Differential migration of epidermal and dermal dendritic cells during skin infection

The Monocytic Cell Line THP-1 as a Validated and Robust Surrogate Model for Human Dendritic Cells

We have implemented an improved, cost-effective, and highly reproducible protocol for a simple and rapid differentiation of the human leukemia monocytic cell line THP-1 into surrogates for immature dendritic cells (iDCs) or mature dendritic cells (mDCs). The successful differentiation of THP-1 cells into iDCs was determined by high numbers of cells expressing the DC activation markers CD54 (88%) and CD86 (61%), and the absence of the maturation marker CD83. The THP-1-derived mDCs are characterized by high numbers of cells expressing CD54 (99%), CD86 (73%), and the phagocytosis marker CD11b (49%) and, in contrast to THP-1-derived iDCs, CD83 (35%) and the migration marker CXCR4 (70%). Treatment of iDCs with sensitizers, such as NiSO4 and DNCB, led to high expression of CD54 (97%/98%; GMFI, 3.0/3.2-fold induction) and CD86 (64%/96%; GMFI, 4.3/3.2-fold induction) compared to undifferentiated sensitizer-treated THP-1 (CD54, 98%/98%; CD86, 55%/96%). Thus, our iDCs are highly suitable for toxicological studies identifying potential sensitizing or inflammatory compounds. Furthermore, the expression of CD11b, CD83, and CXCR4 on our iDC and mDC surrogates could allow studies investigating the molecular mechanisms of dendritic cell maturation, phagocytosis, migration, and their use as therapeutic targets in various disorders, such as sensitization, inflammation, and cancer.

Isolating and cryopreserving pig skin cells for single-cell RNA sequencing study

The pig skin architecture and physiology are similar to those of humans. Thus, the pig model is very valuable for studying skin biology and testing therapeutics. The single-cell RNA sequencing (scRNA-seq) technology allows quantitatively analyzing cell types, compositions, states, signaling, and receptor-ligand interactome at single-cell resolution and at high throughput. scRNA-seq has been used to study mouse and human skins. However, studying pig skin with scRNA-seq is still rare. A critical step for successful scRNA-seq is to obtain high-quality single cells from the pig skin tissue. Here we report a robust method for isolating and cryopreserving pig skin single cells for scRNA-seq. We showed that pig skin could be efficiently dissociated into single cells with high cell viability using the Miltenyi Human Whole Skin Dissociation kit and the Miltenyi gentleMACS Dissociator. Furthermore, the obtained single cells could be cryopreserved using 90% FBS + 10% DMSO without causing additional cell death, cell aggregation, or changes in gene expression profiles. Using the developed protocol, we were able to identify all the major skin cell types. The protocol and results from this study are valuable for the skin research scientific community.

Nociceptive sensory neurons promote CD8 T cell responses to HSV-1 infection

Host protection against cutaneous herpes simplex virus 1 (HSV-1) infection relies on the induction of a robust adaptive immune response. Here, we show that Nav1.8+ sensory neurons, which are involved in pain perception, control the magnitude of CD8 T cell priming and expansion in HSV-1-infected mice. The ablation of Nav1.8-expressing sensory neurons is associated with extensive skin lesions characterized by enhanced inflammatory cytokine and chemokine production. Mechanistically, Nav1.8+ sensory neurons are required for the downregulation of neutrophil infiltration in the skin after viral clearance to limit the severity of tissue damage and restore skin homeostasis, as well as for eliciting robust CD8 T cell priming in skin-draining lymph nodes by controlling dendritic cell responses. Collectively, our data reveal an important role for the sensory nervous system in regulating both innate and adaptive immune responses to viral infection, thereby opening up possibilities for new therapeutic strategies.

Herpes Simplex Virus type 1 infects Langerhans cells and the novel epidermal dendritic cell, Epi-cDC2s, via different entry pathways

Skin mononuclear phagocytes (MNPs) provide the first interactions of invading viruses with the immune system. In addition to Langerhans cells (LCs), we recently described a second epidermal MNP population, Epi-cDC2s, in human anogenital epidermis that is closely related to dermal conventional dendritic cells type 2 (cDC2) and can be preferentially infected by HIV. Here we show that in epidermal explants topically infected with herpes simplex virus (HSV-1), both LCs and Epi-cDC2s interact with HSV-1 particles and infected keratinocytes. Isolated Epi-cDC2s support higher levels of infection than LCs in vitro, inhibited by acyclovir, but both MNP subtypes express similar levels of the HSV entry receptors nectin-1 and HVEM, and show similar levels of initial uptake. Using inhibitors of endosomal acidification, actin and cholesterol, we found that HSV-1 utilises different entry pathways in each cell type. HSV-1 predominantly infects LCs, and monocyte-derived MNPs, via a pH-dependent pathway. In contrast, Epi-cDC2s are mainly infected via a pH-independent pathway which may contribute to the enhanced infection of Epi-cDC2s. Both cells underwent apoptosis suggesting that Epi-cDC2s may follow the same dermal migration and uptake by dermal MNPs that we have previously shown for LCs. Thus, we hypothesize that the uptake of HSV and infection of Epi-cDC2s will stimulate immune responses via a different pathway to LCs, which in future may help guide HSV vaccine development and adjuvant targeting.

Herpes Simplex Virus Type-2 Paralyzes the Function of Monocyte-Derived Dendritic Cells

Herpes simplex viruses not only infect a variety of different cell types, including dendritic cells (DCs), but also modulate important cellular functions in benefit of the virus. Given the relevance of directed immune cell migration during the initiation of potent antiviral immune responses, interference with DC migration constitutes a sophisticated strategy to hamper antiviral immunity. Notably, recent reports revealed that HSV-1 significantly inhibits DC migration in vitro. Thus, we aimed to investigate whether HSV-2 also modulates distinct hallmarks of DC biology. Here, we demonstrate that HSV-2 negatively interferes with chemokine-dependent in vitro migration capacity of mature DCs (mDCs). Interestingly, rather than mediating the reduction of the cognate chemokine receptor expression early during infection, HSV-2 rapidly induces β2 integrin (LFA-1)-mediated mDC adhesion and thereby blocks mDC migration. Mechanistically, HSV-2 triggers the proteasomal degradation of the negative regulator of β2 integrin activity, CYTIP, which causes the constitutive activation of LFA-1 and thus mDC adhesion. In conclusion, our data extend and strengthen recent findings reporting the reduction of mDC migration in the context of a herpesviral infection. We thus hypothesize that hampering antigen delivery to secondary lymphoid organs by inhibition of mDC migration is an evolutionary conserved strategy among distinct members of Herpesviridae.

