In vivo targeting of DC-SIGN-positive antigen-presenting cells in a nonhuman primate model

Canvassing Prospects of Glyco-Nanovaccines for Developing Cross-Presentation Mediated Anti-Tumor Immunotherapy

In view of the severe downsides of conventional cancer therapies, the quest of developing alternative strategies still remains of critical importance. In this regard, antigen cross-presentation, usually employed by dendritic cells (DCs), has been recognized as a potential solution to overcome the present impasse in anti-cancer therapeutic strategies. It has been established that an elevated cytotoxic T lymphocyte (CTL) response against cancer cells can be achieved by targeting receptors expressed on DCs with specific ligands. Glycans are known to serve as ligands for C-type lectin receptors (CLRs) expressed on DCs, and are also known to act as a tumor-associated antigen (TAA), and, thus, can be harnessed as a potential immunotherapeutic target. In this scenario, integrating the knowledge of cross-presentation and glycan-conjugated nanovaccines can help us to develop so called 'glyco-nanovaccines' (GNVs) for targeting DCs. Here, we briefly review and analyze the potential of GNVs as the next-generation anti-tumor immunotherapy. We have compared different antigen-presenting cells (APCs) for their ability to cross-present antigens and described the potential nanocarriers for tumor antigen cross-presentation. Further, we discuss the role of glycans in targeting of DCs, the immune response due to pathogens, and imitative approaches, along with parameters, strategies, and challenges involved in cross-presentation-based GNVs for cancer immunotherapy. It is known that the effectiveness of GNVs in eradicating tumors by inducing strong CTL response in the tumor microenvironment (TME) has been largely hindered by tumor glycosylation and the expression of different lectin receptors (such as galectins) by cancer cells. Tumor glycan signatures can be sensed by a variety of lectins expressed on immune cells and mediate the immune suppression which, in turn, facilitates immune evasion. Therefore, a sound understanding of the glycan language of cancer cells, and glycan-lectin interaction between the cancer cells and immune cells, would help in strategically designing the next-generation GNVs for anti-tumor immunotherapy.

Therapeutic Interventions Targeting Innate Immune Receptors: A Balancing Act

The innate immune system is an organism's first line of defense against an onslaught of internal and external threats. The downstream adaptive immune system has been a popular target for therapeutic intervention, while there is a relative paucity of therapeutics targeting the innate immune system. However, the innate immune system plays a critical role in many human diseases, such as microbial infection, cancer, and autoimmunity, highlighting the need for ongoing therapeutic research. In this review, we discuss the major innate immune pathways and detail the molecular strategies underpinning successful therapeutics targeting each pathway as well as previous and ongoing efforts. We will also discuss any recent discoveries that could inform the development of novel therapeutic strategies. As our understanding of the innate immune system continues to develop, we envision that therapies harnessing the power of the innate immune system will become the mainstay of treatment for a wide variety of human diseases.

Glycan-Lectin Interactions in Cancer and Viral Infections and How to Disrupt Them

Glycan-lectin interactions play an essential role in different cellular processes. One of their main functions is involvement in the immune response to pathogens or inflammation. However, cancer cells and viruses have adapted to avail themselves of these interactions. By displaying specific glycosylation structures, they are able to bind to lectins, thus promoting pathogenesis. While glycan-lectin interactions promote tumor progression, metastasis, and/or chemoresistance in cancer, in viral infections they are important for viral entry, release, and/or immune escape. For several years now, a growing number of investigations have been devoted to clarifying the role of glycan-lectin interactions in cancer and viral infections. Various overviews have already summarized and highlighted their findings. In this review, we consider the interactions of the lectins MGL, DC-SIGN, selectins, and galectins in both cancer and viral infections together. A possible transfer of ways to target and disrupt them might lead to new therapeutic approaches in different pathological backgrounds.

