PRKCSH contributes to TNFSF resistance by extending IGF1R half-life and activation in lung cancer.

Tumor necrosis factor superfamily (TNFSF) resistance contributes to the development and progression of tumors and resistance to various cancer therapies. Tumor-intrinsic alterations involved in the adaptation to the TNFSF response remain largely unknown. Here, we demonstrate that protein kinase C substrate 80K-H (PRKCSH) abundance in lung cancers boosts oncogenic IGF1R activation, leading to TNFSF resistance. PRKCSH abundance is correlated with IGF1R upregulation in lung cancer tissues. Specifically, PRKCSH interacts with IGF1R and extends its half-life. The PRKCSH-IGF1R axis in tumor cells impairs caspase-8 activation, increases Mcl-1 expression, and inhibits caspase-9, leading to an imbalance between cell death and survival. PRKCSH deficiency augmented the antitumor effects of natural killer (NK) cells, representative TNFSF effector cells, in a tumor xenograft IL-2Rg-deficient NOD/SCID (NIG) mouse model. Our data suggest that PRKCSH plays a critical role in TNFSF resistance and may be a potential target to improve the efficacy of NK cell-based cancer therapy.

CD115- monocytic myeloid-derived suppressor cells are precursors of OLFM4high polymorphonuclear myeloid-derived suppressor cells.

Myeloid-derived suppressor cells (MDSCs) consist of monocytic (M-) MDSCs and polymorphonuclear (PMN-) MDSCs that contribute to an immunosuppressive environment in tumor-bearing hosts. However, research on the phenotypic and functional heterogeneity of MDSCs in tumor-bearing hosts and across different disease stage is limited. Here we subdivide M-MDSCs based on CD115 expression and report that CD115- M-MDSCs are functionally distinct from CD115+ M-MDSCs. CD115- M-MDSCs increased in bone marrow and blood as tumors progressed. Transcriptome analysis revealed that CD115- M-MDSCs expressed higher levels of neutrophil-related genes. Moreover, isolated CD115- M-MDSCs had higher potential to be differentiated into PMN-MDSCs compared with CD115+ M-MDSCs. Of note, CD115- M-MDSCs were able to differentiate into both olfactomedin 4 (OLFM4)hi and OLFM4lo PMN-MDSCs, whereas CD115+ M-MDSCs differentiated into a smaller proportion of OLFM4lo PMN-MDSCs. In vivo, M-MDSC to PMN-MDSC differentiation occurred most frequently in bone marrow while M-MDSCs preferentially differentiated into tumor-associated macrophages in the tumor mass. Our study reveals the presence of previously unrecognized subtypes of CD115- M-MDSCs in tumor-bearing hosts and demonstrates their cellular plasticity during tumorigenesis.

Paraoxonase-2 contributes to promoting lipid metabolism and mitochondrial function via autophagy activation.

Non-alcoholic fatty liver disease (NAFLD) is an increasingly prevalent immuno-metabolic disease that can progress to hepatic cirrhosis and cancer. NAFLD pathogenesis is extremely complex and is characterized by oxidative stress, impaired mitochondrial function and lipid metabolism, and cellular inflammation. Thus, in-depth research on its underlying mechanisms and subsequent investigation into a potential drug target that has overarching effects on these features will help in the discovery of effective treatments for NAFLD. Our study examines the role of endogenous paraoxonase-2 (PON2), a membrane protein with reported antioxidant activity, in an in vitro cell model of NAFLD. We found that the hepatic loss of PON2 activity aggravated steatosis and oxidative stress under lipotoxic conditions, and our transcriptome analysis revealed that the loss of PON2 disrupts the activation of numerous functional pathways closely related to NAFLD pathogenesis, including mitochondrial respiratory capacity, lipid metabolism, and hepatic fibrosis and inflammation. We found that PON2 promoted the activation of the autophagy pathway, specifically the mitophagy cargo sequestration, which could potentially aid PON2 in alleviating oxidative stress, mitochondrial dysfunction, lipid accumulation, and inflammation. These results provide a mechanistic foundation for the prospect of PON2 as a drug target, leading to the development of novel therapeutics for NAFLD.

