Migratory properties of pulmonary dendritic cells are determined by their developmental lineage

Complement receptor C5aR1/CD88 and dipeptidyl peptidase-4/CD26 define distinct hematopoietic lineages of dendritic cells

Differential display of the integrins CD103 and CD11b are widely used to distinguish two major dendritic cell (DC) subsets in nonlymphoid tissues. CD103(+) DCs arise from FLT3-dependent DC precursors (preDCs), whereas CD11b(hi) DCs can arise either from preDCs or FLT3-independent monocytes. Functional characterization of these two lineages of CD11b(hi) DCs has been hindered by the lack of a widely applicable method to distinguish between them. We performed gene expression analysis of fractionated lung DCs from C57BL/6 mice and found that monocyte-derived DCs (moDCs), including CD11b(hi)Ly-6C(lo) tissue-resident and CD11b(hi)Ly-6C(hi) inflammatory moDCs, express the complement 5a receptor 1/CD88, whereas preDC-derived conventional DCs (cDCs), including CD103(+) and CD11b(hi) cDCs, express dipeptidyl peptidase-4/CD26. Flow cytometric analysis of multiple organs, including the kidney, liver, lung, lymph nodes, small intestine, and spleen, confirmed that reciprocal display of CD88 and CD26 can reliably distinguish FLT3-independent moDCs from FLT3-dependent cDCs in C57BL/6 mice. Similar results were obtained when DCs from BALB/c mice were analyzed. Using this novel approach to study DCs in mediastinal lymph nodes, we observed that most blood-derived lymph node-resident DCs, as well as tissue-derived migratory DCs, are cDCs. Furthermore, cDCs, but not moDCs, stimulated naive T cell proliferation. We anticipate that the use of Abs against CD88 and CD26 to distinguish moDCs and cDCs in multiple organs and mouse strains will facilitate studies aimed at assigning specific functions to distinct DC lineages in immune responses.

The development and function of lung-resident macrophages and dendritic cells

Gas exchange is the vital function of the lungs. It occurs in the alveoli, where oxygen and carbon dioxide diffuse across the alveolar epithelium and the capillary endothelium surrounding the alveoli, separated only by a fused basement membrane 0.2-0.5 μm in thickness. This tenuous barrier is exposed to dangerous or innocuous particles, toxins, allergens and infectious agents inhaled with the air or carried in the blood. The lung immune system has evolved to ward off pathogens and restrain inflammation-mediated damage to maintain gas exchange. Lung-resident macrophages and dendritic cells are located in close proximity to the epithelial surface of the respiratory system and the capillaries to sample and examine the air-borne and blood-borne material. In communication with alveolar epithelial cells, they set the threshold and the quality of the immune response.

Trif-dependent induction of Th17 immunity by lung dendritic cells

Allergic asthma is thought to stem largely from maladaptive T helper 2 (Th2) responses to inhaled allergens, which in turn lead to airway eosinophilia and airway hyperresponsiveness (AHR). However, many individuals with asthma have airway inflammation that is predominantly neutrophilic and resistant to treatment with inhaled glucocorticoids. An improved understanding of the molecular basis of this form of asthma might lead to improved strategies for its treatment. Here, we identify novel roles of the adaptor protein, TRIF (TIR-domain-containing adapter-inducing interferon-β), in neutrophilic responses to inhaled allergens. In different mouse models of asthma, Trif-deficient animals had marked reductions in interleukin (IL)-17, airway neutrophils, and AHR compared with wild-type (WT) mice, whereas airway eosinophils were generally similar in these two strains. Compared with lung dendritic cells (DCs) from WT mice, lung DCs from Trif-deficient mice displayed impaired lipopolysaccharide (LPS)-induced migration to regional lymph nodes, lower levels of the costimulatory molecule, CD40, and produced smaller amounts of the T helper 17 (Th17)-promoting cytokines, IL-6, and IL-1β. When cultured with allergen-specific, naive T cells, Trif-deficient lung DCs stimulated robust Th2 cell differentiation but very weak Th1 and Th17 cell differentiation. Together, these findings reveal a TRIF-CD40-Th17 axis in the development of IL-17-associated neutrophilic asthma.

