Non-myeloablative busulfan chimeric mouse models are less pro-inflammatory than head-shielded irradiation for studying immune cell interactions in brain tumours

Regulation of muscle hypertrophy through granulin: Relayed communication among mesenchymal progenitors, macrophages, and satellite cells

Skeletal muscles exert remarkable regenerative or adaptive capacities in response to injuries or mechanical loads. However, the cellular networks underlying muscle adaptation are poorly understood compared to those underlying muscle regeneration. We employed single-cell RNA sequencing to investigate the gene expression patterns and cellular networks activated in overloaded muscles and compared these results with those observed in regenerating muscles. The cellular composition of the 4-day overloaded muscle, when macrophage infiltration peaked, closely resembled that of the 10-day regenerating muscle. In addition to the mesenchymal progenitor-muscle satellite cell (MuSC) axis, interactome analyses or targeted depletion experiments revealed communications between mesenchymal progenitors-macrophages and macrophages-MuSCs. Furthermore, granulin, a macrophage-derived factor, inhibited MuSC differentiation, and Granulin-knockout mice exhibited blunted muscle hypertrophy due to the premature differentiation of overloaded MuSCs. These findings reveal the critical role of granulin through the relayed communications of mesenchymal progenitors, macrophages, and MuSCs in facilitating efficient muscle hypertrophy.

Chimeric antigen receptor macrophages target and resorb amyloid plaques

Substantial evidence suggests a role for immunotherapy in treating Alzheimer's disease (AD). While the precise pathophysiology of AD is incompletely understood, clinical trials of antibodies targeting aggregated forms of β amyloid (Aβ) have shown that reducing amyloid plaques can mitigate cognitive decline in patients with early-stage AD. Here, we describe what we believe to be a novel approach to target and degrade amyloid plaques by genetically engineering macrophages to express an Aβ-targeting chimeric antigen receptor (CAR-Ms). When injected intrahippocampally, first-generation CAR-Ms have limited persistence and fail to significantly reduce plaque load, which led us to engineer next-generation CAR-Ms that secrete M-CSF and self-maintain without exogenous cytokines. Cytokine secreting "reinforced CAR-Ms" have greater survival in the brain niche and significantly reduce plaque load locally in vivo. These findings support CAR-Ms as a platform to rationally target, resorb, and degrade pathogenic material that accumulates with age, as exemplified by targeting Aβ in AD.

CNS Repopulation by Hematopoietic-Derived Microglia-Like Cells Corrects Progranulin deficiency

Hematopoietic stem cell transplantation can deliver therapeutic proteins to the CNS through donor-derived hematopoietic cells that become microglia-like cells. However, using standard conditioning approaches, hematopoietic stem cell transplantation is currently limited by low and slow engraftment of microglia-like cells. We report an efficient conditioning regimen based on Busulfan and a six-day course of microglia depletion using the colony-stimulating factor receptor 1 inhibitor PLX3397. Combining Busulfan-myeloablation and transient microglia depletion results in robust, rapid, and persistent microglia replacement by bone marrow-derived microglia-like cells throughout the CNS. Adding PLX3397 does not affect neurobehavior or has adverse effects on hematopoietic reconstitution. Through single-cell RNA sequencing and high-dimensional CyTOF mass cytometry, we show that microglia-like cells are a heterogeneous population and describe six distinct subpopulations. Though most bone-marrow-derived microglia-like cells can be classified as homeostatic microglia, their gene signature is a hybrid of homeostatic/embryonic microglia and border associated-macrophages. Busulfan-myeloablation and transient microglia depletion induce specific cytokines in the brain, ultimately combining myeloid proliferative and chemo-attractive signals that act locally to repopulate microglia from outside the niche. Importantly, this conditioning approach demonstrates therapeutic efficacy in a mouse model of GRN deficiency. Transplanting wild-type bone marrow into Grn-/- mice conditioned with Busulfan plus PLX3397 results in high engraftment of microglia-like cells in the brain and retina, restoring GRN levels and normalizing lipid metabolism.

