Coordinate stimulation of macrophages by microparticles and TLR ligands induces foam cell formation

Aberrant activation of macrophages in arterial walls by oxidized lipoproteins can lead to atherosclerosis. Oxidized lipoproteins convert macrophages to foam cells through lipid uptake and TLR signaling. To investigate the relative contributions of lipid uptake and TLR signaling in foam cell formation, we established an in vitro assay using liposomes of defined lipid compositions. We found that TLRs signaling through Toll/IL-1R domain-containing adapter inducing IFN-β promoted foam cell formation by inducing both NF-κB signaling and type I IFN production, whereas TLRs that do not induce IFN, like TLR2, did not enhance foam cell formation. Addition of IFN-α to TLR2 activator promoted robust foam cell formation. TLR signaling further required peroxisome proliferator-activated receptor α, as inhibition of peroxisome proliferator-activated receptor α blocked foam cell formation. We then investigated the ability of endogenous microparticles (MP) to contribute to foam cell formation. We found that lipid-containing MP promoted foam cell formation, which was enhanced by TLR stimulation or IFN-α. These MP also stimulated foam cell formation in a human skin model. However, these MP suppressed TNF-α production and T cell activation, showing that foam cell formation can occur by immunosuppressive MP. Taken together, the data reveal novel signaling requirements for foam cell formation and suggest that uptake of distinct types of MP in the context of activation of multiple distinct TLR can induce foam cell formation.

Syndecan-2 is a novel target of insulin-like growth factor binding protein-3 and is over-expressed in fibrosis

Extracellular matrix deposition and tissue scarring characterize the process of fibrosis. Transforming growth factor beta (TGFβ) and Insulin-like growth factor binding protein-3 (IGFBP-3) have been implicated in the pathogenesis of fibrosis in various tissues by inducing mesenchymal cell proliferation and extracellular matrix deposition. We identified Syndecan-2 (SDC2) as a gene induced by TGFβ in an IGFBP-3-dependent manner. TGFβ induction of SDC2 mRNA and protein required IGFBP-3. IGFBP-3 independently induced production of SDC2 in primary fibroblasts. Using an ex-vivo model of human skin in organ culture expressing IGFBP-3, we demonstrate that IGFBP-3 induces SDC2 ex vivo in human tissue. We also identified Mitogen-activated protein kinase-interacting kinase (Mknk2) as a gene induced by IGFBP-3. IGFBP-3 triggered Mknk2 phosphorylation resulting in its activation. Mknk2 independently induced SDC2 in human skin. Since IGFBP-3 is over-expressed in fibrotic tissues, we examined SDC2 levels in skin and lung tissues of patients with systemic sclerosis (SSc) and lung tissues of patients with idiopathic pulmonary fibrosis (IPF). SDC2 levels were increased in fibrotic dermal and lung tissues of patients with SSc and in lung tissues of patients with IPF. This is the first report describing elevated levels of SDC2 in fibrosis. Increased SDC2 expression is due, at least in part, to the activity of two pro-fibrotic factors, TGFβ and IGFBP-3.

Suppression of autoimmune diabetes by soluble galectin-1

Type 1 diabetes (T1D) is a T cell-mediated autoimmune disease that targets the beta-cells of the pancreas. We investigated the ability of soluble galectin-1 (gal-1), an endogenous lectin that promotes T cell apoptosis, to down-regulate the T cell response that destroys the pancreatic beta-cells. We demonstrated that in nonobese diabetic (NOD) mice, gal-1 therapy reduces significantly the amount of Th1 cells, augments the number of T cells secreting IL-4 or IL-10 specific for islet cell Ag, and causes peripheral deletion of beta-cell-reactive T cells. Administration of gal-1 prevented the onset of hyperglycemia in NOD mice at early and subclinical stages of T1D. Preventive gal-1 therapy shifted the composition of the insulitis into an infiltrate that did not invade the islets and that contained a significantly reduced number of Th1 cells and a higher percentage of CD4(+) T cells with content of IL-4, IL-5, or IL-10. The beneficial effects of gal-1 correlated with the ability of the lectin to trigger apoptosis of the T cell subsets that cause beta-cell damage while sparing naive T cells, Th2 lymphocytes, and regulatory T cells in NOD mice. Importantly, gal-1 reversed beta-cell autoimmunity and hyperglycemia in NOD mice with ongoing T1D. Because gal-1 therapy did not cause major side effects or beta-cell toxicity in NOD mice, the use of gal-1 to control beta-cell autoimmunity represents a novel alternative for treatment of subclinical or ongoing T1D.

