Dendritic cells amplify T cell-mediated immune responses in the central nervous system

Neuroinflammation often starts with the invasion of T lymphocytes into the CNS leading to recruitment of macrophages and amplification of inflammation. In this study, we show that dendritic cells (DCs) facilitate T-T cell help in the CNS and contribute to the amplification of local neuroinflammation. We adoptively transferred defined amounts of naive TCR-transgenic (TCR) recombination-activating gene-1-deficient T cells into another TCR-transgenic mouse strain expressing different Ag specificity. Following adoptive transfers, we coinjected DCs that presented one or multiple Ags into the brain and followed the activation of T cells with defined specificities simultaneously. Injection of DCs presenting both Ags simultaneously led to significantly higher infiltration of T cells into the brain compared with injection of a mixture of DCs pulsed with two Ags separately. DCs mediated either cooperative or competitive interactions between T cell populations with different specificities depending upon their MHC-restricting element usage. These results suggest that DC-mediated cooperation between brain-infiltrating T cells of different Ag specificities in the CNS plays an important role in regulation of neuroinflammation. This work also implies that blocking Ag-specific responses may block not only the targeted specificities, but may also effectively block their cooperative assistance to other T cells. Therefore, these data justify more attention to Ag-specific therapeutic approaches for neuroinflammation.     

Role of the C-C chemokine, TCA3, in the protective anticryptococcal cell-mediated immune response

Activated T lymphocytes play a crucial role in orchestrating cellular infiltration during a cell-mediated immune (CMI) reaction. TCA3, a C-C chemokine, is produced by Ag-activated T cells and is chemotactic for neutrophils and macrophages, two cell types in a murine CMI reaction. Using a gelatin sponge model for delayed-type hypersensitivity (DTH), we show that TCA3 is a component of the expression phase of an anticryptococcal CMI response in mice. TCA3 mRNA levels are augmented in anticryptococcal DTH reactions at the same time peak influxes of neutrophils and lymphocytes are observed. Neutralization of TCA3 in immunized mice results in reduced numbers of neutrophils and lymphocytes at DTH reaction sites. However, when rTCA3 is injected into sponges in naive mice, only neutrophils are attracted into the sponges, indicating TCA3 is chemotactic for neutrophils, but not lymphocytes. We show that TCA3 is indirectly attracting lymphocytes into DTH-reactive sponges by affecting at least one other chemokine that is chemotactic for lymphocytes. Of the two lymphocyte-attracting chemokines assessed, monocyte-chemotactic protein-1 and macrophage-inflammatory protein-1alpha (MIP-1alpha), only MIP-1alpha was reduced when TCA3 was neutralized, indicating that TCA3 affects the levels of MIP-1alpha, which attracts lymphocytes into the sponges. TCA3 also plays a role in protection against Cryptococcus neoformans in the lungs and brains of infected mice, as evidenced by the fact that neutralization of TCA3 results in increased C. neoformans CFU in those two organs.     

Oral beta-glucan adjuvant therapy converts nonprotective Th2 response to protective Th1 cell-mediated immune response in mammary tumor-bearing mice

Beta (1-3)-D-glucans were identified almost 40 years ago as biological response modifiers that stimulated tumor rejection. In vitro studies have shown that beta-glucans bind to a lectin domain within complement receptor type 3 (CR3), or to, more recently described dectin-1 a beta-glucan specific receptor, acting mainly on phagocytic cells. In this study, we assessed the intracellular cytokine profiles of peripheral blood lymphocytes from mice bearing mammary tumors receiving i.v. anti-tumor mAbs combined or not with whole glucan particle suspension given orally (WGP, 400 microg every 24 hours). The proportions of T cells producing IL-4 and IFNgamma were determined by flow cytometry. The proportion of T cells producing IL-4 was significantly higher in tumor-bearing mice not receiving beta-glucan-enhanced therapy. Conversely, T cells from mice undergoing beta-glucan-enhanced therapy showed increased production of the Th1 cytokine IFNgamma. The switch from a Th2 to a Th1 response after WGP therapy was possibly mediated by intestinal mucosal macrophages releasing IL-12.     

