Fate mapping reveals origins and dynamics of monocytes and tissue macrophages under homeostasis

Generation of an Inducible Destabilized-Domain Cre Mouse Line to Target Disease Associated Microglia

The function of microglia during progression of Alzheimer's disease (AD) can be investigated using mouse models that enable genetic manipulation of microglial subpopulations in a temporal manner. We developed mouse lines that express either Cre recombinase (Cre) for constitutive targeting, or destabilized-domain Cre recombinase (DD-Cre) for inducible targeting from the Cst7 locus (Cst7 DD-Cre) to specifically manipulate disease associated microglia (DAM) and crossed with Ai14 tdTomato cre-reporter line mice. Cst7Cre was found to target all brain resident myeloid cells, due to transient developmental expression of Cst7, but no expression was found in the inducible Cst7 DD-Cre mice. Further crossing of this line with 5xFAD mice combined with dietary administration of trimethoprim to induce DD-Cre activity produces long-term labeling in DAM without evidence of leakiness, with tdTomato-expression restricted to cells surrounding plaques. Using this model, we found that DAMs are a subset of plaque-associated microglia (PAMs) and their transition to DAM increases with age and disease stage. Spatial transcriptomic analysis revealed that tdTomato+ cells show higher expression of disease and inflammatory genes compared to other microglial populations, including non-labeled PAMs. These models allow either complete cre-loxP targeting of all brain myeloid cells (Cst7Cre), or inducible targeting of DAMs, without leakiness (Cst7 DD-Cre).

Label-free quantitative shotgun analysis of bis(monoacylglycero)phosphate lipids

Interest in the role of bis(monoacylglycero)phosphate (BMP) lipids in lysosomal function has significantly grown in recent years. Emerging evidence highlights BMPs as critical players not only in Niemann-Pick disease type C (NPC) but also in other pathologies such as neurodegeneration, cardiovascular diseases, and cancers. However, the selective analysis of BMPs is significantly hindered by isomeric phosphatidylglycerol (PG) lipids. While this can be addressed by chromatographic separation, it poses a significant challenge for shotgun lipidomics approaches. Here, we present a shotgun lipidomics strategy to detect and separate BMPs from PGs using differential fragmentation of sodiated ions. This approach, including isotope correction, is integrated into an existing quantitative shotgun lipidomics workflow (Lipidyzer combined with Shotgun Lipidomics Assistant software) that simultaneously quantifies >1400 lipids. Validation using K-562 cell extracts demonstrated acceptable linearity, trueness, repeatability, and a limit of quantification of 0.12 µM, confirming robust analytical performance. Finally, characteristic accumulation of BMP lipids is shown in bone marrow-derived macrophages from NPC mice, demonstrating its applicability. Our method presents a quantitative, selective, rapid, and robust solution for shotgun-based BMP analysis without the need for extensive chromatographic separation or derivatization. The integration of BMP lipid detection into the Lipidyzer platform, alongside the recently launched iSODA data visualization tool, empowers chemists and biologists to gain deeper insights into BMP lipid biology.

Impaired CAMK4 Activity Limits Atherosclerosis and Reprograms Myelopoiesis

BACKGROUND

Chronic inflammation is a major driver of atherosclerotic cardiovascular disease, and therapeutics that target inflammation reduce cardiac events beyond levels seen with strategies targeting cholesterol alone. RNA sequencing revealed increased expression of CaMK4 (calcium/calmodulin-dependent protein kinase IV) in advanced/unstable human carotid artery plaque. We validated this finding in mouse and human atherosclerotic lesions, demonstrating increased CaMK4 in plaque macrophages. Therefore, we hypothesized that CaMK4 would promote inflammation and impair resolution in atherosclerosis.

METHODS

We obtained mice in which exon 3 within the kinase domain of CaMK4 is deleted, leading to degradation and deletion of the gene (Camk4-/-). Control and Camk4-/- mice were injected with a gain-of-function AAV (adeno-associated virus) 8-PCSK9 (proprotein convertase subtilisin/kexin type 9) virus, rendering them hypercholesterolemic, and fed a high-fat/high-cholesterol diet for 12 weeks.

RESULTS

Hypercholesterolemic Camk4-/- mice developed smaller and more stable lesions compared with control mice. Surprisingly, Camk4-/- mice had a peripheral monocytosis with skewing of monocyte populations toward the nonclassical Ly6clow subset, suggesting a less inflammatory monocyte population. Silencing or inhibition of CaMK4 in human monocytes recapitulated this phenotype. In response to hypercholesterolemia, which promotes myelopoiesis, Camk4-/- mice had markedly more myeloid progenitors. Camk4-/- monocytes expressed higher levels of genes associated with myeloid differentiation and recruitment of ATF6 (activating transcription factor 6) to conserved binding sites. In addition, Camk4-/- monocytes expressed higher levels of Nr4a1, which promotes conversion of Ly6chigh to Ly6clow monocytes. Camk4-/- monocytes failed to efficiently traffic in vitro and in vivo. Bone marrow-derived macrophages generated from Camk4-/- marrow had a more proreparative phenotype than control macrophages, consistent with our in vivo observations in the plaque.

CONCLUSIONS

These findings suggest that CaMK4 is an important regulator of the myelopoietic response to hypercholesterolemia through ATF6-mediated transcriptional regulation and that loss of functional CaMK4 promotes a proreparative phenotype in myeloid cells. Therefore, targeting CaMK4 may offer a unique way to target the progression of atherosclerosis.

Macrophage polarization in cancer and beyond: from inflammatory signaling pathways to potential therapeutic strategies

Macrophages are innate immune cells distributed throughout the body that play vital roles in organ development, tissue homeostasis, and immune surveillance. Macrophages acquire a binary M1/M2 polarized phenotype through signaling cascades upon sensing different signaling molecules in the environment, thereby playing a core role in a series of immune tasks, rendering precise regulation essential. M1/M2 macrophage phenotypes regulate inflammatory responses, while controlled activation of inflammatory signaling pathways is involved in regulating macrophage polarization. Among the relevant signaling pathways, we focus on the six well-characterized NF-κB, MAPK, JAK-STAT, PI3K/AKT, inflammasome, and cGAS-STING inflammatory pathways, and elucidate their roles and crosstalk in macrophage polarization. Furthermore, the effects of many environmental signals that influence macrophage polarization are investigated by modulating these pathways in vivo and in vitro. We thus detail the physiological and pathophysiological status of these six inflammatory signaling pathways and involvement in regulating macrophage polarization in cancer and beyond, as well as describe potential therapeutic approaches targeting these signaling pathways. In this review, the latest research advances in inflammatory signaling pathways regulating macrophage polarization are reviewed, as targeting these inflammatory signaling pathways provides suitable strategies to intervene in macrophage polarization and various tumor and non-tumor diseases.

The role of macrophages in asthma-related fibrosis and remodelling

Airway remodelling significantly contributes to the progressive loss of lung function and heightened symptom severity in chronic asthma. Additionally, it often persists and demonstrates reduced responsiveness to the mainstay treatments. The excessive deposition of collagen and extracellular matrix proteins leads to subepithelial fibrosis and airway remodelling, resulting in increased stiffness and decreased elasticity in the airway. Studies have emphasized the crucial role of subepithelial fibrosis in the pathogenesis of asthma. Fibrotic processes eventually cause airway narrowing, reduced lung function, and exacerbation of asthma symptoms. Macrophages play a crucial role in this process by producing pro-fibrotic cytokines, growth factors, and enzymes such as matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs). Additionally, identification of novel genetic markers has provided evidence for a strong genetic component in fibrosis within macrophage regulated fibrosis. Although macrophages contribute to the progression of airway remodelling and subepithelial fibrosis, interventions targeting macrophage-driven fibrotic changes have not yet been developed. This review synthesizes research on the intricate pathways through which macrophages contribute to subepithelial fibrosis in chronic asthma and its' pathological features. Understanding the interplay between macrophages, fibrosis, and asthma pathogenesis is essential for developing effective therapeutic strategies to manage severe asthma and improve patient outcomes.

Endothelial dysfunction drives atherosclerotic plaque macrophage-dependent abdominal aortic aneurysm formation

Currently there is no effective pharmacotherapy to prevent the growth and rupture of abdominal aortic aneurysms. Using a mouse model that combines cigarette smoke exposure and hypercholesterolemia, we demonstrated that cigarette smoke exacerbated atherosclerosis, leading to elastin fragmentation, aneurysm formation, rupture and death. Arterial injury was driven by macrophages that accumulated within atherosclerotic plaques and exhibited tissue-degrading proteolytic activity in vivo (a process dependent on the endothelial cell-derived macrophage growth factor CSF-1). Single-nucleus RNA sequencing revealed that cigarette smoke-induced endothelial cell dysfunction promoted monocyte recruitment and inflammatory signaling and amplified vascular injury. Furthermore, single-cell transcriptomic analysis identified conserved macrophage responses across mouse and human abdominal aortic aneurysm, including TREM2+ macrophages, which were key mediators of arterial damage. These findings established atherosclerotic plaque macrophages as critical drivers of aneurysm pathology and provide key insights into the mechanisms underlying aneurysm progression and rupture.