Various Tastes of Sugar: The Potential of Glycosylation in Targeting and Modulating Human Immunity via C-Type Lectin Receptors

C-type lectin receptors (CLRs) are important in several immune regulatory processes. These receptors recognize glycans expressed by host cells or by pathogens. Whereas pathogens are recognized through their glycans, which leads to protective immunity, aberrant cellular glycans are now increasingly recognized as disease-driving factors in cancer, auto-immunity, and allergy. The vast variety of glycan structures translates into a wide spectrum of effects on the immune system ranging from immune suppression to hyper-inflammatory responses. CLRs have distinct expression patterns on antigen presenting cells (APCs) controlling their role in immunity. CLRs can also be exploited to selectively target specific APCs, modulate immune responses and enhance antigen presentation. Here we will discuss the role of glycans and their receptors in immunity as well as potential strategies for immune modulation. A special focus will be given to different dendritic cell subsets as these APCs are crucial orchestrators of immune responses in infections, cancer, auto-immunity and allergies. Furthermore, we will highlight the potential use of nanoscale lipid bi-layer structures (liposomes) in targeted immunotherapy.

Targeting C-type lectin receptors: a high-carbohydrate diet for dendritic cells to improve cancer vaccines

There is a growing understanding of why certain patients do or do not respond to checkpoint inhibition therapy. This opens new opportunities to reconsider and redevelop vaccine strategies to prime an anticancer immune response. Combination of such vaccines with checkpoint inhibitors will both provide the fuel and release the brake for an efficient anticancer response. Here, we discuss vaccine strategies that use C-type lectin receptor (CLR) targeting of APCs, such as dendritic cells and macrophages. APCs are a necessity for the priming of antigen-specific cytotoxic and helper T cells. Because CLRs are natural carbohydrate-recognition receptors highly expressed by multiple subsets of APCs and involved in uptake and processing of Ags for presentation, these receptors seem particularly interesting for targeting purposes.

Influence of Core β-1,2-Xylosylation on Glycoprotein Recognition by Murine C-type Lectin Receptors and Its Impact on Dendritic Cell Targeting

Targeting antigens to dendritic cell subsets is a promising strategy to enhance the efficacy of vaccines. C-type lectin receptors (CLRs) expressed by dendritic cells are particularly attractive candidates since CLR engagement may promote cell uptake and may further stimulate antigen presentation and subsequent T cell activation. While most previous approaches have involved antibody-mediated CLR-targeting, glycan-based CLR targeting has become more and more attractive in recent years. In the present study, we show that small structural glycan modifications may markedly influence CLR recognition, dendritic cell targeting, and subsequent T cell activation. A biantennary N-glycan (G0) and its analogous O-2 core xylosylated N-glycan (XG0) were synthesized, covalently conjugated to the model antigen ovalbumin, and analyzed for binding to a set of murine CLR-Fc fusion proteins using lectin microarray. To evaluate whether the differential binding of G0 and XG0 to CLRs impacted dendritic cell targeting, uptake studies using murine dendritic cells were performed. Finally, effects of the ovalbumin glycoconjugates on T cell activation were measured in a dendritic cell/T cell cocultivation assay. Our results highlight the utility of glycan-based dendritic cell targeting and demonstrate that small structural differences may have a major impact on dendritic cell targeting efficacy.

Immunity, immunopathology, and human vaccine development against sexually transmitted Chlamydia trachomatis

This review examines the immunity, immunopathology, and contemporary problems of vaccine development against sexually transmitted Chlamydia trachomatis. Despite improved surveillance and treatment initiatives, the incidence of C. trachomatis infection has increased dramatically over the past 30 years in both the developed and developing world. Studies in animal models have shown that protective immunity to C. trachomatis is largely mediated by Th1 T cells producing IFN-γ which is needed to prevent dissemination of infection. Similar protection appears to develop in humans but in contrast to mice, immunity in humans may take years to develop. Animal studies and evidence from human infection indicate that immunity to C. trachomatis is accompanied by significant pathology in the upper genital tract. Although no credible evidence is currently available to indicate that autoimmunity plays a role, nevertheless, this underscores the necessity to design vaccines strictly based on chlamydial-specific antigens and to avoid those displaying even minimal sequence homologies with host molecules. Current advances in C. trachomatis vaccine development as well as alternatives for designing new vaccines for this disease are discussed. A novel approach for chlamydia vaccine development, based on targeting endogenous dendritic cells, is described.

