Up-regulation of alpha 4 integrin on activated Langerhans cells: analysis of adhesion molecules on Langerhans cells relating to their migration from skin to draining lymph nodes

Integrin α5 regulates motility of human monocyte-derived Langerhans cells during immune response

Langerhans cells (LCs) are mainly present in the epidermis and mucosa, and have important roles during skin infection. Migration of LCs to lymph nodes is essential for antigen presentation. However, due to the difficulties in isolating and culturing human LCs, it is not fully understood how LCs move and interact with the extracellular matrix (ECM) through their adhesion molecules such as integrin, during the immune responses. In this study, we aimed to investigate LC motility, cell shape and the role of integrin under inflammatory conditions using monocyte-derived Langerhans cells (moLCs) as a model. As a result, lipopolysaccharide (LPS) stimulation increased adhesion on fibronectin coated substrate and integrin α5 expression in moLCs. Time-lapse imaging of moLCs revealed that stimulation with LPS elongated cell shape, whilst decreasing their motility. Additionally, this decrease in motility was not observed when pre-treated with a neutralising antibody targeting integrin α5. Together, our data suggested that activation of LCs decreases their motility by promoting integrin α5 expression to enhance their affinity to the fibronectin, which may contribute to their migration during inflammation.

Cell-permeable transgelin-2 as a potent therapeutic for dendritic cell-based cancer immunotherapy

Background

Transgelin-2 is a 22 kDa actin-binding protein that has been proposed to act as an oncogenic factor, capable of contributing to tumorigenesis in a wide range of human malignancies. However, little is known whether this tiny protein also plays an important role in immunity, thereby keeping body from the cancer development and metastasis. Here, we investigated the functions of transgelin-2 in dendritic cell (DC) immunity. Further, we investigated whether the non-viral transduction of cell-permeable transgelin-2 peptide potentially enhance DC-based cancer immunotherapy.

Methods

To understand the functions of transgelin-2 in DCs, we utilized bone marrow-derived DCs (BMDCs) purified from transgelin-2 knockout (Tagln2^−/−) mice. To observe the dynamic cellular mechanism of transgelin-2, we utilized confocal microscopy and flow cytometry. To monitor DC migration and cognate T–DC interaction in vivo, we used intravital two-photon microscopy. For the solid and metastasis tumor models, OVA⁺ B16F10 melanoma were inoculated into the C57BL/6 mice via intravenously (i.v.) and subcutaneously (s.c.), respectively. OTI TCR T cells were used for the adoptive transfer experiments. Cell-permeable, de-ubiquitinated recombinant transgelin-2 was purified from Escherichia coli and applied for DC-based adoptive immunotherapy.

Results

We found that transgelin-2 is remarkably expressed in BMDCs during maturation and lipopolysaccharide activation, suggesting that this protein plays a role in DC-based immunity. Although Tagln2^−/− BMDCs exhibited no changes in maturation, they showed significant defects in their abilities to home to draining lymph nodes (LNs) and prime T cells to produce antigen-specific T cell clones, and these changes were associated with a failure to suppress tumor growth and metastasis of OVA⁺ B16F10 melanoma cells in mice. Tagln2^−/− BMDCs had defects in filopodia-like membrane protrusion and podosome formation due to the attenuation of the signals that modulate actin remodeling in vitro and formed short, unstable contacts with cognate CD4⁺ T cells in vivo. Strikingly, non-viral transduction of cell-permeable, de-ubiquitinated recombinant transgelin-2 potentiated DC functions to suppress tumor growth and metastasis.

Conclusion

This work demonstrates that transgelin-2 is an essential protein for both cancer and immunity. Therefore, transgelin-2 can act as a double-edged sword depending on how we apply this protein to cancer therapy. Engineering and clinical application of this protein may unveil a new era in DC-based cancer immunotherapy. Our findings indicate that cell-permeable transgelin-2 have a potential clinical value as a cancer immunotherapy based on DCs.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13045-021-01058-6.

Interleukin-32 promotes detachment and activation of human Langerhans cells in a human skin explant model

BACKGROUND

Cross-talk between skin keratinocytes (KCs) and Langerhans cells (LCs) plays a fundamental role in the body's first line of immunological defences. However, the mechanism behind the interaction between these two major epidermal cells is unknown. Interleukin (IL)-32 is produced in inflammatory skin disorders. We questioned the role of IL-32 in the epidermis.

OBJECTIVES

We aimed to determine the role of IL-32 produced by KCs on surrounding LCs.

METHODS

We used an ex vivo human explant model from healthy donors and investigated the role of IL-32 on LC activation using imaging, flow cytometry, reverse transcriptase quantitative polymerase chain reaction and small interfering (si)RNA treatment.

RESULTS

Modified vaccinia virus ankara (MVA) infection induced KC death alongside the early production of the proinflammatory cytokine IL-32. We demonstrated that IL-32 produced by MVA-infected KCs induced modest but significant morphological changes in LCs and downregulation of adhesion molecules, such as epithelial cell adhesion molecule and very late antigen-4, and CXCL10 production. The treatment of KCs with IL-32-specific siRNA, and anti-IL-32 blocking antibody significantly inhibited LC activation, demonstrating the role of IL-32 in LC activation. We also found that some Toll-like receptor ligands induced a very high level of IL-32 production by KCs, which initiated LC activation.

CONCLUSIONS

We propose, for the first time, that IL-32 is a molecular link between KCs and LCs in healthy skin, provoking LC migration from the epidermis to the dermis prior to their migration to the draining lymph nodes.

Dendritic cells: importance in allergy

In this review we discuss the role of dendritic cells (DC) in the pathogenesis of allergic contact hypersensitivity (ACH) and atopic disorders, such as asthma and atopic eczema. In ACH patients, DC recognize the invasion of simple chemicals such as haptens, and trigger antigen-specific T cell responses leading to the characteristic histological and clinical changes such as spongiosis and papulovesicular eruptions. During atopic disorders, it is well known that the Th2-deviated immune response plays a crucial role in their pathogenesis. DC provide T cells with antigen and costimulatory signals (signals 1 and 2, respectively), as well as with a polarizing signal (signal 3). When studying ACH, it is important to understand how simple chemicals induce the activation of DC and their migration to the draining lymph nodes where they supply signals 1 and 2 to naive T cells. The mechanisms by which DC induce the Th2-deviated immune response, namely via the Th2-deviated signal 3, are central topics in the pathogenesis of atopic disorders.

