Selective expansion of the monocytic lineage directed by bacterial infection

An early regulatory mechanism of hyperinflammation by restricting monocyte contribution

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.

Made to order: emergency myelopoiesis and demand-adapted innate immune cell production

Definitive haematopoiesis is the process by which haematopoietic stem cells, located in the bone marrow, generate all haematopoietic cell lineages in healthy adults. Although highly regulated to maintain a stable output of blood cells in health, the haematopoietic system is capable of extensive remodelling in response to external challenges, prioritizing the production of certain cell types at the expense of others. In this Review, we consider how acute insults, such as infections and cytotoxic drug-induced myeloablation, cause molecular, cellular and metabolic changes in haematopoietic stem and progenitor cells at multiple levels of the haematopoietic hierarchy to drive accelerated production of the mature myeloid cells needed to resolve the initiating insult. Moreover, we discuss how dysregulation or subversion of these emergency myelopoiesis mechanisms contributes to the progression of chronic inflammatory diseases and cancer.

ExoS Effector in Pseudomonas aeruginosa Hyperactive Type III Secretion System Mutant Promotes Enhanced Plasma Membrane Rupture in Neutrophils

Pseudomonas aeruginosa is an opportunistic bacterial pathogen responsible for a large percentage of airway infections that cause morbidity and mortality in immunocompromised patients, especially those with cystic fibrosis (CF). One important P. aeruginosa virulence factor is a type III secretion system (T3SS) that translocates effectors into host cells. ExoS is a T3SS effector with ADP ribosyltransferase (ADPRT) activity. The ADPRT activity of ExoS promotes P. aeruginosa virulence by inhibiting phagocytosis and limiting the oxidative burst in neutrophils. The P. aeruginosa T3SS also translocates flagellin, which can activate the NLRC4 inflammasome, resulting in: 1) gasdermin-D (GSDMD) pores, release of IL-1β and pyroptosis; and 2) histone 3 citrullination (CitH3) and decondensation and expansion of nuclear DNA into the cytosol. However, recent studies with the P. aeruginosa laboratory strain PAO1 indicate that ExoS ADPRT activity inhibits activation of the NLRC4 inflammasome in neutrophils. Here, an ExoS+ CF clinical isolate of P. aeruginosa with a hyperactive T3SS was identified. Variants of the hyperactive T3SS mutant or PAO1 were used to infect neutrophils from C57BL/6 mice or mice engineered to have a CF genotype or a defect in inflammasome assembly. Responses to NLRC4 inflammasome assembly or ExoS ADPRT activity were assayed, results of which were found to be similar for C57BL/6 or CF neutrophils. The hyperactive T3SS mutant had enhanced resistance to neutrophil killing, like previously identified hypervirulent P. aeruginosa isolates. ExoS ADPRT activity in the hyperactive T3SS mutant regulated inflammasome and nuclear DNA decondensation responses like PAO1 but promoted enhanced CitH3 and plasma membrane rupture (PMR). Glycine supplementation inhibited PMR caused by the hyperactive T3SS mutant, suggesting ninjurin-1 is required for this process. These results identify enhanced neutrophil PMR as a pathogenic activity of ExoS ADPRT in a hypervirulent P. aeruginosa isolate.

Modulation of haematopoiesis by protozoal and helminth parasites

During inflammation, haematopoietic stem cells (HSCs) in the bone marrow (BM) and periphery rapidly expand and preferentially differentiate into myeloid cells that mediate innate immune responses. HSCs can be directed into quiescence or differentiation by sensing alterations to the haematopoietic niche, including cytokines, chemokines, and pathogen-derived products. Most studies attempting to identify the mechanisms of haematopoiesis have focused on bacterial and viral infections. From intracellular protozoan infections to large multicellular worms, parasites are a global health burden and represent major immunological challenges that remain poorly defined in the context of haematopoiesis. Immune responses to parasites vary drastically, and parasites have developed sophisticated immunomodulatory mechanisms that allow development of chronic infections. Recent advances in imaging, genomic sequencing, and mouse models have shed new light on how parasites induce unique forms of emergency haematopoiesis. In addition, parasites can modify the haematopoiesis in the BM and periphery to improve their survival in the host. Parasites can also induce long-lasting modifications to HSCs, altering future immune responses to infection, inflammation or transplantation, a term sometimes referred to as central trained immunity. In this review, we highlight the current understanding of parasite-induced haematopoiesis and how parasites target this process to promote chronic infections.

Proliferation of monocytes and macrophages in homeostasis, infection, injury, and disease

Monocytes (Mo) and macrophages (Mφ) play important roles in the function of tissues, organs, and systems of all animals during homeostasis, infection, injury, and disease. For decades, conventional wisdom has dictated that Mo and Mφ are end-stage cells that do not proliferate and that Mφ accumulation in tissues is the result of infiltration of Mo from the blood and subsequent differentiation to Mφ. However, reports from the early 1900s to the present describe evidence of Mo and Mφ proliferation in different tissues and contexts. The purpose of this review is to summarize both historical and current evidence for the contribution of Mφ proliferation to their accumulation in different tissues during homeostasis, infection, injury, and disease. Mφ proliferate in different organs and tissues, including skin, peritoneum, lung, heart, aorta, kidney, liver, pancreas, brain, spinal cord, eye, adipose tissue, and uterus, and in different species including mouse, rat, rabbit, and human. Mφ can proliferate at different stages of differentiation with infiltrating Mo-like cells proliferating in certain inflammatory contexts (e.g. skin wounding, kidney injury, bladder and liver infection) and mature resident Mφ proliferating in other inflammatory contexts (e.g. nematode infection, acetaminophen liver injury) and during homeostasis. The pathways involved in stimulating Mφ proliferation also may be context dependent, with different cytokines and transcription factors implicated in different studies. Although Mφ are known to proliferate in health, injury, and disease, much remains to be learned about the regulation of Mφ proliferation in different contexts and its impact on the homeostasis, injury, and repair of different organs and tissues.

TLR7 activation at epithelial barriers promotes emergency myelopoiesis and lung antiviral immunity

Monocytes are heterogeneous innate effector leukocytes generated in the bone marrow and released into circulation in a CCR2-dependent manner. During infection or inflammation, myelopoiesis is modulated to rapidly meet the demand for more effector cells. Danger signals from peripheral tissues can influence this process. Herein we demonstrate that repetitive TLR7 stimulation via the epithelial barriers drove a potent emergency bone marrow monocyte response in mice. This process was unique to TLR7 activation and occurred independently of the canonical CCR2 and CX3CR1 axes or prototypical cytokines. The monocytes egressing the bone marrow had an immature Ly6C-high profile and differentiated into vascular Ly6C-low monocytes and tissue macrophages in multiple organs. They displayed a blunted cytokine response to further TLR7 stimulation and reduced lung viral load after RSV and influenza virus infection. These data provide insights into the emergency myelopoiesis likely to occur in response to the encounter of single-stranded RNA viruses at barrier sites.

ThPOK is a critical multifaceted regulator of myeloid lineage development

The transcription factor ThPOK (encoded by Zbtb7b) is well known for its role as a master regulator of CD4 lineage commitment in the thymus. Here, we report an unexpected and critical role of ThPOK as a multifaceted regulator of myeloid lineage commitment, differentiation and maturation. Using reporter and knockout mouse models combined with single-cell RNA-sequencing, progenitor transfer and colony assays, we show that ThPOK controls monocyte-dendritic cell versus granulocyte lineage production during homeostatic differentiation, and serves as a brake for neutrophil maturation in granulocyte lineage-specified cells through transcriptional regulation of lineage-specific transcription factors and RNA via altered messenger RNA splicing to reprogram intron retention.