Diffuse traumatic brain injury induced stimulator of interferons (STING) signaling in microglia drives cortical neuroinflammation, neuronal dysfunction, and impaired cognition

Neuropsychiatric complications including depression and cognitive impairment develop, persist, and worsen in the years after traumatic brain injury (TBI), negatively affecting life and lifespan. Inflammatory responses mediated by microglia are associated with the transition from acute to chronic neuroinflammation after TBI. Moreover, type I interferon (IFN-I) signaling is a key mediator of inflammation during this transition. Thus, the purpose of this study was to determine the degree to which a microglia-specific knockout of the stimulator of interferons (STING) influenced TBI-induced neuroinflammation, neuronal dysfunction, and cognitive impairment. Here, microglial inducible STING knockout (CX₃CR1Cre/ERT2 x STINGfl/fl) mice were created and validated (mSTING-/-). Diffuse brain injury (midline fluid percussion) in male and female mice increased STING expression in microglia, promoted microglial morphological restructuring, and induced robust cortical inflammation and pathology 7 days post injury (dpi). These TBI-associated responses were attenuated in mSTING-/- mice. Increased cortical astrogliosis and rod-shaped microglia induced by TBI were independent of mSTING-/-. 7 dpi, TBI induced 237 differentially expressed genes (DEG) in the cortex of functionally wildtype (STINGfl/fl) associated with STING, NF-κB, and Interferon Alpha signaling and 85% were attenuated by mSTING-/-. Components of neuronal injury including reduced NeuN expression, increased cortical lipofuscin, and increased neurofilament light chain in plasma were increased by TBI and dependent on mSTING. TBI-associated cognitive tasks (novel object recognition/location, NOR/NOL) at 7 dpi were dependent on mSTING. Notably, the TBI-induced cognitive deficits in NOR/NOL and increased cortical inflammation 7 dpi were unaffected in global interferon-α/β receptor 1 knockout (IFNAR1) mice. In the final study, the RNA profile of neurons after TBI in STINGfl/fl and mSTING-/- mice was assessed 7 dpi by single nucleus RNA-sequencing. There was a TBI-dependent suppression of cortical neuronal homeostasis with reductions in CREB signaling, synaptogenesis, and oxytocin signaling and increases in cilium assembly and PTEN signaling. Overall, mSTING-/- prevented 50% of TBI-induced DEGs in cortical neurons. Collectively, ablation of STING in microglia attenuates TBI-induced interferon responses, cortical inflammation, neuronal dysfunction, neuronal pathology, and cognitive impairment.

Monocytes can efficiently replace all brain macrophages and fetal liver monocytes can generate bona fide SALL1+ microglia

Microglia and border-associated macrophages (BAMs) are critical for brain health, and their dysfunction is associated to disease. Replacing brain macrophages holds substantial therapeutic promise but remains challenging. Here, we demonstrate that monocytes can efficiently replace all brain macrophages. Monocytes readily replaced embryonal BAMs upon their depletion and engrafted as monocyte-derived microglia (Mo-Microglia) upon more sustained niche availability. Mo-Microglia expanded comparably to their embryonic counterparts and showed similar longevity. However, monocytes were unable to replicate the distinct identity of embryonically derived BAMs and microglia. Using xenotransplantation, we found that human monocytes exhibited similar behavior, enabling identification of putative Mo-Microglia in Alzheimer's disease individuals. In mice and humans, monocyte ontogeny shaped their identity as brain macrophages. Importantly, mouse fetal liver monocytes exhibited a distinct epigenetic landscape and could develop a bona fide microglial identity. Our results illuminate brain macrophage development and highlight monocytes as an abundant progenitor source for brain macrophage replacement therapies.

Inflammasome signaling in astrocytes modulates hippocampal plasticity

Emerging evidence indicates that a baseline level of controlled innate immune signaling is required to support proper brain function. However, little is known about the function of most innate immune pathways in homeostatic neurobiology. Here, we report a role for astrocyte-dependent inflammasome signaling in regulating hippocampal plasticity. Inflammasomes are multiprotein complexes that promote caspase-1-mediated interleukin (IL)-1 and IL-18 production in response to pathogens and tissue damage. We observed that inflammasome complex formation was regularly detected under homeostasis in hippocampal astrocytes and that its assembly is dynamically regulated in response to learning and regional activity. Conditional ablation of caspase-1 in astrocytes limited hyperexcitability in an acute seizure model and impacted hippocampal plasticity via modulation of synaptic protein density, neuronal activity, and perineuronal net coverage. Caspase-1 and IL-18 regulated hippocampal IL-33 production and related plasticity. These findings reveal a homeostatic function for astrocyte inflammasome activity in regulating hippocampal physiology in health and disease.

Acute and Long-Term Consequences of Neonatal NMDA Blockade in the Cx3cr1 Knock-Out Mouse

Neuron-microglia communication through the fractalkine pathway is a critical factor mediating microglial proliferation, migration, release of mediators, and clearance of cellular debris, as well as the function of neuronal NMDA receptors. Disruption of the fractalkine-mediated microglia-neuron communication is associated with divergent outcomes, from damaging to protective, in different neurological conditions (including schizophrenia and epilepsy). In the present work we explore the impact of the absence of the fractalkine receptor (CX3CR1) after neonatal blockade of NMDA receptors, which induces acute and long-term alterations in behavior, neuronal integrity and excitability. Wild-type (WT) and Cx3cr1-/- (KO) mice of both sexes randomly received either a low (0.5 mg/kg) or high dose (1 mg/kg) of MK-801 (NMDA receptor antagonist) or saline, for five consecutive days, during early postnatal development. Neuronal apoptosis was assessed at a midpoint of the pharmacological protocol. Survival and growth rates were determined up to adulthood when innate behaviors, unconditioned anxiety, contextual memory and seizure susceptibility were evaluated, as well as hippocampal local field potential and sensory gating. CX3CR1 depletion and neonatal MK-801 treatment had a synergistic acute effect, increasing neuronal apoptosis and overall mortality. Both factors independently induced long-lasting impairments in the wide array of behavioral tasks assessed during adulthood. However, low MK-801 dose treatment greatly augmented the mortality of pentylenetetrazol-induced seizures in WT mice, an effect prevented by CX3CR1 depletion. MK-801 treatment induced a shift in the power spectrum of the hippocampal local field potential towards higher frequencies that was averted in Cx3cr1-/- mice by an opposite shift. Our results reveal that CX3CR1 depletion severely increases the vulnerability to neonatal NMDA antagonism with additional complex interactions regarding cognitive and neurophysiological effects, which should be considered in the context of neuron-microglia miscommunication in many neurological disorders including schizophrenia and epilepsy.

Interleukin-34-dependent perivascular macrophages promote vascular function in the brain

The development of most macrophages depends on the colony-stimulating factor 1 (CSF-1) receptor, which has two ligands: CSF-1 and interleukin-34 (IL-34). While IL-34 is required for the homeostasis of microglia, the parenchymal macrophages in the central nervous system (CNS), it is unclear whether brain border-associated macrophages (BAMs) also depend on this cytokine. Here, we demonstrated that the embryonic development of murine BAMs in the choroid plexus, leptomeninges, and perivascular spaces required CSF-1, while IL-34 was critical for their maintenance in adulthood. In the brain, Il34 was expressed by mural cells and perivascular fibroblasts, and its transgenic deletion in these cells interrupted BAM maintenance. Il34 deficiency coincided with transcriptional changes in vascular cells, leading to increased flow velocity and vasomotion in pial and penetrating arterioles. Similarly, Mrc1CreCsf1rfl/fl mice lacking CD206+ perivascular BAMs exhibited increased hemodynamics in arterial networks. These findings reveal a crosstalk between vascular cells and CNS macrophages regulating cerebrovascular function.