Distinct subsets of human skin dendritic cells differ on their ability to initiate Th17 responses (90.24)

Activated human cutaneous dendritic cells (DCs) have the ability to initiate Th1 and Th2 immunity. However, whether human cutaneous DCs are capable of biasing pro-inflammatory Th17 responses remains little understood. Utilizing human skin explants composed by both epidermis and dermis or epidermal or dermal sheets to collect activated skin-migratory DCs (smiDCs), we demonstrated that smiDCs stimulate allogeneic naïve CD4+ T cells to differentiate simultaneously into two distinct effector Th17 and Th1 populations with ability of skin homing and inducing severe tissue damage. Of the two main myeloid DC populations resident in the skin, the subset of epidermal skin-migratory Langerhans cells (smiLCs) were capable of inducing Th17 responses. This effect depended on the combined effects of IL-15 and stabilized IL-6 trans-signaling of naïve CD4+ T cells. Purified skin-migratory DDCs did not synthesize IL-15 and were unable to bias Th17 responses however, they acquired the ability to bias Th17 cells in co-cultures with CD4+ T cells supplemented with IL-15 and stabilized IL-6. Overall, our data demonstrate that human cutaneous LCs induce Th17 responses by mechanisms different from those previously described for mouse DCs and human monocytes and highlight the need to target clinical treatments based on these variations.

Supported by NIH grants: R01 CA100893 (ATL)

Dendritic cell (DC)- derived exosomes spread donor allo-antigen between recipient's DC following cardiac transplantation (141.11)

Exosomes are nanovesicles generated in multivesicular endosomes that are released to the extracellular space by different cell types. Since DC-derived exosomes (dexosomes) express MHC-peptide and costimulatory molecules, we investigated the role of dexosomes in elicitation of the anti-donor response in transplantation. Dexosomes were generated from bone marrow-derived DC and labeled with PKH67 for traffic studies. CD4+ TCRtg T cells (Thy1.1+) specific for the IEα52-68 (BALB/c)-IAb (B10) complex were CFSE-labeled and transferred i.v. to host Thy1.2+ B6 mice. Graft infiltrating DC were isolated from BALB/c (CD45.2+) cardiac grafts 3 days after transplantation in B6 (CD45.2+) recipients and genetically-engineered ex vivo to release exosomes expressing the reporter marker eGFP, and then transferred i.v. into host CD45.1+ B6 mice. We demonstrated that although graft-infiltrating leukocytes release exosomes ex vivo, they do not secrete enough exosomes in circulation to stimulate donor-reactive T-cells in lymphoid organs. Instead, migrating DCs (generated in vitro or isolated from allografts), once they home in the spleen, they transfer exosomes expressing eGFP to spleen-resident DCs. Thus, exchange of exosomes between DCs in lymphoid organs constitutes a mechanism by which passenger leukocytes transfer alloAg to recipient's APC and amplify generation of donor-reactive T-cells.

