Severe Listeria monocytogenes infection induces development of monocytes with distinct phenotypic and functional features

Validation of CSF-1 receptor (CD115) staining for analysis of murine monocytes by flow cytometry

CD115, the receptor for colony stimulating factor 1, is essential for survival and differentiation of monocytes and macrophages and is therefore frequently used to define monocyte subsets and their progenitors in immunological assays. However, CD115 surface expression and detection by flow cytometry is greatly influenced by cell isolation and processing methods, organ source, and disease context. In a systematic analysis of murine monocytes, we define experimental conditions that preserve or limit CD115 surface expression and staining by flow cytometry. We also find that, independent of conditions, CD115 surface levels are consistently lower in Ly6Clo monocytes than in Ly6Chi monocytes, with the exception of Ly6Clo monocytes in the bone marrow. Furthermore, in contrast to IL-34, the presence of colony stimulating factor 1 impairs CD115 antibody staining in a dose-dependent manner, which, in a model of ischemic kidney injury with elevated levels of colony stimulating factor 1, influenced quantification of kidney monocytes. Thus, staining and experimental conditions affect quantitative and qualitative analysis of monocytes and may influence experimental conclusions.

Negative Regulation of the IL-1 System by IL-1R2 and IL-1R8: Relevance in Pathophysiology and Disease

Interleukin-1 (IL-1) is a primary cytokine of innate immunity and inflammation. IL-1 belongs to a complex family including ligands with agonist activity, receptor antagonists, and an anti-inflammatory cytokine. The receptors for these ligands, the IL-1 Receptor (IL-1R) family, include signaling receptor complexes, decoy receptors, and negative regulators. Agonists and regulatory molecules co-evolved, suggesting the evolutionary relevance of a tight control of inflammatory responses, which ensures a balance between amplification of innate immunity and uncontrolled inflammation. IL-1 family members interact with innate immunity cells promoting innate immunity, as well as with innate and adaptive lymphoid cells, contributing to their differentiation and functional polarization and plasticity. Here we will review the properties of two key regulatory receptors of the IL-1 system, IL-1R2, the first decoy receptor identified, and IL-1R8, a pleiotropic regulator of different IL-1 family members and co-receptor for IL-37, the anti-inflammatory member of the IL-1 family. Their complex impact in pathology, ranging from infections and inflammatory responses, to cancer and neurologic disorders, as well as clinical implications and potential therapeutic exploitation will be presented.

Critical role of synovial tissue-resident macrophage niche in joint homeostasis and suppression of chronic inflammation

Little is known about the mechanisms regulating the transition of circulating monocytes into pro- or anti-inflammatory macrophages in chronic inflammation. Here, we took advantage of our novel mouse model of rheumatoid arthritis, in which Flip is deleted under the control of a CD11c promoter (HUPO mice). During synovial tissue homeostasis, both monocyte-derived F4/80^int and self-renewing F4/80^hi tissue-resident, macrophage populations were identified. However, in HUPO mice, decreased synovial tissue-resident macrophages preceded chronic arthritis, opened a niche permitting the influx of activated monocytes, with impaired ability to differentiate into F4/80^hi tissue-resident macrophages. In contrast, Flip-replete monocytes entered the vacated niche and differentiated into tissue-resident macrophages, which suppressed arthritis. Genes important in macrophage tissue residency were reduced in HUPO F4/80^hi macrophages and in leukocyte-rich rheumatoid arthritis synovial tissue monocytes. Our observations demonstrate that the macrophage tissue-resident niche is necessary for suppression of chronic inflammation and may contribute to the pathogenesis of rheumatoid arthritis.