IL-10 sensing by lung interstitial macrophages prevents bacterial dysbiosis-driven pulmonary inflammation and maintains immune homeostasis

Crosstalk between the immune system and the microbiome is critical for maintaining immune homeostasis. Here, we examined this communication and the impact of immune-suppressive IL-10 signaling on pulmonary homeostasis. We found that IL-10 sensing by interstitial macrophages (IMs) is required to prevent spontaneous lung inflammation. Loss of IL-10 signaling in IMs initiated an inflammatory cascade through the activation of classical monocytes and CD4+ T cell subsets, leading to chronic lung inflammation with age. Analyses of antibiotic-treated and germ-free mice established that lung inflammation in the animals lacking IL-10 signaling was triggered by commensal bacteria. 16S rRNA sequencing revealed Delftia acidovorans and Rhodococcus erythropolis as potential drivers of lung inflammation. Intranasal administration of these bacteria or transplantation of human fecal microbiota elicited lung inflammation in gnotobiotic Il10-deficient mice. These findings highlight that IL-10 sensing by IMs contributes to pulmonary homeostasis by preventing lung inflammation caused by commensal dysbiosis.

BNIP3-mediated mitophagy in macrophages regulates obesity-induced adipose tissue metaflammation

Adipose tissue macrophages (ATMs) are key cellular components that respond to nutritional excess, contributing to obesity-induced inflammation and insulin resistance. However, the mechanisms underlying macrophage polarization and recruitment in adipose tissue during obesity remain unclear. In this study, we investigated mitophagy-dependent metabolic reprogramming in ATMs and identified a crucial role of the mitophagy receptor BNIP3 in regulating macrophage polarization in response to obesity. Mitophagic flux in ATMs increased following 12 weeks of high-fat diet (HFD) feeding, with Bnip3 levels upregulated in a HIF1A dependent manner, without affecting other mitophagy receptors. Macrophage-specific bnip3 knockout reduced HFD-induced adipose tissue inflammation and improved glucose tolerance and insulin sensitivity. Mechanistically, hypoxic conditions in vitro induced HIF1A-BNIP3-mediated mitophagy and glycolytic shift in macrophages. Furthermore, HIF1A-BNIP3 signaling-enhanced lipopolysaccharide-induced pro-inflammatory activation in macrophages. These findings demonstrate that BNIP3-mediated mitophagy regulates the glycolytic shift and pro-inflammatory polarization in macrophages and suggest that BNIP3 could be a therapeutical target for obesity-related metabolic diseases.Abbreviation: 2-DG: 2-deoxyglucose; ACADM/MCAD: acyl-CoA dehydrogenase medium chain; ADGRE1/F4/80: adhesion G protein-coupled receptor E1; ATMs: adipose tissue macrophages; BNIP3: BCL2 interacting protein 3; BNIP3L/NIX: BCL2 interacting protein 3 like; CLS: crown-like structure; CoCl2: cobalt(II) chloride; COX4/COXIV: cytochrome c oxidase subunit 4; ECAR: extracellular acidification rate; ECM: extraceullular matrix; gWAT: gonadal white adipose tissue; HFD: high-fat diet; HIF1A/HIF-1 α: hypoxia inducible factor 1 subunit alpha; IL1B/IL-1β: interleukin 1 beta; ITGAM/CD11B: integrin subunit alpha M; KO: knockout; LAMs: lipid-associated macrophages; LPS: lipopolysaccharide; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MRC1/CD206: mannose receptor C-type 1; mtDNA: mitochondrial DNA; NCD: normal chow diet; OCR: oxygen consumption rate; OXPHOS: oxidative phosphorylation; PINK1: PTEN induced kinase 1; PRKN/Parkin: parkin RBR E3 ubiquitin protein ligase; PTPRC/CD45: protein tyrosine phosphatase receptor type C; SVFs: stromal vascular fractions; TEM: transmission electron microscopy; TMRM: tetramethylrhodamine methyl ester; TOMM20: Translocase of outer mitochondrial membrane 20; TREM2: triggering receptor expressed on myeloid cells 2; WT: wild-type.

Altered maturation and activation state of circulating monocytes is associated with their enhanced recruitment in pulmonary arterial hypertension

BACKGROUND

It is well-established that patients with pulmonary arterial hypertension (PAH) exhibit increased recruitment of circulating monocytes to their pulmonary arteries. However, it remains unclear whether these monocytes have intrinsic abnormalities that contribute to their recruitment and to PAH pathogenesis. This study aimed to characterize the gene expression profiles of circulating classical, intermediate, and non-classical monocytes and assess their maturation trajectory in patients with idiopathic (I) PAH compared to control subjects. Additionally, it sought to explore the relationship between the observed IPAH abnormalities and deficiencies in bone morphogenetic receptor 2 (BMPR2), the most frequently mutated gene in PAH, and to assess adhesion and transendothelial migration, key processes in monocyte infiltration of pulmonary arteries.

METHODS

Differentially expressed genes and maturation trajectories of circulating monocytes from patients with IPAH vs. control subjects were compared using single cell RNA sequencing (scRNAseq), followed by FACS analysis. Observations from IPAH and control cells were related to reduced BMPR2 using a THP1 monocyte cell line with BMPR2 reduced by siRNA as well as induced pluripotent stem cell (iPSC) derived monocytes (iMono) from hereditary (H) PAH patients with a BMPR2 mutation and monocytes from mice with Bmpr2 deleted (MON-Bmpr2-/-).

RESULTS

Classical IPAH monocytes have decreased CD14 mRNA leading to a deviation in their maturation trajectory and early terminal fate, which is not rescued by cytokine treatment. Monocytes that evade early cell death show elevated STAT1, PPDPF and HLA-B, and an interferon (IFN) signature indicative of an altered activation state. A strong link between decreased BMPR2 and CD14 was observed in THP1 cells and in HPAH iMono with a BMPR2 mutation associated with STAT1 and IFN related genes, and in monocytes from MON-Bmpr2-/- mice. Increased adhesion to iPSC-derived endothelial cells (iECs) in HPAH-BMPR2 mutant iMono was associated with elevated ICAM1 expression. Enhanced transendothelial migration of these cells was associated with the reduction in endothelial VE-cadherin (CDH5).

CONCLUSIONS

IPAH monocytes exhibit an altered activation state associated with reduced BMPR2 and CD14, along with elevated STAT1-IFN expression. These changes are linked to intrinsic functional abnormalities that contribute to the monocytes' increased propensity to invade the pulmonary circulation.

Immune checkpoint TIM-3 regulates microglia and Alzheimer's disease

Microglia are the resident immune cells in the brain and have pivotal roles in neurodevelopment and neuroinflammation1,2. This study investigates the function of the immune-checkpoint molecule TIM-3 (encoded by HAVCR2) in microglia. TIM-3 was recently identified as a genetic risk factor for late-onset Alzheimer's disease3, and it can induce T cell exhaustion4. However, its specific function in brain microglia remains unclear. We demonstrate in mouse models that TGFβ signalling induces TIM-3 expression in microglia. In turn, TIM-3 interacts with SMAD2 and TGFBR2 through its carboxy-terminal tail, which enhances TGFβ signalling by promoting TGFBR-mediated SMAD2 phosphorylation, and this process maintains microglial homeostasis. Genetic deletion of Havcr2 in microglia leads to increased phagocytic activity and a gene-expression profile consistent with the neurodegenerative microglial phenotype (MGnD), also referred to as disease-associated microglia (DAM). Furthermore, microglia-targeted deletion of Havcr2 ameliorates cognitive impairment and reduces amyloid-β pathology in 5×FAD mice (a transgenic model of Alzheimer's disease). Single-nucleus RNA sequencing revealed a subpopulation of MGnD microglia in Havcr2-deficient 5×FAD mice characterized by increased pro-phagocytic and anti-inflammatory gene expression alongside reduced pro-inflammatory gene expression. These transcriptomic changes were corroborated by single-cell RNA sequencing data across most microglial clusters in Havcr2-deficient 5×FAD mice. Our findings reveal that TIM-3 mediates microglia homeostasis through TGFβ signalling and highlight the therapeutic potential of targeting microglial TIM-3 in Alzheimer's disease.

Loss of ATG7 in microglia impairs UPR, triggers ferroptosis, and weakens amyloid pathology control

Microglia impact brain development, homeostasis, and pathology. One important microglial function in Alzheimer's disease (AD) is to contain proteotoxic amyloid-β (Aβ) plaques. Recent studies reported the involvement of autophagy-related (ATG) proteins in this process. Here, we found that microglia-specific deletion of Atg7 in an AD mouse model impaired microglia coverage of Aβ plaques, increasing plaque diffusion and neurotoxicity. Single-cell RNA sequencing, biochemical, and immunofluorescence analyses revealed that Atg7 deficiency reduces unfolded protein response (UPR) while increasing oxidative stress. Cellular assays demonstrated that these changes lead to lipoperoxidation and ferroptosis of microglia. In aged mice without Aβ buildup, UPR reduction and increased oxidative damage induced by Atg7 deletion did not impact microglia numbers. We conclude that reduced UPR and increased oxidative stress in Atg7-deficient microglia lead to ferroptosis when exposed to proteotoxic stress from Aβ plaques. However, these microglia can still manage misfolded protein accumulation and oxidative stress as they age.