Microbiota-derived lactate promotes hematopoiesis and erythropoiesis by inducing stem cell factor production from leptin receptor+ niche cells.

Although functional interplay between intestinal microbiota and distant sites beyond the gut has been identified, the influence of microbiota-derived metabolites on hematopoietic stem cells (HSCs) remains unclear. This study investigated the role of microbiota-derived lactate in hematopoiesis using mice deficient in G-protein-coupled receptor (Gpr) 81 (Gpr81-/-), an established lactate receptor. We detected significant depletion of total HSCs in the bone marrow (BM) of Gpr81-/- mice compared with heterogenic (Gpr81+/-) mice in a steady state. Notably, the expression levels of stem cell factor (SCF), which is required for the proliferation of HSCs, decreased significantly in leptin receptor-expressing (LepR+) mesenchymal stromal cells (MSCs) around the sinusoidal vessels of the BM from Gpr81-/- mice compared with Gpr81+/- mice. Hematopoietic recovery and activation of BM niche cells after irradiation or busulfan treatment also required Gpr81 signals. Oral administration of lactic acid-producing bacteria (LAB) activated SCF secretion from LepR+ BM MSCs and subsequently accelerated hematopoiesis and erythropoiesis. Most importantly, LAB feeding accelerated the self-renewal of HSCs in germ-free mice. These results suggest that microbiota-derived lactate stimulates SCF secretion by LepR+ BM MSCs and subsequently activates hematopoiesis and erythropoiesis in a Gpr81-dependent manner.

RUVBL1/2 Complex Regulates Pro-Inflammatory Responses in Macrophages via Regulating Histone H3K4 Trimethylation.

Macrophages play an important role in the host defense mechanism. In response to infection, macrophages activate a genetic program of pro-inflammatory response to kill any invading pathogen, and initiate an adaptive immune response. We have identified RUVBL2 - an ATP-binding protein belonging to the AAA+ (ATPase associated with diverse cellular activities) superfamily of ATPases - as a novel regulator in pro-inflammatory response of macrophages. Gene knockdown of Ruvbl2, or pharmacological inhibition of RUVBL1/2 activity, compromises type-2 nitric oxide synthase (Nos2) gene expression, nitric oxide production and anti-bacterial activity of mouse macrophages in response to lipopolysaccharides (LPS). RUVBL1/2 inhibitor similarly inhibits pro-inflammatory response in human monocytes, suggesting functional conservation of RUVBL1/2 in humans. Transcriptome analysis further revealed that major LPS-induced pro-inflammatory pathways in macrophages are regulated in a RUVBL1/2-dependent manner. Furthermore, RUVBL1/2 inhibition significantly reduced the level of histone H3K4me3 at the promoter region of Nos2 and Il6, two prototypical pro-inflammatory genes, and diminished the recruitment of NF-kappaB to the corresponding enhancers. Our study reveals RUVBL1/2 as an integral component of macrophage pro-inflammatory responses through epigenetic regulations, and the therapeutic potentials of RUVBL1/2 inhibitors in the treatment of diseases caused by aberrant activation of pro-inflammatory pathways.

Endogenous DEL-1 restrains melanoma lung metastasis by limiting myeloid cell-associated lung inflammation.