The Liver-Stage Plasmodium Infection Is a Critical Checkpoint for Development of Experimental Cerebral Malaria

Cerebral malaria is a life-threatening complication of malaria in humans, and the underlying pathogenic mechanisms are widely analyzed in a murine model of experimental cerebral malaria (ECM). Here, we show abrogation of ECM by hemocoel sporozoite-induced infection of a transgenic Plasmodium berghei line that overexpresses profilin, whereas these parasites remain fully virulent in transfusion-mediated blood infection. We, thus, demonstrate the importance of the clinically silent liver-stage infection for modulating the onset of ECM. Even though both parasites triggered comparable splenic immune cell expansion and accumulation of antigen-experienced CD8⁺ T cells in the brain, infection with transgenic sporozoites did not lead to cerebral vascular damages and suppressed the recruitment of overall lymphocyte populations. Strikingly, infection with the transgenic strain led to maintenance of CD115⁺Ly6C⁺ monocytes, which disappear in infected animals prone to ECM. An early induction of IL-10, IL-12p70, IL-6, and TNF at the time when parasites emerge from the liver might lead to a diminished induction of hepatic immunity. Collectively, our study reveals the essential role of early host interactions in the liver that may dampen the subsequent pro-inflammatory immune responses and influence the occurrence of ECM, highlighting a novel checkpoint in this fatal pathology.

Neuroinvasive Listeria monocytogenes Infection Triggers IFN-Activation of Microglia and Upregulates Microglial miR-155

MicroRNA (miR) miR-155 modulates microglial activation and polarization, but its role in activation of microglia during bacterial brain infection is unclear. We studied miR-155 expression in brains of C57BL/6 (B6.WT) mice infected i.p. with the neuro-invasive bacterial pathogen Listeria monocytogenes (L. monocytogenes). Infected mice were treated with ampicillin starting 2 days (d) post-infection (p.i.) and analyzed 3d, 7d, and 14d p.i. Virulent L. monocytogenes strains EGD and 10403s upregulated miR-155 in whole brain 7 d p.i. whereas infection with avirulent, non-neurotropic Δhly or ΔactA L. monocytogenes mutants did not. Similarly, infection with virulent but not mutated bacteria upregulated IFN-γ mRNA in the brain at 7 d p.i. Upregulation of miR-155 in microglia was confirmed by qPCR of flow cytometry-sorted CD45^intCD11b^pos brain cells. Subsequently, brain leukocyte influxes and gene expression in sorted microglia were compared in L. monocytogenes-infected B6.WT and B6.Cg-Mir155tm1.1Rsky/J (B6.miR-155^−/−) mice. Brain influxes of Ly-6C^high monocytes and upregulation of IFN-related genes in microglia were similar to B6.WT mice at 3 d p.i. In contrast, by d 7 p.i. expressions of microglial IFN-related genes, including markers of M1 polarization, were significantly lower in B6.miR-155^−/− mice and by 14 d p.i., influxes of activated T-lymphocytes were markedly reduced. Notably, CD45^highCD11b^pos brain cells from B6.miR-155^−/− mice isolated at 7 d p.i. expressed 2-fold fewer IFN-γ transcripts than did cells from B6.WT mice suggesting reduced IFN-γ stimulation contributed to dampened gene expression in B6.miR-155^−/− microglia. Lastly, in vitro stimulation of 7 d p.i. brain cells with heat-killed L. monocytogenes induced greater production of TNF in B6.miR-155^−/− microglia than in B6.WT microglia. Thus, miR-155 affects brain inflammation by multiple mechanisms during neuroinvasive L. monocytogenes infection. Peripheral miR-155 promotes brain inflammation through its required role in optimal development of IFN-γ-secreting lymphocytes that enter the brain and activate microglia. Microglial miR-155 promotes M1 polarization, and also inhibits inflammatory responses to stimulation by heat-killed L. monocytogenes, perhaps by targeting Tab2.

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

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

Burkholderia pseudomallei-loaded cells act as a Trojan horse to invade the brain during endotoxemia