Fast, accurate, and versatile data analysis platform for the quantification of molecular spatiotemporal signals

Optical recording of intricate molecular dynamics is becoming an indispensable technique for biological studies, accelerated by the development of new or improved biosensors and microscopy technology. This creates major computational challenges to extract and quantify biologically meaningful spatiotemporal patterns embedded within complex and rich data sources, many of which cannot be captured with existing methods. Here, we introduce activity quantification and analysis (AQuA2), a fast, accurate, and versatile data analysis platform built upon advanced machine-learning techniques. It decomposes complex live-imaging-based datasets into elementary signaling events, allowing accurate and unbiased quantification of molecular activities and identification of consensus functional units. We demonstrate applications across a wide range of biosensors, cell types, organs, animal models, microscopy techniques, and imaging approaches. As exemplar findings, we show how AQuA2 identified drug-dependent interactions between neurons and astroglia, as well as distinct sensorimotor signal propagation patterns in the mouse spinal cord.

MK5 Regulates Microglial Activation and Neuroinflammation in Experimental Stroke Models

OBJECTIVE

Microglial activation plays a crucial role in neuroinflammation following ischemic stroke. This study was conducted to investigate the role and potential mechanisms of MK5 within microglial cells in the inflammatory response following ischemic stroke in mice in vivo and in vitro.

METHODS

Microglia-specific conditional MK5 knockout (MK5 cKO) mice and their control mice (MK5f/f) were subjected to middle cerebral artery occlusion (MCAO). BV2 cells (a mouse microglial cell line) were transfected with small interfering RNA (siRNA) to knock down MK5 levels and subsequently exposed to oxygen-glucose deprivation/reperfusion (OGD/R) to simulate ischemic conditions in vitro. Following MCAO, behavioral tests and infarct volume measurements were conducted. Levels of cytokines and microglial markers were evaluated using qPCR and Western blotting, while immunofluorescence was employed to observe microglial activation. Additionally, Western blotting was performed to assess the phosphorylation of HSP27 and NF-κB.

RESULTS

Compared to the control group, the knockout of the MK5 gene in microglia significantly exacerbated neurological deficits and increased infarct volume in MCAO mice. The loss of the MK5 promoted inflammation by upregulating pro-inflammatory factors and downregulating anti-inflammatory factors, while also enhancing microglial activation in both MCAO mice and BV2 microglial cells subjected to OGD/R. Furthermore, the knockout of the MK5 gene in microglia reduced the phosphorylation levels of HSP27 and increased the phosphorylation levels of NF-κB in the aforementioned models.

CONCLUSION

Microglial MK5 plays a critical role in the ischemic neuroinflammatory response by regulating the phosphorylation of HSP27 and NF-κB, positioning it as a potential target for stroke treatment.

Comparison of human macrophages derived from peripheral blood and bone marrow

Macrophage differentiation, phenotype, and function have been assessed extensively in vitro by predominantly deriving human macrophages from peripheral blood. It is accepted that there are differences between macrophages isolated from different human tissues; however, the importance of anatomical source for in vitro differentiation and characterization is less clear. Here, phenotype and function were evaluated between human macrophages derived from bone marrow or peripheral blood. Macrophages were differentiated by adherence of heterogenous cell populations or CD14 isolation and polarized with IFNγ and LPS or IL-4 and IL-13 for 48 hours before evaluation of phenotype and phagocytic capacity. The presence of stromal cells in bone marrow heterogenous cultures resulted in a reduction in macrophage purity compared to peripheral blood, which was negated after CD14 isolation. Phenotypically, monocyte-derived macrophages (MDMs) derived from peripheral blood and bone marrow resulted in similar expression of classical and polarized macrophages markers, including CD14, HLA-DR, CD38, and CD40 (increased after IFNγ/LPS), and CD11b and CD206 (elevated after IL-4/IL-13). Functionally, these cells also showed similar levels of Fc-independent and Fc-dependent phagocytosis, although there was a nonsignificant reduction of Fc-dependent phagocytosis in the bone marrow derived macrophages after IFNγ/LPS stimulation. In summary, we have identified that human MDMs differentiated from peripheral blood and bone marrow showed similar characteristics and functionality, suggesting that isolating cells from different anatomical niches does not affect macrophage differentiation after CD14 isolation. Consequently, due to high yield and ready availability peripheral blood derived macrophages are still the most suitable source.

Redefining Macrophage Heterogeneity in Atherosclerosis: A Focus on Possible Therapeutic Implications

Atherosclerosis is a lipid disorder where modified lipids (especially oxidized LDL) induce macrophage foam cell formation in the aorta. Its pathogenesis involves a continuum of persistent inflammation accompanied by dysregulated anti-inflammatory responses. Changes in the immune cell status due to differences in the lesional microenvironment are crucial in terms of plaque development, its progression, and plaque rupture. Ly6Chi monocytes generated through both medullary and extramedullary cascades act as one of the major sources of plaque macrophages and thereby foam cells. Both monocytes and monocyte-derived macrophages also participate in pathological events in atherosclerosis-associated multiple organ systems through inter-organ communications. For years, macrophage phenotypes M1 and M2 have been shown to perpetuate inflammatory and resolution responses; nevertheless, such a dualistic classification is too simplistic and contains severe drawbacks. As the lesion microenvironment is enriched with multiple mediators that possess the ability to activate macrophages to diverse phenotypes, it is obvious that such cells should demonstrate substantial heterogeneity. Considerable research in this regard has indicated the presence of additional macrophage phenotypes that are exclusive to atherosclerotic plaques, namely Mox, M4, Mhem, and M(Hb) type. Furthermore, although the concept of macrophage clusters has come to the fore in recent years with the evolution of high-dimensional techniques, classifications based on such 'OMICS' approaches require extensive functional validation as well as metabolic phenotyping. Bearing this in mind, the current review provides an overview of the status of different macrophage populations and their role during atherosclerosis and also outlines possible therapeutic implications.

The liver as a central "hub" of the immune system: pathophysiological implications

The purpose of this review is to describe the immune function of the liver, guiding the reader from the homeostatic tolerogenic status to the aberrant activation demonstrated in chronic liver disease. An extensive description of the pathways behind the inflammatory modulation of the healthy liver will be provided focusing on the complex immune cell network residing within the liver. The limit of tolerance will be presented in the context of organ transplantation, seizing the limits of homeostatic mechanisms that fail in accepting the graft, progressing eventually toward rejection. The triggers and mechanisms behind chronic activation in metabolic liver conditions and viral hepatitis will be discussed. The last part of the review will be dedicated to one of the greatest paradoxes for a tolerogenic organ, developing autoimmunity. Through the description of the three most common autoimmune liver diseases, the autoimmune reaction against hepatocytes and biliary epithelial cells will be dissected.

Central Nervous System Macrophages in Health and Disease

The central nervous system (CNS) has a unique set of macrophages that seed the tissue early during embryonic development. Microglia reside in the parenchyma, and border-associated macrophages are present in border regions, including the meninges, perivascular spaces, and choroid plexus. CNS-resident macrophages support brain homeostasis during development and steady state. In the diseased brain, however, the immune landscape is altered, with phenotypic and transcriptional changes in resident macrophages and the invasion of blood-borne monocytes, which differentiate into monocyte-derived macrophages upon entering the CNS. In this review, we focus on the fate and function of the macrophage compartment in health, neurodegenerative conditions such as amyloidosis, and neuroinflammation as observed in multiple sclerosis and infection. We discuss our current understanding that monocyte-derived macrophages contribute to neuropathology whereas native macrophages play a neuroprotective role in disease.

Immune-Infiltrated Cancer Spheroid Model with Vascular Recirculation Reveals Temporally Dependent and Tissue-Specific Macrophage Recruitment

Immune cell infiltration in tumors has been reported to influence tumor progression and clinical outcomes. Considerable efforts have been made to understand interactions between tumors and the immune system. However, current models are either not comprehensive or limited to short-term studies. Recognizing thedynamic and long-term nature of tumor-immune interactions, an immune-infiltrated cancer spheroid model is developed by continuously perfusing and recirculating immune cells with gravity-driven flow through a tubular blood vessel adjacent to a cancer spheroid. Fibroblasts and pericytes are embedded in the gel matrix to support endothelial cells and enhance the vascular barrier. With continuous monocyte recirculation, monocyte adhesion, transendothelium migration, differentiation, and macrophage recruitment into breast carcinoma and hepatoma spheroids is successfully demonstrated over a week. The macrophage recruitment process is temporally dependent and tissue-specific, leading to the formation of cancer-macrophage heterospheroids. Elevated secretion of granulocyte-macrophage colony-stimulating factor (GM-CSF), which regulates monocyte recruitment and macrophage activation, is observed in the breast carcinoma model. Increased levels of Interleukin 6 (IL-6) and Interleukin 8 (IL-8) are detected, indicating a pro-inflammatory environment associated with tumor progression and metastasis. This platform provides a valuable framework for investigating immune cell infiltration and differentiation within the tumor microenvironment, supporting the advancement of cancer immunotherapies.