Direct Delivery of Antigens to Dendritic Cells via Antibodies Specific for Endocytic Receptors as a Promising Strategy for Future Therapies

Dendritic cells (DCs) are the most potent professional antigen presenting cells and are therefore indispensable for the control of immunity. The technique of antibody mediated antigen targeting to DC subsets has been the basis of intense research for more than a decade. Many murine studies have utilized this approach of antigen delivery to various kinds of endocytic receptors of DCs both in vitro and in vivo. Today, it is widely accepted that different DC subsets are important for the induction of select immune responses. Nevertheless, many questions still remain to be answered, such as the actual influence of the targeted receptor on the initiation of the immune response to the delivered antigen. Further efforts to better understand the induction of antigen-specific immune responses will support the transfer of this knowledge into novel treatment strategies for human diseases. In this review, we will discuss the state-of-the-art aspects of the basic principles of antibody mediated antigen targeting approaches. A table will also provide a broad overview of the latest studies using antigen targeting including addressed DC subset, targeted receptors, outcome, and applied coupling techniques.

Trial watch: Dendritic cell-based anticancer therapy

The use of patient-derived dendritic cells (DCs) as a means to elicit therapeutically relevant immune responses in cancer patients has been extensively investigated throughout the past decade. In this context, DCs are generally expanded, exposed to autologous tumor cell lysates or loaded with specific tumor-associated antigens (TAAs), and then reintroduced into patients, often in combination with one or more immunostimulatory agents. As an alternative, TAAs are targeted to DCs in vivo by means of monoclonal antibodies, carbohydrate moieties or viral vectors specific for DC receptors. All these approaches have been shown to (re)activate tumor-specific immune responses in mice, often mediating robust therapeutic effects. In 2010, the first DC-based preparation (sipuleucel-T, also known as Provenge®) has been approved by the US Food and Drug Administration (FDA) for use in humans. Reflecting the central position occupied by DCs in the regulation of immunological tolerance and adaptive immunity, the interest in harnessing them for the development of novel immunotherapeutic anticancer regimens remains high. Here, we summarize recent advances in the preclinical and clinical development of DC-based anticancer therapeutics.

Antibodies and carbohydrate ligands binding to DC-SIGN differentially modulate receptor trafficking

DCs are regarded as key APCs that initiate humoral and cellular immune responses. Consequently, targeted delivery of Ag toward DC-specific receptors enhances vaccine efficacy. DC-SIGN is a C-type lectin receptor that facilitates DC-specific delivery of Ag. This is accomplished by conjugating Ag to receptor-specific Ab or carbohydrate ligands that bind to its carbohydrate recognition domain. Here, we investigated the fate of DC-SIGN following receptor triggering with Ab. Both whole and single-chain Ab induced rapid internalization of about half of the surface receptor molecules. Biochemical studies showed that about half of the receptor molecules were still intracellular after 3 h, while minimal or no resurfacing of internalized or newly synthesized unbound DC-SIGN molecules was observed. Prolonged exposure of DCs to DC-SIGN Ab, but not carbohydrate ligands, resulted in reduced receptor expression levels, which lasted up to 2 days following removal of the Ab. In addition, exposure to DC-SIGN Ab reduced the ability of the receptor to internalize. Consequently, DC-SIGN showed a poor ability to accumulate targeting Abs within DCs. Vaccine efficacy may therefore be enhanced by strategies increasing the amount of Ag entering via a single receptor molecule, such as the use of targeting moieties allowing DC-SIGN recycling or Ab-coated vaccine carriers.

A simple approach for enhanced immune response using engineered dendritic cell targeted nanoparticles

In this study, we demonstrate a simple strategy for enhanced immune response using a two-component dendritic cell (DC) targeted antigen delivery system. One component consists of a recombinant bifunctional fusion protein (bfFp) used for DC targeting, whereas, the other component is made of biotinylated PLGA nanoparticles that encapsulate the antigen. The fusion protein (bfFp) made of a truncated core-streptavidin fused to anti-DEC-205 single chain antibody (scFv) was mixed with ovalbumin-loaded biotinylated NPs that were formulated using biotin-PEG (2000)-PLGA, and the combination, bfFp functionalized NPs was used for DC targeted antigen delivery. In vitro DC uptake studies revealed a 2-fold higher receptor-mediated uptake of bfFp functionalized NPs when compared to non-targeted NPs. Immunization of the mice with the bfFp functionalized NPs in conjunction with DC maturation stimulus (anti-CD40 mAb) enhanced OVA-specific IgG and IgG subclass responses. Splenocytes of these mice secreted significantly higher levels of Th1 (IFN-γ and IL-2) cytokines upon ex vivo restimulation with OVA. The promising outcomes of the bfFp functionalized DC targeted system support its use as a versatile vaccine delivery system for the design of monovalent or polyvalent vaccines.

