A postmigrational switch among skin-derived dendritic cells to a macrophage-like phenotype is predetermined by the intracutaneous cytokine balance

Multifaceted role of CD14 in innate immunity and tissue homeostasis

CD14 is a co-receptor of Toll-like receptor (TLR)- 4, with a critical role in innate immune responses. CD14 recognizes bacterial lipopolysaccharides, pathogen-, and damage-associated molecular patterns, thereby facilitating inflammatory immune responses. In addition to its well-established association with TLR4, CD14 is also implicated in TLR4-independent signaling, which leads to the apoptotic death of differentiated dendritic cells and activation of the noncanonical inflammasome pathway. CD14 also has a role beyond that of the immune responses. It contributes to tissue homeostasis by promoting the clearance of various apoptotic cells via recognizing externalized phosphatidylinositol phosphates. CD14 also has context-dependent roles, particularly in barrier tissues that include the skin and gastrointestinal tract. For example, CD14+ dendritic cells in the skin can induce immunostimulatory or immunosuppressive responses. In the gastrointestinal system, CD14 is involved in producing inflammatory cytokines in inflammatory bowel disease and maintaining of intestinal integrity. This review focuses on the multifaceted roles of CD14 in innate immunity and its potential regulatory functions in barrier tissues characterized by rapid cell renewal. By providing insights into the diverse functions of CD14, this review offers potential therapeutic implications for this versatile molecule in immune modulation and tissue homeostasis.

Single-cell RNAseq provides insight into altered immune cell populations in human fracture nonunions

Nonunion describes bone fractures that fail to heal, resulting in the fracture callus failing to fully ossify or, in atrophic cases, not forming altogether. Fracture healing is regulated, in part, by the balance of proinflammatory and anti-inflammatory processes occurring within the bone marrow and surface cell populations. We sought to further understand the role of osteoimmunology (i.e., study of the close relationship between the immune system and bone) by examining immune cell gene expression via single-cell RNA sequencing of intramedullary canal tissue obtained from human patients with femoral nonunions. Intramedullary canal tissue samples obtained by reaming were collected at the time of surgical repair for femur fracture nonunion (n = 5) or from native bone controls when harvesting autologous bone graft (n = 4). Cells within the samples were isolated and analyzed using the Chromium Single-Cell System (10x Genomics Inc.) and Illumina sequencers. Twenty-three distinct cell clusters were identified, with higher cell proportions in the nonunion samples for monocytes and CD14 + dendritic cells (DCs), and lower proportions of T cells, myelocytes, and promyelocytes in nonunion samples. Gene expression differences were identified in each of the cell clusters from cell types associated with osteoimmunology, including CD14 + DC, monocytes, T cells, promyelocytes, and myelocytes. These results provide human-derived gene profiles that can further our understanding of pathways that may be a cause or a consequence of nonunion, providing the clinical rationale to focus on specific components of osteoimmunology. Clinical significance: The novel single-cell approach may lead to clinically relevant diagnostic biomarkers during earlier stages of nonunion development and/or investigation into therapeutic options.

Transfer of Cellular Content from the Allogeneic Cell-Based Cancer Vaccine DCP-001 to Host Dendritic Cells Hinges on Phosphatidylserine and Is Enhanced by CD47 Blockade

DCP-001 is a cell-based cancer vaccine generated by differentiation and maturation of cells from the human DCOne myeloid leukemic cell line. This results in a vaccine comprising a broad array of endogenous tumor antigens combined with a mature dendritic cell (mDC) costimulatory profile, functioning as a local inflammatory adjuvant when injected into an allogeneic recipient. Intradermal DCP-001 vaccination has been shown to be safe and feasible as a post-remission therapy in acute myeloid leukemia. In the current study, the mode of action of DCP-001 was further characterized by static and dynamic analysis of the interaction between labelled DCP-001 and host antigen-presenting cells (APCs). Direct cell-cell interactions and uptake of DCP-001 cellular content by APCs were shown to depend on DCP-001 cell surface expression of calreticulin and phosphatidylserine, while blockade of CD47 enhanced the process. Injection of DCP-001 in an ex vivo human skin model led to its uptake by activated skin-emigrating DCs. These data suggest that, following intradermal DCP-001 vaccination, local and recruited host APCs capture tumor-associated antigens from the vaccine, become activated and migrate to the draining lymph nodes to subsequently (re)activate tumor-reactive T-cells. The improved uptake of DCP-001 by blocking CD47 rationalizes the possible combination of DCP-001 vaccination with CD47 blocking therapies.

Immune landscape in vulvar cancer-draining lymph nodes indicates distinct immune escape mechanisms in support of metastatic spread and growth

Background

Therapeutic immune intervention is highly dependent on the T-cell priming and boosting capacity of tumor-draining lymph nodes (TDLN). In vulvar cancer, in-depth studies on the immune status of (pre)metastatic TDLN is lacking.

Methods

We have phenotyped and enumerated various T-cell and myeloid subsets in tumor-free (LN−, n=27) and metastatic TDLN (LN+, n=11) using flow cytometry. Additionally, we studied chemokine and cytokine release profiles and assessed expression of indoleamine 2,3-dioxygenase (IDO) in relation to plasmacytoid dendritic cell (pDC) or myeloid subsets.

Results

Metastatic involvement of TDLN was accompanied by an inflamed microenvironment with immune suppressive features, marked by hampered activation of migratory DC, increased cytokine/chemokine release, and closely correlated elevations of pDC and LN-resident conventional DC (LNR-cDC) activation state and frequencies, as well as of terminal CD8⁺ effector-memory T-cell (TemRA) differentiation, regulatory T-cell (Treg) rates, T-cell activation, and expression of cytotoxic T-lymphocyte protein-4 (CTLA-4) and programmed cell death protein-1 (PD-1) immune checkpoints. In addition, high indoleamine 2,3-dioxygenase (IDO) expression and increased frequencies of monocytic myeloid-derived suppressor cells (mMDSC) were observed. Correlation analyses with primary and metastatic tumor burden suggested respective roles for Tregs and suppression of inducible T cell costimulator (ICOS)⁺ T helper cells in early metastatic niche formation and for CD14⁺ LNR-cDC and terminal T-cell differentiation in later stages of metastatic growth.

Conclusions

Metastatic spread in vulvar TDLN is marked by an inflamed microenvironment with activated effector T cells, which are likely kept in check by an interplay of suppressive feedback mechanisms. Our data support (neoadjuvant) TDLN-targeted therapeutic interventions based on CTLA-4 and PD-1 blockade, to reinvigorate memory T cells and curb early metastatic spread and growth.

Modified Plasma Cytokine Profile in Occupational Dermatitis

BACKGROUND

Despite significant progress in the diagnosis of contact dermatitis, the identification by specific tests or biomarkers remains an unsolved issue, particularly when needed for the confirmation of the occupational origin of the disease.

OBJECTIVE

To characterize the plasma proteome profile in occupational dermatitis in workers of paint industry.

METHODS

The study has a case-control design, comparing exposed workers with and without occupational contact dermatitis, matched for age, gender, occupational history, and comorbidities. An immunological assay (Human XL Cytokine Array Kit - ARY022B, R&D Systems) was used to measure the plasma levels of 105 cytokines and chemokines in a pooled sample of the cases and a pooled sample of the controls.

