CD169+ macrophages orchestrate innate immune responses by regulating bacterial localization in the spleen

Lymph node stromal cells vary in susceptibility to infection but can support the intracellular growth of Listeria monocytogenes

Lymph node stromal cells (LNSCs) are an often overlooked component of the immune system but play a crucial role in maintaining tissue homeostasis and orchestrating immune responses. Our understanding of the functions these cells serve in the context of bacterial infections remains limited. We previously showed that Listeria monocytogenes, a facultative intracellular foodborne bacterial pathogen, must replicate within an as-yet-unidentified cell type in the mesenteric lymph node (MLN) to spread systemically. Here, we show that L. monocytogenes could invade, escape from the vacuole, replicate exponentially, and induce a type I interferon response in the cytosol of 2 LNSC populations infected in vitro, fibroblastic reticular cells (FRCs) and blood endothelial cells (BECs). Infected FRCs and BECs also produced a significant chemokine and proinflammatory cytokine response after in vitro infection. Flow cytometric analysis confirmed that GFP+  L. monocytogenes were associated with a small percentage of MLN stromal cells in vivo following foodborne infection of mice. Using fluorescent microscopy, we showed that these cell-associated bacteria were intracellular L. monocytogenes and that the number of infected FRCs and BECs changed over the course of a 3-day infection in mice. Ex vivo culturing of these infected LNSC populations revealed viable, replicating bacteria that grew on agar plates. These results highlight the unexplored potential of FRCs and BECs to serve as suitable growth niches for L. monocytogenes during foodborne infection and to contribute to the proinflammatory environment within the MLN that promotes clearance of listeriosis.

PEG-conjugated bovine haemoglobin enhances efficiency of chemotherapeutic agent doxorubicin with alleviating DOX-induced splenocardiac toxicity in the breast cancer

Doxorubicin (DOX) is an effective chemotherapeutic agent widely used for cancer treatment. However, hypoxia in tumour tissue and obvious adverse effects particularly cardiotoxicity restricts the clinical usage of DOX. Our study is based on the co-administration of haemoglobin-based oxygen carriers (HBOCs) and DOX in a breast cancer model to investigate HBOCs' ability to enhance chemotherapeutic effectiveness and its capabilities to alleviate the side effects induced by DOX. In an in-vitro study, the results suggested the cytotoxicity of DOX was significantly improved when combined with HBOCs in a hypoxic environment, and produced more γ-H2AX indicating higher DNA damage than free DOX did. Compared with administration of free DOX, combined therapy exhibited a stronger tumour suppressive effect in an in-vivo study. Further mechanism studies showed that the expression of various proteins such as hypoxia-inducible factor-1α (HIF-1α), CD31, CD34, and vascular endothelial growth factor (VEGF) in tumour tissues was also significantly reduced in the combined treatment group. In addition, HBOCs can significantly reduce the splenocardiac toxicity induced by DOX, according to the results of the haematoxylin and eosin (H&E) staining and histological investigation. This study suggested that PEG-conjugated bovine haemoglobin may not only reduce the hypoxia in tumours and increase the efficiency of chemotherapeutic agent DOX, but also alleviate the irreversible heart toxicity caused by DOX-inducted splenocardiac dysregulation.

KEAP1 mutation in lung adenocarcinoma promotes immune evasion and immunotherapy resistance

Lung cancer treatment has benefited greatly through advancements in immunotherapies. However, immunotherapy often fails in patients with specific mutations like KEAP1, which are frequently found in lung adenocarcinoma. We established an antigenic lung cancer model and used it to explore how Keap1 mutations remodel the tumor immune microenvironment. Using single-cell technology and depletion studies, we demonstrate that Keap1-mutant tumors diminish dendritic cell and T cell responses driving immunotherapy resistance. This observation was corroborated in patient samples. CRISPR-Cas9-mediated gene targeting revealed that hyperactivation of the NRF2 antioxidant pathway is responsible for diminished immune responses in Keap1-mutant tumors. Importantly, we demonstrate that combining glutaminase inhibition with immune checkpoint blockade can reverse immunosuppression, making Keap1-mutant tumors susceptible to immunotherapy. Our study provides new insight into the role of KEAP1 mutations in immune evasion, paving the way for novel immune-based therapeutic strategies for KEAP1-mutant cancers.

Platelets interact with CD169+ macrophages and cDC1 and enhance liposome-induced CD8+ T cell responses

Historically platelets are mostly known for their crucial contribution to hemostasis, but there is growing understanding of their role in inflammation and immunity. The immunomodulatory role of platelets entails interaction with pathogens, but also with immune cells including macrophages and dendritic cells (DCs), to activate adaptive immune responses. In our previous work, we have demonstrated that splenic CD169+ macrophages scavenge liposomes and collaborate with conventional type 1 DCs (cDC1) to induce expansion of CD8+ T cells. Here, we show that platelets associate with liposomes and bind to DNGR-1/Clec9a and CD169/Siglec-1 receptors in vitro. In addition, platelets interacted with splenic CD169+ macrophages and cDC1 and further increased liposome internalization by cDC1. Most importantly, platelet depletion prior to liposomal immunization resulted in significantly diminished antigen-specific CD8+ T cell responses, but not germinal center B cell responses. Previously, complement C3 was shown to be essential for platelet-mediated CD8+ T cell activation during bacterial infection. However, after liposomal vaccination CD8+ T cell priming was not dependent on complement C3. While DCs from platelet-deficient mice exhibited unaltered maturation status, they did express lower levels of CCR7. In addition, in the absence of platelets, CCL5 plasma levels were significantly reduced. Overall, our findings demonstrate that platelets engage in a cross-talk with CD169+ macrophages and cDC1 and emphasize the importance of platelets in induction of CD8+ T cell responses in the context of liposomal vaccination.

Factor VIII trafficking to CD4+ T cells shapes its immunogenicity and requires several types of antigen-presenting cells

Despite >80 years of clinical experience with coagulation factor VIII (FVIII) inhibitors, surprisingly little is known about the in vivo mechanism of this most serious complication of replacement therapy for hemophilia A. These neutralizing antidrug alloantibodies arise in ∼30% of patients. Inhibitor formation is T-cell dependent, but events leading up to helper T-cell activation have been elusive because of, in part, the complex anatomy and cellular makeup of the spleen. Here, we show that FVIII antigen presentation to CD4+ T cells critically depends on a select set of several anatomically distinct antigen-presenting cells, whereby marginal zone B cells and marginal zone and marginal metallophilic macrophages but not red pulp macrophages (RPMFs) participate in shuttling FVIII to the white pulp in which conventional dendritic cells (DCs) prime helper T cells, which then differentiate into follicular helper T (Tfh) cells. Toll-like receptor 9 stimulation accelerated Tfh cell responses and germinal center and inhibitor formation, whereas systemic administration of FVIII alone in hemophilia A mice increased frequencies of monocyte-derived and plasmacytoid DCs. Moreover, FVIII enhanced T-cell proliferation to another protein antigen (ovalbumin), and inflammatory signaling-deficient mice were less likely to develop inhibitors, indicating that FVIII may have intrinsic immunostimulatory properties. Ovalbumin, which, unlike FVIII, is absorbed into the RPMF compartment, fails to elicit T-cell proliferative and antibody responses when administered at the same dose as FVIII. Altogether, we propose that an antigen trafficking pattern that results in efficient in vivo delivery to DCs and inflammatory signaling, shape the immunogenicity of FVIII.

Age-dependent differences in efferocytosis determine the outcome of opsonophagocytic protection from invasive pathogens

In early life, susceptibility to invasive infection skews toward a small subset of microbes, whereas other pathogens associated with diseases later in life, including Streptococcus pneumoniae (Spn), are uncommon among neonates. To delineate mechanisms behind age-dependent susceptibility, we compared age-specific mouse models of invasive Spn infection. We show enhanced CD11b-dependent opsonophagocytosis by neonatal neutrophils improved protection against Spn during early life. The augmented function of neonatal neutrophils was mediated by higher CD11b surface expression at the population level due to dampened efferocytosis, which also resulted in more CD11bhi "aged" neutrophils in peripheral blood. Dampened efferocytosis during early life could be attributed to the lack of CD169+ macrophages in neonates and reduced systemic expressions of multiple efferocytic mediators, including MerTK. On experimentally impairing efferocytosis later in life, CD11bhi neutrophils increased and protection against Spn improved. Our findings reveal how age-dependent differences in efferocytosis determine infection outcome through the modulation of CD11b-driven opsonophagocytosis and immunity.