Ultraviolet B radiation suppresses Langerhans cell migration in the dermis by down-regulation of alpha4 integrin

BACKGROUND/PURPOSE

Ultraviolet B (UVB) radiation affects the migration and function of epidermal Langerhans cells (LC) and causes immunosuppression of contact hypersensitivity. It is known that LC leaves the epidermis after exposure to UVB. To know the behavior of LC in the dermis after UVB radiation, we studied the effect of UVB radiation on the expression of integrin families on freshly isolated or cultured murine LC. We also examined whether UVB radiation affects the migration of LC to secondary lymphoid tissue chemokine (SLC/6Ckine).

METHODS

Integrin expressions of murine LC cultured in epidermal cell suspension were analyzed using flowcytometry. We used murine LC sorted flowcytometrically for binding assay to extracellular matrix and for migration assay to chemokine. Skin explant assay and immnohistochemical staining for 'cords formation' were performed as previously described.

RESULTS

Twenty and 40 mJ/cm2 of UVB radiation down-regulated the expression of alpha4 integrin on 24 h-cultured LC, but not that of alpha6, beta1, or beta4 integrin. The number of cultured LC adhered to fibronectin, a ligand for alpha4 integrin, was decreased after UVB irradiation, while that to laminin, a ligand for alpha6 integrin, was not influenced. UVB radiation reduced the number of migrating LC to SLC. Furthermore, skin sheet explant experiments showed that UVB radiation inhibited the 'cords' formation in dermal vessels of the 48 h-cultured skin.

CONCLUSIONS

These data suggest that UVB radiation may suppress the migration of LC from the dermis to lymphatic vessels. UVB radiation may downregulate the adherence of LC to dermal fibronectin and migration to SLC, and consequently suppress the migration of LC from the UVB-irradiated dermis to lymphatics.

Adhesion capacity and integrin expression by dendritic-like cells generated from acute myeloid leukemia blasts by calcium ionophore treatment

OBJECTIVE

Dendritic cells (DC) play a key role in initiation of immune responses. In vitro modified acute myeloid leukemia (AML) blasts acquire certain specific features of DC and are suggested as a potential source of anti-leukemia vaccines. AML-DC have been characterized in terms of costimulatory molecule expression and cytokine production. In contrast, migratory capacity of AML-DC, which is a major attribute of DC required for their in vivo function, remains unknown. Here we present data on adhesion properties and profile of integrin expression of AML-DC.

MATERIALS AND METHODS

Blasts from nine patients were used to generate AML-DC by calcium ionophore treatment. Adhesion of AML-DC to the major components of the extracellular matrix and the profile of integrin expression was studied using flow cytometry.

RESULTS

Similar to their normal counterparts, calcium ionophore-induced AML-DC acquired the ability to bind to fibronectin and in 4 of 7 studied cases to bind to denatured collagen. Adhesion to native collagen remained unchanged during DC-type differentiation of AML blasts. AML-DC and DC obtained from monocytes of healthy donors expressed CD49d, CD49e, alphavbeta3, and alphavbeta5. However, AML-DC from 3 of 8 patients down-regulated CD49d, which plays an important role in cell-to-cell and cell-to-matrix interactions and normally is coexpressed with CD83.

CONCLUSION

The results provide further evidence that AML blasts can be induced to display functional properties characteristic for DC and may prove useful for in vivo delivery and presentation of tumor antigens to the immune system. Abnormal CD49d expression and variability in AML-DC adhesion to denatured collagen indicate that motility of AML-DC from individual patients may vary, and a customized approach is essential for evaluating leukemic cell feasibility for vaccine design.

Activation of Discoidin Domain Receptor 1 Facilitates the Maturation of Human Monocyte-Derived Dendritic Cells Through the TNF Receptor Associated Factor 6/TGF-β-Activated Protein Kinase 1 Binding Protein 1β/p38α Mitogen-Activated Protein Kinase Signaling Cascade

Maturation of dendritic cells (DCs) is critical for their ability to stimulate resting naive T cells in primary immune responses. Previous studies demonstrated that collagen, such as type I collagen, could facilitate DC maturation; however, the basis of collagen-mediated DC maturation remains unclear. Discoidin domain receptor 1 (DDR1) is a nonintegrin collagen receptor constitutively expressed in a variety of epithelial cells, including tumor cells, and is inducible in leukocytes. In this study, we evaluated the role of DDR1 in DC maturation using human monocyte-derived DCs. Two DDR1 isoforms, DDR1a and DDR1b, were expressed in both immature and mature DCs. Activation of DDR1 on immature DCs resulted in their partial maturation; however, DDR1 activation markedly amplified TNF-alpha- and LPS-induced phenotypic and functional maturation of DCs through activation of p38 mitogen-activated protein kinase (MAPK), suggesting the involvement of DDR1b in this process. Activation of DDR1b on differentiated DDR1b-overexpressing THP-1 cells or DDR1 on mature DCs induced the formation of TNFR associated factor 6 (TRAF6)/TGF-beta-activated kinase 1 binding protein 1beta/p38alpha MAPK complex and p38alpha autophosphorylation. Transfection of differentiated DDR1b-overexpressing THP-1 cells with dominant negative TRAF6 completely abrogated DDR1b-mediated p38 MAPK phosphorylation, indicating a critical role of TRAF6 in DDR1b-mediated p38 MAPK activation. Taken together, our data suggest that DDR1b-collagen interaction augments the maturation of DCs in a tissue microenvironment through a unique TRAF6/TGF-beta-activated kinase 1 binding protein 1beta/p38alpha MAPK signaling cascade and contributes to the development of adaptive immune responses.

p38 Mitogen-activated protein kinase and extracellular signal-regulated kinases play distinct roles in the activation of dendritic cells by two representative haptens, NiCl2 and 2,4-dinitrochlorobenzene