Agonistic signaling via the neurokinin 1 receptor and CD40 have a synergistic effect to promote dendritic cell survival and potent CTL responses (135.56)

Immune competent organs are richly innervated and pro-inflammatory neuropeptides released by nerve endings favor the initiation of innate and adaptive immune responses. By binding the neurokinin 1 receptor (NK1R), the pro-inflammatory neuropeptide substance P promotes immune cell survival and potent cellular immunity. Dendritic cells (DCs) express surface NK1R and the priming of T cell responses requires Ag presentation by DCs able to withstand apoptotic signaling. We hypothesized that signaling DCs via the NK1R prevents apoptosis of DCs favoring sustained DC-T cell contact and robust CTL responses. Using murine bone marrow-derived DCs (BMDCs) cultured with the NK1R agonist [Sar9Met(02)11]-SP (SarSP-DCs), we demonstrate that signaling via the NK1R utilizes the PI3K-Akt pathway to prolong the expression of anti-apoptotic molecules. Additionally, SarSP-DCs showed increased expression of surface CD40. Adoptive transfer of Ag-loaded SarSP-DCs showed enhanced longevity in local draining lymph nodes vs. control DCs, which was a result of a combination of signaling via the NK1R and CD40 molecules. Importantly, SarSP-DCs elicited potent CTL responses compared to control DCs. We conclude that agonistic signaling via the NK1R and CD40 have a synergistic effect that enhances DC survival and favors the generation of potent CTL responses.

Supported by NIH grant: R01 CA100893 (ATL)

Quiescent recipient splenic dendritic cells (DC) re-process therapeutic DC into alloantigen for presentation to and down-regulation of anti-donor T cells to prolong allograft survival (141.31)

Current dogma in transplantation presumes that therapeutic maturation-resistant dendritic cells (MR-DC) prevent allograft rejection by directly interacting with donor-reactive T-cells in vivo. However, this assumption remains untested. Our study therefore aimed to elucidate the mechanism by which donor MR-DC down-regulate the anti-donor response to prolong allograft survival in mice. We employed as prototypic donor MR-DC, bone marrow-derived DC treated with the active form of vitamin D3, 1α,25(OH)2D3. We demonstrate that donor-derived MR-DC prolonged cardiac allograft survival similarly as donor apoptotic or viable splenocytes, suggesting that different donor-derived cellular therapies share a common mechanism of action. We determined that once injected i.v., MR-DC were short-lived and failed to directly tolerize donor-reactive T-cells. Rather, apoptotic MR-DC were internalized and re-processed by quiescent recipient splenic CD11chiCD8α+ and CD11chiCD8- DC, which in turn induced deletion of effector T-cells and outgrowth of regulatory T-cells via the indirect pathway. Importantly, activated recipient DC induced robust T-cell activation via indirect presentation. Our data suggest that therapeutic DC, like other more practical cellular therapies, serve as a source of alloantigen for recipient quiescent DC that in turn down-regulate the anti-donor response to prolong allograft survival.

Differential capability of human cutaneous dendritic cell subsets to initiate Th17 responses

Human skin-migratory dendritic cells (DCs) have the ability to prime and bias Th1 and Th2 CD4+ T lymphocytes. However, whether human cutaneous DCs are capable of initiating proinflammatory Th17 responses remains undetermined. We report that skin-migratory DCs stimulate allogeneic naive CD4+ T cells that differentiate simultaneously into two distinct effector Th17 and Th1 populations capable of homing to the skin, where they induce severe cutaneous damage. Skin-migratory Langerhans cells (smiLCs) were the main cutaneous DC subset capable of inducing Th17 responses dependent on the combined effects of IL-15 and stabilized IL-6, which resulted in IL-6 trans-signaling of naive CD4+ T cells. Different from smiLCs, purified skin-migratory dermal DCs did not synthesize IL-15 and were unable to bias Th17 responses. Nevertheless, these dermal DCs were capable of differentiating Th17 cells in mixed leukocyte cultures supplemented with IL-15 and stabilized IL-6. Overall, our data demonstrate that human epidermal smiLCs induce Th17 responses by mechanisms different from those previously described and highlight the need to target clinical treatments based on these variations.

