Selective depletion of CD11c+CD11b+ dendritic cells partially abrogates tolerogenic effects of intravenous MOG in murine EAE

Intravenous (i.v.) injection of a soluble myelin antigen can induce tolerance, which effectively ameliorates experimental autoimmune encephalomyelitis (EAE). We have previously shown that i.v. myelin oligodendrocyte glycoprotein (MOG) induces tolerance in EAE and expands a subpopulation of tolerogenic CD11c⁺CD11b⁺ dendritic cells (DCs) with an immature phenotype having low expression of IA and co‐stimulatory molecules CD40, CD86, and CD80. Here, we further investigate the role of tolerogenic DCs in i.v. tolerance by injecting clodronate‐loaded liposomes, which selectively deplete CD11c⁺CD11b⁺ and immature DCs, but not CD11c⁺CD8⁺ DCs and mature DCs. I.v. MOG‐induced suppression of EAE was partially, yet significantly, blocked by CD11c⁺CD11b⁺ DC depletion. While i.v. MOG inhibited IA, CD40, CD80, CD86 expression and induced TGF‐β, IL‐27, IL‐10 production in CD11c⁺CD11b⁺ DCs, these effects were abrogated after injection of clodronate‐loaded liposomes. Depletion of CD11c⁺CD11b⁺ DCs also precluded i.v. autoantigen‐induced T‐cell tolerance, such as decreased production of IL‐2, IFN‐γ, IL‐17 and numbers of IL‐2⁺, IFN‐γ⁺, and IL‐17⁺ CD4⁺ T cells, as well as an increased proportion of CD4⁺CD25⁺Foxp3⁺ regulatory T cells and CD4⁺IL‐10⁺Foxp3^? Tr1 cells. CD11c⁺CD11b⁺ DCs, through low expression of IA and costimulatory molecules as well as high expression of TGF‐β, IL‐27, and IL‐10, play an important role in i.v. tolerance‐induced EAE suppression.     

Dual role of monocyte‐derived dendritic cells in Trypanosoma cruzi infection

Pathogens can cause inflammation when inoculated into the skin. The vector‐transmitted protozoan parasite Trypanosoma cruzi induces poor cellular‐infiltration and disseminates, causing high mortality in the experimental model. Here, we characterized the inflammatory foci at the parasite inoculation site and secondary lymphoid organs using a murine model. While no macrophages and few neutrophils and monocytes (Mo) were recruited into the skin, T. cruzi infection elicited the mobilization of Ly6C⁺ Mo to draining lymph nodes and spleen. Over time, this population became enriched in CD11b⁺Ly6C⁺CD11c⁺MHCII⁺CD86⁺ cells resembling inflammatory dendritic cells (DCs). Adoptive transfer of Ly6C⁺ Mo purified from the bone marrow of CD11c‐GFP transgenic mice confirmed the monocytic origin of Ly6C⁺ DCs found in the spleen of infected animals. Isolated Mo‐derived cells not only produced TNF‐α and nitric oxide, but also IL‐10 and displayed a poor capacity to induce lymphoproliferation. Ablation of Mo‐derived cells by 5‐fluorouracil confirmed their dual role during infection, limiting the parasite load by inducible nitric oxide synthase‐related mechanisms and negatively affecting the development of anti‐parasite T‐cell response. This study demonstrated that consistent with their antagonistic properties, these cells not only control the parasite spreading but also its persistence in the host.     

Renal‐infiltrating CD11c+ cells are pathogenic in murine lupus nephritis through promoting CD4+ T cell responses

