Comparative analysis of cytotoxic T lymphocyte response induced by dendritic cells loaded with hepatocellular carcinoma -derived RNA or cell lysate

The choice of the tumor antigen preparation used for dendritic cell (DC) loading is important for optimizing DC vaccines. In the present study, we compared DCs pulsed with hepatocellular carcinoma (HCC) total RNA or cell lysates for their capacity to activate T cells. We showed here that HCC total RNA pulsed-DCs induced effector T lymphocyte responses which showed higher killing ability to HCC cell lines, as well as higher frequency of IFN-γ producing of CD4+ and CD8+ T cells when compared with lysate pulsed-DCs. Both of RNA and lysate loading did not influence the changes of mature DC phenotype and the capacity of inducing T cell proliferation. However, HCC lysate loading significantly inhibited the production of inflammatory cytokines IL-12p70, IFN-γ and enhanced the secretion of anti-inflammatory cytokines IL-10 of mature DCs. Our results indicated that DCs loaded with HCC RNA are superior to that loaded with lysate in priming anti-HCC CTL response, suggesting that total RNA may be a better choice for DCs-based HCC immunotherapy.     

Mature dendritic cells derived from human monocytes within 48 hours: a novel strategy for dendritic cell differentiation from blood precursors

It is widely believed that generation of mature dendritic cells (DCs) with full T cell stimulatory capacity from human monocytes in vitro requires 5-7 days of differentiation with GM-CSF and IL-4, followed by 2-3 days of activation. Here, we report a new strategy for differentiation and maturation of monocyte-derived DCs within only 48 h of in vitro culture. Monocytes acquire immature DC characteristics by day 2 of culture with GM-CSF and IL-4; they down-regulate CD14, increase dextran uptake, and respond to the inflammatory chemokine macrophage inflammatory protein-1alpha. To accelerate DC development and maturation, monocytes were incubated for 24 h with GM-CSF and IL-4, followed by activation with proinflammatory mediators for another 24 h (FastDC). FastDC expressed mature DC surface markers as well as chemokine receptor 7 and secreted IL-12 (p70) upon CD40 ligation in the presence of IFN-gamma. The increase in intracellular calcium in response to 6Ckine showed that chemokine receptor 7 expression was functional. When FastDC were compared with mature monocyte-derived DCs generated by a standard 7-day protocol, they were equally potent in inducing Ag-specific T cell proliferation and IFN-gamma production as well as in priming autologous naive T cells using tetanus toxoid as a model Ag. These findings indicate that FastDC are as effective as monocyte-derived DCs in stimulating primary, Ag-specific, Th 1-type immune responses. Generation of FastDC not only reduces labor, cost, and time required for in vitro DC development, but may also represent a model more closely resembling DC differentiation from monocytes in vivo.     

Downregulation of endogenous STAT3 augments tumoricidal activity of interleukin 15 activated dendritic cell against lymphoma and leukemia via TRAIL

Effector functions in tumor resistance by dendritic cells (DCs) are less well characterized. In this study, we describe that the murine DCs upon stimulation with recombinant IL-15 in vitro or in vivo, expresses TNF superfamily member TRAIL which mediates cytotoxicity and growth inhibition against a murine lymphoma called Dalton lymphoma (DL) via apoptosis. Presence of tumor lysate or intact tumor cells significantly reduces the DC mediated tumoricidal effect, possibly via masking and down-regulating TRAIL in DCs. The antitumor effect of DC derived TRAIL was further augmented by deactivation of STAT3 in tumor cells by cucurbitacin I, which makes it more susceptible to DC derived TRAIL Treatment of tumor cells with cucurbitacin I upregulates TRAIL receptor expression in addition to activation of caspases. Compared to naïve DCs, DCs from tumor bearing mice are significantly impaired in TRAIL expression and consequent antitumor functions against DL which was partially restored by activation with IL-15 or LPS. Priming with recombinant IL-15 prolongs the survival of tumor bearing mice treated with cucurbitacin I. Naïve peripheral blood DCs derived from chronic myeloid leukemia (CML) patients have significant impairment in expression of TRAIL and consequent tumoricidal properties against TRAIL sensitive lymphoma cell lines and primary tumor cells compared to normal control.     

