Evaluation of in vitro cross-reactivity to avian H5N1 and pandemic H1N1 2009 influenza following prime boost regimens of seasonal influenza vaccination in healthy human subjects: a randomised trial.

INTRODUCTION: Recent studies have demonstrated that inactivated seasonal influenza vaccines (IIV) may elicit production of heterosubtypic antibodies, which can neutralize avian H5N1 virus in a small proportion of subjects. We hypothesized that prime boost regimens of live and inactivated trivalent seasonal influenza vaccines (LAIV and IIV) would enhance production of heterosubtypic immunity and provide evidence of cross-protection against other influenza viruses. METHODS: In an open-label study, 26 adult volunteers were randomized to receive one of four vaccine regimens containing two doses of 2009-10 seasonal influenza vaccines administered 8 (±1) weeks apart: 2 doses of LAIV; 2 doses of IIV; LAIV then IIV; IIV then LAIV. Humoral immunity assays for avian H5N1, 2009 pandemic H1N1 (pH1N1), and seasonal vaccine strains were performed on blood collected pre-vaccine and 2 and 4 weeks later. The percentage of cytokine-producing T-cells was compared with baseline 14 days after each dose. RESULTS: Subjects receiving IIV had prompt serological responses to vaccine strains. Two subjects receiving heterologous prime boost regimens had enhanced haemagglutination inhibition (HI) and neutralization (NT) titres against pH1N1, and one subject against avian H5N1; all three had pre-existing cross-reactive antibodies detected at baseline. Significantly elevated titres to H5N1 and pH1N1 by neuraminidase inhibition (NI) assay were observed following LAIV-IIV administration. Both vaccines elicited cross-reactive CD4+ T-cell responses to nucleoprotein of avian H5N1 and pH1N1. All regimens were safe and well tolerated. CONCLUSION: Neither homologous nor heterologous prime boost immunization enhanced serum HI and NT titres to 2009 pH1N1 or avian H5N1 compared to single dose vaccine. However heterologous prime-boost vaccination did lead to in vitro evidence of cross-reactivity by NI; the significance of this finding is unclear. These data support the strategy of administering single dose trivalent seasonal influenza vaccine at the outset of an influenza pandemic while a specific vaccine is being developed. TRIAL REGISTRATION: ClinicalTrials.gov NCT01044095.

Pathogen-host-environment interplay and disease emergence.

Gaining insight in likely disease emergence scenarios is critical to preventing such events from happening. Recent focus has been on emerging zoonoses and on identifying common patterns and drivers of emerging diseases. However, no overarching framework exists to integrate knowledge on all emerging infectious disease events. Here, we propose such a conceptual framework based on changes in the interplay of pathogens, hosts and environment that lead to the formation of novel disease patterns and pathogen genetic adjustment. We categorize infectious disease emergence events into three groups: (i) pathogens showing up in a novel host, ranging from spill-over, including zoonoses, to complete species jumps; (ii) mutant pathogens displaying novel traits in the same host, including an increase in virulence, antimicrobial resistance and host immune escape; and (iii) disease complexes emerging in a new geographic area, either through range expansion or through long distance jumps. Each of these categories is characterized by a typical set of drivers of emergence, matching pathogen trait profiles, disease ecology and transmission dynamics. Our framework may assist in disentangling and structuring the rapidly growing amount of available information on infectious diseases. Moreover, it may contribute to a better understanding of how human action changes disease landscapes globally.

Antiviral immune responses in H5N1-infected human lung tissue and possible mechanisms underlying the hyperproduction of interferon-inducible protein IP-10.

Information on the immune response against H5N1 within the lung is lacking. Here we describe the sustained antiviral immune responses, as indicated by the expression of MxA protein and IFN-alpha mRNA, in autopsy lung tissue from an H5N1-infected patient. H5N1 infection of primary bronchial/tracheal epithelial cells and lung microvascular endothelial cells induced IP-10, and also up-regulated the retinoic acid-inducible gene-I (RIG-I). Down-regulation of RIG-I gene expression decreased IP-10 response. Co-culturing of H5N1-infected pulmonary cells with TNF-alpha led to synergistically enhanced production of IP-10. In the absence of viral infection, TNF-alpha and IFN-alpha also synergistically enhanced IP-10 response. Methylprednisolone showed only a partial inhibitory effect on this chemokine response. Our findings strongly suggest that both the H5N1 virus and the locally produced antiviral cytokines; IFN-alpha and TNF-alpha may have an important role in inducing IP-10 hyperresponse, leading to inflammatory damage in infected lung.