Macrophage ATF6 accelerates corticotomy-assisted orthodontic tooth movement through promoting Tnfα transcription

Corticotomy is a clinical procedure to accelerate orthodontic tooth movement characterized by the regional acceleratory phenomenon (RAP). Despite its therapeutic effects, the surgical risk and unclear mechanism hamper the clinical application. Numerous evidences support macrophages as the key immune cells during bone remodeling. Our study discovered that the monocyte-derived macrophages primarily exhibited a pro-inflammatory phenotype that dominated bone remodeling in corticotomy by CX3CR1CreERT2; R26GFP lineage tracing system. Fluorescence staining, flow cytometry analysis, and western blot determined the significantly enhanced expression of binding immunoglobulin protein (BiP) and emphasized the activation of sensor activating transcription factor 6 (ATF6) in macrophages. Then, we verified that macrophage specific ATF6 deletion (ATF6f/f; CX3CR1CreERT2 mice) decreased the proportion of pro-inflammatory macrophages and therefore blocked the acceleration effect of corticotomy. In contrast, macrophage ATF6 overexpression exaggerated the acceleration of orthodontic tooth movement. In vitro experiments also proved that higher proportion of pro-inflammatory macrophages was positively correlated with higher expression of ATF6. At the mechanism level, RNA-seq and CUT&Tag analysis demonstrated that ATF6 modulated the macrophage-orchestrated inflammation through interacting with Tnfα promotor and augmenting its transcription. Additionally, molecular docking simulation and dual-luciferase reporter system indicated the possible binding sites outside of the traditional endoplasmic reticulum-stress response element (ERSE). Taken together, ATF6 may aggravate orthodontic bone remodeling by promoting Tnfα transcription in macrophages, suggesting that ATF6 may represent a promising therapeutic target for non-invasive accelerated orthodontics.

Macrophage Differentiation and Metabolic Adaptation in Mycobacterial Infections

The adaptation of macrophages-the most common tissue-resident immune cells-to metabolic and microbial cues with high local variability is essential for the maintenance of organ integrity. In homeostasis, macrophages show largely predictable tissue-specific differentiation, as recently revealed by multidimensional methods. However, chronic infections with human-adapted pathogens substantially contribute to the differentiation complexity of tissue macrophages, which has been only partially resolved. Specifically, the response to mycobacterial species-which range from Mycobacterium tuberculosis (with highest specificity for humans, broad organ tropism, yet tissue-specific disease phenotypes) to environmental mycobacteria with humans as accidental hosts-may serve as a paradigm of tissue macrophage adaptation mechanisms. While mycobacterial species-specific tissue preferences are partially related to the mode of acquisition and pathogen characteristics, evolutionary convergence with macrophages driven by metabolic features of the target organ likely contributes to infection resistance and immunopathology. In this review, we unravel the mechanisms of tissue-specific macrophage differentiation and its limitations in mycobacterial infections.

Cellular and Molecular Mechanisms of Innate Memory Responses

There has been an increasing effort to understand the memory responses of a complex interplay among innate, adaptive, and structural cells in peripheral organs and bone marrow. Trained immunity is coined as the de facto memory of innate immune cells and their progenitors. These cells acquire epigenetic modifications and shift their metabolism to equip an imprinted signature to a persistent fast-responsive functional state. Recent studies highlight the contribution of noncoding RNAs and modulation of chromatin structures in establishing this epigenetic readiness for potential immune perturbations. In this review, we discuss recent studies that highlight trained immunity-mediated memory responses emerging intrinsically in innate immune cells and as a complex interplay with other cells at the organ level. Lastly, we survey epigenetic contributors to trained immunity phenotypes-specifically, a recently discovered regulatory circuit coordinating the regulation of a key driver of trained immunity.

Enhancing immunity during ageing by targeting interactions within the tissue environment

Immunity declines with age. This results in a higher risk of age-related diseases, diminished ability to respond to new infections and reduced response to vaccines. The causes of this immune dysfunction are cellular senescence, which occurs in both lymphoid and non-lymphoid tissue, and chronic, low-grade inflammation known as 'inflammageing'. In this Review article, we highlight how the processes of inflammation and senescence drive each other, leading to loss of immune function. To break this cycle, therapies are needed that target the interactions between the altered tissue environment and the immune system instead of targeting each component alone. We discuss the relative merits and drawbacks of therapies that are directed at eliminating senescent cells (senolytics) and those that inhibit inflammation (senomorphics) in the context of tissue niches. Furthermore, we discuss therapeutic strategies designed to directly boost immune cell function and improve immune surveillance in tissues.

An aging bone marrow exacerbates lung fibrosis by fueling profibrotic macrophage persistence

Pulmonary fibrosis is an incurable disease that manifests with advanced age. Yet, how hematopoietic aging influences immune responses and fibrosis progression remains unclear. Using heterochronic bone marrow transplant mouse models, we found that an aged bone marrow exacerbates lung fibrosis irrespective of lung tissue age. Upon lung injury, there was an increased accumulation of monocyte-derived alveolar macrophages (Mo-AMs) driven by cell-intrinsic hematopoietic aging. These Mo-AMs exhibited an enhanced profibrotic profile and stalled maturation into a homeostatic, tissue-resident phenotype. This delay was shaped by cell-extrinsic environmental signals such as reduced pulmonary interleukin-10 (IL-10), perpetuating a profibrotic macrophage state. We identified regulatory T cells (Tregs) as critical providers of IL-10 upon lung injury that promote Mo-AM maturation and attenuate fibrosis progression. Our study highlights the impact of an aging bone marrow on lung immune regulation and identifies Treg-mediated IL-10 signaling as a promising target to mitigate fibrosis and promote tissue repair.

Myeloid cell path to malignancy: insights into liver cancer

Clinically approved treatments for advanced liver cancer often lack potency because of the heterogeneous characteristics of hepatocellular carcinoma (HCC). This complexity is largely driven by context-dependent inflammatory responses brought on by diverse etiologies, such as metabolic dysfunction-associated steatohepatitis (MASH), the genetic makeup of cancer cells, and the versatile adaptability of immune cells, such as myeloid cells. In this review, we discuss the evolutionary dynamics of the immune landscape, particularly that of liver-resident Kupffer cells (KCs), TREM2+, and SPP1+ macrophages with an active role during liver disease progression, which eventually fuels hepatocarcinogenesis. We highlight exploitable immunomodulatory avenues amenable to mitigate both the inherent pathological characteristics of liver cancers and the associated external factors that favor malignancy, paving a roadmap toward improving the management and therapeutic outcome for patients with HCC.

Radial glia integrin avb8 regulates cell autonomous microglial TGFβ1 signaling that is necessary for microglial identity

Microglial diversity arises from the interplay between inherent genetic programs and external environmental signals. However, the mechanisms by which these processes develop and interact within the growing brain are not yet fully understood. Here, we show that radial glia-expressed integrin beta 8 (ITGB8) activates microglia-expressed TGFβ1 to drive microglial development. Domain-restricted deletion of Itgb8 in these progenitors results in regionally restricted and developmentally arrested microglia that persist into adulthood. In the absence of autocrine TGFβ1 signaling, microglia adopt a similar phenotype, leading to neuromotor symptoms almost identical to Itgb8 mutant mice. In contrast, microglia lacking the canonical TGFβ signal transducers Smad2 and Smad3 have a less polarized dysmature phenotype and correspondingly less severe neuromotor dysfunction. Our study describes the spatio-temporal regulation of TGFβ activation and signaling in the brain necessary to promote microglial development, and provides evidence for the adoption of microglial developmental signaling pathways in brain injury or disease.

Selective deletion of interleukin-1 alpha in microglia does not modify acute outcome but may regulate neurorepair processes after experimental ischemic stroke

Inflammation is a key contributor to stroke pathogenesis and exacerbates brain damage leading to poor outcome. Interleukin-1 (IL-1) is an important regulator of post-stroke inflammation, and blocking its actions is beneficial in pre-clinical stroke models and safe in the clinical setting. However, the distinct roles of the two major IL-1 receptor type 1 agonists, IL-1α and IL-1β, and the specific role of IL-1α in ischemic stroke remain largely unknown. Here we show that IL-1α and IL-1β have different spatio-temporal expression profiles in the brain after experimental stroke, with early microglial IL-1α expression (4 h) and delayed IL-1β expression in infiltrated neutrophils and a small microglial subset (24-72 h). We examined for the first time the specific role of microglial-derived IL-1α in experimental permanent and transient ischemic stroke through microglial-specific tamoxifen-inducible Cre-loxP-mediated recombination. Microglial IL-1α deletion did not influence acute outcome after ischemic stroke. However, microglial IL-1α knock out (KO) mice showed reduced peri-infarct vessel density and reactive astrogliosis at 14 days post-stroke, alongside long-term impaired functional recovery. Our study identifies for the first time a critical role for microglial IL-1α on post-stroke neurorepair and recovery, highlighting the importance of targeting specific IL-1 mechanisms in brain injury to develop effective therapies.