IL-23 is increased in dendritic cells in multiple sclerosis and down-regulation of IL-23 by antisense oligos increases dendritic cell IL-10 production

IL-23 is a heterodimeric cytokine comprising a p19 subunit associated with the IL-12/23p40 subunit. Like IL-12, IL-23 is expressed predominantly by activated dendritic cells (DCs) and phagocytic cells, and both cytokines induce IFN-gamma secretion by T cells. The induction of experimental autoimmune encephalitis, the animal model of multiple sclerosis (MS), occurs in mice lacking IL-12, but not in mice with targeted disruption of IL-23 or both IL-12 and IL-23. Thus, IL-23 expression in DCs may play an important role in the pathogenesis of human autoimmune diseases such as MS. We quantified the expression of IL-23 in monocyte-derived DCs in MS patients and healthy donors and found that DCs from MS patients secrete elevated amounts of IL-23 and express increased levels of IL-23p19 mRNA. Consistent with this abnormality, we found increased IL-17 production by T cells from MS patients. We then transfected monocyte-derived DCs from healthy donors with antisense oligonucleotides specific for the IL-23p19 and IL-12p35 genes and found potent suppression of gene expression and blockade of bioactive IL-23 and IL-12 production without affecting cellular viability or DCs maturation. Inhibition of IL-23 and IL-12 was associated with increased IL-10 and decreased TNF-alpha production. Furthermore, transfected DCs were poor allostimulators in the MLR. Our results demonstrate that an abnormal Th1 bias in DCs from MS patients related to IL-23 exists, and that antisense oligonucleotides specific to IL-23 can be used for immune modulation by targeting DC gene expression.     

Nrf2-dependent repression of interleukin-12 expression in human dendritic cells exposed to inorganic arsenic

Inorganic arsenic, a well-known Nrf2 inducer, exerts immunosuppressive properties. In this context, we recently reported that the differentiation of human blood monocytes into immature dendritic cells (DCs), in the presence of low and noncytotoxic concentrations of arsenic, represses the ability of DCs to release key cytokines in response to different stimulating agents. Particularly, arsenic inhibits the expression of human interleukin-12 (IL-12, also named IL-12p70), a major proinflammatory cytokine that controls the differentiation of Th1 lymphocytes. In the present study, we determined if Nrf2 could contribute to these arsenic immunotoxic effects. To this goal, human monocyte-derived DCs were first differentiated in the absence of metalloid and then pretreated with arsenic just before DC stimulation with lipopolysaccharide (LPS). Under these experimental conditions, arsenic rapidly and stably activates Nrf2 and increases the expression of Nrf2 target genes. It also significantly inhibits IL-12 expression in activated DCs, at both mRNA and protein levels. Particularly, arsenic reduces mRNA levels of IL12A and IL12B genes which encodes the p35 and p40 subunits of IL-12p70, respectively. tert-Butylhydroquinone (tBHQ), a reference Nrf2 inducer, mimics arsenic effects and potently inhibits IL-12 expression. Genetic inhibition of Nrf2 expression markedly prevents the repression of both IL12 mRNA and IL-12 protein levels triggered by arsenic and tBHQ in human LPS-stimulated DCs. In addition, arsenic significantly reduces IL-12 mRNA levels in LPS-activated bone marrow-derived DCs from Nrf2^+/+ mice but not in DCs from Nrf2^−/− mice. Finally, we show that, besides IL-12, arsenic significantly reduces the expression of IL-23, another heterodimer containing the p40 subunit. In conclusion, our study demonstrated that arsenic represses IL-12 expression in human-activated DCs by specifically stimulating Nrf2 activity.     

Infection of dendritic cells by a gamma2-herpesvirus induces functional modulation

The murine gamma-herpesvirus-68 (gammaHV68) establishes viral latency in dendritic cells (DCs). In the present study, we examined the specific consequences of DC infection by gammaHV68, both in vivo and in vitro. Ex vivo analysis of infected mice showed that the virus colonizes respiratory DCs very early after infection and that all subsets of splenic DCs analyzed are viral targets. We have developed and characterized an in vitro model of gammaHV68 infection of DCs. Using this model, we demonstrated that viral infection neither induces full DC maturation nor interferes with exogenous activation, which is assessed by cell surface phenotypic changes. However, whereas gammaHV68 infection alone failed to elicit cytokine secretion, IL-10 secretion of exogenously activated DCs was enhanced. Furthermore, gammaHV68-infected DCs efficiently stimulated virus-specific T cell hybridomas but failed to induce alloreactive stimulation of normal T cells. These data indicate that viral infection doesn't interfere with Ag processing and presentation but does interfere with the ability of DCs to activate T cells. The inhibition of T cell activation was partially reversed by blocking IL-10. Analysis of infected mice shows elevated levels of IL-10 expression in DCs and that lack of endogenous IL-10 is associated with decreased gammaHV68 long-term latency. Taken together, these observations indicate that gamma2-herpesvirus infection of DCs is a mechanism of viral immune evasion, partially mediated by IL-10.     