RESULTS

A 1.5-fold increase was noticed for interleukin 3, interleukin 10, and leptin in cases, as compared to controls. Fibroblast growth factor-7 and growth/differentiation factor-15 showed a 1.4-fold increase, while interleukin 19, interleukin 31, and macrophage inflammatory protein 3a.had only a 1.3- fold increase. The leukemia inhibitory factor was the only plasma cytokine that showed a 1.3-fold decrease. All other cytokines had a variation of less than 1.2-fold between cases and controls.

CONCLUSION

The recognition of the molecular signatures is very important for an accurate and indisputable diagnosis of occupational contact dermatitis. In workers from the paint industry, plasma levels of interleukins 3, 10, 13 and 19, fibroblast growth factor-7, and growth/differentiation factor-15, together with leukemia inducible factor, may differentiate subjects with contact dermatitis from those without skin lesions.

Progress on Reconstructed Human Skin Models for Allergy Research and Identifying Contact Sensitizers

Contact with the skin is inevitable or desirable for daily life products such as cosmetics, hair dyes, perfumes, drugs, household products, and industrial and agricultural products. Whereas the majority of these products are harmless, a number can become metabolized and/or activate the immunological defense via innate and adaptive mechanisms resulting in sensitization and allergic contact dermatitis upon following exposures to the same substance. Therefore, strict safety (hazard) assessment of actives and ingredients in products and drugs applied to the skin is essential to determine I) whether the chemical is a potential sensitizer and if so II) what is the safe concentration for human exposure to prevent sensitization from occurring. Ex vivo skin is a valuable model for skin penetration studies but due to logistical and viability limitations the development of in vitro alternatives is required. The aim of this review is to give a clear overview of the organotypic in vitro skin models (reconstructed human epidermis, reconstructed human skin, immune competent skin models incorporating Langerhans Cells and T-cells, skin-on-chip) that are currently commercially available or which are being used in a laboratory research setting for hazard assessment of potential sensitizers and for investigating the mechanisms (sensitization key events 1-4) related to allergic contact dermatitis. The limitations of the models, their current applications, and their future potential in replacing animals in allergy-related science are discussed.

Palmitoylated antigens for the induction of anti-tumor CD8+ T cells and enhanced tumor recognition

Induction of tumor-specific cytotoxic CD8⁺ T cells (CTLs) via immunization relies on the presentation of tumor-associated peptides in major histocompatibility complex (MHC) class I molecules by dendritic cells (DCs). To achieve presentation of exogenous peptides into MHC class I, cytosolic processing and cross-presentation are required. Vaccination strategies aiming to induce tumor-specific CD8⁺ T cells via this exogenous route therefore pose a challenge. In this study, we describe improved CD8⁺ T cell induction and in vivo tumor suppression of mono-palmitic acid-modified (C16:0) antigenic peptides, which can be attributed to their unique processing route, efficient receptor-independent integration within lipid bilayers, and continuous intracellular accumulation and presentation through MHC class I. We propose that this membrane-integrating feature of palmitoylated peptides can be exploited as a tool for quick and efficient antigen enrichment and MHC class I loading. Importantly, both DCs and non-professional antigen-presenting cells (APCs), similar to tumor cells, facilitate anti-tumor immunity by efficient CTL priming via DCs and effective recognition of tumors through enhanced presentation of antigens.

Titanium salts tested in reconstructed human skin with integrated MUTZ-3-derived Langerhans cells show an irritant rather than a sensitizing potential

BACKGROUND

The nature of clinically related adverse reactions to titanium is still unknown.

OBJECTIVE

To determine whether titanium salts have irritant or sensitizing potential in a reconstructed human skin (RHS) model with integrated Langerhans cells (LCs).

METHODS

RHS-LCs (ie, reconstructed epidermis) containing primary differentiated keratinocytes and CFSE⁺ CD1a⁺ -LCs generated from the MUTZ-3 cell line on a primary fibroblast-populated collagen hydrogel (dermis) were topically exposed to titanium(IV) bis(ammonium lactato)dihydroxide (TiALH). LC migration and plasticity were determined.

RESULTS

TiALH resulted in CFSE⁺ CD1a⁺ -LC migration out of the epidermis. Neutralizing antibodies to CCL5 and CXCL12 showed that LC migration was CCL5 and not CXCL12 mediated. LCs accumulating within the dermis after TiALH exposure were CFSE⁺ Lang⁺ CD68⁺ which is characteristic of a phenotypic switch of MUTZ-LC to a macrophage-like cell. Furthermore, TiALH did not result in increased interleukin (IL)-1β or CCR7 messenger RNA (mRNA) in the dermis, but did result in increased IL-10 mRNA. In addition, monocultures of MUTZ-LCs failed to increase LC maturation biomarkers CD83, CD86, and CXCL-8 when exposed to noncytotoxic concentrations of four different titanium salts.

CONCLUSION

These results classify titanium salts as irritants rather than sensitizers and indicate that titanium implant-related complaints could be due to localized irritant-mediated inflammation arising from leachable agents rather than a titanium metal allergy.

Lipo-Based Vaccines as an Approach to Target Dendritic Cells for Induction of T- and iNKT Cell Responses

In this study we developed a liposome-based vaccine containing palmitoylated synthetic long peptides (SLP) and alpha galactosylceramide (αGC) to specifically target dendritic cells (DC) for activation of both innate (invariant natural killer T-cells [iNKT]) and adaptive (CD8⁺ T-cells) players of the immune system. Combination of model tumor specific antigens (gp100/MART-1) formulated as a SLP and αGC in one liposome results in strong activation of CD8⁺ and iNKT, as measured by IFNγ secretion. Moreover, addition of lipo-Lewis Y (Le^Y) to the liposomes for C-type lectin targeting increased not only uptake by monocyte-derived dendritic cells (moDC), dermal dendritic cells and Langerhans cells but also enhanced gp100-specific CD8⁺ T- and iNKT cell activation by human skin-emigrated antigen presenting cells in an ex vivo explant model. Loading of moDC with liposomes containing Le^Y also showed priming of MART-1_26−35L specific CD8⁺ T-cells. In conclusion, chemically linking a lipid tail to a glycan-based targeting moiety and SLP combined with αGC in one liposome allows for easy generation of vaccine formulations that target multiple skin DC subsets and induce tumor antigen specific CD8⁺ T- and iNKT cells. These liposomes present a new vaccination strategy against tumors.

In the mix: the potential benefits of adding GM-CSF to CpG-B in the local treatment of patients with early-stage melanoma

Whereas TLR9 agonists are recognized as powerful stimulators of antitumor immunity, GM-CSF has had mixed reviews. In previously reported randomized trials we assessed the effects of local immune modulation in early-stage melanoma with CpG-B alone or with GM-CSF. Here we discuss the added value of GM-CSF and show sex-related differences.

