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Severe infections are a major stress on haematopoiesis, where the consequences for haematopoietic stem cells (HSCs) have only recently started to emerge. HSC function critically depends on the integrity of complex bone marrow (BM) niches; however, what role the BM microenvironment plays in mediating the effects of infection on HSCs remains an open question. Here, using a murine model of malaria and combining single-cell RNA sequencing, mathematical modelling, transplantation assays and intravital microscopy, we show that haematopoiesis is reprogrammed upon infection, whereby the HSC compartment turns over substantially faster than at steady-state and HSC function is drastically affected. Interferon is found to affect both haematopoietic and mesenchymal BM cells and we specifically identify a dramatic loss of osteoblasts and alterations in endothelial cell function. Osteo-active parathyroid hormone treatment abolishes infection-triggered HSC proliferation and—coupled with reactive oxygen species quenching—enables partial rescuing of HSC function.Haltalli et al. show that Plasmodium berghei infection induces interferon release, and affects haematopoietic stem cell proliferation and function, as well as osteoblasts and vascular integrity, in the bone marrow niche.

The malignant microenvironment plays a major role in the development of resistance to therapies and the occurrence of relapses in acute myeloid leukemia (AML). We previously showed that interactions of AML blasts with bone marrow macrophages (MΦ) shift their polarization towards a protumoral (M2-like) phenotype, promoting drug resistance; we demonstrated that inhibiting the colony-stimulating factor-1 receptor (CSF1R) repolarizes MΦ towards an antitumoral (M1-like) phenotype and that other factors may be involved. We investigated here macrophage migration inhibitory factor (MIF) as a target in AML blast survival and protumoral interactions with MΦ. We show that pharmacologically inhibiting MIF secreted by AML blasts results in their apoptosis. However, this effect is abrogated when blasts are co-cultured in close contact with M2-like MΦ. We next demonstrate that pharmacological inhibition of MIF secreted by MΦ, in the presence of granulocyte macrophage-colony stimulating factor (GM-CSF), efficiently reprograms MΦ to an M1-like phenotype that triggers apoptosis of interacting blasts. Furthermore, contact with reprogrammed MΦ relieves blast resistance to venetoclax and midostaurin acquired in contact with CD163 + protumoral MΦ. Using intravital imaging in mice, we also show that treatment with MIF inhibitor 4-IPP and GM-CSF profoundly affects the tumor microenvironment in vivo : it strikingly inhibits tumor vasculature, reduces protumoral MΦ, and slows down leukemia progression. Thus, our data demonstrate that MIF plays a crucial role in AML MΦ M2-like protumoral phenotype that can be reversed by inhibiting its activity and suggest the therapeutic targeting of MIF as an avenue towards improved AML treatment outcomes.

Severe infections such as malaria are on the rise worldwide, driven by both climate change and increasing drug resistance. It is therefore paramount that we better understand how the host responds to severe infection. Hematopoiesis is particularly of interest in this context because hematopoietic stem and progenitor cells (HSPCs) maintain the turnover of all blood cells, including all immune cells. Severe infections have been widely acknowledged to affect HSPCs; however, this disruption has been mainly studied during the acute phase, and the process and level of HSPC recovery remain understudied. Using a self-resolving model of natural rodent malaria, infection by Plasmodium chabaudi, here we systematically assess phenotypically defined HSPCs’ acute response and recovery upon pathogen clearance. We demonstrate that during the acute phase of infection the most quiescent and functional stem cells are depleted, multipotent progenitor compartments are drastically enlarged, and oligopotent progenitors virtually disappear, underpinned by dramatic, population-specific and sometimes unexpected changes in proliferation rates. HSPC populations return to homeostatic size and proliferation rate again through specific patterns of recovery. Overall, our data demonstrate that HSPC populations adopt different responses to cope with severe infection and suggest that the ability to adjust proliferative capacity becomes more restricted as differentiation progresses.

Adoptive cell therapy with chimeric antigen receptor (CAR) T cells has transformed standard-of-care for selected hematologic malignancies, but relapses are frequent and efficacy against solid tumors remains limited1,2. The tumor microenvironment (TME) plays a key role in tumor progression3, and both soluble and cellular TME components can limit CAR-T cell function and persistence4. Targeting soluble TME factors to enhance anti-tumor responses of engineered T cells through chimeric receptors is not yet broadly explored due to the unpredictable signaling characteristics of synthetic protein receptors. Here we developed a protein design platform for the de novo bottom-up assembly of allosteric receptors with programmable input-output behaviors that respond to soluble TME factors with co-stimulation and cytokine signals in T cells, called T-SenSER (TME-sensing switch receptor for enhanced response to tumors). We developed two sets of T-SenSERs targeting vascular endothelial growth factor (VEGF) or colony stimulating factor 1 (CSF1), that are both selectively enriched in a variety of tumors. Combination of CAR and T-SenSER in human T cells enhanced anti-tumor responses in models of lung cancer and multiple myeloma, in a VEGF or CSF1-dependent manner. Our study sets the stage for the accelerated development of synthetic biosensors with custom-built sensing and responses for basic and translational cell engineering applications.