Lupus nephritis (LN) is a major manifestation of systemic lupus erythematosus (SLE), causing morbidity and mortality in 40–60% of SLE patients. The pathogenic mechanisms of LN are not completely understood. Recent studies have demonstrated the presence of various immune cell populations in lupus nephritic kidneys of both SLE patients and lupus‐prone mice. These cells may play important pathogenic or regulatory roles in situ to promote or sustain LN. Here, using lupus‐prone mouse models, we showed the pathogenic role of a kidney‐infiltrating CD11c⁺ myeloid cell population in LN. These CD11c⁺ cells accumulated in the kidneys of lupus‐prone mice as LN progressed. Surface markers of this population suggest their dendritic cell identity and differentiation from lymphocyte antigen 6 complex (Ly6C)^low mature monocytes. The cytokine/chemokine profile of these renal‐infiltrating CD11c⁺ cells suggests their roles in promoting LN, which was confirmed further in a loss‐of‐function in‐vivo study by using an antibody‐drug conjugate (ADC) strategy targeting CX₃CR1, a chemokine receptor expressed highly on these CD11c⁺ cells. However, CX₃CR1 was dispensable for the homing of CD11c⁺ cells into lupus nephritic kidneys. Finally, we found that these CD11c⁺ cells co‐localized with infiltrating T cells in the kidney. Using an ex‐ vivo co‐culture system, we showed that renal‐infiltrating CD11c⁺ cells promoted the survival, proliferation and interferon‐γ production of renal‐infiltrating CD4⁺ T cells, suggesting a T cell‐dependent mechanism by which these CD11c⁺ cells promote LN. Together, our results identify a pathogenic kidney‐infiltrating CD11c⁺ cell population promoting LN progression, which could be a new therapeutic target for the treatment of LN.     

IL‐10 promotes homeostatic proliferation of human CD8+ memory T cells and,when produced by CD1c+ DCs,shapes naive CD8+ T‐cell priming

IL‐10 is an anti‐inflammatory cytokine that inhibits maturation and cytokine production of dendritic cells (DCs). Although mature DCs have the unique capacity to prime CD8⁺ CTL, IL‐10 can promote CTL responses. To understand these paradoxic findings, we analyzed the role of IL‐10 produced by human APC subsets in T‐cell responses. IL‐10 production was restricted to CD1c⁺ DCs and CD14⁺ monocytes. Interestingly, it was differentially regulated, since R848 induced IL‐10 in DCs, but inhibited IL‐10 in monocytes. Autocrine IL‐10 had only a weak inhibitory effect on DC maturation, cytokine production, and CTL priming with high‐affinity peptides. Nevertheless, it completely blocked cross‐priming and priming with low‐affinity peptides of a self/tumor‐antigen. IL‐10 also inhibited CD1c⁺ DC‐induced CD4⁺ T‐cell priming and enhanced Foxp3 induction, but was insufficient to induce T‐cell IL‐10 production. CD1c⁺ DC‐derived IL‐10 had also no effect on DC‐induced secondary expansions of memory CTL. However, IL‐15‐driven, TCR‐independent proliferation of memory CTL was enhanced by IL‐10. We conclude that DC‐derived IL‐10 selects high‐affinity CTL upon priming. Moreover, IL‐10 preserves established CTL memory by enhancing IL‐15‐dependent homeostatic proliferation. These combined effects on CTL priming and memory maintenance provide a plausible mechanism how IL‐10 promotes CTL responses in humans.     

CD103+CD11b− salivary gland dendritic cells have antigen cross‐presenting capacity

Dendritic cells (DCs) in lymphoid and non‐lymphoid tissues are professional antigen‐presenting cells that are essential for effective immunity and tolerance. However, the presence and characteristics of DCs in steady‐state salivary glands (SGs) currently remain unknown. We herein identified CD64^?CD11c⁺ classical DCs (cDCs) as well as CD64⁺ macrophages among CD45⁺MHC class II⁺ antigen‐presenting cells in steady‐state murine SGs. SG cDCs were divided into CD103⁺CD11b^? and CD103^?CD11b⁺ cDCs. CD103⁺CD11b^? cDCs expressed XCR1, CLEC9A, and interferon regulatory factor 8, whereas CD103^?CD11b⁺ cDCs strongly expressed CD172a. Both cDC subsets in SGs markedly expanded in response to the Flt3 ligand (Flt3L), were replenished by bone marrow‐derived precursors, and differentiated from common DC precursors, but not monocytes. Furthermore, ovalbumin‐pulsed SG CD103⁺CD11b^? cDCs induced the proliferation of naïve ovalbumin‐specific CD8⁺ T cells and production of interferon‐γ from proliferating T cells. SG CD103⁺CD11b^? cDCs expanded by Flt3L in vivo exhibited the same properties. These results indicate that bona fide cDCs reside in steady‐state murine SGs and cDCs with the CD103⁺CD11b^? phenotype possess antigen cross‐presenting capacity. Moreover, Flt3L enhances protective immunity by expanding cDCs. Taken together, SG cDCs might play an important role in maintaining immune homeostasis in the tissues.     