Targeting Dendritic Cells in vivo for Cancer Therapy

Monoclonal antibodies that recognize cell surface molecules have been used deliver antigenic cargo to dendritic cells (DC) for induction of immune responses. The encouraging anti-tumor immunity elicited using this immunization strategy suggests its suitability for clinical trials. This review discusses the complex network of DC, the functional specialization of DC subsets, the immunological outcomes of targeting different DC subsets and their cell surface receptors, and the requirements for the induction of effective anti-tumor CD4 and CD8 T cell responses that can recognize tumor-specific antigens. Finally, we review preclinical experiments and the progress toward targeting human DC in vivo.

Boosting antibody responses by targeting antigens to dendritic cells

Delivering antigens directly to dendritic cells (DCs) in situ, by injecting antigens coupled to antibodies specific for DC surface molecules, is a promising strategy for enhancing vaccine efficacy. Enhanced cytotoxic T cell responses are obtained if an adjuvant is co-administered to activate the DC. Such DC targeting is also effective at enhancing humoral immunity, via the generation of T follicular helper cells. Depending on the DC surface molecule targeted, antibody production can be enhanced even in the absence of adjuvants. In the case of Clec9A as the DC surface target, enhanced antibody production is a consequence of the DC-restricted expression of the target molecule. Few other cells absorb the antigen-antibody construct, therefore, it persists in the bloodstream, allowing sustained antigen presentation, even by non-activated DCs.

Non-human primate dendritic cells

Non-human primates (NHP) are essential translational models for biomedical research. Dendritic cells (DC) are a group of antigen presenting cells (APC) that play pivotal roles in the immunobiology of health and disease and are attractive cells for adoptive immunotherapy to stimulate and suppress immunity. DC have been studied extensively in humans and mice but until recently, have not been well characterized in NHP. This review considers the available data about DC across a range of NHP species and summarizes the understanding of in vitro-propagated DC and in vivo-isolated DC, which is now established. It is clear that although NHP DC exist within the paradigm of human DC, there are important functional and phenotypic differences when compared with human DC subsets. These differences need to be taken into account when designing preclinical, translational studies of DC therapy using NHP models.

The influence of PEG chain length and targeting moiety on antibody-mediated delivery of nanoparticle vaccines to human dendritic cells

Targeted delivery of nanoparticles (NPs) carrying vaccine components to dendritic cells (DCs) is a promising strategy to initiate antigen-specific immune responses. Improving the interactions between nanoparticle-carried ligands and receptors on DCs is a major challenge. These NPs are generally coated with poly(ethylene glycol) (PEG), to shield non-specific interactions, and antibodies, to facilitate specific delivery to DC surface receptors. We have devised a strategy to covalently link PEG molecules of various chain length (Mw 2000-20000 g/moL) to poly(lactic-co-)glycolic acid (PLGA) NP vaccines. We coated these NPs with various antibodies recognizing the DC-specific receptor DC-SIGN to study the effects of shielding and antibody type on antibody--receptor interactions. Chemical attachment of PEG to the particle surface was followed by detailed zeta potential, DLS and NMR studies, and analyzed by analytical chemistry. Increasing the PEG chain length increased particle size and polydispersity index and reduced the intracellular degradation rate of encapsulated antigens. Binding and uptake of NPs by human DCs was affected by both PEG chain length and antibody type. NPs coated with PEG-3000 had the optimal chain length for antibody--receptor interactions and induction of antigen-specific T-cell responses. Interestingly, clear differences were observed upon targeting distinct epitopes of the same receptor. Binding and uptake of NPs carrying antibodies recognizing the carbohydrate recognition domain of DC-SIGN was enhanced when compared to those carrying antibodies recognizing the receptor's neck region. In conclusion, our data show that PEG chains cannot be extended beyond a certain length for shielding purposes without compromising the efficacy of targeted delivery. Thereby, the implications of our findings are not limited to the future design of nanovaccines specifically targeted to DC-SIGN, but apply to the general design of targeted nanocarriers.