Previous studies have demonstrated that haptens induce several phenotypic and functional changes of dendritic cells in vivo as well as in vitro. Although recently, the crucial role of p38 mitogen-activated protein kinase has been reported in the activation of dendritic cells by haptens, the signal transduction elements involved in each phenotypic and functional changes that occur in the activation of dendritic cells by haptens remain unknown. Therefore, we examined the role of mitogen-activated protein kinases and nuclear factor-kappaB in the signal transduction of dendritic cells stimulated with two representative haptens, i.e., NiCl2 and 2,4-dinitrochlorobenzene. Human monocyte-derived dendritic cells stimulated with 2,4-dinitrochlorobenzene induced the phosphorylation of p38 and stress-activated protein kinase/c-jun N-terminal kinases, whereas NiCl2 induced that of p44/42 extracellular signal-regulated kinases, p38, and stress-activated protein kinase/c-jun N-terminal kinases. In addition, NiCl2 phosphorylated inhibitor kappaB and activated nuclear factor-kappaB. In contrast, primary irritants, e.g., benzalkonium chloride, or sodium lauryl sulfate, did not activate these signal transduction pathways. By using specific inhibitors for extracellular signal-regulated kinases and p38 pathways, PD98059 and SB203580, respectively, we demonstrated that the augmentation of CD86, HLA-DR, and CD83, and the production of interleukin-8 along with its increased mRNA expression by monocyte-derived dendritic cells stimulated with 2,4-dinitrochlorobenzene, and the augmentation of CD83 and the interleukin-12 p40 production by monocyte-derived dendritic cells stimulated with NiCl2, were suppressed by SB203580, whereas PD98059 suppressed the production of interleukin-1beta and tumor necrosis factor-alpha, together with their increased mRNA expression by monocyte-derived dendritic cells treated with NiCl2. On the other hand, in spite of the activation of nuclear factor-kappaB by monocyte-derived dendritic cells stimulated with NiCl2, nuclear factor-kappaB inhibitor did not significantly affect the phenotypic and functional changes in the activation of monocyte-derived dendritic cells. These data indicate that NiCl2 and 2,4-dinitrochlorobenzene stimulate different signal transduction pathways in monocyte-derived dendritic cells, and subsequently induce different phenotypic and functional changes in them.

Synthesis and surface expression of hyaluronan by dendritic cells and its potential role in antigen presentation

Hyaluronan (HA) is a large glycosaminoglycan consisting of repeating disaccharide units of glucuronic acid and N-acetylglucosamine. HA is known to act as a filling material of extracellular matrices and as an adhesive substrate for cellular migration. Here we report that dendritic cells (DC) express mRNAs for HA synthases and hyaluronidases, actively synthesize HA, and display HA on their surfaces. Interestingly, HA expression levels on DC were not significantly altered by their maturation states. With respect to physiological function, three specific HA inhibitors, i.e., bovine proteoglycan, a 12-mer HA-binding peptide (GAHWQFNALTVR) termed Pep-1, and an oligomeric Pep-1 formulation, all interfered with DC-induced activation of CD4(+) T cells isolated from DO11.10 TCR transgenic mice. For example, Pep-1 oligomer efficiently inhibited DC-dependent cluster formation, IL-2 and IFN-gamma production, and proliferation by DO11.10 T cells in vitro without affecting the viabilities of DC or T cells, DC function to uptake exogenous proteins, or DC-T cell conjugate formation at earlier time points. These observations suggest a paracrine mechanism by which DC-associated HA facilitates some of the late changes in T cell activation. Although T cells constitutively expressed mRNAs for HA synthases and hyaluronidases, their surface HA expression became detectable only after activation. Oligomeric Pep-1 and bovine proteoglycan both inhibited mitogen-triggered T cell activation in the absence of DC, suggesting an autocrine mechanism by which HA expressed by T cells assists their own activation processes. Finally, adoptively transferred DO11.10 T cells showed progressive mitosis when stimulated with Ag-pulsed DC in living animals, and this clonal expansion was inhibited significantly by administration of Pep-1 oligomer. Our findings may introduce a new concept that relatively simple carbohydrate moieties expressed on DC and perhaps T cells play an important immunomodulatory role during Ag presentation.

Multistep navigation of Langerhans/dendritic cells in and out of the skin

Langerhans cells (LCs) are specialized antigen-presenting cells that reside in the epidermis as sentinels of the immune system. LCs constantly monitor the epidermal microenvironment by taking up antigen and processing it into fragments that can be recognized by cells of the adaptive immune response. Because of their unique migratory ability, LCs can transport antigen from the epidermis to regional lymph nodes, where they can initiate systemic immune responses. The mechanisms of LC trafficking thus seem to be of particular relevance for the induction and maintenance of cutaneous immunity. LCs or their putative precursors express surface molecules that allow them to home to skin and localize in the epidermis for prolonged periods of time. Tissue injury, microbial infection, and other perturbants of epidermal homeostasis (eg, contact allergens) provide danger signals, leading to a local production of proinflammatory cytokines that induce LC mobilization to the lymphoid tissue. At the same time, signals are generated that recruit LC precursors into the skin to maintain the epidermal LC population. Distinct pairs of chemokines and their receptors control the migration from blood to epidermis and from there to the regional lymphatics. In addition, trafficking is controlled at the level of cell adhesion, where LCs downregulate some adhesion molecules to exit the epidermis and upregulate others to migrate across the extracellular matrix and home to T-cell areas of regional lymphoid tissue. The improved understanding of mechanisms that regulate LC trafficking might offer new opportunities for therapeutic interventions to suppress, stimulate, or deviate cutaneous immune responses.

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.

Maturation-dependent expression and function of the CD49d integrin on monocyte-derived human dendritic cells

Dendritic cells (DC) are highly specialized APC that are critical for the initiation of T cell-dependent immune responses. DC exert a sentinel function while immature and, after activation by inflammatory stimuli or infectious agents, mature and migrate into lymphoid organs to prime T cells. We have analyzed integrin expression on monocyte-derived DC (MDDC) and found that expression of CD49d integrins (CD49d/CD29 and CD49d/beta7) was induced/up-regulated during TNF-alpha- or LPS-initiated MDDC maturation, reflecting the induction/up-regulation of CD49d and beta7 mRNA. CD49d mRNA steady-state level increased more than 10 times during maturation, with the highest levels observed 24 h after TNF-alpha treatment. CD49d integrin expression conferred mature MDDC with an elevated capacity to adhere to the CS-1 fragment of fibronectin, and also mediated transendothelial migration of mature MDDC. Up-regulation of CD49d integrin expression closely paralleled that of the mature DC marker CD83. CD49d integrin expression was dependent on cell maturation, as its induction was abrogated by N:-acetylcysteine, which inhibits NF-kappaB activation and the functional and phenotypic maturation of MDDC. Moreover, CD49d integrin up-regulation and MDDC maturation were prevented by SB203580, a specific inhibitor of p38 mitogen-activated protein kinase, but were almost unaffected by the mitogen-activated protein/extracellular signal-related kinase kinase 1/2 inhibitor PD98059. Our results support the existence of a link between functional and phenotypic maturation of MDDC and CD49d integrin expression, thus establishing CD49d as a maturation marker for MDDC. The differential expression of CD49d on immature and mature MDDC might contribute to their distinct motility capabilities and mediate mature DC migration into lymphoid organs.