CD169+ macrophage intrinsic IL-10 production regulates immune homeostasis during sepsis

Macrophages facilitate critical functions in regulating pathogen clearance and immune homeostasis in tissues. The remarkable functional diversity exhibited by macrophage subsets is dependent on tissue environment and the nature of the pathological insult. Our current knowledge of the mechanisms that regulate the multifaceted counter-inflammatory responses mediated by macrophages remains incomplete. Here, we report that CD169+ macrophage subsets are necessary for protection under excessive inflammatory conditions. We show that in the absence of these macrophages, even under mild septic conditions, mice fail to survive and exhibit increased production of inflammatory cytokines. Mechanistically, CD169+ macrophages control inflammatory responses via interleukin-10 (IL-10), as CD169+ macrophage-specific deletion of IL-10 was lethal during septic conditions, and recombinant IL-10 treatment reduced lipopolysaccharide (LPS)-induced lethality in mice lacking CD169+ macrophages. Collectively, our findings show a pivotal homeostatic role for CD169+ macrophages and suggest they may serve as an important target for therapy under damaging inflammatory conditions.

Gasdermin D protects against Streptococcus equi subsp. zooepidemicus infection through macrophage pyroptosis

Streptococcus equi subsp. zooepidemicus (S. zooepidemicus, SEZ) is an essential zoonotic bacterial pathogen that can cause various inflammation, such as meningitis, endocarditis, and pneumonia. Gasdermin D (GSDMD) is involved in cytokine release and cell death, indicating an important role in controlling the microbial infection. This study investigated the protective role of GSDMD in mice infected with SEZ and examined the role of GSDMD in peritoneal macrophages in the infection. GSDMD-deficient mice were more susceptible to intraperitoneal infection with SEZ, and the white pulp structure of the spleen was seriously damaged in GSDMD-deficient mice. Although the increased proportion of macrophages did not depend on GSDMD in both spleen and peritoneal lavage fluid (PLF), deficiency of GSDMD caused the minor release of the pro-inflammatory cytokines interleukin-1β (IL-1β) and interleukin-18 (IL-18) during the infection in vivo. In vitro, SEZ infection induced more release of IL-1β, IL-18, and lactate dehydrogenase (LDH) in wild-type macrophages than in GSDMD-deficient macrophages. Finally, we demonstrated that pore formation and pyroptosis of macrophages depended on GSDMD. Our findings highlight the host defense mechanisms of GSDMD against SEZ infection, providing a potential therapeutic target in SEZ infection.

Innate immune surveillance of the circulation: A review on the removal of circulating virions from the bloodstream

Many viruses utilize the lymphohematogenous route for dissemination; however, they may not freely use this highway unchecked. The reticuloendothelial system (RES) is an innate defense system that surveys circulating blood, recognizing and capturing viral particles. Examination of the literature shows that the bulk of viral clearance is mediated by the liver; however, the precise mechanism(s) mediating viral vascular clearance vary between viruses and, in many cases, remains poorly defined. Herein, we summarize what is known regarding the recognition and capture of virions from the circulation prior to the generation of a specific antibody response. We also discuss the consequences of viral capture on viral pathogenesis and the fate of the captor cell. Finally, this understudied topic has implications beyond viral pathogenesis, including effects on arbovirus ecology and the application of virus-vectored gene therapies.

Listeriosis, a model infection to study host-pathogen interactions in vivo

Listeria monocytogenes (Lm) is a foodborne pathogen and the etiological agent of listeriosis. This facultative intracellular Gram-positive bacterium has the ability to colonize the intestinal lumen, cross the intestinal, blood-brain and placental barriers, leading to bacteremia, neurolisteriosis and maternal-fetal listeriosis. Lm is a model microorganism for the study of the interplay between a pathogenic microbe, host tissues and microbiota in vivo. Here we review how animal models permissive to Lm-host interactions allow deciphering some of the key steps of the infectious process, from the intestinal lumen to the crossing of host barriers and dissemination within the host. We also highlight recent investigations using tagged Lm and clinically relevant strains that have shed light on within-host dynamics and the purifying selection of Lm virulence factors. Studying Lm infection in vivo is a way forward to explore host biology and unveil the mechanisms that have selected its capacity to closely associate with its vertebrate hosts.

Memory CD8+ T cells mediate early pathogen-specific protection via localized delivery of chemokines and IFNγ to clusters of monocytes

While cognate antigen drives clonal expansion of memory CD8+ T (CD8+ TM) cells to achieve sterilizing immunity in immunized hosts, not much is known on how cognate antigen contributes to early protection before clonal expansion occurs. Here, using distinct models of immunization, we establish that cognate antigen recognition by CD8+ TM cells on dendritic cells initiates their rapid and coordinated production of a burst of CCL3, CCL4, and XCL1 chemokines under the transcriptional control of interferon (IFN) regulatory factor 4. Using intravital microscopy imaging, we reveal that CD8+ TM cells undergo antigen-dependent arrest in splenic red pulp clusters of CCR2+Ly6C+ monocytes to which they deliver IFNγ and chemokines. IFNγ enables chemokine-induced microbicidal activities in monocytes for protection. Thus, rapid and effective CD8+ TM cell responses require spatially and temporally coordinated events that quickly restrict microbial pathogen growth through the local delivery of activating chemokines to CCR2+Ly6C+ monocytes.

Arsenene Nanodots with Selective Killing Effects and their Low-Dose Combination with ß-Elemene for Cancer Therapy

Arsenical drugs have achieved hallmark success in treating patients with acute promyelocytic leukemia, but expanding their clinical utility to solid tumors has proven difficult with the contradiction between the therapeutic efficacy and the systemic toxicity. Here, leveraging efforts from materials science, biocompatible PEGylated arsenene nanodots (AsNDs@PEG) with high monoelemental arsenic purity that can selectively and effectively treat solid tumors are synthesized. The intrinsic selective killing effect of AsNDs@PEG is closely related to high oxidative stress in tumor cells, which leads to an activated valence-change of arsenic (from less toxic As⁰ to severely toxic oxidation states), followed by decreased superoxide dismutase activity and massive reactive oxygen species (ROS) production. These effects occur selectively within cancer cells, causing mitochondrial damage, cell-cycle arrest, and DNA damage. Moreover, AsNDs@PEG when applied in a multi-drug combination strategy with β-elemene, a plant-derived anticancer drug, achieves synergistic antitumor outcomes, and its newly discovered on-demand photothermal properties facilitate the elimination of the tumors without recurrence, potentially further expanding its clinical utility. In line of the practicability for a large-scale fabrication and negligible systemic toxicity of AsNDs@PEG (even at high doses and with repetitive administration), a new-concept arsenical drug with high therapeutic efficacy for selective solid tumor therapy is provided.

Imaging Infection Across Scales of Size: From Whole Animals to Single Molecules

Infectious diseases are a leading cause of global morbidity and mortality, and the threat of infectious diseases to human health is steadily increasing as new diseases emerge, existing diseases reemerge, and antimicrobial resistance expands. The application of imaging technology to the study of infection biology has the potential to uncover new factors that are critical to the outcome of host-pathogen interactions and to lead to innovations in diagnosis and treatment of infectious diseases. This article reviews current and future opportunities for the application of imaging to the study of infectious diseases, with a particular focus on the power of imaging objects across a broad range of sizes to expand the utility of these approaches. Expected final online publication date for the Annual Review of Microbiology, Volume 75 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

In vivo imaging of retrovirus infection reveals a role for Siglec-1/CD169 in multiple routes of transmission

Early events in retrovirus transmission are determined by interactions between incoming viruses and frontline cells near entry sites. Despite their importance for retroviral pathogenesis, very little is known about these events. We developed a bioluminescence imaging (BLI)-guided multiscale imaging approach to study these events in vivo. Engineered murine leukemia reporter viruses allowed us to monitor individual stages of retrovirus life cycle including virus particle flow, virus entry into cells, infection and spread for retroorbital, subcutaneous, and oral routes. BLI permitted temporal tracking of orally administered retroviruses along the gastrointestinal tract as they traversed the lumen through Peyer’s patches to reach the draining mesenteric sac. Importantly, capture and acquisition of lymph-, blood-, and milk-borne retroviruses spanning three routes was promoted by a common host factor, the I-type lectin CD169, expressed on sentinel macrophages. These results highlight how retroviruses co-opt the immune surveillance function of tissue-resident sentinel macrophages for establishing infection.