Differential expression of S100A10 protein in leukocytes and its effects on monocyte emigration from bone marrow

Although the importance of the unique member of S100 EF-hand family, S100A10 in health and disease is well appreciated, a precise characterization of S100A10 expression still remains elusive. To this purpose, we generated a knock-in mouse line in which downstream of the coding sequence of the S100a10 gene was inserted IRES-mCherry-pA sequence. Interestingly, mCherry fluorescence was widely distributed in splenic myeloid and lymphoid cells, whereas neutrophils showed a negligible mCherry level. By taking advantage of these reporter mice, we found Ly6C+ monocytes expressed the highest levels of S100A10 and bound significantly more plasminogen compared with the other respective leukocyte subsets. Furthermore, we demonstrated that S100A10 was required for emigration of Ly6C+ monocytes from bone marrow by mainly affecting CCR2 cell surface presentation. S100a10-/- mice had fewer circulating Ly6C+ monocytes and, after challenged with thioglycolate, accumulated less CCR2+ monocytes in bone marrow. However, S100A10 was not necessary for efficient neutrophil recruitment from the blood to inflamed tissue. These findings provide evidence that S100A10 is critical for monocyte mobilization and suggest its differential regulatory roles for monocyte and neutrophil chemoattractants in leukocyte homeostasis. Thus, targeting the S100A10-CCR2 pathway may be an attractive approach to regulate inflammatory responses and infectious diseases.

Tracing LYVE1+ peritoneal fluid macrophages unveils two paths to resident macrophage repopulation with differing reliance on monocytes

Mouse resident peritoneal macrophages, called large cavity macrophages (LCM), arise from embryonic progenitors that proliferate as mature, CD73+Gata6+ tissue-specialized macrophages. After injury from irradiation or inflammation, monocytes are thought to replenish CD73+Gata6+ LCMs through a CD73-LYVE1+ LCM intermediate. Here, we show that CD73-LYVE1+ LCMs indeed yield Gata6+CD73+ LCMs through integrin-mediated interactions with mesothelial surfaces. CD73-LYVE1+ LCM repopulation of the peritoneum was reliant upon and quantitatively proportional to recruited monocytes. Unexpectedly, fate mapping indicated that only ~10% of Gata6-dependent LCMs that repopulated the peritoneum after injury depended on the LYVE1+ LCM stage. Further supporting nonoverlapping lifecycles of CD73-LYVE1+ and CD73+Gata6+ LCMs, in mice bearing a paucity of monocytes, Gata6+CD73+ LCMs rebounded after ablative irradiation substantially more efficiently than their presumed LYVE1+ or CD73- LCM upstream precursors. Thus, after inflammatory insult, two temporally parallel pathways, each generating distinct differentiation intermediates with varying dependencies on monocytes, contribute to the replenish hment of Gata6+ resident peritoneal macrophages.

Healthy and premature aging of monocytes and macrophages

Aging is associated with immunosenescence, a decline in immune functions, but also with inflammaging, a chronic, low-grade inflammation, contributing to immunosenescence. Monocytes and macrophages belong to the innate immune system and aging has a profound impact on these cells, leading to functional changes and most importantly, to the secretion of pro-inflammatory cytokines and thereby contributing to inflammaging. Rheumatoid arthritis (RA) is an autoimmune disease and age is an important risk factor for developing RA. RA is associated with the early development of age-related co-morbidities like cardiovascular manifestations and osteoporosis. The immune system of RA patients shows signs of premature aging like age-inappropriate increased production of myeloid cells, accelerated telomeric erosion, and the uncontrolled production of pro-inflammatory cytokines. In this review we discuss the influence of aging on monocytes and macrophages during healthy aging and premature aging in rheumatoid arthritis.

Are monocytes a preferable option to develop myeloid cell-based therapies for solid tumors?

In the last two decades, novel and promising cell-based therapies have populated the treatment landscape for haematological tumors. However, commonly exploited T and NK cell-based therapies show limited applicability to solid tumors. This is mainly given by the impaired tumor trafficking capability and limited effector activity of these cells within a highly immunosuppressive tumor microenvironment. Myeloid cells spontaneously home to tumors and can thus be reprogrammed and/or engineered to directly attack tumor cells or locally and selectively deliver therapeutically relevant payloads that may improve the efficacy of immunotherapy against difficult-to-access solid tumors. In the context of myeloid cell-based therapies, adoptive transfer of monocytes has often been overshadowed by infusion of differentiated macrophages or hematopoietic stem cell transplantation despite their promising therapeutic potential. Here, we summarize the recent improvements and benefits of using monocytes for the treatment of solid tumors, their current clinical applications and the challenges of their use as well as some possible strategies to overcome them.

The Role of Choroid Plexus in Hydrocephalus from the Perspective of Structure and Function: a Therapeutic Target

Hydrocephalus is one of the most common neurological diseases, characterized by abnormal excessive accumulation of cerebrospinal fluid (CSF) in the ventricular system. Its pathophysiological mechanism is believed to be related to the imbalance of CSF circulation and homeostasis. As the main source of CSF secretion, the choroid plexus is closely related to hydrocephalus. The choroid plexus is a specialized vascularized tissue located within the cerebral ventricles. It has multiple physiological functions including regulating CSF, immune response, endocrine metabolism, etc. Strategies that reduce choroid plexus CSF secretion have been shown to be effective in the treatment of hydrocephalus. However, the role of other physiological functions of the choroid plexus in hydrocephalus is still unclear. Recent studies on the choroid plexus and the blood-CSF barrier have deepened our understanding of the structure and function of the choroid plexus. The idea of targeting the choroid plexus to treat hydrocephalus has spawned many branches: choroid plexus epithelial cells, choroid plexus immune cells, choroid plexus peptides, and choroid plexus cilia, etc. This review introduces the basic structure and function of the choroid plexus, summarizes their changes in hydrocephalus, and analyzes the possibility of the choroid plexus as a therapeutic target for hydrocephalus.

A humanized antibody against mucormycosis targets angioinvasion and augments the host immune response

Mucormycosis is a fungal infection caused by Mucorales fungi that cause severe disease and fatality, especially in immunocompromised individuals. Although vaccines and immunotherapeutics have been successful in combating viral and bacterial infections, approved antifungal immunotherapies are yet to be realized. To address this gap, monoclonal antibodies targeting invasive fungal infections have emerged as a promising approach, particularly for immunocompromised patients who are unlikely to maximally benefit from vaccines. The Mucorales spore coat (CotH) proteins have been identified as crucial fungal invasins that bind to glucose-regulated protein 78 (GRP78) and integrins of host barrier cells. Previously, we described a murine monoclonal antibody, anti-CotH C2, which protected diabetic ketoacidosis (DKA) and neutropenic mice from mucormycosis. Here, we advanced the development of the C2 immunoglobulin G1 (IgG1) by humanizing it, establishing a stable Chinese hamster ovary cell line producing the antibody at commercial yields, and carried out optimization of the upstream and downstream manufacturing processes. The resultant humanized IgG1 (VX-01) exhibited a 10-fold increase in binding affinity to CotH proteins and conferred comparable in vitro and in vivo efficacy when compared to C2 antibody. The mechanism of protection was reliant on prevention of angioinvasion and enhancing opsonophagocytic killing. VX-01 demonstrated acceptable safety profiles with no detectable damage to host cells in vitro and weak or moderate binding to only cytoplasmic proteins in ex vivo good laboratory practice-human tissue cross-reactivity studies. Our studies warrant continued development of VX-01 as a promising adjunctive immunotherapy.