The role of the microbiota in myelopoiesis during homeostasis and inflammation

The microbiota engages in the development and maintenance of the host immune system. The microbiota affects not only mucosal tissues where it localizes but also the distal organs. Myeloid cells are essential for host defense as first responders of the host immune system. Their generation, called myelopoiesis, is regulated by environmental signals, including commensal microbiota. Hematopoietic stem and progenitor cells in bone marrow can directly or indirectly sense microbiota-derived signals, thereby giving rise to myeloid cell lineages at steady-state and during inflammation. In this review, we discuss the role of commensal microorganisms in the homeostatic regulation of myelopoiesis in the bone marrow. We also outline the effects of microbial signals on myelopoiesis during inflammation and infection, with a particular focus on the development of innate immune memory. Studying the relationship between the microbiota and myelopoiesis will help us understand how the microbiota regulates immune responses at a systemic level beyond the local mucosa.

Type I Interferon Orchestrates Demand-Adapted Monopoiesis during Influenza A Virus Infection via STAT1-Mediated Upregulation of Macrophage Colony-Stimulating Factor Receptor Expression

Whether and how a local virus infection affects the hematopoietic system in the bone marrow is largely unknown, unlike with systemic infection. In this study, we showed that influenza A virus (IAV) infection leads to demand-adapted monopoiesis in the bone marrow. The beta interferon (IFN-β) promoter stimulator 1 (IPS-1)-type I IFN-IFN-α receptor 1 (IFNAR1) axis-mediated signaling was found to induce the emergency expansion of the granulocyte-monocyte progenitor (GMP) population and upregulate the expression of the macrophage colony-stimulating factor receptor (M-CSFR) on bipotent GMPs and monocyte progenitors via the signal transducer and activator of transcription 1 (STAT1), leading to a scaled-back proportion of granulocyte progenitors. To further address the influence of demand-adapted monopoiesis on IAV-induced secondary bacterial infection, IAV-infected wild-type (WT) and Stat1-/- mice were challenged with Streptococcus pneumoniae. Compared with WT mice, Stat1-/- mice did not demonstrate demand-adapted monopoiesis, had more infiltrating granulocytes, and were able to effectively eliminate the bacterial infection. IMPORTANCE Our findings show that influenza A virus infection induces type I interferon (IFN)-mediated emergency hematopoiesis to expand the GMP population in the bone marrow. The type I IFN-STAT1 axis was identified as being involved in mediating the viral-infection-driven demand-adapted monopoiesis by upregulating M-CSFR expression in the GMP population. As secondary bacterial infections often manifest during a viral infection and can lead to severe or even fatal clinical complications, we further assessed the impact of the observed monopoiesis on bacterial clearance. Our results suggest that the resulting decrease in the proportion of granulocytes may play a role in diminishing the IAV-infected host's ability to effectively clear secondary bacterial infection. Our findings not only provide a more complete picture of the modulatory functions of type I IFN but also highlight the need for a more comprehensive understanding of potential changes in hematopoiesis during local infections to better inform clinical interventions.

Cutting Edge: Neutrophils License the Maturation of Monocytes into Effective Antifungal Effectors

Neutrophils are critical for the direct eradication of Aspergillus fumigatus conidia, but whether they mediate antifungal defense beyond their role as effectors is unclear. In this study, we demonstrate that neutrophil depletion impairs the activation of protective antifungal CCR2+ inflammatory monocytes. In the absence of neutrophils, monocytes displayed limited differentiation into monocyte-derived dendritic cells, reduced formation of reactive oxygen species, and diminished conidiacidal activity. Upstream regulator analysis of the transcriptional response in monocytes predicted a loss of STAT1-dependent signals as the potential basis for the dysfunction seen in neutrophil-depleted mice. We find that conditional removal of STAT1 on CCR2+ cells results in diminished antifungal monocyte responses, whereas exogenous administration of IFN-γ to neutrophil-depleted mice restores monocyte-derived dendritic cell maturation and reactive oxygen species production. Altogether, our findings support a critical role for neutrophils in antifungal immunity not only as effectors but also as important contributors to antifungal monocyte activation, in part by regulating STAT1-dependent functions.

Gingival monocytes: Lessons from other barriers

Unlike other non-lymphoid tissues monocytes comprise a large proportion of mononuclear phagocytes present within the gingiva. Their functions and fate remain poorly understood. The oral mucosa faces challenges common to all barrier surfaces, including constant exposure to antigens and the resident commensal bacteria, but also experiences ongoing mechanical damage from mastication. Gingiva monocytes may therefore possess both myeloid functions observed at other barrier sites, such as hypo-responsiveness to bacterial stimulation, and distinctive functions tailored by their unique environment. In this review, we discuss the establishment and function of monocytes and macrophages at several mucosal tissues, and posit potential functions of monocytes within the gingiva tissue.

Role of the Yersinia pseudotuberculosis Virulence Plasmid in Pathogen-Phagocyte Interactions in Mesenteric Lymph Nodes

Yersinia pseudotuberculosis is an Enterobacteriaceae family member that is commonly transmitted by the fecal-oral route to cause infections. From the small intestine, Y. pseudotuberculosis can invade through Peyer's patches and lymph vessels to infect the mesenteric lymph nodes (MLNs). Infection of MLNs by Y. pseudotuberculosis results in the clinical presentation of mesenteric lymphadenitis. MLNs are important for immune responses to intestinal pathogens and microbiota in addition to their clinical relevance to Y. pseudotuberculosis infections. A characteristic of Y. pseudotuberculosis infection in MLNs is the formation of pyogranulomas. Pyogranulomas are composed of neutrophils, inflammatory monocytes, and lymphocytes surrounding extracellular microcolonies of Y. pseudotuberculosis. Key elements of the complex pathogen-host interaction in MLNs have been identified using mouse infection models. Y. pseudotuberculosis requires the virulence plasmid pYV to induce the formation of pyogranulomas in MLNs. The YadA adhesin and the Ysc-Yop type III secretion system (T3SS) are encoded on pYV. YadA mediates bacterial binding to host receptors, which engages the T3SS to preferentially translocate seven Yop effectors into phagocytes. The effectors promote pathogenesis by blocking innate immune defenses such as superoxide production, degranulation, and inflammasome activation, resulting in survival and growth of Y. pseudotuberculosis. On the other hand, certain effectors can trigger immune defenses in phagocytes. For example, YopJ triggers activation of caspase-8 and an apoptotic cell death response in monocytes within pyogranulomas that limits dissemination of Y. pseudotuberculosis from MLNs to the bloodstream. YopE can be processed as an antigen by phagocytes in MLNs, resulting in T and B cell responses to Y. pseudotuberculosis. Immune responses to Y. pseudotuberculosis in MLNs can also be detrimental to the host in the form of chronic lymphadenopathy. This review focuses on interactions between Y. pseudotuberculosis and phagocytes mediated by pYV that concurrently promote pathogenesis and host defense in MLNs. We propose that MLN pyogranulomas are immunological arenas in which opposing pYV-driven forces determine the outcome of infection in favor of the pathogen or host.

Systemic bacterial infections affect dendritic cell development and function

Dendritic cells (DCs) are critical in host defense against infection. DC depletion is an early event in the course of sepsis that may impair the host defense mechanisms. Here, we addressed whether DC depletion and dysfunction are pathogen-independent, mediated via pattern recognition receptors, and are due to impaired DC development upon systemic infection with the Gram-negative bacterium Escherichia coli and the Gram-positive pathogen Staphylococcus aureus. Infection with E. coli and S. aureus led to reduced numbers of splenic DC subsets and of DC progenitors in the bone marrow (BM) with this effect persisting significantly longer in mice infected with S. aureus than with E. coli. The reduction of DC subsets and their progenitors was mainly TLR-independent as was the infection-induced monopoiesis. Moreover, de novo DC development was impaired in mice infected with S. aureus, and BM cells from E. coli or S. aureus infected mice favored macrophage differentiation in vitro. As a consequence of reduced DC numbers and their reduced expression of MHC II less CD4⁺ and CD8⁺ T cells, especially Th1 and IFN-γ producing CD8⁺ T cells, could be detected in S. aureus compared to E. coli infected mice. These differences are reflected in the rapid killing of E. coli as opposed to an increase in bacterial load in S. aureus. In summary, our study supports the idea that systemic bacterial infections generally affect the number and development of DCs and thereby the T cell responses, but the magnitude is pathogen-dependent.

