HIV-1 subverts antigen processing in dendritic cells (DCs) resulting in viral uptake, infection, and transfer to T cells. Although DCs bound monomeric gp120 and HIV-1 similarly, virus rarely colocalized with endolysosomal markers, unlike gp120, suggesting HIV-1 alters endolysosomal trafficking. Virus within DC intracellular compartments rapidly moved to DC-CD4 ؉ lymphocyte synapses when introduced to CD4 ؉ lymphocyte cultures.Although viral harboring and transfer from nonlysosomal compartments was transient, given DC-associated virus protein, nucleic acids, and infectious HIV-1 transfer to CD4 ؉ , lymphocytes decayed within 24 hours. However a second long-term transfer phase was apparent in immature DCs after 48 hours as a zidovudinesensitive rise in proviral DNA. Therefore, DCs transfer HIV-1 to CD4 ؉ lymphocytes in 2 distinct phases. Immature and mature DCs first divert virus from the endolysosomal pathway to the DC-T-cell synapse. Secondly, the later transfer phase from immature DCs is through de novo HIV-1 production. Thus, the controversy of DCs being infected or not infected for the mechanics of viral transfer to CD4 ؉ lymphocytes can be addressed as a function of time.

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Dendritic cells are professional antigen-presenting cells that initiate primary immunity. Migration from sites of antigen uptake to lymphoid organs is crucial for the generation of immune responses. We investigated the migratory pathways specifically of epidermal Langerhans cells by tracing them from the epidermis to the draining lymph nodes. This was possible with a new monoclonal antibody, directed against murine Langerin/CD207, a type II lectin specific for Langerhans cells. In situ, resident, and activated Langerhans cells express Langerin in the epidermis and on their way through dermal lymphatic vessels. Both emigrated and trypsinization-derived Langerhans cells expressed high levels of Langerin intracellularly but reduced it upon prolonged culture periods. Sizeable numbers of Langerin+ cells were found in skin draining lymph nodes but not in mesenteric nodes. Langerin+ cells localized to the T cells areas and rarely to B cell zones. Numbers of Langerin-expressing cells increased after application of a contact sensitizer. In the steady state, Langerhans cells in the skin-draining nodes expressed maturation markers, such as 2A1 and costimulatory molecules CD86 and CD40. These molecules, CD86 and CD40, were further upregulated upon inflammatory stimuli such as contact sensitization. Thus, the novel anti-Langerin monoclonal antibody permits the unequivocal visualization of migratory Langerhans cells in the lymph nodes for the first time and thereby allows to dissect the relative immunogenic or tolerogenic contributions of Langerhans cells and other types of dendritic cells.

Langerhans cells of the epidermis and dermal dendritic cells screen the skin for invading antigens. They initiate primary immune responses after migrating from sites of antigen uptake to lymphoid organs. The skin is a feasible model to study the morphology and regulation of dendritic cell migration. We therefore used murine skin explant cultures for tracking the pathways of dendritic cell migration by electron microscopy. Several novel observations are reported. (i) In 48 h cultures of epidermal sheets numerous Langerhans cells migrated out between keratinocytes extending long and thin cytoplasmic processes ("veils"). (ii) Langerhans cells in transition from epidermis to dermis were observed by transmission electron microscopy. Where Langerhans cells penetrated the basement membrane, the lamina densa was focally absent. (iii) This was highlighted by scanning electron microscopy, which presented the basement membrane as a tightly packed and dense network of fibrils. (iv) Scanning electron microscopy of the dermis revealed dendritic cells extending their cytoplasmic processes and clinging to collagen fibrils. (v) Entry of dendritic cells into dermal lymphatics was observed by transmission electron microscopy. It occurred by transmigration through intercellular spaces of adjacent endothelial cells. Entry through wide gaps between endothelial cells also seemed to take place. (vi) Dendritic cells inside the afferent lymphatics frequently carried material such as melanosomes and apoptotic bodies. These observations visualize the cumbersome pathway that dendritic cells have to take when they generate immunity.

An important property of dendritic cells (DC), which contributes crucially to their strong immunogenic function, is their capacity to migrate from sites of antigen capture to the draining lymphoid organs. Here we studied in detail the migratory pathway and the differentiation of DC during migration in a skin organ culture model and, for comparison, in the conventional contact hypersensitivity system. We report several observations on the capacity of cutaneous DC to migrate in mouse ear skin. (i) Upon application of contact allergens in vivo the density of Langerhans cells in epidermal sheets decreased, as determined by immunostaining for major histocompatibility complex class II, ADPase, F4/80, CD11b, CD32, NLDC-145/DEC-205, and the cytoskeleton protein vimentin. Evaluation was performed by computer assisted morphometry. (ii) Chemically related nonsensitizing or tolerizing compounds left the density of Langerhans cells unchanged. (iii) Immunohistochemical double-staining of dermal sheets from skin organ cultures for major histocompatibility complex class II and CD54 excluded blood vessels as a cutaneous pathway of DC migration. (iv) Electron microscopy of organ cultures revealed dermal accumulations of DC (including Birbeck granule containing Langerhans cells) within typical lymphatic vessels. (v) Populations of migrating DC in organ cultures upregulated markers of maturity (the antigen recognized by monoclonal antibody 2A1, CD86), but retained indicators of immaturity (invariant chain, residual antigen processing function). These data provide additional evidence that during both the induction of contact hypersensitivity and in skin organ culture, Langerhans cells physically leave the epidermis. Both Langerhans cells and dermal DC enter lymphatic vessels. DC mature while they migrate through the skin.