Exosomes as a short-range mechanism to spread alloantigen between dendritic cells during T cell allorecognition

Exosomes are nanovesicles released by different cell types including dendritic cells (DCs). The fact that exosomes express surface MHC-peptide complexes suggests that they could function as Ag-presenting vesicles or as vehicles to spread allogeneic Ags for priming of anti-donor T cells during elicitation of graft rejection or induction/maintenance of transplant tolerance. We demonstrate that circulating exosomes transporting alloantigens are captured by splenic DCs of different lineages. Internalization of host-derived exosomes transporting allopeptides by splenic DCs leads to activation of anti-donor CD4 T cells by the indirect pathway of allorecognition, a phenomenon that requires DC-derived, instead of exosome-derived, MHC class II molecules. By contrast, allogeneic exosomes are unable to stimulate direct-pathway T cells in vivo. We demonstrate in mice that although graft-infiltrating leukocytes release exosomes ex vivo, they do not secrete enough concentrations of exosomes into circulation to stimulate donor-reactive T cells in secondary lymphoid organs. Instead, our findings indicate that migrating DCs (generated in vitro or isolated from allografts), once they home in the spleen, they transfer exosomes expressing the reporter marker GFP to spleen-resident DCs. Our results suggest that exchange of exosomes between DCs in lymphoid organs might constitute a potential mechanism by which passenger leukocytes transfer alloantigens to recipient's APCs and amplify generation of donor-reactive T cells following transplantation.

Human skin culture as an ex vivo model for assessing the fibrotic effects of insulin-like growth factor binding proteins

Systemic sclerosis (SSc) is a connective tissue disease of unknown etiology. A hallmark of SSc is fibrosis of the skin and internal organs. We recently demonstrated increased expression of IGFBP-3 and IGFBP-5 in primary cultures of fibroblasts from the skin of patients with SSc. In vitro, IGFBP-3 and IGFBP-5 induced a fibrotic phenotype and IGFBP-5 triggered dermal fibrosis in mice. To assess the ability of IGFBPs to trigger fibrosis, we used an ex vivo human skin organ culture model. Our findings demonstrate that IGFBP-3 and IGFBP-5, but not IGFBP-4, increase dermal and collagen bundle thickness in human skin explants, resulting in substantial dermal fibrosis and thickening. These fibrotic effects were sustained for at least two weeks. Our findings demonstrate that human skin ex vivo is an appropriate model to assess the effects of fibrosis-inducing factors such as IGFBPs, and for evaluating the efficacy of inhibitors/therapies to halt the progression of fibrosis and potentially reverse it.

Helper function of memory CD8+ T cells: heterologous CD8+ T cells support the induction of therapeutic cancer immunity

In contrast to the well-established efficacy of preventive vaccines, the effectiveness of therapeutic vaccines remains limited. To develop effective vaccination regimens against cancer, we have analyzed the effect of effector and memory CD8+ T cells on the ability of dendritic cells to mediate the immunologic and antitumor effects of vaccination. We show that in contrast to effector CD8+ T cells that kill antigen-carrying dendritic cells, IFNgamma-producing memory CD8+ T cells act as "helper" cells, supporting the ability of dendritic cells to produce interleukin-12 (IL-12) p70. Promoting the interaction of tumor antigen-carrying dendritic cells with memory-type "heterologous" (tumor-irrelevant) CD8+ T cells strongly enhances the IL-12p70-dependent immunogenic and therapeutic effects of vaccination in the animals bearing established tumors. Our data show that the suppressive and helper functions of CD8+ T cells are differentially expressed at different phases of CD8+ T-cell responses. Selective performance of helper functions by memory (in contrast to effector) CD8+ T cells helps to explain the phenomenon of immune memory and facilitates the design of effective therapeutic vaccines against cancer and chronic infections.