T-cell-restricted T-bet overexpression induces aberrant hematopoiesis of myeloid cells and impairs function of macrophages in the lung

Although overexpression of T-bet, a master transcription factor in type-1 helper T lymphocytes, has been reported in several hematologic and immune diseases, its role in their pathogenesis is not fully understood. In the present study, we used transgenic model mice (T-bet(tg/wt) and T-bet(tg/tg)) to investigate the effects of T-bet overexpression selectively in T lymphocytes on the development of hematologic and immune diseases. The results showed that T-bet overexpression in T cells spontaneously induced maturation arrest in the mononuclear phagocyte lineage, as well as spontaneous dermatitis and pulmonary alveolar proteinosis (PAP)-like disease in T-bet(tg/wt) and T-bet(tg/tg) mice, respectively. T-bet(tg/tg) alveoli with the PAP phenotype showed remarkable reorganization of alveolar mononuclear phagocyte subpopulations and impaired function, in addition to augmented T-cell infiltration. In addition, PAP development in T-bet(tg/tg) mice was found to be associated with increased migration of myeloid cells from the bone marrow into the peripheral blood. These findings reveal an unexpected link between T-bet overexpression in T lymphocytes and the development of PAP caused by reorganization of mononuclear phagocytes in the lung, and provide new insight into the molecular pathogenesis of secondary PAP accompanied by hematologic disorders.

Epigenetic control of Ccr7 expression in distinct lineages of lung dendritic cells

Adaptive immune responses to inhaled allergens are induced following CCR7-dependent migration of precursor of dendritic cell (pre-DC)-derived conventional DCs (cDCs) from the lung to regional lymph nodes. However, monocyte-derived (moDCs) in the lung express very low levels of Ccr7 and consequently do not migrate efficiently to LN. To investigate the molecular mechanisms that underlie this dichotomy, we studied epigenetic modifications at the Ccr7 locus of murine cDCs and moDCs. When expanded from bone marrow precursors, moDCs were enriched at the Ccr7 locus for trimethylation of histone 3 lysine 27 (H3K27me3), a modification associated with transcriptional repression. Similarly, moDCs prepared from the lung also displayed increased levels of H3K27me3 at the Ccr7 promoter compared with migratory cDCs from that organ. Analysis of DC progenitors revealed that epigenetic modification of Ccr7 does not occur early during DC lineage commitment because monocytes and pre-DCs both had low levels of Ccr7-associated H3K27me3. Rather, Ccr7 is gradually silenced during the differentiation of monocytes to moDCs. Thus, epigenetic modifications of the Ccr7 locus control the migration and therefore the function of DCs in vivo. These findings suggest that manipulating epigenetic mechanisms might be a novel approach to control DC migration and thereby improve DC-based vaccines and treat inflammatory diseases of the lung.

Distinct dendritic cell subsets actively induce Th2 polarization

The mechanisms by which dendritic cells induce Th2 polarization (DC(Th2) cells) have been controversial. Many have argued that DC(Th2) cells are not a distinct functional DC subset, but rather, DC-induced polarization of Th2 cells is a default pathway that occurs in the absence of inflammatory signals leading to DC-induced polarization of Th1/Th17 cells. However, recent studies demonstrate that distinct subsets of tissue DCs actively polarize Th2 cells after stimulation with type-2 inducing stimuli. DC(Th2) cells development is marked by the upregulation of specific transcription factors, cell surface molecules, and cytokines. These findings counter previous hypotheses that Th2 skewing by DCs is a passive response and support a model in which DCs are actively programmed to induce Th2 differentiation.

Dendritic cells in asthma

The lungs are constantly exposed to antigens, most of which are non-pathogenic and do not require the induction of an immune response. Dendritic cells (DCs) are situated at the basolateral site of the lungs and continuously scan the environment to detect the presence of pathogens and subsequently initiate an immune response. They are a heterogeneous population of antigen-presenting cells that exert specific functions. Compelling evidence is now provided that DCs are both sufficient and necessary to induce allergic responses against several inhaled harmless allergens. How various DC subsets exactly contribute to the induction of allergic asthma is currently a subject of intense investigation. We here review the current progress in this field.