Airway Macrophage and Dendritic Cell Subsets in the Resting Human Lung

Dendritic cells (DCs) and macrophages (MΦs) are antigen-presenting phagocytic cells found in many peripheral tissues of the human body, including the blood, lymph nodes, skin, and lung. They are vital to maintaining steady-state respiration in the human lung based on their ability to clear airways while also directing tolerogenic or inflammatory responses based on specific stimuli. Over the past three decades, studies have determined that there are multiple subsets of these two general cell types that exist in the airways and interstitium. Identifying these numerous subsets has proven challenging, especially with the unique microenvironments present in the lung. Cells found in the vasculature are not the same subsets found in the skin or the lung, as demonstrated by surface marker expression. By transcriptional profiling, these subsets show similarities but also major differences. Primary human lung cells and/ or tissues are difficult to acquire, particularly in a healthy condition. Additionally, surface marker screening and transcriptional profiling are continually identifying new DC and MΦ subsets. While the overall field is moving forward, we emphasize that more attention needs to focus on replicating the steady-state microenvironment of the lung to reveal the physiological functions of these subsets.

Interactions between Autophagy and DNA Viruses

Autophagy is a catabolic biological process in the body. By targeting exogenous microorganisms and aged intracellular proteins and organelles and sending them to the lysosome for phagocytosis and degradation, autophagy contributes to energy recycling. When cells are stimulated by exogenous pathogenic microorganisms such as viruses, activation or inhibition of autophagy is often triggered. As autophagy has antiviral effects, many viruses may escape and resist the process by encoding viral proteins. At the same time, viruses can also use autophagy to enhance their replication or increase the persistence of latent infections. Here, we give a brief overview of autophagy and DNA viruses and comprehensively review the known interactions between human and animal DNA viruses and autophagy and the role and mechanisms of autophagy in viral DNA replication and DNA virus-induced innate and acquired immunity.

Toll-like receptor 4 attenuates a murine model of atopic dermatitis through inhibition of langerin-positive DCs migration

Atopic dermatitis (AD) is a common chronic inflammatory skin disease that is often associated with skin barrier dysfunction leading to a higher frequency of bacterial and viral skin infections. Toll-like receptor (TLR) 4 on resident skin cells was involved in sensing pathogens and eliciting pathogen-specific innate and adaptive immune responses. Previous studies have demonstrated that TLR4 was linked to AD severity in context of pathogen infection. However, the immune regulatory role of TLR4 in AD remains to be defined. We here investigated the immune regulatory function of TLR4 in AD induced by repeated epicutaneous application of a hapten, 2,4-dinitrochlorobenzene (DNCB). Our results showed that TLR4-deficient (TLR4^-/- ) mice exhibited more severe AD symptoms than WT mice after DNCB challenge. The DNCB-treated TLR4^-/- mice also displayed higher expression levels of inflammatory cytokines and stronger Th2 response than WT counterparts. Moreover, the skin expression of thymic stromal lymphopoietin (TSLP), an important potential contributor to allergic inflammation, was significantly elevated in TLR4^-/- mice compared with that in WT mice upon DNCB administration. Furthermore, we demonstrated that the migration of langerin-positive dendritic cells (DCs) into draining lymph nodes was enhanced in TLR4^-/- mice following DNCB challenge, which is partially dependent on the production of pro-inflammatory cytokine TNF-α. Together, these results determined that TLR4 affected the hapten-induced skin inflammation in the absence of exogenous pathogen infection, suggesting that TLR4 not only regulates infection but also may serve as a modulator of the immune response during AD development.

Human Langerhans Cells with Pro-inflammatory Features Relocate within Psoriasis Lesions

Psoriasis is a common skin disease that presents with well-demarcated patches of inflammation. Recurrent disease in fixed areas of the skin indicates a localized disease memory that is preserved in resolved lesions. In line with such concept, the involvement of tissue-resident immune cells in psoriasis pathology is increasingly appreciated. Langerhans cells (LCs) are perfectly placed to steer resident T cells and local tissue responses in psoriasis. Here, we present an overview of the current knowledge of LCs in human psoriasis, including findings that highlight pro-inflammatory features of LCs in psoriasis lesions. We also review the literature on conflicting data regarding LC localization and functionality in psoriasis. Our review highlights that further studies are needed to elucidate the molecular mechanisms that drive LCs functionality in inflammatory diseases.

Effective Priming of Herpes Simplex Virus-Specific CD8+ T Cells In Vivo Does Not Require Infected Dendritic Cells

Resolution of virus infections depends on the priming of virus-specific CD8⁺ T cells by dendritic cells (DC). While this process requires major histocompatibility complex (MHC) class I-restricted antigen presentation by DC, the relative contribution to CD8⁺ T cell priming by infected DC is less clear. We have addressed this question in the context of a peripheral infection with herpes simplex virus 1 (HSV). Assessing the endogenous, polyclonal HSV-specific CD8⁺ T cell response, we found that effective in vivo T cell priming depended on the presence of DC subsets specialized in cross-presentation, while Langerhans cells and plasmacytoid DC were dispensable. Utilizing a novel mouse model that allows for the in vivo elimination of infected DC, we also demonstrated in vivo that this requirement for cross-presenting DC was not related to their infection but instead reflected their capacity to cross-present HSV-derived antigen. Taking the results together, this study shows that infected DC are not required for effective CD8⁺ T cell priming during a peripheral virus infection.IMPORTANCE The ability of some DC to present viral antigen to CD8⁺ T cells without being infected is thought to enable the host to induce killer T cells even when viruses evade or kill infected DC. However, direct experimental in vivo proof for this notion has remained elusive. The work described in this study characterizes the role that different DC play in the induction of virus-specific killer T cell responses and, critically, introduces a novel mouse model that allows for the selective elimination of infected DC in vivo Our finding that HSV-specific CD8⁺ T cells can be fully primed in the absence of DC infection shows that cross-presentation by DC is indeed sufficient for effective CD8⁺ T cell priming during a peripheral virus infection.

