Interplay between Keratinocytes and Myeloid Cells Drives Dengue Virus Spread in Human Skin

The Dynamic Relationship between Dengue Virus and the Human Cutaneous Innate Immune Response

Dengue virus (DENV) is a continuing global threat that puts half of the world's population at risk for infection. This mosquito-transmitted virus is endemic in over 100 countries. When a mosquito takes a bloodmeal, virus is deposited into the epidermal and dermal layers of human skin, infecting a variety of permissive cells, including keratinocytes, Langerhans cells, macrophages, dermal dendritic cells, fibroblasts, and mast cells. In response to infection, the skin deploys an array of defense mechanisms to inhibit viral replication and prevent dissemination. Antimicrobial peptides, pattern recognition receptors, and cytokines induce a signaling cascade to increase transcription and translation of pro-inflammatory and antiviral genes. Paradoxically, this inflammatory environment recruits skin-resident mononuclear cells that become infected and migrate out of the skin, spreading virus throughout the host. The details of the viral-host interactions in the cutaneous microenvironment remain unclear, partly due to the limited body of research focusing on DENV in human skin. This review will summarize the functional role of human skin, the cutaneous innate immune response to DENV, the contribution of the arthropod vector, and the models used to study DENV interactions in the cutaneous environment.

A mosquito salivary protein-driven influx of myeloid cells facilitates flavivirus transmission

Mosquitoes transmit many disease-relevant flaviviruses. Efficient viral transmission to mammalian hosts requires mosquito salivary factors. However, the specific salivary components facilitating viral transmission and their mechanisms of action remain largely unknown. Here, we show that a female mosquito salivary gland-specific protein, here named A. aegypti Neutrophil Recruitment Protein (AaNRP), facilitates the transmission of Zika and dengue viruses. AaNRP promotes a rapid influx of neutrophils, followed by virus-susceptible myeloid cells toward mosquito bite sites, which facilitates establishment of local infection and systemic dissemination. Mechanistically, AaNRP engages TLR1 and TLR4 of skin-resident macrophages and activates MyD88-dependent NF-κB signaling to induce the expression of neutrophil chemoattractants. Inhibition of MyD88-NF-κB signaling with the dietary phytochemical resveratrol reduces AaNRP-mediated enhancement of flavivirus transmission by mosquitoes. These findings exemplify how salivary components can aid viral transmission, and suggest a potential prophylactic target.

Dengue Virus Infects Human Skin Langerhans Cells through Langerin for Dissemination to Dendritic Cells

Dengue virus (DENV) is the most disease-causative flavivirus worldwide. DENV as a mosquito-borne virus infects human hosts through the skin; however, the initial target cells in the skin remain unclear. In this study, we have investigated whether epidermal Langerhans cells (LCs) play a role in DENV acquisition and dissemination. We have used a human epidermal ex vivo infection model as well as isolated LCs to investigate infection by DENV. Notably, both immature and mature LCs were permissive to DENV infection in vitro and ex vivo, and infection was dependent on C-type lectin receptor langerin because blocking antibodies against langerin significantly reduced DENV infection in vitro and ex vivo. DENV-infected LCs efficiently transmitted DENV to target cells such as dendritic cells. Moreover, DENV exposure increased the migration of LCs from epidermal explants. These results strongly suggest that DENV targets epidermal LCs for infection and dissemination in the human host. These findings could provide potential drug targets to combat the early stage of DENV infection.

Ly6C+ monocytes in the skin promote systemic alphavirus dissemination

Alphaviruses are mosquito-transmitted pathogens that induce high levels of viremia, which facilitates dissemination and vector transmission. One prevailing paradigm is that, after skin inoculation, alphavirus-infected resident dendritic cells migrate to the draining lymph node (DLN), facilitating further rounds of infection and dissemination. Here, we assess the contribution of infiltrating myeloid cells to alphavirus spread. We observe two phases of virus transport to the DLN, one that occurs starting at 1 h post infection and precedes viral replication, and a second that requires replication in the skin, enabling transit to the bloodstream. Depletion of Ly6C+ monocytes reduces local chikungunya (CHIKV) or Ross River virus (RRV) infection in the skin, diminishes the second phase of virus transport to the DLN, and delays spread to distal sites. Our data suggest that infiltrating monocytes facilitate alphavirus infection at the initial infection site, which promotes more rapid spread into circulation.