Tissue macrophages: origin, heterogenity, biological functions, diseases and therapeutic targets

Macrophages are immune cells belonging to the mononuclear phagocyte system. They play crucial roles in immune defense, surveillance, and homeostasis. This review systematically discusses the types of hematopoietic progenitors that give rise to macrophages, including primitive hematopoietic progenitors, erythro-myeloid progenitors, and hematopoietic stem cells. These progenitors have distinct genetic backgrounds and developmental processes. Accordingly, macrophages exhibit complex and diverse functions in the body, including phagocytosis and clearance of cellular debris, antigen presentation, and immune response, regulation of inflammation and cytokine production, tissue remodeling and repair, and multi-level regulatory signaling pathways/crosstalk involved in homeostasis and physiology. Besides, tumor-associated macrophages are a key component of the TME, exhibiting both anti-tumor and pro-tumor properties. Furthermore, the functional status of macrophages is closely linked to the development of various diseases, including cancer, autoimmune disorders, cardiovascular disease, neurodegenerative diseases, metabolic conditions, and trauma. Targeting macrophages has emerged as a promising therapeutic strategy in these contexts. Clinical trials of macrophage-based targeted drugs, macrophage-based immunotherapies, and nanoparticle-based therapy were comprehensively summarized. Potential challenges and future directions in targeting macrophages have also been discussed. Overall, our review highlights the significance of this versatile immune cell in human health and disease, which is expected to inform future research and clinical practice.

The role of myeloid cell heterogeneity during spontaneous choroidal neovascularization in Vldlr knockout mice

BACKGROUND

Myeloid cells are heterogeneous cells that are critical for spontaneous choroidal neovascularization (CNV) in the Vldlr-/- mouse model. However, the specific myeloid cell subtype necessary for CNV remains unknown.

METHODS AND RESULTS

To investigate the role of monocytes, we bred Ccr2-/- and Nr4a1-/- mice into the Vldlr-/- background. We found that Ccr2 and Nr4a1 deficiency had no effect upon macrophage counts, CNV lesion number, or total CNV area. Next, we investigated the role of microglia by generating Vldlr-/-Tmem119CreER/+Rosa26DTR/+ mice. Diphtheria toxin (DT) treatment reduced macrophage counts at CNV lesions and CNV lesion number, but did not affect total CNV lesion area. To target microglia via a second strategy, we generated Vldlr-/-Cx3cr1CreERCsf1riDTR mice and treated them with a single low dose of tamoxifen to target microglia without affecting choroidal macrophages. DT treatment in Vldlr-/-Cx3cr1CreERCsf1riDTR mice decreased macrophage counts at CNV lesions and CNV lesion number but again had no effect upon total CNV lesion area. To target choroidal macrophages and microglia, we treated Vldlr-/-Cx3cr1CreERCsf1riDTR mice with 9 tamoxifen treatments. DT-treated mice showed dramatic reductions in macrophage counts, CNV number, and total lesion area.

CONCLUSIONS

These data suggest that monocytes and monocyte-derived macrophages are dispensable, microglia are likely initiators for CNV development, and choroidal macrophages are potential key contributors to CNV growth and/or maintenance in the Vldlr-/- model.

The contribution of the monocyte-macrophage lineage to immunotherapy outcomes

ABSTRACT

Macrophages execute core functions in maintaining tissue homeostasis, in which their extensive plasticity permits a spectrum of functions from tissue remodeling to immune defense. However, perturbations to tissue-resident macrophages during disease, and the subsequent emergence of monocyte-derived macrophages, can hinder tissue recovery and promote further damage through inflammatory and fibrotic programs. Gaining a fundamental understanding of the critical pathways defining pathogenic macrophage populations enables the development of targeted therapeutic approaches to improve disease outcomes. In the setting of chronic graft-versus-host disease (cGVHD), which remains the major complication of allogeneic hematopoietic stem cell transplantation, colony-stimulating factor 1 (CSF1)-dependent donor-derived macrophages have been identified as key pathogenic mediators of fibrotic skin and lung disease. Antibody blockade of the CSF1 receptor (CSF1R) to induce macrophage depletion showed remarkable capacity to prevent fibrosis in preclinical models and has subsequently demonstrated impressive efficacy for improving cGVHD in ongoing clinical trials. Similarly, macrophage depletion approaches are currently under investigation for their potential to augment responses to immune checkpoint inhibition. Moreover, both monocyte and tissue-resident macrophage populations have recently been implicated as mediators of the numerous toxicities associated with chimeric antigen receptor T-cell therapy, further highlighting potential avenues of macrophage-based interventions to improve clinical outcomes. Herein, we examine the current literature on basic macrophage biology and contextualize this in the setting of cellular and immunotherapy. Additionally, we highlight mechanisms by which macrophages can be targeted, largely by interfering with the CSF1/CSF1R signaling axis, for therapeutic benefit in the context of both cellular and immunotherapy.

Optic Nerve Crush Does Not Induce Retinal Ganglion Cell Loss in the Contralateral Eye

Purpose

Optic nerve crush (ONC) is a model for studying optic nerve trauma. Unilateral ONC induces massive retinal ganglion cell (RGC) degeneration in the affected eye, leading to vision loss within a month. A common assumption has been that the non-injured contralateral eye is unaffected due to the minimal retino-retinal projections of the RGCs at the chiasm. Yet, recently, microglia, the brain-resident macrophages, have shown a responsive phenotype in the contralateral eye after ONC. Whether RGC loss accompanies this phenotype is still controversial.

Methods

Using the available RGCode algorithm and developing our own RGC-Quant deep-learning-based tool, we quantify RGC's total number and density across the entire retina after ONC.

Results

We confirm a short-term microglia response in the contralateral eye after ONC, but this did not affect the microglia number. Furthermore, we cannot confirm the previously reported RGC loss between naïve and contralateral retinas 5 weeks after ONC induction across the commonly used Cx3cr1creERT2 and C57BL6/J mouse models. Neither sex nor the direct comparison of the RGC markers Brn3a and RBPMS, with Brn3a co-labeling, on average, 89% of the RBPMS+-cells, explained this discrepancy, suggesting that the early microglia-responsive phenotype does not have immediate consequences on the RGC number.

Conclusions

Our results corroborate that unilateral optic nerve injury elicits a microglial response in the uninjured contralateral eye but without RGC loss. Therefore, the contralateral eye should be treated separately and not as an ONC control.

Crosstalk between lipid metabolism and macrophages in atherosclerosis: therapeutic potential of natural products

Atherosclerosis is a highly prevalent cardiovascular condition that affects individuals worldwide. Despite ongoing research into its treatment and prevention, atherosclerotic cardiovascular disease continues to exhibit high morbidity and mortality rates. The accumulation of low-density lipoprotein cholesterol is considered a major contributor to the development of atherosclerosis, with abnormalities in lipid metabolism playing a significant role in its pathogenesis. Lipid metabolism and macrophage function are intricately interconnected, with lipid metabolism being influenced by macrophage inflammatory responses, while macrophage activity is regulated by alterations in lipid metabolism. The interaction between these two processes plays a critical role in the progression of atherosclerosis. Natural products have shown considerable promise in treating a variety of diseases, including atherosclerosis. Moreover, the modulation of lipid metabolism and macrophage crosstalk represents a key mechanism through which natural products may exert their effects. This research aims to provide new insights into the current state of research on the role of natural products in regulating this pathway and the interplay between lipid metabolism and macrophages in the context of atherosclerosis, offering potential directions for the future.

The phagosome-mediated anti-bacterial immunity is governed by the proton-activated chloride channel in peritoneal macrophages

Phagosome degradation is an evolutionally conserved and highly effective innate immune response against pathogen infections. The success of this process relies on the ability of phagocytes to regulate the maturation of phagosomes. However, the underlying molecular mechanisms and its roles in shaping downstream immune activation remain poorly understood. Here, we identify the proton-activated chloride (PAC) channel as a key negative regulator of phagosome maturation. PAC deletion enhanced phagosomal acidification and protease activities, leading to augmented bacterial killing in large peritoneal macrophages (LPMs) upon peritoneal Escherichia coli infection in mice. Surprisingly, phagosome bacterial degradation also stimulated STING-IRF3-interferon responses and inflammasome activation in LPMs, both of which are enhanced upon PAC deletion. The increased inflammasome activation and pyroptosis induced an unexpected release of cleaved gasdermin D, which localized to the surface of bacteria in the peritoneum and further contributed to their killing. Finally, enhanced bacterial clearance by PAC-deficient LPMs reduced proinflammatory immune cell infiltration and overall peritoneal inflammation, resulting in improved survival in mice. Our study thus provides new insights into the molecular mechanism of phagosome maturation and the dynamics of host defense response following phagosome-mediated bacterial degradation in peritoneal macrophages. It also highlights the potential of targeting the PAC channel as a therapeutic strategy for treating bacterial infections.

