microRNA regulation of inflammatory responses.

The mammalian inflammatory response is a rapid and complex physiological reaction to noxious stimuli including microbial pathogens. Although inflammation plays a valuable role in combating infection, its dysregulation often occurs in people and can cause a variety of pathologies, ranging from chronic inflammation, to autoimmunity, to cancer. In recent years, our understanding of both the cellular and molecular networks that regulate inflammation has improved dramatically. Although much of the focus has been on the study of protein regulators of inflammation, recent evidence also points to a critical role for a specific class of noncoding RNAs, called microRNAs (miRNAs), in managing certain features of the inflammatory process. In this review, we discuss recent advances in our understanding of miRNAs and their connection to inflammatory responses. Additionally, we consider the link between perturbations in miRNA levels and the onset of human inflammatory diseases.

Adaptive immunity induces mutualism between commensal eukaryotes.

Pathogenic fungi reside in the intestinal microbiota but rarely cause disease. Little is known about the interactions between fungi and the immune system that promote commensalism. Here we investigate the role of adaptive immunity in promoting mutual interactions between fungi and host. We find that potentially pathogenic Candida species induce and are targeted by intestinal immunoglobulin A (IgA) responses. Focused studies on Candida albicans reveal that the pathogenic hyphal morphotype, which is specialized for adhesion and invasion, is preferentially targeted and suppressed by intestinal IgA responses. IgA from mice and humans directly targets hyphal-enriched cell-surface adhesins. Although typically required for pathogenesis, C. albicans hyphae are less fit for gut colonization1,2 and we show that immune selection against hyphae improves the competitive fitness of C. albicans. C. albicans exacerbates intestinal colitis3 and we demonstrate that hyphae and an IgA-targeted adhesin exacerbate intestinal damage. Finally, using a clinically relevant vaccine to induce an adhesin-specific immune response protects mice from C. albicans-associated damage during colitis. Together, our findings show that adaptive immunity suppresses harmful fungal effectors, with benefits to both C. albicans and its host. Thus, IgA uniquely uncouples colonization from pathogenesis in commensal fungi to promote homeostasis.

MicroRNA-155 Plays Selective Cell-Intrinsic Roles in Brain-Infiltrating Immune Cell Populations during Neuroinflammation.

The proinflammatory microRNA-155 (miR-155) is highly expressed in the serum and CNS lesions of patients with multiple sclerosis (MS). Global knockout (KO) of miR-155 in mice confers resistance to a mouse model of MS, experimental autoimmune encephalomyelitis (EAE), by reducing the encephalogenic potential of CNS-infiltrating Th17 T cells. However, cell-intrinsic roles for miR-155 during EAE have not been formally determined. In this study, we use single-cell RNA sequencing and cell-specific conditional miR-155 KOs to determine the importance of miR-155 expression in distinct immune cell populations. Time-course single-cell sequencing revealed reductions in T cells, macrophages, and dendritic cells (DCs) in global miR-155 KO mice compared with wild-type controls at day 21 after EAE induction. Deletion of miR-155 in T cells, driven by CD4 Cre, significantly reduced disease severity similar to global miR-155 KOs. CD11c Cre-mediated deletion of miR-155 in DCs also resulted in a modest yet significant reduction in the development of EAE, with both T cell- and DC-specific KOs showing a reduction in Th17 T cell infiltration into the CNS. Although miR-155 is highly expressed in infiltrating macrophages during EAE, deletion of miR-155 using LysM Cre did not impact disease severity. Taken together, these data show that although miR-155 is highly expressed in most infiltrating immune cells, miR-155 has distinct roles and requirements depending on the cell type, and we have demonstrated this using the gold standard conditional KO approach. This provides insights into which functionally relevant cell types should be targeted by the next generation of miRNA therapeutics.

miR-155-SOCS1 as a Functional Axis: Satisfying the Burden of Proof.

The importance of individual target genes for miRNA activity has been difficult to establish. In this issue of Immunity, Lu et al. (2015) disrupt the miR-155 binding site in the SOCS1 3' UTR in the mouse germline and show that this axis is important for T and NK cell function.

miR-155 promotes T follicular helper cell accumulation during chronic, low-grade inflammation.