Immunomodulatory Effects of Asiaticoside Against Shigella flexneri-Infected Macrophages

Macrophages provide the first line of defense against Shigella flexneri infection in the gastrointestinal tract by inducing a variety of inflammatory and antimicrobial responses. Secondary metabolites of plants are used as drugs against infections that are resistant to common antibiotics. In this study, the innate effects of asiaticoside on the proinflammatory activity of mouse macrophages infected with S. flexneri were investigated. The viability of the infected mouse macrophages were examined using viability assay, while the pro-inflammatory cytokines productions were determined using the enzyme-linked immunosorbent assay (ELISA) for determination of IL-1β, IL-12 p40 and TNF-α levels. The production of nitric oxide (NO) and the expression of inducible nitric oxide synthase (iNOS) protein were determined using the Griess assay and western blot, respectively. Statistical analyses were performed using the Statistical Package of Social Sciences (SPSS) software, version 20. The data obtained from independent experiments (n = 3) were presented as the mean ± standard error of mean (SEM). The results showed that, asiaticoside stimulated the infected macrophages by stimulating increased production of TNF-α, IL-12 p40 and NO as well as increased expression of iNOS in a dose-dependent manner. In contrast the viability of the cells and the production of IL-1β and were reduced also in a dose-dependent manner when compared to untreated cells. These results indicate that asiaticoside has immunomodulatory effects on the innate immune function of infected macrophages, showing the potential use of this compound to reduce the clinical symptoms of the infections.

IFN-β Acts on Monocytes to Ameliorate CNS Autoimmunity by Inhibiting Proinflammatory Cross-Talk Between Monocytes and Th Cells

IFN-β has been the treatment for multiple sclerosis (MS) for almost three decades, but understanding the mechanisms underlying its beneficial effects remains incomplete. We have shown that MS patients have increased numbers of GM-CSF+ Th cells in circulation, and that IFN-β therapy reduces their numbers. GM-CSF expression by myelin-specific Th cells is essential for the development of experimental autoimmune encephalomyelitis (EAE), an animal model of MS. These findings suggested that IFN-β therapy may function via suppression of GM-CSF production by Th cells. In the current study, we elucidated a feedback loop between monocytes and Th cells that amplifies autoimmune neuroinflammation, and found that IFN-β therapy ameliorates central nervous system (CNS) autoimmunity by inhibiting this proinflammatory loop. IFN-β suppressed GM-CSF production in Th cells indirectly by acting on monocytes, and IFN-β signaling in monocytes was required for EAE suppression. IFN-β increased IL-10 expression by monocytes, and IL-10 was required for the suppressive effects of IFN-β. IFN-β treatment suppressed IL-1β expression by monocytes in the CNS of mice with EAE. GM-CSF from Th cells induced IL-1β production by monocytes, and, in a positive feedback loop, IL-1β augmented GM-CSF production by Th cells. In addition to GM-CSF, TNF and FASL expression by Th cells was also necessary for IL-1β production by monocyte. IFN-β inhibited GM-CSF, TNF, and FASL expression by Th cells to suppress IL-1β secretion by monocytes. Overall, our study describes a positive feedback loop involving several Th cell- and monocyte-derived molecules, and IFN-β actions on monocytes disrupting this proinflammatory loop.

The Role of Trogocytosis in the Modulation of Immune Cell Functions

Trogocytosis is an active process, in which one cell extracts the cell fragment from another cell, leading to the transfer of cell surface molecules, together with membrane fragments. Recent reports have revealed that trogocytosis can modulate various biological responses, including adaptive and innate immune responses and homeostatic responses. Trogocytosis is evolutionally conserved from protozoan parasites to eukaryotic cells. In some cases, trogocytosis results in cell death, which is utilized as a mechanism for antibody-dependent cytotoxicity (ADCC). In other cases, trogocytosis-mediated intercellular protein transfer leads to both the acquisition of novel functions in recipient cells and the loss of cellular functions in donor cells. Trogocytosis in immune cells is typically mediated by receptor–ligand interactions, including TCR–MHC interactions and Fcγ receptor-antibody-bound molecule interactions. Additionally, trogocytosis mediates the transfer of MHC molecules to various immune and non-immune cells, which confers antigen-presenting activity on non-professional antigen-presenting cells. In this review, we summarize the recent advances in our understanding of the role of trogocytosis in immune modulation.

Tissue-resident macrophages: guardians of organ homeostasis

Tissue-resident macrophages (M_TR) have recently emerged as a key rheostat capable of regulating the balance between organ health and disease. In most organs, ontogenetically and functionally distinct macrophage subsets fulfill a plethora of functions specific to their tissue environment. In this review, we summarize recent findings regarding the ontogeny and functions of macrophage populations in different mammalian tissues, describing how these cells regulate tissue homeostasis and how they can contribute to inflammation. Furthermore, we highlight new developments concerning certain general principles of tissue macrophage biology, including the importance of metabolism for understanding macrophage activation states and the influence of intrinsic and extrinsic factors on macrophage metabolic control. We also shed light on certain open questions in the field and how answering these might pave the way for tissue-specific therapeutic approaches.

Jaw Periosteum-Derived Mesenchymal Stem Cells Regulate THP-1-Derived Macrophage Polarization

Mesenchymal stem cells from bone marrow have powerful immunomodulatory capabilities. The interactions between jaw periosteal cells (JPCs) and macrophages are not only relevant for the application of JPCs in regenerative medicine, but this understanding could also help treating diseases like osteonecrosis of the jaw. In previous studies, we analyzed, for the first time, immunomodulatory features of 2D- and 3D-cultured JPCs. In the present work, the effects of JPCs on the polarization state of macrophages in contact coculture were analyzed. To improve the macrophage polarization study, different concentrations of PMA (5 nM, 25 nM, and 150 nM) or different medium supplementations (10% FBS, 10% hPL and 5% hPL) were compared. Further, in order to analyze the effects of JPCs on macrophage polarization, JPCs and PMA-stimulated THP-1 cells were cocultured under LPS/IFN-γ or IL-4/IL-13 stimulatory conditions. Surface marker expression of M1 and M2 macrophages were analyzed under the different culture supplementations in order to investigate the immunomodulatory properties of JPCs. Our results showed that 5 nM PMA can conduct an effective macrophage polarization. The analyses of morphological parameters and surface marker expression showed more distinct M1/M2 phenotypes over FBS supplementation when using 5% hPL during macrophage polarization. In the coculture, immunomodulatory properties of JPCs improved significantly under 5% hPL supplementation compared to other supplementations. We concluded that, under the culture condition with 5% hPL, JPCs were able to effectively induce THP-1-derived macrophage polarization.

Pulmonary insults exacerbate susceptibility to oral Listeria monocytogenes infection through the production of IL-10 by NK cells

Most individuals who consume foods contaminated with the bacterial pathogen Listeria monocytogenes (Lm) develop mild symptoms, while others are susceptible to life-threatening systemic infections (listeriosis). Although it is known that the risk of severe disease is increased in certain human populations, including the elderly, it remains unclear why others who consume contaminated food develop listeriosis. Here, we used a murine model to discover that pulmonary coinfections can impair the host’s ability to adequately control and eradicate systemic Lm that cross from the intestines to the bloodstream. We found that the resistance of mice to oral Lm infection was dramatically reduced by coinfection with Streptococcus pneumoniae (Spn), a bacterium that colonizes the respiratory tract and can also cause severe infections in the elderly. Exposure to Spn or microbial products, including a recombinant Lm protein (L1S) and lipopolysaccharide (LPS), rendered otherwise resistant hosts susceptible to severe systemic Lm infection. In addition, we show that this increase in susceptibility was dependent on an increase in the production of interleukin-10 (IL-10) from Ncr1+ cells, including natural killer (NK) cells. Lastly, the ability of Ncr1+ cell derived IL-10 to increase disease susceptibility correlated with a dampening of both myeloid cell accumulation and myeloid cell phagocytic capacity in infected tissues. These data suggest that efforts to minimize inflammation in response to an insult at the respiratory mucosa render the host more susceptible to infections by Lm and possibly other pathogens that access the oral mucosa.