Severe infections such as malaria are on the rise worldwide, driven by both climate change and increasing drug-resistance. It is therefore paramount that we better understand how the host’s organism responds to severe infection. Hematopoiesis is particularly of interest in this context because hematopoietic stem and progenitor cells (HSPCs) maintain the turnover of all blood cells, including all immune cells. Severe infections have been widely acknowledged to affecting HSPCs, however this disruption has been mainly studied during the acute phase, and the process and level of HSPC recovery remains understudied. Using a self-resolving model of natural rodent malaria, infection by Plasmodium chabaudi, here we systematically assess HSPCs’ acute response and recovery upon pathogen clearance. We demonstrate that during the acute phase of infection the most quiescent and functional stem cells are depleted, multipotent progenitor compartments are drastically enlarged, and oligopotent progenitors virtually disappear, underpinned by dramatic, population-specific and sometimes unexpected changes in proliferation rates. HSPC populations return to homeostatic size and proliferation rate again through specific patterns of recovery. Overall, our data demonstrate that HSPC populations adopt different responses to cope with severe infection and suggest that the ability to adjust proliferative capacity becomes more restricted as differentiation progresses.

Hematopoietic stem cells (HSCs) sustain lifelong haematopoiesis as their progeny differentiate into all blood cell lineages. Homeostatic HSCs are mostly quiescent and only rarely divide, however their proliferation and differentiation rates can be modulated by external factors. Acute and chronic infections from a wide range of pathogens are known to challenge HSCs at the population level, being forced to respond to inflammation-mediated organismal demand to replenish the myeloid cell pool. However, less is known about the degree of heterogeneity in the HSCs’ response to inflammation at the single cell level. Here, using a natural murine malaria model and an NHS-ester biotin dilution assay we identify two subsets of HSCs, BiotinLo and BiotinHi, with distinct proliferation kinetics. Using combined functional, single-cell transcriptomics and phenotypic analyses, we uncover that BiotinHi HSCs remain highly functional despite expressing strong interferon response signatures. These infection-resistant HSCs express high levels of MHC II and are metabolically distinct from the remaining HSCs as they maintain less active mitochondria. These findings demonstrate that a likely reserve pool of HSCs remains highly functional during Plasmodium infection not because cells are shielded, but because they maintain a stemness associated metabolic profile despite effectively sensing inflammation.

Acute myeloid leukemia (AML) continues to have a poor prognosis due to its ability to relapse following initial response to chemotherapy. While immunotherapies hold the promise to revolutionize cancer treatment, AML has been particularly challenging to target. It is therefore important to better understand the relationship between AML cells and immune cells within the bone marrow (BM) microenvironment, where this disease grows. Here we focus on non-malignant BM macrophages, and using a combination of intravital microscopy, flow cytometry, transcriptomics and functional analyses we identify a subpopulation of immunomodulatory BM macrophages (IMMs) with a unique profile and function during AML progression. While the majority of macrophages are already being lost at early infiltration, IMMs are locally enriched. They are capable of efferocytosis and support AML growth through inhibition of T cells. Enrichment of IMMs in the BM of patients developing early relapse indicates that future development of interventions that target IMMs’ development and function may improve AML patients’ outcome.

Abstract Acute Myeloid Leukemia, a hematological malignancy with poor clinical outcome, is composed of hierarchically heterogeneous cells. We examine the contribution of this heterogeneity to disease progression in the context of anti-tumor immune responses and investigate whether these responses regulate the balance between stemness and differentiation in AML. Combining phenotypic analysis with proliferation dynamics and fate-mapping of AML cells in a murine AML model, we demonstrate the presence of a terminally differentiated, chemoresistant population expressing high levels of PDL1. We show that PDL1 upregulation in AML cells, following exposure to IFNγ from activated T cells, is coupled with AML differentiation and the dynamic balance between proliferation, versus differentiation and immunosuppression, facilitates disease progression in the presence of immune responses. This microenvironment-responsive hierarchical heterogeneity in AML may be key in facilitating disease growth at the population level at multiple stages of disease, including following bone marrow transplantation and immunotherapy. Competing Interest Statement The authors have declared no competing interest.

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