Human dendritic cells and macrophages exhibit different intracellular processing pathways for soluble and liposome-encapsulated antigens.

The intracellular fates of soluble and liposomal antigens in human macrophages and dendritic cells are not well defined. Previous studies using murine macrophages have demonstrated that liposomal antigens can enter the MHC class I pathway. The Golgi complex is a major organelle in this pathway. Phagocytosis of the antigens is followed by translocation of antigen-derived peptides to the trans-Golgi where they can complex with MHC class I molecules. In contrast, soluble antigens are normally processed through the MHC class II pathway. Therefore, in the present study, ovalbumin and a synthetic Ebola peptide were used either in a soluble form or encapsulated in liposomes to investigate the intracellular trafficking and localization of these antigens to the Golgi complex in human macrophages and dendritic cells. While liposome-encapsulated antigens were transported to the trans-Golgi region in 59-78% of macrophages, soluble antigens remained diffuse throughout the cytoplasm with only 3-11% of the macrophages exhibiting trans-Golgi localization. The majority of dendritic cells localized both soluble (Ebola, 75%; ovalbumin, 84%) and liposomal antigens (58% and 65%), and irradiated Ebola virus to the trans-Golgi. These studies demonstrate that the intracellular fate of soluble and liposomal antigens can differ depending upon the antigen-presenting cell.

Functional microtubules are required for antigen processing by macrophages and dendritic cells.

Antigen-presenting cells readily phagocytose antigens and channel them through various membrane-bound organelles within the cell. In previous studies, we demonstrated that macrophages concentrated and localized particulate antigens to the trans-Golgi prior to displaying the MHC-class I-antigenic peptides on the cell surface. In this study, we evaluated the importance of cytoskeletal elements in the intracellular trafficking of soluble and liposome-encapsulated ovalbumin in murine bone marrow-derived macrophages and human dendritic cells. F-actin, as identified by staining with fluorescein phalloidin, was observed at the point of contact between soluble or liposomal antigen and the cell membrane, suggesting that a rearrangement of the cytoskeleton occurs to facilitate the uptake of the antigens. Cells were incubated with colchicine, a microtubule depolymerizing agent, or paclitaxel, a microtubule polymerizing agent, before the addition of Texas Red-labeled ovalbumin or liposome-encapsulated Texas Red-labeled ovalbumin. Colchicine disrupted the trans-Golgi, whereas the trans-Golgi complexes were intact in paclitaxel treated cells. In either paclitaxel or colchicine-treated macrophages, internalized liposomal ovalbumin was not concentrated in the area of the trans-Golgi as determined by staining with fluorescent ceramide. In contrast, soluble ovalbumin was concentrated in the region of the trans-Golgi in 15% of the dendritic cells treated with paclitaxel, whereas 6% of the dendritic cells were able to concentrate liposomal antigen. In colchicine-treated dendritic cells, both soluble and liposomal antigens were internalized but did not localize to the area of the trans-Golgi. These data suggest that trafficking of soluble and liposome-encapsulated ovalbumin requires a functional microtubule-dependent translocation system.

Modulation of dengue virus infection of dendritic cells by Aedes aegypti saliva.

Dengue virus (DV) is a flavivirus carried by the Aedes aegypti mosquito that causes a spectrum of illnesses in the tropics, including dengue fever, dengue hemorrhagic fever, and dengue shock syndrome. Dendritic cells (DCs) are professional antigen presenting cells recently shown to be permissive for DV, and implicated as the primary targets of initial DV infection. DV is transmitted to human host by infected mosquitoes during a blood meal, but it is currently unknown whether transmission is modified by vector saliva that is also deposited in the host's skin during feeding. Previous studies evaluated only the outcome of DV infection of DCs, and did not address the influence of mosquito saliva. To more fully characterize natural transmission of DV, we evaluated the effects of Ae. aegypti saliva on DV infection of human myeloid DCs. We found that saliva inhibited DV infection in DCs. Moreover, pre-sensitization of DCs with saliva, prior to DV infection, enhanced inhibition. In addition, enhanced production of IL-12p70 and TNF-alpha were detected in DV-infected DC cultures exposed to mosquito saliva. The proportion of dead cells was also significantly reduced in these cultures. These data contribute to the overall understanding of the natural pathogenesis of DV infection and suggest that there is a protective role for mosquito saliva that limits viral uptake by DCs.