Constitutively active GSK3β as a means to bolster dendritic cell functionality in the face of tumour-mediated immune suppression

In patients with cancer, the functionality of Dendritic Cells (DC) is hampered by high levels of tumor-derived suppressive cytokines, which interfere with DC development and maturation. Poor DC development can limit the efficacy of immune checkpoint blockade and in vivo vaccination approaches. Interference in intracellular signaling cascades downstream from the receptors of major tumor-associated suppressive cytokines like IL-10 and IL-6, might improve DC development and activation, and thus enhance immunotherapy efficacy. We performed exploratory functional screens on arrays consisting of >1000 human kinase peptide substrates to identify pathways involved in DC development and its inhibition by IL-10 or IL-6. The resulting alterations in phosphorylation of the kinome substrate profile pointed to glycogen-synthase kinase-3β (GSK3β) as a pivotal kinase in both DC development and suppression. GSK3β inhibition blocked human DC differentiation in vitro, which was accompanied by decreased levels of IL-12p70 secretion, and a reduced capacity for T cell priming. More importantly, adenoviral transduction of monocytes with a constitutively active form of GSK3β induced resistance to the suppressive effects of IL-10 and melanoma-derived supernatants alike, resulting in improved DC development, accompanied by up-regulation of co-stimulatory markers, an increase in CD83 expression levels in mature DC, and diminished release of IL-10. Moreover, adenovirus-mediated intratumoral manipulation of this pathway in an in vivo melanoma model resulted in DC activation and recruitment, and in improved immune surveillance and tumor control. We propose the induction of constitutive GSK3β activity as a novel therapeutic means to bolster DC functionality in the tumor microenvironment.

Investigating Human Dendritic Cell Immune Responses to Borrelia burgdorferi

Dendritic cells (DCs) are professional antigen-presenting cells that recognize and phagocytose pathogens, and help to orchestrate adaptive immune responses to combat them. DCs are abundant in the skin where Borrelia burgdorferi first enters the body during a tick bite, and are thus critical in determining the initial stages of the innate and adaptive immune responses against Borrelia. Here, we describe two methods to study the response of DCs to Borrelia; an in vitro approach using monocyte-derived DCs (moDCs) and an ex vivo approach using a human skin model.

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

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

Dendritic Cells in Systemic Lupus Erythematosus: From Pathogenic Players to Therapeutic Tools

System lupus erythematosus (SLE) is a multifactorial systemic autoimmune disease with a wide variety of presenting features. SLE is believed to result from dysregulated immune responses, loss of tolerance of CD4 T cells and B cells to ubiquitous self-antigens, and the subsequent production of anti-nuclear and other autoreactive antibodies. Recent research has associated lupus development with changes in the dendritic cell (DC) compartment, including altered DC subset frequency and localization, overactivation of mDCs and pDCs, and functional defects in DCs. Here we discuss the current knowledge on the role of DC dysfunction in SLE pathogenesis, with the focus on DCs as targets for interventional therapies.

Expression of a mutant prohibitin from the aP2 gene promoter leads to obesity-linked tumor development in insulin resistance-dependent manner

A critical unmet need for the study of obesity-linked cancer is the lack of preclinical models that spontaneously develop obesity and cancer sequentially. Prohibitin (PHB) is a pleiotropic protein that has a role in adipose and immune functions. We capitalized on this attribute of PHB to develop a mouse model for obesity-linked tumor. We achieved this by expressing Y114F-PHB (m-PHB) from the aP2 gene promoter for simultaneous manipulation of adipogenic and immune signaling functions. The m-PHB mice develop obesity in a sex-neutral manner, but only male mice develop impaired glucose homeostasis and hyperinsulinemia similar to transgenic mice expressing PHB. Interestingly, only male m-PHB mice develop histiocytosis with lymphadenopathy, suggesting that metabolic dysregulation or m-PHB alone is not sufficient for the tumor development and that both are required for tumorigenesis. Moreover, ovariectomy in female m-PHB mice resulted in impaired glucose homeostasis, hyperinsulinemia and consequently tumor development similar to male m-PHB mice. These changes were not observed in sham-operated control m-Mito-Ob mice, further confirming the role of obesity-related metabolic dysregulation in tumor development in m-PHB mice. Our data provide a proof-of-concept that obesity-associated hyperinsulinemia promotes tumor development by facilitating dormant mutant to manifest and reveals a sex-dimorphic role of PHB in adipose-immune interaction or immunometabolism. Targeting PHB may provide a unique opportunity for the modulation of immunometabolism in obesity, cancer and in immune diseases.

Phenotypic and Functional Properties of Human Steady State CD14+ and CD1a+ Antigen Presenting Cells and Epidermal Langerhans Cells

Cutaneous antigen presenting cells (APCs) are critical for the induction and regulation of skin immune responses. The human skin contains phenotypically and functionally distinct APCs subsets that are present at two separated locations. While CD1ahigh LCs form a dense network in the epidermis, the CD14+ and CD1a+ APCs reside in the dermal compartment. A better understanding of the biology of human skin APC subsets is necessary for the improvement of vaccine strategies that use the skin as administration route. In particular, progress in the characterization of uptake and activatory receptors will certainly improve APC-targeting strategies in vaccination. Here we performed a detailed analysis of the expression and function of glycan-binding and pattern-recognition receptors in skin APC subsets. The results demonstrate that under steady state conditions human CD1a+ dermal dendritic cells (DCs) were phenotypically most mature as measured by the expression of CD83 and CD86, whereas the CD14+ cells showed a higher expression of the CLRs DC-SIGN, mannose receptor and DCIR and had potent antigen uptake capacity. Furthermore, steady state LCs showed superior antigen cross-presentation as compared to the dermal APC subsets. Our results also demonstrate that the TLR3 ligand polyribosinic-polyribocytidylic acid (pI:C) was the most potent stimulator of cytokine production by both LCs and dDCs. These studies warrant further exploration of human CD1a+ dDCs and LCs as target cells for cancer vaccination to induce anti-tumor immune responses.

Gingiva Equivalents Secrete Negligible Amounts of Key Chemokines Involved in Langerhans Cell Migration Compared to Skin Equivalents

Both oral mucosa and skin have the capacity to maintain immune homeostasis or regulate immune responses upon environmental assault. Whereas much is known about key innate immune events in skin, little is known about oral mucosa. Comparative studies are limited due to the scarce supply of oral mucosa for ex vivo studies. Therefore, we used organotypic tissue equivalents (reconstructed epithelium on fibroblast-populated collagen hydrogel) to study cross talk between cells. Oral mucosa and skin equivalents were compared regarding secretion of cytokines and chemokines involved in LC migration and general inflammation. Basal secretion, representative of homeostasis, and also secretion after stimulation with TNFα, an allergen (cinnamaldehyde), or an irritant (SDS) were assessed. We found that proinflammatory IL-18 and chemokines CCL2, CCL20, and CXCL12, all involved in LC migration, were predominantly secreted by skin as compared to gingiva. Furthermore, CCL27 was predominantly secreted by skin whereas CCL28 was predominantly secreted by gingiva. In contrast, general inflammatory cytokines IL-6 and CXCL8 were secreted similarly by skin and gingiva. These results indicate that the cytokines and chemokines triggering innate immunity and LC migration are different in skin and gingiva. This differential regulation should be figured into novel therapy or vaccination strategies in the context of skin versus mucosa.