Potential Tamoxifen Repurposing to Combat Infections by Multidrug-Resistant Gram-Negative Bacilli

The development of new strategic therapies for multidrug-resistant bacteria, like the use of non-antimicrobial approaches and/or drugs repurposed to be used as monotherapies or in combination with clinically relevant antibiotics, has become urgent. A therapeutic alternative for infections by multidrug-resistant Gram-negative bacilli (MDR-GNB) is immune system modulation to improve the infection clearance. We showed that immunocompetent mice pretreated with tamoxifen at 80 mg/kg/d for three days and infected with Acinetobacter baumannii, Pseudomonas aeruginosa, or Escherichia coli in peritoneal sepsis models showed reduced release of the monocyte chemotactic protein-1 (MCP-1) and its signaling pathway interleukin-18 (IL-18), and phosphorylated extracellular signal-regulated kinase 1/2 (ERK1/2). This reduction of MCP-1 induced the reduction of migration of inflammatory monocytes and neutrophils from the bone marrow to the blood. Indeed, pretreatment with tamoxifen in murine peritoneal sepsis models reduced the bacterial load in tissues and blood, and increased mice survival from 0% to 60–100%. Together, these data show that tamoxifen presents therapeutic efficacy against MDR A. baumannii, P. aeruginosa, and E. coli in experimental models of infection and may be a new candidate to be repurposed as a treatment for GNB infections.

In situ mapping identifies distinct vascular niches for myelopoiesis

In contrast to nearly all other tissues, the anatomy of cell differentiation in the bone marrow remains unknown. This is owing to a lack of strategies for examining myelopoiesis-the differentiation of myeloid progenitors into a large variety of innate immune cells-in situ in the bone marrow. Such strategies are required to understand differentiation and lineage-commitment decisions, and to define how spatial organizing cues inform tissue function. Here we develop approaches for imaging myelopoiesis in mice, and generate atlases showing the differentiation of granulocytes, monocytes and dendritic cells. The generation of granulocytes and dendritic cells-monocytes localizes to different blood-vessel structures known as sinusoids, and displays lineage-specific spatial and clonal architectures. Acute systemic infection with Listeria monocytogenes induces lineage-specific progenitor clusters to undergo increased self-renewal of progenitors, but the different lineages remain spatially separated. Monocyte-dendritic cell progenitors (MDPs) map with nonclassical monocytes and conventional dendritic cells; these localize to a subset of blood vessels expressing a major regulator of myelopoiesis, colony-stimulating factor 1 (CSF1, also known as M-CSF)1. Specific deletion of Csf1 in endothelium disrupts the architecture around MDPs and their localization to sinusoids. Subsequently, there are fewer MDPs and their ability to differentiate is reduced, leading to a loss of nonclassical monocytes and dendritic cells during both homeostasis and infection. These data indicate that local cues produced by distinct blood vessels are responsible for the spatial organization of definitive blood cell differentiation.

Lymph node stromal CCL2 limits antibody responses

Nonhematopoietic stromal cells in lymph nodes such as fibroblastic reticular cells (FRCs) can support the survival of plasmablasts and plasma cells [together, antibody-forming cells (AFCs)]. However, a regulatory function for the stromal compartment in AFC accumulation has not been appreciated. Here, we show that chemokine ligand 2 (CCL2)-expressing stromal cells limit AFC survival. FRCs express high levels of CCL2 in vessel-rich areas of the T cell zone and the medulla, where AFCs are located. FRC CCL2 is up-regulated during AFC accumulation, and we use lymph node transplantation to show that CCL2 deficiency in BP3⁺ FRCs and lymphatic endothelial cells increases AFC survival without affecting B or germinal center cell numbers. Monocytes are key expressers of the CCL2 receptor CCR2, as monocyte depletion and transfer late in AFC responses increases and decreases AFC accumulation, respectively. Monocytes express reactive oxygen species (ROS) in an NADPH oxidase 2 (NOX2)-dependent manner, and NOX2-deficient monocytes fail to reduce AFC numbers. Stromal CCL2 modulates both monocyte accumulation and ROS production, and is regulated, in part, by manipulations that modulate vascular permeability. Together, our results reveal that the lymph node stromal compartment, by influencing monocyte accumulation and functional phenotype, has a regulatory role in AFC survival. Our results further suggest a role for inflammation-induced vascular activity in tuning the lymph node microenvironment. The understanding of stromal-mediated AFC regulation in vessel-rich environments could potentially be harnessed to control antibody-mediated autoimmunity.

Identification of distinct immune activation profiles in adult humans

Latent infectious agents, microbial translocation, some metabolites and immune cell subpopulations, as well as senescence modulate the level and quality of activation of our immune system. Here, we tested whether various in vivo immune activation profiles may be distinguished in a general population. We measured 43 markers of immune activation by 8-color flow cytometry and ELISA in 150 adults, and performed a double hierarchical clustering of biomarkers and volunteers. We identified five different immune activation profiles. Profile 1 had a high proportion of naïve T cells. By contrast, Profiles 2 and 3 had an elevated percentage of terminally differentiated and of senescent CD4+ T cells and CD8+ T cells, respectively. The fourth profile was characterized by NK cell activation, and the last profile, Profile 5, by a high proportion of monocytes. In search for etiologic factors that could determine these profiles, we observed a high frequency of naïve Treg cells in Profile 1, contrasting with a tendency to a low percentage of Treg cells in Profiles 2 and 3. Moreover, Profile 5 tended to have a high level of 16s ribosomal DNA, a direct marker of microbial translocation. These data are compatible with a model in which specific causes, as the frequency of Treg or the level of microbial translocation, shape specific profiles of immune activation. It will be of interest to analyze whether some of these profiles drive preferentially some morbidities known to be fueled by immune activation, as insulin resistance, atherothrombosis or liver steatosis.

The Role of the Bone Marrow Microenvironment in the Response to Infection

Hematopoiesis in the bone marrow (BM) is the primary source of immune cells. Hematopoiesis is regulated by a diverse cellular microenvironment that supports stepwise differentiation of multipotent stem cells and progenitors into mature blood cells. Blood cell production is not static and the bone marrow has evolved to sense and respond to infection by rapidly generating immune cells that are quickly released into the circulation to replenish those that are consumed in the periphery. Unfortunately, infection also has deleterious effects injuring hematopoietic stem cells (HSC), inefficient hematopoiesis, and remodeling and destruction of the microenvironment. Despite its central role in immunity, the role of the microenvironment in the response to infection has not been systematically investigated. Here we summarize the key experimental evidence demonstrating a critical role of the bone marrow microenvironment in orchestrating the bone marrow response to infection and discuss areas of future research.

Macrophages Orchestrate Hematopoietic Programs and Regulate HSC Function During Inflammatory Stress

The bone marrow contains distinct cell types that work in coordination to generate blood and immune cells, and it is the primary residence of hematopoietic stem cells (HSCs) and more committed multipotent progenitors (MPPs). Even at homeostasis the bone marrow is a dynamic environment where billions of cells are generated daily to replenish short-lived immune cells and produce the blood factors and cells essential for hemostasis and oxygenation. In response to injury or infection, the marrow rapidly adapts to produce specific cell types that are in high demand revealing key insight to the inflammatory nature of "demand-adapted" hematopoiesis. Here we focus on the role that resident and monocyte-derived macrophages play in driving these hematopoietic programs and how macrophages impact HSCs and downstream MPPs. Macrophages are exquisite sensors of inflammation and possess the capacity to adapt to the environment, both promoting and restraining inflammation. Thus, macrophages hold great potential for manipulating hematopoietic output and as potential therapeutic targets in a variety of disease states where macrophage dysfunction contributes to or is necessary for disease. We highlight essential features of bone marrow macrophages and discuss open questions regarding macrophage function, their role in orchestrating demand-adapted hematopoiesis, and mechanisms whereby they regulate HSC function.