Differential intracellular fate of Burkholderia pseudomallei 844 and Burkholderia thailandensis UE5 in human monocyte-derived dendritic cells and macrophages.

BACKGROUND: Burkholderia pseudomallei (Bp) is a category B biothreat organism that causes a potentially fatal disease in humans and animals, namely melioidosis. Burkholderia thailandensis (Bt) is another naturally occurring species that is very closely related to Bp. However, despite this closely related genotype, Bt is considered avirulent as it does not cause the disease. In the present study, we compared the growth kinetics of B. pseudomallei strain 844 (Bp-844) in human monocyte-derived dendritic cells (MoDCs) and macrophages (Mphis), as well as its ability to stimulate host cell responses with those of B. thailandensis strain UE5 (Bt-UE5). RESULTS: Primary human MoDCs and Mphis were infected with Bp-844 and its intracellular growth kinetics and ability to induce host cell responses were evaluated. The results were compared with those obtained using the Bt-UE5. In human MoDCs, both bacteria were similar in respect to their ability to survive and replicate intracellularly, induce upregulation of costimulatory molecules and cytokines and bias T helper cell differentiation toward a Th1 phenotype. By contrast, the two bacteria exhibited different growth kinetics in human Mphis, where the intracellular growth of Bt-UE5, but not Bp-844, was significantly suppressed. Moreover, the ability of Mphis to kill Bp-844 was markedly enhanced following stimulation with IFN-gamma. CONCLUSION: The data presented showed that while both strains were similar in their ability to survive and replicate in human MoDCs, only Bp-844 could readily replicate in human Mphis. Both bacteria induced similar host cellular responses, particularly with regard to their ability to bias T cell differentiation toward a Th1 phenotype.

Leptospira interrogans is recognized through DC-SIGN and induces maturation and cytokine production by human dendritic cells.

Leptospirosis is a global zoonotic disease, caused by pathogenic Leptospira species including Leptospira interrogans, that causes public health and livestock problems. Pathogenesis, immune response and cellular receptors for Leptospira are not well understood. Interaction of dendritic cells (DCs) with L. interrogans serovar Autumnalis L-643 and BL-6 isolated from leptospirosis patients, and both virulent and avirulent serovar Pyrogenes 2317 strains isolated from animal were investigated. Carbohydrate analysis using lectins showed that all of these leptospires contained high mannose components as a common backbone and DC-SIGN was involved in leptospires' attachment. Interaction of the L. interrogans strains with DCs induced maturation, but had different effects on IL-10, IL-12p70 and tumor necrosis factor (TNF)-alpha production. Both virulent and avirulent Pyrogenes 2317 and Autumnalis BL-6 but not L-643 strains induced IL-12p70 and TNF-alpha production, but minimal IL-10 secretion. These data demonstrated that L. interrogans binds DC-SIGN and induces DCs maturation and cytokine production, which should provide new insights into cellular immune processes during leptospirosis.

Expression and function of Toll-like receptors on dendritic cells and other antigen presenting cells from non-human primates.

Antigen presenting cells (APCs), especially dendritic cells (DCs), play a crucial role in immune responses against infections by sensing microbial invasion through Toll-like receptors (TLRs). In this regard, TLR ligands are attractive candidates for use in humans and animal models as vaccine adjuvants. So far, no studies have been performed on TLR expression in non-human primates such as rhesus macaques. Therefore, we studied the TLR expression patterns in different subsets of APC in rhesus macaques and compared them to similar APC subsets in human. Also, expression was compared with corresponding DC subsets from different organs from mice. Here we show by semi-quantitative RT-PCR, that blood DC subsets of rhesus macaque expressed the same sets of TLRs as those of human but substantially differed from mouse DC subsets. Macaque myeloid DCs (MDCs) expressed TLR3, 4, 7 and 8 whereas macaque plasmacytoid DCs (PDCs) expressed only TLR7 and 9. Additionally, TLR expression patterns in macaque monocyte-derived dendritic cells (mo-DCs) (i.e., TLR3, 4, 8 and 9), monocytes (i.e., TLR4, 7, and 8) and B cells (i.e., TLR4, 7, 8, and 9) were also similar to their human counterparts. However, the responsiveness of macaque APCs to certain TLR ligands partially differed from that of human in terms of phenotype differentiation and cytokine production. Strikingly, in contrast to human mo-DCs, no IL-12p70 production was observed when macaque mo-DCs were stimulated with TLR ligands. In addition, CD40 and CD86 phenotypic responses to TLR8 ligand (poly U) in mo-DCs of macaque were higher than that of human. Despite these functional differences, our results provide important information for a rational design of animal models in evaluating TLR ligands as adjuvant in vivo.