The bacterial pathogen Listeria monocytogenes (Lm) causes food-borne infections in humans and animals. Most humans who consume Lm-contaminated foods develop mild symptoms, but in a subset of individuals Lm causes severe systemic infections that are often lethal. Although the factors that predispose individuals to develop severe Lm infection are not well understood, systemic infections require bacteria to disseminate from the intestines to the bloodstream and peripheral tissues. Here we show in a murine model of infection that feeding of Lm alone results in the dissemination of only small numbers of bacteria that are contained and fail to cause symptoms. However, feeding of Lm in mice that also encounter a second infection in the lungs, or have exposure to microbial products in the lungs, results in a severe infection with large numbers of systemic Lm. These lung exposures increase the survival and expansion of Lm that disseminate from the intestines to peripheral tissues by stimulating release of regulatory proteins that dampen the ability of myeloid cells to kill Lm. This study thus reveals how the dampening of inflammation upon microbial exposure at one mucosal tissue can impair the immune response to pathogens entering at a different site and how secondary exposures impact severity of infection in animals that consume Lm-contaminated foods.

Lung Marginated and Splenic Murine Resident Neutrophils Constitute Pioneers in Tissue-Defense During Systemic E. coli Challenge

The rapid response of neutrophils throughout the body to a systemic challenge is a critical first step in resolution of bacterial infection such as Escherichia coli (E. coli). Here we delineated the dynamics of this response, revealing novel insights into the molecular mechanisms using lung and spleen intravital microscopy and 3D ex vivo culture of living precision cut splenic slices in combination with fluorescent labelling of endogenous leukocytes. Within seconds after challenge, intravascular marginated neutrophils and lung endothelial cells (ECs) work cooperatively to capture pathogens. Neutrophils retained on lung ECs slow their velocity and aggregate in clusters that enlarge as circulating neutrophils carrying E. coli stop within the microvasculature. The absolute number of splenic neutrophils does not change following challenge; however, neutrophils increase their velocity, migrate to the marginal zone (MZ) and form clusters. Irrespective of their location all neutrophils capturing heat-inactivated E. coli take on an activated phenotype showing increasing surface CD11b. At a molecular level we show that neutralization of ICAM-1 results in splenic neutrophil redistribution to the MZ under homeostasis. Following challenge, splenic levels of CXCL12 and ICAM-1 are reduced allowing neutrophils to migrate to the MZ in a CD29-integrin dependent manner, where the enlargement of splenic neutrophil clusters is CXCR2-CXCL2 dependent. We show directly molecular mechanisms that allow tissue resident neutrophils to provide the first lines of antimicrobial defense by capturing circulating E. coli and forming clusters both in the microvessels of the lung and in the parenchyma of the spleen.

Why is Listeria monocytogenes such a potent inducer of CD8+ T-cells?

Listeria monocytogenes is a rapidly growing, Gram-positive, facultative intracellular pathogen that has been used for over 5 decades as a model to study basic aspects of infection and immunity. In a murine intravenous infection model, immunisation with a sublethal infection of L. monocytogenes initially leads to rapid intracellular multiplication followed by clearance of the bacteria and ultimately culminates in the development of long-lived cell-mediated immunity (CMI) mediated by antigen-specific CD8+ cytotoxic T-cells. Importantly, effective immunisation requires live, replicating bacteria. In this review, we summarise the cell and immunobiology of L. monocytogenes infection and discuss aspects of its pathogenesis that we suspect lead to robust CMI. We suggest five specific features of L. monocytogenes infection that positively impact the development of CMI: (a) the bacteria have a predilection for professional antigen-presenting cells; (b) the bacteria escape from phagosomes, grow, and secrete antigens into the host cell cytosol; (c) bacterial-secreted proteins enter the major histocompatibility complex (MHC) class I pathway of antigen processing and presentation; (d) the bacteria do not induce rapid host cell death; and (e) cytosolic bacteria induce a cytokine response that favours CMI. Collectively, these features make L. monocytogenes an attractive vaccine vector for both infectious disease applications and cancer immunotherapy.

Novel Mouse Model Reveals That Serine Phosphorylation of L-Plastin Is Essential for Effective Splenic Clearance of Pneumococcus

Asplenia imparts susceptibility to life-threatening sepsis with encapsulated bacteria, such as the pneumococcus. However, the cellular components within the splenic environment that guard against pneumococcal bacteremia have not been defined. The actin-bundling protein L-plastin (LPL) is essential for the generation of marginal zone B cells and for anti-pneumococcal host defense, as revealed by a mouse model of genetic LPL deficiency. In independent studies, serine phosphorylation of LPL at residue 5 (S5) has been described as a key "switch" in regulating LPL actin binding and subsequent cell motility, although much of the data are correlative. To test the importance of S5 phosphorylation in LPL function, and to specifically assess the requirement of LPL S5 phosphorylation in anti-pneumococcal host defense, we generated the "S5A" mouse, expressing endogenous LPL bearing a serine-to-alanine mutation at this position. S5A mice were bred to homozygosity, and LPL was expressed at levels equivalent to wild-type, but S5 phosphorylation was absent. S5A mice exhibited specific impairment in clearance of pneumococci following i.v. challenge, with 10-fold-higher bacterial bloodstream burden 24 h after challenge compared with wild-type or fully LPL-deficient animals. Defective bloodstream clearance correlated with diminished population of marginal zone macrophages and with reduced phagocytic capacity of multiple innate immune cells. Development and function of other tested leukocyte lineages, such as T and B cell motility and activation, were normal in S5A mice. The S5A mouse thus provides a novel system in which to elucidate the precise molecular control of critical immune cell functions in specific host-pathogen defense interactions.

TAP dysfunction in dendritic cells enables noncanonical cross-presentation for T cell priming

Classic major histocompatibility complex class I (MHC-I) presentation relies on shuttling cytosolic peptides into the endoplasmic reticulum (ER) by the transporter associated with antigen processing (TAP). Viruses disable TAP to block MHC-I presentation and evade cytotoxic CD8+ T cells. Priming CD8+ T cells against these viruses is thought to rely solely on cross-presentation by uninfected TAP-functional dendritic cells. We found that protective CD8+ T cells could be mobilized during viral infection even when TAP was absent in all hematopoietic cells. TAP blockade depleted the endosomal recycling compartment of MHC-I molecules and, as such, impaired Toll-like receptor-regulated cross-presentation. Instead, MHC-I molecules accumulated in the ER-Golgi intermediate compartment (ERGIC), sequestered away from Toll-like receptor control, and coopted ER-SNARE Sec22b-mediated vesicular traffic to intersect with internalized antigen and rescue cross-presentation. Thus, when classic MHC-I presentation and endosomal recycling compartment-dependent cross-presentation are impaired in dendritic cells, cell-autonomous noncanonical cross-presentation relying on ERGIC-derived MHC-I counters TAP dysfunction to nevertheless mediate CD8+ T cell priming.

Enteric helminth coinfection enhances host susceptibility to neurotropic flaviviruses via a tuft cell-IL-4 receptor signaling axis

Although enteric helminth infections modulate immunity to mucosal pathogens, their effects on systemic microbes remain less established. Here, we observe increased mortality in mice coinfected with the enteric helminth Heligmosomoides polygyrus bakeri (Hpb) and West Nile virus (WNV). This enhanced susceptibility is associated with altered gut morphology and transit, translocation of commensal bacteria, impaired WNV-specific T cell responses, and increased virus infection in the gastrointestinal tract and central nervous system. These outcomes were due to type 2 immune skewing, because coinfection in Stat6-/- mice rescues mortality, treatment of helminth-free WNV-infected mice with interleukin (IL)-4 mirrors coinfection, and IL-4 receptor signaling in intestinal epithelial cells mediates the susceptibility phenotypes. Moreover, tuft cell-deficient mice show improved outcomes with coinfection, whereas treatment of helminth-free mice with tuft cell-derived cytokine IL-25 or ligand succinate worsens WNV disease. Thus, helminth activation of tuft cell-IL-4-receptor circuits in the gut exacerbates infection and disease of a neurotropic flavivirus.

Stable colony-stimulating factor 1 fusion protein treatment increases hematopoietic stem cell pool and enhances their mobilisation in mice

Background

Prior chemotherapy and/or underlying morbidity commonly leads to poor mobilisation of hematopoietic stem cells (HSC) for transplantation in cancer patients. Increasing the number of available HSC prior to mobilisation is a potential strategy to overcome this deficiency. Resident bone marrow (BM) macrophages are essential for maintenance of niches that support HSC and enable engraftment in transplant recipients. Here we examined potential of donor treatment with modified recombinant colony-stimulating factor 1 (CSF1) to influence the HSC niche and expand the HSC pool for autologous transplantation.