Skin tropism during Usutu virus and West Nile virus infection: an amplifying and immunological role

Usutu virus (USUV) and West Nile virus (WNV) are closely related emerging arboviruses belonging to the Flavivirus genus and posing global public health concerns. Although human infection by these viruses is mainly asymptomatic, both have been associated with neurological disorders such as encephalitis and meningoencephalitis. Since USUV and WNV are transmitted through the bite of an infected mosquito, the skin represents the initial site of virus inoculation and provides the first line of host defense. Although some data on the early stages of WNV skin infection are available, very little is known about USUV. Herein, USUV-skin resident cell interactions were characterized. Using primary human keratinocytes and fibroblasts, an early replication of USUV during the first 24 hours was shown in both skin cells. In human skin explants, a high viral tropism for keratinocytes was observed. USUV infection of these models induced type I and III interferon responses associated with upregulated expression of various interferon-stimulated genes as well as pro-inflammatory cytokine and chemokine genes. Among the four USUV lineages studied, the Europe 2 strain replicated more efficiently in skin cells and induced a higher innate immune response. In vivo, USUV and WNV disseminated quickly from the inoculation site to distal cutaneous tissues. In addition, viral replication and persistence in skin cells were associated with an antiviral response. Taken together, these results provide a better understanding of the pathophysiology of the early steps of USUV infection and suggest that the skin constitutes a major amplifying organ for USUV and WNV infection.IMPORTANCEUsutu virus (USUV) and West Nile virus (WNV) are closely related emerging Flaviviruses transmitted through the bite of an infected mosquito. Since they are directly inoculated within the upper skin layers, the interactions between the virus and skin cells are critical in the pathophysiology of USUV and WNV infection. Here, during the early steps of infection, we showed that USUV can efficiently infect two human resident skin cell types at the inoculation site: the epidermal keratinocytes and the dermal fibroblasts, leading to the induction of an antiviral innate immune response. Moreover, following cutaneous inoculation, we demonstrated that both viruses can rapidly spread, replicate, and persist in all distal cutaneous tissues in mice, a phenomenon associated with a generalized skin inflammatory response. These results highlight the key amplifying and immunological role of the skin during USUV and WNV infection.

Dengue Vaccination: Towards a New Dawn of Curbing Dengue Infection

Dengue is an infectious disease caused by dengue virus (DENV) and is a serious global burden. Antibody-dependent enhancement and the ability of DENV to infect immune cells, along with other factors, lead to fatal Dengue Haemorrhagic Fever and Dengue Shock Syndrome. This necessitates the development of a robust and efficient vaccine but vaccine development faces a number of hurdles. In this review, we look at the epidemiology, genome structure and cellular targets of DENV and elaborate upon the immune responses generated by human immune system against DENV infection. The review further sheds light on various challenges in development of a potent vaccine against DENV which is followed by presenting a current account of different vaccines which are being developed or have been licensed.