Neurologic melioidosis occurs in both human and animals; however, the mechanism by which the pathogen Burkholderia pseudomallei invades the central nervous system (CNS) remains unclear. B. pseudomallei-loaded Ly6C cells have been suggested as a putative portal; however, during melioidosis, lipopolysaccharide (LPS) can drive disruption of the blood-brain barrier (BBB). This study aims to test whether the Trojan horse-like mechanism occurs during endotoxemia. The expression levels of cerebral cytokines, chemokines and cell adhesion molecules; the activation of astrocytes, microglia and endothelial cells; and the increased vascular permeability and brain-infiltrating leukocytes were evaluated using B. pseudomallei, B. thailandensis, B. cenocepacia and B. multivorans LPS-induced brains. Accordingly, different degrees of BBB damage in those brains with endotoxemia were established. The B. multivorans LPS-induced brain exhibited the highest levels of disruptive BBB according to the above mediators/indicators. Into these distinct groups of endotoxemic mice, B. pseudomallei-loaded Ly6C cells or free B. pseudomallei were adoptively transferred at equal bacterial concentrations (10³ CFU). The bacterial load and number of cases of meningeal neutrophil infiltration in the brains of animals treated with B. pseudomallei-loaded Ly6C cells were higher than those in brains induced by free B. pseudomallei in any of the endotoxemic groups. In particular, these results were reproducible in B. multivorans LPS-induced brains. We suggest that B. pseudomallei-loaded cells can act as a Trojan horse and are more effective than free B. pseudomallei in invading the CNS under septic or endotoxemic conditions even when there is a high degree of BBB disruption.

Tuning inflammation and immunity by the negative regulators IL-1R2 and IL-1R8

Interleukin-1 receptor family members (ILRs) and Toll-Like Receptors (TLRs) are key players in immunity and inflammation and are tightly regulated at different levels. Most cell types, including cells of the innate and adaptive immune system express ILRs and TLRs. In addition, IL-1 family members are emerging as key players in the differentiation and function of innate and adaptive lymphoid cells. IL-1R2 and IL-1R8 (also known as TIR8 or SIGIRR) are members of the ILR family acting as negative regulators of the IL-1 system. IL-1R2 binds IL-1 and the accessory protein IL-1RAcP without activating signaling and can be released as a soluble form (sIL-1R2), thus modulating IL-1 availability for the signaling receptor. IL-1R8 dampens ILR- and TLR-mediated cell activation and it is a component of the receptor recognizing human IL-37. Here, we summarize our current understanding of the structure and function of IL-1R2 and IL-1R8, focusing on their role in different pathological conditions, ranging from infectious and sterile inflammation, to autoimmunity and cancer-related inflammation. We also address the emerging evidence regarding the role of IL-1R8 as a crucial checkpoint molecule in NK cells in anti-cancer and antiviral activity and the potential therapeutic implications of IL-1R8 blockade in specific pathological contexts.

Adhesion of Toxoplasma gondii tachyzoite-infected vehicle leukocytes to capillary endothelial cells triggers timely parasite egression

Intracellular pathogens have numerous strategies for effective dissemination within the host. Many intracellular pathogens first infect leukocytes, which they use as a vehicle to transport them to target organs. Once at the target organ, intracellular parasite Toxoplasma gondii can cross the capillary wall in extracellular form by infecting endothelial cells. However, after egression from leukocytes, extracellular parasites face the risk of host immune attack. In this study, observation of infected mouse organs, using a method that renders tissue transparent, revealed that adhesion of tachyzoite-infected leukocytes to endothelial cells triggers immediate egression of the parasite. This signal enables the parasite to time egression from its vehicle leukocyte to coincide with arrival at a target organ, minimizing the opportunity for immune attack during the transition from a vehicle leukocyte to capillary endothelial cells.

The role of monocytes in models of infection by protozoan parasites

The confirmation of developmental differences between tissue macrophages and peripheral monocytes has changed our view of the functions and dynamics of these two important components of the innate immune system. It has been demonstrated conclusively that homeostasis of tissue resident macrophages is maintained by a low proliferative turn over. During an inflammatory response, bone marrow derived monocytes enter the tissue in large numbers and take part in the defense against the pathogens. After the destruction of invading pathogens, these cells disappear and tissue resident macrophages can be detected again. This new appreciation of the innate immune response has not only answered many outstanding questions regarding the role of the different myeloid cell types in inflammation, but also opened up new areas of research relating to the tissue- and pathogen-specific fate of the inflammatory macrophages or dendritic cells (DCs), and the transfer of this knowledge from mouse models to the human immune system. Nevertheless, there is still confusion in infection models, and especially in studies of human infections, as to what extent these recent observations and findings influence previous interpretations of data. This review will focus on insights from mouse models, summarize the literature on the ontogeny of macrophages and monocytes, explain the role of frequently used monocyte markers and effector molecules, and finally, discuss the role of inflammatory monocytes/macrophages/DCs in two experimental parasitic diseases.