Chronic inflammation is a contributing factor to most life-shortening human diseases. However, the molecular and cellular mechanisms that sustain chronic inflammatory responses remain poorly understood, making it difficult to treat this deleterious condition. Using a mouse model of age-dependent inflammation that results from a deficiency in miR-146a, we demonstrate that miR-155 contributed to the progressive inflammatory disease that emerged as Mir146a(-/-) mice grew older. Upon analyzing lymphocytes from inflamed versus healthy middle-aged mice, we found elevated numbers of T follicular helper (Tfh) cells, germinal center (GC) B cells, and autoantibodies, all occurring in a miR-155-dependent manner. Further, Cd4-cre Mir155(fl/fl) mice were generated and demonstrated that miR-155 functions in T cells, in addition to its established role in B cells, to promote humoral immunity in a variety of contexts. Taken together, our study discovers that miR-146a and miR-155 counterregulate Tfh cell development that drives aberrant GC reactions during chronic inflammation.

miR-125a-5p regulates megakaryocyte proplatelet formation via the actin-bundling protein L-plastin.

There is heritability to interindividual variation in platelet count, and better understanding of the regulating genetic factors may provide insights for thrombopoiesis. MicroRNAs (miRs) regulate gene expression in health and disease, and megakaryocytes (MKs) deficient in miRs have lower platelet counts, but information about the role of miRs in normal human MK and platelet production is limited. Using genome-wide miR profiling, we observed strong correlations among human bone marrow MKs, platelets, and differentiating cord blood-derived MK cultures, and identified MK miR-125a-5p as associated with human platelet number but not leukocyte or hemoglobin levels. Overexpression and knockdown studies showed that miR-125a-5p positively regulated human MK proplatelet (PP) formation in vitro. Inhibition of miR-125a-5p in vivo lowered murine platelet counts. Analyses of MK and platelet transcriptomes identified LCP1 as a miR-125a-5p target. LCP1 encodes the actin-bundling protein, L-plastin, not previously studied in MKs. We show that miR-125a-5p directly targets and reduces expression of MK L-plastin. Overexpression and knockdown studies show that L-plastin promotes MK progenitor migration, but negatively correlates with human platelet count and inhibits MK PP formation (PPF). This work provides the first evidence for the actin-bundling protein, L-plastin, as a regulator of human MK PPF via inhibition of the late-stage MK invagination system, podosome and PPF, and PP branching. We also provide resources of primary and differentiating MK transcriptomes and miRs associated with platelet counts. miR-125a-5p and L-plastin may be relevant targets for increasing in vitro platelet manufacturing and for managing quantitative platelet disorders.

T Cell-Expressed microRNA-155 Reduces Lifespan in a Mouse Model of Age-Related Chronic Inflammation.

Aging-related chronic inflammation is a risk factor for many human disorders through incompletely understood mechanisms. Aged mice deficient in microRNA (miRNA/miR)-146a succumb to life-shortening chronic inflammation. In this study, we report that miR-155 in T cells contributes to shortened lifespan of miR-146a-/- mice. Using single-cell RNA sequencing and flow cytometry, we found that miR-155 promotes the activation of effector T cell populations, including T follicular helper cells, and increases germinal center B cells and autoantibodies in mice aged over 15 months. Mechanistically, aerobic glycolysis genes are elevated in T cells during aging, and upon deletion of miR-146a, in a T cell miR-155-dependent manner. Finally, skewing T cell metabolism toward aerobic glycolysis by deleting mitochondrial pyruvate carrier recapitulates age-dependent T cell phenotypes observed in miR-146a-/- mice, revealing the sufficiency of metabolic reprogramming to influence immune cell functions during aging. Altogether, these data indicate that T cell-specific miRNAs play pivotal roles in regulating lifespan through their influences on inflammaging.

Anti-inflammatory microRNA-146a protects mice from diet-induced metabolic disease.