IRAK-M Regulates Monocyte Trafficking to the Lungs in Response to Bleomycin Challenge

Idiopathic pulmonary fibrosis is a deadly disease characterized by excessive extracellular matrix deposition in the lungs, resulting in decreased pulmonary function. Although epithelial cells and fibroblasts have long been the focus of idiopathic pulmonary fibrosis research, the role of various subpopulations of macrophages in promoting a fibrotic response is an emerging target. Healthy lungs are composed of two macrophage populations, tissue-resident alveolar macrophages and interstitial macrophages, which help to maintain homeostasis. After injury, tissue-resident alveolar macrophages are depleted, and monocytes from the bone marrow (BM) traffic to the lungs along a CCL2/CCR2 axis and differentiate into monocyte-derived alveolar macrophages (Mo-AMs), which is a cell population implicated in murine models of pulmonary fibrosis. In this study, we sought to determine how IL-1R-associated kinase-M (IRAK-M), a negative regulator of TLR signaling, modulates monocyte trafficking into the lungs in response to bleomycin. Our data indicate that after bleomycin challenge, mice lacking IRAK-M have decreased monocyte trafficking and reduced Mo-AMs in their lungs. Although IRAK-M expression did not regulate differences in chemokines, cytokines, or adhesion molecules associated with monocyte recruitment, IRAK-M was necessary for CCR2 upregulation following bleomycin challenge. This finding prompted us to develop a competitive BM chimera model, which demonstrated that expression of BM-derived IRAK-M was necessary for monocyte trafficking into the lung and for subsequent enhanced collagen deposition. These data indicate that IRAK-M regulates monocyte trafficking by increasing the expression of CCR2, resulting in enhanced monocyte translocation into the lung, Mo-AM differentiation, and development of pulmonary fibrosis.

Dendritic cell development in infection

The immune system protects from infections primarily by detecting and eliminating invading pathogens. This is predominantly mediated by innate immune cells like neutrophils, monocytes and dendritic cells (DCs) expressing specific receptors recognizing pathogen-associated molecular patterns. DC activation by pathogens leads to the initiation of antigen-specific adaptive immune responses, thereby bridging the innate and adaptive immune systems. However, various pathogens have evolved immune evasion strategies to ensure their survival. In this review, we highlight recent findings on how various microorganisms or their structural features affect or modulate DC development and whether this has any consequences for a protective immune response.

Emergence of immunoregulatory Ym1+Ly6Chi monocytes during recovery phase of tissue injury

Ly6C^hi monocytes migrate to injured sites and induce inflammation in the acute phase of tissue injury. However, once the causes of tissue injury are eliminated, monocyte-derived macrophages contribute to the resolution of inflammation and tissue repair. It remains unclear whether the emergence of these immunoregulatory macrophages is attributed to the phenotypic conversion of inflammatory monocytes in situ or to the recruitment of bone marrow-derived regulatory cells de novo. Here, we identified a subpopulation of Ly6C^hi monocytes that contribute to the resolution of inflammation and tissue repair. Ym1⁺Ly6C^hi monocytes greatly expanded in bone marrow during the recovery phase of systemic inflammation or tissue injury. Ym1⁺Ly6C^hi monocytes infiltrating into an injured site exhibited immunoregulatory and tissue-reparative phenotypes. Deletion of Ym1⁺Ly6C^hi monocytes resulted in delayed recovery from colitis. These results demonstrate that a distinct monocyte subpopulation destined to act in immunoregulation is generated in bone marrow and participates in resolution of inflammation and tissue repair.

Candida/Staphylococcal Polymicrobial Intra-Abdominal Infection: Pathogenesis and Perspectives for a Novel Form of Trained Innate Immunity

Polymicrobial sepsis is difficult to diagnose and treat and causes significant morbidity and mortality, especially when fungi are involved. In vitro, synergism between Candida albicans and various bacterial species has been described for many years. Our laboratory has developed a murine model of polymicrobial intra-abdominal infection with Candida albicans and Staphylococcus aureus, demonstrating that polymicrobial infections cause high levels of mortality, while monoinfections do not. By contrast, closely related Candida dubliniensis does not cause synergistic lethality and rather provides protection against lethal polymicrobial infection. This protection is thought to be driven by a novel form of trained innate immunity mediated by myeloid-derived suppressor cells (MDSCs), which we are proposing to call "trained tolerogenic immunity". MDSC accumulation has been described in patients with sepsis, as well as in in vivo sepsis models. However, clinically, MDSCs are considered detrimental in sepsis, while their role in in vivo models differs depending on the sepsis model and timing. In this review, we will discuss the role of MDSCs in sepsis and infection and summarize our perspectives on their development and function in the spectrum of trained innate immune protection against fungal-bacterial sepsis.

Nonclassical Monocytes in Health and Disease

Monocytes are innate blood cells that maintain vascular homeostasis and are early responders to pathogens in acute infections. There are three well-characterized classes of monocytes: classical (CD14+CD16−in humans and Ly6Chiin mice), intermediate (CD14+CD16+in humans and Ly6C+Treml4+in mice), and nonclassical (CD14−CD16+in humans and Ly6Cloin mice). Classical monocytes are critical for the initial inflammatory response. Classical monocytes can differentiate into macrophages in tissue and can contribute to chronic disease. Nonclassical monocytes have been widely viewed as anti-inflammatory, as they maintain vascular homeostasis. They are a first line of defense in recognition and clearance of pathogens. However, their roles in chronic disease are less clear. They have been shown to be protective as well as positively associated with disease burden. This review focuses on the state of the monocyte biology field and the functions of monocytes, particularly nonclassical monocytes, in health and disease.

Inflammatory monocytes are detrimental to the host immune response during acute infection with Cryptococcus neoformans

Cryptococcus neoformans is a leading cause of invasive fungal infections among immunocompromised patients. However, the cellular constituents of the innate immune response that promote clearance versus progression of infection upon respiratory acquisition of C. neoformans remain poorly defined. In this study, we found that during acute C. neoformans infection, CCR2⁺ Ly6C^hi inflammatory monocytes (IM) rapidly infiltrate the lungs and mediate fungal trafficking to lung-draining lymph nodes. Interestingly, this influx of IM is detrimental to the host, since ablating IM or impairing their recruitment to the lungs improves murine survival and reduces fungal proliferation and dissemination. Using a novel conditional gene deletion strategy, we determined that MHC class II expression by IM did not mediate their deleterious impact on the host. Furthermore, although ablation of IM reduced the number of lymphocytes, innate lymphoid cells, and eosinophils in the lungs, the effects of IM were not dependent on these cells. We ascertained that IM in the lungs upregulated transcripts associated with alternatively activated (M2) macrophages in response to C. neoformans, consistent with the model that IM assume a cellular phenotype that is permissive for fungal growth. We also determined that conditional knockout of the prototypical M2 marker arginase 1 in IM and deletion of the M2-associated transcription factor STAT6 were not sufficient to reverse the harmful effects of IM. Overall, our findings indicate that C. neoformans can subvert the fungicidal potential of IM to enable the progression of infection through a mechanism that is not dependent on lymphocyte priming, eosinophil recruitment, or downstream M2 macrophage polarization pathways. These results give us new insight into the plasticity of IM function during fungal infections and the level of control that C. neoformans can exert on host immune responses.

Cryptococcus neoformans is a fungus that is prevalent throughout the environment and can cause a fatal infection of the central nervous system when inhaled into the lungs by patients with impaired immune systems. Our understanding of the immune responses that either help clear C. neoformans from the lungs or permit development of disease remains limited. In this study, we used a mouse model of lethal C. neoformans infection to determine that inflammatory monocytes, immune cells that are often among the first responders to infections, actually facilitate the progression of infection rather than clearance. These findings establish a foundation for future work to target the immune response of inflammatory monocytes as a strategy to improve the outcomes of patients that develop C. neoformans infections.