Langerin is a C-type lectin expressed by a subset of dendritic leukocytes, the Langerhans cells (LC).Langerin is a cell surface receptor that induces the formation of an LC-specific organelle, the Birbeck granule (BG). We generated a langerin ؊/؊ mouse on a C57BL/6 background which did not display any macroscopic aberrant development. In the absence of langerin, LC were detected in normal numbers in the epidermis but the cells lacked BG. LC of langerin ؊/؊ mice did not present other phenotypic alterations compared to wild-type littermates. Functionally, the langerin ؊/؊ LC were able to capture antigen, to migrate towards skin draining lymph nodes, and to undergo phenotypic maturation. In addition, langerin ؊/؊ mice were not impaired in their capacity to process native OVA protein for I- Dendritic cells (DC) are the most potent leukocytes to mediate the rapid initiation of a primary immune response (7). DC are bone marrow-derived leukocytes, localized in most tissues including primary and secondary lymphoid organs. In the periphery, most DC are in an immature state and are able to capture antigenic molecules via unique endocytic receptors or by fluid-phase macropinocytosis. This process generally leads to a first step of DC maturation, concomitant with their migration to secondary lymphoid organs. DC can subsequently activate naive CD4 ϩ T or CD8 ϩ T lymphocytes if peptides processed from native antigenic molecules are displayed on cell surface major histocompatibility complex (MHC) class II or I molecules in conjunction with cosignaling molecules (40).DC heterogeneity is a common feature of mice and humans. Precursor cell populations, anatomical localization, morphology, phenotype, and functions determine the type of DC. However, the origin of different DC subsets is still controversial (4). A particular subset of DC is represented by Langerhans cells (LC), which are immature DC present in the epidermis and mucosal epithelium (53, 71). LC can be generated either from myeloid precursors (77, 78) or from CD4 low lymphoid precursors (2). LC express a number of cell surface receptors including CD205/DEC205, Fc␥ and Fcε receptors, and langerin/ CD207 (70,72).Langerin is a C-type lectin oriented in a type II configuration and featuring a single carbohydrate recognition domain in its extracellular region (72). Langerin molecules oligomerize as trimers at the cell surface and display Ca 2ϩ -dependent binding specificity for mannose, N-acetyl-glucosamine, and fucose (61, 72). Langerin is a potent inducer of Birbeck granules (BG), the hallmark organelles of LC, which consist of pentalamellar and zippered membranes at the electron microscopic level (12,75). In addition to inducing BG formation, langerin is an endocytic receptor involved in the trafficking of exogenous mannosylated ligands from the cell surface into intracellular BG compartments (71).To further explore the role of langerin and BG, we generated C57BL/6 mice with a targeted disruption of the langerin gene. Although MHC class II-positive LC were detected in n...

The challenge in observing de novo virus production in human immunodeficiency virus (HIV)-infected dendritic cells (DCs) is the lack of resolution between cytosolic immature and endocytic mature HIV gag protein. To track HIV production, we developed an infectious HIV construct bearing a diothiol-resistant tetracysteine motif (dTCM) at the C terminus of HIV p17 matrix within the HIV gag protein. Using this construct in combination with biarsenical dyes, we observed restricted staining of the dTCM to de novo-synthesized uncleaved gag in the DC cytosol. Co-staining with HIV gag antibodies, reactive to either p17 matrix or p24 capsid, preferentially stained mature virions and thus allowed us to track the virus at distinct stages of its life cycle within DCs and upon transfer to neighboring DCs or T cells. Thus, in staining HIV gag with biarsenical dye system in situ, we characterized a replication-competent virus capable of being tracked preferentially within infected leukocytes and observed in detail the dynamic nature of the HIV production and transfer in primary DCs.

The capacity to migrate from peripheral tissues, where antigen is encountered, to lymphoid organs, where the primary immune response is initiated, is crucial to the immunogenic function of dendritic cells (DC). The skin is a suitable tissue to study migration. DC were observed to gather in distinct nonrandom arrays ("cords") in the dermis upon culture of murine whole skin explants. It is assumed that cords represent lymphatic vessels. Using a similar organ culture model with human split-thickness skin explants, we investigated migration pathways in human skin. We made the following observations. 1) Spontaneous emigration of Langerhans cells took place in skin cultured for 1-3 d. Nonrandom distribution patterns of strongly major histocompatibility complex class II-expressing DC (cords) occurred in cultured dermis. A variable, yet high (>50%) percentage of these DC coexpressed the Birbeck granule-associated antigen "Lag." Ultrastructurally, the cells corresponded to mature DC. 2) Electron microscopy proved that the dermal structures harboring the accumulations of DC (i.e., cords) were typical lymph vessels. Moreover, markers for blood endothelia (monoclonal antibody PAL-E, Factor VIII-related antigen) and markers for cords (strong major histocompatibility complex class II expression on nonrandomly arranged, hairy-appearing cells) were expressed in a mutually exclusive pattern. 3) On epidermal sheets we failed to detect gross changes in the levels of expression of adhesion molecules (CD44, CD54/ ICAM-1, E-cadherin) on keratinocytes in the course of the culture period. The reactivity of a part of the DC in the dermal cords with Birbeck granule-specific monoclonal antibody "Lag" suggests that the migratory population is composed of both epidermal Langerhans cells and dermal DC. We conclude that this organ culture model may prove helpful in resolving pathways and mechanisms of DC migration.