Microbial lipopeptides stimulate dendritic cell maturation via Toll-like receptor 2

The ability of dendritic cells (DC) to initiate immune responses in naive T cells is dependent upon a maturation process that allows the cells to develop their potent Ag-presenting capacity. Although immature DC can be derived in vitro by treatment of peripheral blood monocytes with GM-CSF and IL-4, additional signals such as those provided by TNF-alpha, CD40 ligand, or LPS are required for complete maturation and maximum APC function. Because we recently found that microbial lipoproteins can activate monocytes and DC through Toll-like receptor (TLR) 2, we also investigated whether lipoproteins can drive DC maturation. Immature DC were cultured with or without lipoproteins and were monitored for expression of cell surface markers indicative of maturation. Stimulation with lipopeptides increased expression of CD83, MHC class II, CD80, CD86, CD54, and CD58, and decreased CD32 expression and endocytic activity; these lipopeptide-matured DC also displayed enhanced T cell stimulatory capacity in MLR, as measured by T cell proliferation and IFN-gamma secretion. The lipid moiety of the lipopeptide was found to be essential for induction of maturation. Preincubation of maturing DC with an anti-TLR2 blocking Ab before addition of lipopeptide blocked the phenotypic and functional changes associated with DC maturation. These results demonstrate that lipopeptides can stimulate DC maturation via TLR2, providing a mechanism by which products of bacteria can participate in the initiation of an immune response.     

CD1 molecules efficiently present antigen in immature dendritic cells and traffic independently of MHC class II during dendritic cell maturation

Upon exposure to Ag and inflammatory stimuli, dendritic cells (DCs) undergo a series of dynamic cellular events, referred to as DC maturation, that involve facilitated peptide Ag loading onto MHC class II molecules and their subsequent transport to the cell surface. Besides MHC molecules, human DCs prominently express molecules of the CD1 family (CD1a, -b, -c, and -d) and mediate CD1-dependent presentation of lipid and glycolipid Ags to T cells, but the impact of DC maturation upon CD1 trafficking and Ag presentation is unknown. Using monocyte-derived immature DCs and those stimulated with TNF-alpha for maturation, we observed that none of the CD1 isoforms underwent changes in intracellular trafficking that mimicked MHC class II molecules during DC maturation. In contrast to the striking increase in surface expression of MHC class II on mature DCs, the surface expression of CD1 molecules was either increased only slightly (for CD1b and CD1c) or decreased (for CD1a). In addition, unlike MHC class II, DC maturation-associated transport from lysosomes to the plasma membrane was not readily detected for CD1b despite the fact that both molecules were prominently expressed in the same MIIC lysosomal compartments before maturation. Consistent with this, DCs efficiently presented CD1b-restricted lipid Ags to specific T cells similarly in immature and mature DCs. Thus, DC maturation-independent pathways for lipid Ag presentation by CD1 may play a crucial role in host defense, even before DCs are able to induce maximum activation of peptide Ag-specific T cells.     

1 Alpha,25-dihydroxyvitamin D3 inhibits differentiation, maturation, activation, and survival of dendritic cells leading to impaired alloreactive T cell activation

1 Alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3), the active form of vitamin D3, is a potent immunomodulatory agent. Here we show that dendritic cells (DCs) are major targets of 1,25(OH)2D3-induced immunosuppressive activity. 1,25(OH)2D3 prevents the differentiation in immature DCs of human monocytes cultured with GM-CSF and IL-4. Addition of 1,25(OH)2D3 during LPS-induced maturation maintains the immature DC phenotype characterized by high mannose receptor and low CD83 expression and markedly inhibits up-regulation of the costimulatory molecules CD40, CD80, and CD86 and of class II MHC molecules. This is associated with a reduced capacity of DCs to activate alloreactive T cells, as determined by decreased proliferation and IFN-gamma secretion in mixed leukocyte cultures. 1, 25(OH)2D3 also affects maturing DCs, leading to inhibition of IL-12p75 and enhanced IL-10 secretion upon activation by CD40 ligation. In addition, 1,25(OH)2D3 promotes the spontaneous apoptosis of mature DCs. The modulation of phenotype and function of DCs matured in the presence of 1,25(OH)2D3 induces cocultured alloreactive CD4+ cells to secrete less IFN-gamma upon restimulation, up-regulate CD152, and down-regulate CD154 molecules. The inhibition of DC differentiation and maturation as well as modulation of their activation and survival leading to T cell hyporesponsiveness may explain the immunosuppressive activity of 1, 25(OH)2D3.     