Regulation of Dendritic Cell Function by Vitamin D

Studies over the last two decades have revealed profound immunomodulatory aspects of vitamin D on various aspects of the immune system. This review will provide an overview of Vitamin D metabolism, a description of dendritic cell subsets, and highlight recent advances on the effects of vitamin D on dendritic cell function, maturation, cytokine production and antigen presentation. The active form of vitamin D, 1,25(OH)₂D₃, has important immunoregulatory and anti-inflammatory effects. Specifically, the 1,25(OH)₂D₃-Vitamin D₃ complex can affect the maturation and migration of many dendritic cell subsets, conferring a special immunoregulatory role as well as tolerogenic properties affecting cytokine and chemokine production. Furthermore, there have been many recent studies demonstrating the effects of Vitamin D on allergic disease and autoimmunity. A clear understanding of the effects of the various forms of Vitamin D will provide new opportunities to improve human health.

MUTZ-3 derived Langerhans cells in human skin equivalents show differential migration and phenotypic plasticity after allergen or irritant exposure

After allergen or irritant exposure, Langerhans cells (LC) undergo phenotypic changes and exit the epidermis. In this study we describe the unique ability of MUTZ-3 derived Langerhans cells (MUTZ-LC) to display similar phenotypic plasticity as their primary counterparts when incorporated into a physiologically relevant full-thickness skin equivalent model (SE-LC). We describe differences and similarities in the mechanisms regulating LC migration and plasticity upon allergen or irritant exposure. The skin equivalent consisted of a reconstructed epidermis containing primary differentiated keratinocytes and CD1a(+) MUTZ-LC on a primary fibroblast-populated dermis. Skin equivalents were exposed to a panel of allergens and irritants. Topical exposure to sub-toxic concentrations of allergens (nickel sulfate, resorcinol, cinnamaldehyde) and irritants (Triton X-100, SDS, Tween 80) resulted in LC migration out of the epidermis and into the dermis. Neutralizing antibody to CXCL12 blocked allergen-induced migration, whereas anti-CCL5 blocked irritant-induced migration. In contrast to allergen exposure, irritant exposure resulted in cells within the dermis becoming CD1a(-)/CD14(+)/CD68(+) which is characteristic of a phenotypic switch of MUTZ-LC to a macrophage-like cell in the dermis. This phenotypic switch was blocked with anti-IL-10. Mechanisms previously identified as being involved in LC activation and migration in native human skin could thus be reproduced in the in vitro constructed skin equivalent model containing functional LC. This model therefore provides a unique and relevant research tool to study human LC biology in situ under controlled in vitro conditions, and will provide a powerful tool for hazard identification, testing novel therapeutics and identifying new drug targets.

Allogeneic Mature Human Dendritic Cells Generate Superior Alloreactive Regulatory T Cells in the Presence of IL-15

Expansion of Ag-specific naturally occurring regulatory T cells (nTregs) is required to obtain sufficient numbers of cells for cellular immunotherapy. In this study, different allogeneic stimuli were studied for their capacity to generate functional alloantigen-specific nTregs. A highly enriched nTreg fraction (CD4(+)CD25(bright)CD127(-) T cells) was alloantigen-specific expanded using HLA-mismatched immature, mature monocyte-derived dendritic cells (moDCs), or PBMCs. The allogeneic mature moDC-expanded nTregs were fully characterized by analysis of the demethylation status within the Treg-specific demethylation region of the FOXP3 gene and the expression of both protein and mRNA of FOXP3, HELIOS, CTLA4, and cytokines. In addition, the Ag-specific suppressive capacity of these expanded nTregs was tested. Allogeneic mature moDCs and skin-derived DCs were superior in inducing nTreg expansion compared with immature moDCs or PBMCs in an HLA-DR- and CD80/CD86-dependent way. Remarkably, the presence of exogenous IL-15 without IL-2 could facilitate optimal mature moDC-induced nTreg expansion. Allogeneic mature moDC-expanded nTregs were at low ratios (<1:320), potent suppressors of alloantigen-induced proliferation without significant suppression of completely HLA-mismatched, Ag-induced proliferation. Mature moDC-expanded nTregs were highly demethylated at the Treg-specific demethylation region within the FOXP3 gene and highly expressed of FOXP3, HELIOS, and CTLA4. A minority of the expanded nTregs produced IL-10, IL-2, IFN-γ, and TNF-α, but few IL-17-producing nTregs were found. Next-generation sequencing of mRNA of moDC-expanded nTregs revealed a strong induction of Treg-associated mRNAs. Human allogeneic mature moDCs are highly efficient stimulator cells, in the presence of exogenous IL-15, for expansion of stable alloantigen-specific nTregs with superior suppressive function.

In situ Delivery of Antigen to DC-SIGN(+)CD14(+) Dermal Dendritic Cells Results in Enhanced CD8(+) T-Cell Responses

CD14(+) dendritic cells (DCs) present in the dermis of human skin represent a large subset of dermal DCs (dDCs) that are considered macrophage-like cells with poor antigen (cross)-presenting capacity and limited migratory potential to the lymph nodes. CD14(+) dDC highly express DC-specific ICAM-3-grabbing non-integrin (DC-SIGN), a receptor containing potent endocytic capacity, facilitating intracellular routing of antigens to major histocompatibility complex I and II (MHC-I andII) loading compartments for the presentation to antigen-specific CD8(+) and CD4(+) T cells. Here we show using a human skin explant model that the in situ targeting of antigens to DC-SIGN using glycan-modified liposomes enhances the antigen-presenting capacity of CD14(+) dDCs. Intradermal vaccination of liposomes modified with the DC-SIGN-targeting glycan Lewis(X), containing melanoma antigens (MART-1 or Gp100), accumulated in CD14(+) dDCs and resulted in enhanced Gp100- or MART-1-specific CD8(+) T-cell responses. Simultaneous intradermal injection of the cytokines GM-CSF and IL-4 as adjuvant enhanced the migration of the skin DCs and increased the expression of DC-SIGN on the CD14(+) and CD1a(+) dDCs. These data demonstrate that human CD14(+) dDCs exhibit potent cross-presenting capacity when targeted in situ through DC-SIGN.

Functional characterization of a STAT3-dependent dendritic cell-derived CD14+ cell population arising upon IL-10-driven maturation

Interleukin (IL)-10 is a major cancer-related immunosuppressive factor, exhibiting a unique ability to hamper the maturation of dendritic cells (DCs). We have previously reported that IL-10 induces the conversion of activated, migratory CD1a⁺ DCs found in the human skin to CD14⁺CD141⁺ macrophage-like cells. Here, as a model of tumor-conditioned DC maturation, we functionally assessed CD14^- and CD14⁺ DCs that matured in vitro upon exposure to IL-10. IL-10-induced CD14⁺ DCs were phenotypically characterized by a low maturation state as well as by high levels of BDCA3 and DC-SIGN, and as such they closely resembled CD14⁺ cells infiltrating melanoma metastases. Compared with DC matured under standard conditions, CD14⁺ DCs were found to express high levels of B7-H1 on the cell surface, to secrete low levels of IL-12p70, to preferentially induce TH2 cells, to have a lower allogeneic TH cell and tumor antigen-specific CD8⁺ T-cell priming capacity and to induce proliferative T-cell anergy. In contrast to their CD14⁺ counterparts, CD14^- monocyte-derived DCs retained allogeneic TH priming capacity but induced a functionally anergic state as they completely abolished the release of effector cytokines. Transcriptional and cytokine release profiling studies indicated a more profound angiogenic and pro-invasive signature of CD14⁺ DCs as compared with DCs matured in standard conditions or CD14⁻ DCs matured in the presence of IL-10. Importantly, signal transducer and activator of transcription 3 (STAT3) depletion by RNA interference prevented the development of the IL-10-associated CD14⁺ phenotype, allowing for normal DC maturation and providing a potential means of therapeutic intervention.