In vivo signaling through the neurokinin 1 receptor favors transgene expression by Langerhans cells and promotes the generation of Th1- and Tc1-biased immune responses

The proinflammatory capacities of the skin and the presence of high numbers of resident dendritic cells (DCs) constitute an ideal microenvironment for successful immunizations. Regardless of the ability of DCs to respond to local inflammatory signals in an immunostimulatory fashion, the immune functions of skin-resident DCs remain controversial, and epidermal Langerhans cells (LCs) have been referred to recently as anti-inflammatory/protolerogenic APCs. Substance P (SP), released by skin nerve fibers, is a potent proinflammatory neuropeptide that favors development of skin-associated cellular immunity. SP exerts its proinflammatory functions by binding with high affinity to the neurokinin 1 receptor (NK1R). In this study, we tested whether signaling skin cells via the NK1R promotes humoral and cellular immunity during skin genetic immunizations. We used the gene gun to deliver transgenic (tg) Ag to the skin of C57BL/6 mice and the selective NK1R agonist [Sar(9)Met (O(2)) (11)]-SP as a potential proinflammatory Th1-biasing adjuvant. Our strategy expressed tg Ag exclusively in the epidermis and induced a preferential migration of activated LCs to skin-draining lymph nodes. Local administration of the NK1R agonist during skin genetic immunizations increased significantly the expression of tg Ag by a mechanism involving the translocation of NF-kappaB into the nuclei of cutaneous DCs homing to skin-draining lymph nodes. Importantly, our immunization approach resulted in Th1 and T cytotoxic (CTL)-1 bias of effector T cells that supported cellular and Ab-mediated immune responses. We demonstrate that signaling skin cells via the NK1R provides the adjuvant effect which favors the immunostimulatory functions of LCs.

Professional antigen-presenting cells of the skin

The skin functions as an important pro-inflammatory and immune organ. Accordingly, the epidermis and dermis are highly populated by dendritic cells (DC), which are potent antigen-presenting cells (APC) with important immunostimulatory and migratory activities. Whereas the biological characteristics and immunological functions of epidermal DC known as Langernahs cells (LC) have been the focus of intense research in the past, less is known regarding their dermal counterparts named dermal dendritic cells (DDC). Although it has been widely accepted that LC are the more relevant skin-resident APC, recent experimental evidence challenges this concept and proposes a different role for these important cell populations. In this article we compile recent scientific advances regarding the function of different skin-resident DC and we try to reconcile the new observations with the previously established paradigm.

Genetic immunizations in the presence of a specific neurokinin 1 receptor agonist triggers Th1-Tc1 biased immune responses that protect against B16 melanomas (48.13)

Antigen (Ag) specific Th1-Tc1 cellular immunity induced by genetic immunizations is critical to prevent / eradicate tumors. The high number of resident dendritic cells (DCs) makes the skin an ideal microenvironment for genetic immunizations against tumors. Skin DCs become potent stimulators of Ag specific T cells by signals triggered during acute inflammation in peripheral tissues. The secretion of the pro-inflammatory neuropeptide substance P signaling via the neurokinin 1 receptor (NK1R) represents an early danger signal that stimulates APC functions of DCs. Here we tested whether signaling via the NK1R during skin genetic immunizations favors Th1-Tc1 immunity in C57BL/6 mice injected with B16 melanoma cells. We utilized the gene gun to deliver transgenic (tg) OVA or TRP-2 Ag to mouse skin and the NK1R agonist Sar9Met (O2)11-SP as a potential Th1 adjuvant. Our strategy expressed Ag exclusively in the epidermis and induced substantial migration of activated Langerhans cells (LCs) transporting tg Ag to skin draining lymph nodes, which resulted in generation of Th1-Tc1 immunity. Importantly delivery of tg TRP-2 plus NK1R agonist was able to protect mice against B16 melanoma tumor cell growth. Our data demonstrate that NK1R signaling provides the Th1 adjuvant effect that positively modulates the immunostimulatory functions of LCs during skin genetic immunizations leading to productive immune responses.