Targeted antigen delivery and activation of dendritic cells in vivo: steps towards cost effective vaccines

During the past decade, the immunotherapeutic potential of ex vivo generated professional antigen presenting dendritic cells (DCs) has been explored in the clinic. Albeit safe, clinical results have thus far been limited. A major disadvantage of current cell-based dendritic cell (DC) therapies, preventing universal implementation of this form of immunotherapy, is the requirement that vaccines need to be tailor made for each individual. Targeted delivery of antigens to DC surface receptors in vivo would circumvent this laborious and expensive ex vivo culturing steps involved with these cell-based therapies. In addition, the opportunity to target natural and often rare DC subsets in vivo might have advantages over loading more artificial ex vivo cultured DCs. Preclinical studies show targeting antigens to DCs effectively induces humoral responses, while cellular responses are induced provided a DC maturation or activation stimulus is co-administered. Here, we discuss strategies to target antigens to distinct DC subsets and to simultaneously employ adjuvants to activate these cells to induce immunity.

Efficient generation of a monoclonal antibody against the human C-type lectin receptor DCIR by targeting murine dendritic cells

Dendritic cells (DCs) are very important for the generation of long lasting immune responses against pathogens or the induction of anti-tumor responses. Targeting antigen to dendritic cells via monoclonal antibodies specific for DC cell surface receptors such as DEC205 was shown to elicit potent cellular and humoral immune responses in vivo. Therefore, we investigated whether this novel strategy might also be useful for the generation of new monoclonal antibodies against molecules of choice. We show, that by targeting the extracellular domain of the human C-type lectin receptor ClecSF6/DCIR/LLIR (hDCIR) to DEC205 on DCs in vivo, we were able to generate highly specific monoclonal antibodies against hDCIR.

Redirecting lentiviral vectors pseudotyped with Sindbis virus-derived envelope proteins to DC-SIGN by modification of N-linked glycans of envelope proteins

Redirecting the tropism of viral vectors enables specific transduction of selected cells by direct administration of vectors. We previously developed targeting lentiviral vectors by pseudotyping with modified Sindbis virus envelope proteins. These modified Sindbis virus envelope proteins have mutations in their original receptor-binding regions to eliminate their natural tropisms, and they are conjugated with targeting proteins, including antibodies and peptides, to confer their tropisms on target cells. We investigated whether our targeting vectors interact with DC-SIGN, which traps many types of viruses and gene therapy vectors by binding to the N-glycans of their envelope proteins. We found that these vectors do not interact with DC-SIGN. When these vectors were produced in the presence of deoxymannojirimycin, which alters the structures of N-glycans from complex to high mannose, these vectors used DC-SIGN as their receptor. Genetic analysis demonstrated that the N-glycans at E2 amino acid (aa) 196 and E1 aa 139 mediate binding to DC-SIGN, which supports the results of a previous report of cryoelectron microscopy analysis. In addition, we investigated whether modification of the N-glycan structures could activate serum complement activity, possibly by the lectin pathway of complement activation. DC-SIGN-targeted transduction occurs in the presence of human serum complement, demonstrating that high-mannose structure N-glycans of the envelope proteins do not activate human serum complement. These results indicate that the strategy of redirecting viral vectors according to alterations of their N-glycan structures would enable the vectors to target specific cells types expressing particular types of lectins.

Targeted PLGA nano- but not microparticles specifically deliver antigen to human dendritic cells via DC-SIGN in vitro

Vaccine efficacy is strongly enhanced by antibody-mediated targeting of vaccine components to dendritic cells (DCs), which are professional antigen presenting cells. However, the options to link antigens or immune modulators to a single antibody are limited. Here, we engineered versatile nano- and micrometer-sized slow-release vaccine delivery vehicles that specifically target human DCs to overcome this limitation. The nano- (NPs) and microparticles (MPs), with diameters of approximately 200nm and 2microm, consist of a PLGA core coated with a polyethylene glycol-lipid layer carrying the humanized targeting antibody hD1, which does not interact with complement or Fc receptors and recognizes the human C-type lectin receptor DC-SIGN on DCs. We studied how these particles interact with human DCs and blood cells, as well as the kinetics of PLGA-encapsulated antigen degradation within DCs. Encapsulation of antigen resulted in almost 38% degradation for both NPs and MPs 6days after particle ingestion by DCs, compared to 94% when nonencapsulated, soluble antigen was used. In contrast to the MPs, which were taken up rather nonspecifically, the NPs effectively targeted human DCs. Consequently, targeted delivery only improved antigen presentation of NPs and induced antigen-dependent T cell responses at 10-100 fold lower concentrations than nontargeted NPs.