Polypropylene mesh implantation for hernia repair causes myeloid cell-driven persistent inflammation

Polypropylene meshes that are commonly used for inguinal hernia repair may trigger granulomatous foreign body reactions. Here, we show that asymptomatic patients display mesh-associated inflammatory granulomas long after surgery, which are dominated by monocyte-derived macrophages expressing high levels of inflammatory activation markers. In mice, mesh implantation by the onlay technique induced rapid and strong myeloid cell accumulation, without substantial attenuation for up to 90 days. Myeloid cells segregated into distinct macrophage subsets with separate spatial distribution, activation profiles, and functional properties, showing a stable inflammatory phenotype in the tissue surrounding the biomaterial and a mixed, wound-healing phenotype in the surrounding stromal tissue. Protein mass spectrometry confirmed the inflammatory nature of the foreign body reaction, as characterized by cytokines, complement activation, and matrix-modulating factors. Moreover, immunoglobulin deposition increased over time around the implant, arguing for humoral immune responses in association with the cell-driven inflammation. Intravital multiphoton microscopy revealed a high motility and continuous recruitment of myeloid cells, which is partly dependent on the chemokine receptor CCR2. CCR2-dependent macrophages are particular drivers of fibroblast proliferation. Thus, our work functionally characterizes myeloid cell-dependent inflammation following mesh implantation, thereby providing insights into the dynamics and mechanisms of foreign body reactions to implanted biomaterials.

Alteration of Flt3-Ligand-dependent de novo generation of conventional dendritic cells during influenza infection contributes to respiratory bacterial superinfection

Secondary bacterial infections contribute to the excess morbidity and mortality of influenza A virus (IAV) infection. Disruption of lung integrity and impaired antibacterial immunity during IAV infection participate in colonization and dissemination of the bacteria out of the lungs. One key feature of IAV infection is the profound alteration of lung myeloid cells, characterized by the recruitment of deleterious inflammatory monocytes. We herein report that IAV infection causes a transient decrease of lung conventional dendritic cells (cDCs) (both cDC1 and cDC2) peaking at day 7 post-infection. While triggering emergency monopoiesis, IAV transiently altered the differentiation of cDCs in the bone marrow, the cDC1-biaised pre-DCs being particularly affected. The impaired cDC differentiation during IAV infection was independent of type I interferons (IFNs), IFN-γ, TNFα and IL-6 and was not due to an intrinsic dysfunction of cDC precursors. The alteration of cDC differentiation was associated with a drop of local and systemic production of Fms-like tyrosine kinase 3 ligand (Flt3-L), a critical cDC differentiation factor. Overexpression of Flt3-L during IAV infection boosted the cDC progenitors' production in the BM, replenished cDCs in the lungs, decreased inflammatory monocytes' infiltration and lowered lung damages. This was associated with partial protection against secondary pneumococcal infection, as reflected by reduced bacterial dissemination and prolonged survival. These findings highlight the impact of distal viral infection on cDC genesis in the BM and suggest that Flt3-L may have potential applications in the control of secondary infections.

Type III interferon is a critical regulator of innate antifungal immunity

Type III interferons (IFN-λs) are the most recently found members of the IFN cytokine family and engage IFNLR1 and IL10R2 receptor subunits to activate innate responses against viruses. We have identified IFN-λs as critical instructors of antifungal neutrophil responses. Using Aspergillus fumigatus (Af) as a model to study antifungal immune responses, we found that depletion of CCR2⁺ monocytes compromised the ability of neutrophils to control invasive fungal growth. Using an unbiased approach, we identified type I and III IFNs as critical regulators of the interplay between monocytes and neutrophils responding to Af We found that CCR2⁺ monocytes are an important early source of type I IFNs that prime optimal expression of IFN-λ. Type III IFNs act directly on neutrophils to activate their antifungal response, and mice with neutrophil-specific deletion of IFNLR1 succumb to invasive aspergillosis. Dysfunctional neutrophil responses in CCR2-depleted mice were rescued by adoptive transfer of pulmonary CCR2⁺ monocytes or by exogenous administration of IFN-α and IFN-λ. Thus, CCR2⁺ monocytes promote optimal activation of antifungal neutrophils by initiating a coordinated IFN response. We have identified type III IFNs as critical regulators of neutrophil activation and type I IFNs as early stimulators of IFN-λ expression.

Understanding the Cellular Origin of the Mononuclear Phagocyte System Sheds Light on the Myeloid Postulate of Immune Paralysis in Sepsis

Sepsis, in essence, is a serious clinical condition that can subsequently result in death as a consequence of a systemic inflammatory response syndrome including febrile leukopenia, hypotension, and multiple organ failures. To date, such life-threatening organ dysfunction remains one of the leading causes of death in intensive care units, with an increasing incidence rate worldwide and particularly within the rapidly growing senior population. While most of the clinical trials are aimed at dampening the overwhelming immune response to infection that spreads through the bloodstream, based on several human immunological investigations, it is now widely accepted that susceptibility to nosocomial infections and long-term sepsis mortality involves an immunosuppressive phase that is characterized by a decrease in some subsets of dendritic cells (DCs). Only recently substantial advances have been made in terms of the origin of the mononuclear phagocyte system that is now likely to allow for a better understanding of how the paralysis of DCs leads to sepsis-related death. Indeed, the unifying view of each subset of DCs has already improved our understanding of the pivotal pathways that contribute to the shift in commitment of their progenitors that originate from the bone marrow. It is quite plausible that this anomaly in sepsis may occur at the single level of DC-committed precursors, and elucidating the immunological basis for such a derangement during the ontogeny of each subset of DCs is now of particular importance for restoring an adequate cell fate decision to their vulnerable progenitors. Last but not least, it provides a direct perspective on the development of sophisticated myelopoiesis-based strategies that are currently being considered for the treatment of immunosenescence within different tissue microenvironments, such as the kidney and the spleen.

The F-Box Protein Fbp1 Shapes the Immunogenic Potential of Cryptococcus neoformans

Cryptococcus neoformans is the main etiologic agent of cryptococcal meningitis and causes a significant number of deadly infections per year. Although it is well appreciated that host immune responses are crucial for defense against cryptococcosis, our understanding of factors that control the development of effective immunity to this fungus remains incomplete. In previous studies, we identified the F-box protein Fbp1 as a novel determinant of C. neoformans virulence. In this study, we found that the hypovirulence of the fbp1Δ mutant is linked to the development of a robust host immune response. Infection with the fbp1Δ mutant induces a rapid influx of CCR2⁺ monocytes and their differentiation into monocyte-derived dendritic cells (mo-DCs). Depletion of CCR2⁺ monocytes and their derivative mo-DCs resulted in impaired activation of a protective inflammatory response and the rapid death of mice infected with the fbp1Δ mutant. Mice lacking B and T cells also developed fungal meningitis and succumbed to infection with the fbp1Δ mutant, demonstrating that adaptive immune responses to the fbp1Δ mutant help to maintain the long-term survival of the host. Adaptive immune responses to the fbp1Δ mutant were characterized by enhanced differentiation of Th1 and Th17 CD4⁺ T cells together with diminished Th2 responses compared to the H99 parental strain. Importantly, we found that the enhanced immunogenicity of fbp1Δ mutant yeast cells can be harnessed to confer protection against a subsequent infection with the virulent H99 parental strain. Altogether, our findings suggest that Fbp1 functions as a novel virulence factor that shapes the immunogenicity of C. neoformansIMPORTANCECryptococcus neoformans is the most common cause of deadly fungal meningitis, with over 270,000 infections per year. Immune responses are critically required for the prevention of cryptococcosis, and patients with impaired immunity and low CD4⁺ T cell numbers are at high risk of developing these deadly infections. Although it is well appreciated that the development of protective immunity is shaped by the interactions of the host immune system with fungal cells, our understanding of fungal products that influence this process remains poor. In this study, we found that the activity of F-box protein 1 (Fbp1) in highly virulent C. neoformans clinical strain H99 shapes its immunogenicity and thus affects the development of protective immune responses in the host. The identification of this new mechanism of virulence may facilitate the future development of therapeutic interventions aimed at boosting antifungal host immunity.