Human skin migratory dendritic cells that do not secrete IL-12p70 stimulate both Th17 and Th1 allogeneic responses (36.20)

We analyzed the ability of human skin migratory dendritic cells (smiDC) to stimulate allogeneic (allo)-CD4+ T helper cell responses and to induce differentiation of Th1 and Th17 cells. SmiDCs obtained from cultures of human skin explants consisted of Langerhans cells and dermal DCs, which synthesized IL-8, IL-15, IL-23, IL-10 and TGF-β1, but not IL-12p70, even after exposure to DC1-driving stimuli. Regardless of the lack of IL-12p70 smiLC and DDC induced differentiation of effector memory Th cells that expressed CD45RO, the skin homing molecule CLA, and decreased CD62L and CCR7. SmiDCs stimulated proliferation of allo-CD4+ T cells that differentiated into Th1 and Th17 cells during 5 day-MLCs. T cell proliferation depended on the expression of MHC-II, costimulatory and adhesion molecules by smiDCs. The Th1 biasing function of smiDCs depended on the production of IL-23. The Th17 response was supported by the proinflammatory cytokines IL-8 and IL-15 and was significantly increased by the blockade of IL-6, whereas the blockade of IL-23 did not affect the secretion of IL-17. Levels of IL-17 and IFN-γ secretion reached their maximum at day 4 and 5, respectively. IFN-γ (Th1) and IL-17 (Th17) was produced by two distinct Th cell subsets as demonstrated by flow cytometry. Therefore, human smiDCs support the induction and coexistence of Th1 and Th17 effector T cells by mechanisms distinct from those described in the mouse model.

Transgenic Galectin-1 Induces Maturation of Dendritic Cells That Elicit Contrasting Responses in Naive and Activated T Cells

Dendritic cells (DC) are professional APC that control the balance between T cell immunity and tolerance. Genetic engineering of DC to regulate the outcome of the immune response is an area of intense research. Galectin (gal)-1 is an endogenous lectin that binds to glycoproteins and exerts potent regulatory effects on T cells. Consequently, gal-1 participates in central deletion of thymocytes and exerts therapeutic effects on experimental models of T cell-mediated autoimmune disorders and graft-vs-host disease. Together, these observations strongly indicate that engineering DC to express transgenic (tg) gal-1 may be beneficial to treat T cell-mediated disorders. In this study, we have investigated the impact of the expression of high levels of tg gal-1 on maturation/activation of DC and on their T cell stimulatory function. Murine DC were transduced with a recombinant adenovirus encoding hu gal-1 (gal-1-DC). Tg gal-1 was exported by a nonclassical pathway through exosomes and was retained on the DC surface inducing segregation of its ligand CD43. Expression of tg gal-1 triggered activation of DC determined by induction of a more mature phenotype, increased levels of mRNA for proinflammatory cytokines, and enhanced ability to stimulate naive T cells. Conversely, gal-1-DC induced rapid apoptosis of activated T cells. In vivo, gal-1-DC increased significantly the sensitization phase of contact hypersensitivity assays while inducing a drastic inhibition of the elicitation phase by triggering apoptosis of activated T cells in the dermis. Gal-1-DC represent a novel tool to control differentially the afferent and efferent arms of the T cell response.

Use of the inhibitory effect of apoptotic cells on dendritic cells for graft survival via T-cell deletion and regulatory T cells

Tolerance induction against donor allo-antigens (allo-Ag) remains one of the most challenging aspects of transplant immunology. The ability of dendritic cells (DC) to participate in immunity and tolerance makes them an excellent tool for tolerance induction. Here, we employed the immunosuppressive properties of apoptotic cells to deliver simultaneously an inhibitory signal and donor allo-Ag to recipient DC for treatment of allograft rejection. DC that captured apoptotic cells remained immature and activated deficiently anti-donor CD4(+) T cells that were unable to upregulate T-cell activation markers, to secrete IL-2 and IFN-gamma and to survive under homeostatic conditions due to low expression of Bcl-X(L), IL-7R and IL-15R. Administration of donor apoptotic cells decreased the systemic anti-donor T- and B-cell response and prolonged cardiac allograft survival in mice. The effect was donor specific and required the interaction of donor apoptotic cells with recipient quiescent CD8alpha(+) DC. When combined with CD40-CD154-blockade, administration of donor apoptotic cells resulted in indefinite graft survival mediated by generation of regulatory T cells. The use of the inhibitory effects of apoptotic cells on the anti-donor response provides a new approach to treat transplant rejection.