IL-4 blocks TH1-polarizing/inflammatory cytokine gene expression during monocyte-derived dendritic cell differentiation through histone hypoacetylation

BACKGROUND

Whereas recent research has characterized the mechanism by which dendritic cells (DCs) induce T(H)1/T(H)17 responses, the functional specialization enabling DCs to polarize T(H)2 responses remains undefined. Because IL-4 is essential during T(H)2 responses not only by acting on CD4(+) T cells through the activation of GATA-3 but also by regulating IgE class-switching, epithelial cell permeability, and muscle contractility, we hypothesized that IL-4 could also have a role in the conditioning of DCs during T(H)2 responses.

OBJECTIVE

We sought to analyze whether IL-4 exerts an immunomodulatory function on DCs during their differentiation, leading to their functional specialization for the induction of T(H)2 responses.

METHODS

Monocyte-derived DCs (moDCs) conditioned by IL-4 during their differentiation (IL-4-conditioned moDCs [IL-4-moDCs]) were analyzed for T(H)1-polarizing/inflammatory cytokine production in response to Toll-like receptor stimulation. The acetylation level of the promoters of the genes encoding these cytokines was analyzed by using chromatin immunoprecipitation. Gene expression profiling of IL-4-moDCs was defined by using mouse genome microarrays. IL-4-moDCs were tested for their capacity to induce house dust mite-mediated allergic reactions.

RESULTS

Our data suggest that IL-4 inhibits T(H)1-polarizing/inflammatory cytokine gene expression on IL-4-moDCs through the deacetylation of the promoters of these genes, leading to their transcriptional repression. Microarray analyses confirmed that IL-4 upregulated T(H)2-related genes as eosinophil-associated ribonucleases, eosinophil/basophil chemokines, and M2 genes. IL-4 licensed moDCs for the induction of T(H)2 responses, causing house dust mite-mediated allergic airway inflammation.

CONCLUSION

This study describes a new role for IL-4 by demonstrating that moDCs are conditioned by IL-4 for the induction of T(H)2 responses by blocking T(H)1-polarizing/inflammatory cytokine production through histone hypoacetylation and upregulating T(H)2-related genes.

Migratory conventional dendritic cells in the induction of peripheral T cell tolerance

Conventional DCs are an extremely heterogeneous cell population that comprises several different subsets. A major distinction can be made between lymphoid-resident DCs that are present in the lymphoid tissues and the blood and migratory DCs that reside in the nonlymphoid tissues and migrate to the lymph nodes, both in homeostatic conditions and during the course of an infection. Migratory DCs differ from tissue to tissue but share the unique ability to transport antigens to the draining lymph nodes-in particular, tissue-restricted antigens in homeostatic conditions and microbial antigens after an infection. Recently, steady-state migratory DCs have gained much attention after the discovery of their high tolerogenic potential. The purpose of this review is to give a picture of the recent finding regarding steady-state migratory DCs with particular interest in their role in inducing T cell tolerance.

PGI₂ signaling inhibits antigen uptake and increases migration of immature dendritic cells

PGI₂ signaling through IP inhibits allergen-induced inflammatory responses in mice. We reported previously that PGI₂ analogs decreased proinflammatory cytokine and chemokine production by mature BMDCs. However, whether PGI₂ modulates the function of immature DCs has not been investigated. We hypothesized that PGI2 negatively regulates immature DC function and investigated the effect of PGI2 analogs on immature BMDC antigen uptake and migration in vitro and in vivo. Immature BMDCs were obtained from WT and IPKO mice, both on a C57BL/6 background. The PGI2 analog cicaprost decreased FITC-OVA uptake by immature BMDCs. In addition, cicaprost increased immature BMDC podosome dissolution, pro-MMP-9 production, cell surface CCR7 expression, and chemotactic migration toward CCL19 and CCL21, as well as chemokinesis, in an IP-specific fashion. These in vitro results suggested that cicaprost promotes migration of immature DCs from mucosal surface to draining LNs. This concept was supported by the finding that migration of immature GFP⁺ BMDCs to draining LNs was enhanced by pretreatment with cicaprost. Further, migration of immature lung DCs labeled with PKH26 was enhanced by intranasal cicaprost administration. Our results suggest PGI2-IP signaling increases immature DC migration to the draining LNs and may represent a novel mechanism by which this eicosanoid inhibits immune responses.