Differential role for p38 mitogen-activated protein kinase in regulating CD40-induced gene expression in dendritic cells and B cells

We investigated whether human monocyte-derived dendritic cells (DCs) differed from tonsillar B cells in the set of cell fate genes they express constitutively and in the way these genes are affected after CD40 ligation. In particular, Bcl-2, TNF receptor-associated factor-2 (TRAF2), and TRAF4 were clearly inducible via CD40 in B cells but not in DCs. DCs, unlike B cells, were induced to increase expression of IL-1beta, IL-1Ra, IL-8, IL-12 p40, RANTES, macrophage inflammatory protein-1alpha, and monocyte chemoattractant protein-1 after CD40 ligation. We next tested whether CD40-induced signaling pathways were different in DCs vs B cells. In DCs, as in B cells, CD40 ligation activated p38 mitogen-activated protein kinase (MAPK), its downstream target, MAPKAPK-2, and the c-Jun N-terminal kinase. The p38 MAPK-specific inhibitor, SB203580, blocked CD40-induced MAPKAPK-2 activation, but did not affect activation of c-Jun N-terminal kinase. Furthermore, unlike in B cells, extracellular signal-regulated kinase-1 and -2 were activated after CD40 ligation in DCs. SB203580 strongly blocked CD40-induced IL-12 p40 production in DCs at both mRNA and protein levels, while having minimal effect on CD40-induced expression of the chemokine RANTES. In contrast, no detectable IL-12 p40 protein was secreted in CD40-stimulated B cells. Furthermore, CD40-induced mRNA expression of cellular inhibitor of apoptosis protein-2 was also dependent on the p38 MAPK pathway in DCs and differed compared with that in B cells. In conclusion, CD40 induces distinct programs in DCs and B cells, and the set of p38 MAPK-dependent genes in DCs (IL-12 p40 and cellular inhibitor of apoptosis protein-2) is different from that in B cells (IL-10 and IL-1beta).     

Mature human Langerhans cells derived from CD34+ hematopoietic progenitors stimulate greater cytolytic T lymphocyte activity in the absence of bioactive IL-12p70, by either single peptide presentation or cross-priming, than do dermal-interstitial or monocyte-derived dendritic cells

The emerging heterogeneity of dendritic cells (DCs) mirrors their increasingly recognized division of labor at myriad control points in innate and acquired cellular immunity. We separately generated blood monocyte-derived DCs (moDCs), as well as Langerhans cells (LCs) and dermal-interstitial DCs (DDC-IDCs) from CD34(+) hematopoietic progenitor cells. Differential expression of CD11b, CD52, CD91, and the CD1 isoforms proved useful in distinguishing these three DC types. All mature DCs uniformly expressed comparable levels of HLA-DR, CD83, CD80, and CD86, and were potent stimulators of allogeneic T cells after exposure either to recombinant human CD40L trimer or a combination of inflammatory cytokines with PGE(2). moDCs, however, required 0.5-1 log greater numbers than LCs or DDC-IDCs to stimulate comparable T cell proliferation. Only moDCs secreted the bioactive heterodimer IL-12p70, and moDCs phagocytosed significantly more dying tumor cells than did either LCs or DDC-IDCs. LCs nevertheless proved superior to moDCs and DDC-IDCs in stimulating CTL against a recall viral Ag by presenting passively loaded peptide or against tumor Ag by cross-priming autologous CD8(+) T cells. LCs also secreted significantly more IL-15 than did either moDCs or DDC-IDCs, which is especially important to the generation of CTL. These findings merit further comparisons in clinical trials designed to determine the physiologic relevance of these distinctions in activity between LCs and other DCs.     