CD11c+ macrophages and levels of TNF‐α and MMP‐3 are increased in synovial and adipose tissues of osteoarthritic mice with hyperlipidaemia

To understand more clearly the link between osteoarthritis and hyperlipidaemia, we investigated the inflammatory macrophage subsets and macrophage‐regulated matrix metalloprotease‐3 (MMP‐3) and A disintegrin and metalloprotease with thrombospondin motifs‐4 (ADAMTS4) in synovial (ST) and adipose tissues (AT) of osteoarthritic mice with hyperlipidaemia (STR/Ort). CD11c⁺F4/80⁺CD11b⁺ macrophage populations in the ST and AT of 9‐month‐old STR/Ort and C57BL/6J mice were characterized and compared by flow cytometry and real‐time polymerase chain reaction (PCR) analyses. Expression of tumour necrosis factor (TNF)‐α, MMP‐3 and ADAMTS4, and the response of these factors to anionic liposomal clodronate induced‐macrophage depletion were also evaluated by real‐time PCR. Expression of TNF‐α in CD11c⁺ cells, which were isolated by magnetic beads, was compared to CD11c^– cells. In addition, the effect of TNF‐α on cultured synovial fibroblasts and adipocytes was investigated. CD11c⁺F4/80⁺CD11b⁺ macrophages were increased in ST and AT of STR/Ort mice. The CD11c⁺ cell fraction highly expressed TNF‐α. Expression of TNF‐α and MMP3 was increased in ST and AT, and was decreased upon macrophage depletion. TNF‐α treatment of cultured synovial fibroblasts and adipocytes markedly up‐regulated MMP‐3. CD11c⁺F4/80⁺CD11b⁺ macrophages were identified as a common inflammatory subset in the AT and ST of STR/Ort mice with hyperlipidaemia. The induction of inflammation in AT and ST may be part of a common mechanism that regulates MMP3 expression through TNF‐α. Our findings suggest that increased numbers of CD11c⁺ macrophages and elevated levels of TNF‐α and MMP‐3 in AT and ST may explain the relationship between hyperlipidaemia and OA.     

Interleukin‐2‐mediated inhibition of dendritic cell development correlates with decreased CD135 expression and increased monocyte/macrophage precursors

We have previously shown that interleukin-2 (IL-2) inhibits dendritic cell (DC) development from mouse bone marrow (BM) precursors stimulated with the ligand for FMS-like tyrosine kinase 3 receptor (Flt3L), and have provided evidence that this inhibition occurs at the monocyte DC precursor stage of DC development. Here, we explored the mechanism of IL-2-mediated inhibition of DC development. First, we showed that these in vitro cultures accurately model DCs that develop in vivo by comparing gene and protein expression of the three main Flt3L-induced DC subsets from the BM, CD11b⁺ and CD24⁺ conventional DCs (cDCs) and plasmacytoid DCs (pDCs) with their respective ex vivo spleen DC subsets (CD11b⁺, CD8⁺ and pDCs). Next, gene expression changes were quantified in Flt3L DC subsets that developed in the presence of IL-2. These changes included increased expression of Bcl2l11, which encodes the apoptosis-inducing protein Bim, and decreased expression of Flt3 (CD135), the receptor that initiates DC development. Interleukin-2 also significantly reduced Flt3 protein expression on all three Flt3L DC subsets, and attenuated Flt3L-induced STAT3 phosphorylation in DCs. Based on these data, we hypothesized that decreased Flt3 signalling may divert BM precursors down monocyte and macrophage lineages. Indeed, addition of IL-2 led to increases in Flt3⁻ cells, including cKit⁺ Ly6C⁺ CD11b⁻ populations consistent with the recently identified committed monocyte/macrophage progenitor. Therefore, IL-2 can inhibit DC development via decreased signalling through Flt3 and increased monocyte/macrophage development.     