Dengue virus infection promotes translocation of high mobility group box 1 protein from the nucleus to the cytosol in dendritic cells, upregulates cytokine production and modulates virus replication.

High mobility group box 1 (HMGB1) protein functions in regulation of transcription, cellular activation and pro-inflammatory responses. However, the potential role of HMGB1 during viral infection has not been investigated. This study attempted to elucidate whether the HMGB1-mediated inflammatory response contributes to the pathogenesis of dengue virus (DENV) infection. Our data showed that HMGB1 was released at low DENV infection levels (m.o.i. of 1) under non-necrotic conditions by human dendritic cells (DCs). When DENV-infected DCs were co-cultured with autologous T cells, there was increased production of HMGB1 by both cell types. HMGB1 regulated tumour necrosis factor alpha, interleukin (IL)-6, IL-8 and alpha interferon secretion in DENV-infected DCs. Additionally, increased HMGB1 production was associated with reduced DENV replication titres in DCs. These results suggest that HMGB1 production influences DENV infection in susceptible hosts.

Functional characterization of ex vivo blood myeloid and plasmacytoid dendritic cells after infection with dengue virus.

Myeloid and plasmacytoid dendritic cells (mDC and pDC) are naturally distinctive subsets. We exposed both subsets to dengue virus (DV) in vitro and investigated their functional characteristics. High levels of DV replication in mDC were found to correlate with DC-SIGN expression. Production of inflammatory cytokines by mDC increased gradually after DV-infection, which was dependent on DV replication. Co-stimulatory markers were upregulated on mDC upon DV-infection. On the contrary, lower levels of DV-replication were observed in pDC, but the cytokine production in pDC was quicker and stronger. This cytokine response was not dependent on viral replication, but dependent on cell endosomal activity and TLR7, and could be also induced by purified DV genome RNA. These results clearly suggested functional differences between mDC and pDC in response to DV infection. Additionally, the TLR7-mediated recognition of DV RNA may be involved in pDC functional activation.

Phenotypic analysis of dengue virus isolates associated with dengue fever and dengue hemorrhagic fever for cellular attachment, replication and interferon signaling ability.

Eighteen dengue viruses (DENVs) representing all four serotypes, isolated from pediatric patients at children's hospital, Queen Sirikit National Institute of Child Health, Bangkok, Thailand exhibiting a diverse spectrum of disease ranging from uncomplicated dengue fever (DF) to severe dengue hemorrhagic fever (DHF), were tested for their ability to attach to host cells, replicate and interfere with the IFNalpha signaling pathway by interfering with signal transducer and activator of transcription 1 (STAT-1) function. Although most isolates suppressed IFNalpha-induced STAT-1 phosphorylation, our results showed no difference between DENV strains associated with DF and those associated with DHF. However, the DHF isolates tended replicate to higher titers in dendritic cells (DCs) than the DF isolates, but this ability was independent of their cell-binding capability. Our results suggest that the emergence early in infection of viruses with a high degree of replication fitness may play an important role in DENV pathogenesis.

Potential role for Toll-like receptor 4 in mediating Escherichia coli maltose-binding protein activation of dendritic cells.