Methods

We administered an acute treatment regimen of CSF1 Fc fusion protein (CSF1-Fc, daily injection for 4 consecutive days) to naive C57Bl/6 mice. Treatment impacts on macrophage and HSC number, HSC function and overall hematopoiesis were assessed at both the predicted peak drug action and during post-treatment recovery. A serial treatment strategy using CSF1-Fc followed by granulocyte colony-stimulating factor (G-CSF) was used to interrogate HSC mobilisation impacts. Outcomes were assessed by in situ imaging and ex vivo standard and imaging flow cytometry with functional validation by colony formation and competitive transplantation assay.

Results

CSF1-Fc treatment caused a transient expansion of monocyte-macrophage cells within BM and spleen at the expense of BM B lymphopoiesis and hematopoietic stem and progenitor cell (HSPC) homeostasis. During the recovery phase after cessation of CSF1-Fc treatment, normalisation of hematopoiesis was accompanied by an increase in the total available HSPC pool. Multiple approaches confirmed that CD48⁻CD150⁺ HSC do not express the CSF1 receptor, ruling out direct action of CSF1-Fc on these cells. In the spleen, increased HSC was associated with expression of the BM HSC niche macrophage marker CD169 in red pulp macrophages, suggesting elevated spleen engraftment with CD48⁻CD150⁺ HSC was secondary to CSF1-Fc macrophage impacts. Competitive transplant assays demonstrated that pre-treatment of donors with CSF1-Fc increased the number and reconstitution potential of HSPC in blood following a HSC mobilising regimen of G-CSF treatment.

Conclusion

These results indicate that CSF1-Fc conditioning could represent a therapeutic strategy to overcome poor HSC mobilisation and subsequently improve HSC transplantation outcomes.

Identification of a nerve-associated, lung-resident interstitial macrophage subset with distinct localization and immunoregulatory properties

Tissue-resident macrophages are a diverse population of cells that perform specialized functions including sustaining tissue homeostasis and tissue surveillance. Here, we report an interstitial subset of CD169⁺ lung-resident macrophages that are transcriptionally and developmentally distinct from alveolar macrophages (AMs). They are primarily localized around the airways and are found in close proximity to the sympathetic nerves in the bronchovascular bundle. These nerve- and airway-associated macrophages (NAMs) are tissue resident, yolk sac derived, self-renewing, and do not require CCR2⁺ monocytes for development or maintenance. Unlike AMs, the development of NAMs requires CSF1 but not GM-CSF. Bulk population and single-cell transcriptome analysis indicated that NAMs are distinct from other lung-resident macrophage subsets and highly express immunoregulatory genes under steady-state and inflammatory conditions. NAMs proliferated robustly after influenza infection and activation with the TLR3 ligand poly(I:C), and in their absence, the inflammatory response was augmented, resulting in excessive production of inflammatory cytokines and innate immune cell infiltration. Overall, our study provides insights into a distinct subset of airway-associated pulmonary macrophages that function to maintain immune and tissue homeostasis.

Myeloid Cell CK2 Regulates Inflammation and Resistance to Bacterial Infection

Kinase activity plays an essential role in the regulation of immune cell defenses against pathogens. The protein kinase CK2 (formerly casein kinase II) is an evolutionarily conserved kinase with hundreds of identified substrates. CK2 is ubiquitously expressed in somatic and immune cells, but the roles of CK2 in regulation of immune cell function remain largely elusive. This reflects the essential role of CK2 in organismal development and limited prior work with conditional CK2 mutant murine models. Here, we generated mice with a conditional (floxed) allele of Csnk2a, which encodes the catalytic CK2α subunit of CK2. When crossed to Lyz2-cre mice, excision of Csnk2a sequence impaired CK2α expression in myeloid cells but failed to detectably alter myeloid cell development. By contrast, deficiency for CK2α increased inflammatory myeloid cell recruitment, activation, and resistance following systemic Listeria monocytogenes (Lm) infection. Results from mixed chimera experiments indicated that CK2α deficiency in only a subset of myeloid cells was not sufficient to reduce bacterial burdens. Nor did cell-intrinsic deficiency for CK2α suffice to alter accumulation or activation of monocytes and neutrophils in infected tissues. These data suggest that CK2α expression by Lyz2-expressing cells promotes inflammatory and anti-bacterial responses through effects in trans. Our results highlight previously undescribed suppressive effects of CK2 activity on inflammatory myeloid cell responses and illustrate that cell-extrinsic effects of CK2 can shape inflammatory and protective innate immune responses.

Mucosal CD8 T Cell Responses Are Shaped by Batf3-DC After Foodborne Listeria monocytogenes Infection

While immune responses have been rigorously examined after intravenous Listeria monocytogenes (Lm) infection, less is understood about its dissemination from the intestines or the induction of adaptive immunity after more physiologic models of foodborne infection. Consequently, this study focused on early events in the intestinal mucosa and draining mesenteric lymph nodes (MLN) using foodborne infection of mice with Lm modified to invade murine intestinal epithelium (InlA^MLm). InlA^MLm trafficked intracellularly from the intestines to the MLN and were associated with Batf3-independent dendritic cells (DC) in the lymphatics. Consistent with this, InlA^MLm initially disseminated from the gut to the MLN normally in Batf3^–/– mice. Activated migratory DC accumulated in the MLN by 3 days post-infection and surrounded foci of InlA^MLm. At this time Batf3^–/– mice displayed reduced InlA^MLm burdens, implicating cDC1 in maximal bacterial accumulation in the MLN. Batf3^–/– mice also exhibited profound defects in the induction and gut-homing of InlA^MLm-specific effector CD8 T cells. Restoration of pathogen burden did not rescue antigen-specific CD8 T cell responses in Batf3^–/– mice, indicating a critical role for Batf3 in generating anti-InlA^MLm immunity following foodborne infection. Collectively, these data suggest that DC play diverse, dynamic roles in the early events following foodborne InlA^MLm infection and in driving the establishment of intestinal Lm-specific effector T cells.

The intrinsic and extrinsic elements regulating inflammation

Inflammation is a sophisticated biological tissue response to both extrinsic and intrinsic stimuli. Although the pathological aspects of inflammation are well appreciated, there are still rooms for understanding the physiological functions of the inflammation. Recent studies have focused on mechanisms, context and the role of physiological inflammation. Besides, there have been progress in the comprehension of commensal microbiota, immunometabolism, cancer and intracellular signaling events' roles that impact on the regulation of inflammation. Despite the fact that inflammatory responses are vital through tissue damage, understanding the mechanisms to turn off the finished or unnecessary inflammation is crucial for restoring homeostasis. Inflammation seems to be a smart process that acts like two edges of a sword, meaning that it has both protective and deleterious consequences. Knowing both edges and the regulation processes will help the future understanding and therapy for various diseases.

TLR2 and endosomal TLR-mediated secretion of IL-10 and immune suppression in response to phagosome-confined Listeria monocytogenes

Listeria monocytogenes is a facultative intracellular bacterial pathogen that escapes from phagosomes and induces a robust adaptive immune response in mice, while mutants unable to escape phagosomes fail to induce a robust adaptive immune response and suppress the immunity to wildtype bacteria when co-administered. The capacity to suppress immunity can be reversed by blocking IL-10. In this study, we sought to understand the host receptors that lead to secretion of IL-10 in response to phagosome-confined L. monocytogenes (Δhly), with the ultimate goal of generating strains that fail to induce IL-10. We conducted a transposon screen to identify Δhly L. monocytogenes mutants that induced significantly more or less IL-10 secretion in bone marrow-derived macrophages (BMMs). A transposon insertion in lgt, which encodes phosphatidylglycerol-prolipoprotein diacylglyceryl transferase and is essential for the formation of lipoproteins, induced significantly reduced IL-10 secretion. Mutants with transposon insertions in pgdA and oatA, which encode peptidoglycan N-acetylglucosamine deacetylase and O-acetyltransferase, are sensitive to lysozyme and induced enhanced IL-10 secretion. A ΔhlyΔpgdAΔoatA strain was killed in BMMs and induced enhanced IL-10 secretion that was dependent on Unc93b1, a trafficking molecule required for signaling of nucleic acid-sensing TLRs. These data revealed that nucleic acids released by bacteriolysis triggered endosomal TLR-mediated IL-10 secretion. Secretion of IL-10 in response to infection with the parental strain was mostly TLR2-dependent, while IL-10 secretion in response to lysozyme-sensitive strains was dependent on TLR2 and Unc93b1. In mice, the IL-10 response to vacuole-confined L. monocytogenes was also dependent on TLR2 and Unc93b1. Co-administration of Δhly and ΔactA resulted in suppressed immunity in WT mice, but not in mice with mutations in Unc93b1. These data revealed that secretion of IL-10 in response to L. monocytogenes infection in vitro is mostly TLR2-dependent and immune suppression by phagosome-confined bacteria in vivo is mostly dependent on endosomal TLRs.