Activation of human monocyte-derived dendritic cells by Burkholderia pseudomallei does not require binding to the C-type lectin DC-SIGN.

Dendritic cells (DCs) are essential in regulating adaptive immunity. DC-SIGN (DC-specific ICAM-grabbing nonintegrin) is a C-type lectin receptor that is expressed mainly by DCs. Accumulating evidence supports that certain pathogens target DC-SIGN to escape host immunity. To investigate a possible role of DC-SIGN in Burkholderia pseudomallei infection, we initially screened its DC-SIGN binding activity by an ELISA method utilizing a DC-SIGN-Fc chimeric protein and found that all of the B. pseudomallei strains tested failed to bind DC-SIGN. However, one strain, the LPS mutant SRM117, which lacks the type II O-polysaccharide expression, actually bound DC-SIGN, in contrast to its wild-type counterpart 1026b (P<0.001). We also found that, although the LPS mutant could readily activate monocyte-derived human DCs, it induced lower levels of IL-12p70 and IL-10 production than its wild-type counterpart (P<0.01). By contrast, the wild-type and the LPS mutants were indistinguishable from one another in terms of T(H)1/T(H)2 differentiation. Altogether, these data suggest that, unlike other certain host pathogen interactions, activation of DCs by B. pseudomallei is not dependent on DC-SIGN. We also found evidence that the LPS mutant that binds DC-SIGN has a suppressive effect on DC cytokine production.

High susceptibility of human dendritic cells to avian influenza H5N1 virus infection and protection by IFN-alpha and TLR ligands.

There is worldwide concern that the avian influenza H5N1 virus, with a mortality rate of >50%, might cause the next influenza pandemic. Unlike most other influenza infections, H5N1 infection causes a systemic disease. The underlying mechanisms for this effect are still unclear. In this study, we investigate the interplay between avian influenza H5N1 and human dendritic cells (DC). We showed that H5N1 virus can infect and replicate in monocyte-derived and blood myeloid DC, leading to cell death. These results suggest that H5N1 escapes viral-specific immunity, and could disseminate via DC. In contrast, blood pDC were resistant to infection and produced high amounts of IFN-alpha. Addition of this cytokine to monocyte-derived DC or pretreatment with TLR ligands protected against infection and the cytopathic effects of H5N1 virus.

Schistosoma mansoni soluble egg antigens are internalized by human dendritic cells through multiple C-type lectins and suppress TLR-induced dendritic cell activation.

In schistosomiasis, a parasitic disease caused by helminths, the parasite eggs induce a T helper 2 cell (T(H)2) response in the host. Here, the specific role of human monocyte-derived dendritic cells (DCs) in initiation and polarization of the egg-specific T cell responses was examined. We demonstrate that immature DCs (iDCs) pulsed with schistosome soluble egg antigens (SEA) do not show an increase in expression of co-stimulatory molecules or cytokines, indicating that no conventional maturation was induced. The ability of SEA to affect the Toll-like receptor (TLR) induced maturation of iDCs was examined by copulsing the DCs with SEA and TLR-ligands. SEA suppressed both the maturation of iDCs induced by poly-I:C and LPS, as indicated by a decrease in co-stimulatory molecule expression and production of IL-12, IL-6 and TNF-alpha. In addition, SEA suppressed T(H)1 responses induced by the poly-I:C-pulsed DCs, and skewed the LPS-induced mixed response towards a T(H)2 response. Immature DCs rapidly internalized SEA through the C-type lectins DC-SIGN, MGL and the mannose receptor and the antigens were targeted to MHC class II-positive lysosomal compartments. The internalization of SEA by multiple C-type lectins may be important to regulate the response of the iDCs to TLR-induced signals.

Neisseria meningitidis expressing lgtB lipopolysaccharide targets DC-SIGN and modulates dendritic cell function.