The Escherichia coli maltose-binding protein (MBP) is used to increase the stability and solubility of proteins in bacterial protein expression systems and is increasingly being used to facilitate the production and delivery of subunit vaccines against various pathogenic bacteria and viruses. The MBP tag is presumed inert, with minimum effects on the bioactivity of the tagged protein or its biodistribution. However, few studies have characterized the immunological attributes of MBP. Here, we analyze the phenotypic and functional outcomes of MBP-treated dendritic cells (DCs) and show that MBP induces DC activation and production of proinflammatory cytokines (interleukin-1beta [IL-1beta], IL-6, IL-8, tumor necrosis factor alpha, and IL-12p70) within 24 h and strongly increases Ikappabeta phosphorylation in treated cells. Interestingly, phosphorylation of Ikappabeta was largely abrogated by the addition of anti-human Toll-like receptor 4 (TLR4) antibodies, indicating that MBP activates signaling for DC maturation via TLR4. Consistent with this hypothesis, MBP activated the TLR4-expressing cell line 293-hTLR4A but not control cultures to secrete IL-8. The observed data were independent of lipopolysaccharide contamination and support a role for TLR4 in mediating the effects of MBP. These results provide insight into a mechanism by which MBP might enhance immune responses to vaccine fusion proteins.

Restricted replication and lysosomal trafficking of yellow fever 17D vaccine virus in human dendritic cells.

The yellow fever virus attenuated 17D vaccine strain is a safe and effective vaccine and a valuable model system for evaluating immune responses against attenuated viral variants. This study compared the in vitro interactions of the commercially available yellow fever vaccine (YF-VAX), Dengue virus and the live-attenuated dengue vaccine PDK50 with dendritic cells (DCs), the main antigen-presenting cells at the initiation of immune responses. Similar to PDK50, infection with YF-VAX generated activated DCs; however, for YF-VAX, activation occurred with limited intracellular virus replication. The majority of internalized virus co-localized with endolysosomal markers within 90 min, suggesting that YF-VAX is processed rapidly in DCs. These results indicate that restricted virus replication and lysosomal compartmentalization may be important contributing factors to the success of the YF-VAX vaccine.

Phase I safety and immunogenicity trial of FMP1/AS02A, a Plasmodium falciparum MSP-1 asexual blood stage vaccine.

We report the first safety and immunogenicity trial of the Plasmodium falciparum malaria blood stage vaccine candidate, FMP1/AS02A consisting of the FMP1 antigen, an Escherichia coli-expressed His-tagged fusion protein from the 42 kDa C-terminal fragment from the 3D7 clone of the merozoite surface protein 1 formulated in the AS02A adjuvant. An open label, prospective, single-center Phase I dose escalation trial of FMP1/AS02A was conducted in 15 adult malaria-naïve human volunteers to assess safety, reactogenicity, and immunogenicity. The vaccine was safe and well-tolerated and no serious adverse events were observed. The vaccine induced high-titer ELISA and IFA responses in all volunteers. Proliferative and ELISPOT responses were induced to vaccine antigen. Biologically active antibodies were induced as measured by GIA. This study establishes the foundation to further evaluate and measure the vaccine's ability to reduce morbidity and mortality in target populations directly affected by P. falciparum malaria.

Differential effects of dengue virus on infected and bystander dendritic cells.

Dendritic cells (DCs) play a central role as major targets of dengue virus (DV) infections and initiators of antiviral immune responses. Previous observations showed that DCs are activated by infection, presumably acquiring the capacity to promote cell-mediated immunity. However, separate evaluations of the maturation profiles of infected and uninfected bystander cells show that infection impairs the ability of DCs to upregulate cell surface expression of costimulatory, maturation, and major histocompatibility complex molecules, resulting in reduced T-cell stimulatory capacity. Infected DCs failed to respond to tumor necrosis factor alpha as an additional maturation stimulus and were apoptotic. Interleukin 10 (IL-10) was detected in supernatants from cultures of DV-infected DCs and cocultures of DCs and T cells. Taken together, these results constitute an immune evasion strategy used by DV that directly impairs antigen-presenting cell function by maturation blockade and induction of apoptosis.

IFN-gamma and IL-10 mediate parasite-specific immune responses of cord blood cells induced by pregnancy-associated Plasmodium falciparum malaria.