CD11c+ cells primed with unrelated antigens facilitate an accelerated immune response to influenza virus in mice

Recent evidence suggests that an individual's unique history and sequence of exposures to pathogens and antigens may dictate downstream immune responses to disparate antigens. We show that the i.n. delivery of nonreplicative virus‐like particles (VLPs), which bear structural but no antigenic similarities to respiratory pathogens, acts to prime the lungs of both C56BL/6 and BALB/c mice, facilitating heightened and accelerated primary immune responses to high‐dose influenza challenge, thus providing a nonpathogenic model of innate imprinting. These responses correspond closely to those observed following natural infection with the opportunistic fungus, Pneumocystis murina, and are characterized by accelerated antigen processing by DCs and alveolar macrophages, an enhanced influx of cells to the local tracheobronchial lymph node, and early upregulation of T‐cell co‐stimulatory/adhesion molecules. CD11c⁺ cells, which have been directly exposed to VLPs or Pneumocystis are necessary in facilitating enhanced clearance of influenza virus, and the repopulation of the lung by Ly‐6C⁺ precursors relies on CCR2 expression. Thus, immune imprinting 72 h after VLP‐priming, or 2 weeks after Pneumocystis‐priming is CCR2‐mediated and results from the enhanced antigen processing, maturation, and trafficking abilities of DCs and alveolar macrophages, which cause accelerated influenza‐specific primary immune responses and result in superior viral clearance.     

New insights into the role of the aryl hydrocarbon receptor in the function of CD11c+ cells during respiratory viral infection

The aryl hydrocarbon receptor (AHR) has garnered considerable attention as a modulator of CD4⁺ cell lineage development and function. It also regulates antiviral CD8⁺ T‐cell responses, but via indirect mechanisms that have yet to be determined. Here, we show that during acute influenza virus infection, AHR activation skews dendritic‐cell (DC) subsets in the lung‐draining lymph nodes, such that there are fewer conventional CD103⁺ DCs and CD11b⁺ DCs. Sorting DC subsets reveals AHR activation reduces immunostimulatory function of CD103⁺ DCs in the mediastinal lymph nodes, and decreases their frequency in the lung. DNA‐binding domain Ahr mutants demonstrate that alterations in DC subsets require the ligand‐activated AHR to contain its inherent DNA‐binding domain. To evaluate the intrinsic role of AHR in DCs, conditional knockouts were created using Cre‐LoxP technology, which revealed that AHR in CD11c⁺ cells plays a key role in controlling the acquisition of effector CD8⁺ T cells in the infected lung. However, AHR within other leukocyte lineages contributes to diminished naïve CD8⁺ T‐cell activation in the draining lymphoid nodes. These findings indicate DCs are among the direct targets of AHR ligands in vivo, and AHR signaling modifies host responses to a common respiratory pathogen by affecting the complex interplay of multiple cell types.     

CCL2, but not its receptor,is essential to restrict immune privileged central nervous system‐invasion of Japanese encephalitis virus via regulating accumulation of CD11b+ Ly‐6Chi monocytes

Japanese encephalitis virus (JEV) is a re‐emerging zoonotic flavivirus that poses an increasing threat to global health and welfare due to rapid changes in climate and demography. Although the CCR2–CCL2 axis plays an important role in trafficking CD11b⁺ Ly‐6C^hi monocytes to regulate immunopathological diseases, little is known about their role in monocyte trafficking during viral encephalitis caused by JEV infection. Here, we explored the role of CCR2 and its ligand CCL2 in JE caused by JEV infection using CCR2‐ and CCL2‐ablated murine models. Somewhat surprisingly, the ablation of CCR2 and CCL2 resulted in starkly contrasting susceptibility to JE. CCR2 ablation induced enhanced resistance to JE, whereas CCL2 ablation highly increased susceptibility to JE. This contrasting regulation of JE progression by CCR2 and CCL2 was coupled to central nervous system (CNS) infiltration of Ly‐6C^hi monocytes and Ly‐6G^hi granulocytes. There was also enhanced expression of CC and CXC chemokines in the CNS of CCL2‐ablated mice, which appeared to induce CNS infiltration of these cell populations. However, our data revealed that contrasting regulation of JE in CCR2‐ and CCL2‐ablated mice was unlikely to be mediated by innate natural killer and adaptive T‐cell responses. Furthermore, CCL2 produced by haematopoietic stem cell‐derived leucocytes played a dominant role in CNS accumulation of Ly‐6C^hi monocytes in infected bone marrow chimeric models, thereby exacerbating JE progression. Collectively, our data indicate that CCL2 plays an essential role in conferring protection against JE caused by JEV infection. In addition, blockage of CCR2, but not CCL2, will aid in the development of strategies for prophylactics and therapeutics of JE.     

