RESUMEN
Innate immune cells play a key role in inflammation as a source of pro-inflammatory cytokines. However, it remains unclear how innate immunity-mediated inflammation is fine-tuned to minimize tissue damage and assure the host's survival at the early phase of systemic inflammation. The results of this study with mouse models demonstrate that the supply of monocytes is restricted depending on the magnitude of inflammation. During the acute phase of severe inflammation, monocytes, but not neutrophils, were substantially reduced by apoptosis and the remaining monocytes were dysfunctional in the bone marrow. Monocyte-specific ablation of Casp3/7 prevented monocyte apoptosis but promoted monocyte necrosis in the bone marrow, leading to elevated levels of pro-inflammatory cytokines and the increased mortality of mice during systemic inflammation. Importantly, the limitation of monocyte supply was dependent on pro-inflammatory cytokines in vivo. Consistently, a reduction of monocytes was observed in the peripheral blood during cytokine-release syndrome (CRS) patients, a pathogen-unrelated systemic inflammation induced by chimeric antigen receptor-T cell (CAR-T cell) therapy. Thus, monocytes act as a safety valve to alleviate tissue damage caused by inflammation and ensure host survival, which may be responsible for a primitive immune-control mechanism that does not require intervention by acquired immunity.
Asunto(s)
Citocinas , Inflamación , Monocitos , Animales , Monocitos/inmunología , Ratones , Humanos , Inflamación/inmunología , Citocinas/metabolismo , Apoptosis , Síndrome de Liberación de Citoquinas/inmunología , Síndrome de Liberación de Citoquinas/patología , Inmunidad Innata , Ratones Endogámicos C57BL , Modelos Animales de Enfermedad , Masculino , FemeninoRESUMEN
Invariant natural killer T (iNKT) cells are innate-like T lymphocytes that express an invariant T cell receptor α chain and contribute to bridging innate and acquired immunity with rapid production of large amounts of cytokines after stimulation. Among effecter subsets of iNKT cells, follicular helper NKT (NKTFH) cells are specialized to help B cells. However, the mechanisms of NKTFH cell differentiation remain to be elucidated. In this report, we studied the mechanism of NKTFH cell differentiation induced by pneumococcal surface protein A and α-galactosylceramide (P/A) vaccination. We found that Gr-1+ cells helped iNKT cell proliferation and NKTFH cell differentiation in the spleen by producing interleukin-27 (IL-27) in the early phase after vaccination. The neutralization of IL-27 impaired NKTFH cell differentiation, which resulted in compromised antibody production and diminished protection against Streptococcus pneumoniae infection by the P/A vaccine. Our data indicated that Gr-1+ cell-derived IL-27 stimulated mitochondrial metabolism, meeting the energic demand required for iNKT cells to differentiate into NKTFH cells. Interestingly, Gr-1+ cell-derived IL-27 was induced by iNKT cells via interferon-γ production. Collectively, our findings suggest that optimizing the metabolism of iNKT cells was essential for acquiring specific effector functions, and they provide beneficial knowledge on iNKT cell-mediated vaccination-mediated therapeutic strategies.
Asunto(s)
Interleucina-27 , Células T Asesinas Naturales , Animales , Ratones , Interleucina-27/metabolismo , Linfocitos T Colaboradores-Inductores , Citocinas/metabolismo , Diferenciación Celular , Ratones Endogámicos C57BLRESUMEN
Despite the importance of IL-23 in mucosal host defense and disease pathogenesis, the mechanisms regulating the development of IL-23-producing mononuclear phagocytes remain poorly understood. Here, we employed an Il23aVenus reporter strain to investigate the developmental identity and functional regulation of IL-23-producing cells. We showed that flagellin stimulation or Citrobacter rodentium infection led to robust induction of IL-23-producing EpCAM+ DCIR2+ CD103- cDC2s, termed cDCIL23, which was confined to gut-associated lymphoid tissues, including the mesenteric lymph nodes, cryptopatches, and isolated lymphoid follicles. Furthermore, we demonstrated that Notch2 signaling was crucial for the development of EpCAM+ DCIR2+ cDC2s, and the combination of Notch2 signaling with retinoic acid signaling controlled their terminal differentiation into cDCIL23, supporting a two-step model for the development of gut cDCIL23. Our findings provide fundamental insights into the developmental pathways and cellular dynamics of IL-23-producing cDC2s at steady state and during pathogen infection.
