Disappearance of certain acidic organelles (endosomes and Langerhans cell granules) accompanies loss of antigen processing capacity upon culture of epidermal Langerhans cells.

Freshly isolated epidermal Langerhans cells (LC) can actively process native protein antigens, but are weak in sensitizing helper T cells. During culture, when LC mature into potent immunostimulatory dendritic cells, T cell sensitizing capacity develops but antigen processing capacity is downregulated. Processing of exogenous antigens for class II-restricted antigen presentation involves acidic organelles. We used the DAMP-technique to monitor acidic organelles at the ultrastructural level in fresh, as well as cultured, mouse and human LC. We observed that the loss of antigen processing capacity with culture of LC was reflected by the disappearance of certain acidic organelles, namely endosomes (particularly early ones), and the hitherto enigmatic LC granules ("Birbeck Granules"). Our findings support the notion that endosomes are critical for antigen processing and suggest that LC granules might be involved as well.

A fusion inhibitor prevents spread of immunodeficiency viruses, but not activation of virus-specific T cells, by dendritic cells.

Dendritic cells (DCs) play a key role in innate immune responses, and their interactions with T cells are critical for the induction of adaptive immunity. However, immunodeficiency viruses are efficiently captured by DCs and can be transmitted to and amplified in CD4(+) T cells, with potentially deleterious effects on the induction of immune responses. In DC-T-cell cocultures, contact with CD4(+), not CD8(+), T cells preferentially facilitated virus movement to and release at immature and mature DC-T-cell contact sites. This occurred within 5 min of DC-T-cell contact. While the fusion inhibitor T-1249 did not prevent virus capture by DCs or the release of viruses at the DC-T-cell contact points, it readily blocked virus transfer to and amplification in CD4(+) T cells. Higher doses of T-1249 were needed to block the more robust replication driven by mature DCs. Virus accumulated in DCs within T-1249-treated cocultures but these DCs were actually less infectious than DCs isolated from untreated cocultures. Importantly, T-1249 did not interfere with the stimulation of virus-specific CD4(+) and CD8(+) T-cell responses when present during virus-loading of DCs or for the time of the DC-T-cell coculture. These results provide clues to identifying strategies to prevent DC-driven virus amplification in CD4(+) T cells while maintaining virus-specific immunity, an objective critical in the development of microbicides and therapeutic vaccines.

Migration of dendritic cells into lymphatics-the Langerhans cell example: routes, regulation, and relevance.

Dendritic cells are leukocytes of bone marrow origin. They are central to the control of the immune response. Dendritic cells are highly specialized in processing and presenting antigens (microbes, proteins) to helper T lymphocytes. Thereby, they critically regulate further downstream processes such as the development of cytotoxic T lymphocytes, the production of antibodies by B lymphocytes, or the activation of macrophages. A new field of dendritic cell biology is the study of their potential role in inducing peripheral tolerance. The immunogenic/tolerogenic potential of dendritic cells is increasingly being utilized in immunotherapy, particularly for the elicitation of antitumor responses. One very important specialization of dendritic cells is their outstanding capacity to migrate from sites of antigen uptake to lymphoid organs. Much has been learned about this process from studying one particular type of dendritic cell, namely, the Langerhans cell of the epidermis. Therefore, the migratory properties of Langerhans cells are reviewed. Knowledge about this "prototype dendritic cell" may help researchers to understand migration of other types of dendritic cells.

Two populations of splenic dendritic cells detected with M342, a new monoclonal to an intracellular antigen of interdigitating dendritic cells and some B lymphocytes.

A monoclonal has been isolated that labels an intracellular antigen in dendritic cells and some B cells. The M342 hamster immunoglobulin was selected because it stained cells in the periarterial sheaths of spleen, the deep cortex of lymph node, and the thymic medulla--the same regions in which one finds interdigitating cells, the presumptive in situ counterparts of isolated lymphoid dendritic cells. M342 labeled an antigen within granules of isolated dendritic cells, but only in cells that had been cultured for a day and not in fresh isolates. This extends recent findings that most freshly isolated spleen dendritic cells are located in the periphery of the white pulp nodule and may serve as precursors for the periarterial pool of interdigitating cells, the site for M342 staining in situ. By electron microscopic immunolabeling, the M342 antigen was found exclusively in a type of multivesicular body. M342 staining was not found in mononuclear phagocytes from blood and peritoneal cavity. Peritoneal B cells expressed M342+ granules, and upon appropriate stimulation splenic B cells developed reactive granules as well. We conclude that M342 is a strong marker for interdigitating cells. Its existence reveals intracellular specializations in the vacuolar system of antigen-presenting cells including subsets of dendritic cells.