Monocytes Are the Predominant Cell Type Associated with Listeria monocytogenes in the Gut, but They Do Not Serve as an Intracellular Growth Niche

After foodborne transmission of the facultative intracellular bacterial pathogen Listeria monocytogenes, most of the bacterial burden in the gut is extracellular. However, we previously demonstrated that intracellular replication in an as yet unidentified cell type was essential for dissemination and systemic spread of L. monocytogenes In this article, we show that the vast majority of cell-associated L. monocytogenes in the gut were adhered to Ly6C^hi monocytes, a cell type that inefficiently internalized L. monocytogenes With bone marrow-derived in vitro cultures, high multiplicity of infection or the use of opsonized bacteria enhanced uptake of L. monocytogenes in CD64^- monocytes, but very few bacteria reached the cell cytosol. Surprisingly, monocytes that had upregulated CD64 expression in transition toward becoming macrophages fully supported intracellular growth of L. monocytogenes In contrast, inflammatory monocytes that had increased CD64 expression in the bone marrow of BALB/c/By/J mice prior to L. monocytogenes exposure in the gut did not support L. monocytogenes growth. Thus, contrary to the perception that L. monocytogenes can infect virtually all cell types, neither naive nor inflammatory Ly6C^hi monocytes served as a productive intracellular growth niche for L. monocytogenes. These results have broad implications for innate immune recognition of L. monocytogenes in the gut and highlight the need for additional studies on the interaction of extracellular, adherent L. monocytogenes with the unique subsets of myeloid-derived inflammatory cells that infiltrate sites of infection.

Cytokine decoy and scavenger receptors as key regulators of immunity and inflammation

IL-1R2 was the first decoy receptor to be described. Subsequently receptors which act as pure decoys or scavengers or trigger dampening of cytokine signaling have been described for cytokines and chemokines. Here we review the current understanding of the mode of action and significance in pathology of the chemokine atypical receptor ACKR2, the IL-1 decoy receptor IL-1R2 and the atypical IL-1 receptor family IL-1R8. Decoy and scavenger receptors with no or atypical signaling have emerged as a general strategy conserved in evolution to tune the action of cytokines, chemokines and growth factors.

Blood Monocytes and Their Subsets: Established Features and Open Questions

In contrast to the past reliance on morphology, the identification and enumeration of blood monocytes are nowadays done with monoclonal antibodies and flow cytometry and this allows for subdivision into classical, intermediate, and non-classical monocytes. Using specific cell surface markers, dendritic cells in blood can be segregated from these monocytes. While in the past, changes in monocyte numbers as determined in standard hematology counters have not had any relevant clinical impact, the subset analysis now has uncovered informative changes that may be used in management of disease.

Insights how monocytes and dendritic cells contribute and regulate immune defense against microbial pathogens

The immune system protects from infections primarily by detecting and eliminating invading pathogens. Beside neutrophils, monocytes and dendritic cells (DCs) have been recently identified as important sentinels and effectors in combating microbial pathogens. In the steady state mononuclear phagocytes like monocytes and DCs patrol the blood and the tissues. Mammalian monocytes contribute to antimicrobial defense by supplying tissues with macrophage and DC precursors. DCs recognize pathogens and are essential in presenting antigens to initiate antigen-specific adaptive immune responses, thereby bridging the innate and adaptive immune systems. Both, monocytes and DCs play distinct roles in the shaping of immune response. In this review we will focus on the contributions of monocytes and lymphoid organ DCs to immune defense against microbial pathogens in the mouse and their dynamic regulation from steady state to infection.