The multifaceted roles of macrophages in the transition from hepatitis to hepatocellular carcinoma: From mechanisms to therapeutic strategies

Macrophages are central to the progression from hepatitis to hepatocellular carcinoma (HCC), with their remarkable plasticity and ability to adapt to the changing liver microenvironment. Chronic inflammation, fibrosis, and ultimately tumorigenesis are driven by macrophage activation, making them key regulators of liver disease progression. This review explores the diverse roles of macrophages in the transition from hepatitis to HCC. In the early stages of hepatitis, macrophages are essential for pathogen clearance and tissue repair. However, chronic activation leads to prolonged inflammation, which exacerbates liver damage and promotes fibrosis. As the disease progresses to liver fibrosis, macrophages interact with hepatic stellate cells, fostering a pro-tumorigenic microenvironment that supports HCC development. In hepatocarcinogenesis, macrophages contribute to tumor initiation, growth, metastasis, immune evasion, cancer stem cell maintenance, and angiogenesis. Their functional plasticity enables them to adapt to the tumor microenvironment, thereby promoting tumor progression and resistance to therapy. Targeting macrophages represents a promising strategy for preventing and treating HCC. Therapeutic approaches, including reprogramming macrophage phenotypes to enhance anti-tumor immunity, blocking macrophage recruitment and activation, and utilizing nanoparticle-based drug delivery systems, may provide new avenues for combating HCC by modulating macrophage functions and tumor microenvironment dynamics.

Vascular-Associated Mononuclear Phagocytes: First-Line Soldiers Ambushing Metastasis

Mononuclear phagocytes (MPs), which consist of dendritic cells, monocytes, and macrophages, are distributed throughout the body and actively eliminate invading microorganisms and abnormal cells. Depending on the local microenvironment, MPs manifest considerably various lifespans and phenotypes to maintain tissue homeostasis. Vascular-associated mononuclear phagocytes (VaMPs) are the special subsets of MPs that are localized either within the lumen side or on the apical surface of vessels, acting as the critical sentinels to recognize and defend against disseminated tumor cells. In this review, we introduce three major types of VaMPs, patrolling monocytes, Kupffer cells, and perivascular macrophages, and discuss their emerging roles in immunosurveillance during incipient metastasis. We also explore the roles of lineage-determining transcription factors and cell surface receptors that endow VaMPs with potent anti-tumor activity. Finally, we highlight the molecular and cellular mechanisms that drive the phenotypic plasticity of VaMPs and summarize combinatory strategies for targeting VaMPs in overt metastasis.

Kupffer cell diversity maintains liver function in alcohol-associated liver disease

BACKGROUND AND AIMS

Liver macrophages are heterogeneous and play an important role in alcohol-associated liver disease (ALD) but there is limited understanding of the functions of specific macrophage subsets in the disease. We used a Western diet alcohol (WDA) mouse model of ALD to examine the hepatic myeloid cell compartment by single cell RNAseq and targeted KC ablation to understand the diversity and function of liver macrophages in ALD.

APPROACH AND RESULTS

In the WDA liver, KCs and infiltrating monocytes/macrophages each represented about 50% of the myeloid pool. Five major KC clusters all expressed genes associated with receptor-mediated endocytosis and lipid metabolism, but most were predicted to be noninflammatory and antifibrotic with 1 minor KC cluster having a proinflammatory and extracellular matrix degradation gene signature. Infiltrating monocyte/macrophage clusters, in contrast, were predicted to be proinflammatory and profibrotic. In vivo, diphtheria toxin-based selective KC ablation during alcohol exposure resulted in a liver failure phenotype with increases in PT/INR and bilirubin, loss of differentiated hepatocyte gene expression, and an increase in expression of hepatocyte progenitor markers such as EpCAM, CK7, and Igf2bp3. Gene set enrichment analysis of whole-liver RNAseq from the KC-ablated WDA mice showed a similar pattern as seen in human alcoholic hepatitis.

CONCLUSIONS

In this ALD model, KCs are anti-inflammatory and are critical for the maintenance of hepatocyte differentiation. Infiltrating monocytes/macrophages are largely proinflammatory and contribute more to liver fibrosis. Future targeting of specific macrophage subsets may provide new approaches to the treatment of liver failure and fibrosis in ALD.

Unveiling the immunological landscape of disseminated tuberculosis: a single-cell transcriptome perspective

Introduction

Hematogenous disseminated tuberculosis (DTB) has an unclear etiology that likely involves multiple factors. Understanding the underlying immunological characteristics of DTB is crucial for elucidating its pathogenesis.

Methods

We conducted single-cell RNA transcriptome and T cell receptor (TCR) sequencing on samples from seven DTB patients. Additionally, we integrated and analyzed data from two published profiles of latent TB infection, three active TB cases, and two healthy controls.

Results

Our analysis revealed a significantly higher proportion of inflammatory immune cells (e.g., monocytes and macrophages) in DTB patients, along with a notably lower abundance of various lymphocytes (including T cells, B cells, and plasma cells), suggesting that lymphopenia is a prominent feature of the disease. T cell pseudotime analysis indicated a decrease in the expression of most hypervariable genes over time, pointing to T cell functional exhaustion. Furthermore, a marked absence of mucosal-associated invariant T (MAIT) cells was observed in the peripheral blood of DTB patients. In the TCR repertoire, specific polymorphisms (TRAV9-2, TRAV13-1, TRBV20-1, and TRBV5-1) and dominant clones (TRAJ49, TRBJ2-7, and TRBJ2-1) were identified. Analysis of the complementarity determining region 3 (CDR3) showed that the most frequent combination was TRAV1-2/TRAJ33, with the motif "CAAMD" being significantly reduced in DTB patients.

Discussion

These findings suggest that lymphopenia and T cell exhaustion, along with unique TCR signatures, may play critical roles in DTB pathogenesis. The reduced "CAAMD" motif and altered TCR clonotypes provide novel insights into the complex cellular dynamics associated with the disease, potentially offering new avenues for targeted immunological interventions.

Differences in glycolytic metabolism between tissue-resident alveolar macrophages and recruited lung macrophages

Immunometabolism has emerged as a key area of focus in immunology and has the potential to lead to new treatments for immune-related diseases. It is well-established that glycolytic metabolism is essential for adaptation to hypoxia and for macrophage inflammatory function. Macrophages have been shown to upregulate their glycolytic metabolism in response to pathogens and pathogen-associated molecular patterns such as LPS. As a direct link to the external environment, the lungs' distinctive nutrient composition and multiple macrophage subtypes provide a unique opportunity to study macrophage metabolism. This review aims to highlight how the steady-state airway and severely inflamed airway offer divergent environments for macrophage glycolytic metabolism. We describe the differences in glycolytic metabolism between tissue-resident alveolar macrophages, and other lung macrophages at steady-state and during inflammation/injury. We also provide an overview of experimental guidelines on how to assess metabolism at the cellular level using Seahorse-based bioenergetic analysis including a review of pharmacologic agents used to inhibit or activate glycolysis.

Comprehensive analysis of myeloid reporter mice

Macrophages are a pivotal cell type within the synovial lining and sub-lining of the joint, playing a crucial role in maintaining homeostasis of synovium. Although fate-mapping techniques have been employed to differentiate synovial macrophages from other synovial myeloid cells, no comprehensive study has yet been conducted within the mouse synovial macrophage compartment. In this study, we present, for the first time, lineage tracing results from 18 myeloid-specific fate-mapping models in mouse peripheral blood (PB) and synovial tissue. The identification of synovial macrophages and monocyte-lineage cells through flow cytometry was further validated using cellular indexing of transcriptomes and epitopes by sequencing (CITE-seq) datasets. These findings provide a valuable methodological tool for researchers to select appropriate models for studying the function of synovial myeloid cells and serve as a reference for investigations in other tissue types.

The various roles of TREM2 in cardiovascular disease

Triggering receptor expressed on myeloid cells-2 (TREM2) is a transmembrane immune receptor that is expressed mainly on macrophages. As a pathology-induced immune signaling hub, TREM2 senses tissue damage and activates immune remodeling in response. Previous studies have predominantly focused on the TREM2 signaling pathway in Alzheimer's disease, metabolic syndrome, and cancer. Recent research has indicated that TREM2 signaling is also activated in various cardiovascular diseases. In this review, we summarize the current understanding and the unanswered questions regarding the role of TREM2 signaling in mediating the metabolism and function of macrophages in atherosclerosis and various models of heart failure. In the context of atherosclerosis, TREM2 signaling promotes foam cell formation and is crucial for maintaining macrophage survival and plaque stability through efferocytosis and cholesterol efflux. Recent studies on myocardial infarction, sepsis-induced cardiomyopathy, and hypertensive heart failure also implicated the protective role of TREM2 signaling in cardiac macrophages through efferocytosis and paracrine functions. Additionally, we discuss the clinical significance of elevated soluble TREM2 (sTREM2) in cardiovascular disease and propose potential therapies targeting TREM2. The overall aim of this review is to highlight the various roles of TREM2 in cardiovascular diseases and to provide a framework for therapeutic strategies targeting TREM2.