Carlos Ardavín

Origin, precursors and differentiation of mouse dendritic cells

Functional specialization allows defined dendritic-cell (DC) subsets to induce efficient defence mechanisms against pathogens and tumour cells, and maintain T-cell tolerance by inducing the inactivation of autoreactive T cells. A crucial question, which has important implications for both our understanding of the induction and control of immunity by DCs, as well as the use of DCs for immunotherapy, is whether the functional diversity of DCs results from the existence of developmentally independent DC subpopulations, or whether DC subsets that share a common differentiation origin acquire specific functions in response to environmental signals. This review discusses recent findings on mouse DC development.

Origin and differentiation of dendritic cells

Despite extensive, recent research on the development of dendritic cells (DCs), their origin is a controversial issue in immunology, with important implications regarding their use in cancer immunotherapy. Although, under defined experimental conditions, DCs can be generated from myeloid or lymphoid precursors, the differentiation pathways that generate DCs in vivo remain unknown largely. Indeed, experimental results suggest that the in vivo differentiation of a particular DC subpopulation could be unrelated to its possible experimental generation. Nevertheless, the analysis of DC differentiation by in vivo and in vitro experimental systems could provide important insights into the control of the physiological development of DCs and constitutes the basis of a model of common DC differentiation that we propose.

Characterization of a common precursor population for dendritic cells

Dendritic cells (DCs) are essential for the establishment of immune responses against pathogens and tumour cells, and thus have great potential as tools for vaccination and cancer immunotherapy trials. Experimental evidence has led to a dual DC differentiation model, which involves the existence of both myeloid- and lymphoid-derived DCs. But this concept has been challenged by recent reports demonstrating that both CD8- and CD8+ DCs, considered in mice as archetypes of myeloid and lymphoid DCs respectively, can be generated from either lymphoid or myeloid progenitors. The issue of DC physiological derivation therefore remains an open question. Here we report the characterization of a DC-committed precursor population, which has the capacity to generate all the DC subpopulations present in mouse lymphoid organs—including CD8- and CD8+ DCs, as well as the B220+ DC subset—but which is devoid of myeloid or lymphoid differentiation potential. These data support an alternative model of DC development, in which there is an independent, common DC differentiation pathway.

Differentiation and function of mouse monocyte‐derived dendritic cells in steady state and inflammation

Summary:  Although monocytes were originally described as precursors to all the different subpopulations of macrophages found in the steady state and formed under inflammatory and infectious conditions, recent data have demonstrated conclusively that monocytes can also differentiate into dendritic cells (DCs). Monocytes are the precursors to different subsets of DCs, such as Langerhans cells and DCs found in the lamina propria of the gastrointestinal, respiratory, and urogenital tracts. In addition, monocyte‐derived DCs (moDCs), newly formed during inflammatory reactions, appear to fulfill an essential role in defense mechanisms against pathogens by participating in the induction of both adaptive and innate immune responses. In this regard, moDCs have the capacity to activate antigen‐specific CD4+ T‐cell responses and to cross‐prime CD8+ T cells, during viral, bacterial, and parasitic infections. In addition, monocytes have been recently described as the precursors to a subset of DCs specialized in innate immunity against pathogens, named TipDCs [for TNF‐α (tumor necrosis factor‐α)‐iNOS (inducible nitric oxide synthase)‐producing DCs] that display a remarkable microbicidal activity and also provide iNOS‐dependent help for antibody production by B cells. Importantly, in contrast to DCs developing in the steady state, moDCs formed during inflammatory and infectious processes are subjected to diverse soluble mediators that determine the multiple functional specificities displayed by moDCs, as a result of the remarkable developmental plasticity of monocytes. In this review, we discuss recent findings dealing with the differentiation and functional relevance of moDCs that have widened the frontiers of DC immunobiology in relation to innate and adaptive immunity and the etiology of chronic inflammatory diseases.

Monocyte-derived dendritic cells in innate and adaptive immunity.

Monocytes have been classically considered essential elements in relation with innate immune responses against pathogens, and inflammatory processes caused by external aggressions, infection and autoimmune disease. However, although their potential to differentiate into dendritic cells (DCs) was discovered 14 years ago, their functional relevance with regard to adaptive immune responses has only been uncovered very recently. Studies performed over the last years have revealed that monocyte‐derived DCs play an important role in innate and adaptive immunity, due to their microbicidal potential, capacity to stimulate CD4+ and CD8+ T‐cell responses and ability to regulate Immunoglobulin production by B cells. In addition, monocyte‐derived DCs not only constitute a subset of DCs formed at inflammatory foci, as previously thought, but also comprise different subsets of DCs located in antigen capture areas, such as the skin and the intestinal, respiratory and reproductive tracts.