Host Derivation of Sindbis Virus Influences Mammalian Type I Interferon Response to Infection

Arboviruses are defined by their ability to replicate in both mosquito vectors and mammalian hosts. There is good evidence that arboviruses "prime" their progeny for infection of the next host, such as via differential glycosylation of their outer glycoproteins or packaging of host ribosomal subunits. We and others have previously shown that mosquito-derived viruses more efficiently infect mammalian cells than mammalian-derived viruses. These observations are consistent with arboviruses acquiring host-specific adaptations, and we hypothesized that a virus derived from either the mammalian host or mosquito vector elicits different responses when infecting the mammalian host. Here, we perform an RNA-sequencing analysis of the transcriptional response of Human Embryonic Kidney 293 (HEK-293) cells to infection with either mosquito (Aedes albopictus, C7/10)- or mammalian (Baby Hamster Kidney, BHK-21)-derived Sindbis virus (SINV). We show that the C7/10-derived virus infection leads to a more robust transcriptional response in HEK-293s compared to infection with the BHK-derived virus. Surprisingly, despite more efficient infection, we found an increase in interferon-β (IFN-β) and interferon-stimulated gene (ISG) transcripts in response to the C7/10-derived virus infection versus the BHK-derived virus infection. However, translation of interferon-stimulated genes was lower in HEK-293s infected with the C7/10-derived virus, starkly contrasting with the transcriptional response. This inhibition of ISG translation is reflective of a more rapid overall shut-off of host cell translation following infection with the C7/10-derived virus. Finally, we show that the C7/10-derived virus infection of HEK-293 cells leads to elevated levels of phosphorylated eukaryotic translation elongation factor-2 (eEF2), identifying a potential mechanism leading to the more rapid shut-off of host translation. We postulate that the rapid shut-off of host translation in mammalian cells infected with the mosquito-derived virus acts to counter the IFN-β-stimulated transcriptional response.

Zika virus spreads through infection of lymph node-resident macrophages

To disseminate through the body, Zika virus (ZIKV) is thought to exploit the mobility of myeloid cells, in particular monocytes and dendritic cells. However, the timing and mechanisms underlying shuttling of the virus by immune cells remains unclear. To understand the early steps in ZIKV transit from the skin, at different time points, we spatially mapped ZIKV infection in lymph nodes (LNs), an intermediary site en route to the blood. Contrary to prevailing hypotheses, migratory immune cells are not required for the virus to reach the LNs or blood. Instead, ZIKV rapidly infects a subset of sessile CD169+ macrophages in the LNs, which release the virus to infect downstream LNs. Infection of CD169+ macrophages alone is sufficient to initiate viremia. Overall, our experiments indicate that macrophages that reside in the LNs contribute to initial ZIKV spread. These studies enhance our understanding of ZIKV dissemination and identify another anatomical site for potential antiviral intervention.

Discrepant Activation Pattern of Inflammation and Pyroptosis Induced in Dermal Fibroblasts in Response to Dengue Virus Serotypes 1 and 2 and Nonstructural Protein 1

Four serotypes of dengue virus (DENV-1 to DENV-4) cause mild to severe disease in humans through infected mosquito bites. Dermal fibroblasts were found to be susceptible to DENV, and this may play a critical role in establishing the initial infection stage. However, the cellular response induced by the different DENV serotypes in dermal fibroblasts during the early stage of infection remains unclear. To determine this, normal human dermal fibroblast WS1 cells were infected with DENV-1 or DENV-2. Compared with the response elicited by DENV-1 infection, DENV-2 induced a stronger innate inflammatory response and cell death in the WS1 cells. However, DENV-1 activated a higher level of pyroptosis signaling than did DENV-2, which was associated with higher virion production. Caspase-1 inhibitor Ac-YVAD-cmk and imipramine, an antidepressant drug, reduced DENV-mediated caspase-1 and interleukin 1β (IL-β) cleavage in the pyroptosis pathway. Ac-YVAD-cmk and imipramine downregulated DENV virion production in WS1 cells. Furthermore, DENV-1 and DENV-2 NS1 proteins promoted diverse activation levels of cell death, inflammatory response, and activation of caspase-1 and IL-β in dermal fibroblasts at different time points. Collectively, these data suggest that DENV-1, DENV-2, and their nonstructural protein 1 (NS1) induce discrepant activation patterns of inflammation and pyroptosis in dermal fibroblasts. The pyroptosis caused by virus and NS1 may facilitate DENV replication in dermal fibroblasts. IMPORTANCE Skin fibroblasts are the primary cells of DENV infection through mosquito bites. Establishing a successful infection in dermal fibroblasts might be critical for dengue disease. However, the cellular response induced by DENV in dermal fibroblasts remains unclear. In this in vitro study, we found that DENV-2 and DENV-1 showed different time course patterns of virus replication and inflammation in dermal fibroblasts. We demonstrated that DENV-1 and DNEV-2 and their viral protein NS1 activate the cellular pyroptosis response to regulate virus replication in dermal fibroblasts. This finding suggests that pyroptosis activation in the DENV primary inoculation site plays a role in the establishment of a DENV infection.