Central IRF4/5 Signaling Are Critical for Microglial Activation and Impact on Stroke Outcomes

Microglia and monocytes play a critical role in immune responses to cerebral ischemia. Previous studies have demonstrated that interferon regulatory factor 4 (IRF4) and IRF5 direct microglial polarization after stroke and impact outcomes. However, IRF4/5 are expressed by both microglia and monocytes, and it is not clear if it is the microglial (central) or monocytic (peripheral) IRF4-IRF5 regulatory axis that functions in stroke. In this work, young (8-12 weeks) male pep boy (PB), IRF4 or IRF5 flox, and IRF4 or IRF5 conditional knockout (CKO) mice were used to generate 8 types of bone marrow chimeras, to differentiate the role of central (PB-to-IRF CKO) vs. peripheral (IRF CKO-to-PB) phagocytic IRF4-IRF5 axis in stroke. Chimeras generated from PB and flox mice were used as controls. All chimeras were subjected to 60-min middle cerebral artery occlusion (MCAO) model. Three days after the stroke, outcomes and inflammatory responses were analyzed. We found that PB-to-IRF4 CKO chimeras had more robust microglial pro-inflammatory responses than IRF4 CKO-to-PB chimeras, while ameliorated microglial response was seen in PB-to-IRF5 CKO vs. IRF5 CKO-to-PB chimeras. PB-to-IRF4 or IRF5 CKO chimeras had worse or better stroke outcomes respectively than their controls, whereas IRF4 or 5 CKO-to-PB chimeras had similar outcomes compared to controls. We conclude that the central IRF4/5 signaling is responsible for microglial activation and mediates stroke outcomes.

Panhematopoietic RNA barcoding enables kinetic measurements of nucleate and anucleate lineages and the activation of myeloid clones following acute platelet depletion

BACKGROUND

Platelets and erythrocytes constitute over 95% of all hematopoietic stem cell output. However, the clonal dynamics of HSC contribution to these lineages remains largely unexplored.

RESULTS

We use lentiviral genetic labeling of mouse hematopoietic stem cells to quantify output from all lineages, nucleate, and anucleate, simultaneously linking these with stem and progenitor cell transcriptomic phenotypes using single-cell RNA-sequencing. We observe dynamic shifts of clonal behaviors through time in same-animal peripheral blood and demonstrate that acute platelet depletion shifts the output of multipotent hematopoietic stem cells to the exclusive production of platelets. Additionally, we observe the emergence of new myeloid-biased clones, which support short- and long-term production of blood cells.

CONCLUSIONS

Our approach enables kinetic studies of multi-lineage output in the peripheral blood and transcriptional heterogeneity of individual hematopoietic stem cells. Our results give a unique insight into hematopoietic stem cell reactivation upon platelet depletion and of clonal dynamics in both steady state and under stress.

Chimeric Antigen Receptor Macrophages Target and Resorb Amyloid Plaques in a Mouse Model of Alzheimer's Disease

Substantial evidence suggests a role for immunotherapy in treating Alzheimer's disease (AD). Several monoclonal antibodies targeting aggregated forms of beta amyloid (Aβ), have been shown to reduce amyloid plaques and in some cases, mitigate cognitive decline in early-stage AD patients. We sought to determine if genetically engineered macrophages could improve the targeting and degradation of amyloid plaques. Chimeric antigen receptor macrophages (CAR-Ms), which show promise as a cancer treatment, are an appealing strategy to enhance target recognition and phagocytosis of amyloid plaques in AD. We genetically engineered macrophages to express a CAR containing the anti-amyloid antibody aducanumab as the external domain and the Fc receptor signaling domain internally. CAR-Ms recognize and degrade Aβ in vitro and on APP/PS1 brain slices ex vivo; however, when injected intrahippocampally, these first-generation CAR-Ms have limited persistence and fail to reduce plaque load. We overcame this limitation by creating CAR-Ms that secrete M-CSF and self-maintain without exogenous cytokines. These CAR-Ms have greater survival in the brain niche, and significantly reduce plaque load locally in vivo. These proof-of-principle studies demonstrate that CAR-Ms, previously only applied to cancer, may be utilized to target and degrade unwanted materials, such as amyloid plaques in the brains of AD mice.

Macrophages and microglia in glioblastoma: heterogeneity, plasticity, and therapy

Glioblastoma (GBM) is the most aggressive tumor in the central nervous system and contains a highly immunosuppressive tumor microenvironment (TME). Tumor-associated macrophages and microglia (TAMs) are a dominant population of immune cells in the GBM TME that contribute to most GBM hallmarks, including immunosuppression. The understanding of TAMs in GBM has been limited by the lack of powerful tools to characterize them. However, recent progress on single-cell technologies offers an opportunity to precisely characterize TAMs at the single-cell level and identify new TAM subpopulations with specific tumor-modulatory functions in GBM. In this Review, we discuss TAM heterogeneity and plasticity in the TME and summarize current TAM-targeted therapeutic potential in GBM. We anticipate that the use of single-cell technologies followed by functional studies will accelerate the development of novel and effective TAM-targeted therapeutics for GBM patients.