Asunto(s)
Células Dendríticas , Infecciones por Enterobacteriaceae , Interleucina-23 , Animales , Ratones , Molécula de Adhesión Celular Epitelial , Flagelina , TretinoinaRESUMEN
Natural killer (NK) cells are innate immune cells critical for protective immune responses against infection and cancer. Although NK cells differentiate in the bone marrow (BM) in an interleukin-15 (IL-15)-dependent manner, the cellular source of IL-15 remains elusive. Using NK cell reporter mice, we show that NK cells are localized in the BM in scattered and clustered manners. NK cell clusters overlap with monocyte and dendritic cell accumulations, whereas scattered NK cells require CXCR4 signaling. Using cell-specific IL-15-deficient mice, we show that hematopoietic cells, but not stromal cells, support NK cell development in the BM through IL-15. In particular, IL-15 produced by monocytes and dendritic cells appears to contribute to NK cell development. These results demonstrate that hematopoietic cells are the IL-15 niche for NK cell development in the BM and that BM NK cells are present in scattered and clustered compartments by different mechanisms, suggesting their distinct functions in the immune response.
Asunto(s)
Médula Ósea , Interleucina-15 , Ratones , Animales , Células de la Médula Ósea , Diferenciación Celular , Células Asesinas NaturalesRESUMEN
Emergency myelopoiesis (EM) is a hematopoietic response against systemic infections that quickly supplies innate immune cells. As lymphopoiesis is strongly suppressed during EM, the role of lymphocytes in that process has not received much attention. Here, we found that myeloid-like B cells (M-B cells), which express myeloid markers, emerge in the bone marrow (BM) after the induction of EM. M-B cells were mainly derived from pre-B cells and preferentially expressed IL-10, which directly stimulates hematopoietic progenitors to enhance their survival and myeloid-biased differentiation. Indeed, lacking IL-10 in B cells, blocking IL-10 in the BM with a neutralizing antibody, and deleting the IL-10 receptor in hematopoietic progenitors significantly suppressed EM, which failed to clear microbes in a cecal ligation and puncture model. Thus, a distinct B cell subset generated during infection plays a pivotal role in boosting EM, which suggests the on-demand reinforcement of EM by adaptive immune cells.
Asunto(s)
Linfocitos B , Interleucina-10 , Mielopoyesis , Médula Ósea/fisiología , Células de la Médula Ósea , Hematopoyesis , Células MieloidesRESUMEN
Invariant natural killer T (iNKT) cells are a group of innate-like T lymphocytes that recognize lipid antigens. They are supposed to be tissue resident and important for systemic and local immune regulation. To investigate the heterogeneity of iNKT cells, we recharacterized iNKT cells in the thymus and peripheral tissues. iNKT cells in the thymus were divided into three subpopulations by the expression of the natural killer cell receptor CD244 and the chemokine receptor CXCR6 and designated as C0 (CD244-CXCR6-), C1 (CD244-CXCR6+), or C2 (CD244+CXCR6+) iNKT cells. The development and maturation of C2 iNKT cells from C0 iNKT cells strictly depended on IL-15 produced by thymic epithelial cells. C2 iNKT cells expressed high levels of IFN-γ and granzymes and exhibited more NK cell-like features, whereas C1 iNKT cells showed more T cell-like characteristics. C2 iNKT cells were influenced by the microbiome and aging and suppressed the expression of the autoimmune regulator AIRE in the thymus. In peripheral tissues, C2 iNKT cells were circulating that were distinct from conventional tissue-resident C1 iNKT cells. Functionally, C2 iNKT cells protected mice from the tumor metastasis of melanoma cells by enhancing antitumor immunity and promoted antiviral immune responses against influenza virus infection. Furthermore, we identified human CD244+CXCR6+ iNKT cells with high cytotoxic properties as a counterpart of mouse C2 iNKT cells. Thus, this study reveals a circulating subset of iNKT cells with NK cell-like properties distinct from conventional tissue-resident iNKT cells.