Human Dendritic Cells Transmit Enterovirus A71 via Heparan Sulfates to Target Cells Independent of Viral Replication

Enterovirus A71 (EV-A71) is a causative agent of life-threatening neurological diseases in young children. EV-A71 is highly infectious but it remains unclear how the virus disseminates from primary entry sites-the mucosa of the respiratory tract or the intestine-to secondary replication sites-skin or brain. Here, we investigated the role of dendritic cells (DCs) in EV-A71 dissemination. DCs reside in the mucosa of the airway and gut, and migrate to lymphoid tissues upon activation and, therefore, could facilitate EV-A71 dissemination to secondary replication sites. Monocyte-derived DCs were not permissive to different genotypes of EV-A71 but, notably, coculture with EV-A71-susceptiblle RD99 cells led to very efficient infection of RD99 cells. Notably, EV-A71 transmission of DCs to RD99 was independent of viral replication as a replication inhibitor did not affect transmission. Soluble heparin blocked EV-A71 transmission by DCs to RD99 cells, in contrast to antibodies against known attachment receptor DC-SIGN. These results strongly suggest that DCs might be a first target for EV-A71 and involved in viral dissemination via heparan sulfates and heparin derivatives might be an effective treatment to attenuate dissemination. IMPORTANCE EV-A71 is an emerging neurotropic virus that is of emerging concern and can result in polio-like illness. The exact mechanism of how EV-A71 results in neurological symptoms are unknown. In particular, the early dissemination of the virus from primary replication sites (airway and intestine) to secondary sites (central nervous system and skin) needs to be elucidated. There is evidence pointing toward a role for dendritic cells (DC) in EV-A71 transmission. Moreover, heparan sulfate (HS) binding mutations are observed in patients with severe diseases. Therefore, we evaluated the potential role of HS on DC in transmission. We find that HS are critical for transmitting EV-A71 by DC to target cells. Our data are consistent with other clinical and in vitro observations highlighting the importance of HS in EV-A71-induced disease.

Innate and adaptive immune evasion by dengue virus

Dengue is a mosquito-borne disease which causes significant public health concerns in tropical and subtropical countries. Dengue virus (DENV) has evolved various strategies to manipulate the innate immune responses of the host such as ‘hiding’ in the ultrastructure of the host, interfering with the signaling pathway through RNA modifications, inhibiting type 1 IFN production, as well as inhibiting STAT1 phosphorylation. DENV is also able to evade the adaptive immune responses of the host through antigenic variation, antigen-dependent enhancement (ADE), partial maturation of prM proteins, and inhibition of antigen presentation. miRNAs are important regulators of both innate and adaptive immunity and they have been shown to play important roles in DENV replication and pathogenesis. This makes them suitable candidates for the development of anti-dengue therapeutics. This review discusses the various strategies employed by DENV to evade innate and adaptive immunity. The role of miRNAs and DENV non-structural proteins (NS) are promising targets for the development of anti-dengue therapeutics.

