F as well as combined blockade of platelet GPIIBIIIA and glycoprotein VI reduce platelet PA in vivo and aggravate pulmonary inflammatory hemorrhage. Our findings illustrate a novel role of procoagulant platelets in the prevention of inflammatory bleeding and provide evidence that PA of patrolling platelet sentinels effectively targets and confines activation of coagulation to breaches of vascular integrity.

Colorectal cancer is one of the most common cancers and a major cause of mortality. Proinflammatory and antitumor immune responses play critical roles in colitis-associated colon cancer. CCL17, a chemokine of the C-C family and ligand for CCR4, is expressed by intestinal dendritic cells in the steady state and is upregulated during colitis in mouse models and inflammatory bowel disease patients. In this study, we investigated the expression pattern and functional relevance of CCL17 for colitis-associated colon tumor development using CCL17-enhanced GFP-knockin mice. CCL17 was highly expressed by dendritic cells but also upregulated in macrophages and intermediary monocytes in colon tumors induced by exposure to azoxymethane and dextran sodium sulfate. Despite a similar degree of inflammation in the colon, CCL17-deficient mice developed fewer tumors than did CCL17-competent mice. This protective effect was abrogated by cohousing, indicating a dependency on the microbiota. Changes in microbiota diversity and composition were detected in separately housed CCL17-deficient mice, and these mice were more susceptible to azoxymethane-induced early apoptosis in the colon affecting tumor initiation. Immune cell infiltration in colitis-induced colon tumors was not affected by the lack of CCL17. Taken together, our results indicate that CCL17 promotes colitis-associated tumorigenesis by influencing the composition of the intestinal microbiome and reducing apoptosis during tumor initiation.

Plasmacytoid dendritic cells (pDCs) display an increased abundance in visceral adipose tissue (VAT) of humans with obesity. In the current study, we set out to decipher the molecular mechanisms of their recruitment to VAT and the functional relevance of this process. We observed increased pDC numbers in murine blood, liver, spleen, and VAT after feeding a high-fat diet (HFD) for 3 wk when compared with a standard diet. pDCs were enriched in fat-associated lymphoid clusters representing highly specific lymphoid regions within VAT. HFD led to an enlargement of fat-associated lymphoid clusters with an increased density and migratory speed of pDCs as shown by intravital multiphoton microscopy. For their recruitment into VAT, pDCs employed P-selectin with E-selectin and L-selectin being only critical in response to HFD, indicating that the molecular cues underlying pDC trafficking were dependent on the nutritional state. Subsequent recruitment steps required α4β1 and α4β7 integrins and engagement of CCR7. Application of fingolimod (FTY720) abrogated egress of pDCs from VAT, indicating the involvement of sphingosine-1-phosphate in this process. Furthermore, HFD altered pDC functions by promoting their activation and type 1 IFN expression. Blocking pDC infiltration into VAT prevented weight gain and improved glucose tolerance during HFD. In summary, a HFD fundamentally alters pDC biology by promoting their trafficking, retention, and activation in VAT, which in turn seems to regulate metabolism.

Infection with SARS-CoV-2 is associated with thromboinflammation, involving thrombotic and inflammatory responses, in many COVID-19 patients. In addition, immune dysfunction occurs in patients characterized by T cell exhaustion and severe lymphopenia. We investigated the distribution of phosphatidylserine (PS), a marker of dying cells, activated platelets, and platelet-derived microparticles (PMP), during the clinical course of COVID-19. We found an unexpectedly high amount of blood cells loaded with PS+ PMPs for weeks after the initial COVID-19 diagnosis. Elevated frequencies of PS+PMP+ PBMCs correlated strongly with increasing disease severity. As a marker, PS outperformed established laboratory markers for inflammation, leucocyte composition, and coagulation, currently used for COVID-19 clinical outcome prognosis. PS+ PMPs preferentially bound to CD8+ T cells with gene expression signatures of proliferating effector rather than memory T cells. As PS+ PMPs carried programmed death-ligand 1 (PD-L1), they may affect T cell expansion or function. Our data provide a novel marker for disease severity and show that PS, which can trigger the blood coagulation cascade, the complement system, and inflammation, resides on activated immune cells. Therefore, PS may serve as a beacon to attract thromboinflammatory processes toward lymphocytes and cause immune dysfunction in COVID-19.

Loss of appetite (anorexia) is a typical behavioral response to infectious diseases that often reduces body weight. Also, anorexia can be observed in cancer and trauma patients, causing poor quality of life and reduced prospects of positive therapeutic outcomes. Although anorexia is an acute symptom, its initiation and endocrine regulation during antiviral immune responses are poorly understood. During viral infections, plasmacytoid dendritic cells (pDCs) produce abundant type I interferon (IFN-I) to initiate first-line defense mechanisms. Here, by targeted ablation of pDCs and various in vitro and in vivo mouse models of viral infection and inflammation, we identified that IFN-I is a significant driver of somatostatin (SST). Consequently, SST suppressed the hunger hormone ghrelin that led to severe metabolic changes, anorexia, and rapid body weight loss. Furthermore, during vaccination with Modified Vaccinia Ankara virus (MVA), the SST-mediated suppression of ghrelin was critical to viral immune response, as ghrelin restrained the production of early cytokines by natural killer (NK) cells and pDCs, and impaired the clonal expansion of CD8+ T cells. Thus, the hormonal modulation of ghrelin through SST and the cytokine IFN-I is fundamental for optimal antiviral immunity, which comes at the expense of calorie intake.

Gut microbiota and the immune system are in constant exchange shaping both host immunity and microbial communities. Here, improper immune regulation can cause inflammatory bowel disease (IBD) and colitis. Antibody therapies blocking signaling through the CD40–CD40L axis showed promising results as these molecules are deregulated in certain IBD patients. To better understand the mechanism, we used transgenic DC-LMP1/CD40 animals with a constitutive CD40-signal in CD11c+ cells, causing a lack of intestinal CD103+ dendritic cells (DCs) and failure to induce regulatory T (iTreg) cells. These mice rapidly develop spontaneous fatal colitis, accompanied by dysbiosis and increased inflammatory IL-17+IFN-γ+ Th17/Th1 and IFN-γ + Th1 cells. In the present study, we analyzed the impact of the microbiota on disease development and detected elevated IgA- and IgG-levels in sera from DC-LMP1/CD40 animals. Their serum antibodies specifically bound intestinal bacteria, and by proteome analysis, we identified a 60 kDa chaperonin GroEL (Hsp60) from Helicobacter hepaticus (Hh) as the main specific antigen targeted in the absence of iTregs. When re-derived to a different Hh-free specific-pathogen-free (SPF) microbiota, mice showed few signs of disease, normal microbiota, and no fatality. Upon recolonization of mice with Hh, the disease developed rapidly. Thus, the present work identifies GroEL/Hsp60 as a major Hh-antigen and its role in disease onset, progression, and outcome in this colitis model. Our results highlight the importance of CD103+ DC- and iTreg-mediated immune tolerance to specific pathobionts to maintain healthy intestinal balance.