Cultured human Langerhans cells resemble lymphoid dendritic cells in phenotype and function.

Freshly isolated murine epidermal Langerhans cells (LC) are weak stimulators of resting T cells. Upon culture their phenotype changes, their stimulatory activity increases significantly, and they come to resemble lymphoid dendritic cells. Resident murine LC, therefore, might represent a reservoir of immature dendritic cells. We have now used enzyme cytochemistry, a panel of some 80 monoclonal antibodies, and immunofluorescence microscopy or two-color flow cytometry, as well as transmission electron microscopy, to analyse the phenotype and morphology of human LC before and after 2-4 d of bulk epidermal cell culture. In addition, LC were enriched from bulk epidermal cell cultures, and their stimulatory capacity was tested in the allogeneic mixed leukocyte reaction and the oxidative mitogenesis assay. Cultured human LC resembled human lymphoid dendritic cells in morphology, phenotype, and function. Specifically, LC became non-adherent upon culture and developed sheet-like processes (so-called "veils"), decreased their surface ATP/ADP'ase activity, and lost nonspecific esterase activity. As in the mouse, surface expression of MHC class I and II antigens increased significantly, and FcII receptors were significantly reduced. Markers that are expressed by dendritic cells (like CD40) appeared on LC following culture. Cultured human LC were potent T-cell stimulators. Our findings support the view that resident human LC, like murine LC, represent immature precursors of lymphoid dendritic cells in skin-draining lymph nodes.

Entry into afferent lymphatics and maturation in situ of migrating murine cutaneous dendritic cells.

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.

Human cutaneous dendritic cells migrate through dermal lymphatic vessels in a skin organ culture model.

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.

Global degranulation of rat mast cells stimulated with DNP-polystyrene.

Mediator release was studied in rat peritoneal mast cells sensitized with a mouse monoclonal anti-DNP IgE antibody, and stimulated with DNP-ornithine covalently attached to radio-derivatized polystyrene petri dishes. Cells releasing serotonin at maximal rates were investigated by transmission electron microscopy. Generalized exocytosis of granules could be observed, suggesting non-directional release of mediators, and non-compartmentalized action of second messengers in mast cells stimulated with polystyrene-bound DNP. Stimulation of sensitized mast cells by DNP covalently bound to the rigid polystyrene surface is consistent with extrinsic mechanisms proposed for Fc(epsilon)RI receptor action, and suggests that internalization of Fc(epsilon)RI is not needed for triggering cell degranulation.

Human herpesvirus-8 infection of umbilical cord-blood-derived CD34+ stem cells enhances the immunostimulatory function of their dendritic cell progeny.

CD34(+) progenitor cells carrying human herpesvirus-8, Kaposi's sarcoma-associated herpesvirus (HHV-8/KSHV), have been described in the peripheral blood of AIDS patients suffering from Kaposi's sarcoma (KS). In this study, we investigated the influence of HHV-8 on the differentiation of CD34(+) progenitor cells. Native CD34(+) cells derived from cord blood could be infected by a laboratory strain of HHV-8, as shown by immunofluorescence staining and polymerase chain reaction, but no significant initial maturation/differentiation effects were observed. In addition, these infected cells were differentiated into immature and mature dendritic cells (DCs) using cytokine induction with recombinant human granulocyte-macrophage colony-stimulating factor (rhGm-CSF), recombinant human tumor necrosis factor (rhTNF-alpha) and recombinant human stem cell factor (rhSCF). Double immunofluorescence and flow cytometry studies demonstrated that virus infection did not impair the development of immature and mature DC populations. Subsequently, the immunostimulating capacity of DC populations was tested in a mixed lymphocyte reaction using allogeneic T-cells. The HHV-8-infected CD34(+) progenitor cell-derived mature DC population showed a significantly enhanced antigen-presenting capacity, compared to non-infected DCs, which was not observed with the immature DCs. This suggests stimulation of DC function by HHV-8 infection. Because there are only a small percentage of HHV-8-positive DCs in the preparations and because it is not clear whether infection is abortive or productive to some extent, this seems to be most likely due to an indirect viral effect.