N-Linked Glycans Shape Skin Immune Responses during Arthritis and Myositis after Intradermal Infection with Ross River Virus

Arthritogenic alphaviruses are mosquito-borne arboviruses that include several re-emerging human pathogens, including the chikungunya (CHIKV), Ross River (RRV), Mayaro (MAYV), and o'nyong-nyong (ONNV) virus. Arboviruses are transmitted via a mosquito bite to the skin. Herein, we describe intradermal RRV infection in a mouse model that replicates the arthritis and myositis seen in humans with Ross River virus disease (RRVD). We show that skin infection with RRV results in the recruitment of inflammatory monocytes and neutrophils, which together with dendritic cells migrate to draining lymph nodes (LN) of the skin. Neutrophils and monocytes are productively infected and traffic virus from the skin to LN. We show that viral envelope N-linked glycosylation is a key determinant of skin immune responses and disease severity. RRV grown in mammalian cells elicited robust early antiviral responses in the skin, while RRV grown in mosquito cells stimulated poorer early antiviral responses. We used glycan mass spectrometry to characterize the glycan profile of mosquito and mammalian cell-derived RRV, showing deglycosylation of the RRV E2 glycoprotein is associated with curtailed skin immune responses and reduced disease following intradermal infection. Altogether, our findings demonstrate skin infection with an arthritogenic alphavirus leads to musculoskeletal disease and envelope glycoprotein glycosylation shapes disease outcome. IMPORTANCE Arthritogenic alphaviruses are transmitted via mosquito bites through the skin, potentially causing debilitating diseases. Our understanding of how viral infection starts in the skin and how virus systemically disseminates to cause disease remains limited. Intradermal arbovirus infection described herein results in musculoskeletal pathology, which is dependent on viral envelope N-linked glycosylation. As such, intradermal infection route provides new insights into how arboviruses cause disease and could be extended to future investigations of skin immune responses following infection with other re-emerging arboviruses.

Host immune response against DENV and ZIKV infections

Dengue is a major public health concern, affecting almost 400 million people worldwide, with about 70% of the global burden of disease in Asia. Despite revised clinical classifications of dengue infections by the World Health Organization, the wide spectrum of the manifestations of dengue illness continues to pose challenges in diagnosis and patient management for clinicians. When the Zika epidemic spread through the American continent and then later to Africa and Asia in 2015, researchers compared the characteristics of the Zika infection to Dengue, considering both these viruses were transmitted primarily through the same vector, the Aedes aegypti female mosquitoes. An important difference to note, however, was that the Zika epidemic diffused in a shorter time span compared to the persisting feature of Dengue infections, which is endemic in many Asian countries. As the pathogenesis of viral illnesses is affected by host immune responses, various immune modulators have been proposed as biomarkers to predict the risk of the disease progression to a severe form, at a much earlier stage of the illness. However, the findings for most biomarkers are highly discrepant between studies. Meanwhile, the cross-reactivity of CD8+ and CD4+ T cells response to Dengue and Zika viruses provide important clues for further development of potential treatments. This review discusses similarities between Dengue and Zika infections, comparing their disease transmissions and vectors involved, and both the innate and adaptive immune responses in these infections. Consideration of the genetic identity of both the Dengue and Zika flaviviruses as well as the cross-reactivity of relevant T cells along with the actions of CD4+ cytotoxic cells in these infections are also presented. Finally, a summary of the immune biomarkers that have been reported for dengue and Zika viral infections are discussed which may be useful indicators for future anti-viral targets or predictors for disease severity. Together, this information appraises the current understanding of both Zika and Dengue infections, providing insights for future vaccine design approaches against both viruses.