Interferon Alpha Favors Macrophage Infection by Visceral Leishmania Species Through Upregulation of Sialoadhesin Expression

Type I interferons (IFNs) induced by an endogenous Leishmania RNA virus or exogenous viral infections have been shown to exacerbate infections with New World Cutaneous Leishmania parasites, however, the impact of type I IFNs in visceral Leishmania infections and implicated mechanisms remain to be unraveled. This study assessed the impact of type I IFN on macrophage infection with L. infantum and L. donovani and the implication of sialoadhesin (Siglec-1/CD169, Sn) as an IFN-inducible surface receptor. Stimulation of bone marrow-derived macrophages with type I IFN (IFN-α) significantly enhanced susceptibility to infection of reference laboratory strains and a set of recent clinical isolates. IFN-α particularly enhanced promastigote uptake. Enhanced macrophage susceptibility was linked to upregulated Sn surface expression as a major contributing factor to the infection exacerbating effect of IFN-α. Stimulation experiments in Sn-deficient macrophages, macrophage pretreatment with a monoclonal anti-Sn antibody or a novel bivalent anti-Sn nanobody and blocking of parasites with soluble Sn restored normal susceptibility levels. Infection of Sn-deficient mice with bioluminescent L. infantum promastigotes revealed a moderate, strain-dependent role for Sn during visceral infection under the used experimental conditions. These data indicate that IFN-responsive Sn expression can enhance the susceptibility of macrophages to infection with visceral Leishmania promastigotes and that targeting of Sn may have some protective effects in early infection.

Structure and function of the immune system in the spleen

The spleen is the largest secondary lymphoid organ in the body and, as such, hosts a wide range of immunologic functions alongside its roles in hematopoiesis and red blood cell clearance. The physical organization of the spleen allows it to filter blood of pathogens and abnormal cells and facilitate low-probability interactions between antigen-presenting cells (APCs) and cognate lymphocytes. APCs specific to the spleen regulate the T and B cell response to these antigenic targets in the blood. This review will focus on cell types, cell organization, and immunologic functions specific to the spleen and how these affect initiation of adaptive immunity to systemic blood-borne antigens. Potential differences in structure and function between mouse and human spleen will also be discussed.

Sleep-restriction Inhibits Neurogenesis Through Decreasing the Infiltration of CD169+ Macrophages to Ischemic Brain After Stroke

Chronic sleep-restriction (SR) is shown to be correlated with neurodevelopmental disorders. However, the effects of SR during stroke recovery on neurorepair remain unclear. In this study, mice were subjected to 60 min of cerebral ischemia followed by reperfusion. The SR protocol was accomplished by depriving mice of sleep for 20 h/day for 14 days starting at 14 days post-ischemia. We found that SR increased CD169⁺ macrophages infiltration into the ischemic brain parenchyma and inhibited neurogenesis and functional recovery. SR decreased CD169⁺ macrophages infiltration into the choroid plexus (CP) and cerebrospinal fluid (CSF), accompanied by increased expression of Chemokine C-X3-C-Motif Ligand 1 (CX3CL1) and intercellular adhesion molecule (ICAM-1) via IFN-γ/IFN-γR signaling in the CP. When splenic CD169⁺ macrophages sorted from Kaede transgenic mice were administered into CSF of C57BL/6 mice, they homed to the ischemic brain parenchyma. Moreover, blockade of IFN-γ/IFN-γR signaling, CX3CL1 or ICAM-1 decreased CD169⁺ macrophages infiltration into the CP, CSF and ischemic brain parenchyma, as well as decreasing neurogenesis and functional recovery after SR. The promoting roles of infiltrated CD169⁺ macrophages in post-stroke neurogenesis were due to increasing regulatory T cells (Tregs) in the ischemic brain parenchyma. Furthermore, dexmedetomidine treatment during SR increased CD169⁺ macrophages infiltration into the CP, CSF and ischemic brain parenchyma, and promoted neurogenesis and functional recovery. Taken together, our results showed that SR during stroke recovery decreased Tregs in the ischemic brain parenchyma by decreasing CD169⁺ macrophages infiltration to the ischemic brain parenchyma across the CP, which inhibited neurogenesis and functional recovery.

An Inducible Cre-lox System to Analyze the Role of LLO in Listeria monocytogenes Pathogenesis

Listeriolysin O (LLO) is a pore-forming cytolysin that allows Listeria monocytogenes to escape from phagocytic vacuoles and enter the host cell cytosol. LLO is expressed continuously during infection, but it has been a challenge to evaluate the importance of LLO secreted in the host cell cytosol because deletion of the gene encoding LLO (hly) prevents localization of L. monocytogenes to the cytosol. Here, we describe a L. monocytogenes strain (hlyfl) in which hly is flanked by loxP sites and Cre recombinase is under the transcriptional control of the L. monocytogenesactA promoter, which is highly induced in the host cell cytosol. In less than 2 h after infection of bone marrow-derived macrophages (BMMs), bacteria were 100% non-hemolytic. hlyfl grew intracellularly to levels 10-fold greater than wildtype L. monocytogenes and was less cytotoxic. In an intravenous mouse model, 90% of bacteria were non-hemolytic within three hours in the spleen and eight hours in the liver. The loss of LLO led to a 2-log virulence defect in the spleen and a 4-log virulence defect in the liver compared to WT L. monocytogenes. Thus, the production of LLO in the cytosol has significant impact on the pathogenicity of L. monocytogenes.

Serum Amyloid A Proteins Induce Pathogenic Th17 Cells and Promote Inflammatory Disease

Lymphoid cells that produce interleukin (IL)-17 cytokines protect barrier tissues from pathogenic microbes but are also prominent effectors of inflammation and autoimmune disease. T helper 17 (Th17) cells, defined by RORγt-dependent production of IL-17A and IL-17F, exert homeostatic functions in the gut upon microbiota-directed differentiation from naive CD4⁺ T cells. In the non-pathogenic setting, their cytokine production is regulated by serum amyloid A proteins (SAA1 and SAA2) secreted by adjacent intestinal epithelial cells. However, Th17 cell behaviors vary markedly according to their environment. Here, we show that SAAs additionally direct a pathogenic pro-inflammatory Th17 cell differentiation program, acting directly on T cells in collaboration with STAT3-activating cytokines. Using loss- and gain-of-function mouse models, we show that SAA1, SAA2, and SAA3 have distinct systemic and local functions in promoting Th17-mediated inflammatory diseases. These studies suggest that T cell signaling pathways modulated by the SAAs may be attractive targets for anti-inflammatory therapies.

Breast cancer cells promote CD169+ macrophage-associated immunosuppression through JAK2-mediated PD-L1 upregulation on macrophages

Macrophages are recognized as one of the major cell types in tumor microenvironment, and macrophage infiltration has been predominantly associated with poor prognosis among patients with breast cancer. Using the murine models of triple-negative breast cancer in CD169-DTR mice, we found that CD169⁺ macrophages support tumor growth and metastasis. CD169⁺ macrophage depletion resulted in increased accumulation of CD8⁺ T cells within tumor, and produced significant expansion of CD8⁺ T cells in circulation and spleen. In addition, we observed that CD169⁺ macrophage depletion alleviated tumor-induced splenomegaly in mice, but had no improvement in bone loss and repression of bone marrow erythropoiesis in tumor-bearing mice. Cancer cells and tumor associated macrophages exploit the upregulation of the immunosuppressive protein PD-L1 to subvert T cell-mediated immune surveillance. Within the tumor microenvironment, our understanding of the regulation of PD-L1 protein expression is limited. We showed that there was a 5-fold higher relative expression of PD-L1 on macrophages as compared with 4T1 tumor cells; coculture of macrophages with 4T1 cells augmented PD-L1 levels on macrophages, but did not upregulate the expression of PD-L1 on 4T1 cells. JAK2/STAT3 signaling pathway was activated in macrophages after coculture, and we further identified the JAK2 as a critical regulator of PD-L1 expression in macrophages during coculture with 4T1 cells. Collectively, our data reveal that breast cancer cells and CD169⁺ macrophages exhibit bidirectional interactions that play a critical role in tumor progression, and inhibition of JAK2 signaling pathway in CD169⁺ macrophages may be potential strategy to block tumor microenvironment-derived immune escape.