Mesenteric lymph node CD11b− CD103+ PD‐L1High dendritic cells highly induce regulatory T cells

Dendritic cells (DCs) in mesenteric lymph nodes (MLNs) induce Foxp3⁺ regulatory T cells to regulate immune responses to beneficial or non‐harmful agents in the intestine, such as commensal bacteria and foods. Several studies in MLN DCs have revealed that the CD103⁺ DC subset highly induces regulatory T cells, and another study has reported that MLN DCs from programmed death ligand 1 (PD‐L1) ‐deficient mice could not induce regulatory T cells. Hence, the present study investigated the expression of these molecules on MLN CD11c⁺ cells. Four distinct subsets expressing CD103 and/or PD‐L1 were identified, namely CD11b⁺ CD103⁺ PD‐L1^High, CD11b⁻ CD103⁺ PD‐L1^High, CD11b⁻ CD103⁺ PD‐L1^Low and CD11b⁺ CD103⁻ PD‐L1^Int. Among them, the CD11b⁻ CD103⁺ PD‐L1^High DC subset highly induced Foxp3⁺ T cells. This subset expressed Aldh1a2 and Itgb8 genes, which are involved in retinoic acid metabolism and transforming growth factor‐β (TGF‐β) activation, respectively. Exogenous TGF‐β supplementation equalized the level of Foxp3⁺ T‐cell induction by the four subsets whereas retinoic acid did not, which suggests that high ability to activate TGF‐β is determinant for the high Foxp3⁺ T‐cell induction by CD11b⁻ CD103⁺ PD‐L1^High DC subset. Finally, this subset exhibited a migratory DC phenotype and could take up and present orally administered antigens. Collectively, the MLN CD11b⁻ CD103⁺ PD‐L1^High DC subset probably takes up luminal antigens in the intestine, migrates to MLNs, and highly induces regulatory T cells through TGF‐β activation.     

Supplementation of CXCL12 (CXCL12) induces homing of CD11c+ dendritic cells to the spleen and enhances control of Plasmodium berghei malaria in BALB/c mice

In malaria, parasitaemia is controlled in the spleen, a multicomponent organ that undergoes changes in its cellular constituents to control the parasite. During this process, dendritic cells (DCs) orchestrate the positioning of effector cells in a timely manner for optimal parasite clearance. We have recently demonstrated that CXCL12 [stromal cell‐derived factor‐1 (CXCL12)] supplementation partially restores the ability to control parasitaemia in Plasmodium berghei‐infected mice. In the present study, we investigated the nature of the DCs involved by flow cytometry and immunohistochemistry of CD11c⁺ cells. Flow cytometry of bone marrow cells showed that infection with P. berghei did not alter the proportion of CD11c⁺ cells present in this haematopoietic compartment, while CXCL12 supplementation of naïve uninfected mice induced only minor increases in the population of CD11c⁺ cells. In the spleen, P. berghei infection alone resulted in an increase in CD11c⁺ cells as compared with naïve animals. Exogenously administered CXCL12 in the absence of infection resulted in a significant expansion of the splenic CD11c⁺ population, and this effect was even more pronounced in infected and supplemented mice. Immunohistochemistry revealed that CD11c⁺ cells infiltrated the perivascular areas and marginal zone of the spleen in infected animals treated with CXCL12, suggesting that this chemokine induces homing of CD11c⁺ dendritic cells to the splenic compartment. Our results show that small amounts of CXCL12 supplementation are effective in recruiting DCs to the spleens of both uninfected and infected mice, suggesting the participation of CXCL12 and CD11c⁺ cells in the establishment of an adequate environment in the spleen for malaria control.     

Expansion of CD11b+Ly-6C+ myeloid-derived suppressor cells (MDSCs) driven by galectin-9 attenuates CVB3-induced myocarditis

Galectin-9 is known to play a role in the modulation of innate and adaptive immunity to ameliorate CVB3-induced myocarditis. In the present study, we found that galectin-9 induced the expansion of CD11b⁺Ly-6C⁺ myeloid-derived suppressor cells (MDSCs) in the heart from CVB3-infected mice. Adoptive transfer of CD11b⁺Ly-6C⁺ MDSCs significantly alleviated myocarditis accompanied by increased Th2 and Treg frequency and anti-inflammatory cytokines expression in the heart tissue. Moreover, Ly6C⁺ MDSCs, but not Ly6G⁺ cells, expressed Arg-1 and NOS2, and suppressed CD4⁺ T cell proliferation in vitro in an Arg-1-dependent mechanism; an event that was reversed with treatment of either an Arg-1 inhibitor or addition of excess l-arginine. Furthermore, Ly6C⁺ MDSCs co-expressed higher levels of F4/80, Tim-3, and IL-4Rα, and had the plasticity to up-regulate NOS2 or Arg-1 in response to IFN-γ or IL-4 treatment. The present results indicate that galectin-9 expands CD11b⁺Ly-6C⁺ MDSCs to ameliorate CVB3-induced myocarditis.     

