Dendritic cells (DC) produce interleukin-12 (IL-12) in response to Toll-like receptor (TLR) activation. Two major TLR signaling pathways participate in the response to pathogens: the nuclear factor-κB (NF-κB)–dependent pathway leading to inflammatory cytokine secretion including IL-12 and the interferon (IFN)-dependent pathway inducing type I IFN and IFN-regulated genes. Here we show that the two pathways cooperate and are likely both necessary for inducing an optimal response to pathogens. R-848/Resiquimod (TLR7 ligand in the mouse and TLR7/8 ligand in human) synergized with poly(I:C) (TLR3 ligand) or lipopolysaccharide (LPS; TLR4 ligand) in inducing high levels of bioactive IL-12p70 secretion and IFN-β mRNA accumulation by mouse bone marrow–derived DC (BM-DC). Strikingly, IL-12p70 but not IL-12p40 secretion was strongly reduced in BM-DC from STAT1^−/− and IFNAR^−/− mice. STAT1 tyrosine-phosphorylation, IL-12p35, and IFN-β mRNA accumulation were strongly inhibited in IFNAR^−/− BM-DC activated with the TLR ligand combinations. Similar observation were obtained in human TLR8-expressing monocyte-derived DC (moDC) using neutralizing anti-IFNAR2 antibodies, although results also pointed to a possible involvement of IFN-λ1 (also known as IL-29). This suggests that TLR engagement on DC induces endogenous IFNs that further synergize with the NF-κB pathway for optimal IL-12p70 secretion. Moreover, analysis of interferon regulatory factors (IRF) regulation in moDC suggests a role for IRF7/8 in mediating IRF3-independent type I IFN and possibly IL-12p35 synthesis in response to TLR7/8.

The recruitment of selected dendritic cell (DC) subtypes conditions the class of the immune response. Here we show that the migration of human plasmacytoid DCs (pDCs), the blood natural interferon α–producing cells, is induced upon the collective action of inducible and constitutive chemokines. Despite expression of very high levels of CXCR3, pDCs do not respond efficiently to CXCR3 ligands. However, they migrate in response to the constitutive chemokine stromal cell–derived factor 1 (SDF-1)/CXCL12 and CXCR3 ligands synergize with SDF-1/CXCL12 to induce pDC migration. This synergy reflects a sensitizing effect of CXCR3 ligands, which, independently of a gradient and chemoattraction, decrease by 20–50-fold the threshold of sensitivity to SDF-1/CXCL12. Thus, the ability of the constitutive chemokine SDF-1/CXCL12 to induce pDC recruitment might be controlled by CXCR3 ligands released during inflammation such as in virus infection. SDF-1/CXCL12 and the CXCR3 ligands Mig/CXCL9 and ITAC/CXCL1 display adjacent expression both in secondary lymphoid organs and in inflamed epithelium from virus-induced pathologic lesions. Because pDCs express both the lymph node homing molecule l-selectin and the cutaneous homing molecule cutaneous lymphocyte antigen, the cooperation between inducible CXCR3 ligands and constitutive SDF-1/CXCL12 may regulate recruitment of pDCs either in lymph nodes or at peripheral sites of inflammation.

Progressing tumors in man and mouse are often infiltrated by dendritic cells (DCs). Deficient antitumor immunity could be related to a lack of tumor-associated antigen (TAA) presentation by tumor-infiltrating DCs (TIDCs) or to a functional defect of TIDCs. Here we investigated the phenotype and function of TIDCs in transplantable and transgenic mouse tumor models. Although TIDCs could encompass various known DC subsets, most had an immature phenotype. We observed that TIDCs were able to present TAA in the context of major histocompatibility complex class I but that they were refractory to stimulation with the combination of lipopolysaccharide, interferon γ, and anti-CD40 antibody. We could revert TIDC paralysis, however, by in vitro or in vivo stimulation with the combination of a CpG immunostimulatory sequence and an anti-interleukin 10 receptor (IL-10R) antibody. CpG or anti–IL-10R alone were inactive in TIDCs, whereas CpG triggered activation in normal DCs. In particular, CpG plus anti–IL-10R enhanced the TAA-specific immune response and triggered de novo IL-12 production. Subsequently, CpG plus anti–IL-10R treatment showed robust antitumor therapeutic activity exceeding by far that of CpG alone, and elicited antitumor immune memory.