Neisseria meningitidis lipopolysaccharide (LPS) has been identified as a major determinant of dendritic cell (DC) function. Here we report that one of a series of meningococcal mutants with defined truncations in the lacto-N-neotetraose outer core of the LPS exhibited unique strong adhesion and internalization properties towards DC. These properties were mediated by interaction of the GlcNAc(beta1-3)-Gal(beta1-4)-Glc-R oligosaccharide outer core of lgtB LPS with the dendritic-cell-specific ICAM-3 grabbing non-integrin (DC-SIGN) lectin receptor. Activation of DC-SIGN with this novel oligosaccharide ligand skewed T-cell responses driven by DC towards T helper type 1 activity. Thus, the use of lgtB LPS may provide a powerful instrument to selectively induce the desired arm of the immune response and potentially increase vaccine efficacy.

Selective probiotic bacteria induce IL-10-producing regulatory T cells in vitro by modulating dendritic cell function through dendritic cell-specific intercellular adhesion molecule 3-grabbing nonintegrin.

BACKGROUND: Lactobacilli are probiotic bacteria that are frequently tested in the management of allergic diseases or gastroenteritis. It is hypothesized that these probiotics have immunoregulatory properties and promote mucosal tolerance, which is in part mediated by regulatory T cells (Treg cells). On the basis of pathogenic or tissue-specific priming, dendritic cells (DC) acquire different T cell-instructive signals and drive the differentiation of naive T H cells into either T H 1, T H 2, or regulatory effector T cells. OBJECTIVE: We studied in what way different species of lactobacilli prime human DCs for their ability to drive Treg cells. METHODS: Human monocyte-derived DCs were cultured in vitro with lactobacilli of different species. RESULTS: Two different species of lactobacilli, Lactobacillus reuteri and Lactobacillus casei , but not Lactobacillus plantarum, prime monocyte-derived DCs to drive the development of Treg cells. These Treg cells produced increased levels of IL-10 and were capable of inhibiting the proliferation of bystander T cells in an IL-10-dependent fashion. Strikingly, both L reuteri and L casei , but not L plantarum , bind the C-type lectin DC-specific intercellular adhesion molecule 3-grabbing non-integrin (DC-SIGN). Blocking antibodies to DC-SIGN inhibited the induction of the Treg cells by these probiotic bacteria, stressing that ligation of DC-SIGN can actively prime DCs to induce Treg cells. CONCLUSIONS: The targeting of DC-SIGN by certain probiotic bacteria might explain their beneficial effect in the treatment of a number of inflammatory diseases, including atopic dermatitis and Crohn's disease.

Helicobacter pylori modulates the T helper cell 1/T helper cell 2 balance through phase-variable interaction between lipopolysaccharide and DC-SIGN.

The human gastric pathogen Helicobacter pylori spontaneously switches lipopolysaccharide (LPS) Lewis (Le) antigens on and off (phase-variable expression), but the biological significance of this is unclear. Here, we report that Le+ H. pylori variants are able to bind to the C-type lectin DC-SIGN and present on gastric dendritic cells (DCs), and demonstrate that this interaction blocks T helper cell (Th)1 development. In contrast, Le- variants escape binding to DCs and induce a strong Th1 cell response. In addition, in gastric biopsies challenged ex vivo with Le+ variants that bind DC-SIGN, interleukin 6 production is decreased, indicative of increased immune suppression. Our data indicate a role for LPS phase variation and Le antigen expression by H. pylori in suppressing immune responses through DC-SIGN.

Pathogens use carbohydrates to escape immunity induced by dendritic cells.

Dendritic cells (DCs) play a central role in balancing immune responses between tolerance induction and immune activation. Under steady state conditions DCs continuously sample antigens, leading to tolerance, whereas inflammatory conditions activate DCs, inducing immune activation. DCs express C-type lectin receptors (CLRs) for antigen capture and presentation, whereas Toll-like receptors (TLRs) are involved in pathogen recognition and DC activation. Recent data demonstrate that communication between TLRs and CLRs can affect the direction of immune responses. Several pathogens specifically target CLRs to subvert this communication to escape immune surveillance, either by inducing tolerance or skewing the protective immune responses.

Dynamic populations of dendritic cell-specific ICAM-3 grabbing nonintegrin-positive immature dendritic cells and liver/lymph node-specific ICAM-3 grabbing nonintegrin-positive endothelial cells in the outer zones of the paracortex of human lymph nodes.