Herpes simplex virus 1 interferes with autophagy of murine dendritic cells and impairs their ability to stimulate CD8+ T lymphocytes

The MHC class I presentation is responsible for the presentation of viral proteins to CD8⁺ T lymphocytes and mainly depends on the classical antigen processing pathway. Recently, a second pathway involving autophagy has been implicated in this process. Here, we show an increase in the capacity of murine dendritic cells (DCs) to present viral antigens on MHC class I after infection with a mutant herpes simplex virus 1 (HSV-1-Δ34.5), lacking infected cell protein 34.5 (ICP34.5), when compared to its parental HSV-1 strain. The ICP34.5 protein counteracts host cell translational arrest and suppresses macroautophagy, and the lack of this protein resulted in a low viral protein abundance, which was processed and presented in an efficient way. Our study demonstrates an important role of autophagy in processing endogenous viral proteins in HSV-1-infected DCs.

Sustained accumulation of antigen-presenting cells after infection promotes local T-cell immunity

Antigen-presenting cells (APC), such as dendritic cells (DC) and macrophages, are critical for T-cell-mediated immunity. Although it is established that memory T cells accumulate and persist in peripheral tissues after the resolution of infection, whether this is also the case for APC remains unclear. Here, we report that CCR2-dependent cells infiltrate skin during acute infection with herpes simplex virus (HSV)-1 and subsequently give rise to localized populations of DCs and macrophages. These APC are found at elevated numbers at sites of resolved infection or inflammation compared with unaffected regions of skin. Importantly, this local accumulation of APC is sustained for prolonged periods of time and has important functional consequences, as it promotes interferon-γ responses by virus-specific CD4⁺ T cells upon localized challenge infection with HSV-1. Thus, our results highlight how infection history determines long-term changes in immune cell composition in skin and how different types of immune cells accumulate, persist and co-operate to provide optimal immunity at this critical barrier site.

Fibered Confocal Fluorescence Microscopy for the Noninvasive Imaging of Langerhans Cells in Macaques

Purpose

We developed a new approach to visualize skin Langerhans cells by in vivo fluorescence imaging in nonhuman primates.

Procedures

Macaques were intradermally injected with a monoclonal, fluorescently labeled antibody against HLA-DR molecule and were imaged for up to 5 days by fibered confocal microscopy (FCFM).

Results

The network of skin Langerhans cells was visualized by in vivo fibered confocal fluorescence microscopy. Quantification of Langerhans cells revealed no changes to cell density with time. Ex vivo experiments confirmed that injected fluorescent HLA-DR antibody specifically targeted Langerhans cells in the epidermis.

Conclusions

This study demonstrates the feasibility of single-cell, in vivo imaging as a noninvasive technique to track Langerhans cells in nontransgenic animals.

Dynamic Changes in Resident and Infiltrating Epidermal Dendritic Cells in Active and Resolved Psoriasis

Epidermal Langerhans cells (LCs) are spatially separated from dermal dendritic cells (DCs) in healthy human skin. In active psoriasis, maintained by local production of IL-23 and IL-17, inflammatory DCs infiltrate both skin compartments. Here we show that CCR2⁺ epidermal DCs (eDCs) were confined to lesional psoriasis and phenotypically distinct from dermal DCs. The eDCs exceeded the number of LCs and displayed high expression of genes involved in neutrophil recruitment and the activation of keratinocytes and T cells. Resident LCs responded to toll-like receptor 4 and toll-like receptor 7/8 activation with increased IL-23 production, whereas eDCs additionally produced IL-1β together with IL-23 and tumor necrosis factor. Psoriasis typically recur in fixed skin lesions. eDCs were absent from resolved psoriasis. Instead, LCs from anti-tumor necrosis factor-treated lesions retained high IL23A expression and responded to toll-like receptor stimulation by producing IL-23. Our results reveal phenotypic and functional properties of eDCs and resident LCs in different clinical phases of psoriasis, and the capacity of these cells to amplify the epidermal microenvironment through the secretion of IL-17 polarizing cytokines.

Duality at the gate: Skin dendritic cells as mediators of vaccine immunity and tolerance

Since Edward Jenner's discovery that intentional exposure to cowpox could provide lifelong protection from smallpox, vaccinations have been a major focus of medical research. However, while the protective benefits of many vaccines have been successfully translated into the clinic, the cellular and molecular mechanisms that differentiate effective vaccines from sub-optimal ones are not well understood. Dendritic cells (DCs) are the gatekeepers of the immune system, and are ultimately responsible for the generation of adaptive immunity and lifelong protective memory through interactions with T cells. In addition to lymph node and spleen resident DCs, a number of tissue resident DC populations have been identified at barrier tissues, such as the skin, which migrate to the local lymph node (migDC). These populations have unique characteristics, and play a key role in the function of cutaneous vaccinations by shuttling antigen from the vaccination site to the draining lymph node, rapidly capturing freely draining antigens in the lymph node, and providing key stimuli to T cells. However, while migDCs are responsible for the generation of immunity following exposure to certain pathogens and vaccines, recent work has identified a tolerogenic role for migDCs in the steady state as well as during protein immunization. Here, we examine the roles and functions of skin DC populations in the generation of protective immunity, as well as their role as regulators of the immune system.

Understanding natural herpes simplex virus immunity to inform next-generation vaccine design

Incremental advances in our knowledge of how natural immune control of herpes simplex virus (HSV) develops have yielded insight as to why previous vaccine attempts have only been partially successful, however, our understanding of these pathways, particularly in humans, is still incomplete. Further elucidation of the innate immune events that are responsible for stimulating these effector responses is required to accurately inform vaccine design. An enhanced understanding of the mechanism of action of novel adjuvants will also facilitate the rational choice of adjuvant to optimise such responses. Here we review the reasons for the hitherto partial HSV vaccine success and align these with our current knowledge of how natural HSV immunity develops. In particular, we focus on the innate immune response and the role of dendritic cells in inducing protective T-cell responses and how these pathways might be recapitulated in a vaccine setting.

Dendritic cells and the extracellular matrix: A challenge for maintaining tolerance/homeostasis

The importance of the extracellular matrix (ECM) in contributing to structural, mechanical, functional and tissue-specific features in the body is well appreciated. While the ECM was previously considered to be a passive bystander, it is now evident that it plays active, dynamic and flexible roles in shaping cell survival, differentiation, migration and death to varying extents depending on the specific site in the body. Dendritic cells (DCs) are recognized as potent antigen presenting cells present in many tissues and in blood, continuously scrutinizing the microenvironment for antigens and mounting local and systemic host responses against harmful agents. DCs also play pivotal roles in maintaining homeostasis to harmless self-antigens, critical for preventing autoimmunity. What is less understood are the complex interactions between DCs and the ECM in maintaining this balance between steady-state tissue residence and DC activation during inflammation. DCs are finely tuned to inflammation-induced variations in fragment length, accessible epitopes and post-translational modifications of individual ECM components and correspondingly interpret these changes appropriately by adjusting their profiles of cognate binding receptors and downstream immune activation. The successful design and composition of novel ECM-based mimetics in regenerative medicine and other applications rely on our improved understanding of DC-ECM interplay in homeostasis and the challenges involved in maintaining it.