Dendritic cells (DCs) form a network comprising different populations that initiate and differentially regulate immune responses. Langerhans cells (LCs) represent a unique population of DCs colonizing epithelium, and we present here observations suggesting that macrophage inflammatory protein (MIP)-3α plays a central role in LC precursor recruitment into the epithelium during inflammation. (a) Among DC populations, MIP-3α was the most potent chemokine inducing the selective migration of in vitro–generated CD34⁺ hematopoietic progenitor cell–derived LC precursors and skin LCs in accordance with the restricted MIP-3α receptor (CC chemokine receptor 6) expression to these cells. (b) MIP-3α was mainly produced by epithelial cells, and the migration of LC precursors induced by the supernatant of activated skin keratinocytes was completely blocked with an antibody against MIP-3α. (c) In vivo, MIP-3α was selectively produced at sites of inflammation as illustrated in tonsils and lesional psoriatic skin where MIP-3α upregulation appeared associated with an increase in LC turnover. (d) Finally, the secretion of MIP-3α was strongly upregulated by cells of epithelial origin after inflammatory stimuli (interleukin 1β plus tumor necrosis factor α) or T cell signals. Results of this study suggest a major role of MIP-3α in epithelial colonization by LCs under inflammatory conditions and immune disorders, and might open new ways to control epithelial immunity.

DCs (dendritic cells) function as sentinels of the immune system. They traffic from the blood to the tissues where, while immature, they capture antigens. They then leave the tissues and move to the draining lymphoid organs where, converted into mature DC, they prime naive T cells. This suggestive link between DC traffic pattern and functions led us to investigate the chemokine responsiveness of DCs during their development and maturation. DCs were differentiated either from CD34⁺ hematopoietic progenitor cells (HPCs) cultured with granulocyte/macrophage colony–stimulating factor (GM-CSF) plus tumor necrosis factor (TNF)-α or from monocytes cultured with GM-CSF plus interleukin 4. Immature DCs derived from CD34⁺ HPCs migrate most vigorously in response to macrophage inflammatory protein (MIP)-3α, but also to MIP-1α and RANTES (regulated on activation, normal T cell expressed and secreted). Upon maturation, induced by either TNF-α, lipopolysaccharide, or CD40L, DCs lose their response to these three chemokines when they acquire a sustained responsiveness to a single other chemokine, MIP-3β. CC chemokine receptor (CCR)6 and CCR7 are the only known receptors for MIP-3α and MIP-3β, respectively. The observation that CCR6 mRNA expression decreases progressively as DCs mature, whereas CCR7 mRNA expression is sharply upregulated, provides a likely explanation for the changes in chemokine responsiveness. Similarly, MIP-3β responsiveness and CCR7 expression are induced upon maturation of monocyte- derived DCs. Furthermore, the chemotactic response to MIP-3β is also acquired by CD11c⁺ DCs isolated from blood after spontaneous maturation. Finally, detection by in situ hybridization of MIP-3α mRNA only within inflamed epithelial crypts of tonsils, and of MIP-3β mRNA specifically in T cell–rich areas, suggests a role for MIP-3α/CCR6 in recruitment of immature DCs at site of injury and for MIP-3β/CCR7 in accumulation of antigen-loaded mature DCs in T cell–rich areas.

Dendritic cells initiate immune responses by ferrying antigen from the tissues to the lymphoid organs for presentation to lymphocytes. Little is known about the molecular mechanisms underlying this migratory behavior. We have identified a chemokine receptor which appears to be selectively expressed in human dendritic cells derived from CD34⁺ cord blood precursors, but not in dendritic cells derived from peripheral blood monocytes. When stably expressed as a recombinant protein in a variety of host cell backgrounds, the receptor shows a strong interaction with only one chemokine among 25 tested: the recently reported CC chemokine macrophage inflammatory protein 3α. Thus, we have designated this receptor as the CC chemokine receptor 6. The cloning and characterization of a dendritic cell CC chemokine receptor suggests a role for chemokines in the control of the migration of dendritic cells and the regulation of dendritic cell function in immunity and infection.