Antigen processing in populations of mature murine dendritic cells is caused by subsets of incompletely matured cells.

Immature dendritic cells (DC), such as freshly isolated Langerhans cells (LC), are excellent at processing native protein Ag. During short term culture they shut off MHC class II synthesis and down-regulate their processing capacity. They retain, however, the MHC/peptide complexes, up-regulate adhesion and costimulatory molecules, and acquire the ability to sensitize T cells. Two reports describing substantial processing activity in populations of mature DC prompted us to undertake an extensive comparative study of the Ag-processing capacities of immature vs mature DC. We used a panel of 17 peptide-specific T cell hybridomas restricted by six different MHC class II molecules: I-Ab, I-A(d), I-E(d), hybrid I-A beta dE alpha, I-Ak, and I-Ek. Side by side comparisons revealed in all cases that freshly isolated LC were superior to cultured mature LC in their ability to process native proteins. With some hybridomas, however, we found a considerable degree of processing by populations of cultured LC at high doses of Ag or Ag-presenting cells. This activity, however, did not correlate with the MHC haplotype. Direct comparison over wide ranges of DC doses or Ag doses showed that it was always less than that of corresponding fresh immature LC. Immunoperoxidase staining of cytospins and flow cytometry with mAb In1 disclosed a small (20% maximum) subset of cultured LC expressing the MHC class II-associated invariant chain, indicating ongoing biosynthesis of this molecule and, thus, incomplete maturation of these LC. Therefore, the residual processing activity observed in populations of mature DC may be explained by small subpopulations of incompletely matured DC.

Ultrastructural analysis of thymic nurse cell epithelium.

Thymic nurse cells (TNC), a paradigmatic cell type of cortical epithelium, are large lymphoid-epithelial cell complexes of thymocytes enclosed within vacuoles lined by the epithelial cell membrane. TNC express major histocompatibility complex (MHC) class I and class II molecules on their surface and vacuole-lining membranes at high density and it was suggested that TNC provide an optimal microenvironment for positive selection of T cells. In this report we present electron microscopical data demonstrating that chicken TNC display morphological structures of exocytosis previously shown for hormone-secreting cells. In TNC, however, exocytosis is restricted to the capillary cleft between the epithelial cell and engulfed thymocytes. Thus, besides physical contact between the epithelial cell and enclosed thymocytes, TNC may additionally influence the development of thymocytes through release of soluble factors in a restricted microenvironment. By employing the 3-(2,4-dinitroanilino)-3'-amino-N-methyl-propylamine technique which at the ultrastructural level detects acidic organelles involved in processing of antigens presented by MHC class II molecules, we also show that TNC contain acidic compartments similar to classical antigen-presenting cells, i.e. early and late endosomes and lysosomes, albeit in a lower amount than in thymic dendritic cells. This fact provides evidence that TNC not only are capable of antigen presentation but also possess the intracellular machinery for antigen processing.

Human immunodeficiency virus type 1 derived from cocultures of immature dendritic cells with autologous T cells carries T-cell-specific molecules on its surface and is highly infectious.

During the budding process, human immunodeficiency virus type 1 (HIV-1) acquires cell surface molecules; thus, the viral surface of HIV-1 reflects the antigenic pattern of the host cell. To determine the source of HIV-1 released from cocultures of dendritic cells (DC) with T cells, immature DC (imDC), mature DC (mDC), T cells, and their cocultures were infected with different HIV-1 isolates. The macrophage-tropic HIV-1 isolate Ba-L allowed viral replication in both imDC and mDC, whereas the T-cell-line-tropic primary isolate PI21 replicated in mDC only. By a virus capture assay, HIV-1 was shown to carry a T-cell- or DC-specific cell surface pattern after production by T cells or DC, respectively. Upon cocultivation of HIV-1-pulsed DC with T cells, HIV-1 exclusively displayed a typical T-cell pattern. Additionally, functional analysis revealed that HIV-1 released from imDC-T-cell cocultures was more infectious than HIV-1 derived from mDC-T-cell cocultures and from cultures of DC, T cells, or peripheral blood mononuclear cells alone. Therefore, we conclude that the interaction of HIV-1-pulsed imDC with T cells in vivo might generate highly infectious virus which primarily originates from T cells.