NR4A1‐dependent Ly6Clow monocytes contribute to reducing joint inflammation in arthritic mice through Treg cells

Monocytes are central to the physiopathology of arthritis, but their roles in progression and resolution of the disease remain to be clarified. Using NR4A1^?/? mice, which lack patrolling lymphocyte antigen 6C (Ly6C^low) monocytes, we found that inflammatory Ly6C^high monocytes contribute to rapid development of arthritis in a serum transfer‐induced arthritis (STIA) model. Our experiments suggest that patrolling monocytes do not promote the initiation and progression of arthritis in mice, as severity of symptoms was amplified in NR4A1^?/? mice. Moreover, we show that treatment of arthritic wild type (WT) mice with cytosporone B (Csn‐B), a NR4A1‐specific agonist, significantly reduces severity of disease. Effects of Csn‐B were absent in monocyte‐depleted mice treated with clodronate until Ly6C^low monocytes were restored. Adoptive transfer of Ly6C^low monocytes in arthritic NR4A1^?/? mice treated with Csn‐B reduces joint inflammation, supporting the regulatory role of Ly6C^low subset on disease development. Our results also reveal that administration of Csn‐B to arthritic mice enhances levels of circulating CD4⁺CD25⁺FoxP3⁺ Treg cells, a process requiring the presence of Ly6C^low monocytes. Together, these data indicate that Ly6C^high monocytes are involved in the initiation and progression of arthritis and Ly6C^low monocytes contribute to reduce joint inflammation through the mobilization of Treg cells.     

Origins and functional basis of regulatory CD11c+CD8+ T cells

Previously, we showed that CD11c defines a novel subset of CD8⁺ T cells whose in vivo activity is therapeutic for arthritis; however, the mechanisms directing their development, identity of their precursors, and basis of their effector function remain unknown. Here, we show that the novel subset develops from CD11c^surface?CD8⁺ T cells and undergoes robust expansion in an antigen‐ and 4‐1BB (CD137)‐dependent manner. CD11c⁺CD8⁺ T cells exist in naïve mice (<3%) with limited suppressive activity. Once activated, they suppress CD4⁺ T cells in vivo and in vitro. Suppression of CD4⁺ by CD11c⁺CD8⁺ T cells is related to an increase in IDO activity induced in competent cells via the general control non‐derepressible‐2 pathway. CD11c⁺CD8⁺ T cells are refractory to the effect of IDO but constrict in a novel 1‐methyl D ,L ‐tryptophan‐dependent mechanism resulting in reversal of their suppressive effects. Thus, our data uncover, for the first time, the origin, development, and basis of the suppressive function of this novel CD11c⁺CD8⁺ T‐cell subpopulation that has many signature features of Treg.     

Antigen-specific cytotoxic T lymphocytes target airway CD103+ and CD11b+ dendritic cells to suppress allergic inflammation

Allergic airway inflammation is driven by the recognition of inhaled allergen by T helper type 2 (Th2) cells in the airway and lung. Allergen-specific cytotoxic T lymphocytes (CTLs) can strongly reduce airway inflammation, however, the mechanism of their inhibitory activity is not fully defined. We used mouse models to show that allergen-specific CTLs reduced early cytokine production by Th2 cells in lung, and their subsequent accumulation and production of interleukin (IL)-4 and IL-13. In addition, treatment with specific CTLs also increased the proportion of caspase⁺ dendritic cells (DCs) in mediastinal lymph node (MLN), and decreased the numbers of CD103⁺ and CD11b⁺ DCs in the lung. This decrease required expression of the cytotoxic mediator perforin in CTLs and of the appropriate MHC-antigen ligand on DCs, suggesting that direct CTL-DC contact was necessary. Lastly, lung imaging experiments revealed that in airway-challenged mice XCR1-GFP⁺ DCs, corresponding to the CD103⁺ DC subset, and XCR1-GFP⁻ CD11c⁺ cells, which include CD11b⁺ DCs and alveolar macrophages, both clustered in the areas surrounding the small airways and were closely associated with allergen-specific CTLs. Thus, allergen-specific CTLs reduce allergic airway inflammation by depleting CD103⁺ and CD11b⁺ DC populations in the lung, and may constitute a mechanism through which allergic immune responses are regulated.     