Immune regulation by monocytes

Monocytes emerging from the bone marrow are the progenitors of monocyte-derived macrophages. An essential function of monocytes is to seed tissues with sufficient macrophages to replace loss from infection and tissue damage. Recent work from diverse inflammatory and homeostatic settings has shown monocytes also possess direct protective and pathogenic activities. Thus, monocytes are not simply needed to generate macrophages, but instead contribute to the overall orchestration of immunity. Some recently described properties of monocytes are both surprising and mechanistically specific; for example, inflammatory monocytes are required for the efficacy of transferred activated cytotoxic T cells, but can have potent tissue damaging effects while patrolling monocytes are required for anti-tumor immunity in some cases, but in another example provokes resistance to chemotherapy and thereby aid tumor growth. This summary will therefore focus on new findings about the regulatory activities of monocytes themselves.

Granulocyte-Monocyte Progenitors and Monocyte-Dendritic Cell Progenitors Independently Produce Functionally Distinct Monocytes

Granulocyte-monocyte progenitors (GMPs) and monocyte-dendritic cell progenitors (MDPs) produce monocytes during homeostasis and in response to increased demand during infection. Both progenitor populations are thought to derive from common myeloid progenitors (CMPs), and a hierarchical relationship (CMP-GMP-MDP-monocyte) is presumed to underlie monocyte differentiation. Here, however, we demonstrate that mouse MDPs arose from CMPs independently of GMPs, and that GMPs and MDPs produced monocytes via similar but distinct monocyte-committed progenitors. GMPs and MDPs yielded classical (Ly6C^hi) monocytes with gene expression signatures that were defined by their origins and impacted their function. GMPs produced a subset of "neutrophil-like" monocytes, whereas MDPs gave rise to a subset of monocytes that yielded monocyte-derived dendritic cells. GMPs and MDPs were also independently mobilized to produce specific combinations of myeloid cell types following the injection of microbial components. Thus, the balance of GMP and MDP differentiation shapes the myeloid cell repertoire during homeostasis and following infection.

Type I interferons drive inflammasome-independent emergency monocytopoiesis during endotoxemia

Emergency monocytopoiesis is an inflammation-driven hematological process that supplies the periphery with monocytes and subsequently with macrophages and monocyte-derived dendritic cells. Yet, the regulatory mechanisms by which early bone marrow myeloid progenitors commit to monocyte-derived phagocytes during endotoxemia remains elusive. Herein, we show that type I interferons signaling promotes the differentiation of monocyte-derived phagocytes at the level of their progenitors during a mouse model of endotoxemia. In this model, we characterized early changes in the numbers of conventional dendritic cells, monocyte-derived antigen-presenting cells and their respective precursors. While loss of caspase-1/11 failed to impair a shift toward monocytopoiesis, we observed sustained type-I-IFN-dependent monocyte progenitors differentiation in the bone marrow correlated to an accumulation of Mo-APCs in the spleen. Importantly, IFN-alpha and -beta were found to efficiently generate the development of monocyte-derived antigen-presenting cells while having no impact on the precursor activity of conventional dendritic cells. Consistently, the LPS-driven decrease of conventional dendritic cells and their direct precursor occurred independently of type-I-IFN signaling in vivo. Our characterization of early changes in mononuclear phagocytes and their dependency on type I IFN signaling during sepsis opens the way to the development of treatments for limiting the immunosuppressive state associated with sepsis.

Chronic Inflammation in Immune Aging: Role of Pattern Recognition Receptor Crosstalk with the Telomere Complex?

Age-related decline in immunity is characterized by stem cell exhaustion, telomere shortening, and disruption of cell-to-cell communication, leading to increased patient risk of disease. Recent data have demonstrated that chronic inflammation exerts a strong influence on immune aging and is closely correlated with telomere length in a range of major pathologies. The current review discusses the impact of inflammation on immune aging, the likely molecular mediators of this process, and the various disease states that have been linked with immunosenescence. Emerging findings implicate NF-κB, the major driver of inflammatory signaling, in several processes that regulate telomere maintenance and/or telomerase activity. While prolonged triggering of pattern recognition receptors is now known to promote immunosenescence, it remains unclear how this process is linked with the telomere complex or telomerase activity. Indeed, enzymatic control of telomere length has been studied for many decades, but alternative roles of telomerase and potential influences on inflammatory responses are only now beginning to emerge. Crosstalk between these pathways may prove to be a key molecular mechanism of immunosenescence. Understanding how components of immune aging interact and modify host protection against pathogens and tumors will be essential for the design of new vaccines and therapies for a wide range of clinical scenarios.

Activated Mesenchymal Stem Cells Interact with Antibiotics and Host Innate Immune Responses to Control Chronic Bacterial Infections

Chronic bacterial infections associated with biofilm formation are often difficult to resolve without extended courses of antibiotic therapy. Mesenchymal stem cells (MSC) exert antibacterial activity in vitro and in acute bacterial infection models, but their activity in chronic infection with biofilm models has not been previously investigated. Therefore, we studied the effects of MSC administration in mouse and dog models of chronic infections associated with biofilms. Mice with chronic Staphylococcus aureus implant infections were treated by i.v. administration of activated or non-activated MSC, with or without antibiotic therapy. The most effective treatment protocol was identified as activated MSC co-administered with antibiotic therapy. Activated MSC were found to accumulate in the wound margins several days after i.v. administration. Macrophages in infected tissues assumed an M2 phenotype, compared to untreated infections which contained predominately M1 macrophages. Bacterial killing by MSC was found to be mediated in part by secretion of cathelicidin and was significantly increased by antibiotics. Studies in pet dogs with spontaneous chronic multi drug-resistant wound infections demonstrated clearance of bacteria and wound healing following repeated i.v. administration of activated allogeneic canine MSC. Thus, systemic therapy with activated MSC may be an effective new, non-antimicrobial approach to treatment of chronic, drug-resistant infections.

Analysis of monocyte infiltration in MPTP mice reveals that microglial CX3CR1 protects against neurotoxic over-induction of monocyte-attracting CCL2 by astrocytes

Background

Evidence from mice suggests that brain infiltrating immune cells contribute to neurodegeneration, and we previously identified a deleterious lymphocyte infiltration in Parkinson’s disease mice. However, this remains controversial for monocytes, due to artifact-prone techniques used to distinguish them from microglia. Our aim was to reassess this open question, by taking advantage of the recent recognition that chemokine receptors CCR2 and CX3CR1 can differentiate between inflammatory monocytes and microglia, enabling to test whether CCR2⁺ monocytes infiltrate the brain during dopaminergic (DA) neurodegeneration and whether they contribute to neuronal death. This revealed unexpected insights into possible regulation of monocyte-attracting CCL2 induction.

Methods

We used acute 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mice and assessed monocyte infiltration by combining laser microdissection-guided chemokine RNA profiling of the substantia nigra (SN) with immunohistochemistry and CCR2-GFP reporter mice. To determine contribution to neuronal loss, we used CCR2-deletion and CCL2-overexpression, to reduce and increase CCR2⁺ monocyte infiltration, and CX3CR1-deletion to assess a potential implication in CCL2 regulation.

Results

Nigral chemokine profiling revealed early CCL2/7/12-CCR2 axis induction, suggesting monocyte infiltration in MPTP mice. CCL2 protein showed early peak induction in nigral astrocytes, while CCR2-GFP mice revealed early but limited nigral monocyte infiltration. However, blocking infiltration by CCR2 deletion did not influence DA neuronal loss. In contrast, transgenic astrocytic CCL2 over-induction increased CCR2⁺ monocyte infiltration and DA neuronal loss in MPTP mice. Surprisingly, CCL2 over-induction was also detected in MPTP intoxicated CX3CR1-deleted mice, which are known to present increased DA neuronal loss. Importantly, CX3CR1/CCL2 double-deletion suggested that increased neurotoxicity was driven by astrocytic CCL2 over-induction.