In vitro treatment of human transforming growth factor-beta1-treated monocyte-derived dendritic cells with haptens can induce the phenotypic and functional changes similar to epidermal Langerhans cells in the initiation phase of allergic contact sensitivity reaction

Human monocyte-derived dendritic cells (MoDCs) obtained from peripheral blood monocytes (PBMC) cultured with granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-4 (IL-4) can be activated in vitro by a variety of simple chemicals such as haptens and several metals. Recently, it has been demonstrated that transforming growth factor-beta1 (TGF-beta1) can induce further differentiation of MoDCs to the cells that share some characteristics with epidermal Langerhans cells, i.e. they contain Birbeck granules and express E-cadherin. In this study, using such TGF-beta1-treated dendritic cells (TGF-beta1+ DCs), we examined the in vitro effects of representative haptens, i.e. NiCl2 and dinitrochlorobenzene (DNCB), on their phenotypic and functional characteristics, comparing with those reported in vivo in epidermal Langerhans cells during the sensitization phase of a contact sensitivity reaction. Treatment of TGF-beta1+ DCs with NiCl2 increased their expression of the molecules related to antigen presentation such as CD86, major histocompatibility complex class I and class II, and CD83, although weakly, in addition to that of those essential for their migration to the regional lymph nodes, such as CD49e, CD44 and its variant 6, while it down-regulated the expression of the molecules required for homing to the skin and staying in the epidermis, such as cutaneous leucocyte antigen (CLA) and E-cadherin. It also increased the production of tumour necrosis factor-alpha, but not that of IL-1beta or IL-12. DNCB also increased their CD86 expression and down-regulated E-cadherin and CLA, but did not affect other phenotypic changes that were observed in TGF-beta1+ DCs treated with NiCl2. TGF-beta1+ DCs treated with either NiCl2 or DNCB increased their allogeneic T-cell stimulatory function. In addition, reverse transcribed polymerase chain reaction revealed augmented expression of chemokine receptor 7 mRNA by TGF-beta1+ DCs when treated with either NiCl2 or DNCB. Moreover, consistent with this data, TGF-beta1+ DCs treated with these chemicals chemotactically responded to macrophage inflammatory protein-3beta. These data suggest the possibility that TGF-beta1+ DCs present a good in vitro model to study the biology of epidermal Langerhans cells.

Langerhans cells migrate to local lymph nodes following cutaneous infection with an arbovirus

Whereas there has been recent interest in interactions between dendritic cells and pathogenic viruses, the role of dendritic cells in the initiation of protective immunity to such organisms has not been elucidated. The aim of this study was to examine whether a resident dendritic cell population in the skin, Langerhans cells, respond to cutaneous viral infections which are effectively cleared by the immune system. We therefore characterized the ability of Langerhans cells to migrate to local draining lymph nodes following infection with the arthropod-borne viruses, West Nile virus or Semliki Forest virus. The data show that major histocompatibility complex class II+/NLDC145+/E-cadherin+ Langerhans cell numbers are increased in the draining lymph nodes of infected mice and this increase is accompanied by a concomitant decrease in the Langerhans cell density in the epidermis. Langerhans cell migration is associated with an accumulation of leukocytes in the lymph node, which is one of the earliest events in the initiation of an immune response. Both the migratory response and the draining lymph node leukocyte accumulation were abrogated if ultraviolet-inactivated instead of live viruses were used, suggesting the activation and subsequent migration of Langerhans cells requires a live, replicating antigen. Our findings are likely to have wider implications for the development of epidermally delivered vaccines and suggest that mobilization of dendritic cells may be involved in the development of immune responses to arthropod-borne viruses.

Generation of Mature Dendritic Cells from a CD14+ Cell Line (XS52) by IL-4, TNF-α, IL-1β, and Agonistic Anti-CD40 Monoclonal Antibody

We established a model system to generate mature dendritic cells (DC) from a GM-CSF-dependent cell line, XS52, which had been isolated from the epidermis of newborn BALB/c mice. Screening of various soluble factors revealed that IL-4 induces phenotypic maturation of XS52 (as evaluated by enhanced expression of class II, CD40, CD80, CD86, CD11c, and loss of expression of CD14) in a time-dependent manner. The addition of TNF-α, IL-1β, and agonistic anti-CD40 mAb further enhanced expression of these maturation markers. Consistent with their phenotypic maturation, these cells (termed XS-DC) exhibited potent Ag-presenting capacity to both naive and primed T cells. In addition, injection of hapten-conjugated XS-DC induced contact hypersensitivity in vivo, suggesting their potential as tools for vaccination. Expression of CD14 by the starting cell population, the requirement for GM-CSF and IL-4, and the relatively long culture period are the common characteristics shared between our cells and human monocyte-derived DC, whose analogues in mice have not been identified. Because large numbers of skin-associated mature DC devoid of other cell lineages are easily obtained, this model system may facilitate the study of molecular events associated with maturation of DC and the use of DC for immunization.

Apoptosis in dendritic cell biology

Apoptosis or programmed cell death regulates many aspects in immunological homeostasis and, thus, controls the initiation, magnitude, duration, and termination of immune responses. Recent studies on dendritic cells (DC), including Langerhans cells (LC), have reinforced this concept by documenting that these antigen presenting cells express surface receptors and ligands that are known to mediate apoptotic cell death and that they are highly susceptible to apoptotic signals. In this review article, four major topics concerning apoptosis in the biology of DC will be overviewed: (a) molecular mechanisms of apoptosis; (b) DC apoptosis induced by various stimuli; (c) regulation of DC apoptosis; and (d) cross-priming and cross-tolerance induced by DC ingesting apoptotic bodies.