Available evidence suggests that immune cells from neonates born to mothers with placental Plasmodium falciparum (Pf) infection are sensitized to parasite Ag in utero but have reduced ability to generate protective Th1 responses. In this study, we detected Pf Ag-specific IFN-gamma(+) T cells in cord blood from human neonates whose mothers had received treatment for malaria or who had active placental Pf infection at delivery, with responses being significantly reduced in the latter group. Active placental malaria at delivery was also associated with reduced expression of monocyte MHC class I and II in vivo and following short term in vitro coculture with Pf Ag compared with levels seen in neonates whose mothers had received treatment during pregnancy. Given that APC activation and Th1 responses are driven in part by IFN-gamma and down-regulated by IL-10, we examined the role of these cytokines in modulating the Pf Ag-specific immune responses in cord blood samples. Exogenous recombinant human IFN-gamma and neutralizing anti-human IL-10 enhanced T cell IFN-gamma production, whereas recombinant human IFN-gamma also restored MHC class I and II expression on monocytes from cord blood mononuclear cells cocultured with Pf Ag. Accordingly, active placental malaria at delivery was associated with increased frequencies of Pf Ag-specific IL-10(+)CD4(+) T cells in cord blood mononuclear cell cultures from these neonates. Generation and maintenance of IL-10(+) T cells in utero may thus contribute to suppression of APC function and Pf Ag-induced Th1 responses in newborns born to mothers with placental malaria at delivery, which may increase susceptibility to infection later in life.

Cellular and molecular requirements for the recall of IL-4-producing memory CD4(+)CD45RO(+)CD27(-) T cells during protection induced by attenuated Plasmodium falciparum sporozoites.

The requirements for maintenance of antigen (Ag)-specific memory T cells in protection to malaria is poorly understood. We have previously demonstrated a recall of IL-4-producing memory CD4(+)CD45RO(+) T cells with parasitized red blood cells (pRBC) in persons protected by radiation-attenuated Plasmodium falciparum sporozoites (gamma-spz). Using the CD27 marker, we have now identified two subsets of CD4(+)CD45RO(+) T cells: CD4(+)CD45RO(+)CD27(+) T cells representing an early memory and CD4(+)CD45RO(+)CD27() T cells representing a terminally differentiated memory cells. A small subset of CD4(+)CD45RO(+)CD27(-) T cells also expressed CD70, the CD27 ligand. The addition of anti-CD70 monoclonal antibody (mAb) to pRBC-stimulated cultures significantly inhibited the conversion of CD27(+) to CD27(-) subset without profoundly affecting IL-4 production. In contrast, the inclusion of anti-CD27 mAb in parallel cultures abrogated IL-4 production without interfering with conscription of T cells into the CD27(-) T cell set. We propose that the persistence of memory CD4(+) T cells depends on Ag-driven conscription of a mature memory phenotype through co-ligation of CD27 and CD70 expressed, respectively, on CD27(+) and CD27(-) T cells. Hence, protracted protection in malaria depends in part on memory CD4(+) T cells that require specific Ag presumably from the repositories of liver-and blood-stage antigens and the delivery of a second signal from the CD27:CD70 interaction.

Induction of T helper type 1 and 2 responses to 19-kilodalton merozoite surface protein 1 in vaccinated healthy volunteers and adults naturally exposed to malaria.

Plasmodium falciparum malaria is a major cause of death in the tropics. The 19-kDa subunit of P. falciparum merozoite surface protein 1 (MSP-1(19)), a major blood stage vaccine candidate, is the target of cellular and humoral immune responses in animals and humans. In this phase I trial of MSP-1(19), immunization of nonexposed human volunteers with either of the two allelic forms of recombinant MSP-1(19) induced high levels of antigen-specific Th1 (gamma interferon) and Th2 (interleukin 4 [IL-4] and IL-10) type lymphokines. The adjustment of the antigen dose and number of immunizations regulated the level of specificity of immune responses and Th1/Th2 bias of responses induced by vaccination. Novel conserved and allelic T-cell epitopes which induced cross-strain immune responses were identified. Importantly, responses to many of these novel epitopes were also present in adults exposed to malaria, both in east (Kenya) and west Africa (The Gambia). These data suggest that epitope-specific naturally acquired MSP-1(19) immune responses in endemic populations can be boosted by vaccination.