Distant metastasis represents the primary cause of cancer-associated death. Pulmonary metastasis is most frequently seen in many cancers, largely driven by lung inflammation. Components from primary tumor or recruited leukocytes are known to facilitate metastasis formation. However, contribution of target site-specific host factor to metastasis is poorly understood. Here, we show that developmental endothelial locus-1 (DEL-1), an anti-inflammatory factor abundant in the lung and down-regulated by inflammatory insults, protects from melanoma lung metastasis independently of primary tumor development and systemic immunosurveillance. DEL-1 deficiency is associated with gene profiles that favor metastatic progression with inflammation and defective immunosurveillance. Mechanistically, DEL-1 deficiency primarily influences Ly6G+ neutrophil accumulation in lung metastatic niche, leading to IL-17A up-regulation from γδ T cells and reduced antimetastatic NK cells. In support, neutrophil depletion or recombinant DEL-1 treatment profoundly reverses these effects. Thus, our results identify DEL-1 as a previously unrecognized link between tumor-induced inflammation and pulmonary metastasis.

Plasmacytoid dendritic cells regulate colitis-associated tumorigenesis by controlling myeloid-derived suppressor cell infiltration.

Toll-like receptor (TLR)3 and TLR7 are important for stimulating plasmacytoid dendritic cells (pDCs), which secrete type I interferon. Mice deficient for TLR3 and TLR7 (TLR3-/-TLR7-/-) reportedly exhibit deteriorated colitis because of impaired pDCs. However, the role of pDCs in tumorigenesis-associated inflammation progression has not been studied. We treated wild-type or TLR3-/-TLR7-/- mice with dextran sulfate sodium (DSS) and/or azoxymethane (AOM) and examined colon mucosa, measured body weight and colon length of mice, and examined pDC and myeloid-derived suppressor cell (MDSC) accumulation. Further, we depleted pDCs in AOM/DSS-treated wild-type mice by treating them with anti-PDCA-1 antibodies. We found that MDSCs significantly increased, while pDCs decreased in TLR3-/-TLR7-/- mice. Moreover, TLR3-/-TLR7-/- mice developed colitis-associated colon cancer following AOM/DSS treatment. Additionally, we showed that a defect in TLR7 of pDCs is responsible for the aggravation of colitis-associated colon cancer. Further, we showed that TLR7 ligand mitigates colitis-associated colon cancer. Collectively, our results demonstrate that gut pDCs play a crucial role in reducing colorectal cancer development via the regulation of infiltrating MDSCs.

Peripheral natural killer cells and myeloid-derived suppressor cells correlate with anti-PD-1 responses in non-small cell lung cancer.

Inhibition of immune checkpoint proteins like programmed death 1 (PD-1) is a promising therapeutic approach for several cancers, including non-small cell lung cancer (NSCLC). Although PD-1 ligand (PD-L1) expression is used to predict anti-PD-1 therapy responses in NSCLC, its accuracy is relatively less. Therefore, we sought to identify a more accurate predictive blood biomarker for evaluating anti-PD-1 response. We evaluated the frequencies of T cells, B cells, natural killer (NK) cells, polymorphonuclear myeloid-derived suppressor cells (PMN-MDSCs), mononuclear myeloid-derived suppressor cells (M-MDSCs), and Lox-1+ PMN-MDSCs in peripheral blood samples of 62 NSCLC patients before and after nivolumab treatment. Correlation of immune-cell population frequencies with treatment response, progression-free survival, and overall survival was also determined. After the first treatment, the median NK cell percentage was significantly higher in responders than in non-responders, while the median Lox-1+ PMN-MDSC percentage showed the opposite trend. NK cell frequencies significantly increased in responders but not in non-responders. NK cell frequency inversely correlated with that of Lox-1+ PMN-MDSCs after the first treatment cycle. The NK cell-to-Lox-1+ PMN-MDSC ratio (NMR) was significantly higher in responders than in non-responders. Patients with NMRs ≥ 5.75 after the first cycle had significantly higher objective response rates and longer progression-free and overall survival than those with NMRs <5.75. NMR shows promise as an early predictor of response to further anti-PD-1 therapy.

Recognition of the microbiota by Nod2 contributes to the oral adjuvant activity of cholera toxin through the induction of interleukin-1ß.