Interleukin 2 in cell-mediated immune responses

The lymphokine Interleukin 2 (IL2) restores T cell responses in a number of in vitro systems where immunogenicity has been compromised. UV irradiation of the stimulating allogeneic cells in a mixed leukocyte culture eliminates the production of cytotoxic T lymphocytes and greatly reduces the DNA synthesis response. IL2 restores both parameters. UV-irradiated stimulators are also unable to induce the normal production of IL2 which is observed in a mixed leukocyte culture. The cytotoxic activity of allogeneically stimulated thymocytes is almost completely lost within 24 hours after removal of IL2 at 5 days, indicating that the lymphokine is continuously required to maintain CTL. Thymocytes in 4-day cultures do not adsorb IL2 unless they are simultaneously activated with a mitogen. Finally, IL2 does not adequately restore a secondary response to the purified protein derivative of tuberculin (PPD) in adherent-cell-depleted cultures, indicating that macrophages, in addition to being required for IL2 production, have other functions. These probably include the presentation of soluble antigens to responding cells.     

Dendritic cell-mediated induction of mucosal cytotoxic responses following intravaginal immunization with the nontoxic B subunit of cholera toxin

The use of the nontoxic B subunit of cholera toxin (CTB) as mucosal adjuvant and carrier-delivery system for inducing secretory Ab responses has been documented previously with different soluble Ags. In this study, we have evaluated this approach for inducing CTL responses against a prototype Ag, OVA, in the female genital mucosa. We report here the ability of an immunogen comprised of CTB conjugated to OVA (CTB-OVA) given by intravaginal (ivag) route to induce genital OVA-specific CTLs in mice. Using adoptive transfer models, we demonstrate that ivag application of CTB-OVA activates OVA-specific IFN-gamma-producing CD4 and CD8 T cells in draining lymph nodes (DLN). Moreover, ivag CTB induces an expansion of IFN-gamma-secreting CD8+ T cells in DLN and genital mucosa and promotes Ab responses to OVA. In contrast, ivag administration of OVA alone or coadministered with CTB failed to induce such responses. Importantly, we demonstrate that ivag CTB-OVA generates OVA-specific CTLs in DLN and the genital mucosa. Furthermore, genital CD11b+ CD11c+ dendritic cells (DCs), but not CD8+ CD11c+ or CD11c- APCs, present MHC class I epitopes acquired after ivag CTB-OVA, suggesting a critical role of this DC subset in the priming of genital CTLs. Inhibition studies indicate that the presentation of OVA MHC class I epitopes by DCs conditioned with CTB-OVA involves a proteasome-dependent and chloroquine-sensitive mechanism. These results demonstrate that CTB is an efficient adjuvant-delivery system for DC-mediated induction of genital CTL responses and may have implications for the design of vaccines against sexually transmitted infections.     

Vehicles for genetic vaccines against human immunodeficiency virus: induction of T cell-mediated immune responses

Success of a candidate vaccine against human immunodeficiency virus (HIV) depends on the type, site, strength, longevity and specificity of the immune responses it induces. The specificity of a vaccine is determined by the HIV-derived immunogens it employs in its formulation. Central to the other features is a correct and efficient delivery of the immunogens to the relevant cells of the immune system, which leads to orchestrated actions of millions of cells of several types and functions at multiple sites in the body. Thus, for elicitation of cytotoxic T lymphocyte responses, immunogens have to be delivered to the so called 'professional' antigen-presenting cells in a way that leads to a specific activation and expansion of na?ve or precursor T cells, subsequent maturation of effector functions and, importantly, generation of a potent immunological memory. Many aspects of theseprocesses are currently unknown. However, it is very likely that the immunogenicities of genetic vaccines, i.e. vaccines delivering genes coding for immunogens rather than purified possibly adjuvanted proteins or peptides themselves, are in great part determined by the choice of vaccine vehicles and route of administration. In addition, vaccine immunogenicities can be augmented semi-rationally by immunogen engineering and co-delivering immunomodulatory molecules, and empirically by combining different vehicles expressing the same immunogen in heterologous prime-boost protocols. In any case, a successful vaccination strategy against HIV as well as other chronic viral infections has to elicit better immune responses than the natural infections do.     