A Dual Role for Corneal Dendritic Cells in Herpes Simplex Keratitis: Local Suppression of Corneal Damage and Promotion of Systemic Viral Dissemination

The cornea is the shield to the foreign world and thus, a primary site for peripheral infections. However, transparency and vision are incompatible with inflammation and scarring that may result from infections. Thus, the cornea is required to perform a delicate balance between fighting infections and preserving vision. To date, little is known about the specific role of antigen-presenting cells in viral keratitis. In this study, utilizing an established murine model of primary acute herpes simplex virus (HSV)-1 keratitis, we demonstrate that primary HSV keratitis results in increased conventional dendritic cells (cDCs) and macrophages within 24 hours after infection. Local depletion of cDCs in CD11c-DTR mice by subconjuntival diphtheria toxin injections, led to increased viral proliferation, and influx of inflammatory cells, resulting in increased scarring and clinical keratitis. In addition, while HSV infection resulted in significant corneal nerve destruction, local depletion of cDCs resulted in a much more severe loss of corneal nerves. Further, local cDC depletion resulted in decreased corneal nerve infection, and subsequently decreased and delayed systemic viral transmission in the trigeminal ganglion and draining lymph node, resulting in decreased mortality of mice. In contrast, sham depletion or depletion of macrophages through local injection of clodronate liposomes had neither a significant impact on the cornea, nor an effect on systemic viral transmission. In conclusion, we demonstrate that corneal cDCs may play a primary role in local corneal defense during viral keratitis and preserve vision, at the cost of inducing systemic viral dissemination, leading to increased mortality.

Mononuclear phagocytes of the intestine, the skin, and the lung

Tissues that are in direct contact with the outside world face particular immunological challenges. The intestine, the skin, and the lung possess important mononuclear phagocyte populations to deal with these challenges, but the cellular origin of these phagocytes is strikingly different from one subset to another, with some cells derived from embryonic precursors and some from bone marrow-derived circulating monocytes. Here, we review the current knowledge regarding the developmental pathways that control the differentiation of mononuclear phagocytes in these barrier tissues. We have also attempted to build a theoretical model that could explain the distinct cellular origin of mononuclear phagocytes in these tissues.

A subset of human plasmacytoid dendritic cells expresses CD8α upon exposure to herpes simplex virus type 1

Classical and plasmacytoid dendritic cells (DC) play important roles in the defense against murine and human infections with herpes simplex virus (HSV). So far, CD8α expression has only been reported for murine DC. CD8α⁺ DC have prominent cross-presenting activities, which are enhanced by murine CD8α⁺ PDC. The human orthologue of murine CD8α⁺ DC, the CD141 (BDCA3)⁺ DC, mainly cross-present after TLR3 ligation. We report here the serendipitous finding that a subset of human PDC upregulates CD8α upon HSV-1 stimulation, as shown by gene array and flow cytometry analyses. CD8α, not CD8ß, was expressed upon exposure. Markers of activation, migration, and costimulation were upregulated on CD8α-expressing human PDC. In these cells, increased cytokine and chemokine levels were detected that enhance development and function of T, B, and NK cells, and recruit immature DC, monocytes, and Th1 cells, respectively. Altogether, human CD8α⁺ PDC exhibit a highly activated phenotype and appear to recruit other immune cells to the site of inflammation. Further studies will show whether CD8α-expressing PDC contribute to antigen cross-presentation, which may be important for immune defenses against HSV infections in vitro and in vivo.

Dendritic cells as Achilles' heel and Trojan horse during varicella zoster virus infection

Varicella zoster virus (VZV), a human alphaherpesvirus, causes varicella and subsequently establishes latency within sensory nerve ganglia. Later in life VZV can reactivate to cause herpes zoster. A reduced frequency of VZV-specific T cells is strongly associated with herpes zoster illustrating that these immune cells are central to control latency. Dendritic cells (DCs) are required for the generation of VZV-specific T cells. However, DCs can also be infected in vitro and in vivo allowing VZV to evade the antiviral immune response. Thus, DCs represent the immune systems' Achilles heel. Uniquely among the human herpesviruses, VZV infects both DCs and T cells, and exploits both as Trojan horses. During primary infection VZV-infected DCs traffic to the draining lymph nodes and tonsils, where the virus is transferred to T cells. VZV-infected T cells subsequently spread infection throughout the body to give the typical varicella skin rash. The delicate interplay between VZV and DCs and its consequences for viral immune evasion and viral dissemination will be discussed in this article.

Relay of herpes simplex virus between Langerhans cells and dermal dendritic cells in human skin

The mechanism by which immunity to Herpes Simplex Virus (HSV) is initiated is not completely defined. HSV initially infects mucosal epidermis prior to entering nerve endings. In mice, epidermal Langerhans cells (LCs) are the first dendritic cells (DCs) to encounter HSV, but it is CD103⁺ dermal DCs that carry viral antigen to lymph nodes for antigen presentation, suggesting DC cross-talk in skin. In this study, we compared topically HSV-1 infected human foreskin explants with biopsies of initial human genital herpes lesions to show LCs are initially infected then emigrate into the dermis. Here, LCs bearing markers of maturation and apoptosis formed large cell clusters with BDCA3⁺ dermal DCs (thought to be equivalent to murine CD103⁺ dermal DCs) and DC-SIGN⁺ DCs/macrophages. HSV-expressing LC fragments were observed inside the dermal DCs/macrophages and the BDCA3⁺ dermal DCs had up-regulated a damaged cell uptake receptor CLEC9A. No other infected epidermal cells interacted with dermal DCs. Correspondingly, LCs isolated from human skin and infected with HSV-1 in vitro also underwent apoptosis and were taken up by similarly isolated BDCA3⁺ dermal DCs and DC-SIGN⁺ cells. Thus, we conclude a viral antigen relay takes place where HSV infected LCs undergo apoptosis and are taken up by dermal DCs for subsequent antigen presentation. This provides a rationale for targeting these cells with mucosal or perhaps intradermal HSV immunization.

