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Mast cell degranulation is important in the pathogenesis of anaphylaxis and allergic disorders. Many animal venoms contain components that can induce mast cell degranulation, and this has been thought to contribute to the pathology and mortality caused by envenomation. However, we recently reported evidence that mast cells can enhance the resistance of mice to the venoms of certain snakes and that mouse mast cell-derived carboxypeptidase A3 (CPA3) can contribute to this effect. Here, we investigated whether mast cells can enhance resistance to the venom of the Gila monster, a toxic component of that venom (helodermin), and the structurally similar mammalian peptide, vasoactive intestinal polypeptide (VIP). Using 2 types of mast cell-deficient mice, as well as mice selectively lacking CPA3 activity or the chymase mouse mast cell protease-4 (MCPT4), we found that mast cells and MCPT4, which can degrade helodermin, can enhance host resistance to the toxicity of Gila monster venom. Mast cells and MCPT4 also can limit the toxicity associated with high concentrations of VIP and can reduce the morbidity and mortality induced by venoms from 2 species of scorpions. Our findings support the notion that mast cells can enhance innate defense by degradation of diverse animal toxins and that release of MCPT4, in addition to CPA3, can contribute to this mast cell function.

FOXP3+ regulatory T cells (Treg) are indispensable for immune homoeostasis and for the prevention of autoimmune diseases. Interleukin-2 (IL-2) signalling is critical in all aspects of Treg biology. Consequences of defective IL-2 signalling are insufficient numbers or dysfunction of Treg and hence autoimmune disorders in human and mouse. The restoration and maintenance of immune homoeostasis remain central therapeutic aims in the field of autoimmunity. Historically, broadly immunosuppressive drugs with serious side-effects have been used for the treatment of autoimmune diseases or prevention of organ-transplant rejection. More recently, ex vivo expanded or in vivo stimulated Treg have been shown to induce effective tolerance in clinical trials supporting the clinical benefit of targeting natural immunosuppressive mechanisms. Given the central role of exogenous IL-2 in Treg homoeostasis, a new and promising focus in drug development are IL-2-based approaches for in vivo targeted expansion of Treg or for enhancement of their suppressive activity. In this review, we summarise the role of IL-2 in Treg biology and consequences of dysfunctional IL-2 signalling pathways. We then examine evidence of efficacy of IL-2-based biological drugs targeting Treg with specific focus on therapeutic candidates in clinical trials and discuss their limitations.

Inducible genetic labelling of haematopoietic stem cells (HSCs) and linked mathematical modelling show that at least 30% of all HSCs are productive, and that adult haematopoiesis is largely sustained by ‘short-term’ downstream stem cells that operate near self-renewal in the steady state; HSC fate mapping provides a quantitative model for better understanding of HSC functions in health and disease. Following haematopoiesis in vivo Most of what we know of the properties of haematopoietic stem cells (HSCs) is derived from transplantation and reconstitution of an emptied blood and immune system. Relatively little is known about how HSCs behave under physiological conditions. It was reported recently that normal haematopoeisis in adults is driven by thousands of long-lived progenitors rather than classic HSCs. Hans-Reimer Rodewald and colleagues have used inducible genetic labelling of primitive HSCs in a mouse model, combined with mathematical modelling, to show that although HSCs participate in establishment of the blood system in early life, steady-state haematopoiesis depends mainly on progenitors that are able to self-renew but also receive rare input from long-term HSCs. This input is increased following physiological challenges. Haematopoietic stem cells (HSCs) are widely studied by HSC transplantation into immune- and blood-cell-depleted recipients. Single HSCs can rebuild the system after transplantation 1 , 2 , 3 , 4 , 5 . Chromosomal marking 6 , viral integration 7 , 8 , 9 and barcoding 10 , 11 , 12 of transplanted HSCs suggest that very low numbers of HSCs perpetuate a continuous stream of differentiating cells. However, the numbers of productive HSCs during normal haematopoiesis, and the flux of differentiating progeny remain unknown. Here we devise a mouse model allowing inducible genetic labelling of the most primitive Tie2 + HSCs in bone marrow, and quantify label progression along haematopoietic development by limiting dilution analysis and data-driven modelling. During maintenance of the haematopoietic system, at least 30% or ∼5,000 HSCs are productive in the adult mouse after label induction. However, the time to approach equilibrium between labelled HSCs and their progeny is surprisingly long, a time scale that would exceed the mouse’s life. Indeed, we find that adult haematopoiesis is largely sustained by previously designated ‘short-term’ stem cells downstream of HSCs that nearly fully self-renew, and receive rare but polyclonal HSC input. By contrast, in fetal and early postnatal life, HSCs are rapidly used to establish the immune and blood system. In the adult mouse, 5-fluoruracil-induced leukopenia enhances the output of HSCs and of downstream compartments, thus accelerating haematopoietic flux. Label tracing also identifies a strong lineage bias in adult mice, with several-hundred-fold larger myeloid than lymphoid output, which is only marginally accentuated with age. Finally, we show that transplantation imposes severe constraints on HSC engraftment, consistent with the previously observed oligoclonal HSC activity under these conditions. Thus, we uncover fundamental differences between the normal maintenance of the haematopoietic system, its regulation by challenge, and its re-establishment after transplantation. HSC fate mapping and its linked modelling provide a quantitative framework for studying in situ the regulation of haematopoiesis in health and disease.