The Dynamics of Tumor-Infiltrating Myeloid Cell Activation and the Cytokine Expression Profile in a Glioma Resection Site during the Post-Surgical Period in Mice

Glioblastoma is the most aggressive brain cancer and is highly infiltrated with cells of myeloid lineage (TIM) that support tumor growth and invasion. Tumor resection is the primary treatment for glioblastoma; however, the activation state of TIM at the site of tumor resection and its impact on glioma regrowth are poorly understood. Using the C57BL/6/GL261 mouse glioma implantation model, we investigated the state of TIM in the tumor resection area during the post-surgical period. TIM isolated from brain tissue at the resection site were analyzed at 0, 1, 4, 7, 14, and 21 days after tumor resection. An increase in expression of CD86 during the first 7 days after surgical resection and then upregulation of arginase 1 from the 14th to 21st days after resection were detected. Cytokine expression analysis combined with qRT-PCR revealed sustained upregulation of IL4, IL5, IL10, IL12, IL17, vascular endothelial growth factor (VEGF), and monocyte chemoattractant protein 1 (MCP1/CCL2) in TIM purified from regrown tumors compared with primary implanted tumors. Flow cytometry analysis revealed increased CD86+/CD206+ population in regrown tumors compared with primary implanted tumors. Overall, we found that TIM in primary implanted tumors and tumors regrown after resection exhibited different phenotypes and cytokine expression patterns.

Improved engraftment and therapeutic efficacy by human genome-edited hematopoietic stem cells with Busulfan-based myeloablation

Autologous hematopoietic stem cell transplantation using genome-edited cells can become a definitive therapy for hematological and non-hematological disorders with neurological involvement. Proof-of-concept studies using human genome-edited hematopoietic stem cells have been hindered by the low efficiency of engraftment of the edited cells in the bone marrow and their modest efficacy in the CNS. To address these challenges, we tested a myeloablative conditioning regimen based on Busulfan in an immunocompromised model of mucopolysaccharidosis type 1. Compared with sub-lethal irradiation, Busulfan conditioning enhanced the engraftment of edited CD34+ cells in the bone marrow, as well the long-term homing and survival of bone-marrow-derived cells in viscera, and in the CNS, resulting in higher transgene expression and biochemical correction in these organs. Edited cell selection using a clinically compatible marker resulted in a population with low engraftment potential. We conclude that conditioning can impact the engraftment of edited hematopoietic stem cells. Furthermore, Busulfan-conditioned recipients have a higher expression of therapeutic proteins in target organs, particularly in the CNS, constituting a better conditioning approach for non-hematological diseases with neurological involvement.

A cardioimmunologist's toolkit: genetic tools to dissect immune cells in cardiac disease

Cardioimmunology is a field that encompasses the immune cells and pathways that modulate cardiac function in homeostasis and regulate the temporal balance between tissue injury and repair in disease. Over the past two decades, genetic fate mapping and high-dimensional sequencing techniques have defined increasing functional heterogeneity of innate and adaptive immune cell populations in the heart and other organs, revealing a complexity not previously appreciated and challenging established frameworks for the immune system. Given these rapid advances, understanding how to use these tools has become crucial. However, cardiovascular biologists without immunological expertise might not be aware of the strengths and caveats of immune-related tools and how they can be applied to examine the pathogenesis of myocardial diseases. In this Review, we guide readers through case-based examples to demonstrate how tool selection can affect data quality and interpretation and we provide critical analysis of the experimental tools that are currently available, focusing on their use in models of ischaemic heart injury and heart failure. The goal is to increase the use of relevant immunological tools and strategies among cardiovascular researchers to improve the precision, translatability and consistency of future studies of immune cells in cardiac disease.

Treatment of a genetic brain disease by CNS-wide microglia replacement

Hematopoietic cell transplantation after myeloablative conditioning has been used to treat various genetic metabolic syndromes but is largely ineffective in diseases affecting the brain presumably due to poor and variable myeloid cell incorporation into the central nervous system. Here, we developed and characterized a near-complete and homogeneous replacement of microglia with bone marrow cells in mice without the need for genetic manipulation of donor or host. The high chimerism resulted from a competitive advantage of scarce donor cells during microglia repopulation rather than enhanced recruitment from the periphery. Hematopoietic stem cells, but not immediate myeloid or monocyte progenitor cells, contained full microglia replacement potency equivalent to whole bone marrow. To explore its therapeutic potential, we applied microglia replacement to a mouse model for Prosaposin deficiency, which is characterized by a progressive neurodegeneration phenotype. We found a reduction of cerebellar neurodegeneration and gliosis in treated brains, improvement of motor and balance impairment, and life span extension even with treatment started in young adulthood. This proof-of-concept study suggests that efficient microglia replacement may have therapeutic efficacy for a variety of neurological diseases.