Herpes Simplex Virus (HSV) is a highly prevalent virus that causes cold sores and genital herpes but also increases the chance of contracting HIV by several folds. In fact, most new cases of HIV in Africa occur in people infected with HSV. Thus, a protective HSV vaccine would have a large impact on public health. Currently, the process by which immunity to HSV is generated is incompletely understood. Paradoxically, the first immune cells to become infected, Langerhans cells in the epidermis, are not the cells that initiate the immune response, while the dermal dendritic cells thought to be responsible for initiating the immune response are not likely to be infected. Here, we have shown, in human skin models and genital herpes lesion biopsies, an interaction between these dendritic cells that could relay HSV to the lymph node. HSV is taken up by the epidermal Langerhans cells that then migrate into the dermis, die and are taken up by another subset of dermal dendritic cells—the homologs of those in mice which stimulate HSV-specific T cells in the lymph node. Thus, a mucosal or intradermal vaccine targeting these two dendritic cells may be required.

Cross-Presentation in Mouse and Human Dendritic Cells

Cross-presentation designates the presentation of exogenous antigens on major histocompatibility complex class I molecules and is essential for the initiation of cytotoxic immune responses. It is now well established that dendritic cells (DCs) are the best cross-presenting cells. In this chapter, we will discuss recent advances in our understanding of the molecular mechanisms of cross-presentation. We will also describe the different DC subsets identified in mouse and human, and their functional specialization for cross-presentation. Finally, we will summarize the current knowledge of the role of cross-presentation in pathological situations.

Herpes simplex virus type 2-infected dendritic cells produce TNF-α, which enhances CCR5 expression and stimulates HIV production from adjacent infected cells

Prior HSV-2 infection enhances the acquisition of HIV-1 >3-fold. In genital herpes lesions, the superficial layers of stratified squamous epithelium are disrupted, allowing easier access of HIV-1 to Langerhans cells (LC) in the epidermis and perhaps even dendritic cells (DCs) in the outer dermis, as well as to lesion infiltrating activated T lymphocytes and macrophages. Therefore, we examined the effects of coinfection with HIV-1 and HSV-2 on monocyte-derived DCs (MDDC). With simultaneous coinfection, HSV-2 significantly stimulated HIV-1 DNA production 5-fold compared with HIV-1 infection alone. Because <1% of cells were dually infected, this was a field effect. Virus-stripped supernatants from HSV-2-infected MDDCs were shown to enhance HIV-1 infection, as measured by HIV-1-DNA and p24 Ag in MDDCs. Furthermore these supernatants markedly stimulated CCR5 expression on both MDDCs and LCs. TNF-α was by far the most prominent cytokine in the supernatant and also within HSV-2-infected MDDCs. HSV-2 infection of isolated immature epidermal LCs, but not keratinocytes, also produced TNF-α (and low levels of IFN-β). Neutralizing Ab to TNF-α and its receptor, TNF-R1, on MDDCs markedly inhibited the CCR5-stimulating effect of the supernatant. Therefore, these results suggest that HSV-2 infection of DCs in the skin during primary or recurrent genital herpes may enhance HIV-1 infection of adjacent DCs, thus contributing to acquisition of HIV-1 through herpetic lesions.

Monocyte recruitment to the dermis and differentiation to dendritic cells increases the targets for dengue virus replication

Dengue virus (DENV) causes the most prevalent arthropod-borne viral disease in humans. Although Aedes mosquitoes transmit DENV when probing for blood in the skin, no information exists on DENV infection and immune response in the dermis, where the blood vessels are found. DENV suppresses the interferon response, replicates, and causes disease in humans but not wild-type mice. Here, we used mice lacking the interferon-α/β receptor (Ifnar(-/-)), which had normal cell populations in the skin and were susceptible to intradermal DENV infection, to investigate the dynamics of early DENV infection of immune cells in the skin. CD103(+) classical dendritic cells (cDCs), Ly6C(-) CD11b(+) cDCs, and macrophages in the steady-state dermis were initial targets of DENV infection 12-24 hours post-inoculation but then decreased in frequency. We demonstrated recruitment of adoptively-transferred Ly6C(high) monocytes from wild-type and Ifnar(-/-) origin to the DENV-infected dermis and differentiation to Ly6C(+) CD11b(+) monocyte-derived DCs (moDCs), which became DENV-infected after 48 hours, and were then the major targets for virus replication. Ly6C(high) monocytes that entered the DENV-infected dermis expressed chemokine receptor CCR2, likely mediating recruitment. Further, we show that ∼ 100-fold more hematopoietic cells in the dermis were DENV-infected compared to Langerhans cells in the epidermis. Overall, these results identify the dermis as the main site of early DENV replication and show that DENV infection in the skin occurs in two waves: initial infection of resident cDCs and macrophages, followed by infection of monocytes and moDCs that are recruited to the dermis. Our study reveals a novel viral strategy of exploiting monocyte recruitment to increase the number of targets for infection at the site of invasion in the skin and highlights the skin as a potential site for therapeutic action or intradermal vaccination.

The role of dendritic cells in immunity against primary herpes simplex virus infections

Herpes simplex virus (HSV) is a DNA virus with tropism for infecting skin and mucosal epithelia during the lytic stages of its complex life cycle. The immune system has evolved a multitude of strategies to respond to primary HSV infections. These include rapid innate immune responses largely driven by pattern recognition systems and protective anti-viral immunity. Dendritic cells (DC) represent a versatile and heterogenic group of antigen presenting cells that are important for pathogen recognition at sites of infection and for priming of protective HSV-specific T cells. Here we will review the current knowledge on the role of DCs in the host immune response to primary HSV infection. We will discuss how DCs integrate viral cues into effective innate immune responses, will dissect how HSV infection of DCs interferes with their capacity to migrate from sites of infection to the draining lymph nodes and will outline how migratory DCs can make antigens available to lymph node resident DCs. The role of distinct DC subsets and their relevant contribution to antigen presentation on MHC class I and MHC class II molecules will be detailed in the context of T cell priming in the lymph node and the elicitation of effector function in infected tissues. An improved understanding of the fundamental mechanisms of how DCs recognize HSV, process and present its antigens to naïve and effector T cells will not only assist in the improvement of vaccine-based preventions of this important viral disease, but also serves as a paradigm to resolve basic immunological principles.