The signals controlling T reg cell homeostasis and survival are still being determined. Liston and colleagues demonstrate that peripheral T reg cells depend critically on IL-2 and the survival factor Mcl-1 but not on Bcl-2. Foxp3 + regulatory T (T reg ) cells are a crucial immunosuppressive population of CD4 + T cells, yet the homeostatic processes and survival programs that maintain the T reg cell pool are poorly understood. Here we report that peripheral T reg cells markedly alter their proliferative and apoptotic rates to rapidly restore numerical deficit through an interleukin 2–dependent and costimulation-dependent process. By contrast, excess T reg cells are removed by attrition, dependent on the Bim-initiated Bak- and Bax-dependent intrinsic apoptotic pathway. The antiapoptotic proteins Bcl-x L and Bcl-2 were dispensable for survival of T reg cells, whereas Mcl-1 was critical for survival of T reg cells, and the loss of this antiapoptotic protein caused fatal autoimmunity. Together, these data define the active processes by which T reg cells maintain homeostasis via critical survival pathways.

MicroRNA (miR) are short non-coding RNA sequences of 19–24 nucleotides that regulate gene expression by binding to mRNA target sequences. The miR-29 family of miR ( miR-29a, b-1, b-2 and c ) is a key player in T-cell differentiation and effector function, with deficiency causing thymic involution and a more inflammatory T-cell profile. However, the relative roles of different miR-29 family members in these processes have not been dissected. We studied the immunological role of the individual members of the miR-29 family using mice deficient for miR-29a / b-1 or miR-29b-2 / c in homeostasis and during collagen-induced arthritis. We found a definitive hierarchy of immunological function, with the strong phenotype of miR-29a -deficiency in thymic involution and T-cell activation being reduced or absent in miR-29c -deficient mice. Strikingly, despite elevating the Th1 and Th17 responses, loss of miR-29a conferred near-complete protection from collagen-induced arthritis (CIA), with profound defects in B-cell proliferation and antibody production. Our results identify the hierarchical structure of the miR-29 family in T-cell biology, and identify miR-29a in B cells as a potential therapeutic target in arthritis.

Adrian Liston and colleagues use a transgenic mouse model to demonstrate that beta cell failure is a mechanistic commonality in type 1 and type 2 diabetes. They find that the changes in the molecular pathways identified as contributing to beta cell loss are paralleled in human islets from patients with type 2 diabetes. Type 1 (T1D) and type 2 (T2D) diabetes share pathophysiological characteristics, yet mechanistic links have remained elusive. T1D results from autoimmune destruction of pancreatic beta cells, whereas beta cell failure in T2D is delayed and progressive. Here we find a new genetic component of diabetes susceptibility in T1D non-obese diabetic (NOD) mice, identifying immune-independent beta cell fragility. Genetic variation in Xrcc4 and Glis3 alters the response of NOD beta cells to unfolded protein stress, enhancing the apoptotic and senescent fates. The same transcriptional relationships were observed in human islets, demonstrating the role of beta cell fragility in genetic predisposition to diabetes.