Human skin dendritic cells in health and disease

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Human skin dendritic cells (DCs) are heterogenous and functionally specialised.

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Factor XIIIa+ dermal dendrocytes are resident dermal macrophages.

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Dermal CD14+ cells, previously defined as DCs, are monocyte-derived macrophages.

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Dynamic changes occur in the composition of recruited ‘inflammatory’ DCs and resident DCs in inflamed skin.

Dendritic cells (DCs) are specialized antigen presenting cells abundant in peripheral tissues such as skin where they function as immune sentinels. Skin DCs migrate to draining lymph node where they interact with naïve T cells to induce immune responses to microorganisms, vaccines, tumours and self-antigens. In this review, we present the key historical developments and recent advances in human skin DC research. We also integrate the current understanding on the origin and functional specializations of DC subsets in healthy skin with findings in inflammatory skin diseases focusing on psoriasis and atopic eczema. A comprehensive understanding of the dynamic changes in DC subsets in health and disease will form a strong foundation to facilitate the clinical translation of DC-based therapeutic and vaccination strategies.

Human dermal CD14⁺ cells are a transient population of monocyte-derived macrophages

Dendritic cells (DCs), monocytes, and macrophages are leukocytes with critical roles in immunity and tolerance. The DC network is evolutionarily conserved; the homologs of human tissue CD141^hiXCR1⁺CLEC9A⁺ DCs and CD1c⁺ DCs are murine CD103⁺ DCs and CD64⁻CD11b⁺ DCs. In addition, human tissues also contain CD14⁺ cells, currently designated as DCs, with an as-yet unknown murine counterpart. Here we have demonstrated that human dermal CD14⁺ cells are a tissue-resident population of monocyte-derived macrophages with a short half-life of <6 days. The decline and reconstitution kinetics of human blood CD14⁺ monocytes and dermal CD14⁺ cells in vivo supported their precursor-progeny relationship. The murine homologs of human dermal CD14⁺ cells are CD11b⁺CD64⁺ monocyte-derived macrophages. Human and mouse monocytes and macrophages were defined by highly conserved gene transcripts, which were distinct from DCs. The demonstration of monocyte-derived macrophages in the steady state in human tissue supports a conserved organization of human and mouse mononuclear phagocyte system.

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Human dermal CD14⁺ cells are a transient population of macrophages

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Dermal CD14⁺ cells are derived from circulating blood monocytes

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Human CD14⁺ cells are homologous to murine CD11b⁺CD64⁺ monocyte-derived macrophages

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Human and mouse mononuclear phagocyte network organization is conserved

In situ loading of skin dendritic cells with apoptotic bleb-derived antigens for the induction of tumor-directed immunity

The generation and loading of dendritic cells (DC) ex-vivo for tumor vaccination purposes is laborious and costly. Direct intradermal (i.d.) administration of tumor-associated antigens could be an attractive alternative approach, provided that efficient uptake and cross-presentation by appropriately activated skin DCs can be achieved. Here, we compare the efficiency of i.d. delivery of relatively small apoptotic blebs (diameter ∼0.1-1 μm) derived from MART-1 transduced acute myeloid leukemia (AML) HL60 cells, to that of larger apoptotic cell remnants (ACR; 2-10 μm) in a physiologically highly relevant human skin explant model. Injection of either fluorescently-labelled ACRs or blebs alone did not affect the number or distribution of migrated DC subsets from skin biopsies after 48 hours, but resulted in a general up-regulation of the co-stimulatory molecules CD83 and CD86 on skin DCs that had ingested apoptotic material. We have previously shown that i.d. administration of GM-CSF and IL-4 resulted in preferential migration of a mature and highly T cell-stimulatory CD11^hiCD1a⁺CD14^- dermal DC subset. Here, we found that co-injection of GM-CSF and IL-4 together with either ACRs or blebs resulted in uptake efficiencies within this dermal DC subset of 7.6% (±6.1%) and 19.1% (±15.9%), respectively, thus revealing a significantly higher uptake frequency of blebs (P < 0.02). Intradermal delivery of tumor-derived blebs did not affect the T-cell priming and T_H-skewing abilities of migratory skin DC. Nevertheless, in contrast to i.d. administration of ACR, the injection of blebs lead to effective cross-presentation of MART-1 to specific CD8⁺ effector T cells. We conclude that apoptotic bleb-based vaccines delivered through the skin may offer an attractive, and broadly applicable, cancer immunotherapy.

Induction of dendritic cell maturation in the skin microenvironment by soluble factors derived from colon carcinoma

Autologous tumor cell-based vaccines provide a wide range of tumor antigens and personalized neo-epitopes based on individual tumors' unique antigenic mutanome signatures. However, tumor-derived factors may hamper in situ maturation of dendritic cells (DC) and thus interfere with the generation of effective anti-tumor immunity. As the skin is a preferred site for tumor vaccine delivery, we investigated the influence of primary colon carcinoma-derived soluble factors on the maturation state of migrating DC in a human skin explant model. Primary tumor-derived supernatants (TDSN) enhanced the phenotypic maturation state of skin-emigrated DC, resulting in an increased T-cell stimulatory ability in an allogeneic mixed leukocyte response. In case of monocyte-derived DC a similar TDSN-induced maturation induction was found to entirely depend on cyclooxygenase (COX)-regulated prostaglandins. In contrast, the increase in skin-emigrated DC maturation was completely prostaglandin-independent, as evidenced by the inability of the COX inhibitor indomethacin to abrogate this TDSN-induced effect. Although TDSN conditioning affected a drop in IL-12p70 release by the skin-emigrated DC and induced a predominant Th17/Th22 transcriptional profile in subsequently stimulated T-cells, Th cell subset differentiation, as assessed by intracellular cytokine expression upon polyclonal priming and re-stimulation, was not affected. Comparative analysis of phenotypic and transcriptional profiles suggests that the observed maturational effects in skin-derived DC may have been induced by tumor-derived GM-CSF. In conclusion, soluble factors derived from whole-cell colon tumor vaccines will not negatively impact DC migration and maturation in human skin, but rather induce DC maturation that will facilitate the priming of a poly-functional Th cell response.

Vitamin D₃ and phenylbutyrate promote development of a human dendritic cell subset displaying enhanced antimicrobial properties

A promising strategy in the fight against multidrug-resistant pathogens is the induction of endogenous AMPs, with compounds such as VitD₃ and PBA. These compounds display an array of immunomodulatory effects that remain to be investigated in further detail to establish their role in the clearance of infection and possible modulation of AMP expression. Here, we have investigated the effects of VitD₃ and PBA on human monocyte-DC differentiation and found that VitD₃ and PBA promote the development of a stretched CD14⁺/CD1a⁻ DC subset. This subset produced enhanced levels of ROS and human cathelicidin; furthermore, it displayed enhanced killing capacity of Staphylococcus aureus compared with control DCs. When experiments were performed in WT and cathelicidin-deficient mice, we established that a ROS-producing, stretched DC subset was also induced in mouse-derived cells, independent of cathelicidin expression. However, in contrast to the human DCs, enhanced cathelicidin expression and enhanced antimicrobial activities were not found in the murine VitD₃/PBA DC subset. In conclusion, the results of this study show that VitD₃ and PBA induce a human DC subset that is effective against infection. These results promote further research into the use of these compounds as an antimicrobial treatment strategy.