Negative regulatory receptors of the IL-1 family

The IL-1 family of ligands and receptors has a central role in both innate and adaptive immune responses and is tightly controlled by antagonists, decoy receptors, scavengers, dominant negative molecules, miRNAs and other mechanisms, acting extracellularly or intracellularly. During evolution, the development of multiple mechanisms of negative regulation reveals the need for tight control of the biological consequences of IL-1 family ligands in order to balance local and systemic inflammation and limit immunopathology. Indeed, studies with gene targeted mice for negative regulators and genetic studies in humans provide evidence for their non-redundant role in controlling inflammation, tissue damage and adaptive responses. In addition, studies have revealed the need of negative regulation of the IL-1 family not only in disease, but also in homeostatic conditions. In this review, the negative regulation mediated by decoy receptors are presented and include IL-1R2 and IL-IL-18BP as well as atypical receptors, which include TIR8/SIGIRR, IL-1RAcPb, TIGIRR-1 and IL-1RAPL. Particular emphasis is given to IL-1R2, since its discovery is the basis for the formulation of the decoy paradigm, now considered a general strategy to counter the primary inflammatory activities of cytokines and chemokines. Emphasis is also given to TIR8, a prototypical negative regulatory receptor having non-redundant roles in limiting inflammation and adaptive responses.

MicroRNA-Mediated Down-Regulation of M-CSF Receptor Contributes to Maturation of Mouse Monocyte-Derived Dendritic Cells

Dendritic cell (DC) maturation is a tightly regulated process that requires coordinated and timed developmental cues. Here we investigate whether microRNAs are involved in this process. We identify microRNAs in mouse GM-CSF-generated, monocyte-related DC (GM-DC) that are differentially expressed during both spontaneous and LPS-induced maturation and characterize M-CSF receptor (M-CSFR), encoded by the Csf1r gene, as a key target for microRNA-mediated regulation in the final step toward mature DC. MicroRNA-22, -34a, and -155 are up-regulated in mature MHCII^hi CD86^hi DC and mediate Csf1r mRNA and protein down-regulation. Experimental inhibition of Csf1r-targeting microRNAs in vitro results not only in sustained high level M-CSFR protein expression but also in impaired DC maturation upon stimulation by LPS. Accordingly, over-expression of Csf1r in GM-DC inhibits terminal differentiation. Taken together, these results show that developmentally regulated microRNAs control Csf1r expression, supplementing previously identified mechanisms that regulate its transcription and protein surface expression. Furthermore, our data indicate a novel function for Csf1r in mouse monocyte-derived DC, showing that down-regulation of M-CSFR expression is essential for final DC maturation.

Mouse neutrophils express the decoy type 2 interleukin-1 receptor (IL-1R2) constitutively and in acute inflammatory conditions

The proinflammatory activities of IL-1 are tightly controlled at different levels. IL-1R2 acts as a decoy receptor and has been shown to regulate the biological effects of IL-1 in vitro and in vivo. However, little is known about its natural expression in the mouse in physiologic and pathologic conditions. In this study, we examined IL-1R2 mRNA and protein expression in isolated cells and tissues in response to different stimulatory conditions. Data obtained using ex vivo CD11b(+)Ly6G(+) peripheral blood cells and in vitro-differentiated CD11b(+)Ly6G(+) BMG indicated that neutrophils are the major source of constitutively expressed IL-1R2 in the mouse. The expression of IL-1R2 on BMG and ex vivo Ly6G(+) peripheral blood cells was highly up-regulated by HC. IL-1R2 pull-down experiments showed that mouse rIL-1β binds to BMG IL-1R2, whereas binding of IL-1Ra could not be detected. Furthermore, LPS treatment induced shedding of IL-1R2 from the neutrophil membrane in vitro and in vivo, executed mainly by ADAM17. Finally, in in vivo models of inflammation, including thioglycolate-induced acute peritonitis and acute lung injury, infiltrating Ly6G(+) neutrophils, expressed IL-1R2. Our data show that in the mouse, neutrophils mainly express the decoy receptor IL-1R2 under naïve and inflammatory conditions. These data suggest that neutrophils may contribute to the resolution of acute inflammation.