The role of symbiotic bacteria in the development of antigen-specific immunity remains poorly understood. Previous studies showed that sensing of symbiotic bacteria by nucleotide-binding oligomerization domain-containing protein 2 (Nod2) regulates antibody responses in response to nasal immunization with antigen and cholera toxin (CT). In this study, we examined the role of the microbiota in the adjuvant activity of CT induced after oral immunization with antigen. Germ-free (GF) mice showed impaired production of antibody responses and T-cell-specific cytokines after oral immunization when compared with that observed in conventionally raised mice. Similar to GF mice, Nod2-deficient mice showed reduced humoral responses upon oral immunization with antigen and CT. Treatment with CT enhanced the production of interleukin-1ß (IL-1ß), but not tumor necrosis factor-α or IL-12p40, induced by stimulation of dendritic cells with muramyl dipeptide, the Nod2 ligand. Mechanistically, the enhanced production of IL-1ß induced by muramyl dipeptide and CT stimulation required Nod2 and was mediated by both increased synthesis of pro-IL-1ß and caspase-1 activation. Furthermore, antigen-specific antibody and cytokine responses induced by CT were impaired in orally immunized IL-1ß-deficient mice. Collectively, our results indicate that Nod2 stimulation by symbiotic bacteria contributes to optimal CT-mediated antigen-specific oral vaccination through the induction of IL-1ß production.

Macrophages-Triggered Sequential Remodeling of Endothelium-Interstitial Matrix to Form Pre-Metastatic Niche in Microfluidic Tumor Microenvironment.

The primed microenvironment of future metastatic sites, called the pre-metastatic niche, is a prerequisite for overt metastasis. However, a mechanistic understanding of the contributions of recruited cells to the niche is hindered by complex in vivo systems. Herein, a microfluidic platform that incorporates endothelial cells and extracellular matrix (ECM) scaffolds is developed, and the distinct role of recruited monocytes and macrophages in establishing pre-metastatic niches is delineated. It is observed that monocyte-derived matrix metalloproteinase 9 facilitates cancer cell extravasation through destruction of endothelial tight junctions. Furthermore, subsequent cancer cell invasiveness is significantly enhanced. Close examination of ECM structures reveals that cancer cells move within characteristic "microtracks" generated by macrophages, suggesting that macrophages could serve as a compensatory mechanism for the reduced migratory capacity of cancer cells. Thus, the first evidence of monocyte/macrophage-induced remodeling is shown, and these findings will open up new horizons for improving characterization of the pre-metastatic niche and corresponding immunotherapies.

Real-Time Monitoring of Cancer Cells in Live Mouse Bone Marrow.

Disseminated tumor cells in the bone marrow environment are the main cause of systemic metastasis after curative treatment for major solid tumors. However, the detailed biological processes of tumor biology in bone marrow have not been well defined in a real-time manner, because of a lack of a proper in vivo experimental model thereof. In this study, we established intravital imaging models of the bone marrow environment to enable real-time observation of cancer cells in the bone marrow. Using these novel imaging models of intact bone marrow and transplanted bone marrow of mice, respectively, via two-photon microscopy, we could first successfully track and analyze both the distribution and the phenotype of cancer cells in bone marrow of live mouse. Therefore, these novel in vivo imaging models for the bone marrow would provide a valuable tool to identify the biologic processes of cancer cells in a real-time manner in a live animal model.

Mesenchymal Cell-Specific MyD88 Signaling Promotes Systemic Dissemination of Salmonella Typhimurium via Inflammatory Monocytes.