Asunto(s)
Células T Asesinas Naturales , Ratones , Humanos , Animales , Células T Asesinas Naturales/metabolismo , Células T Asesinas Naturales/patología , Interleucina-15 , Antivirales , Granzimas , Receptores de Células Asesinas Naturales , Receptores de Quimiocina/metabolismo , LípidosRESUMEN
Dendritic cells (DCs) and monocytes are widely conserved immune cells in vertebrates that arise from hematopoietic stem cells via intermediate progenitors. The progenitors that strictly give rise to DCs or monocytes have been recently identified both in humans and in mice, thereby revealing their differentiation pathways. Advances in analysis technologies have further deepened our understanding of the development of DCs and monocytes from progenitor population-based to individual progenitor cell-based commitment. Since DC-committed progenitors, common DC progenitors (CDPs) and precursor conventional DCs (pre-cDCs) do not differentiate into monocytes, DCs are a distinct lineage from monocytes, although monocytes can acquire DC functions upon activation at tissues where they arrive.
Asunto(s)
Células Dendríticas/inmunología , Monocitos/inmunología , Animales , HumanosRESUMEN
BACKGROUND: Epidemiologic studies have yielded conflicting results regarding the influence of a single bout of prolonged high-intensity exercise on viral infection. OBJECTIVE: We sought to learn whether prolonged high-intensity exercise either exacerbates or ameliorates herpes simplex virus type 2 (HSV-2) infection according to the interval between virus exposure and exercise. METHODS: Mice were intravaginally infected with HSV-2 and exposed to run on the treadmill. RESULTS: Prolonged high-intensity exercise 17 hours after infection impaired the clearance of HSV-2, while exercise 8 hours after infection enhanced the clearance of HSV-2. These impaired or enhanced immune responses were related to a transient decrease or increase in the number of blood-circulating plasmacytoid dendritic cells. Exercise-induced glucocorticoids transiently decreased the number of circulating plasmacytoid dendritic cells by facilitating their homing to the bone marrow via the CXCL12-CXCR4 axis, which led to their subsequent increase in the blood. CONCLUSION: A single bout of prolonged high-intensity exercise can be either deleterious or beneficial to antiviral immunity.
Asunto(s)
Células Dendríticas/inmunología , Glucocorticoides/metabolismo , Herpes Simple/inmunología , Herpesvirus Humano 2/fisiología , Animales , Quimiocina CXCL12/metabolismo , Ejercicio Físico , Humanos , Inmunidad , Ratones , Ratones Endogámicos C57BL , Modelos Animales , Condicionamiento Físico Animal , Receptores CXCR4/metabolismoRESUMEN
As hematopoietic progenitors supply a large number of blood cells, therapeutic strategies targeting hematopoietic progenitors are potentially beneficial to eliminate unwanted blood cells, such as leukemic cells and immune cells causing diseases. However, due to their pluripotency, targeting those cells may impair the production of multiple cell lineages, leading to serious side effects such as anemia and increased susceptibility to infection. To minimize those side effects, it is important to identify monopotent progenitors that give rise to a particular cell lineage. Monocytes and monocyte-derived macrophages play important roles in the development of inflammatory diseases and tumors. Recently, we identified human monocyte-restricted progenitors, namely, common monocyte progenitors and pre-monocytes, both of which express high levels of CD64, a well-known monocyte marker. Here, we introduce a dimeric pyrrolobenzodiazepine (dPBD)-conjugated anti-CD64 antibody (anti-CD64-dPBD) that selectively induces the apoptosis of proliferating human monocyte-restricted progenitors but not non-proliferating mature monocytes. Treatment with anti-CD64-dPBD did not affect other types of hematopoietic cells including hematopoietic stem and progenitor cells, neutrophils, lymphocytes and platelets, suggesting that its off-target effects are negligible. In line with these findings, treatment with anti-CD64-dPBD directly killed proliferating monocytic leukemia cells and prevented monocytic leukemia cell generation from bone marrow progenitors of chronic myelomonocytic leukemia patients in a patient-derived xenograft model. Furthermore, by depleting the source of monocytes, treatment with anti-CD64-dPBD ultimately eliminated tumor-associated macrophages and significantly reduced tumor size in humanized mice bearing solid tumors. Given the selective action of anti-CD64-dPBD on proliferating monocyte progenitors and monocytic leukemia cells, it should be a promising tool to target cancers and other monocyte-related inflammatory disorders with minimal side effects on other cell lineages.