Identifying regulatory mechanisms that influence inflammation in metabolic tissues is critical for developing novel metabolic disease treatments. Here, we investigated the role of microRNA-146a (miR-146a) during diet-induced obesity in mice. miR-146a is reduced in obese and type 2 diabetic patients and our results reveal that miR-146a-/- mice fed a high-fat diet (HFD) have exaggerated weight gain, increased adiposity, hepatosteatosis, and dysregulated blood glucose levels compared to wild-type controls. Pro-inflammatory genes and NF-κB activation increase in miR-146a-/- mice, indicating a role for this miRNA in regulating inflammatory pathways. RNA-sequencing of adipose tissue macrophages demonstrated a role for miR-146a in regulating both inflammation and cellular metabolism, including the mTOR pathway, during obesity. Further, we demonstrate that miR-146a regulates inflammation, cellular respiration and glycolysis in macrophages through a mechanism involving its direct target Traf6. Finally, we found that administration of rapamycin, an inhibitor of mTOR, was able to rescue the obesity phenotype in miR-146a-/- mice. Altogether, our study provides evidence that miR-146a represses inflammation and diet-induced obesity and regulates metabolic processes at the cellular and organismal levels, demonstrating how the combination of diet and miRNA genetics influences obesity and diabetic phenotypes.

Megakaryocyte-specific knockout of the Mir-99b/let7e/125a cluster lowers platelet count without altering platelet function.

The purpose of this research was to assess the effects of a microRNA (miRNA) cluster on platelet production. Human chromosome 19q13.41 harbors an evolutionarily conserved cluster of three miRNA genes (MIR99B, MIRLET7E, MIR125A) within 727 base-pairs. We now report that levels of miR-99b-5p, miR-let7e-5p and miR-125a-5p are strongly correlated in human platelets, and all are positively associated with platelet count, but not white blood count or hemoglobin level. Although the cluster regulates hematopoietic stem cell proliferation, the function of this genomic locus in megakaryocyte (MK) differentiation and platelet production is unknown. Furthermore, studies of individual miRNAs do not represent broader effects in the context of a cluster. To address this possibility, MK/platelet lineage-specific Mir-99b/let7e/125a knockout mice were generated. Compared to wild type littermates, cluster knockout mice had significantly lower platelet counts and reduced MK proplatelet formation, but no differences in MK numbers, ploidy, maturation or ultra-structural morphology, and no differences in platelet function. Compared to wild type littermates, knockout mice showed similar survival after pulmonary embolism. The major conclusions are that the effect of the Mir-99b/let7e/125a cluster is confined to a late stage of thrombopoiesis, and this effect on platelet number is uncoupled from platelet function.

Single-Cell RNA Sequencing Reveals the Diversity of the Immunological Landscape following Central Nervous System Infection by a Murine Coronavirus.

Intracranial (i.c.) infection of susceptible C57BL/6 mice with the neurotropic JHM strain of mouse hepatitis virus (JHMV) (a member of the Coronaviridae family) results in acute encephalomyelitis and viral persistence associated with an immune-mediated demyelinating disease. The present study was undertaken to better understand the molecular pathways evoked during innate and adaptive immune responses as well as the chronic demyelinating stage of disease in response to JHMV infection of the central nervous system (CNS). Using single-cell RNA sequencing analysis (scRNAseq) on flow-sorted CD45-positive (CD45+) cells enriched from brains and spinal cords of experimental mice, we demonstrate the heterogeneity of the immune response as determined by the presence of unique molecular signatures and pathways involved in effective antiviral host defense. Furthermore, we identify potential genes involved in contributing to demyelination as well as remyelination being expressed by both microglia and macrophages. Collectively, these findings emphasize the diversity of the immune responses and molecular networks at defined stages following viral infection of the CNS.IMPORTANCE Understanding the immunological mechanisms contributing to both host defense and disease following viral infection of the CNS is of critical importance given the increasing number of viruses that are capable of infecting and replicating within the nervous system. With this in mind, the present study was undertaken to evaluate the molecular signatures of immune cells within the CNS at defined times following infection with a neuroadapted murine coronavirus using scRNAseq. This approach has revealed that the immunological landscape is diverse, with numerous immune cell subsets expressing distinct mRNA expression profiles that are, in part, dictated by the stage of infection. In addition, these findings reveal new insight into cellular pathways contributing to control of viral replication as well as to neurologic disease.

CIPR: a web-based R/shiny app and R package to annotate cell clusters in single cell RNA sequencing experiments.

