Accumulating evidence indicates that gut microbiota dysbiosis is associated with increased blood–brain barrier (BBB) permeability and contributes to Alzheimer's disease (AD) pathogenesis. In contrast, the influence of gut microbiota on the blood‐cerebrospinal fluid (CSF) barrier has not yet been studied. Here, we report that mice lacking gut microbiota display increased blood‐CSF barrier permeability associated with disorganized tight junctions (TJs), which can be rescued by recolonization with gut microbiota or supplementation with short‐chain fatty acids (SCFAs). Our data reveal that gut microbiota is important not only for the establishment but also for the maintenance of a tight barrier. Also, we report that the vagus nerve plays an important role in this process and that SCFAs can independently tighten the barrier. Administration of SCFAs in App ^NL‐G‐F mice improved the subcellular localization of TJs at the blood‐CSF barrier, reduced the β‐amyloid (Aβ) burden, and affected microglial phenotype. Altogether, our results suggest that modulating the microbiota and administering SCFAs might have therapeutic potential in AD via blood‐CSF barrier tightening and maintaining microglial activity and Aβ clearance.

Dendritic cells (DCs) mature in an immunogenic or tolerogenic manner depending on the context in which an antigen is perceived, preserving the balance between immunity and tolerance. Whereas the pathways driving immunogenic maturation in response to infectious insults are well-characterized, the signals that drive tolerogenic maturation during homeostasis are still poorly understood. We found that the engulfment of apoptotic cells triggered homeostatic maturation of type 1 conventional DCs (cDC1s) within the spleen. This maturation process could be mimicked by engulfment of empty, nonadjuvanted lipid nanoparticles (LNPs), was marked by intracellular accumulation of cholesterol, and was highly specific to cDC1s. Engulfment of either apoptotic cells or cholesterol-rich LNPs led to the activation of the liver X receptor (LXR) pathway, which promotes the efflux of cellular cholesterol, and repressed genes associated with immunogenic maturation. In contrast, simultaneous engagement of TLR3 to mimic viral infection via administration of poly(I:C)-adjuvanted LNPs repressed the LXR pathway, thus delaying cellular cholesterol efflux and inducing genes that promote T cell–mediated immunity. These data demonstrate that conserved cellular cholesterol efflux pathways are differentially regulated in tolerogenic versus immunogenic cDC1s and suggest that administration of nonadjuvanted cholesterol-rich LNPs may be an approach for inducing tolerogenic DC maturation.

The choroid plexus (ChP), located in the brain ventricles, is largely composed of ChP epithelial cells that produce the cerebrospinal fluid (CSF) and form the blood–CSF barrier. At the ChP–brain attachment sites, we have discovered unique fibroblasts referred to as ChP base barrier cells (BBCs). We show that ChP BBCs originate from meningeal mesenchymal precursors, arrive early during development, remain throughout life and are conserved across species. ChP BBCs are transcriptionally similar to meningeal arachnoid barrier cells and are interconnected by both adherens and tight junctions. Notably, we provide evidence that the BBCs function as a barrier, controlling communication between the periphery and central nervous system. Moreover, during inflammatory insult, we observed a loss of barrier integrity and immune cell crossing. Altogether, our research revealed a barrier at the ChP base, crucial in protecting the central nervous system by compartmentalizing the ChP stroma, brain parenchyma and CSF.

Fc receptors mediate antibody effector functions. Immunoglobulin G (IgG), the predominant antibody in circulation and in clinical use, engages diverse Fc gamma (Fcγ) receptors differentially expressed across cell types. Here, we provide a comprehensive overview of Fcγ receptor and neonatal Fc receptor (FcRn) expression in humans, macaques, and mice. This analysis revealed substantial differences in Fcγ receptor diversity, cell-specific expression, and regulatory mechanisms that compromise the translation of mouse and macaque models for antibody research. To improve preclinical modeling, we generated a mouse in which humanized Fcγ receptors (FcγRI/CD64, FcγRIIA/CD32A, FcγRIIB/CD32B, FcγRIIIA/CD16A, and FcγRIIIB/CD16B), expressed under control of human promotors, replace their murine counterparts. This model also incorporates human FcRn to improve antibody pharmacokinetics. Humanization resulted in more faithful Fcγ receptor expression. We validated receptor functionality and demonstrated how cytokines modulate their expression. Together, this cross-species Fcγ receptor atlas and humanized mouse model can improve the preclinical evaluation of antibody-based therapeutics.

Dendritic cells (DCs) are critical inducers of anti-tumor immunity. To achieve a comprehensive mapping of mouse and human DC subsets and states in a cancer context, here we generate pan-cancer mouse and human tumor-associated DC (TADC) scRNA-seq atlases, encompassing 14 mouse tumor models and 10 human cancer types, within which we identify several lineage-defined DC subsets along with maturation/functional states. We show that TADCs acquire an inflammatory profile with tumor progression and that tumor-mediated reprogramming occurs within the DCs from lymph nodes of tumor-bearing mice. Importantly, we demonstrate that TADCs are broadly conserved between mice and humans, although species-specific differences may exist in some subsets and states. Moreover, we present a comprehensive assessment of how different human TADC clusters associate with patient survival outcomes. Overall, we provide an in-depth characterization of the TADC compartment in mouse and human cancers, which can improve our understanding of the tumor microenvironment and contribute to the development of new anti-cancer therapies.

The continuous engulfment of apoptotic cells initiates a homeostatic maturation program in conventional type I dendritic cells (cDC1s), hallmarked by the activation of the transcription factor LXRb, which mediates cholesterol efflux and dampens interferon stimulated gene expression. cDC1s are characterized by a high basal activation of the unfolded protein response (UPR) sensor IRE1, without concomitant induction of a proper UPR gene signature, a finding that has puzzled the field. Here we show that in absence of IRE1, the homeostatic maturation of cDC1s is blocked, while homeostatic maturation of cDC2s remains unaffected. IRE1 activation is strictly dependent on apoptotic cell engulfment and cholesterol influx, explaining its cDC1 subset specific activity. Stimulation of IRE1 endonuclease activity in cDC1s leads to a Regulated IRE1 Dependent Decay (RIDD) response, targeting miRNAs rather than mRNAs. This causes the degradation of miRNA-92a, which targets the cholesterol efflux transporter Abcg1. Loss of IRE1 leads to defects in cholesterol efflux in mature cDC1s and concomitant cell death, while cDC2s do not show any defects. Blocking miRNA synthesis or enforcing cholesterol efflux by treatment with reconstituted high-density lipoproteins rescues cDC1s from cell death. These data highlight the central role of IRE1 as a sensor of cholesterol influx in the ER, extending IRE1’s function beyond its canonical role in protein folding. Furthermore, they underscore the tight control of cholesterol metabolism during cDC1 maturation, uncovering a second pathway to coordinate cholesterol efflux that acts in parallel to LXRb.