Dendritic cells trigger tumor cell death by a nitric oxide-dependent mechanism

Dendritic cells (DCs) are well known for their capacity to induce adaptive antitumor immune response through Ag presentation and tumor-specific T cell activation. Recent findings reveal that besides this role, DCs may display additional antitumor effects. In this study, we provide evidence that LPS- or IFN-gamma-activated rat bone marrow-derived dendritic cells (BMDCs) display killing properties against tumor cells. These cytotoxic BMDCs exhibit a mature DC phenotype, produce high amounts of IL-12, IL-6, and TNF-alpha, and retain their phagocytic properties. BMDC-mediated tumor cell killing requires cell-cell contact and depends on NO production, but not on perforin/granzyme or on death receptors. Furthermore, dead tumor cells do not exhibit characteristics of apoptosis. Thus, intratumoral LPS injections induce an increase of inducible NO synthase expression in tumor-infiltrating DCs associated with a significant arrest of tumor growth. Altogether, these results suggest that LPS-activated BMDCs represent powerful tumoricidal cells which enforce their potential as anticancer cellular vaccines.     

B lymphocyte stimulator regulates adaptive immune responses by directly promoting dendritic cell maturation

B lymphocyte stimulator (BLyS) is a well-known direct costimulator of adaptive immune cells, particularly B lineage cells. However, we have reported recently that BLyS is also able to activate monocytes. Other innate immune cells, such as dendritic cells (DCs), play a key role in the initiation of adaptive immune responses and the purpose of the current study was to assess whether there is a direct role for BLyS in modulating human DC functions. In this study, we show that BLyS induces DC activation and maturation. Thus, BLyS strongly induced up-regulation of surface costimulatory molecule expression and secretion of specific cytokines and chemokines in DCs. BLyS-stimulated DCs (BLyS-DCs) were also able to augment allogeneic CD4 T cell proliferation to a greater extent than control DCs. BLyS-DCs secreted elevated levels of the major Th1-polarizing cytokine, IL-12p70, and they promoted naive CD4 T cell differentiation into Th1 T cells. Regarding BLyS receptor expression, DCs primarily express cytoplasmic transmembrane activator and CAML interactor; however, low levels of cell surface transmembrane activator and CAML interactor are expressed as well. Collectively, our data suggest that BLyS may modulate adaptive immune cells indirectly by inducing DC maturation.     

Human tumor antigen MUC1 is chemotactic for immature dendritic cells and elicits maturation but does not promote Th1 type immunity

The immunostimulatory outcome of the interactions of many pathogens with dendritic cells (DCs) has been well characterized. There are many fewer examples of similar interactions between DCs and self-molecules, especially the abnormal self-proteins such as many tumor Ags, and their effects on DC function and the immune response. We show that human epithelial cell Ag MUC1 mucin is recognized in its aberrantly glycosylated form on tumor cells by immature human myeloid DCs as both a chemoattractant (through its polypeptide core) and a maturation and activation signal (through its carbohydrate moieties). On encounter with MUC1, similar to the encounter with LPS, immature DCs increase cell surface expression of CD80, CD86, CD40, and CD83 molecules and the production of IL-6 and TNF-alpha cytokines but fail to make IL-12. When these DCs are cocultured with allogeneic CD4+ T cells, they induce production of IL-13 and IL-5 and lower levels of IL-2, thus failing to induce a type 1 response. Our data suggest that, in vivo in cancer patients, MUC1 attracts immature DCs to the tumor through chemotaxis and subverts their function by negatively affecting their ability to stimulate type 1 helper T cell responses important for tumor rejection.     

Cutting edge: Expression of functional CD137 receptor by dendritic cells

Interaction between dendritic cells (DCs) and T cells is a prerequisite for the initiation of a T cell response. The molecular nature of this interaction remains to be fully characterized. We report in this work that freshly isolated mouse splenic DCs and bone marrow-derived DCs express CD137 on the cell surface and in soluble form. Triggering CD137 increased the secretion of IL-6 and IL-12 from DCs. More importantly, infusion of an agonistic mAb to CD137 into naive mice enhanced the ability of DCs to stimulate T cell proliferation in response to both alloantigens and a nominal Ag in vitro. This enhancement of DC function is not mediated through activation of T cells, because the effect was also observed in RAG-1 knockout mice that lack T cells. Our findings implicate CD137 as an important receptor involved in the modulation of DC function.     