Dendritic Cell Plasticity in Tumor-Conditioned Skin: CD14(+) Cells at the Cross-Roads of Immune Activation and Suppression

Tumors abuse myeloid plasticity to re-direct dendritic cell (DC) differentiation from T cell stimulatory subsets to immune-suppressive subsets that can interfere with anti-tumor immunity. Lined by a dense network of easily accessible DC the skin is a preferred site for the delivery of DC-targeted vaccines. Various groups have recently been focusing on functional aspects of DC subsets in the skin and how these may be affected by tumor-derived suppressive factors. IL-6, Prostaglandin-E2, and IL-10 were identified as factors in cultures of primary human tumors responsible for the inhibited development and activation of skin DC as well as monocyte-derived DC. IL-10 was found to be uniquely able to convert fully developed DC to immature macrophage-like cells with functional M2 characteristics in a physiologically highly relevant skin explant model in which the phenotypic and functional traits of “crawl-out” DC were studied. Mostly from mouse studies, the JAK2/STAT3 signaling pathway has emerged as a “master switch” of tumor-induced immune suppression. Our lab has additionally identified p38-MAPK as an important signaling element in human DC suppression, and recently validated it as such in ex vivo cultures of single-cell suspensions from melanoma metastases. Through the identification of molecular mechanisms and signaling events that drive myeloid immune suppression in human tumors, more effective DC-targeted cancer vaccines may be designed.

Human dendritic cell subsets

Dendritic cells are highly adapted to their role of presenting antigen and directing immune responses. Developmental studies indicate that DCs originate independently from monocytes and tissue macrophages. Emerging evidence also suggests that distinct subsets of DCs have intrinsic differences that lead to functional specialisation in the generation of immunity. Comparative studies are now allowing many of these properties to be more fully understood in the context of human immunology.

Effects of antigen-expressing immunostimulatory liposomes on chemotaxis and maturation of dendritic cells in vitro and in human skin explants

PURPOSE

Antigen-Expressing Immunostimulatory Liposomes (AnExILs) represent a novel DNA vaccination platform based on the production of protein antigens from DNA templates inside liposomes mediated by an in vitro transcription and translation (IVTT) mix. The aim of this study was to analyze the effects of AnExILs on different dendritic cells (DCs) models and to better understand the role of the different components of this formulation on its adjuvanticity.

METHODS

The effect of β-galactosidase-expressing AnExILs on maturation and particle uptake by murine DC cell line, fresh human monocyte-derived DCs or human dermal DCs in skin explants was investigated and compared to the effects of either plain liposomes or IVTT mix alone.

RESULTS

AnExILs induced efficient DC chemotaxis and promoted up-regulation of maturation markers on murine DCs, due to the presence of IVTT in the formulation. Furthermore, the amount of active βGal associated with DCs was higher for AnExILs than for free βGal expressed in IVTT or βGal encapsulated into non-adjuvanted liposomes. Most interestingly, the same trend was observed with human DCs.

CONCLUSIONS

Both IVTT mix and liposomal vehicles were shown to be key components of the AnExIL formulation responsible for its adjuvanticity. AnExILs combine antigen production, adjuvanticity and delivery in one system, and can efficiently activate both murine and human DCs.

IL-10 conditioning of human skin affects the distribution of migratory dendritic cell subsets and functional T cell differentiation

In cancer patients pervasive systemic suppression of Dendritic Cell (DC) differentiation and maturation can hinder vaccination efficacy. In this study we have extensively characterized migratory DC subsets from human skin and studied how their migration and T cell-stimulatory abilities were affected by conditioning of the dermal microenvironment through cancer-related suppressive cytokines. To assess effects in the context of a complex tissue structure, we made use of a near-physiological skin explant model. By 4-color flow cytometry, we identified migrated Langerhans Cells (LC) and five dermis-derived DC populations in differential states of maturation. From a panel of known tumor-associated suppressive cytokines, IL-10 showed a unique ability to induce predominant migration of an immature CD14(+)CD141(+)DC-SIGN(+) DC subset with low levels of co-stimulatory molecules, up-regulated expression of the co-inhibitory molecule PD-L1 and the M2-associated macrophage marker CD163. A similarly immature subset composition was observed for DC migrating from explants taken from skin overlying breast tumors. Whereas predominant migration of mature CD1a(+) subsets was associated with release of IL-12p70, efficient Th cell expansion with a Th1 profile, and expansion of functional MART-1-specific CD8(+) T cells, migration of immature CD14(+) DDC was accompanied by increased release of IL-10, poor expansion of CD4(+) and CD8(+) T cells, and skewing of Th responses to favor coordinated FoxP3 and IL-10 expression and regulatory T cell differentiation and outgrowth. Thus, high levels of IL-10 impact the composition of skin-emigrated DC subsets and appear to favor migration of M2-like immature DC with functional qualities conducive to T cell tolerance.

Intradermal delivery of TLR agonists in a human explant skin model: preferential activation of migratory dendritic cells by polyribosinic-polyribocytidylic acid and peptidoglycans

TLR agonists are attractive candidate adjuvants for therapeutic cancer vaccines as they can induce a balanced humoral and T cell-mediated immune response. With a dense network of dendritic cells (DCs) and draining lymphatics, the skin provides an ideal portal for vaccine delivery. Beside direct DC activation, TLR agonists may also induce DC activation through triggering the release of inflammatory mediators by accessory cells in the skin microenvironment. Therefore, a human skin explant model was used to explore the in vivo potential of intradermally delivered TLR agonists to stimulate Langerhans cells and dermal DCs in their natural complex tissue environment. The skin-emigrated DCs were phenotyped and analyzed for T cell stimulatory capacity. We report that, of six tested TLR-agonists, the TLR2 and -3 agonists peptidoglycan (PGN) and polyribosinic-polyribocytidylic acid (Poly I:C) were uniquely able to enhance the T cell-priming ability of skin-emigrated DCs, which, in the case of PGN, was accompanied by Th1 polarization. The enhanced priming capacity of Poly I:C-stimulated DCs was associated with a strong upregulation of appropriate costimulatory molecules, including CD70, whereas that of PGN-stimulated DCs was associated with the release of a broad array of proinflammatory cytokines. Transcriptional profiling further supported the notion that the PGN- and Poly I:C-induced effects were mediated through binding to TLR2/nucleotide-binding oligomerization domain 2 and TLR3/MDA5, respectively. These data warrant further exploration of PGN and Poly I:C, alone or in combination, as DC-targeted adjuvants for intradermal cancer vaccines.