Dendritic Cells Undergo Rapid Apoptosis In Vitro During Antigen-Specific Interaction with CD4+ T Cells

The terminal fate of dendritic cells (DC) remains relatively uncertain. In this study, we tested the hypothesis that DC undergo apoptosis after Ag-specific interaction with T cells. When splenic DC isolated from BALB/c mice were cocultured with HDK-1 T cells (a keyhole limpet hemocyanin (KLH)-specific CD4+ Th1 clone) in the presence of KLH, they showed conspicuous cell death as measured by propidium iodide (PI) uptake and chromatin condensation, whereas they remained relatively intact when incubated with either T cells or KLH alone. Likewise, the long term DC line XS52, which was established from BALB/c mouse epidermis, also died rapidly (within 2 h), and they exhibited characteristic DNA laddering when cocultured with HDK-1 T cells in the presence of KLH. RT-PCR and FACS analyses revealed the expression of CD95 (Fas) by XS52 DC and of CD95 ligand (CD95L) (Fas ligand) by activated HDK-1 T cells, suggesting a functional role for these molecules. In fact, anti-CD95L mAb inhibited partially (50%) T cell-mediated XS52 cell death, and coupling of surface CD95 with anti-CD95 mAb triggered significant XS52 cell death, but only in the presence of cycloheximide. Thus, ligation of CD95 (on DC) with CD95L (on T cells) is one, but not the only, mechanism by which T cells induce DC death. Finally, DC isolated from the CD95-deficient mice were found to be significantly more efficient than DC from control mice in their capacity to induce delayed type hypersensitivity responses in vivo. We propose that T cell-induced DC apoptosis serves as a unique down-regulatory mechanism that prevents the interminable activation of T cells by Ag-bearing DC.

Adhesive and/or Signaling Functions of CD44 Isoforms in Human Dendritic Cells

The regulation and function of the CD44 family of surface glycoproteins were investigated in human monocyte-derived dendritic cells (DCs). Variant CD44 isoform transcripts encoding exons v3, v6, and v9 are differently regulated during the differentiation of monocytes into DCs. TNF-α treatment, which induces the maturation of DCs, up-regulates the expression of all v3-, v6-, and v9-containing isoforms examined. CD44 molecules are involved in the adhesion of DCs to immobilized hyaluronate (HA), and v3- and v6-containing variants participate in this function, whereas anti-CD44v9 mAbs were unable to inhibit DC adhesion to HA. The consequences of ligand binding to CD44 were examined by culturing DCs on dishes coated with HA or various anti-CD44 mAbs. HA, the anti-pan CD44 mAb J173, and mAbs directed against v6- and v9-containing (but not v3-containing) isoforms provoked DC aggregation, phenotypic and functional maturation, and the secretion of IL-8, TNF-α, IL-1β, and granulocyte-macrophage CSF. In addition, IL-6, IL-10, and IL-12 were released by DCs stimulated with either J173 or HA, although these cytokines were not detected or were found only at low levels in the culture supernatants of DCs treated with anti-CD44v6 or anti-CD44v9 mAbs. Our study points to distinct capacities of the v3-, v6-, and v9-containing isoforms expressed by human DCs to mediate cell adhesion to HA and/or a signal inducing DC maturation and the secretion of cytokines.

Interactions of dendritic cells with fibronectin and endothelial cells

We studied the phenotypic characteristics of spontaneously migrated skin dendritic cells (sDC) and monocyte-derived dendritic cells (moDC), generated under different culture conditions, and their interactions with fibronectin (FN) and endothelial cells. Monocyte-derived dendritic cells were obtained after culturing monocytes with granulocyte-macrophage colony-stimulating factor (GM-CSF) (800 U/ml) and interleukin-4 (IL-4) (500 U/ml) with either 10% fetal bovine serum (FBS) or 10% allogeneic human serum (HS). Regardless of the type of serum used, the majority of moDC expressed human leucocyte antigen-DR (HLA-DR) and CD86. On day 5 of incubation, 20-67% of moDC cultured in the presence of HS (HS-moDC) expressed CD1a, b and c versus 94-97% when cultured in the presence of FBS (FBS-moDC). DC showed a differential gradient of adhesion to FN: FBS-moDC>HS-moDC>sDC approximately monocytes. Both FBS-moDC and HS-moDC were strongly positive for CD49e (alpha5-integrin) and CD29 (beta1-integrin) but negative for CD49d (alpha4-integrin). A monoclonal antibody (mAb) against CD49e blocked the adhesion of both types of moDC to FN. Although both FBS-moDC and HS-moDC attached to endothelium (a 76% and 63% increase, respectively), only HS-moDC were able to migrate through non-activated endothelium. Overall, these results suggest that spontaneously migrated sDC are less adherent to FN than moDC, that HS and FBS induce differences in CD1 expression, that HS-moDC are less adhesive to FN and endothelial cells but more motile than FBS-moDC, and that alpha5beta1-integrin is the molecule involved in moDC adhesion to FN.

Maturation of dendritic cells induced by cytokines and haptens

Recently, elucidation has progressed on a crucial role played by dendritic cells (DCs) in the induction of primary antigen-specific immune reactions. Although mature DCs exhibit potent antigen presenting function, DCs are scattered in nonlymphoid organs throughout the body as immature cells that have only minimum antigen presenting function. When they are stimulated to maturate, they increase their expression of class II major histocompatibility (MHC) antigen and several co-stimulatory molecules, resulting in the augmentation of antigen presenting function. Furthermore, these maturated DCs move to the T-dependent areas of secondary lymphoid organs to sensitize naive T cells for these antigens. Therefore, it is important to understand the mechanism to induce the maturation of DCs. Recent progress in the study of DC biology depicts various factors, such as cytokines, bacterial products and haptens, which are responsible for DC maturation. In this paper, the mechanism of DC maturation induced by cytokines and chemicals is described.