Distinct APC subtypes drive spatially segregated CD4+ and CD8+ T-cell effector activity during skin infection with HSV-1

Efficient infection control requires potent T-cell responses at sites of pathogen replication. However, the regulation of T-cell effector function in situ remains poorly understood. Here, we show key differences in the regulation of effector activity between CD4⁺ and CD8⁺ T-cells during skin infection with HSV-1. IFN-γ-producing CD4⁺ T cells disseminated widely throughout the skin and draining lymph nodes (LN), clearly exceeding the epithelial distribution of infectious virus. By contrast, IFN-γ-producing CD8⁺ T cells were only found within the infected epidermal layer of the skin and associated hair follicles. Mechanistically, while various subsets of lymphoid- and skin-derived dendritic cells (DC) elicited IFN-γ production by CD4⁺ T cells, CD8⁺ T cells responded exclusively to infected epidermal cells directly presenting viral antigen. Notably, uninfected cross-presenting DCs from both skin and LNs failed to trigger IFN-γ production by CD8⁺ T-cells. Thus, we describe a previously unappreciated complexity in the regulation of CD4⁺ and CD8⁺ T-cell effector activity that is subset-specific, microanatomically distinct and involves largely non-overlapping types of antigen-presenting cells (APC).

HSV-1 is a widely distributed pathogen causing a life-long latent infection associated with periodic bouts of reactivation and severe clinical complications. Adaptive immune responses encompassing CD4⁺ and CD8⁺ T-cell activities are key to both the clearance of infectious virus and the control of latent infection. However, precisely how such T-cell responses are regulated, particularly within acutely infected peripheral tissues, remains poorly understood. Using a mouse model of HSV-1 skin infection, we describe a complex regulation of T-cell responses at the site of acute infection. These responses were subset-specific and anatomically distinct, with CD4⁺ and CD8⁺ T-cell activities being directed to distinct anatomical compartments within the skin. While IFN-γ-producing CD4⁺ T cells were broadly distributed, including skin regions a considerable distance away from infected cells, CD8⁺ T-cell activity was strictly confined to directly infected epithelial compartments. This unexpected spatial segregation was a direct consequence of the involvement of largely non-overlapping types of antigen-presenting cells in driving CD4⁺ and CD8⁺ T-cell effector activity. Our results provide novel insights into the cellular regulation of T-cell immunity within peripheral tissues and have the potential to guide the development of T-cell subset-specific approaches for therapeutic and prophylactic intervention in antimicrobial immunity and autoimmunity.

The Skin-Resident Immune Network

The skin provides an effective physical and biological barrier against environmental and pathogenic insults whilst ensuring tolerance against commensal microbes. This protection is afforded by the unique anatomy and cellular composition of the skin, particularly the vast network of skin-associated immune cells. These include the long-appreciated tissue-resident macrophages, dendritic cells, and mast cells, as well as the more recently described dermal γδ T cells and innate lymphoid cells. Collectively, these cells orchestrate the defense against a wide range of pathogens and environmental challenges, but also perform a number of homeostatic functions. Here, we review recent developments in our understanding of the various roles that leukocyte subsets play in cutaneous immunobiology, and introduce the newer members of the skin immune system. Implications for human disease are discussed.

Essential yet limited role for CCR2⁺ inflammatory monocytes during Mycobacterium tuberculosis-specific T cell priming

Defense against infection by Mycobacterium tuberculosis (Mtb) is mediated by CD4 T cells. CCR2⁺ inflammatory monocytes (IMs) have been implicated in Mtb-specific CD4 T cell responses but their in vivo contribution remains unresolved. Herein, we show that transient ablation of IMs during infection prevents Mtb delivery to pulmonary lymph nodes, reducing CD4 T cell responses. Transfer of MHC class II-expressing IMs to MHC class II-deficient, monocyte-depleted recipients, while restoring Mtb transport to mLNs, does not enable Mtb-specific CD4 T cell priming. On the other hand, transfer of MHC class II-deficient IMs corrects CD4 T cell priming in monocyte-depleted, MHC class II-expressing mice. Specific depletion of classical DCs does not reduce Mtb delivery to pulmonary lymph nodes but markedly reduces CD4 T cell priming. Thus, although IMs acquire characteristics of DCs while delivering Mtb to lymph nodes, cDCs but not moDCs induce proliferation of Mtb-specific CD4 T cells.

DOI:http://dx.doi.org/10.7554/eLife.01086.001

Tuberculosis is a disease that kills more than one million people every year. It is caused by mycobacteria, notably Mycobacterium tuberculosis, and the World Health Organization estimates that about one third of the world’s population has latent tuberculosis, although only one person in 10 goes on to develop an active infection. Understanding why some individuals develop active infections, whereas most do not, could help with the development of a vaccine to prevent tuberculosis and/or new treatments for the disease. Disappointing results from vaccine trials and the emergence of drug-resistant strains of tuberculosis have increased the need for more research into the interactions between mycobacteria and the human immune system.

Tuberculosis is spread when an infected person coughs or sneezes and someone else inhales the mycobacteria spread by the first person. When M. tuberculosis first enters the human respiratory tract, the innate immune system tries to identify and destroy cells that have been infected. However, if this initial response is not effective, the M. tuberculosis can persist in the lungs and trigger the adaptive immune response. This involves CD4 T cells working to eliminate the infection, but our understanding of the adaptive immune response is not complete.

Samstein et al. probed the role that immune cells known as inflammatory monocytes play in the adaptive immune response. Previous research has suggested that inflammatory monocytes may develop into dendritic cells that directly prime the CD4 T cells to respond when the lung has been infected. However, Samstein et al. demonstrate that the inflammatory monocytes carry M. tuberculosis from the lungs of infected mice to the draining lymph nodes during the second week of infection. These monocytes develop many of the characteristics of dendritic cells, but they do not activate the CD4 T cells.

Samstein et al. show that dendritic cells, contrary to previous evidence, are not necessary for the transport of the M. tuberculosis from the lungs to the draining lymph nodes. Without the dendritic cells, however, fewer CD4 T cell are primed in the lymph nodes. Samstein et al. suggest that the inflammatory monocytes play a crucial role by transporting the live bacteria to the lymph nodes. And once in the lymph nodes, the monocytes transfer invading antigens to dendritic cells to initiate the production of the CD4 T cells to lead the fight against the infection.