Butyrate inhibits functional differentiation of human monocyte-derived dendritic cells

Dendritic cells (DCs) play a key role in immune function through antigen presentation by MHC and CD1, as well as cytokine production that shapes the immune response. Here we report that butyrate, a histone deacetylase inhibitor, inhibits the functional differentiation of human monocyte-derived DCs. Mature DCs were generated from monocytes in the presence of granulocyte macrophage colony-stimulating factor (GM-CSF) and interleukin-4 (IL-4), followed by 2 day LPS stimulation. Butyrate treatment throughout the culture period inhibited the expression of CD1 molecules, but not on CD83, CD86, and MHC molecules. The suppression was exerted at protein and mRNA levels. Butyrate-treated immature DCs also showed decreased expression of CD1 molecules. Moreover the butyrate-treated immature DCs showed lower production of IL-12 p40 and IL-6 in response to lipopolysaccharides and induced less Th1 cells in allogenic mixed lymphocyte reactions. Our results imply that histone acetylation is involved in regulating immune responses through regulating functional differentiation of DC. Thus HDAC may be one of the targets for controlling the immune response.     

Cutting edge: a novel role for Fas ligand in facilitating antigen acquisition by dendritic cells

Fas ligand (FasL)-expressing tumor cells are found to effectively mediate rejection of the coinoculated FasL negative parental cells while having no effect on the growth of histologically distinct tumor cells. These observations indicate that FasL induces a specific immune response against Ag derived from FasL-bearing tumors and suggest a possible role for FasL in tumor Ag presentation. Indeed, tumor cells expressing FasL can efficiently interact with dendritic cells (DCs) and this interaction requires the expression of membrane-bound FasL on tumors and Fas on DCs. Moreover, DCs cocultured with FasL-expressing tumors are able to elicit a tumor-specific immune response in vivo, suggesting that DCs acquire tumor Ag during the Fas/FasL-mediated DC-tumor contact. These results identify a novel role for FasL in augmenting tumor-DC interactions and subsequent tumor Ag acquisition by DCs, and suggest that FasL-expressing tumor cells could be used to generate tumor-specific DC vaccines.     

Phosphatidylserine regulates the maturation of human dendritic cells

Phosphatidylserine (PS), which is exposed on the surface of apoptotic cells, has been implicated in immune regulation. However, the effects of PS on the maturation and function of dendritic cells (DCs), which play a central role in both immune activation and regulation, have not been described. Large unilamellar liposomes containing PS or phosphatidylcholine were used to model the plasma membrane phospholipid composition of apoptotic and live cells, respectively. PS liposomes inhibited the up-regulation of HLA-ABC, HLA-DR, CD80, CD86, CD40, and CD83, as well as the production of IL-12p70 by human DCs in response to LPS. PS did not affect DC viability directly but predisposed DCs to apoptosis in response to LPS. DCs exposed to PS had diminished capacity to stimulate allogeneic T cell proliferation and to activate IFN-gamma-producing CD4(+) T cells. Exogenous IL-12 restored IFN-gamma production by CD4(+) T cells. Furthermore, activated CTLs proliferated poorly to cognate Ag presented by DCs exposed to PS. Our findings suggest that PS exposure provides a sufficient signal to inhibit DC maturation and to modulate adaptive immune responses.     

Regulated expression of FcgammaR in human dendritic cells controls cross-presentation of antigen-antibody complexes

Receptors for IgG (FcgammaR) expressed in dendritic cells (DCs) influence the initiation of Ab-mediated immunity. Dynamic variations in FcgammaR expression allow DCs to adjust their capacity to capture Ab-opsonized Ag. The current paradigm predicts a progressive decline in FcgammaR-mediated phagocytic function upon DC maturation. Surprisingly, we find that expression of the phagocytic receptor FcgammaRIIa is preserved in immature and mature DCs at comparable levels with macrophages. Moreover, phagocytosis of antigenic peptides directed to FcgammaRIIa on DCs leads to dramatic increases in Ag cross-presentation and T cell activation. In immature DCs, high expression of inhibitory FcgammaRIIb correlates with decreased uptake and cross-presentation of Ab-Ag complexes. In contrast, engagement of FcgammaRIIb is not associated with changes in cross-presentation in mature DCs. We provide evidence that FcgammaRIIb expression is patently reduced in mature DCs, an effect that is modulated by treatment with cytokines. The regulated expression of activating and inhibitory FcgammaRs in DCs emerges as a critical checkpoint in the process of Ag uptake and cross-presentation.     