Alpha 6 integrins are required for Langerhans cell migration from the epidermis

Topical exposure of mice to chemical allergens results in the migration of epidermal Langerhans cells (LCs) from the skin and their accumulation as immunostimulatory dendritic cells (DCs) in draining lymph nodes. Epidermal cell-derived cytokines have been implicated in the maturation and migration of LCs, but the adhesion molecules that regulate LC migration have not been studied. We hypothesized that integrin-mediated interactions with extracellular matrix components of the skin and lymph node may regulate LC/DC migration. We found that alpha 6 integrins and alpha 4 integrins were differentially expressed by epidermal LCs and lymph node DCs. A majority of LCs (70%) expressed the alpha 6 integrin subunit, whereas DCs did not express alpha 6 integrins. In contrast, the alpha 4 integrin subunit was expressed at high levels on DCs but at much lower levels on LCs. The anti-alpha 6 integrin antibody, GoH3, which blocks binding to laminin, completely prevented the spontaneous migration of LCs from skin explants in vitro and the rapid migration of LCs from mouse ear skin induced after intradermal administration of TNF-alpha in vivo. GoH3 also reduced the accumulation of DCs in draining lymph nodes by a maximum of 70% after topical administration of the chemical allergen oxazolone. LCs remaining in the epidermis in the presence of GoH3 adopted a rounded morphology, rather than the interdigitating appearance typical of LCs in naive skin, suggesting that the cells had detached from neighboring keratinocytes and withdrawn cellular processes in preparation for migration, but were unable to leave the epidermis. The anti-alpha 4 integrin antibody PS/2, which blocks binding to fibronectin, had no effect on LC migration from the epidermis either in vitro or in vivo, or on the accumulation of DCs in draining lymph nodes after oxazolone application. RGD-containing peptides were also without effect on LC migration from skin explants. These results identify an important role for alpha 6 integrins in the migration of LC from the epidermis to the draining lymph node by regulating access across the epidermal basement membrane. In contrast, alpha 4 integrins, or other integrin-dependent interactions with fibronectin that are mediated by the RGD recognition sequence, did not influence LC migration from the epidermis. In addition, alpha 4 integrins did not affect the accumulation of LCs as DCs in draining lymph nodes.

CD101 is expressed by skin dendritic cells. Role in T-lymphocyte activation

CD101 was first described in our laboratory using two different monoclonal antibodies, BA27 and BB27, recognizing a 140-kDa disulfide-bonded homodimeric polypeptide on a small subset of circulating T lymphocytes and on most activated T cells in vitro. Further, it has been reported that most intestinal mucosal T lymphocytes expressed CD101. The gene coding for the CD101 antigen has been cloned and found to be identical to the gene coding for the recently described V7 antigen, corresponding to a type I trans-membrane protein with seven immunoglobulin-like loops in its extracellular domain. To define surface proteins that are involved in skin dendritic cell (DC) localization or function, we looked for the expression of CD1O1 on skin DC migrating from human skin explants. The majority of these DC had a phenotype of Langerhans cell (LC)-like mature DC, i.e., HLA-DR+ CD1a+ CD1c+ CD11a+ CD11c+ CD40+ CD50+ CD54+ CD58+ CD80+ CD83+ CD86+. We found that CD101 was expressed by a major subset of these HLA-DR+ CD1a+ CD1c+ LC-like skin DC. Next, we studied the effect of anti-CD101 monoclonal antibodies on primary allogeneic and on soluble antigen-specific mixed skin DC-lymphocyte reactions. We showed that two different monoclonal antibodies, BB27 and V7.1, inhibited the T-lymphocyte proliferative responses and that the inhibitory effect was overcome by high doses of exogenous IL-2. As both DC and T lymphocytes expressed CD101 molecules, we determined that the inhibitory effect was achieved both at the responder T-cell level and at the DC level. Thus, CD101 which is expressed on a subset of circulating T lymphocytes, has also been found on a subpopulation of LC-like DC. This molecule plays a major role in the activation of T cells by skin DC.

Draining lymph node cells of contact-sensitized mice induce suppression of contact sensitivity

The application of hapten to the skin of mice can induce contact sensitivity (CS). It has also been well established that draining lymph node (DLN) cells can induce CS to the hapten used for skin painting when injected into naive mice. This is true for DLN cells recovered about 24 h after skin painting with hapten. It is unclear, however, whether DLN cells recovered shortly after hapten application have the same ability. By using an adoptive transfer assay system, we examined the ability of DLN cells recovered from mice at various times after skin painting with hapten to induce CS. DLN cells harvested 18-24 h after the application of fluorescein isothiocyanate (FITC) or 2,4-dinitrofluorobenzene (DNFB) induced strong CS when injected into naive mice. DLN cells harvested 3-6 h after the application of FITC or DNFB induced either only weak or no CS but induced suppression of the subsequent immunization to the two haptens. The suppression was hapten-specific, MHC restricted, and associated with the appearance of splenic suppressor T lymphocytes. Analyses with antibodies and ultraviolet (UV) B radiation demonstrated that suppression-inducing cells in DLNs were Ia+, Thy-1(-), and functionally UV-sensitive. These data suggest that epicutaneous sensitization with hapten first induces immunologically specific suppressor activity in the draining lymph nodes, whereas immunogenic activity becomes predominant later.

Regulation of CD44 isoform expression and CD44-mediated signaling in human dendritic cells

Dendritic cells (DCs) express CD44, a cell surface receptor for the extracellular matrix ligand hyaluronate, involved in cell-cell interactions and cell migration. Besides the "standard" form of CD44, a variety of splice variants contain an additional extracellular region encoded by 10 "variable" exons termed v1 to v10. The standard form of CD44 as well as variants containing exon v6 (CD44v6) are known to play important roles in the immune system, yet largely unexplored in the DC lineage. In this study, we examined the regulation of CD44 isoforms in human DCs derived from monocytes cultivated in the presence of GM-CSF and IL-4. We found that v3, v6 and v9 variants are all up-regulated upon TNF-alpha stimulation of DCs. In addition, we show that stimulation of DCs using anti-CD44 mAbs can induce DC agregation, up-regulation of accessory molecule expression and secretion of cytokines. A mAb directed against CD44v6 variants was shown to mediate some of these effects.

Tumour necrosis factor receptor II (p75) signalling is required for the migration of Langerhans' cells

Langerhans' cells (LC) represent the major antigen-presenting cells within the epidermis. Following exposure of the skin to antigen, LC take up antigen, migrate into draining lymph nodes (DLN) and present processed antigen to T lymphocytes, thereby initiating an immune response. The molecular mechanisms responsible for LC migration remain unclear. Cytokines, in particular tumour necrosis factor-alpha (TNF-alpha) have been suggested to influence LC migration. There are two distinct membrane receptors for TNF-alpha, TNF receptor I (TNF-R1, p55) and TNF receptor II (TNF-R2, p75), thought to be responsible for distinct TNF-alpha activities. It is believed that most of TNF biological activities are mediated via TNF-R1. In order to examine the role of TNF-R1 signalling in LC migration, we utilized TNF-R1 gene-targeted mutant mice. Following application of the hapten fluorescein isothiocyanate (FITC), FITC-bearing cells in DLN were examined by flow cytometry. A normal number of FITC+/Ia+ cells (LC) were found in DLN from TNF-R1-deficiency mice, suggesting that TNF-R1-dependent signalling is not crucial for LC migration. To investigate the possibility of signalling through TNF-R2, blocking studies using a neutralizing anti-TNF-alpha antibody were performed. The results revealed that anti-TNF-alpha antibody significantly inhibited LC accumulation in DLN in TNF-R1-deficient mice, thus suggesting that TNF-R2 signalling is involved in LC migration from skin to DLN and that murine LC express TNF-R2.