DOI:http://dx.doi.org/10.7554/eLife.01086.002

Langerin+ dermal DC, but not Langerhans cells, are required for effective CD8-mediated immune responses after skin scarification with vaccinia virus

Skin scarification (s.s.) with Vaccinia virus (VACV) is essential for generation of an optimal protective T cell memory immune response. Dendritic Cells (DC), which are professional antigen presenting cells, are required for naïve T cell priming and activation. At least three subsets of skin resident DC have been identified: Langerhans Cells (LC), Dermal Langerin⁺ DC (Lang⁺dDC) and Dermal Langerin⁻ DC (Lang⁻dDC). Using Langerin-diphtheria toxin receptor mice and established mouse model of VACV delivered by s.s., we demonstrated that Lang⁺dDC, but not LC, are absolutely required for the induction of a rapid and robust antigen-specific CD8⁺ T cell response after s.s. with VACV. The depletion of Lang⁺dDC led to a significant delay in the priming and proliferation of antigen-specific CD8⁺ T cells. Moreover CD8⁺ T cells generated after VACV s.s. in the absence of Lang⁺dDC lacked effector cytotoxic functions both in vitro and in vivo. While s.s.-immunized WT and LC depleted mice controlled the progression of OVA_257–264 expressing T cell lymphoma EG7 (injected intradermally), the depletion of Lang⁺dDC led to rapid lymphoma progression and mortality. These data indicate that of all skin DC subsets, Lang⁺dDC the most critical for the generation of robust CD8⁺ T cell immunity after s.s. with VACV.

Immunobiology of dendritic cells and the influence of HIV infection

Recent progress in phenotyping of human dendritic cells (DCs) has allowed a closer alignment of the classification and functions of murine and human dendritic cell subsets. Marked differences in the functions of these human DC subsets and their response to HIV infection have become apparent, relevant to HIV pathogenesis and vaccine and microbicide development. Systems biology approaches to studying HIV uptake and infection of dendritic cells has revealed how markedly HIV subverts their functions, especially in relation to the trafficking pathways and viral transfer to T cells. Furthermore the interactions between DCs and other innate immune cells, NK cells, NKT cells and gamma delta T cells are now known to influence DC and T cell function and are also disturbed by HIV infection in vitro and in vivo. Such cellular interactions are potential targets for vaccine adjuvants and immunotherapy.

Herpes simplex virus γ34.5 interferes with autophagosome maturation and antigen presentation in dendritic cells

The cellular autophagy response induced by herpes simplex virus 1 (HSV-1) is countered by the viral γ34.5 protein. γ34.5 modulates autophagy by binding to the host autophagy protein Beclin-1 and through this binding inhibits the formation of autophagosomes in fibroblasts and neurons. In contrast, in this study dendritic cells (DCs) infected with HSV-1 showed an accumulation of autophagosomes and of the long-lived protein p62. No such accumulations were observed in DCs infected with a γ34.5-null virus or a virus lacking the Beclin-binding domain (BBD) of γ34.5. To explore this further, we established stably transduced DC lines to show that γ34.5 expression alone induced autophagosome accumulation yet prevented p62 degradation. In contrast, DCs expressing a BBD-deleted mutant of γ34.5 were unable to modulate autophagy. DCs expressing γ34.5 were less capable of stimulating T-cell activation and proliferation in response to intracellular antigens, demonstrating an immunological consequence of inhibiting autophagy. Taken together, these data show that in DCs, γ34.5 antagonizes the maturation of autophagosomes and T cell activation in a BBD-dependent manner, illustrating a unique interface between HSV and autophagy in antigen-presenting cells.

Herpes simplex virus 1 (HSV-1) is a highly prevalent pathogen causing widespread morbidity and some mortality. HSV infections are lifelong, and there are no vaccines or antivirals to cure HSV infections. The ability of HSV to modulate host immunity is critical for its virulence. HSV inhibits host autophagy, a pathway with importance in many areas of health and disease. Autophagy is triggered by many microbes, some of which harness autophagy for replication; others evade autophagy or prevent it from occurring. Autophagy is critical for host defense, either by directly degrading the invading pathogen (“xenophagy”) or by facilitating antigen presentation to T cells. In this study, we show that HSV manipulates autophagy through an unsuspected mechanism with a functional consequence of reducing T cell stimulation. These data further our understanding of how HSV evades host immunity to persist for the lifetime of its host, facilitating its spread in the human population.

Early skin immunological disturbance after Plasmodium-infected mosquito bites

Although the role of regulatory T cells (Tregs) during malaria infection has been studied extensively, such studies have focused exclusively on the role of Treg during the blood stage of infection; little is known about the detailed mechanisms of Tregs and sporozoite deposition in the dermis by mosquito bites. In this paper we show that sporozoites introduced into the skin by mosquito bites increase the mobility of skin Tregs and dendritic cells (DCs). We also show differences in MHC class II and/or CD86 expression on skin-resident dendritic cell subtypes and macrophages. From the observed decrease of the number of APCs into draining lymph nodes, suppression of CD28 expression in conventional CD4 T cells, and a low homeostatic proliferation of skin-migrated CD4 T found in nude mice indicate that Tregs may play a fundamental role during the initial phase of malaria parasite inoculation into the mammalian host.

Reactive murine lymph nodes uniquely permit parenchymal access for T cells that enter via the afferent lymphatics

Whereas naïve T cells access the lymph nodes predominantly via the high endothelial venules, their effector counterparts can also enter via the afferent lymphatics. It is unclear if such cells are confined to the lymphatic spaces during their transit through the lymph node or whether they can access the lymphocyte- and dendritic cell-rich parenchyma with its potentially stimulatory environment. We used a flank HSV inoculation model that featured neuronal-mediated movement of virus to distinct areas of skin to study the lymphatic-mediated transit of activated T cells between different skin-draining lymph nodes. These experiments showed that activated T cells released from the brachial lymph node, draining the primary site of inoculation, entered the downstream axillary lymph node. These activated T cells accessed the subcapsular areas of the axillary lymph node via lymphatic vessels exiting the upstream brachial node regardless of whether the former drained skin that was associated with active infection. However, T cells remained within the sinusoidal network of the axillary lymph node unless it was directly associated with peripheral infection. Thus, activated T cells that enter a given lymph node using the afferent lymphatics do not have automatic access to the parenchyma unless it is a reactive node involved with peripheral inflammation or infection.

Diverse microbial interactions with the basement membrane barrier

During primary contact with susceptible hosts, microorganisms face an array of barriers that thwart their invasion process. Passage through the basement membrane (BM), a 50-100-nm-thick crucial barrier underlying epithelia and endothelia, is a prerequisite for successful host invasion. Such passage allows pathogens to reach nerve endings or blood vessels in the stroma and to facilitate spread to internal organs. During evolution, several pathogens have developed different mechanisms to cross this dense matrix of sheet-like proteins. To breach the BM, some microorganisms have developed independent mechanisms, others hijack host cells that are able to transverse the BM (e.g. leukocytes and dendritic cells) and oncogenic microorganisms might even trigger metastatic processes in epithelial cells to penetrate the underlying BM.