Intracellular Salmonella inhibit antigen presentation by dendritic cells

Dendritic cells (DC) are important APCs linking innate and adaptive immunity. During analysis of the intracellular activities of Salmonella enterica in DC, we observed that viable bacteria suppress Ag-dependent T cell proliferation. This effect was dependent on the induction of inducible NO synthase by DC and on the function of virulence genes in Salmonella pathogenicity island 2 (SPI2). Intracellular activities of Salmonella did not affect the viability, Ag uptake, or maturation of DC, but resulted in reduced presentation of antigenic peptides by MHC class II molecules. Increased resistance to reinfection was observed after vaccination of mice with SPI2-deficient Salmonella compared with mice vaccinated with SPI2-proficient Salmonella, and this correlated with an increased amount of CD4(+) as well as CD8(+) T cells. Our study is the first example of interference of an intracellular bacterial pathogen with Ag presentation by DC. The subversion of DC functions is a novel strategy deployed by this pathogen to escape immune defense, colonize host organs, and persist in the infected host.     

The protease inhibitor cystatin C is differentially expressed among dendritic cell populations, but does not control antigen presentation

Dendritic cells (DC) undergo complex developmental changes during maturation. The MHC class II (MHC II) molecules of immature DC accumulate in intracellular compartments, but are expressed at high levels on the plasma membrane upon DC maturation. It has been proposed that the cysteine protease inhibitor cystatin C (CyC) plays a pivotal role in the control of this process by regulating the activity of cathepsin S, a protease involved in removal of the MHC II chaperone Ii, and hence in the formation of MHC II-peptide complexes. We show that CyC is differentially expressed by mouse DC populations. CD8(+) DC, but not CD4(+) or CD4(-)CD8(-) DC, synthesize CyC, which accumulates in MHC II(+)Lamp(+) compartments. However, Ii processing and MHC II peptide loading proceeded similarly in all three DC populations. We then analyzed MHC II localization and Ag presentation in CD8(+) DC, bone marrow-derived DC, and spleen-derived DC lines, from CyC-deficient mice. The absence of CyC did not affect the expression, the subcellular distribution, or the formation of peptide-loaded MHC II complexes in any of these DC types, nor the efficiency of presentation of exogenous Ags. Therefore, CyC is neither necessary nor sufficient to control MHC II expression and Ag presentation in DC. Our results also show that CyC expression can differ markedly between closely related cell types, suggesting the existence of hitherto unrecognized mechanisms of control of CyC expression.     

Human BDCA-1-positive blood dendritic cells differentiate into phenotypically distinct immature and mature populations in the absence of exogenous maturational stimuli: differentiation failure in HIV infection

Current immunological opinion holds that myeloid dendritic cell (mDC) precursors migrate from the blood to the tissues, where they differentiate into immature dermal- and Langerhans-type dendritic cells (DC). Tissue DC require appropriate signals from pathogens or inflammatory cytokines to mature and migrate to secondary lymphoid tissue. We show that purified blood mDC cultured in vitro with GM-CSF and IL-4, but in the absence of added exogenous maturation stimuli, rapidly differentiate into two maturational and phenotypically distinct populations. The major population resembles immature dermal DC, being positive for CD11b, CD1a, and DC-specific ICAM-3-grabbing nonintegrin. They express moderate levels of MHC class II and low levels of costimulatory molecules. The second population is CD11b(-/low) and lacks CD1a and DC-specific ICAM-3-grabbing nonintegrin but expresses high levels of MHC class II and costimulatory molecules. Expression of CCR7 on the CD11b(-/low) population and absence on the CD11b(+) cells further supports the view that these cells are mature and immature, respectively. Differentiation into mature and immature populations was not blocked by polymyxin B, an inhibitor of LPS. Neither population labeled for Langerin, E-cadherin, or CCR6 molecules expressed by Langerhans cells. Stimulation of 48-h cultured DC with LPS, CD40L, or poly(I:C) caused little increase in MHC or costimulatory molecule expression in the CD11b(-/low) DC but caused up-regulated expression in the CD11b(+) cells. In HIV-infected individuals, there was a marked decrease in the viability of cultured blood mDC, a failure to differentiate into the two populations described for normal donors, and an impaired ability to stimulate T cell proliferation.     