Targeting glycan modified OVA to murine DC-SIGN transgenic dendritic cells enhances MHC class I and II presentation

Dendritic cells have gained much interest in the field of anti-cancer vaccine development because of their central function in immune regulation. One of the receptors that facilitate DC-specific targeting of antigens is the DC-specific C-type lectin DC-SIGN. Although DC-SIGN is specifically expressed on human DCs, its murine homologue is not present on any murine DC subsets, which makes in vivo evaluation of potential DC-SIGN targeting vaccines very difficult. Here we describe the use of DC-SIGN transgenic mice, as a good model system to evaluate DC-SIGN targeting vaccines. We demonstrate that glycan modification of OVA with DC-SIGN targeting glycans, targets antigen specifically to bone marrow (BM)** derived DCs and splenic DCs. Glycan modification of OVA with Lewis X or Lewis B oligosaccharides, that target DC-SIGN transgenic DCs, resulted in efficient 10-fold induction of OT-II compared to unmodified OVA. Interestingly, glycan modified OVA proteins were significantly cross-presented to OT-I T cells by wild type DC, 10-fold more than native OVA, and the expression of DC-SIGN further enhanced this cross-presentation. Targeting of glycosylated OVA was neither accompanied with any DC maturation, nor the production of inflammatory or anti-inflammatory cytokines. Thus, we conclude that glycan modification of antigens and targeting to DC-SIGN enhance both CD4 and CD8 T cell responses. Furthermore, our data demonstrate that DC-SIGN transgenic mice are valuable tool for optimisation and efficiency testing of DC vaccination strategies that are designed to target in particular the human DC-SIGN receptor.

A genetically engineered adenovirus vector targeted to CD40 mediates transduction of canine dendritic cells and promotes antigen-specific immune responses in vivo

Targeting viral vectors encoding tumor-associated antigens to dendritic cells (DCs) in vivo is likely to enhance the effectiveness of immunotherapeutic cancer vaccines. We have previously shown that genetic modification of adenovirus (Ad) 5 to incorporate CD40 ligand (CD40L) rather than native fiber allows selective transduction and activation of DCs in vitro. Here, we examine the capacity of this targeted vector to induce immune responses to the tumor antigen CEA in a stringent in vivo canine model. CD40-targeted Ad5 transduced canine DCs via the CD40-CD40L pathway in vitro, and following vaccination of healthy dogs, CD40-targeted Ad5 induced strong anti-CEA cellular and humoral responses. These data validate the canine model for future translational studies and suggest targeting of Ad5 vectors to CD40 for in vivo delivery of tumor antigens to DCs is a feasible approach for successful cancer therapy.

Enhancing immune responses by targeting antigen to DC

mAb that recognise various cell surface receptors have been used to deliver antigen to DC and thereby elicit immune responses. The encouraging data obtained in mouse models suggests that this immunisation strategy is efficient and could lead to clinical trials. We discuss a number of issues pertinent to this vaccination approach. These include which molecules are the best targets for delivering antigen to DC, which DC subtypes should be targeted, the types of immune responses to be generated and whether additional adjuvants are required. Finally, we discuss some progress towards targeting antigen to human DC.

Targeting the porcine immune system--particulate vaccines in the 21st century

During the last decade, the propagation of immunological knowledge describing the critical role of dendritic cells (DC) in the induction of efficacious immune responses has promoted research and development of vaccines systematically targeting DC. Based on the promise for the rational design of vaccine platforms, the current review will provide an update on particle-based vaccines of both viral and synthetic origin, giving examples of recombinant virus carriers such as adenoviruses and biodegradable particulate carriers. The viral carriers carry pathogen-associated molecular patterns (PAMP), used by the original virus for targeting DC, and are particularly efficient and versatile gene delivery vectors. Efforts in the field of synthetic vaccine carriers are focussing on decorating the particle surface with ligands for DC receptors such as heparan sulphate glycosaminoglycan structures, integrins, Siglecs, galectins, C-type lectins and toll-like receptors. The emphasis of this review will be placed on targeting the porcine immune system, but reference will be made to advances with murine and human vaccine delivery systems where information on DC targeting is available.