In the paracortex of lymph nodes, cellular immune responses are generated against antigens captured in peripheral tissues by dendritic cells (DCs). DC-SIGN (dendritic cell-specific ICAM-3 grabbing nonintegrin), a C-type lectin exclusively expressed by DCs, functions as an antigen receptor as well as an adhesion receptor. A functional homologue of DC-SIGN, L-SIGN (liver/lymph node-SIGN, also called DC-SIGN-related), is expressed by liver sinus endothelial cells. In lymph nodes, both DC-SIGN and L-SIGN are expressed. In this study, we analyzed the distribution of these two SIGN molecules in detail in both normal and immunoreactive lymph nodes. DC-SIGN is expressed by mature DCs in paracortical areas and in addition by DCs with an immature phenotype in the outer zones of the paracortex. L-SIGN expression was also detected in the outer zones on sinus endothelial cells characterized by their expression of the lymphatic endothelial markers LYVE-1 and CLEVER-1. During both cellular and humoral immune responses changes in the amount of DC-SIGN+ immature and mature DCs and L-SIGN+ endothelial cells were observed, indicating that the influx or proliferation of these cells is dynamically regulated.

Self- and nonself-recognition by C-type lectins on dendritic cells.

Dendritic cells (DCs) are highly efficient antigen-presenting cells (APCs) that collect antigen in body tissues and transport them to draining lymph nodes. Antigenic peptides are loaded onto major histocompatibility complex (MHC) molecules for presentation to naive T cells, resulting in the induction of cellular and humoral immune responses. DCs take up antigen through phagocytosis, pinocytosis, and endocytosis via different groups of receptor families, such as Fc receptors for antigen-antibody complexes, C-type lectin receptors (CLRs) for glycoproteins, and pattern recognition receptors, such as Toll-like receptors (TLRs), for microbial antigens. Uptake of antigen by CLRs leads to presentation of antigens on MHC class I and II molecules. DCs are well equipped to distinguish between self- and nonself-antigens by the variable expression of cell-surface receptors such as CLRs and TLRs. In the steady state, DCs are not immunologically quiescent but use their antigen-handling capacities to maintain peripheral tolerance. DCs are continuously sampling and presenting self- and harmless environmental proteins to silence immune activation. Uptake of self-components in the intestine and airways are good examples of sites where continuous presentation of self- and foreign antigens occurs without immune activation. In contrast, efficient antigen-specific immune activation occurs upon encounter of DCs with nonself-pathogens. Recognition of pathogens by DCs triggers specific receptors such as TLRs that result in DC maturation and subsequently immune activation. Here we discuss the concept that cross talk between TLRs and CLRs, differentially expressed by subsets of DCs, accounts for the different pathways to peripheral tolerance, such as deletion and suppression, and immune activation.

Differential post-translational modification of CD63 molecules during maturation of human dendritic cells.

The capacity of dendritic cells to initiate T cell responses is related to their ability to redistribute MHC class II molecules from the intracellular MHC class II compartments to the cell surface. This redistribution occurs during dendritic cell development as they are converted from an antigen capturing, immature dendritic cell into an MHC class II-peptide presenting mature dendritic cell. During this maturation, antigen uptake and processing are down-regulated and peptide-loaded class II complexes become expressed in a stable manner on the cell surface. Here we report that the tetraspanin CD63, that associates with intracellularly localized MHC class II molecules in immature dendritic cells, was modified post-translationally by poly N-acetyl lactosamine addition during maturation. This modification of CD63 was accompanied by a change in morphology of MHC class II compartments from typical multivesicular organelles to structures containing densely packed lipid moieties. Post-translational modification of CD63 may be involved in the functional and morphological changes of MHC class II compartments that occur during dendritic cell maturation.

Immune escape through C-type lectins on dendritic cells.

Dendritic cells (DCs) detect different pathogens and elicit tailored anti-microbial immune responses. They express C-type lectins that recognise carbohydrate profiles on microorganisms, resulting in internalisation, processing and presentation. Intracellular sequences of distinct DC-specific lectins point to differences in intracellular routing that influence antigen presentation. Moreover, putative signalling motifs hint to the activation of DCs on carbohydrate recognition. Recent evidence shows that not only pathogens, but also tumour antigens, exploit C-type lectins to escape intracellular degradation resulting in abortive immunity. More insight into ligand specificity, intracellular targeting and signalling will reveal the pathways by which pathogens modulate immunity through C-type lectins.