Enteric pathogens including Salmonella enteric serovar Typhimurium can breach the epithelial barrier of the host and spread to systemic tissues. In response to infection, the host activates innate immune receptors via the signaling molecule MyD88, which induces protective inflammatory and antimicrobial responses. Most of these innate immune responses have been studied in hematopoietic cells, but the role of MyD88 signaling in other cell types remains poorly understood. Surprisingly, we found that Dermo1-Cre;Myd88fl/fl mice with mesenchymal cell-specific deficiency of MyD88 were less susceptible to orogastric and i.p. STyphimurium infection than their Myd88fl/fl littermates. The reduced susceptibility of Dermo1-Cre;Myd88fl/fl mice to infection was associated with lower loads of S. Typhimurium in the liver and spleen. Mutant analyses revealed that S. Typhimurium employs its virulence type III secretion system 2 to promote its growth through MyD88 signaling pathways in mesenchymal cells. Inflammatory monocytes function as a major cell population for systemic dissemination of S. Typhimurium Mechanistically, mesenchymal cell-specific MyD88 signaling promoted CCL2 production in the liver and spleen and recruitment of inflammatory monocytes to systemic organs in response to STyphimurium infection. Consistently, MyD88 signaling in mesenchymal cells enhanced the number of phagocytes including Ly6ChiLy6G- inflammatory monocytes harboring STyphimurium in the liver. These results suggest that S. Typhimurium promotes its systemic growth and dissemination through MyD88 signaling pathways in mesenchymal cells.

Nod2-mediated recognition of the microbiota is critical for mucosal adjuvant activity of cholera toxin.

Cholera toxin (CT) is a potent adjuvant for inducing mucosal immune responses. However, the mechanism by which CT induces adjuvant activity remains unclear. Here we show that the microbiota is critical for inducing antigen-specific IgG production after intranasal immunization. After mucosal vaccination with CT, both antibiotic-treated and germ-free (GF) mice had reduced amounts of antigen-specific IgG, smaller recall-stimulated cytokine responses, impaired follicular helper T (TFH) cell responses and reduced numbers of plasma cells. Recognition of symbiotic bacteria via the nucleotide-binding oligomerization domain containing 2 (Nod2) sensor in cells that express the integrin CD11c (encoded by Itgax) was required for the adjuvanticity of CT. Reconstitution of GF mice with a Nod2 agonist or monocolonization with Staphylococcus sciuri, which has high Nod2-stimulatory activity, was sufficient to promote robust CT adjuvant activity, whereas bacteria with low Nod2-stimulatory activity did not. Mechanistically, CT enhanced Nod2-mediated cytokine production in dendritic cells via intracellular cyclic AMP. These results show a role for the microbiota and the intracellular receptor Nod2 in promoting the mucosal adjuvant activity of CT.

Intestinal macrophages arising from CCR2(+) monocytes control pathogen infection by activating innate lymphoid cells.

Monocytes play a crucial role in antimicrobial host defence, but the mechanisms by which they protect the host during intestinal infection remains poorly understood. Here we show that depletion of CCR2(+) monocytes results in impaired clearance of the intestinal pathogen Citrobacter rodentium. After infection, the de novo recruited CCR2(+) monocytes give rise to CD11c(+)CD11b(+)F4/80(+)CD103(-) intestinal macrophages (MPs) within the lamina propria. Unlike resident intestinal MPs, de novo differentiated MPs are phenotypically pro-inflammatory and produce robust amounts of IL-1ß (interleukin-1ß) through the non-canonical caspase-11 inflammasome. Intestinal MPs from infected mice elicit the activation of RORγt(+) group 3 innate lymphoid cells (ILC3) in an IL-1ß-dependent manner. Deletion of IL-1ß in blood monocytes blunts the production of IL-22 by ILC3 and increases the susceptibility to infection. Collectively, these studies highlight a critical role of de novo differentiated monocyte-derived intestinal MPs in ILC3-mediated host defence against intestinal infection.

Coadministration of Hedera helix L. Extract Enabled Mice to Overcome Insufficient Protection against Influenza A/PR/8 Virus Infection under Suboptimal Treatment with Oseltamivir.