Dendritic cell differentiation state and their interaction with NKT cells determine Th1/Th2 differentiation in the murine model of Leishmania major infection

Recent reports demonstrated that dendritic cells (DC) sense inflammatory and microbial signals differently, redefining their classical subdivision into an immature endocytic and a mature Ag-presenting differentiation stage. Although both signals induce DC maturation by up-regulating MHC class II and costimulatory molecules, only TLR signals such as LPS are able to trigger proinflammatory cytokine secretion by DCs, including Th1-polarizing IL-12. Here, we explored the murine Leishmania major infection model to examine the CD4(+) T cell response induced by differentially matured DCs. When partially matured TNF-DCs were injected into BALB/c mice before infection, the mice failed to control L. major infection and developed a Th2 response which was dependent on IL-4Ralpha signaling. In contrast, injections of fully matured LPS+CD40-DCs induced a Th1 response controlling the infection. Pulsing DCs with a lysate of L. major did not affect DC maturation with TNF-alpha or LPS+anti-CD40. When the expression of different Notch ligands on DCs was analyzed, we found increased expression of Th2-promoting Jagged2 in TNF-DCs, whereas LPS+CD40-DCs up-regulated the Th1-inducing Delta4 and Jagged1 molecules. The Th2 polarization induced by TNF-DCs required interaction with CD1d-restricted NKT cells. However, NKT cell activation by L. major lysate-pulsed DCs was not affected by blockade of the endogenous glycolipid, suggesting exchange with exogenous parasite-derived CD1 glycolipid Ag. In sum, the differentiation stage of DCs as well as their interaction with NKT cells determines Th1/Th2 differentiation. These results have generic implications for the understanding of DC-driven Th cell responses and the development of improved DC vaccines against leishmaniasis.     

Recombinant tumor-associated MUC1 glycoprotein impairs the differentiation and function of dendritic cells

Tumors exploit several strategies to evade immune recognition, including the production of a large number of immunosuppressive factors, which leads to reduced numbers and impaired functions of dendritic cells (DCs) in the vicinity of tumors. We have investigated whether a mucin released by tumor cells could be involved in causing these immunomodulating effects on DCs. We used a recombinant purified form of the MUC1 glycoprotein, an epithelial associated mucin that is overexpressed, aberrantly glycosylated, and shed during cancer transformation. The O-glycosylation profile of the recombinant MUC1 glycoprotein (ST-MUC1) resembled that expressed by epithelial tumors in vivo, consisting of large numbers of sialylated core 1 (sialyl-T, ST) oligosaccharides. When cultured in the presence of ST-MUC1, human monocyte-derived DCs displayed a modified phenotype with decreased expression of costimulatory molecules (CD86, CD40), Ag-presenting molecules (DR and CD1d), and differentiation markers (CD83). In contrast, markers associated with an immature phenotype, CD1a and CD206 (mannose receptor), were increased. This effect was already evident at day 4 of DC culture and was dose dependent. The modified phenotype of DCs corresponded to an altered balance in IL-12/IL-10 cytokine production, with DC expressing an IL-10(high)IL-12(low) phenotype after exposure to ST-MUC1. These DCs were defective in their ability to induce immune responses in both allogeneic and autologous settings, as detected in proliferation and ELISPOT assays. The altered DC differentiation and Ag presentation function induced by the soluble sialylated tumor-associated mucin may represent a mechanism by which epithelial tumors can escape immunosurveillance.     

IL-15 is expressed by dendritic cells in response to type I IFN, double-stranded RNA, or lipopolysaccharide and promotes dendritic cell activation

Cytokines that are induced by infection may contribute to the initiation of immune responses through their ability to stimulate dendritic cells (DCs). In this paper, we have addressed the role of IL-15 in DC activation, investigating its expression by DCs in response to three different signals of infection and examining its ability to stimulate DCs. We report that the expression of both IL-15 and the IL-15 receptor alpha-chain are increased in splenic DCs from mice inoculated with dsRNA (poly(I:C)), LPS, or IFN-alphabeta, and in purified murine splenic DCs treated with IFN-alphabeta in vitro. Furthermore, IL-15 itself was able to activate DCs, as in vivo or in vitro exposure of splenic DCs to IL-15 resulted in an up-regulation of costimulatory molecules, markedly increased production of IFN-gamma by DC and an enhanced ability of DCs to stimulate Ag-specific CD8(+) T cell proliferation. The magnitude of all of the IL-15-induced changes in DCs was reduced in mice deficient for the IFN-alphabeta receptor, suggesting a role for IFN-alphabeta in the stimulation of DCs by IL-15. These results identify IL-15 as a stimulatory cytokine for DCs with the potential for autocrine activity and link its effects to expression of IFN-alphabeta.