CCL2 Shapes Macrophage Polarization by GM-CSF and M-CSF: Identification of CCL2/CCR2-Dependent Gene Expression Profile

The CCL2 chemokine mediates monocyte egress from bone marrow and recruitment into inflamed tissues through interaction with the CCR2 chemokine receptor, and its expression is upregulated by proinflammatory cytokines. Analysis of the gene expression profile in GM-CSF– and M-CSF–polarized macrophages revealed that a high CCL2 expression characterizes macrophages generated under the influence of M-CSF, whereas CCR2 is expressed only by GM-CSF–polarized macrophages. Analysis of the factors responsible for this differential expression identified activin A as a critical factor controlling the expression of the CCL2/CCR2 pair in macrophages, as activin A increased CCR2 expression but inhibited the acquisition of CCL2 expression by M-CSF–polarized macrophages. CCL2 and CCR2 were found to determine the extent of macrophage polarization because CCL2 enhances the LPS-induced production of IL-10, whereas CCL2 blockade leads to enhanced expression of M1 polarization-associated genes and cytokines, and diminished expression of M2-associated markers in human macrophages. Along the same line, Ccr2-deficient bone marrow–derived murine macrophages displayed an M1-skewed polarization profile at the transcriptomic level and exhibited a significantly higher expression of proinflammatory cytokines (TNF-α, IL-6) in response to LPS. Therefore, the CCL2-CCR2 axis regulates macrophage polarization by influencing the expression of functionally relevant and polarization-associated genes and downmodulating proinflammatory cytokine production.

In Vivo Induction of Immune Responses to Pathogens by Conventional Dendritic Cells

Specific defense mechanisms against pathogens are fulfilled by different subsets of nonmucosal conventional dendritic cells (DCs), including migratory Langerhans cells (LCs), dermal DCs, and resident CD8(+) and CD8(-) DCs found in lymphoid organs. Dermal DCs capture antigens in the skin and migrate to lymph nodes, where they can transfer the antigens to CD8(+) DCs and activate CD4(+) T cells. Differential antigen-processing machinery grants CD8(+) DCs a high efficiency in activating CD8(+) T cells through crosspresentation, whereas CD8(-) DCs preferentially trigger CD4(+) T cell responses. Recent findings have revealed the important role played by monocyte-derived DCs (mo-DCs), newly formed during infection, in activating CD4(+) and CD8(+) T cells, regulating immunoglobulin production, and killing pathogens. However, a number of controversial issues regarding the function of different DC subsets during viral, bacterial, and parasitic infections remain to be resolved.

In Vivo Adjuvant-Induced Mobilization and Maturation of Gut Dendritic Cells after Oral Administration of Cholera Toxin

Although dendritic cells (DCs) regulate immune responses, they exhibit functional heterogeneity depending on their anatomical location. We examined the functional properties of intestinal DCs after oral administration of cholera toxin (CT), the most potent mucosal adjuvant. Two CD11c+ DC subsets were identified both in Peyer’s patches and mesenteric lymph nodes (MLN) based on the expression of CD8α (CD8+ and CD8− DCs, respectively). A third subset of CD11c+CD8int was found exclusively in MLN. Feeding mice with CT induced a rapid and transient mobilization of a new CD11c+CD8− DC subset near the intestinal epithelium. This recruitment was associated with an increased production of the chemokine CCL20 in the small intestine and was followed by a massive accumulation of CD8int DCs in MLN. MLN DCs from CT-treated mice were more potent activators of naive T cells than DCs from control mice and induced a Th2 response. This increase in immunostimulating properties was accounted for by CD8int and CD8− DCs, whereas CD8+ DCs remained insensitive to CT treatment. Consistently, the CD8int and CD8− subsets expressed higher levels of costimulatory molecules than CD8+ and corresponding control DCs. Adoptive transfer experiments showed that these two DC subsets, unlike CD8+ DCs, were able to present Ags orally coadministered with CT in an immunostimulating manner. The ability of CT to mobilize immature DCs in the intestinal epithelium and to promote their emigration and differentiation in draining lymph nodes may explain the exceptional adjuvant properties of this toxin on mucosal immune responses.

Molecular cloning, functional characterization and mRNA expression analysis of the murine chemokine receptor CCR6 and its specific ligand MIP‐3α1

We have cloned the murine CCR6 receptor and its ligand, the β‐chemokine mMIP‐3α. Calcium mobilization assays performed with mCCR6 transfectants showed significant responses upon addition of mMIP‐3α. Murine MIP‐3α RNA is expressed in thymus, small intestine and colon, whereas mCCR6 RNA is expressed in spleen and lymph nodes. RT‐PCR analysis of FACS‐sorted lymphoid and antigen presenting cell subsets showed mCCR6 expression mainly in B cells, CD8− splenic dendritic cells and CD4+ T cells. The cloning and functional characterization of the mCCR6 and mMIP‐3α will allow the study of the role of these proteins in mouse models of inflammation and immunity.

Monocyte migration to inflamed skin and lymph nodes is differentially controlled by L-selectin and PSGL-1.

Monocyte recruitment and differentiation into dendritic cells or macrophages play a critical role in defense mechanisms against pathogens and in inflammatory and autoimmune diseases. Important contributions have been made on the molecular events controlling neutrophil and lymphocyte extravasation under steady state or inflammation. However, the molecules involved in monocyte rolling during their migration to antigen capture areas and lymphoid organs during infection remain undefined. Here we have analyzed the homing molecules controlling mouse monocyte rolling in an experimental model of Leishmania major infection. Monocyte migration through inflamed dermal venules was dependent on interactions of PSGL-1 with P- and E-selectins, and of L-selectin with PNAd, whereas migration through lymph node high endothelial venules relied essentially on L-selectin-PNAd interactions. These results might have important implications regarding the induction of immune responses against pathogens and future immunotherapeutic protocols of inflammatory and autoimmune diseases, based on selective inhibition of monocyte migration to specific inflammatory foci.