Several anti-influenza drugs that reduce disease manifestation exist, and although these drugs provide clinical benefits in infected patients, their efficacy is limited by the emergence of drug-resistant influenza viruses. In the current study, we assessed the therapeutic strategy of enhancing the antiviral efficacy of an existing neuraminidase inhibitor, oseltamivir, by coadministering with the leaf extract from Hedera helix L, commonly known as ivy. Ivy extract has anti-inflammatory, antibacterial, antifungal, and antihelminthic properties. In the present study, we investigated its potential antiviral properties against influenza A/PR/8 (PR8) virus in a mouse model with suboptimal oseltamivir that mimics a poor clinical response to antiviral drug treatment. Suboptimal oseltamivir resulted in insufficient protection against PR8 infection. Oral administration of ivy extract with suboptimal oseltamivir increased the antiviral activity of oseltamivir. Ivy extract and its compounds, particularly hedrasaponin F, significantly reduced the cytopathic effect in PR8-infected A549 cells in the presence of oseltamivir. Compared with oseltamivir treatment alone, coadministration of the fraction of ivy extract that contained the highest proportion of hedrasaponin F with oseltamivir decreased pulmonary inflammation in PR8-infected mice. Inflammatory cytokines and chemokines, including tumor necrosis factor-alpha and chemokine (C-C motif) ligand 2, were reduced by treatment with oseltamivir and the fraction of ivy extract. Analysis of inflammatory cell infiltration in the bronchial alveolar of PR8-infected mice revealed that CD11b+Ly6G+ and CD11b+Ly6Cint cells were recruited after virus infection; coadministration of the ivy extract fraction with oseltamivir reduced infiltration of these inflammatory cells. In a model of suboptimal oseltamivir treatment, coadministration of ivy extract fraction that includes hedrasaponin F increased protection against PR8 infection that could be explained by its antiviral and anti-inflammatory activities.

Distinct Commensals Induce Interleukin-1ß via NLRP3 Inflammasome in Inflammatory Monocytes to Promote Intestinal Inflammation in Response to Injury.

The microbiota stimulates inflammation, but the signaling pathways and the members of the microbiota involved remain poorly understood. We found that the microbiota induces interleukin-1ß (IL-1ß) release upon intestinal injury and that this is mediated via the NLRP3 inflammasome. Enterobacteriaceae and in particular the pathobiont Proteus mirabilis, induced robust IL-1ß release that was comparable to that induced by the pathogen Salmonella. Upon epithelial injury, production of IL-1ß in the intestine was largely mediated by intestinal Ly6C(high) monocytes, required chemokine receptor CCR2 and was abolished by deletion of IL-1ß in CCR2(+) blood monocytes. Furthermore, colonization with P. mirabilis promoted intestinal inflammation upon intestinal injury via the production of hemolysin, which required NLRP3 and IL-1 receptor signaling in vivo. Thus, upon intestinal injury, selective members of the microbiota stimulate newly recruited monocytes to induce NLRP3-dependent IL-1ß release, which promotes inflammation in the intestine.

Expansion of Tfh-like cells during chronic Salmonella exposure mediates the generation of autoimmune hypergammaglobulinemia in MyD88-deficient mice.

The role of TLR signaling in linking the innate and adaptive immune systems has been a controversial issue that remains to be solved. Here, we determined whether MyD88-dependent TLR signals are required for the generation of B-cell responses during chronic Salmonella infection. Oral administration of recombinant attenuated Salmonella enterica serovar Typhimurium vaccine (RASV) strain in MyD88(-/-) mice resulted in chronic infection. Infection was accompanied by enlarged germinal centers and hypergammaglobulinemia with anti-double-stranded DNA (dsDNA)-specific Ab in sera, and the deposition of immune complexes in the kidneys, suggesting onset of autoimmunity. CD4(+) T cells expressing PD-1, CXCR5, ICOS, and IL-21 were dramatically increased in chronically infected mice, indicating the expansion of follicular helper T (Tfh)-like cells. Of note, the depletion of CD4(+) T cells completely blocked the generation of polyclonal IgG Ab in sera after oral RASV challenge. Inflammatory myeloid cells expressing CD11b and Gr-1 accumulated in high numbers in the spleen of MyD88(-/-) mice. Interestingly, the blockade of PD-1 or ICOS significantly reduced the hypergammaglobulinemia and dsDNA-specific autoantibody production. Overall, these results suggest that Tfh-like cells in chronic bacterial infection trigger autoimmune hypergammaglobulinemia in a PD-1- and ICOS-dependent manner.