Adaptive immunity to fungi

Only a handful of the more than 100,000 fungal species on our planet cause disease in humans, yet the number of life-threatening fungal infections in patients has recently skyrocketed as a result of advances in medical care that often suppress immunity intensely. This emerging crisis has created pressing needs to clarify immune defense mechanisms against fungi, with the ultimate goal of therapeutic applications. Herein, we describe recent insights in understanding the mammalian immune defenses deployed against pathogenic fungi. The review focuses on adaptive immune responses to the major medically important fungi and emphasizes how dendritic cells and subsets in various anatomic compartments respond to fungi, recognize their molecular patterns, and signal responses that nurture and shape the differentiation of T cell subsets and B cells. Also emphasized is how the latter deploy effector and regulatory mechanisms that eliminate these nasty invaders while also constraining collateral damage to vital tissue.

Early responding dendritic cells direct the local NK response to control herpes simplex virus 1 infection within the cornea

Dendritic cells (DCs) regulate both innate and adaptive immune responses. In this article, we exploit the unique avascularity of the cornea to examine a role for local or very early infiltrating DCs in regulating the migration of blood-derived innate immune cells toward HSV-1 lesions. A single systemic diphtheria toxin treatment 2 d before HSV-1 corneal infection transiently depleted CD11c(+) DCs from both the cornea and lymphoid organs of CD11c-DTR bone marrow chimeric mice for up to 24 h postinfection. Transient DC depletion significantly delayed HSV-1 clearance from the cornea through 6 d postinfection. No further compromise of viral clearance was observed when DCs were continuously depleted throughout the first week of infection. DC depletion did not influence extravasation of NK cells, inflammatory monocytes, or neutrophils into the peripheral cornea, but it did significantly reduce migration of NK cells and inflammatory monocytes, but not neutrophils, toward the HSV-1 lesion in the central cornea. Depletion of NK cells resulted in similar loss of viral control to transient DC ablation. Our findings demonstrate that resident corneal DCs and/or those that infiltrate the cornea during the first 24 h after HSV-1 infection contribute to the migration of NK cells and inflammatory monocytes into the central cornea, and are consistent with a role for NK cells and possibly inflammatory monocytes, but not polymorphonuclear neutrophils, in clearing HSV-1 from the infected cornea.

Constraints for monocyte-derived dendritic cell functions under inflammatory conditions

The activation of TLRs expressed by macrophages or DCs, in the long run, leads to persistently impaired functionality. TLR signals activate a wide range of negative feedback mechanisms; it is not known, however, which of these can lead to long-lasting tolerance for further stimulatory signals. In addition, it is not yet understood how the functionality of monocyte-derived DCs (MoDCs) is influenced in inflamed tissues by the continuous presence of stimulatory signals during their differentiation. Here we studied the role of a wide range of DC-inhibitory mechanisms in a simple and robust model of MoDC inactivation induced by early TLR signals during differentiation. We show that the activation-induced suppressor of cytokine signaling 1 (SOCS1), IL-10, STAT3, miR146a and CD150 (SLAM) molecules possessed short-term inhibitory effects on cytokine production but did not induce persistent DC inactivation. On the contrary, the LPS-induced IRAK-1 downregulation could alone lead to persistent MoDC inactivation. Studying cellular functions in line with the activation-induced negative feedback mechanisms, we show that early activation of developing MoDCs allowed only a transient cytokine production that was followed by the downregulation of effector functions and the preservation of a tissue-resident non-migratory phenotype.

Dendritic cells in tissues: in situ stimulation of immunity and immunopathology

Dendritic cells (DCs) prime and orchestrate naïve T cell immunity in lymphoid organs, but recent data also highlight the importance of DC-effector T cell interactions in tissues. These studies suggest that effector T cells require a second activating step in situ from tissue DCs to become fully competent for effector functions and/or proliferation and survival. DC stimulation of effector T cells within tissues has evolved as a mechanism to ensure that T cells are activated to their full potential only at the site of ongoing infection. Here, we propose that under conditions of uncontrolled inflammation and release of tissue antigens, the same DC-dependent checkpoint perpetuates a destructive response and immunopathology.

Infection of dendritic cells with herpes simplex virus type 1 induces rapid degradation of CYTIP, thereby modulating adhesion and migration

Immune responses require spatial and temporal coordinated interactions between different cell types within distinct microenvironments. This dynamic interplay depends on the competency of the involved cells, predominantly leukocytes, to actively migrate to defined sites of cellular encounters in various tissues. Because of their unique capacity to transport antigen from the periphery to secondary lymphoid tissues for the activation of naive T cells, dendritic cells (DCs) play a key role in the initiation and orchestration of adaptive immune responses. Therefore, pathogen-mediated interference with this process is a very effective way of immune evasion. CYTIP (cytohesin-interacting protein) is a key regulator of DC motility. It has previously been described to control LFA-1 deactivation and to regulate DC adherence. CYTIP expression is up-regulated during DC maturation, enabling their transition from the sessile to the motile state. Here, we demonstrate that on infection of human monocyte-derived DCs with herpes simplex virus type 1 (HSV-1), CYTIP is rapidly degraded and as a consequence β-2 integrins, predominantly LFA-1, are activated. Furthermore, we show that the impairment of migration in HSV-1-infected DCs is in part the result of this increased integrin-mediated adhesion. Thus, we propose a new mechanism of pathogen-interference with central aspects of leukocyte biology.

Accelerated dendritic cell differentiation from migrating Ly6C(lo) bone marrow monocytes in early dermal West Nile virus infection

No study has investigated the participation of Ly6C(+) monocytes in the earliest phase of skin infection with the mosquito-borne West Nile virus. In a novel murine model mimicking natural dermal infection, CCL2-dependent bone marrow (BM)-derived monocyte migration, differentiation into Ly6C(+) dendritic cells (DC), and accumulation around dermal deposits of infected fibroblasts by day 1 postinfection were associated with increasing numbers of monocyte-derived TNF/inducible NO synthase-producing DC by day 2 postinfection in draining auricular lymph nodes (ALN). Adoptive transfer demonstrated simultaneous migration of bone marrow-derived Ly6C(lo) monocytes to virus-infected dermis and ALN, where they first become Ly6C(hi) DC within 24 h and then Ly6C(lo) DC by 72 h. DC migration from the infected dermis to the ALN derived exclusively from Ly6C(lo) BM monocytes. This demonstrates that Ly6C(hi) and Ly6C(lo) BM-derived monocytes have different fates in vivo and suggests that BM may be a reservoir of preinflammatory monocytes for rapid deployment as inflammatory DC during virus infection.