CD64 distinguishes macrophages from dendritic cells in the gut and reveals the Th1‐inducing role of mesenteric lymph node macrophages during colitis

Dendritic cells (DCs) and monocyte‐derived macrophages (MΦs) are key components of intestinal immunity. However, the lack of surface markers differentiating MΦs from DCs has hampered understanding of their respective functions. Here, we demonstrate that, using CD64 expression, MΦs can be distinguished from DCs in the intestine of both mice and humans. On that basis, we revisit the phenotype of intestinal DCs in the absence of contaminating MΦs and we delineate a developmental pathway in the healthy intestine that leads from newly extravasated Ly‐6C^hi monocytes to intestinal MΦs. We determine how inflammation impacts this pathway and show that T cell‐mediated colitis is associated with massive recruitment of monocytes to the intestine and the mesenteric lymph node (MLN). There, these monocytes differentiate into inflammatory MΦs endowed with phagocytic activity and the ability to produce inducible nitric oxide synthase. In the MLNs, inflammatory MΦs are located in the T‐cell zone and trigger the induction of proinflammatory T cells. Finally, T cell‐mediated colitis develops irrespective of intestinal DC migration, an unexpected finding supporting an important role for MLN‐resident inflammatory MΦs in the etiology of T cell‐mediated colitis.     

Depletion of macrophages in mice results in higher dengue virus titers and highlights the role of macrophages for virus control

Monocytes and macrophages are target cells for dengue infection. Besides their potential role for virus replication, activated monocytes/macrophages produce cytokines that may be critical for dengue pathology. To study the in vivo role of monocytes and macrophages for virus replication, we depleted monocytes and macrophages in IFN‐αβγR knockout mice with clodronate liposomes before dengue infection. Although less virus was first recovered in the draining LN in the absence of macrophages, monocyte/macrophage depletion eventually resulted in a ten‐fold higher systemic viral titer. A massive infiltration of CD11b⁺CD11c^lowLy6C^low monocytes into infected organs was observed in parallel with increasing virus titers before viremia was controlled. Depletion of monocytes in the blood before or after local infection had no impact on virus titers, suggesting that monocytes are not required as “virus‐shuttles”. Our data provide evidence that systemic viremia is established independently of tissue macrophages present at the site of infection and blood monocytes. Instead, we demonstrate the importance of monocytes/macrophages for the control of dengue virus.     

Impaired dendritic cell differentiation of CD16‐positive monocytes in tuberculosis: Role of p38 MAPK

Tuberculosis (TB) is one of the world's most pernicious diseases mainly due to immune evasion strategies displayed by its causative agent Mycobacterium tuberculosis (Mtb). Blood monocytes (Mos) represent an important source of DCs during chronic infections; consequently, the alteration of their differentiation constitutes an escape mechanism leading to mycobacterial persistence. We evaluated whether the CD16⁺/CD16^? Mo ratio could be associated with the impaired Mo differentiation into DCs found in TB patients. The phenotype and ability to stimulate Mtb‐specific memory clones DCs from isolated Mo subsets were assessed. We found that CD16^? Mos differentiated into CD1a⁺DC‐SIGN^high cells achieving an efficient recall response, while CD16⁺ Mos differentiated into a CD1a^?DC‐SIGN^low population characterized by a poor mycobacterial Ag‐presenting capacity. The high and sustained phosphorylated p38 expression observed in CD16⁺ Mos was involved in the altered DC profile given that its blockage restored DC phenotype and its activation impaired CD16^? Mo differentiation. Furthermore, depletion of CD16⁺ Mos indeed improved the differentiation of Mos from TB patients toward CD1a⁺DC‐SIGN^high DCs. Therefore, Mos from TB patients are less prone to differentiate into DCs due to their increased proportion of CD16⁺ Mos, suggesting that during Mtb infection Mo subsets may have different fates after entering the lungs.