BACKGROUND: Single cell RNA sequencing (scRNAseq) has provided invaluable insights into cellular heterogeneity and functional states in health and disease. During the analysis of scRNAseq data, annotating the biological identity of cell clusters is an important step before downstream analyses and it remains technically challenging. The current solutions for annotating single cell clusters generally lack a graphical user interface, can be computationally intensive or have a limited scope. On the other hand, manually annotating single cell clusters by examining the expression of marker genes can be subjective and labor-intensive. To improve the quality and efficiency of annotating cell clusters in scRNAseq data, we present a web-based R/Shiny app and R package, Cluster Identity PRedictor (CIPR), which provides a graphical user interface to quickly score gene expression profiles of unknown cell clusters against mouse or human references, or a custom dataset provided by the user. CIPR can be easily integrated into the current pipelines to facilitate scRNAseq data analysis. RESULTS: CIPR employs multiple approaches for calculating the identity score at the cluster level and can accept inputs generated by popular scRNAseq analysis software. CIPR provides 2 mouse and 5 human reference datasets, and its pipeline allows inter-species comparisons and the ability to upload a custom reference dataset for specialized studies. The option to filter out lowly variable genes and to exclude irrelevant reference cell subsets from the analysis can improve the discriminatory power of CIPR suggesting that it can be tailored to different experimental contexts. Benchmarking CIPR against existing functionally similar software revealed that our algorithm is less computationally demanding, it performs significantly faster and provides accurate predictions for multiple cell clusters in a scRNAseq experiment involving tumor-infiltrating immune cells. CONCLUSIONS: CIPR facilitates scRNAseq data analysis by annotating unknown cell clusters in an objective and efficient manner. Platform independence owing to Shiny framework and the requirement for a minimal programming experience allows this software to be used by researchers from different backgrounds. CIPR can accurately predict the identity of a variety of cell clusters and can be used in various experimental contexts across a broad spectrum of research areas.

Microglia influence host defense, disease, and repair following murine coronavirus infection of the central nervous system.

The present study examines functional contributions of microglia in host defense, demyelination, and remyelination following infection of susceptible mice with a neurotropic coronavirus. Treatment with PLX5622, an inhibitor of colony stimulating factor 1 receptor (CSF1R) that efficiently depletes microglia, prior to infection of the central nervous system (CNS) with the neurotropic JHM strain of mouse hepatitis virus (JHMV) resulted in increased mortality compared with control mice that correlated with impaired control of viral replication. Single cell RNA sequencing (scRNASeq) of CD45+ cells isolated from the CNS revealed that PLX5622 treatment resulted in muted CD4+ T cell activation profile that was associated with decreased expression of transcripts encoding MHC class II and CD86 in macrophages but not dendritic cells. Evaluation of spinal cord demyelination revealed a marked increase in white matter damage in PLX5622-treated mice that corresponded with elevated expression of transcripts encoding disease-associated proteins Osteopontin (Spp1), Apolipoprotein E (Apoe), and Triggering receptor expressed on myeloid cells 2 (Trem2) that were enriched within macrophages. In addition, PLX5622 treatment dampened expression of Cystatin F (Cst7), Insulin growth factor 1 (Igf1), and lipoprotein lipase (Lpl) within macrophage populations which have been implicated in promoting repair of damaged nerve tissue and this was associated with impaired remyelination. Collectively, these findings argue that microglia tailor the CNS microenvironment to enhance control of coronavirus replication as well as dampen the severity of demyelination and influence repair.

MicroRNAs: new regulators of immune cell development and function.

Decades of research went into understanding immune cell development and function without awareness that consideration of a key element, microRNA (miRNA), was lacking. The discovery of miRNAs as regulators of developmental events in model organisms suggested to many investigators that miRNA might be involved in the immune system. In the past few years, widespread examination of this possibility has produced notable results. Results have shown that miRNAs affect mammalian immune cell differentiation, the outcome of immune responses to infection and the development of diseases of immunological origin. Some miRNAs repress expression of target proteins with well established functions in hematopoiesis. Here we bring together much of this work, which has so far only scratched the surface of this very fertile field of investigation, and show how the results illuminate many historic questions about hematopoiesis and immune function.

Microbiota promotes systemic T-cell survival through suppression of an apoptotic factor.