Subset of DC-SIGN(+) dendritic cells in human blood transmits HIV-1 to T lymphocytes.

The dendritic cell (DC)-specific molecule DC-SIGN is a receptor for the HIV-1 envelope glycoprotein gp120 and is essential for the dissemination of HIV-1. DC-SIGN is expressed by DCs, both monocyte-derived DCs and DCs in several tissues, including mucosa and lymph nodes. To identify a DC-SIGN(+) DC in blood that may be involved in HIV-1 infection through blood, we have analyzed the expression of DC-SIGN in human blood cells. Here we describe the characterization of a subset of DCs in human blood, isolated from T-/NK-/B-cell-depleted peripheral blood mononuclear cells (PBMCs) on the basis of expression of DC-SIGN. This subset coexpresses CD14, CD16, and CD33 and is thus of myeloid origin. In contrast to CD14(+) monocytes, DC-SIGN(+) blood cells display a DC-like morphology and express markers of antigen-presenting cells, including CD1c, CD11b, CD11c, CD86, and high levels of major histocompatibility complex (MHC) class I and II molecules. This DC population differs from other described CD14(-) blood DC subsets. Functionally, DC-SIGN(+) blood DCs are able to stimulate proliferation of allogeneic T cells and can produce tumor necrosis factor-alpha (TNF-alpha) and interleukin-6 (IL-6) upon activation with lipopolysaccharide (LPS). When they encounter HIV-1, low amounts of these blood DC-SIGN(+) DCs enhance infection of T lymphocytes in trans, whereas blood monocytes and CD14(-) blood DCs are not capable of transmitting HIV-1. Therefore DC-SIGN(+) blood DCs can be the first target for HIV-1 upon transmission via blood; they can capture minute amounts of HIV-1 through DC-SIGN and transfer HIV-1 to infect target T cells in trans.

DC-SIGN, a C-type lectin on dendritic cells that unveils many aspects of dendritic cell biology.

Dendritic cells (DC) are present in essentially every tissue where they operate at the interface of innate and acquired immunity by recognizing pathogens and presenting pathogen-derived peptides to T cells. It is becoming clear that not all C-type lectins on DC serve as antigen receptors recognizing pathogens through carbohydrate structures. The C-type lectin DC-SIGN is unique in that it regulates adhesion processes, such as DC trafficking and T-cell synapse formation, as well as antigen capture. Moreover, even though several C-type lectins have been shown to bind HIV-1, DC-SIGN does not only capture HIV-1 but also protects it in early endosomes allowing HIV-1 transport by DC to lymphoid tissues, where it enhances trans infection of T cells. Here we discuss the carbohydrate/protein recognition profile and other features of DC-SIGN that contribute to the potency of DC to control immunity.

The dendritic cell-specific adhesion receptor DC-SIGN internalizes antigen for presentation to T cells.

Dendritic cells (DCs) capture Ags or viruses in peripheral tissue to transport them to lymphoid organs to induce cellular T cell responses. Recently, a DC-specific C-type lectin was identified, DC-specific ICAM-grabbing non-integrin (DC-SIGN), that functions as cell adhesion receptor mediating both DC migration and T cell activation. DC-SIGN also functions as an HIV-1R that captures HIVgp120 and facilitates DC-induced HIV transmission of T cells. Internalization motifs in the cytoplasmic tail of DC-SIGN hint to a function of DC-SIGN as endocytic receptor. In this study we demonstrate that on DCs DC-SIGN is rapidly internalized upon binding of soluble ligand. Mutating a putative internalization motif in the cytoplasmic tail reduces ligand-induced internalization. Detailed analysis using ratio fluorescence imaging and electron microscopy showed that DC-SIGN-ligand complexes are targeted to late endosomes/lysosomes. Moreover, ligands internalized by DC-SIGN are efficiently processed and presented to CD4+ T cells. The distinct pattern of expression of C-type lectins on DCs in situ and their nonoverlapping Ag recognition profile hint to selective functions of these receptors to allow a DC to recognize a wide variety of Ags and to process these to induce T cell activation. These data point to a novel function of the adhesion receptor DC-SIGN as an efficient DC-specific Ag receptor that can be used as a target to induce viral and antitumor immunity.