Dendritic cells in intestinal immune regulation

A breakdown in intestinal homeostasis can result in chronic inflammatory diseases of the gut including inflammatory bowel disease, coeliac disease and allergy. Dendritic cells, through their ability to orchestrate protective immunity and immune tolerance in the host, have a key role in shaping the intestinal immune response. The mechanisms through which dendritic cells can respond to environmental cues in the intestine and select appropriate immune responses have until recently been poorly understood. Here, we review recent work that is beginning to identify factors responsible for intestinal conditioning of dendritic-cell function and the subsequent decision between tolerance and immunity in the intestine.     

Dendritic cells capture killed tumor cells and present their antigens to elicit tumor-specific immune responses

Due to their capacity to induce primary immune responses, dendritic cells (DC) are attractive vectors for immunotherapy of cancer. Yet the targeting of tumor Ags to DC remains a challenge. Here we show that immature human monocyte-derived DC capture various killed tumor cells, including Jurkat T cell lymphoma, malignant melanoma, and prostate carcinoma. DC loaded with killed tumor cells induce MHC class I- and class II-restricted proliferation of autologous CD8+ and CD4+ T cells, demonstrating cross-presentation of tumor cell-derived Ags. Furthermore, tumor-loaded DC elicit expansion of CTL with cytotoxic activity against the tumor cells used for immunization. CTL elicited by DC loaded with the PC3 prostate carcinoma cell bodies kill another prostate carcinoma cell line, DU145, suggesting recognition of shared Ags. Finally, CTL elicited by DC loaded with killed LNCap prostate carcinoma cells, which express prostate specific Ag (PSA), are able to kill PSA peptide-pulsed T2 cells. This demonstrates that induced CTL activity is not only due to alloantigens, and that alloantigens do not prevent the activation of T cells specific for tumor-associated Ags. This approach opens the possibility of using allogeneic tumor cells as a source of tumor Ag for antitumor therapies.     

Dendritic cells and C-type lectin receptors: coupling innate to adaptive immune responses

Dendritic cells (DCs) have an important function in the initiation and differentiation of immune responses, linking innate information to tailored adaptive responses. Depending on the pathogen invading the body, specific immune responses are built up that are crucial for eliminating the pathogen from the host. Host recognition of invading microorganisms relies on evolutionarily ancient, germline-encoded pattern recognition receptors (PRRs) that are highly expressed on the cell surface of DCs, of which the Toll-like receptors (TLRs) are well characterized and recognize bacterial or viral components. Moreover, they bind a variety of self-proteins released from damaged tissues including several heat-shock proteins. The membrane-associated C-type lectin receptors (CLRs) recognize glycan structures expressed by host cells of the immune system or on specific tissues, which upon recognition allow cellular interactions between DCs and other immune or tissue cells. In addition, CLRs can function as PRRs. In contrast to TLRs, CLRs recognize carbohydrate structures present on the pathogens. Modification of glycan structures on pathogens to mimic host glycans can thereby alter CLR interactions that subsequently modifies DC-induced polarization. In this review, we will discuss in detail how specific glycosylation of antigens can dictate both the innate and adaptive interactions that are mediated by CLRs on DCs and how this balances immune activation and inhibition of DC function.     