The role of dendritic cells in cutaneous immunity

This article reviews the role of dendritic cells in cutaneous immunity. Langerhans cells (LC) found in the epidermis are the best-characterized dendritic cell population. They have the ability to process antigen in the periphery, transport it to the draining lymph nodes (DLN) where they are able to cluster with, and activate, antigen-specific naive T cells. During migration LC undergo phenotypic and functional changes which enable them to perform this function. There are other less well-characterized dendritic cells including dendritic epidermal T cells, dermal dendrocytes and dermal "LC-like' cells. Although there is no evidence that dendritic epidermal T cells (DETC) can present antigen or migrate to lymph nodes, they do influence the intensity of cutaneous immune responses to chemical haptens. Antigen-presenting cells (APC) in the dermis may provide alternative routes of antigen presentation which could be important in the regulation of skin immune responses. Therefore, dendritic cells are vital for the induction of immune responses to antigens encountered via the skin. LC are particularly important in primary immune responses due to their ability to activate naive T cells. The faster kinetics of secondary responses, and the ability of nonprofessional APC to induce effector function in previously activated cells, suggest that antigen presentation in the DLN may be less important in responses to previously encountered antigens. In these secondary responses, dendritic and nondendritic APC in the skin may directly induce effector functions from antigen-specific recirculating cells.

Interleukin-1 beta and granulocyte-macrophage colony-stimulating factor mediate Langerhans cell maturation differently

It has been reported that the in vivo maturation of Langerhans cells after hapten painting is mediated by IL-1 beta while Langerhans cell maturation after in vitro culture is mediated by granulocyte-macrophage colony-stimulating factor (GM-CSF). To clarify the reason for this discrepancy, we examine the expression of Ia antigen and several co-stimulatory molecules on Langerhans cells that were activated by in vitro culture, by hapten painting, or by an intradermal injection of several cytokines. Both cultured Langerhans cells and those activated by hapten painting increased the expression of Ia antigen and all the co-stimulatory molecules (i.e., intercellular adhesion molecule-1 [ICAM-1], B7-1, B7-2, and CD40). In contrast, an intradermal injection of interleukin-1 beta (IL-1 beta) or tumor necrosis factor-alpha (TNF-alpha) increased the expression of Ia antigen, ICAM-1, B7-2, and CD40, but not that of B7-1. These data indicate that IL-1 beta or TNF-alpha is not sufficient to induce B7-1 expression on Langerhans cells in vivo. Subsequently we examined the effect of anti-cytokine antibodies (Abs) on the expression of those molecules on cultured Langerhans cells. While none of the Abs to IL-1 beta, TNF-alpha, or GM-CSF changed the upregulation of Ia antigen, ICAM-1, or CD40 on cultured Langerhans cells, anti-GM-CSF Ab suppressed that of B7-1 and B7-2. Taken together, our present results suggest that IL-1 beta is required for the upregulation of Ia, ICAM-1, B7-2, and CD40, while GM-CSF is required for the upregulation of B7-1 and B7-2, although it still remains unclear why the injected GM-CSF could not augment B7-1 expression on Langerhans cells in vivo and why anti-IL-1 beta Ab did not suppress the upregulation of Ia, ICAM-1, or CD40 on cultured Langerhans cells.

Oral mucosal Langerhans' cells

Langerhans' cells (LC) are dendritic, antigen-presenting cells present within the epithelium of skin and mucosa, including that of the oral cavity. This article reviews the literature on the phenotypic and functional features of oral mucosal Langerhans' cells, and speculates on other aspects by extrapolating from data on their epidermal counterparts.

Epidermal cytokines in allergic contact dermatitis

The understanding of cutaneous immunology has grown significantly during the past decade, particularly regarding the immune function of keratinocytes. Keratinocytes play a major role in immune and inflammatory reactions, mainly through synthesis and release of cytokines. The cytokine network in the skin is an important contributor to normal homeostasis and to the pathogenesis of cutaneous disease. Although cytokine dysregulation has been implicated in the pathogenesis of many cutaneous diseases, allergic contact dermatitis is one that has been the most extensively studied. The aim of this article is to provide a comprehensive current review of the mechanisms of allergic contact dermatitis with particular emphasis on the role of epidermal cytokines.

Different expression of E-cadherin by two cutaneous gamma/delta TcR+ T-cell subsets, V gamma 5- and V gamma 5+ gamma/delta TcR+ T cells

Recently we have demonstrated that, besides V gamma 5+ gamma/delta TcR+ T cells (V gamma 5+ gamma/delta T cells), V gamma 5- gamma/delta TcR+ T cells (V gamma 5- gamma/delta T cells) are also present in murine skin. In the present study, to characterize the functional differences between these two different cutaneous gamma/delta T cells we examined the expression pattern of E-cadherin and its two integrins. After co-culturing of Ly-5+ epidermal cells and migrating cells from organ-cultured murine skin with cutaneous stromal cells, we could expand V gamma 5+ gamma/delta T cells and V gamma 5- gamma/delta T cells, respectively. Flow cytometry demonstrated that cultured V gamma 5+ gamma/delta T cells expressed E-cadherin, but V gamma 5- gamma/delta T cells did not. This difference in E-cadherin expression was also observed in freshly isolated V gamma 5+ and V gamma 5- gamma/delta T cells. On the other hand, both V gamma 5+ and V gamma 5- gamma/delta T cells expressed the alpha chain of the vitronectin receptor, but did not express the alpha 4 integrin. Of these two cutaneous gamma/delta T cells, only V gamma 5+ gamma/delta T cells adhered to murine keratinocyte cell line, PAM 212 cells. Unexpectedly, however, the adhesion of E-cadherin-expressing V gamma 5+ gamma/delta T cells to PAM 212 cells was not inhibited by anti-E-cadherin antibody, which effectively abrogated the adhesion of Langerhans cells to PAM 212 cells. These distinct phenotypic and functional characteristics in the sub-sets of cutaneous gamma/delta T cells may suggest that they reside in different locations in the skin to play different functional roles in skin immunophysiology.