Measles causes a profound immune suppression which is responsible for the high morbidity and mortality induced by secondary infections. Dendritic cells (DC) are professional antigen-presenting cells required for initiation of primary immune responses. To determine whether infection of DC by measles virus (MV) may play a role in virus-induced suppression of cell-mediated immunity, we examined the ability of CD1a⁺ DC derived from cord blood CD34⁺ progenitors and Langerhans cells isolated from human epidermis to support MV replication. Here we show that both cultured CD1a⁺ DC and epidermal Langerhans cells can be infected in vitro by both vaccine and wild type strains of MV. DC infection with MV resulted within 24–48 h in cell–cell fusion, cell surface expression of hemagglutinin, and virus budding associated with production of infectious virus. MV infection of DC completely abrogated the ability of the cells to stimulate the proliferation of naive allogeneic CD4⁺ T cell as early as day 2 of mixed leukocyte reaction (MLR) (i.e., on day 4 of DC infection). Mannose receptor–mediated endocytosis and viability studies indicated that the loss of DC stimulatory function could not be attributed to the death or apoptosis of DC. This total loss of DC stimulatory function required viral replication in the DC since ultraviolet (UV)-inactivated MV or UV-treated supernatant from MV-infected DC did not alter the allostimulatory capacity of DC. As few as 10 MV- infected DC could block the stimulatory function of 10⁴ uninfected DC. More importantly, MV-infected DC, in which production of infectious virus was blocked by UV treatment or paraformaldehyde fixation, actively suppressed allogeneic MLR upon transfer to uninfected DC–T-cultures. Thus, the mechanisms which contribute to the loss of the allostimulatory function of DC include both virus release and active suppression mediated by MV-infected DC, independent of virus production. These data suggest that carriage of MV by DC may facilitate virus spreading to secondary lymphoid organs and that MV replication in DC may play a central role in the general immune suppression observed during measles.

Within T cell–rich areas of secondary lymphoid organs, interdigitating dendritic cells recruit antigen-specific T cells that then induce B cells to secrete Igs. This study investigates the possible role(s) of dendritic cells in the regulation of human B cell responses. In the absence of exogenous cytokines, in vitro generated dendritic cells (referred to as Dendritic Langerhans cells, D-Lc) induced surface IgA expression on ∼10% of CD40-activated naive sIgD⁺ B cells. In the presence of IL-10 and TGF-β, a combination of cytokines previously identified for its capacity to induce IgA switch, D-Lc strongly potentiated the induction of sIgA on CD40-activated naive B cells from 5% to 40–50%. D-Lc alone did not induce the secretion of IgA by CD40-activated naive B cells, which required further addition of IL-10. Furthermore, D-Lc skewed towards the IgA isotype at the expense of IgG, the Ig production of CD40-activated naive B cells cultured in the presence of IL-10 and TGF-β. Importantly, under these culture conditions, both IgA₁ and IgA₂ were detected. In the presence of IL-10, secretion of IgA2 by CD40-activated naive B cells could be detected only in response to D-Lc and was further enhanced by TGF-β. Collectively, these results suggest that in addition to activating T cells in the extrafollicular areas of secondary lymphoid organs, human D-Lc also directly modulate T cell–dependent B cell growth and differentiation, by inducing the IgA isotype switch.

After antigen capture, dendritic cells (DC) migrate into T cell–rich areas of secondary lymphoid organs, where they induce T cell activation, that subsequently drives B cell activation. Here, we investigate whether DC, generated in vitro, can directly modulate B cell responses, using CD40L-transfected L cells as surrogate activated T cells. DC, through the production of soluble mediators, stimulated by 3- to 6-fold the proliferation and subsequent recovery of B cells. Furthermore, after CD40 ligation, DC enhanced by 30–300-fold the secretion of IgG and IgA by sIgD⁻ B cells (essentially memory B cells). In the presence of DC, naive sIgD⁺ B cells produced, in response to interleukin-2, large amounts of IgM. Thus, in addition to activating naive T cells in the extrafollicular areas of secondary lymphoid organs, DC may directly modulate B cell growth and differentiation.