Dendritic cells pulsed with live and dead Legionella pneumophila elicit distinct immune responses

Legionella pneumophila is the causative pathogen of Legionnaires' disease, which is characterized by severe pneumonia. In regard to the pathophysiology of Legionella infection, the role of inflammatory phagocytes such as macrophages has been well documented, but the involvement of dendritic cells (DCs) has not been clarified. In this study, we have investigated the immune responses that DCs generate in vitro and in vivo after contact with L. pneumophila. Heat- and formalin-killed L. pneumophila, but not live L. pneumophila, induced immature DCs to undergo similar phenotypic maturation, but the secreted proinflammatory cytokines showed different patterns. The mechanisms of the DC maturation by heat- or formalin-killed L. pneumophila depended, at least in part, on Toll-like receptor 4 signaling or on Legionella LPS, respectively. After transfer to naive mice, DCs pulsed with dead Legionella produced serum Ig isotype responses specific for Legionella, leading to protective immunity against an otherwise lethal respiratory challenge with L. pneumophila. The in vivo immune responses required the Ag presentation of DCs, especially that on MHC class II molecules, and the immunity yielded cross-protection between clinical and environmental strains of L. pneumophila. Although the DC maturation was impaired by live Legionella, macrophages were activated by live as well as dead L. pneumophila, as evidenced by the up-regulation of MHC class II. Finally, DCs, but not macrophages, exhibited a proliferative response to live L. pneumophila that was consistent with their cell cycle progression. These findings provide a better understanding of the role of DCs in adaptive immunity to Legionella infection.     

Dendritic cell editing by activated natural killer cells results in a more protective cancer-specific immune response

Over the last decade, several studies have extensively reported that activated natural killer (NK) cells can kill autologous immature dendritic cells (DCs) in vitro, whereas they spare fully activated DCs. This led to the proposal that activated NK cells might select a more immunogenic subset of DCs during a protective immune response. However, there is no demonstration that autologous DC killing by NK cells is an event occurring in vivo and, consequently, the functional relevance of this killing remains elusive. Here we report that a significant decrease of CD11c(+) DCs was observed in draining lymph nodes of mice inoculated with MHC-devoid cells as NK cell targets able to induce NK cell activation. This in vivo DC editing by NK cells was perforin-dependent and it was functionally relevant, since residual lymph node DCs displayed an improved capability to induce T cell proliferation. In addition, in a model of anti-cancer vaccination, the administration of MHC-devoid cells together with tumor cells increased the number of tumor-specific CTLs and resulted in a significant increase in survival of mice upon challenge with a lethal dose of tumor cells. Depletion of NK cells or the use of perforin knockout mice strongly decreased the tumor-specific CTL expansion and its protective role against tumor cell challenge. As a whole, our data support the hypothesis that NK cell-mediated DC killing takes place in vivo and is able to promote expansion of cancer-specific CTLs. Our results also indicate that cancer vaccines could be improved by strategies aimed at activating NK cells.     

Plasmacytoid dendritic cells synergize with myeloid dendritic cells in the induction of antigen-specific antitumor immune responses

Plasmacytoid dendritic cells (pDC) are capable of producing high levels of type I IFNs upon viral stimulation, and play a central role in modulating innate and adaptive immunity against viral infections. Whereas many studies have assessed myeloid dendritic cells (mDC) in the induction of antitumor immune responses, the role of pDC in antitumor immunity has not been addressed. Moreover, the interaction of pDC with other dendritic cell subsets has not been evaluated. In this study, we analyzed the capacity of pDC in stimulating an Ag-specific T cell response. Immunization of mice with Ag-pulsed, activated pDC significantly augmented Ag-specific CD8(+) CTL responses, and protected mice from a subsequent tumor challenge. Immunization with a mixture of activated pDC plus mDC resulted in increased levels of Ag-specific CD8(+) T cells and an enhanced antitumor response compared with immunization with either dendritic cell subset alone. Synergy between pDC and mDC in their ability to activate T cells was dependent on MHC I expression by mDC, but not pDC, suggesting that pDC enhanced the ability of mDC to present Ag to T cells. Our results demonstrate that pDC and mDC can interact synergistically to induce an Ag-specific antitumor immune response in vivo.     