Dendritic cells process and present antigens across a range of maturation states

We isolated dendritic cells (DC) from lymphoid organs of mice bearing a transgene for a membrane-bound form of the model protein hen egg white lysozyme (HEL). DC from the spleen had a lower representation of costimulatory molecules and class II MHC molecules than those isolated from lymph nodes and thymi. Splenic DC were capable of further maturation by in vivo treatment of mice with LPS. The immature DC from spleen processed HEL and displayed the chemically dominant epitope as evidenced by FACS analysis. These immature DC also presented this epitope to CD4(+) T cells. Splenic DC from another transgenic mouse (ML-5) containing serum HEL also showed the ability to process and present Ag despite low levels of circulating HEL. In vitro-derived DC from the bone marrow (bone marrow-derived DC) of mHEL mice also displayed immature to mature features and in both cases displayed HEL peptides as well as SDS-stable MHC class II molecules. Immature bone marrow-derived DC also processed exogenous HEL. We conclude that the DC sets normally found in tissue show a scale of maturation features but even the most immature process and present peptides by MHC class II molecules.     

Vitamin D3 affects differentiation, maturation, and function of human monocyte-derived dendritic cells

We studied the effects of 1alpha,25-dihydroxyvitamin D3 (1alpha, 25-(OH)2D3) on differentiation, maturation, and functions of dendritic cells (DC) differentiated from human monocytes in vitro in the presence of GM-CSF and IL-4 for 7 days. Recovery and morphology were not affected by 1alpha,25-(OH)2D3 up to 100 nM. DC differentiated in the presence of 10 nM 1alpha,25-(OH)2D3 (D3-DC) showed a marked decrease in the expression of CD1a, while CD14 remained elevated. Mannose receptor and CD32 were significantly increased, and this correlated with an enhancement of endocytic activity. Costimulatory molecules such as CD40 and CD86 were slightly decreased or nonsignificantly affected (CD80 and MHC II). However, after induction of DC maturation with LPS or incubation with CD40 ligand-transfected cells, D3-DC showed marginal increases in MHC I, MHC II, CD80, CD86, CD40, and CD83. The accessory cell function of D3-DC in classical MLR was also inhibited. Moreover, allogeneic T cells stimulated with D3-DC were poor responders in a second MLR to untreated DC from the same or an unrelated donor, thus indicating the onset of a nonspecific hyporesponsivity. In conclusion, our data suggest that 1alpha,25-(OH)2D3 may modulate the immune system, acting at the very first step of the immune response through the inhibition of DC differentiation and maturation into potent APC.     

Differential susceptibility to CD95 (Apo-1/Fas) and MHC class II-induced apoptosis during murine dendritic cell development

Disappearance of antigen presenting cells (APC) from the lymph node occurs following antigen specific interactions with T cells. We have investigated the regulation of CD95 (Apo-1/Fas) induced apoptosis during murine dendritic cell (DC) development. Consistent with the moderate levels of CD95 surface expression and low, or absent, MHC class II expression, immature DC in bone marrow cultures were highly sensitive to CD95 induced apoptosis, but insensitive to class II mediated apoptosis. In contrast, mature splenic, epidermal and bone marrow derived DC were fully resistant to CD95 induced cell death, but sensitive to class II induced apoptosis. Although caspase 3 and 8 activation was detected in immature DC undergoing CD95L-induced apoptosis, the pan-caspase inhibitor zVAD-fmk did not inhibit the early events of CD95-induced mitochondrial depolarisation or phosphatidyl serine exposure and only partially inhibited the killing of immature DC. In contrast, zVAD-fmk was completely effective in preventing CD95L mediated death of murine thymocytes. Collectively, these data do not support a major role of CD95: CD95L ligation in apoptosis of mature DC, but rather emphasise the existence of distinct pathways for the elimination of DC at different stages of maturation.     