Asunto(s)
Antineoplásicos Inmunológicos/farmacología , Inmunoconjugados/farmacología , Células Precursoras de Monocitos y Macrófagos/efectos de los fármacos , Animales , Antineoplásicos Inmunológicos/uso terapéutico , Humanos , Inmunoconjugados/uso terapéutico , Inmunofenotipificación , Ratones , Ratones Noqueados , Ratones Transgénicos , Células Precursoras de Monocitos y Macrófagos/metabolismo , Monocitos/efectos de los fármacos , Monocitos/metabolismo , Células THP-1 , Macrófagos Asociados a Tumores/efectos de los fármacos , Macrófagos Asociados a Tumores/metabolismoRESUMEN
The physiological stresses that diminish tissue stem-cell characteristics remain largely unknown. We previously reported that type I interferon (IFN), which is essential for host antiviral responses, is a physiological stressor for hematopoietic stem cells (HSCs) and small intestinal stem cells (ISCs) and that interferon regulatory factor-2 (IRF2), which attenuates IFN signaling, maintains their stemness. Here, using a dextran sodium sulfate (DSS)-induced colitis model, we explore the role of IRF2 in maintaining colonic epithelial stem cells (CoSCs). In mice with a conditional Irf2 deletion in the intestinal epithelium (hereafter Irf2ΔIEC mice), both the number and the organoid-forming potential of CoSCs were markedly reduced. Consistent with this finding, the ability of Irf2ΔIEC mice to regenerate colon epithelium after inducing colitis was severely impaired, independently of microbial dysbiosis. Mechanistically, CoSCs differentiated prematurely into transit-amplifying (TA) cells in Irf2ΔIEC mice, which might explain their low CoSC counts. A similar phenotype was induced in wild-type mice by repeated injections of low doses of poly(I:C), which induces type I IFN. Collectively, we demonstrated that chronic IFN signaling physiologically stresses CoSCs. This study provides new insight into the development of colitis and molecular mechanisms that maintain functional CoSCs throughout life.
Asunto(s)
Autorrenovación de las Células , Colitis Ulcerosa/metabolismo , Factor 2 Regulador del Interferón/metabolismo , Interferones/metabolismo , Mucosa Intestinal/metabolismo , Células Madre/metabolismo , Estrés Fisiológico , Animales , Células Cultivadas , Colitis Ulcerosa/patología , Colon/metabolismo , Colon/patología , Factor 2 Regulador del Interferón/genética , Mucosa Intestinal/citología , Mucosa Intestinal/patología , Ratones , Ratones Endogámicos C57BL , Células Madre/fisiologíaRESUMEN
Intestinal stem cells (ISCs) are located at the crypt base and fine-tune the balance of their self-renewal and differentiation1,2, but the physiological mechanism involved in regulating that balance remains unknown. Here we describe a transcriptional regulator that preserves the stemness of ISCs by restricting their differentiation into secretory-cell lineages. Interferon regulatory factor 2 (IRF2) negatively regulates interferon signalling3, and mice completely lacking Irf24 or with a selective Irf2 deletion in their intestinal epithelial cells have significantly fewer crypt Lgr5hi ISCs than control mice. Although the integrity of intestinal epithelial cells was unimpaired at steady state in Irf2-deficient mice, regeneration of their intestinal epithelia after 5-fluorouracil-induced damage was severely impaired. Similarly, extended treatment with low-dose poly(I:C) or chronic infection of lymphocytic choriomeningitis virus clone 13 (LCMV C13)5 caused a functional decline of ISCs in wild-type mice. In contrast, massive accumulations of immature Paneth cells were found at the crypt base of Irf2-/- as well as LCMV C13-infected wild-type mice, indicating that excess interferon signalling directs ISCs towards a secretory-cell fate. Collectively, our findings indicate that regulated interferon signalling preserves ISC stemness by restricting secretory-cell differentiation.
Asunto(s)
Linaje de la Célula , Factor 2 Regulador del Interferón/metabolismo , Mucosa Intestinal/citología , Transducción de Señal , Células Madre/metabolismo , Anciano , Animales , Diferenciación Celular/genética , Linaje de la Célula/genética , Femenino , Regulación de la Expresión Génica , Humanos , Interferones/metabolismo , Mucosa Intestinal/metabolismo , Secreciones Intestinales , Masculino , Ratones , Ratones Endogámicos C57BL , Persona de Mediana Edad , Células Madre/citologíaRESUMEN
Autophagy is a lysosomal degradation pathway with important roles in physiological homeostasis and disease. We previously showed that intrinsic autophagy in intestinal stem cells (ISCs) is important for ISC homeostasis. Here we describe the detailed methods for detecting autophagy in ISCs by observing autophagosomes in GFP-LC3 transgenic mice and quantifying the p62 protein levels. We also describe methods for detecting mitophagy in these cells, by analyzing the mitochondrial transmembrane potential and reactive oxygen species (ROS) level by MitoTracker and CellROX solution, respectively.