Mucosa-associated epithelial chemokine/CCL28 expression in the uterus attracts CCR10+ IgA plasma cells following mucosal vaccination via estrogen control.

Previous studies demonstrated cross talk between mucosal and reproductive organs during secretory IgA Ab induction. In this study, we aimed to clarify the underlying mechanisms of this cross talk. We found significantly higher titers of Ag-specific secretory IgA Ab in the vaginal wash after mucosal vaccination by both the intranasal (i.n.) and the intravaginal routes but not by the s.c. route. Interestingly, Ag-specific IgA Ab-secreting cells (ASCs) were found mainly in the uterus but not in the cervix and vaginal canal after i.n. vaccination. The fact that most Ag-specific IgA ASCs isolated from the uteri of vaccinated mice migrated toward mucosa-associated epithelial chemokine (MEC)/CCL28 suggests dominant expression of CCR10 on the IgA ASCs. Further, IgA ASCs in the uteri of vaccinated mice were reduced drastically in mice treated with neutralizing anti-MEC/CCL28 Ab. Most intriguingly, the female sex hormone estrogen directly regulated MEC/CCL28 expression and was augmented by i.n. vaccination with cholera toxin or stimulators for innate immunity. Further, blockage of estrogen function in the uterus by oral administration of the estrogen antagonist raloxifene significantly inhibited migration of Ag-specific IgA ASCs after i.n. vaccination with OVA plus cholera toxin. Taken together, these data strongly suggest that CCR10(+) IgA ASCs induced by mucosal vaccination via the i.n. route migrate into the uterus in a MEC/CCL28-dependent manner and that estrogen might have a crucial role in the protection against genital infection by regulating MEC/CCL28 expression in the uterus.

Type I interferon signaling regulates Ly6C(hi) monocytes and neutrophils during acute viral pneumonia in mice.

Type I interferon (IFN-I) plays a critical role in the homeostasis of hematopoietic stem cells and influences neutrophil influx to the site of inflammation. IFN-I receptor knockout (Ifnar1⁻/⁻) mice develop significant defects in the infiltration of Ly6C(hi) monocytes in the lung after influenza infection (A/PR/8/34, H1N1). Ly6C(hi) monocytes of wild-type (WT) mice are the main producers of MCP-1 while the alternatively generated Ly6C(int) monocytes of Ifnar1⁻/⁻ mice mainly produce KC for neutrophil influx. As a consequence, Ifnar1⁻/⁻ mice recruit more neutrophils after influenza infection than do WT mice. Treatment of IFNAR1 blocking antibody on the WT bone marrow (BM) cells in vitro failed to differentiate into Ly6C(hi) monocytes. By using BM chimeric mice (WT BM into Ifnar1⁻/⁻ and vice versa), we confirmed that IFN-I signaling in hematopoietic cells is required for the generation of Ly6C(hi) monocytes. Of note, WT BM reconstituted Ifnar1⁻/⁻ chimeric mice with increased numbers of Ly6C(hi) monocytes survived longer than influenza-infected Ifnar1⁻/⁻ mice. In contrast, WT mice that received Ifnar1⁻/⁻ BM cells with alternative Ly6C(int) monocytes and increased numbers of neutrophils exhibited higher mortality rates than WT mice given WT BM cells. Collectively, these data suggest that IFN-I contributes to resistance of influenza infection by control of monocytes and neutrophils in the lung.