Maturation and activation of dendritic cells induced by lymphocyte activation gene-3 (CD223)

Lymphocyte activation gene-3 (LAG-3) is an MHC class II ligand expressed on activated T and NK cells. A LAG-3Ig fusion protein has been used in mice as an adjuvant protein to induce antitumor responses and specific CD8 and CD4 Th1 responses to nominal Ags. In this work we report on the effect of LAG-3Ig on the maturation and activation of human monocyte-derived dendritic cells (DC). LAG-3Ig binds MHC class II molecules expressed in plasma membrane lipid rafts on immature human DC and induces rapid morphological changes, including the formation of dendritic projections. LAG-3Ig markedly up-regulates the expression of costimulatory molecules and the production of IL-12 and TNF-alpha. Consistent with this effect on DC maturation, LAG-3Ig disables DC in their capacity to capture soluble Ags. These events are associated with the acquisition of professional APC function, because LAG-3Ig increases the capacity of DC to stimulate the proliferation and IFN-gamma response by allogeneic T cells. These effects were not observed when using ligation of MHC class II by specific mAb. Class II-mediated signals induced by a natural ligand, LAG-3, lead to complete maturation of DC, which acquire the capacity to trigger naive T cells and drive polarized Th1 responses.     

Soluble fibrinogen-like protein 2/fibroleukin exhibits immunosuppressive properties: suppressing T cell proliferation and inhibiting maturation of bone marrow-derived dendritic cells

Fibrinogen-like protein 2 (fgl2)/fibroleukin is a member of the fibrinogen-related protein superfamily. In addition to its established role in triggering thrombosis, it is known to be secreted by T cells. The soluble fgl2 ((s)fgl2) protein generated in a baculovirus expression system bound to both T cells and bone marrow-derived dendritic cells (DC) in a specific manner. (s)fgl2 exhibited immunomodulatory properties capable of inhibiting T cell proliferation stimulated by alloantigens, anti-CD3/anti-CD28 mAbs, and Con A in a dose-dependent manner; however, it had no inhibitory effects on CTL activity. The time- and dose-dependent inhibitory effect of (s)fgl2 on alloreactive T cell proliferation could be neutralized by a mAb against mouse fgl2. Polarization toward a Th2 cytokine profile with decreased production of IL-2 and IFN-gamma and increased production of IL-4 and IL-10 was observed in (s)fgl2-treated allogeneic cultures. Exposure of immature DC to (s)fgl2 abrogated the expression of CD80(high) and MHC class II(high) molecules and markedly inhibited NF-kappaB nuclear translocation, thus inhibiting their maturation. (s)Fgl2-treated DC had an impaired ability to stimulate allogeneic T cell proliferation. Maximal inhibition of proliferation was observed when allogeneic T cells were cultured with (s)fgl2-treated DC and (s)fgl2 protein was added in the culture. These data provide the first evidence to demonstrate that (s)fgl2 exerts immunosuppressive effects on T cell proliferation and DC maturation.     

Increased extracellular pressure provides a novel adjuvant stimulus for enhancement of conventional dendritic cell maturation strategies

Dendritic cell (DC)-based vaccine strategies have gained increasing popularity in recent years. Methods for ex vivo generation of immunocompetent mature DCs still require optimization. DCs have been shown to phenotypically mature under elevated pressure. We compared the effects of pressure on DC maturation with LPS- and cytokine-stimulation. Human monocyte-derived immature or LPS- and cytokine-matured DCs were exposed to ambient or 40 mmHg increased pressure for 12 h, then assessed for expression of CD80, CD86, CD40, MHC-I/II, and inflammatory cytokine production. DCs were also evaluated for capacity to stimulate T-cell proliferation by co-culture with allogeneic lymphocytes. Pressure significantly increased cytokine production and expression of all surface molecules on immature DC other than MHC-I and CD40. Pressure/LPS-treated DCs displayed further upregulation of MHC-I, CD40, and IL-12p70. Cytokine-matured DCs appeared less responsive to pressure. T-cell proliferation correlated with MHC expression. Results suggest mechanical stimulation of DCs may provide a useful adjuvant to TLR-agonist maturation strategies.     

Differential post-translational modification of CD63 molecules during maturation of human dendritic cells.

The capacity of dendritic cells to initiate T cell responses is related to their ability to redistribute MHC class II molecules from the intracellular MHC class II compartments to the cell surface. This redistribution occurs during dendritic cell development as they are converted from an antigen capturing, immature dendritic cell into an MHC class II-peptide presenting mature dendritic cell. During this maturation, antigen uptake and processing are down-regulated and peptide-loaded class II complexes become expressed in a stable manner on the cell surface. Here we report that the tetraspanin CD63, that associates with intracellularly localized MHC class II molecules in immature dendritic cells, was modified post-translationally by poly N-acetyl lactosamine addition during maturation. This modification of CD63 was accompanied by a change in morphology of MHC class II compartments from typical multivesicular organelles to structures containing densely packed lipid moieties. Post-translational modification of CD63 may be involved in the functional and morphological changes of MHC class II compartments that occur during dendritic cell maturation.     