Asunto(s)
Potencial de la Membrana Mitocondrial/fisiología , Especies Reactivas de Oxígeno/metabolismo , Animales , Autofagosomas/metabolismo , Células Epiteliales/citología , Células Epiteliales/metabolismo , Potencial de la Membrana Mitocondrial/genética , Ratones Transgénicos , Mitocondrias/metabolismo , Receptores Acoplados a Proteínas G/genética , Receptores Acoplados a Proteínas G/metabolismoRESUMEN
The small intestine has a robust regenerative capacity, and various cell types serve as "cells-of-origin" in the epithelial regeneration process after injury. However, how much each population contributes to regeneration remains unclear. Using lineage tracing, we found that Lgr5-expressing cell derivatives contained radioresistant intestinal stem cells (ISCs) crucial for epithelial regeneration in the damaged intestine after irradiation. Single-cell qRT-PCR analysis showed that surviving Lgr5-expressing cell derivatives in the damaged intestine are remarkably heterogeneous, and that the expression levels of a YAP-target gene Sca1 were inversely correlated with their "stemness", suggesting that the YAP/Wnt signal balance in surviving crypt epithelial cells determines the cellular contribution to epithelial regeneration. Single-cell RNA sequencing of Sca1-Lgr5-derivatives revealed that expression of a tetraspanin family member CD81 correlated well with the expression of ISC- and proliferation-related genes. Consistent with these findings, organoid-forming ability was confined to the CD81hiSca1- fraction within the damaged crypt epithelial cells. Characterization of radioresistant epithelial stem cell heterogeneity in the damaged intestine may contribute to therapeutic strategies for gastrointestinal diseases.
Asunto(s)
Células Epiteliales/citología , Intestinos/citología , Células Madre/citología , Animales , Ataxina-1/genética , Ataxina-1/metabolismo , Proliferación Celular , Células Epiteliales/metabolismo , Mucosa Intestinal/metabolismo , Ratones , Receptores Acoplados a Proteínas G/genética , Receptores Acoplados a Proteínas G/metabolismo , Análisis de la Célula Individual , Células Madre/metabolismoRESUMEN
Hematopoiesis is a system that provides red blood cells (RBCs), leukocytes, and platelets, which are essential for oxygen transport, biodefense, and hemostasis; its balance thus affects the outcome of various disorders. Here, we report that stem cell antigen-1 (Sca-1), a cell surface marker commonly used for the identification of multipotent hematopoietic progenitors (Lin-Sca-1+c-Kit+ cells; LSKs), is not suitable for the analysis of hematopoietic responses under biological stresses with interferon production. Lin-Sca-1-c-Kit+ cells (LKs), downstream progenitors of LSKs, acquire Sca-1 expression upon inflammation, which makes it impossible to distinguish between LSKs and LKs. As an alternative and stable marker even under such stresses, we identified CD86 by screening 180 surface markers. The analysis of infection/inflammation-triggered hematopoiesis on the basis of CD86 expression newly revealed urgent erythropoiesis producing stress-resistant RBCs and intact reconstitution capacity of LSKs, which could not be detected by conventional Sca-1-based analysis.
Asunto(s)
Antígeno B7-2/metabolismo , Infecciones Bacterianas/complicaciones , Diferenciación Celular , Hematopoyesis , Células Madre Hematopoyéticas/patología , Inflamación/complicaciones , Animales , Antígenos Ly/metabolismo , Bacterias/metabolismo , Células Cultivadas , Células Madre Hematopoyéticas/inmunología , Células Madre Hematopoyéticas/metabolismo , Células Madre Hematopoyéticas/microbiología , Lipopolisacáridos , Ratones , Ratones Endogámicos BALB C , Ratones Endogámicos C57BL , Proteínas Proto-Oncogénicas c-kit/metabolismoRESUMEN
Propolis possesses several immunological functions. We recently generated a conditional Ca2+ biosensor yellow cameleon (YC3.60) transgenic mouse line and established a five-dimensional (5D) (x, y, z, time, and Ca2+ signaling) system for intravital imaging of lymphoid tissues, including Peyer's patches (PPs). To assess the effects of propolis on immune cells, we analyzed Ca2+ signaling in vitro and in vivo using CD11c-Cre/YC3.60flox transgenic mice, in which CD11c+ dendritic cells (DCs) specifically express YC3.60. We found that propolis induced Ca2+ signaling in DCs in the PPs. Intravital imaging of PPs also showed that an intraperitoneal injection of propolis augmented Ca2+ signaling in CD11c+ cells, suggesting that propolis possesses immune-stimulating activity. Furthermore, CD11c+ cells in PPs in mice administrated propolis indicated an increase in Ca2+ signaling. Our results indicate that propolis induces immunogenicity under physiological conditions.