Microglia and Microglia-Like Cells: Similar but Different

Microglia are the tissue-resident macrophages of the central nervous parenchyma. In mammals, microglia are thought to originate from yolk sac precursors and posteriorly maintained through the entire life of the organism. However, the contribution of microglial cells from other sources should also be considered. In addition to “true” or “bona-fide” microglia, which are of embryonic origin, the so-called “microglia-like cells” are hematopoietic cells of bone marrow origin that can engraft the mature brain mainly under pathological conditions. These cells implement great parts of the microglial immune phenotype, but they do not completely adopt the “true microglia” features. Because of their pronounced similarity, true microglia and microglia-like cells are usually considered together as one population. In this review, we discuss the origin and development of these two distinct cell types and their differences. We will also review the factors determining the appearance and presence of microglia-like cells, which can vary among species. This knowledge might contribute to the development of therapeutic strategies aiming at microglial cells for the treatment of diseases in which they are involved, for example neurodegenerative disorders like Alzheimer’s and Parkinson’s diseases.

Distinction of Microglia and Macrophages in Glioblastoma: Close Relatives, Different Tasks?

For decades, it has been known that the tumor microenvironment is significant for glioma progression, namely the infiltration of myeloid cells like microglia and macrophages. Hence, these cell types and their specific tasks in tumor progression are subject to ongoing research. However, the distribution of the brain resident microglia and the peripheral macrophages within the tumor tissue and their functional activity are highly debated. Results depend on the method used to discriminate between microglia and macrophages, whereby this specification is already difficult due to limited options to distinguish between these both cell populations that show mostly the same surface markers and morphology. Moreover, there are indications about various functions of microglia and macrophages but again varying on the method of discrimination. In our review, we summarize the current literature to determine which methods have been applied to differentiate the brain resident microglia from tumor-infiltrated macrophages. Furthermore, we compiled data about the proportion of microglia and macrophages in glioma tissues and ascertained if pro- or anti-tumoral effects could be allocated to one or the other myeloid cell population. Recent research made tremendous efforts to distinguish microglia from recruited macrophages. For future studies, it could be essential to verify which role these cells play in brain tumor pathology to proceed with novel immunotherapeutic strategies.

Efficient engraftment of genetically modified cells is necessary to ameliorate central nervous system involvement of murine model of mucopolysaccharidosis type II by hematopoietic stem cell targeted gene therapy

Mucopolysaccharidosis type II (MPS II) is a lysosomal storage disease (LSD) caused by a deficiency of the iduronate-2-sulfatase (IDS) that catabolizes glycosaminoglycans (GAGs). Abnormal accumulations of GAGs in somatic cells lead to various manifestations including central nervous system (CNS) disease. Enzyme replacement therapy (ERT) and hematopoietic stem cell transplantation (HSCT) are the currently available therapy for MPS II, but both therapies fail to improve CNS manifestations. We previously showed that hematopoietic stem cell targeted gene therapy (HSC-GT) with lethal irradiation improved CNS involvement in a murine model of MPS II which lacks the gene coding for IDS. However, the strong preconditioning, with lethal irradiation, would cause a high rate of morbidity and mortality. Therefore, we tested milder preconditioning procedures with either low dose irradiation or low dose irradiation plus an anti c-kit monoclonal antibody (ACK2) to assess CNS effects in mice with MPS II after HSC-GT. Mice from all the HSC-GT groups displayed super-physiological levels of IDS enzyme activity and robust reduction of abnormally accumulated GAGs to the wild type mice levels in peripheral organs. However, only the mice treated with lethal irradiation showed significant cognitive function improvement as well as IDS elevation and GAG reduction in the brain. These results suggest that an efficient engraftment of genetically modified cells for HSC-GT requires strong preconditioning to ameliorate CNS involvement in cases with MPS II.

Do haematopoietic stem cells age?

Genetic defects that accumulate in haematopoietic stem cells (HSCs) are thought to be responsible for age-related changes in haematopoiesis that include a decline in lymphopoiesis and skewing towards the myeloid lineage. This HSC-centric view is based largely on studies showing that HSCs from aged mice exhibit these lineage biases following transplantation into irradiated young recipient mice. In this Opinion article, we make the case that the reliance on this approach has led to inaccurate conclusions regarding the effects of ageing on blood-forming stem cells; we suggest instead that changes in the environment contribute to haematopoietic system ageing. We propose that a complete understanding of how ageing affects haematopoiesis depends on the analysis of blood cell production in unperturbed mice. We describe how this can be achieved using in situ fate mapping. This approach indicates that changes in downstream progenitors, in addition to any HSC defects, may explain the reduced lymphopoiesis and sustained myelopoiesis that occur during ageing.