Endogenous cortisol and TGF-beta in human aqueous humor contribute to ocular immune privilege by regulating dendritic cell function

Aqueous humor (AqH) has been shown to have significant immunosuppressive effects on APCs in animal models. We wanted to establish whether, in humans, AqH can regulate dendritic cell (DC) function and to identify the dominant mechanism involved. Human AqH inhibited the capacity of human peripheral blood monocyte-derived DC to induce naive CD4(+) T cell proliferation and cytokine production in vitro, associated with a reduction in DC expression of the costimulatory molecule CD86. This was seen both for DC cultured under noninflammatory conditions (immature DC) and for DC stimulated by proinflammatory cytokines (mature DC). DC expression of MHC classes I/II and CD83 was reduced (mature DC only). Myeloid DC from peripheral blood were similarly sensitive to the effects of human AqH, but only under inflammatory conditions. The addition of α-melanocyte stimulating hormone and vasoactive intestinal peptide did not cause significant inhibition at physiological levels. However, the addition of exogenous cortisol at physiological levels recapitulated the AqH-induced reduction in CD86 and inhibition of DC-induced T cell proliferation, and blockade of cortisol in AqH partially reversed its suppressive effects. TGF-β2 had an additional effect with cortisol, and although simultaneous blockade of cortisol and TGF-β2 in AqH reduced its effectiveness, there was still a cortisol- and TGF-β-independent component. In humans, AqH regulates DC maturation and function by the combined actions of cortisol and TGF-β2, a pathway that is likely to contribute to the maintenance of immune privilege in the eye.     

Cdc42-dependent actin dynamics controls maturation and secretory activity of dendritic cells

Cell division cycle 42 (Cdc42) is a member of the Rho guanosine triphosphatase family and has pivotal functions in actin organization, cell migration, and proliferation. To further study the molecular mechanisms of dendritic cell (DC) regulation by Cdc42, we used Cdc42-deficient DCs. Cdc42 deficiency renders DCs phenotypically mature as they up-regulate the co-stimulatory molecule CD86 from intracellular storages to the cell surface. Cdc42 knockout DCs also accumulate high amounts of invariant chain–major histocompatibility complex (MHC) class II complexes at the cell surface, which cannot efficiently present peptide antigens (Ag’s) for priming of Ag-specific CD4 T cells. Proteome analyses showed a significant reduction in lysosomal MHC class II–processing proteins, such as cathepsins, which are lost from DCs by enhanced secretion. As these effects on DCs can be mimicked by chemical actin disruption, our results propose that Cdc42 control of actin dynamics keeps DCs in an immature state, and cessation of Cdc42 activity during DC maturation facilitates secretion as well as rapid up-regulation of intracellular molecules to the cell surface.     

Brain dendritic cells and macrophages/microglia in central nervous system inflammation

Microglia subpopulations were studied in mouse experimental autoimmune encephalomyelitis and toxoplasmic encephalitis. CNS inflammation was associated with the proliferation of CD11b(+) brain cells that exhibited the dendritic cell (DC) marker CD11c. These cells constituted up to 30% of the total CD11b(+) brain cell population. In both diseases CD11c(+) brain cells displayed the surface phenotype of myeloid DC and resided at perivascular and intraparenchymatic inflammatory sites. By lacking prominent phagocytic organelles, CD11c(+) cells from inflamed brain proved distinct from other microglia, but strikingly resembled bone marrow-derived DC and thus were identified as DC. This brain DC population comprised cells strongly secreting IL-12p70, whereas coisolated CD11c(-) microglia/brain macrophages predominantly produced TNF-alpha, GM-CSF, and NO. In comparison, the DC were more potent stimulators of naive or allogeneic T cell proliferation. Both DC and CD11c(-) microglia/macrophages from inflamed brain primed naive T cells from DO11.10 TCR transgenic mice for production of Th1 cytokines IFN-gamma and IL-2. Resting microglia that had been purified from normal adult brain generated immature DC upon exposure to GM-CSF, while CD40 ligation triggered terminal maturation. Consistently, a functional maturation of brain DC was observed to occur following the onset of encephalitis. In conclusion, these findings indicate that in addition to inflammatory macrophage-like brain cells, intraparenchymatical DC exist in autoimmune and infectious encephalitis. These DC functionally mature upon disease onset and can differentiate from resident microglia. Their emergence, maturation, and prolonged activity within the brain might contribute to the chronicity of intracerebral Th1 responses.     