Chapter six--Adenovirus-based immunotherapy of cancer: promises to keep

Progress in vector design and an increased knowledge of mechanisms underlying tumor-induced immune suppression have led to a new and promising generation of Adenovirus (Ad)-based immunotherapies, which are discussed in this review. As vaccine vehicles Ad vectors (AdVs) have been clinically evaluated and proven safe, but a major limitation of the commonly used Ad5 serotype is neutralization by preexistent or rapidly induced immune responses. Genetic modifications in the Ad capsid can reduce intrinsic immunogenicity and facilitate escape from antibody-mediated neutralization. Further modification of the Ad hexon and fiber allows for liver and scavenger detargeting and selective targeting of, for example, dendritic cells. These next-generation Ad vaccines with enhanced efficacy are now becoming available for testing as tumor vaccines. In addition, AdVs encoding immune-modulating products may be used to convert the tumor microenvironment from immune-suppressive and proinvasive to proinflammatory, thus facilitating cell-mediated effector functions that can keep tumor growth and invasion in check. Oncolytic AdVs, that selectively replicate in tumor cells and induce an immunogenic form of cell death, can also be armed with immune-activating transgenes to amplify primed antitumor immune responses. These novel immunotherapy strategies, employing highly efficacious AdVs in optimized configurations, show great promise and warrant clinical exploration.

Characterization of four conventional dendritic cell subsets in human skin-draining lymph nodes in relation to T-cell activation

To increase (tumor) vaccine efficacy, there is an urgent need for phenotypic and functional characterization of human dendritic cell (DC) subsets residing in lymphoid tissues. In this study we identified and functionally tested 4 human conventional DC (cDC) subsets within skin-draining sentinel lymph nodes (SLNs) from early-stage melanoma patients. These SLNs were all tumor negative and were removed on average 44 days after excision of the primary melanoma. As such, they were considered representative of steady-state conditions. On comparison with skin-migrated cDC, 2 CD1a(+) subsets were identified as most likely skin-derived CD11c(int) Langerhans cells (LC) with intracellular langerin and E-cadherin expression or as CD11c(hi) dermal DCs with variable expression of langerin. Two other CD1a(-) LN-residing cDC subsets were characterized as CD14(-)BDCA3(hi)CD103(-) and CD14(+)BDCA3(lo)CD103(+), respectively. Whereas the CD1a(+) skin-derived subsets displayed greater levels of phenotypic maturation, they were associated with lower levels of inflammatory cytokine release and were inferior in terms of allogeneic T-cell priming and IFNγ induction. Thus, despite their higher maturation state, skin-derived cDCs (and LCs in particular) proved inferior T-cell activators compared with the CD1a(-) cDC subsets residing in melanoma-draining LNs. These observations should be considered in the design of DC-targeting immunotherapies.

The dermis as a portal for dendritic cell-targeted immunotherapy of cutaneous melanoma

Complete surgical excision at an early stage remains the only curative treatment for cutaneous melanoma with few available adjuvant therapy options. Nevertheless, melanoma is a relatively immunogenic tumor type and particularly amenable to immunotherapeutic approaches. A dense network of cutaneous dendritic cells (DC) may account for the reported efficacy of vaccination through the skin and provide an attractive target for the immunotherapy of melanoma. Several phenotypically distinct DC subsets are discernable in the skin, among others, epidermal Langerhans cells and dermal DC. Upon appropriate activation both subsets can efficiently migrate to melanoma-draining lymph nodes (LN) to prime T cell-mediated responses. Unfortunately, from an early stage, melanoma development is characterized by strong immune suppression, facilitating unchecked tumor growth and spread. Particularly the primary tumor site and the first-line tumor-draining LN, the so-called sentinel LN, bear the brunt of this melanoma-induced immune suppression-and these are exactly the sites where anti-melanoma effector T cell responses should be primed by DC in order to prevent early metastasis. Through local immunopotentiation or through DC-targeted vaccination, the dermis may be utilized as a portal to activate DC and kick-start or boost effective T cell-mediated anti-melanoma immunity, even in the face of this immune suppression.

Cross-talk between tumor and myeloid cells: how to tip the balance in favor of antitumor immunity

Myeloid differentiation is often disturbed in cancer, leading to reduced frequencies of immunostimulatory dendritic cells and an over-representation of immunosuppressive immature myeloid cells, granulocytes and macrophages. As a result of this skewed myeloid differentiation, a highly immunosuppressive myeloid subset becomes prevalent during cancer development; these myeloid-derived suppressor cells are also recruited as a collateral to certain protumorigenic inflammatory processes, resulting in an effective downregulation of T-cell-mediated immune surveillance and antitumor immunity. In this article, some of the important myeloid cell subsets and mediators involved in cancer-related immune suppression are reviewed. Furthermore, cross-talk between tumors and the myeloid compartment, and ways in which it can suppress effective cell-mediated immunity, are discussed, as well as possible therapeutic approaches to tip the balance in favor of antitumor immunity.

Insight into the immunobiology of human skin and functional specialization of skin dendritic cell subsets to innovate intradermal vaccination design

Dendritic cells (DC) are the key initiators and regulators of any immune response which determine the outcome of CD4(+) and CD8(+) T-cell responses. Multiple distinct DC subsets can be distinguished by location, phenotype, and function in the homeostatic and inflamed human skin. The function of steady-state cutaneous DCs or recruited inflammatory DCs is influenced by the surrounding cellular and extracellular skin microenvironment. The skin is an attractive site for vaccination given the extended local network of DCs and the easy access to the skin-draining lymph nodes to generate effector T cells and immunoglobulin-producing B cells for long-term protective immunity. In the context of intradermal vaccination we describe in this review the skin-associated immune system, the characteristics of the different skin DC subsets, the mechanism of antigen uptake and presentation, and how the properties of DCs can be manipulated. This knowledge is critical for the development of intradermal vaccine strategies and supports the concept of intradermal vaccination as a superior route to the conventional intramuscular or subcutaneous methods.

Selective transduction of mature DC in human skin and lymph nodes by CD80/CD86-targeted fiber-modified adenovirus-5/3

In vivo targeting of dendritic cells (DC) represents an attractive alternative to currently apply ex vivo DC-based genetic tumor vaccination protocols. Finding the optimal vector for in vivo targeting of DC is important for such strategies. We, therefore, tested a panel of subgroup C/B chimeric and fiber-modified adenoviruses (Ads) for their relative capacity to transduce human DC. We made use of in vitro generated Langerhans cells, and of ex vivo human skin and melanoma-draining lymph node derived DC. Of the tested viruses the C/B-chimeric adenovirus serotype 5 (Ad5)/3 virus most efficiently transduced in vitro generated Langerhans cells. In addition, Ad5/3 preferentially targeted mature myeloid DC from human skin and draining lymph node and transduced them at significantly higher frequencies than Ad5. In addition, Ad5/3 was more specific for mature human skin-derived CD1a+ CD83+ DC than the previously reported DC-transducing C/B-chimeric vector Ad5/35, infecting less bystander cells. It was previously reported that Ad5/3 transduced human monocyte-derived DC by binding to the B7 molecules CD80 and CD86. High-efficiency transduction of mature skin-derived DC was similarly shown to be mediated through binding to CD80/CD86 and not to interfere with subsequent T-cell priming. We conclude that Ad5/3, in combination with DC-activating adjuvants, represents a promising therapeutic tool for the in vivo transduction of mature DC, and may be less likely to induce unwanted side effects such as immune tolerance through the infection of nonprofessional antigen-presenting cells.