The pathogen receptor liver and lymph node sinusoidal endotelial cell C-type lectin is expressed in human Kupffer cells and regulated by PU.1

Human LSECtin (liver and lymph node sinusoidal endothelial cell C-type lectin, CLEC4G) is a C-type lectin encoded within the L-SIGN/DC-SIGN/CD23 gene cluster. LSECtin acts as a pathogen attachment factor for Ebolavirus and the SARS coronavirus, and its expression can be induced by interleukin-4 on monocytes and macrophages. Although reported as a liver and lymph node sinusoidal endothelial cell-specific molecule, LSECtin could be detected in the MUTZ-3 dendritic-like cell line at the messenger RNA (mRNA) and protein level, and immunohistochemistry analysis on human liver revealed its presence in Kupffer cells coexpressing the myeloid marker CD68. The expression of LSECtin in myeloid cells was further corroborated through the analysis of the proximal regulatory region of the human LSECtin gene, whose activity was maximal in LSECtin+ myeloid cells, and which contains a highly conserved PU.1-binding site. PU.1 transactivated the LSECtin regulatory region in collaboration with hematopoietic-restricted transcription factors (Myb, RUNX3), and was found to bind constitutively to the LSECtin proximal promoter. Moreover, knockdown of PU.1 through the use of small interfering RNA led to a decrease in LSECtin mRNA levels in THP-1 and monocyte-derived dendritic cells, thus confirming the involvement of PU.1 in the myeloid expression of the lectin.

CONCLUSION

LSECtin is expressed by liver myeloid cells, and its expression is dependent on the PU.1 transcription factor.

The Janus face of dendritic cells in cancer

On the basis of experimental models and some human data, we can assume that tumor outgrowth results from the balance between immunosurveillance (the extrinsic tumor suppressor mechanisms) and immunosubversion dictated by transformed cells and/or the corrupted surrounding microenvironment. Cancer immunosurveillance relies mainly upon conventional lymphocytes exerting either lytic or secretory functions, whereas immunosubversion results from the activity of regulatory T or suppressor myeloid cells and soluble mediators. Although specific tools to target or ablate dendritic cells (DCs) became only recently available, accumulating evidence points to the critical role of the specialized DC system in dictating most of the conventional and regulatory functions of tumor-specific T lymphocytes. Although DC can be harnessed to silence tumor development, tumors in turn can exploit DC to evade immunity. Indeed, DCs harbor defects in their differentiation and stimulatory functions in cancer-bearing hosts and can actively promote T-cell tolerance to self-tumor antigens. In this review, we will focus on the dual role of DC during tumor progression and discuss pharmacoimmunological strategies to harness DC against cancer.

The dendritic cell subtype-restricted C-type lectin Clec9A is a target for vaccine enhancement

A novel dendritic cell (DC)-restricted molecule, Clec9A, was identified by gene expression profiling of mouse DC subtypes. Based on sequence similarity, a human ortholog was identified. Clec9A encodes a type II membrane protein with a single extracellular C-type lectin domain. Both the mouse Clec9A and human CLEC9A were cloned and expressed, and monoclonal antibodies (mAbs) against each were generated. Surface staining revealed that Clec9A was selective for mouse DCs and was restricted to the CD8(+) conventional DC and plasmacytoid DC subtypes. A subset of human blood DCs also expressed CLEC9A. A single injection of mice with a mAb against Clec9A, which targets antigens (Ags) to the DCs, produced a striking enhancement of antibody responses in the absence of added adjuvants or danger signals, even in mice lacking Toll-like receptor signaling pathways. Such targeting also enhanced CD4 and CD8 T-cell responses. Thus, Clec9A serves as a new marker to distinguish subtypes of both mouse and human DCs. Furthermore, targeting Ags to DCs with antibodies to Clec9A is a promising strategy to enhance the efficiency of vaccines, even in the absence of adjuvants.