The cellular pathway of CD1e in immature and maturing dendritic cells

Dendritic cells (DCs) present antigens to T cells via CD1, HLA class I or class II molecules. During maturation, HLA class II-restricted presentation is optimized. The relocalization of CD1e from Golgi to endosomal compartments during DC maturation suggests also an optimization of the antigen-presentation pathway via CD1 molecules. We here detail the biosynthesis and cellular pathway of CD1e in immature and maturing DCs. Unlike the other CD1 molecules, CD1e was found to reach late endosomes through sorting endosomes, without passing through the plasma membrane in either immature or maturing cells. After induction of DC maturation, CD1e disappeared rapidly from the Golgi and was transiently localized in HLA-DR+ vesicles, while the number of CD1e+/CD1b+ compartments increased for at least 20 h. High-resolution light microscopy showed that, in immature DCs, CD1e+ vesicles were often in close apposition to EEA1+ or HLA-DR+ compartments, while CD1e displayed a nearly exclusive distribution in the lysosomes of mature DCs, a finding corroborated by immunoelectron microscopy. During maturation, CD1e synthesis progressively declined, while the endosomal cleavage of CD1e still occurred. Thus, CD1e displays peculiar properties, suggesting an unexpected role among the family of CD1 antigen-presenting molecules.     

MHC class II-peptide complexes in dendritic cell lipid microdomains initiate the CD4 Th1 phenotype

We investigated differentiation of CD4 T cells responding to Ag presented by bone marrow-derived dendritic cells (DC) in association with MHC class II (MHC II) molecules. Peptides encapsulated in liposomes opsonized by IgG were taken up by endocytosis. MHC II-peptide-specific T cells responding to this Ag were polarized to a Th1 cytokine profile in a CD40-, CD28-, MyD88-, and IL-12-dependent manner. Th2 responses were obtained from the same transgenic T cell population exposed to the same DC on which MHC-peptide complexes had dispersed for 48 h following uptake of FcR-targeted liposomes. DC that took up the same FcR-targeted liposomes and then were exposed to methyl-beta-cyclodextrin, which chelates cholesterol and dissociates lipid microdomains, also stimulated Th2 differentiation. Incubation of T cells with DC incubated with peptides directly binding to MHC II resulted in Th2 responses, whether or not the DC were coincubated with opsonized liposomes as a maturation stimulus. CD4 Th1 polarization thus appears to depend on MHC II-peptide complex clustering in DC lipid microdomains and the time between peptide loading and T cell encounter.     

Histamine induces CD86 expression and chemokine production by human immature dendritic cells

Mast cells and immature dendritic cells (DC) are in close contact in peripheral tissues. Upon activation, mast cells release histamine, a mediator involved in the immediate hypersensitivity reaction. We therefore tested whether histamine could affect human DC activation and maturation. Histamine induces CD86 expression on immature DC in a dose-dependent (significant at 10(-7) M) and transient manner (maximal after 24-h stimulation). Histamine also transiently up-regulates the expression of the costimulatory and accessory molecules, CD40, CD49d, CD54, CD80, and MHC class II. As a consequence, immature DC exposed for 24 h to histamine stimulate memory T cells more efficiently than untreated DC. In addition, histamine induces a potent production of IL-6, IL-8, monocyte chemoattractant protein 1, and macrophage-inflammatory protein 1alpha by immature DC and also up-regulates IL-1beta, RANTES, and macrophage-inflammatory protein 1beta but not TNF-alpha and IL-12 mRNA expression. Histamine activates immature DC through both the H1 and H2 receptors. However, histamine-treated DC do not have a phenotype of fully mature cells, as they do neither show significant changes in the expression of the chemokine receptors, CCR5, CCR7 and CXC chemokine receptor 4, nor expression of CD83 de novo. These data demonstrate that histamine activates immature DC and induces chemokine production, thereby suggesting that histamine, via stimulation of resident DC, may participate locally in T cell stimulation and in the late inflammatory reaction associated with allergic disorders.