CD4+ T cell responses elicited by different subsets of human skin migratory dendritic cells

Skin dendritic cells (DC) are professional APC critical for initiation and control of adaptive immunity. In the present work we have analyzed the CD4+ T cell stimulatory function of different subsets of DC that migrate spontaneously from human skin explants, including CD1a+CD14- Langerhans' cells (LC), CD1a-CD14- dermal DC (DDC), and CD1a-CD14+ LC precursors. Skin migratory DC consisted of APC at different stages of maturation-activation that produced IL-10, TGF-beta1, IL-23p19, and IL-12p40, but did not release IL-12p70 even after exposure to DC1-driving stimuli. LC and DDC migrated as mature/activated APC able to stimulate allogeneic naive CD4+ T cells and to induce memory Th1 cells in the absence of IL-12p70. The potent CD4+ T cell stimulatory function of LC and DDC correlated with their high levels of expression of MHC class II, adhesion, and costimulatory molecules. The Th1-biasing function of LC and DDC depended on their ability to produce IL-23. By contrast, CD1a-CD14+ LC precursors migrated as immature-semimature APC and were weak stimulators of allogeneic naive CD4+ T cells. However, and opposite of a potential tolerogenic role of immature DC, the T cell allostimulatory and Th1-biasing function of CD14+ LC precursors increased significantly by augmenting their cell number, prolonging the time of interaction with responding T cells, or addition of recombinant human IL-23 in MLC. The data presented in this study provide insight into the function of the complex network of skin-resident DC that migrate out of the epidermis and dermis after cutaneous immunizations, pathogen infections, or allograft transplantation.

Tumor cell loaded type-1 polarized dendritic cells induce Th1-mediated tumor immunity

Dendritic cells are professional antigen-presenting cells capable of inducing and regulating innate and antigen-specific immune responses. Therapeutic cancer vaccines using ex vivo engineered or in vivo targeted dendritic cells are being evaluated in clinical trials. T-helper type-1 (Th1)-skewed immune responses are characterized by the preferential induction of antigen-specific IFN-gamma-secreting CD4+ T cells and correlate with effector mechanisms important for tumor and viral immunity. Methods to "polarize" human monocyte-derived dendritic cells for the preferential induction of Th1-skewed immune responses have been developed, and polarized dendritic cells (DC1s) are being evaluated in preclinical and clinical studies. Here, we show that stimulation of bone marrow-derived murine dendritic cell populations with poly(I:C) and CpGs results in phenotypic maturation of dendritic cells and synergistic induction of durable, high-level IL-12p70 secretion characteristic of human type-1 polarized dendritic cells. Functionally, these dendritic cells induce antigen-specific Th1-type CD4+ T-cell activation in vitro and in vivo. Dendritic cell maturation and polarization are not inhibited by the presence of live B16 melanoma tumor cells, and tumor-loaded DC1s induce delayed-type hypersensitivity responses in vivo. DC1s loaded with B16 melanoma cells and injected into tumor-bearing mice induce Th1-skewed tumor-specific CD4+ T cells and a significant reduction in tumor growth. Tumor infiltrates in DC1-immunized animals are characterized by the presence of CD4+ T cells and activated macrophages. These results show a murine model of DC1 function and suggest an important role for CD4+ T cells and macrophages in DC1-induced antitumor immune responses. They have implications for the future development of DC1-based immunotherapies and strategies for clinical immune monitoring of their effectiveness.