Dendritic cells and cytokines in immune rejection of cancer

Dendritic cells (DCs) play a crucial role in linking innate and adaptive immunity and, thus, in the generation of a protective immune response against both infectious diseases and tumors. The ability of DCs to prime and expand an immune response is regulated by signals acting through soluble mediators, mainly cytokines and chemokines. Understanding how cytokines influence DC functions and orchestrate the interactions of DCs with other immune cells is strictly instrumental to the progress in cancer immunotherapy. Herein, we will illustrate how certain cytokines and immune stimulating molecules can induce and sustain the antitumor immune response by acting on DCs. We will also discuss these cytokine-DC interactions in the light of clinical results in cancer patients.     

Evasion from NK cell-mediated immune responses by HIV-1

Human immunodeficiency virus type 1 (HIV-1) mostly owes its success to its ability to evade host immune responses. Understanding viral immune escape mechanisms is a prerequisite to improve future HIV-1 vaccine design. This review focuses on the strategies that HIV-1 has evolved to evade recognition by natural killer (NK) cells.     

Spectrum of in vivo and in vitro cell-mediated immune responses in coccidioidomycosis.

The cell-mediated immune responses of 12 healthy, coccidioidin skin-test positive subjects (Group I) were compared with those of 15 healthy, coccidioidin skin-test positive persons who had primary asymptomatic coccidiodomycosis, (Group II), 12 patients with active, pulmonary coccidioidomycosis (Group III), four patients with disseminated disease (Group IV), and five patients who had been in clinical remission for 1 year or longer (Group V). Lymphocytes from healthy subjects in Groups I and II responded in vitro to Coccidioides immitis antigen by undergoing an increased DNA synthesis (lymphocyte transformation) and/or by producing macrophage migration inhibitory factor (MIF). In contrast, patients in Groups III and IV failed to respond to Coccidioides antigens in vivo (skin tests) or in vitro (lymphocyte transformation and production of MIF). The responses of subjects in Group V with inactive disease fell in between those of healthy donors in Groups I and II and patients in Groups III and IV. The cellular immune defect, in terms of antigen recognition, appeared to be specific for C. immitis in all but one patient.     

Enhancement of in vivo cell-mediated immune responses by three distinct cytokines

The ability of recombinant/purified cytokines to augment delayed-type hypersensitivity (DTH) responses was investigated. Suboptimal doses of haptenized SC were treated in vitro with purified or recombinant derived cytokines and tested for their ability to enhance DTH in vivo. With the use of this protocol, it was shown that both human and mouse rIL-6, as well as mouse rTNF-alpha, potentiated DTH in a dose-dependent manner. In accordance with these data, IL-6/TNF-alpha-containing supernatant from long term nonlymphoid cell lines also possessed the ability to augment DTH. By using the same protocol, we have also identified T cell hybridomas that produce DTH-augmenting activity constitutively. The hybridoma-derived factor, termed the T cell enhancing factor (TCEF), was functionally distinguishable from the defined cytokines IL-1 through IL-6, IFN-gamma, and TNF by bioassay. Furthermore, RNA derived from the hybridoma failed to hybridize with cDNA probes specific for IL-1 to IL-6, IFN-gamma, TNF-alpha, and granulocyte-macrophage CSF. Further characterization of the serum-free conditioned media derived from the hybridoma indicated that the TCEF was a soluble acid labile glycoprotein (Mr greater than 30,000). Finally, we investigated the cellular requirements for DTH augmentation by IL-6, TNF-alpha, and TCEF; all are dependent upon the presence of T cells in the immunizing inoculum. We propose that these cytokines play a critical role in the development of DTH responses in vivo.     