Costimulatory signals by CD28 are critical for thymic regulatory T‐cell (Treg) development. To determine the functional relevance of CD28 for peripheral Treg post thymic selection, we crossed the widely used Forkhead box protein 3 (Foxp3)‐CreYFP mice to mice bearing a conditional Cd28 allele. Treg‐specific CD28 deficiency provoked a severe autoimmune syndrome as a result of a strong disadvantage in competitive fitness and proliferation of CD28‐deficient Tregs. By contrast, Treg survival and lineage integrity were not affected by the lack of CD28. This data demonstrate that, even after the initial induction requirement, Treg maintain a higher dependency on CD28 signalling than conventional T cells for homeostasis. In addition, we found the Foxp3‐CreYFP allele to be a hypomorph, with reduced Foxp3 protein levels. Furthermore, we report here the stochastic activity of the Foxp3‐CreYFP allele in non‐Tregs, sufficient to recombine some conditional alleles (including Cd28) but not others (including R26‐RFP). This hypomorphism and ‘leaky’ expression of the Foxp3‐CreYFP allele should be considered when analysing the conditionally mutated Treg.

BackgroundDespite the success of PD1 blockade in various cancers, overcoming resistance to cancer immunotherapy remains challenging. Targeting CD8+ T cell-associated alternative immune-checkpoints is anticipated to overcome this issue. However, such immune-checkpoints are also expressed on myeloid cells, but their therapeutic and clinical impact remains enigmatic. Thus, the aim of this study was to reveal underappreciated TAMs-ontology enriching immune-inhibitory receptors, to design biomarker-driven immunotherapy.MethodsWe used reverse translational methodologies starting from human tumour multi-omics bioinformatics to inform pre-clinical experimental research, culminating into human multi-omics prognostic/predictive validationResultsWe identified a unique niche of tumour-associated macrophages (TAMs), preferentially co-expressing the TIM3 and VISTA immune-checkpoints, that dominated the human and mouse tumours resistant to PD(L)1 blockade. Subcutaneous epithelial-origin tumours and orthotopic melanoma in mouse showed that TIM3+VISTA+TAMs were sustained by IL4/IL13-enriching tumours with low (neo)antigenic and non-immunogenic milieu. TIM3/VISTA were instrumental in sustaining a hyper-efferocytotic and anti-inflammatory TAM phenotype, and blunting type I interferon (IFN) sensing, thereby fuelling immune subversion. This was established with cancer cells succumbing to immunogenic cell death (ICD). Herein, while dying cancer cells triggered autocrine type I IFN production, yet they also exposed extracellular HMGB1 and surface VISTA as ligands to engage TIM3 and VISTA on TAMs respectively, to suppress paracrine IFN responses. Consequently, TIM3/VISTA blockade preferentially synergized with paclitaxel, an ICD inducing chemotherapy in vivo, to replace the anti-inflammatory TIM3+VISTA+TAMs with pro-inflammatory TAMs-driven cytotoxicity, thus blunting the immuno-resistant tumours. In vivo macrophage-specific genetic knockout of TIM3/VISTA confirmed this synergism, while immune/genetic ablation of type I IFN sensing, macrophages (but not CD8+T cells), or cancer cell associated HMGB1/VISTA disrupted it. Finally, TIM3+VISTA+TAM signature exhibited pan-cancer negative prognostic impact and predicted resistance to immunotherapy in patients.ConclusionsWe discovered that as-yet-uncharacterized TIM3+VISTA+TAMs, enriched by human and mouse non-immunogenic tumours, mediate chemo-immunotherapy resistance. Thus, targeting TIM3+VISTA+TAMs is a novel and conserved strategy to overcome low neo-antigenic, CD8+T cells independent, tumours.Ethics ApprovalMouse Experiments were approved by the animal ethics committee at KU Leuven (project P114/2019 and p195/2020) following the European directive 2010/63/EU as amended by the Regulation (EU) 2019/1010 and the Flemish government decree of 17 February 2017.

Inducible genetic labelling of haematopoietic stem cells (HSCs) and linked mathematical modelling show that at least 30% of all HSCs are productive, and that adult haematopoiesis is largely sustained by 'short-term' downstream stem cells that operate near self-renewal in the steady state; HSC fate mapping provides a quantitative model for better understanding of HSC functions in health and disease.

Inducible genetic labelling of haematopoietic stem cells (HSCs) and linked mathematical modelling show that at least 30% of all HSCs are productive, and that adult haematopoiesis is largely sustained by 'short-term' downstream stem cells that operate near self-renewal in the steady state; HSC fate mapping provides a quantitative model for better understanding of HSC functions in health and disease.