Endocytosis, intracellular sorting, and processing of exosomes by dendritic cells

Exosomes are nanovesicles released by leukocytes and epithelial cells. Although their function remains enigmatic, exosomes are a source of antigen and transfer functional major histocompatibility complex (MHC)-I/peptide complexes to dendritic cells (DCs) for CD8(+) T-cell activation. Here we demonstrate that exosomes also are internalized and processed by immature DCs for presentation to CD4(+) T cells. Endocytosed exosomes are sorted into the endocytic compartment of DCs for processing, followed by loading of exosome-derived peptides in MHC-II molecules for presentation to CD4(+) T cells. Targeting of exosomes to DCs is mediated via milk fat globule (MFG)-E8/lactadherin, CD11a, CD54, phosphatidylserine, and the tetraspanins CD9 and CD81 on the exosome and alpha(v)/beta(3) integrin, and CD11a and CD54 on the DCs. Circulating exosomes are internalized by DCs and specialized phagocytes of the spleen and by hepatic Kupffer cells. Internalization of blood-borne allogeneic exosomes by splenic DCs does not affect DC maturation and is followed by loading of the exosome-derived allopeptide IEalpha(52-68) in IA(b) by host CD8alpha(+) DCs for presentation to CD4(+) T cells. These data imply that exosomes present in circulation or extracellular fluids constitute an alternative source of self- or allopeptides for DCs during maintenance of peripheral tolerance or initiation of the indirect pathway of allorecognition in transplantation.

Highly efficient expression of transgenic proteins by naked DNA-transfected dendritic cells through terminal differentiation

Dendritic cells (DCs) play a key role in the induction and control of immunity. Genetic engineering of DCs is a promising approach for the development of a broad range of immunomodulatory strategies, for purposes ranging from genetic immunization to tolerance induction. The development of DC-based immunotherapies is limited by the inability to efficiently transfect DCs using naked DNA. Here we demonstrate that after plasmid DNA delivery, the transgene expression level controlled by the human immediate-early cytomegalovirus promoter (hIE-CMVp) is higher in mature DCs than in immature DCs and is further increased after terminal differentiation of DCs by agonist anti-CD40 monoclonal antibody (mAb) or after DC interaction with CD4+ T cells. CD40 signaling of DCs resulted in nuclear translocation of the transcription factors nuclear factor-κB (NF-κB), activator of protein-1 (AP-1), and cyclic adenosine monophosphate (cAMP)–responsive element, necessary for the activation of hIE-CMVp. Transgene expression by DCs diminished after the inhibition of these transcription factors or the blockade of adhesion molecules involved in the DC–T-cell synapse. Importantly, CD40 signaling of DCs results in the highly efficient expression and presentation of transgenic antigens and the induction of “in vivo” cytotoxic T-cell (CTL) responses specific for transgenic antigen peptides, demonstrating the functional potential of genetically engineered DCs.

DNA immunisation: altering the cellular localisation of expressed protein and the immunisation route allows manipulation of the immune response

DNA immunisation by intramuscular (IM) injection induces Th1 responses, whereas gene gun (GG) immunisation into the skin stimulates Th2 responses. Three ovalbumin (OVA) cDNA constructs, in which OVA is cytoplasmic (CYT), secreted (SECR), or transmembrane (TM), were compared in immunisation studies using intramuscular injection or biolistic bombardment of the skin. Gene gun immunisation with OVA-CYT or OVA-TM led to strong OVA-specific CTL responses, but not following OVA-SECR immunisation. In contrast, intramuscular immunisation with OVA-SECR or OVA-TM led to potent CTL while immunisation with OVA-CYT was ineffective. OVA-specific antibodies were detected following gene gun immunisation with all three constructs, whereas only the OVA-SECR construct induced antibody production following intramuscular immunisation. These results demonstrate the capacity to manipulate the nature of the immune response by altering the cellular localization of expressed proteins and the route of DNA immunisation.