Development of an ex vivo human skin model for intradermal vaccination: tissue viability and Langerhans cell behaviour

The presence of resident Langerhans cells (LCs) in the epidermis makes the skin an attractive target for DNA vaccination. However, reliable animal models for cutaneous vaccination studies are limited. We demonstrate an ex vivo human skin model for cutaneous DNA vaccination which can potentially bridge the gap between pre-clinical in vivo animal models and clinical studies. Cutaneous transgene expression was utilised to demonstrate epidermal tissue viability in culture. LC response to the culture environment was monitored by immunohistochemistry. Full-thickness and split-thickness skin remained genetically viable in culture for at least 72 h in both phosphate-buffered saline (PBS) and full organ culture medium (OCM). The epidermis of explants cultured in OCM remained morphologically intact throughout the culture duration. LCs in full-thickness skin exhibited a delayed response (reduction in cell number and increase in cell size) to the culture conditions compared with split-thickness skin, whose response was immediate. In conclusion, excised human skin can be cultured for a minimum of 72 h for analysis of gene expression and immune cell activation. However, the use of split-thickness skin for vaccine formulation studies may not be appropriate because of the nature of the activation. Full-thickness skin explants are a more suitable model to assess cutaneous vaccination ex vivo.

Enhanced monocyte Fc phagocytosis by a homologue of interleukin-10 encoded by human cytomegalovirus

Human cytomegalovirus (HCMV) expresses several homologues of human interleukin 10 (hIL-10) possessing immunomodulatory properties which may promote viral infection by modulating the function of myeloid cells. We examined the phenotype and phagocytic capability of human monocytes exposed to hIL-10, an HCMV-encoded hIL-10 homologue expressed during the productive phase of infection (cmvIL-10), and a differentially spliced form of cmvIL-10 expressed during latent and productive phases of infection, (LAcmvIL-10). hIL-10 and cmvIL-10 upregulated expression of Fcgamma receptors, stimulated phagocytosis of IgG-opsonised erythrocytes and decreased MHC class II (HLA-DR) expression on purified monocytes within 24 h. In contrast, LAcmvIL-10 decreased HLA-DR expression at later times (48 h and 72 h) but did not increase Fcgamma receptor expression. We conclude that cmvIL-10 promotes differentiation of monocytes towards a pro-phagocytic phenotype and that LAcmvIL-10 does not affect monocytes by the same mechanism as cmvIL-10. The significance of these properties to cytomegalovirus pathogenesis is discussed.

Highlights from the perspectives in melanoma XII conference

The following meeting highlights are from the Perspectives in Melanoma XII conference, held in Scheveningen/The Hague, the Netherlands, on October 2-4, 2008. Selected reviews are included but further discussions of these and other presentations are posted at http://www.MelanomaCare.org.

Differential rates of replacement of human dermal dendritic cells and macrophages during hematopoietic stem cell transplantation

Animal models of hematopoietic stem cell transplantation have been used to analyze the turnover of bone marrow-derived cells and to demonstrate the critical role of recipient antigen-presenting cells (APC) in graft versus host disease (GVHD). In humans, the phenotype and lineage relationships of myeloid-derived tissue APC remain incompletely understood. It has also been proposed that the risk of acute GVHD, which extends over many months, is related to the protracted survival of certain recipient APC. Human dermis contains three principal subsets of CD45(+)HLA-DR(+) cells: CD1a(+)CD14(-) DC, CD1a(-)CD14(+) DC, and CD1a(-)CD14(+)FXIIIa(+) macrophages. In vitro, each subset has characteristic properties. After transplantation, both CD1a(+) and CD14(+) DC are rapidly depleted and replaced by donor cells, but recipient macrophages can be found in GVHD lesions and may persist for many months. Macrophages isolated from normal dermis secrete proinflammatory cytokines. Although they stimulate little proliferation of naive or memory CD4(+) T cells, macrophages induce cytokine expression in memory CD4(+) T cells and activation and proliferation of CD8(+) T cells. These observations suggest that dermal macrophages and DC are from distinct lineages and that persistent recipient macrophages, although unlikely to initiate alloreactivity, may contribute to GVHD by sustaining the responses of previously activated T cells.

Activin A induces Langerhans cell differentiation in vitro and in human skin explants

Langerhans cells (LC) represent a well characterized subset of dendritic cells located in the epidermis of skin and mucosae. In vivo, they originate from resident and blood-borne precursors in the presence of keratinocyte-derived TGFbeta. In vitro, LC can be generated from monocytes in the presence of GM-CSF, IL-4 and TGFbeta. However, the signals that induce LC during an inflammatory reaction are not fully investigated. Here we report that Activin A, a TGFbeta family member induced by pro-inflammatory cytokines and involved in skin morphogenesis and wound healing, induces the differentiation of human monocytes into LC in the absence of TGFbeta. Activin A-induced LC are Langerin+, Birbeck granules+, E-cadherin+, CLA+ and CCR6+ and possess typical APC functions. In human skin explants, intradermal injection of Activin A increased the number of CD1a+ and Langerin+ cells in both the epidermis and dermis by promoting the differentiation of resident precursor cells. High levels of Activin A were present in the upper epidermal layers and in the dermis of Lichen Planus biopsies in association with a marked infiltration of CD1a+ and Langerin+ cells. This study reports that Activin A induces the differentiation of circulating CD14+ cells into LC. Since Activin A is abundantly produced during inflammatory conditions which are also characterized by increased numbers of LC, we propose that this cytokine represents a new pathway, alternative to TGFbeta, responsible for LC differentiation during inflammatory/autoimmune conditions.

Transcriptional profiling of human skin-resident Langerhans cells and CD1a+ dermal dendritic cells: differential activation states suggest distinct functions

In human skin, two main populations of dendritic cells (DC) can be discriminated: dermal DC (DDC) and epidermal Langerhans cells (LC). Although extensively studied, most of the knowledge about DDC and LC phenotype and function is obtained from studying DDC and LC cultured in vitro or DDC and LC migrated from skin explants. These studies have left the exact relationship between steady-state human LC and DDC unclear: in particular, whether CD1a+ DDC represent migrated LC or whether they constitute a separate subset. To gain further insight in the kinship between skin-resident CD1a+ DDC and LC, we analyzed CD1a+ DDC and LC, isolated from steady-state skin samples, by high-density microarray analysis. Results show that the CD1a+ DDC specifically express markers associated with DDC phenotype, such as the macrophage mannose receptor, DC-specific ICAM-grabbing nonintegrin, the scavenger receptor CD36, coagulation factor XIIIa, and chemokine receptor CCR5, whereas LC specifically express Langerin, membrane ATPase (CD39), and CCR6, all hallmarks of the LC lineage. In addition, under steady-state conditions, both DC subsets display a strikingly different activation status, indicative of distinct functional properties. CD1a+ DDC exhibit a more activated, proinflammatory, migratory, and T cell-stimulatory profile, as compared with LC, whereas LC mainly express molecules involved in cell adhesion and DC retention in the epidermis. In conclusion, transcriptional profiling is consistent with the notion that CD1a+ DDC and LC represent two distinct DC subsets but also that under steady-state conditions, CD1a+ DDC and epidermal LC represent opposites of the DC activation spectrum.