Conclusions

We show that CCR2⁺ monocytes infiltrate the affected CNS, but at the level observed in acute MPTP mice, this does not contribute to DA neuronal loss. In contrast, the underlying astrocytic CCL2 induction seemed to be tightly controled, as already moderate CCL2 over-induction led to increased neurotoxicity in MPTP mice, likely due to the increased CCR2⁺ monocyte infiltration. Importantly, we found evidence suggesting that during DA neurodegeneration, this control was mediated by microglial CX3CR1 signaling, which protects against such neurotoxic CCL2 over-induction by astrocytes, thus hinting at an endogenous mechanism to limit neurotoxic effects of the CCL2-CCR2 axis.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-017-0830-9) contains supplementary material, which is available to authorized users.

BCAP inhibits proliferation and differentiation of myeloid progenitors in the steady state and during demand situations

B-cell adaptor for phosphatidylinositol 3-kinase (BCAP) is a signaling adaptor expressed in mature hematopoietic cells, including monocytes and neutrophils. Here we investigated the role of BCAP in the homeostasis and development of these myeloid lineages. BCAP^-/- mice had more bone marrow (BM) monocytes than wild-type (WT) mice, and in mixed WT:BCAP^-/- BM chimeras, monocytes and neutrophils skewed toward BCAP^-/- origin, showing a competitive advantage for BCAP^-/- myeloid cells. BCAP was expressed in BM hematopoietic progenitors, including lineage^-Sca-1⁺c-kit⁺ (LSK), common myeloid progenitor, and granulocyte/macrophage progenitor (GMP) cells. At the steady state, BCAP^-/- GMP cells expressed more IRF8 and less C/EBPα than did WT GMP cells, which correlated with an increase in monocyte progenitors and a decrease in granulocyte progenitors among GMP cells. Strikingly, BCAP^-/- progenitors proliferated and produced more myeloid cells of both neutrophil and monocyte/macrophage lineages than did WT progenitors in myeloid colony-forming unit assays, supporting a cell-intrinsic role of BCAP in inhibiting myeloid proliferation and differentiation. Consistent with these findings, during cyclophosphamide-induced myeloablation or specific monocyte depletion, BCAP^-/- mice replenished circulating monocytes and neutrophils earlier than WT mice. During myeloid replenishment after cyclophosphamide-induced myeloablation, BCAP^-/- mice had increased LSK proliferation and increased numbers of LSK and GMP cells compared with WT mice. Furthermore, BCAP^-/- mice accumulated more monocytes and neutrophils in the spleen than did WT mice during Listeria monocytogenes infection. Together, these data identify BCAP as a novel inhibitor of myelopoiesis in the steady state and of emergency myelopoiesis during demand conditions.

Macrophage Colony Stimulating Factor Derived from CD4+ T Cells Contributes to Control of a Blood-Borne Infection

Dynamic regulation of leukocyte population size and activation state is crucial for an effective immune response. In malaria, Plasmodium parasites elicit robust host expansion of macrophages and monocytes, but the underlying mechanisms remain unclear. Here we show that myeloid expansion during P. chabaudi infection is dependent upon both CD4+ T cells and the cytokine Macrophage Colony Stimulating Factor (MCSF). Single-cell RNA-Seq analysis on antigen-experienced T cells revealed robust expression of Csf1, the gene encoding MCSF, in a sub-population of CD4+ T cells with distinct transcriptional and surface phenotypes. Selective deletion of Csf1 in CD4+ cells during P. chabaudi infection diminished proliferation and activation of certain myeloid subsets, most notably lymph node-resident CD169+ macrophages, and resulted in increased parasite burden and impaired recovery of infected mice. Depletion of CD169+ macrophages during infection also led to increased parasitemia and significant host mortality, confirming a previously unappreciated role for these cells in control of P. chabaudi. This work establishes the CD4+ T cell as a physiologically relevant source of MCSF in vivo; probes the complexity of the CD4+ T cell response during type 1 infection; and delineates a novel mechanism by which T helper cells regulate myeloid cells to limit growth of a blood-borne intracellular pathogen.

Exogenous remodeling of lung resident macrophages protects against infectious consequences of bone marrow-suppressive chemotherapy

Infection is the single greatest threat to survival during cancer chemotherapy because of depletion of bone marrow-derived immune cells. Phagocytes, especially neutrophils, are key effectors in immunity to extracellular pathogens, which has limited the development of new approaches to protect patients with cancer and chemotherapy-induced neutropenia. Using a model of vaccine-induced protection against lethal Pseudomonas aeruginosa pneumonia in the setting of chemotherapy-induced neutropenia, we found a population of resident lung macrophages in the immunized lung that mediated protection in the absence of neutrophils, bone marrow-derived monocytes, or antibodies. These vaccine-induced macrophages (ViMs) expanded after immunization, locally proliferated, and were closely related to alveolar macrophages (AMs) by surface phenotype and gene expression profiles. By contrast to AMs, numbers of ViMs were stable through chemotherapy, showed enhanced phagocytic activity, and prolonged survival of neutropenic mice from lethal P. aeruginosa pneumonia upon intratracheal adoptive transfer. Thus, induction of ViMs by tissue macrophage remodeling may become a framework for new strategies to activate immune-mediated reserves against infection in immunocompromised hosts.

The Effects of T4 and A3/R Bacteriophages on Differentiation of Human Myeloid Dendritic Cells

Bacteriophages (phages) are viruses of bacteria. Here we evaluated the effects of T4 and A3/R bacteriophages, as well as phage-generated bacterial lysates, on differentiation of human myeloid dendritic cells (DCs) from monocytes. Neither of the phages significantly reduced the expression of markers associated with differentiation of DCs and their role in the activation of T cells (CD40, CD80, CD83, CD86, CD1c, CD11c, MHC II, PD-L1, PD-L2, TLR2, TLR4, and CCR7) and phagocytosis receptors (CD64 and DEC-205). By contrast, bacterial lysate of T4 phage significantly decreased the percentages of DEC-205- and CD1c-positive cells. The percentage of DEC-205-positive cells was also significantly reduced in DCs differentiated in the presence of lysate of A3/R phage. Thus while bacteriophages do not substantially affect differentiation of DCs, some products of phage-induced lysis of bacterial cells may influence the differentiation and potentially also some functions of DCs. Our results have important implications for phage therapy of bacterial infections because during infections monocytes recruited to the site of inflammation are an important source of inflammatory DCs.

2014 Jeffrey M. Hoeg Award Lecture: Transcriptional Control of Monocyte Development

Monocytes and macrophages are key immune cells involved in the early progression of atherosclerosis. Transcription factors that control their development in the bone marrow are important therapeutic targets to control the numbers and functions of these cells in disease. This review highlights what is currently known about the transcription factors that are critical for monocyte development.

Social stress-enhanced severity of Citrobacter rodentium-induced colitis is CCL2-dependent and attenuated by probiotic Lactobacillus reuteri

Psychological stressors are known to affect colonic diseases but the mechanisms by which this occurs, and whether probiotics can prevent stressor effects, are not understood. Because inflammatory monocytes that traffic into the colon can exacerbate colitis, we tested whether CCL2, a chemokine involved in monocyte recruitment, was necessary for stressor-induced exacerbation of infectious colitis. Mice were exposed to a social disruption stressor that entails repeated social defeat. During stressor exposure, mice were orally challenged with Citrobacter rodentium to induce a colonic inflammatory response. Exposure to the stressor during challenge resulted in significantly higher colonic pathogen levels, translocation to the spleen, increases in colonic macrophages, and increases in inflammatory cytokines and chemokines. The stressor-enhanced severity of C. rodentium-induced colitis was not evident in CCL2(-/-) mice, indicating the effects of the stressor are CCL2-dependent. In addition, we tested whether probiotic intervention could attenuate stressor-enhanced infectious colitis by reducing monocyte/macrophage accumulation. Treating mice with probiotic Lactobacillus reuteri reduced CCL2 mRNA levels in the colon and attenuated stressor-enhanced infectious colitis. These data demonstrate that probiotic L. reuteri can prevent the exacerbating effects of stressor exposure on pathogen-induced colitis, and suggest that one mechanism by which this occurs is through downregulation of the chemokine CCL2.