Macrophage Depletion Mitigates Platelet Aggregate Formation in Splenic Marginal Zone and Alleviates LPS-Associated Thrombocytopenia in Rats

Sepsis is often accompanied with thrombocytopenia partly due to platelet sequestration in the lung and liver. The spleen can store up to one-third of circulating platelets and can also significantly affect platelet transfusion outcomes by accumulating platelets. However, in sepsis, it is not clear whether there are platelet changes in the spleen which could contribute to sepsis-associated thrombocytopenia and also influence platelet transfusion outcomes. By using confocal microscopy, we examined endogenous rat platelets and infused human platelets in the spleen of severe combined immune deficient Rag2 KO rats which were injected intraperitoneally with lipopolysaccharide (LPS). LPS-injected Rag2 KO rats developed sepsis as indicated by increased TNFa, IL-6, IL-1b, and IL-10 levels and thrombocytopenia. Large platelet aggregates were observed in the spleen with majority located in the marginal zone and closely associated with CD169+ macrophages. Depletion of macrophages by clodrosome resulted in reduction of LPS-induced cytokine generation and alleviated LPS-induced thrombocytopenia. Macrophage depletion also remarkedly diminished large platelet aggregate formation in the splenic marginal zone but had less effect on those in red pulp. Infusion of human platelets into LPS-injected rats failed to raise platelet counts in the peripheral blood. In LPS-injected rat spleen, human platelets interacted with aggregated rat platelets in the marginal zone. In contrast, human platelets infused into control rats were located outside of splenic marginal zone. This study provides morphological evidence of platelet aggregates in the splenic marginal zone in sepsis which can interact with infused platelets and thus can contribute to platelet infusion refractoriness in sepsis. It indicates that macrophages play an important role in LPS-associated thrombocytopenia. It also suggests that CD169+ macrophages support platelet aggregate formation in the splenic marginal zone.

Effect of dietary stevioside supplementation on growth performance, nutrient digestibility, serum parameters, and intestinal microflora in broilers

Disinhibition of antibiotics promotes the use of probiotics, prebiotics, immune enhancers, and plant extracts. We investigated the effects of stevioside on growth performance, nutrient digestibility, serum parameters, and intestinal microflora in broilers. Eight hundred ninety-six one-day-old male Arbor Acres broiler chicks (average body weight 48.36 ± 0.21 g) were allotted to 1 of 7 experimental treatments. Treatments consisted of: (1) control (basal diet without supplemental stevioside), (2) 100 mg kg-1 supplemental stevioside (S100), (3) 200 mg kg-1 supplemental stevioside (S200), (4) 400 mg kg-1 supplemental stevioside (S400), (5) 800 mg kg-1 supplemental stevioside (S800), (6) 1600 mg kg-1 supplemental stevioside (S1600), and (7) 3200 mg kg-1 supplemental stevioside (S3200). Performance was not affected by stevioside concentration. Dietary stevioside supplementation increased the digestibility of calcium (P < 0.05) and tended to improve phosphorus digestibility (P = 0.0730). There was a linear effect of dietary stevioside on the concentration of serum glucose (P < 0.05). The serum IgG and IgA levels were linearly increased by stevioside supplementation (P < 0.05). In the ileal digesta, the concentration of E. coli decreased with increasing dietary stevioside supplementation (P < 0.05). On the contrary, dietary stevioside supplementation increased the concentration of Bifidobacteria (P < 0.01) and tended to improve the concentration of Lactobacillus (P = 0.0791). In conclusion, our data suggest that stevioside supplementation could improve the calcium and phosphorus digestibility and decrease blood glucose levels of broilers. Additionally, dietary stevioside supplementation significantly increased Bifidobacteria in the cecal digesta, and decreased E. coli.

A Triad of Immune Cells Promotes Infection

The intracellular pathogen L. monocytogenes takes advantage of several myeloid cell populations to establish infection in the spleen. In this issue, Liu et al. (2019) reveal how marginal zone B cells, dendritic cells, and marginal metallophilic macrophages act together with IL-10 to promote L. monocytogenes infection, while simultaneously enabling adaptive CD8⁺ T cell responses.

Systemic inflammation impairs microglial Aβ clearance through NLRP3 inflammasome

Alzheimer's disease is the most prevalent type of dementia and is caused by the deposition of extracellular amyloid‐beta and abnormal tau phosphorylation. Neuroinflammation has emerged as an additional pathological component. Microglia, representing the brain's major innate immune cells, play an important role during Alzheimer's. Once activated, microglia show changes in their morphology, characterized by a retraction of cell processes. Systemic inflammation is known to increase the risk for cognitive decline in human neurogenerative diseases including Alzheimer's. Here, we assess for the first time microglial changes upon a peripheral immune challenge in the context of aging and Alzheimer's in vivo, using 2‐photon laser scanning microscopy. Microglia were monitored at 2 and 10 days post‐challenge by lipopolysaccharide. Microglia exhibited a reduction in the number of branches and the area covered at 2 days, a phenomenon that resolved at 10 days. Systemic inflammation reduced microglial clearance of amyloid‐beta in APP/PS1 mice. NLRP3 inflammasome knockout blocked many of the observed microglial changes upon lipopolysaccharide, including alterations in microglial morphology and amyloid pathology. NLRP3 inhibition may thus represent a novel therapeutic target that may protect the brain from toxic peripheral inflammation during systemic infection.

IL-10-Dependent Crosstalk between Murine Marginal Zone B Cells, Macrophages, and CD8α+ Dendritic Cells Promotes Listeria monocytogenes Infection

Type 1 CD8α⁺ conventional dendritic cells (cDC1s) are required for CD8⁺ T cell priming but, paradoxically, promote splenic Listeria monocytogenes infection. Using mice with impaired cDC2 function, we ruled out a role for cDC2s in this process and instead discovered an interleukin-10 (IL-10)-dependent cellular crosstalk in the marginal zone (MZ) that promoted bacterial infection. Mice lacking the guanine nucleotide exchange factor DOCK8 or CD19 lost IL-10-producing MZ B cells and were resistant to Listeria. IL-10 increased intracellular Listeria in cDC1s indirectly by reducing inducible nitric oxide synthase expression early after infection and increasing intracellular Listeria in MZ metallophilic macrophages (MMMs). These MMMs trans-infected cDC1s, which, in turn, transported Listeria into the white pulp to prime CD8⁺ T cells. However, this also facilitated bacterial expansion. Therefore, IL-10-mediated crosstalk between B cells, macrophages, and cDC1s in the MZ promotes both Listeria infection and CD8⁺ T cell activation.

Glycosylation in health and disease

The glycome describes the complete repertoire of glycoconjugates composed of carbohydrate chains, or glycans, that are covalently linked to lipid or protein molecules. Glycoconjugates are formed through a process called glycosylation and can differ in their glycan sequences, the connections between them and their length. Glycoconjugate synthesis is a dynamic process that depends on the local milieu of enzymes, sugar precursors and organelle structures as well as the cell types involved and cellular signals. Studies of rare genetic disorders that affect glycosylation first highlighted the biological importance of the glycome, and technological advances have improved our understanding of its heterogeneity and complexity. Researchers can now routinely assess how the secreted and cell-surface glycomes reflect overall cellular status in health and disease. In fact, changes in glycosylation can modulate inflammatory responses, enable viral immune escape, promote cancer cell metastasis or regulate apoptosis; the composition of the glycome also affects kidney function in health and disease. New insights into the structure and function of the glycome can now be applied to therapy development and could improve our ability to fine-tune immunological responses and inflammation, optimize the performance of therapeutic antibodies and boost immune responses to cancer. These examples illustrate the potential of the emerging field of 'glycomedicine'.