Amelioration of experimental autoimmune encephalomyelitis by the quinoline-3-carboxamide paquinimod: reduced priming of proinflammatory effector CD4(+) T cells

Quinoline-3-carboxamide compounds (Q compounds) have demonstrated efficacy in treating autoimmune disease in both humans and mice. However, the mode of action of these compounds is poorly understood. Here, we show that preventive treatment with the Q compound paquinimod (ABR-215757) during the first 5 days after induction of experimental autoimmune encephalomyelitis is sufficient to significantly ameliorate disease symptoms. Parallel cell-depletion experiments demonstrated that Ly6C(hi) inflammatory monocytes play an essential role in this phase. The paquinimod-induced amelioration correlated with reduced priming of antigen-specific CD4(+) T cells and reduced frequency of IFN-γ- and IL-17-producing cells in draining lymph nodes. Importantly, the treatment did not inhibit T-cell division per se. In mice with established experimental autoimmune encephalomyelitis, the numbers of Ly6C(hi) CD115(+) inflammatory monocytes and CD11b(+)CD11c(+) dendritic cells (DCs) were reduced in spleen, but not in bone marrow or draining lymph nodes of treated mice. Inflammatory monocyte-derived DCs and CD4(+) T cells were also reduced in the brain. In contrast, there was no decrease in DC subsets previously shown to be critical for effector CD4(+) T-cell development in lymph nodes. Taken together, these data indicate that preventive treatment with paquinimod ameliorates experimental autoimmune encephalomyelitis by reducing effector T-cell priming and, on prolonged treatment, displays a selective effect by decreasing distinct subpopulations of splenic CD11b(+) myeloid cells.

Role of suppressor of cytokine signaling-1 in murine atherosclerosis

Background

While the impact of inflammation as the substantial driving force of atherosclerosis has been investigated in detail throughout the years, the influence of negative regulators of pro-atherogenic pathways on plaque development has remained largely unknown. Suppressor of cytokine signaling (SOCS)-1 potently restricts transduction of various inflammatory signals and, thereby modulates T-cell development, macrophage activation and dendritic cell maturation. Its role in atherogenesis, however has not been elucidated so far.

Methods and Results

Loss of SOCS-1 in the low-density lipoprotein receptor deficient murine model of atherosclerosis resulted in a complex, systemic and ultimately lethal inflammation with increased generation of Ly-6C^hi monocytes and activated macrophages. Even short-term exposure of these mice to high-cholesterol dieting caused enhanced atherosclerotic plaque development with accumulation of M1 macrophages, Ly-6C positive cells and neutrophils.

Conclusion

Our data not only imply that SOCS-1 is athero-protective but also emphasize the fundamental, regulatory importance of SOCS-1 in inflammation-prone organisms.

Control of macrophage lineage populations by CSF-1 receptor and GM-CSF in homeostasis and inflammation

There is recent interest in the role of monocyte/macrophage subpopulations in pathology. How the hemopoietic growth factors, macrophage-colony stimulating factor (M-CSF or CSF-1) and granulocyte macrophage (GM)-CSF, regulate their in vivo development and function is unclear. A comparison is made here on the effect of CSF-1 receptor (CSF-1R) and GM-CSF blockade/depletion on such subpopulations, both in the steady state and during inflammation. In the steady state, administration of neutralizing anti-CSF-1R monoclonal antibody (mAb) rapidly (within 3-4 days) lowered, specifically, the number of the more mature Ly6C(lo) peripheral blood murine monocyte population and resident peritoneal macrophages; it also reduced the accumulation of murine exudate (Ly6C(lo)) macrophages in two peritonitis models and alveolar macrophages in lung inflammation, consistent with a non-redundant role for CSF-1 (or interleukin-34) in certain inflammatory reactions. A neutralizing mAb to GM-CSF also reduced inflammatory macrophage numbers during antigen-induced peritonitis and lung inflammation. In GM-CSF gene-deficient mice, a detailed kinetic analysis of monocyte/macrophage and neutrophil dynamics in antigen-induced peritonitis suggested that GM-CSF was acting, in part, systemically to maintain the inflammatory reaction. A model is proposed in which CSF-1R signaling controls the development of the macrophage lineage at a relatively late stage under steady state conditions and during certain inflammatory reactions, whereas in inflammation, GM-CSF can be required to maintain the response by contributing to the prolonged extravasation of immature monocytes and neutrophils. A correlation has been observed between macrophage numbers and the severity of certain inflammatory conditions, and it could be that CSF-1 and GM-CSF contribute to the control of these numbers in the ways proposed.