CpG-containing oligonucleotides are efficient adjuvants for induction of protective antiviral immune responses with T-cell peptide vaccines

Synthetic nonmethylated oligonucleotides containing CpG dinucleotides (CpG-ODNs) have been shown to exhibit immunostimulatory activity. CpG-ODNs have the capacity to directly activate B cells, macrophages, and dendritic cells, and we show here that this is reflected by cell surface binding of oligonucleotides to these cell subsets. However, T cells are not directly activated by CpG-ODNs, which correlates with the failure to bind to the T-cell surface. Efficient competition for CpG-induced B-cell activation by non-CpG-containing oligonucleotides suggests that oligonucleotides might bind to an as yet undefined sequence-nonspecific receptor prior to cellular activation. Induction of protective T-cell responses against challenge infection with lymphocytic choriomeningitis virus (LCMV) or with recombinant vaccinia virus expressing the LCMV glycoprotein was achieved by immunizing mice with the immunodominant major histocompatibility complex class I-binding LCMV glycoprotein-derived peptide gp33 together with CpG-ODNs. In these experiments, B cells, potentially serving as CpG-ODN-activated antigen-presenting cells (APCs), were not required for induction of protective immunity since CpG-ODN-gp33-immunized B-cell-deficient mice were equally protected against challenge infection with both viruses. This finding suggested that macrophages and/or dendritic cells were sufficiently activated in vivo by CpG-ODNs to serve as potent APCs for the induction of naive T cells. Furthermore, treatment with CpG-ODN alone induced protection against infection with Listeria monocytogenes via antigen-independent activation of macrophages. These data suggest that CpG activation of macrophages and dendritic cells may provide a critical step in CpG-ODN adjuvant activity.     

Dendritic cells and their role in immune reactions of atherosclerosis

Dendritic cells were discovered and recognized as antigen-presenting cells in 1973. Since then, a large volume of information has been accumulated showing the role of dendritic cells as a key element connecting the innate and adaptive immunity. Today, dendritic cells are considered to be dedicated sensors of the immune system that are capable of recognizing both antigen amounts and antigen persistence via complex mechanisms that involve decoding and integration of various signals received in a receptor-dependant manner. The tissue microenvironment plays an important role in the modulation of effector functions of dendritic cells, inducing either activation or suppression of immune reactions. Dendritic cells maintain homeostasis and are involved in a number of diseases, including infectious diseases and cancer. The presence of dendritic cells in arteries was reported in 1995, and, since then, the involvement of dendritic cells in atherogenesis has been evaluated. This review briefly describes the current knowledge of dendritic cells and their role in atherosclerosis.     

Dirofilaria immitis: cell-mediated and humoral immune responses in experimentally-infected dogs.

Four dogs were experimentally infected with 30 Dirofilaria immitis infective larvae, four dogs received two such infections and four dogs served as uninfected controls. A partially-purified D. immitis antigen was used in an indirect hemagglutination assay to determine anti-D. immitis antibody titers. Anti-D. immitis antibody was first detected in infected dogs 4 weeks after infection. Titers were highest 2 weeks after the appearance of microfilariae and diminished to low levels thereafter in the single infection group. Antibody levels in the double infection group decreased similarly but were demonstrable throughout the study. Antibody titers were significantly higher in the infected dogs, but there were no differences in titers between single and double infection groups.The responses of peripheral lymphocytes to phytohemagglutinin P and pokeweed mitogen were significantly depressed in infected dogs. Peripheral blood lymphocyte transformation could not be induced with D. immitis antigens. Differences between groups in T-cell function were not demonstrated by total hemagglutinating antibody or 2-mercaptoethanol labile hemagglutinating antibody following immunization with sheep erythrocytes.     

Dendritic cells directly trigger NK cell functions: cross-talk relevant in innate anti-tumor immune responses in vivo

Cytotoxic T lymphocytes and natural killer cells are essential effectors of anti-tumor immune responses in vivo. Dendritic cells (DC) 'prime' tumor antigen-specific cytotoxic T lymphocytes; thus, we investigated whether DC might also trigger the innate, NK cell-mediated anti-tumor immunity. In mice with MHC class I-negative tumors, adoptively transferred- or Flt3 ligand-expanded DC promoted NK cell-dependent anti-tumor effects. In vitro studies demonstrated a cell-to-cell contact between DC and resting NK cells that resulted in a substantial increase in both NK cell cytolytic activity and IFN-gamma production. Thus, DC are involved in the interaction between innate and adaptive immune responses.