Murine epidermal Langerhans cells express CD48, which is a counter-receptor for mouse CD2

It has recently been demonstrated that CD48, which is expressed on T cells, B cells, thymocytes and splenocytes, is a ligand for mouse CD2 and that it can function as one of the costimulatory molecules in the activation of T cells. In this study, we examined the expression of CD48 on epidermal Langerhans cells (LC), which are potent antigen-presenting cells in the skin. Both freshly isolated and short-term-cultured LC were shown to express CD48 by flow cytometry. In contrast to most of the adhesion molecules expressed on LC, CD48 expression on short-term-cultured LC did not differ significantly from that on freshly isolated LC. We also examined the contribution of CD48 to antigen presentation by LC. We stimulated the myoglobin-specific T-cell clone, TK.G4, and allogeneic splenic T cells with freshly isolated LC and cultured LC, respectively, in the presence of various concentrations of anti-CD48 monoclonal antibody (mAb). Even at the concentration of 30 micrograms/ml, however, the anti-CD48 mAb did not show any inhibitory effects on either allogeneic or antigen-specific T-cell proliferation, whereas at a concentration 10 micrograms/ml, the anti-CD48 mAb significantly suppressed the proliferation of spleen cells stimulated with phytohaemagglutinin (PHA). These findings show that LC persistently express CD48, although its direct role in antigen presentation has not yet been clarified in vitro.

Up-regulation of CD44 expression by tumor necrosis factor-alpha is neutralized by interleukin-10 in Langerhans cells

CD44 is a principal cell-surface receptor for hyaluronate and is found on a wide variety of cells. CD44 plays an important role in lymphocyte homing, lymphohemopoiesis, and T-cell activation as well as in cell motility and migration. CD44 is expressed on the cell surface of epidermal Langerhans cells (LC), and is one of the candidates for molecules that are involved in the migratory capability of LC, but little is known about its regulatory properties. We examined the modulatory effects of tumor necrosis factor (TNF)-alpha and interleukin (IL)-10 on the CD44 expression in LC. We found 1) that TNF-alpha significantly up-regulated the expression of CD44 in a concentration-dependent manner, 2) that IL-10 down-regulated the expression of CD44 in a concentration-dependent manner, 3) that the effect of TNF-alpha or IL-10 was readily detectable as early as 24 h after the initiation of culture, and 4) that the simultaneous addition of TNF-alpha and IL-10 mutually neutralized the effect of each other. These data suggest that in the epidermal microenvironment the expression of CD44 in LC may be reciprocally regulated by TNF-alpha and IL-10, both of which are known to be produced by surrounding keratinocytes.

Specialized functional properties of the integrin alpha 4 cytoplasmic domain

For functional studies of the integrin alpha 4 cytoplasmic domain, we have expressed the following in K562 and Chinese hamster ovary (CHO) cells: 1) wild-type alpha 4 (called X4C4), 2) two chimeric forms of alpha 4 (called X4C2 and X4C5) that contain the cytoplasmic domains of alpha 2 and alpha 5, respectively, and 3) alpha 4 with no cytoplasmic domain (X4C0). Cytoplasmic domain exchange had no effect on VLA-4-dependent static cell adhesion or tethering to VCAM-1 in conditions of shear flow. However, the presence of the alpha 2 or alpha 5 tails markedly enhanced VLA-4-dependent K562 cells spreading (X4C2 > X4C5 > X4C4 > X4C0), increased localization of VLA-4 into focal adhesion-like complexes in CHO cells (X4C2 > X4C5 > X4C4), and strengthened CHO and K562 cell resistance to detachment from VCAM-1 in conditions of shear flow (X4C2 > X4C5 > X4C4 > X4C0). Conversely, the alpha 4 tail supported greater VLA-4-dependent haptotactic and chemotactic cell migration. In the absence of any alpha tail (i.e., X4C0), robust focal adhesions were observed, even though cell spreading and adhesion strengthening were minimal. Thus, such focal adhesions may have relatively little functional importance, and should not be compared with focal adhesions formed when alpha tails are present. Together, these results indicate that all three alpha-chain tails exert defined positive effects (compared with no tail at all), but suggest that the alpha 4 cytoplasmic domain may be specialized to engage in weaker cytoskeletal interactions, leading to diminished focal adhesion formation, cell spreading, and adhesion strengthening, while augmenting cell migration and facilitating rolling under shear flow. These properties of the alpha 4 tail are consistent with the role of alpha 4 integrins on highly motile lymphocytes, monocytes, and eosinophils.

Expression of B7 costimulatory molecule in cultured human epidermal Langerhans cells is regulated at the mRNA level

Langerhans cells (LC) belong to the dendritic cell lineage and are the principal antigen-presenting cells of squamous epithelia. Short-term cultured LC (cLC) exhibit a marked augmented capacity to stimulate allogeneic T cells and acquire the ability to activate naive T cells, probably in relation to enhanced expression of accessory signals. In this study, we evaluated the expression of B7 costimulatory molecule (CD80) in human freshly isolated (fLC) and cLC at both the protein and mRNA level. Staining of frozen skin sections did not reveal any epidermal dendritic cell reactive with either of two different anti-B7 monoclonal antibodies. fLC in suspension did not exhibit any B7 staining as evaluated by two-color flow-cytometry analysis and immunoelectron microscopy. In contrast, LC that were cultured for 24-72 h displayed strong surface B7 reactivity with a characteristic patchy pattern. Treatment with dispase and trypsin did not reduce B7 staining of cLC. Following warming to 37 degrees C, cLC tagged with anti-B7 monoclonal antibody and gold-conjugated secondary antibody could internalize surface B7 by using the organelles of receptor-mediated endocytosis. B7 mRNA, detected by the reverse-transcriptase polymerase chain reaction technique, was expressed at a low level in purified (> 90% HLA-DR+) fLC but not in LC-depleted epidermal cells, and was markedly upregulated in purified cLC. The results indicate that 1) fLC do not express B7 protein on their surface, but acquire B7 during culture, 2) surface B7 is not sensitive to trypsin, 3) B7 expression is regulated primarily at the mRNA level, and 4) membrane B7 can be internalized within cLC. B7 molecule on CLC may be relevant to their increased antigen-presenting cell potency and ability to stimulate naive T lymphocytes.