Symbiotic microbes impact the severity of a variety of diseases through regulation of T-cell development. However, little is known regarding the molecular mechanisms by which this is accomplished. Here we report that a secreted factor, Erdr1, is regulated by the microbiota to control T-cell apoptosis. Erdr1 expression was identified by transcriptome analysis to be elevated in splenic T cells from germfree and antibiotic-treated mice. Suppression of Erdr1 depends on detection of circulating microbial products by Toll-like receptors on T cells, and this regulation is conserved in human T cells. Erdr1 was found to function as an autocrine factor to induce apoptosis through caspase 3. Consistent with elevated levels of Erdr1, germfree mice have increased splenic T-cell apoptosis. RNA sequencing of Erdr1-overexpressing cells identified the up-regulation of genes involved in Fas-mediated cell death, and Erdr1 fails to induce apoptosis in Fas-deficient cells. Importantly, forced changes in Erdr1 expression levels dictate the survival of auto-reactive T cells and the clinical outcome of neuro-inflammatory autoimmune disease. Cellular survival is a fundamental feature regulating appropriate immune responses. We have identified a mechanism whereby the host integrates signals from the microbiota to control T-cell apoptosis, making regulation of Erdr1 a potential therapeutic target for autoimmune disease.

Aging impairs mouse skeletal muscle macrophage polarization and muscle-specific abundance during recovery from disuse.

Impaired recovery of aged muscle following a disuse event is an unresolved issue facing the older adult population. Although investigations in young animals have suggested that rapid regrowth of skeletal muscle following a disuse event entails a coordinated involvement of skeletal muscle macrophages, this phenomenon has not yet been thoroughly tested as an explanation for impaired muscle recovery in aging. To examine this hypothesis, young (4-5 mo) and old (24-26 mo) male mice were examined as controls following 2 wk of hindlimb unloading (HU) and following 4 (RL4) and 7 (RL7) days of reloading after HU. Muscles were harvested to assess muscle weight, myofiber-specifc cross-sectional area, and skeletal muscle macrophages via immunofluorescence. Flow cytometry was used on gastrocnemius and soleus muscle (at RL4) single-cell suspensions to immunophenotype skeletal muscle macrophages. Our data demonstrated impaired muscle regrowth in aged compared with young mice following disuse, which was characterized by divergent muscle macrophage polarization patterns and muscle-specifc macrophage abundance. During reloading, young mice exhibited the classical increase in M1-like (MHC II+CD206-) macrophages that preceeded the increase in percentage of M2-like macrophages (MHC II-CD206+); however, old mice did not demonstrate this pattern. Also, at RL4, the soleus demonstrated reduced macrophage abundance with aging. Together, these data suggest that dysregulated macrophage phenotype patterns in aged muscle during recovery from disuse may be related to impaired muscle growth. Further investigation is needed to determine whether the dysregulated macrophage response in the old during regrowth from disuse is related to a reduced ability to recruit or activate specific immune cells.

MicroRNAs and acute myeloid leukemia: therapeutic implications and emerging concepts.

Acute myeloid leukemia (AML) is a deadly hematologic malignancy characterized by the uncontrolled growth of immature myeloid cells. Over the past several decades, we have learned a tremendous amount regarding the genetic aberrations that govern disease development in AML. Among these are genes that encode noncoding RNAs, including the microRNA (miRNA) family. miRNAs are evolutionarily conserved small noncoding RNAs that display important physiological effects through their posttranscriptional regulation of messenger RNA targets. Over the past decade, studies have identified miRNAs as playing a role in nearly all aspects of AML disease development, including cellular proliferation, survival, and differentiation. These observations have led to the study of miRNAs as biomarkers of disease, and efforts to therapeutically manipulate miRNAs to improve disease outcome in AML are ongoing. Although much has been learned regarding the importance of miRNAs in AML disease initiation and progression, there are many unanswered questions and emerging facets of miRNA biology that add complexity to their roles in AML. Moving forward, answers to these questions will provide a greater level of understanding of miRNA biology and critical insights into the many translational applications for these small regulatory RNAs in AML.

miR-155 promotes FLT3-ITD-induced myeloproliferative disease through inhibition of the interferon response.