RESUMEN
The intestinal mucosa is a physiological barrier for most microbes, including both commensal bacteria and invading pathogens. Under homeostatic conditions, immunoglobulin A (IgA) is the major immunoglobulin isotype in the intestinal mucosa. Microbes stimulate the production of IgA, which controls bacterial translocation and neutralizes bacterial toxins at the intestinal mucosal surface. In the intestinal mucosa, dendritic cells (DCs), specialized antigen-presenting cells, regulate both T-cell-dependent (TD) and -independent (TI) immune responses. The intestinal DCs are a heterogeneous population that includes unique subsets that induce IgA synthesis in B cells. The characteristics of intestinal DCs are strongly influenced by the microenvironment, including the presence of commensal bacterial metabolites and epithelial cell-derived soluble factors. In this review, we summarize the ontogeny, classification, and function of intestinal DCs and how the intestinal microenvironment conditions DCs and their precursors to become the mucosal phenotype, in particular to regulate IgA production, after they arrive at the intestine. Understanding the mechanism of IgA synthesis could provide insights for designing effective mucosal vaccines.
Asunto(s)
Células Dendríticas/inmunología , Inmunoglobulina A/biosíntesis , Mucosa Intestinal/inmunología , Animales , Microambiente Celular , Humanos , Mucosa Intestinal/citologíaRESUMEN
Ulcerative colitis (UC) is a form of inflammatory bowel disease (IBD), which is triggered spontaneously by unknown mechanisms and manifests as chronic and relapsing inflammatory conditions in the colon. Eosinophil infiltration is often observed in the colonic tissue of ulcerative colitis patients. However, the role of eosinophils in the disease has not been well defined. The aim of this study is to investigate the role of eosinophils in colonic inflammation using the murine model of spontaneous colitis. CC chemokine receptor type 3 (CCR3) and interleukin (IL)-10 double knockout mice (CCR3-/-;IL-10-/-) were utilized to evaluate the function of eosinophils in colitis. The levels of colitis were evaluated by colonoscopy, histology, and real-time PCR measurements to determine expression levels of inflammatory cytokines in the colonic tissue. The levels of cytokines produced by T cells in mesenteric lymph nodes were evaluated by ELISA. There was no significant difference in endoscopic and histological scores between the groups of CCR3-/-;IL-10-/- mice and control CCR3+/-;IL-10-/- mice. There was also no significant difference in the expression levels of pro-inflammatory cytokines in the intestinal tissue between the two groups. Similar results were found for IL-17A and interferon gamma (IFN-γ) production from mesenteric lymph node-derived T cells. Our data indicate that eosinophils do not play a significant role in the immunopathology of colitis in IL-10-/- mice.
Asunto(s)
Eosinófilos/patología , Enfermedades Inflamatorias del Intestino/patología , Animales , Células Cultivadas , Colitis/genética , Colitis/patología , Colon/metabolismo , Colon/patología , Modelos Animales de Enfermedad , Eosinófilos/metabolismo , Enfermedades Inflamatorias del Intestino/genética , Interleucina-10/genética , Ratones Endogámicos C57BL , Ratones Noqueados , Receptores CCR3/genéticaRESUMEN
Monocytes are a widely conserved cell population in vertebrates with important roles in both inflammation and homeostasis. Under both settings, monocytes continuously arise from hematopoietic progenitors in the bone marrow and, on demand, migrate into tissues through the bloodstream. Monocytes are classified into three subsets-classical, intermediate and non-classical-based on their cell surface expression of CD14 and CD16 in humans and Ly6C, CX3CR1 and CCR2 in mice. In tissues, monocytes differentiate further into monocyte-derived macrophages and dendritic cells to mediate innate and adaptive immune responses and maintain tissue homeostasis. Recently, the progenitors that strictly give rise to monocytes were identified in both humans and mice, thereby revealing the monocyte differentiation pathways.