Propofol Increases Host Susceptibility to Microbial Infection by Reducing Subpopulations of Mature Immune Effector Cells at Sites of Infection

Anesthetics are known to modulate host immune responses, but separating the variables of surgery from anesthesia when analyzing hospital acquired infections is often difficult. Here, the bacterial pathogen Listeria monocytogenes (Lm) was used to assess the impact of the common anesthetic propofol on host susceptibility to infection. Brief sedation of mice with physiologically relevant concentrations of propofol increased bacterial burdens in target organs by more than 10,000-fold relative to infected control animals. The adverse effects of propofol sedation on immune clearance of Lm persisted after recovery from sedation, as animals given the drug remained susceptible to infection for days following anesthesia. In contrast to propofol, sedation with alternative anesthetics such as ketamine/xylazine or pentobarbital did not increase susceptibility to systemic Lm infection. Propofol altered systemic cytokine and chemokine expression during infection, and prevented effective bacterial clearance by inhibiting the recruitment and/or activity of immune effector cells at sites of infection. Propofol exposure induced a marked reduction in marginal zone macrophages in the spleens of Lm infected mice, resulting in bacterial dissemination into deep tissue. Propofol also significantly increased mouse kidney abscess formation following infection with the common nosocomial pathogen Staphylococcus aureus. Taken together, these data indicate that even brief exposure to propofol severely compromises host resistance to microbial infection for days after recovery from sedation.

An extrafollicular pathway for the generation of effector CD8(+) T cells driven by the proinflammatory cytokine, IL-12

The proinflammatory cytokine IL-12 drives the generation of terminally differentiated KLRG1⁺ effector CD8⁺ T cells. Using a Toxoplasma vaccination model, we delineate the sequence of events that naïve CD8⁺ T cells undergo to become terminal effectors and the differentiation steps controlled by IL-12. We demonstrate that direct IL-12 signaling on CD8⁺ T cells is essential for the induction of KLRG1 and IFN-γ, but the subsequent downregulation of CXCR3 is controlled by IL-12 indirectly through the actions of IFN-γ and IFN-γ-inducible chemokines. Differentiation of nascent effectors occurs in an extrafollicular splenic compartment and is driven by late IL-12 production by DCs distinct from the classical CD8α⁺ DC. Unexpectedly, we also found extensive proliferation of both KLRG1⁻ and KLRG1⁺ CD8⁺ T cells in the marginal zone and red pulp, which ceases prior to the final KLRG1^Hi CXCR3^Lo stage. Our findings highlight the notion of an extrafollicular pathway for effector T cell generation.

DOI:http://dx.doi.org/10.7554/eLife.09017.001

The immune system helps to protect us from cancer, infection by microbes and other diseases. There are several different types of immune cells that each have particular roles. For example, cytotoxic T cells can kill other cells in the body that are damaged or infected. These cells are found in various locations around the body—including a region of the spleen known as the white pulp—where they wait in an inactive state until they detect signals from a damaged or infected cell. These T cells divide and mature to produce populations of active T cells known as effector cytotoxic lymphoid cells (or CTLs for short), a process which is thought to occur within the white pulp.

A small protein called cytokine IL-12 is involved in the production of CTLs. The cytokine is released from other immune cells and causes the activated T cells to divide and mature. It has long been believed that IL-12 produced in the white pulp early on in the process is sufficient to drive this process, but more recent work suggests that sustained production of IL-12 in other areas of the spleen that are accessible to the bloodstream may be needed.

Here, Shah et al. studied the generation of cytotoxic T cells in mice that had been exposed to a vaccine against a disease called Toxoplasmosis. Their experiments show that IL-12 drives both the early and late stages of CTL production. In the early stages, the T cells respond to IL-12 that is secreted by a group of ‘lymphoid dendritic’ cells in the white pulp. However, in the later stages, the T cells move away from the white pulp to other parts of the spleen known as the marginal zone and red pulp, where a distinct group of ‘myeloid dendritic’ cells also produce IL-12 and direct the final maturation of the CTLs.

Shah et al.'s findings also show that the process in which cytotoxic T cells divide and later mature to produce CTLs involves a series of tightly controlled events that mostly occur outside of the white pulp. These observations provide a new perspective on how to develop vaccines and other treatments that more efficiently generate the CTLs needed to protect against infections and cancer.

DOI:http://dx.doi.org/10.7554/eLife.09017.002

Human and Mouse Mononuclear Phagocyte Networks: A Tale of Two Species?

Dendritic cells (DCs), monocytes, and macrophages are a heterogeneous population of mononuclear phagocytes that are involved in antigen processing and presentation to initiate and regulate immune responses to pathogens, vaccines, tumor, and tolerance to self. In addition to their afferent sentinel function, DCs and macrophages are also critical as effectors and coordinators of inflammation and homeostasis in peripheral tissues. Harnessing DCs and macrophages for therapeutic purposes has major implications for infectious disease, vaccination, transplantation, tolerance induction, inflammation, and cancer immunotherapy. There has been a paradigm shift in our understanding of the developmental origin and function of the cellular constituents of the mononuclear phagocyte system. Significant progress has been made in tandem in both human and mouse mononuclear phagocyte biology. This progress has been accelerated by comparative biology analysis between mouse and human, which has proved to be an exceptionally fruitful strategy to harmonize findings across species. Such analyses have provided unexpected insights and facilitated productive reciprocal and iterative processes to inform our understanding of human and mouse mononuclear phagocytes. In this review, we discuss the strategies, power, and utility of comparative biology approaches to integrate recent advances in human and mouse mononuclear phagocyte biology and its potential to drive forward clinical translation of this knowledge. We also present a functional framework on the parallel organization of human and mouse mononuclear phagocyte networks.

Macrophage-Lineage Cells Negatively Regulate the Hematopoietic Stem Cell Pool in Response to Interferon Gamma at Steady State and During Infection

Bone marrow (BM) resident macrophages (Mϕs) regulate hematopoietic stem cell (HSC) mobilization; however, their impact on HSC function has not been investigated. We demonstrate that depletion of BM resident Mϕs increases HSC proliferation as well as the pool of quiescent HSCs. At the same time, during bacterial infection where BM resident Mϕs are selectively increased we observe a decrease in HSC numbers. Moreover, strategies that deplete or reduce Mϕs during infection prevent HSC loss and rescue HSC function. We previously found that the transient loss of HSCs during infection is interferon-gamma (IFNγ)-dependent. We now demonstrate that IFNγ signaling specifically in Mϕs is critical for both the diminished HSC pool and maintenance of BM resident Mϕs during infection. In addition to the IFNγ-dependent loss of BM HSC and progenitor cells (HSPCs) during infection, IFNγ reduced circulating HSPC numbers. Importantly, under infection conditions AMD3100 or G-CSF-induced stem cell mobilization was impaired. Taken together, our data show that IFNγ acts on Mϕs, which are a negative regulator of the HSC pool, to drive the loss in BM and peripheral HSCs during infection. Our findings demonstrate that modulating BM resident Mϕ numbers can impact HSC function in vivo, which may be therapeutically useful for hematologic conditions and refinement of HSC transplantation protocols.

Persistence of Toxoplasma gondii in the central nervous system: a fine-tuned balance between the parasite, the brain and the immune system

Upon infection of humans and animals with Toxoplasma gondii, the parasites persist as intraneuronal cysts that are controlled, but not eliminated by the immune system. In particular, intracerebral T cells are crucial in the control of T. gondii infection and are supported by essential functions from other leukocyte populations. Additionally, brain-resident cells including astrocytes, microglia and neurons contribute to the intracerebral immune response by the production of cytokines, chemokines and expression of immunoregulatory cell surface molecules, such as major histocompatibility (MHC) antigens. However, the in vivo behaviour of these individual cell populations, specifically their interaction during cerebral toxoplasmosis, remains to be elucidated. We discuss here what is known about the function of T cells, recruited myeloid cells and brain-resident cells, with particular emphasis on the potential cross-regulation of these cell populations, in governing cerebral toxoplasmosis.