FLT3-ITD+ acute myeloid leukemia (AML) accounts for ∼25% of all AML cases and is a subtype that carries a poor prognosis. microRNA-155 (miR-155) is specifically overexpressed in FLT3-ITD+ AML compared with FLT3 wild-type (FLT3-WT) AML and is critical for the growth of FLT3-ITD+ AML cells in vitro. However, miR-155's role in regulating FLT3-ITD-mediated disease in vivo remains unclear. In this study, we used a genetic mouse model to determine whether miR-155 influences the development of FLT3-ITD-induced myeloproliferative disease. Results indicate that miR-155 promotes FLT3-ITD-induced myeloid expansion in the bone marrow, spleen, and peripheral blood. Mechanistically, miR-155 increases proliferation of the hematopoietic stem and progenitor cell compartments by reducing the growth-inhibitory effects of the interferon (IFN) response, and this involves targeting of Cebpb. Consistent with our observations in mice, primary FLT3-ITD+ AML clinical samples have significantly higher miR-155 levels and a lower IFN response compared with FLT3-WT AML samples. Further, inhibition of miR-155 in FLT3-ITD+ AML cell lines using CRISPR/Cas9, or primary FLT3-ITD+ AML samples using locked nucleic acid antisense inhibitors, results in an elevated IFN response and reduces colony formation. Altogether, our data reveal that miR-155 collaborates with FLT3-ITD to promote myeloid cell expansion in vivo and that this involves a multitarget mechanism that includes repression of IFN signaling.

MicroRNA-34a perturbs B lymphocyte development by repressing the forkhead box transcription factor Foxp1.

MicroRNAs (miRNAs) can influence lineage choice or affect critical developmental checkpoints during hematopoiesis. We examined the role of the p53-induced microRNA miR-34a in hematopoiesis by gain-of-function analysis in murine bone marrow. Constitutive expression of miR-34a led to a block in B cell development at the pro-B-cell-to-pre-B-cell transition, leading to a reduction in mature B cells. This block appeared to be mediated primarily by inhibited expression of the transcription factor Foxp1. Foxp1 was a direct target of miR-34a in a 3'-untranslated region (UTR)-dependent fashion. Knockdown of Foxp1 by siRNA recapitulated the B cell developmental phenotype induced by miR-34a, whereas cotransduction of Foxp1 lacking its 3' UTR with miR-34a rescued B cell maturation. Knockdown of miR-34a resulted in increased amounts of Foxp1 and mature B cells. These findings identify a role for miR-34a in connecting the p53 network with suppression of Foxp1, a known B cell oncogene.

MicroRNA-155 promotes autoimmune inflammation by enhancing inflammatory T cell development.

Mammalian noncoding microRNAs (miRNAs) are a class of gene regulators that have been linked to immune system function. Here, we have investigated the role of miR-155 during an autoimmune inflammatory disease. Consistent with a positive role for miR-155 in mediating inflammatory responses, Mir155(-/-) mice were highly resistant to experimental autoimmune encephalomyelitis (EAE). miR-155 functions in the hematopoietic compartment to promote the development of inflammatory T cells including the T helper 17 (Th17) cell and Th1 cell subsets. Furthermore, the major contribution of miR-155 to EAE was CD4(+) T cell intrinsic, whereas miR-155 was also required for optimum dendritic cell production of cytokines that promoted Th17 cell formation. Our study shows that one aspect of miR-155 function is the promotion of T cell-dependent tissue inflammation, suggesting that miR-155 might be a promising therapeutic target for the treatment of autoimmune disorders.

Rab27-Dependent Exosome Production Inhibits Chronic Inflammation and Enables Acute Responses to Inflammatory Stimuli.

Extracellular vesicles, including exosomes, have recently been implicated as novel mediators of immune cell communication in mammals. However, roles for endogenously produced exosomes in regulating immune cell functions in vivo are just beginning to be identified. In this article, we demonstrate that Rab27a and Rab27b double-knockout (Rab27DKO) mice that are deficient in exosome secretion have a chronic, low-grade inflammatory phenotype characterized by elevated inflammatory cytokines and myeloproliferation. Upon further investigation, we found that some of these phenotypes could be complemented by wild-type (WT) hematopoietic cells or administration of exosomes produced by GM-CSF-expanded bone marrow cells. In addition, chronically inflamed Rab27DKO mice had a blunted response to bacterial LPS, resembling endotoxin tolerance. This defect was rescued by bone marrow exosomes from WT, but not miR-155-/-, cells, suggesting that uptake of miR-155-containing exosomes is important for a proper LPS response. Further, we found that SHIP1 and IRAK-M, direct targets of miR-155 that are known negative regulators of the LPS response, were elevated in Rab27DKO mice and decreased after treatment with WT, but not miR-155-/-, exosomes. Together, our study finds that Rab27-dependent exosome production contributes to homeostasis within the hematopoietic system and appropriate responsiveness to inflammatory stimuli.