The Silent Undertakers: Macrophages Programmed for Efferocytosis

Two recent Immunity papers provide new insight into efferocytosis by tissue-resident macrophages. Baratin et al. (2017) identify a resident macrophage population in the T cell zone of lymph nodes responsible for the silent uptake of vast numbers of apoptotic cells. Roberts et al. (2017) find that resident macrophages can be programmed by local tissue signals not to respond to the nucleic acid of apoptotic cells.

Innate and Adaptive Immune Responses during Listeria monocytogenes Infection

It could be argued that we understand the immune response to infection with Listeria monocytogenes better than the immunity elicited by any other bacteria. L. monocytogenes are Gram-positive bacteria that are genetically tractable and easy to cultivate in vitro, and the mouse model of intravenous (i.v.) inoculation is highly reproducible. For these reasons, immunologists frequently use the mouse model of systemic listeriosis to dissect the mechanisms used by mammalian hosts to recognize and respond to infection. This article provides an overview of what we have learned over the past few decades and is divided into three sections: "Innate Immunity" describes how the host initially detects the presence of L. monocytogenes and characterizes the soluble and cellular responses that occur during the first few days postinfection; "Adaptive Immunity" discusses the exquisitely specific T cell response that mediates complete clearance of infection and immunological memory; "Use of Attenuated Listeria as a Vaccine Vector" highlights the ways that investigators have exploited our extensive knowledge of anti-Listeria immunity to develop cancer therapeutics.

Obesogenic diet in aging mice disrupts gut microbe composition and alters neutrophil:lymphocyte ratio, leading to inflamed milieu in acute heart failure

Calorie-dense obesogenic diet (OBD) is a prime risk factor for cardiovascular disease in aging. However, increasing age coupled with changes in the diet can affect the interaction of intestinal microbiota influencing the immune system, which can lead to chronic inflammation. How age and calorie-enriched OBD interact with microbial flora and impact leukocyte profiling is currently under investigated. Here, we tested the interorgan hypothesis to determine whether OBD in young and aging mice alters the gut microbe composition and the splenic leukocyte profile in acute heart failure (HF). Young (2-mo-old) and aging (18-mo-old) mice were supplemented with standard diet (STD, ∼4% safflower oil diet) and OBD (10% safflower oil) for 2 mo and then subjected to coronary artery ligation to induce myocardial infarction. Fecal samples were collected pre- and post-diet intervention, and the microbial flora were analyzed using 16S variable region 4 rRNA gene DNA sequencing and Quantitative Insights Into Microbial Ecology informatics. The STD and OBD in aging mice resulted in an expansion of the genus Allobaculum in the fecal microbiota. However, we found a pathologic change in the neutrophil:lymphocyte ratio in aging mice in comparison with their young counterparts. Thus, calorie-enriched OBD dysregulated splenic leukocytes by decreasing immune-responsive F4/80⁺ and CD169⁺ macrophages in aging mice. OBD programmed neutrophil swarming with an increase in isoprostanoid levels, with dysregulation of lipoxygenases, cytokines, and metabolite-sensing receptor expression. In summary, calorie-dense OBD in aging mice disrupted the composition of the gut microbiome, which correlates with the development of integrative and system-wide nonresolving inflammation in acute HF.-Kain, V., Van Der Pol, W., Mariappan, N., Ahmad, A., Eipers, P., Gibson, D. L., Gladine, C., Vigor, C., Durand, T., Morrow, C., Halade, G. V. Obesogenic diet in aging mice disrupts gut microbe composition and alters neutrophil:lymphocyte ratio, leading to inflamed milieu in acute heart failure.

Francisella tularensis enters a double membraned compartment following cell-cell transfer

Previously, we found that phagocytic cells ingest bacteria directly from the cytosol of infected cells without killing the initially infected cell (Steele et al., 2016). Here, we explored the events immediately following bacterial transfer. Francisella tularensis bacteria acquired from infected cells were found within double-membrane vesicles partially composed from the donor cell plasma membrane. As with phagosomal escape, the F. tularensis Type VI Secretion System (T6SS) was required for vacuole escape. We constructed a T6SS inducible strain and established conditions where this strain is trapped in vacuoles of cells infected through bacterial transfer. Using this strain we identified bacterial transfer events in the lungs of infected mice, demonstrating that this process occurs in infected animals. These data and electron microscopy analysis of the transfer event revealed that macrophages acquire cytoplasm and membrane components of other cells through a process that is distinct from, but related to phagocytosis.

Doxorubicin triggers splenic contraction and irreversible dysregulation of COX and LOX that alters the inflammation-resolution program in the myocardium

Doxorubicin (DOX) is a widely used drug for cancer treatment as a chemotherapeutic agent. However, the cellular and integrative mechanism of DOX-induced immunometabolism is unclear. Two-month-old male C57BL/6J mice were divided into high- and low-dose DOX-treated groups with a maintained saline control group. The first group was injected with a high dose of DOX (H-DOX; 15 mg·kg-1·wk-1), and the second group was injected with 7.5 mg·kg-1·wk-1 as a latent low dose of DOX (LL-DOX). H-DOX treatment led to complete mortality in 2 wk and 70% survival in the LL-DOX group compared with the saline control group. Therefore, an additional group of mice was injected with an acute high dose of DOX (AH-DOX) and euthanized at 24 h to compare with LL-DOX and saline control groups. The LL-DOX and AH-DOX groups showed obvious apoptosis and dysfunctional and structural changes in cardiac tissue. Splenic contraction was evident in AH-DOX- and LL-DOX-treated mice, indicating the systems-wide impact of DOX on integrative organs of the spleen, which is essential for cardiac homeostasis and repair. DOX dysregulated splenic-enriched immune-sensitive lipoxygenase and cyclooxygenase in the spleen and left ventricle compared with the saline control group. As a result, lipoxygenase-dependent D- and E-series resolvin precursors, such as 16HDoHE, 4HDoHE, and 12-HEPE, as well as cyclooxygenase-mediated PG species (PGD2, PGE2, and 6-keto-PG2α) were decreased in the left ventricle, suggestive of defective immunometabolism. Both AH-DOX and LL-DOX induced splenic contraction and expansion of red pulp with decreased CD169+ metallophilic macrophages. AH-DOX intoxicated macrophages in the spleen by depleting CD169+ cells in the acute setting and sustained the splenic macrophage loss in the chronic phase in the LL-DOX group. Thus, DOX triggers a vicious cycle of splenocardiac cachexia to facilitate defective immunometabolism and irreversible macrophage toxicity and thereby impaired the inflammation-resolution program. NEW & NOTEWORTHY Doxorubicin (DOX) triggered splenic mass loss and decreased CD169 with germinal center contraction in acute and chronic exposure. Cardiac toxicity of DOX is marked with dysregulation of immunometabolism and thereby impaired resolution of inflammation. DOX suppressed physiological levels of cytokines and chemokines with signs of splenocardiac cachexia.

CD169+ Macrophages Capture and Dendritic Cells Instruct: The Interplay of the Gatekeeper and the General of the Immune System

Since the seminal discovery of dendritic cells (DCs) by Steinman and Cohn in 1973, there has been an ongoing debate to what extent macrophages and DCs are related and perform different functions. The current view is that macrophages and DCs originate from different lineages and that only DCs have the capacity to initiate adaptive immunity. Nevertheless, as we will discuss in this review, lymphoid tissue resident CD169⁺ macrophages have been shown to act in concert with DCs to promote or suppress adaptive immune responses for pathogens and self-antigens, respectively. Accordingly, we propose a functional alliance between CD169⁺ macrophages and DCs in which a division of tasks is established. CD169⁺ macrophages are responsible for the capture of pathogens and are frequently the first cell type infected and thereby provide a confined source of antigen. Subsequently, cross-presenting DCs interact with these antigen-containing CD169⁺ macrophages, pick up antigens and activate T cells. The cross-priming of T cells by DCs is enhanced by the localized production of type I interferons (IFN-I) derived from CD169⁺ macrophages and plasmacytoid DCs (pDCs) that induces DC maturation. The interaction between CD169⁺ macrophages and DCs appears not only to be essential for immune responses against pathogens, but also plays a role in the induction of self-tolerance and immune responses against cancer. In this review we will discuss the studies that demonstrate the collaboration between CD169⁺ macrophages and DCs in adaptive immunity.