Skin Injury Activates a Rapid TRPV1-Dependent Antiviral Protein Response

The skin serves as the interface between the body and the environment and plays a fundamental role in innate antimicrobial host immunity. Antiviral proteins (AVPs) are part of the innate host defense system and provide protection against viral pathogens. How breach of the skin barrier influences innate AVP production remains largely unknown. In this study, we characterized the induction and regulation of AVPs after skin injury and identified a key role of TRPV1 in this process. Transcriptional and phenotypic profiling of cutaneous wounds revealed that skin injury induces high levels of AVPs in both mice and humans. Remarkably, pharmacologic and genetic ablation of TRPV1-mediated nociception abrogated the induction of AVPs, including Oas2, Oasl2, and Isg15 after skin injury in mice. Conversely, stimulation of TRPV1 nociceptors was sufficient to induce AVP production involving the CD301b+ cells‒IL-27‒mediated signaling pathway. Using IL-27 receptor‒knockout mice, we show that IL-27 signaling is required in the induction of AVPs after skin injury. Finally, loss of TRPV1 signaling leads to increased viral infectivity of herpes simplex virus. Together, our data indicate that TRPV1 signaling ensures skin antiviral competence on wounding.

Conditions That Simulate the Environment of Atopic Dermatitis Enhance Susceptibility of Human Keratinocytes to Vaccinia Virus

Individuals with underlying chronic skin conditions, notably atopic dermatitis (AD), are disproportionately affected by infections from members of the herpesviridae, papovaviridae, and poxviridae families. Many patients with AD experience recurrent, widespread cutaneous viral infections that can lead to viremia, serious organ complications, and even death. Little is known about how the type 2 inflammatory environment observed in the skin of AD patients impacts the susceptibility of epidermal cells (keratinocytes) to viral pathogens. Herein, we studied the susceptibility of keratinocytes to the prototypical poxvirus, vaccinia virus (VV)-the causative agent of eczema vaccinatum-under conditions that simulate the epidermal environment observed in AD. Treatment of keratinocytes with type 2 cytokines (IL-4 and -13) to simulate the inflammatory environment or a tight junction disrupting peptide to mirror the barrier disruption observed in AD patients, resulted in a differentiation-dependent increase in susceptibility to VV. Furthermore, pan JAK inhibition was able to diminish the VV susceptibility occurring in keratinocytes exposed to type 2 cytokines. We propose that in AD, the increased viral susceptibility of keratinocytes leads to enhanced virus production in the skin, which contributes to the rampant dissemination and pathology seen within patients.

A Human Skin Model for Assessing Arboviral Infections

Arboviruses such as flaviviruses and alphaviruses cause a significant human healthcare burden on a global scale. Transmission of these viruses occurs during human blood feeding at the mosquito-skin interface. Not only do pathogen immune evasion strategies influence the initial infection and replication of pathogens delivered, but arthropod salivary factors also influence transmission foci. In vitro cell cultures do not provide an adequate environment to study complex interactions between viral, mosquito, and host factors. To address this need for a whole tissue system, we describe a proof of concept model for arbovirus infection using adult human skin ex vivo with Zika virus (flavivirus) and Mayaro virus (alphavirus). Replication of these viruses in human skin was observed up to 4 days after infection. Egressed viruses could be detected in the culture media as well. Antiviral and proinflammatory genes, including chemoattractant chemokines, were expressed in infected tissue. Immunohistochemical analysis showed the presence of virus in the skin tissue 4 days after infection. This model will be useful to further investigate: (i) the immediate molecular mechanisms of arbovirus infection in human skin, and (ii) the influence of arthropod salivary molecules during initial infection of arboviruses in a more physiologically relevant system.

Scavenger Receptors in the Pathogenesis of Viral Infections

Scavenger receptors (SR) are not only pattern recognition receptors involved in the immune response against pathogens but are also important receptors exploited by different virus to enter host cells, and thus represent targets for antiviral therapy. The high mutation rates of viruses, as well as their small genomes are partly responsible for the high rates of virus resistance and effective treatments remain a challenge. Most currently approved formulations target viral-encoded factors. Nevertheless, host proteins may function as additional targets. Thus, there is a need to explore and develop new strategies aiming at cellular factors involved in virus replication and host cell entry. SR-virus interactions have implications in the pathogenesis of several viral diseases and in adenovirus-based vaccination and gene transfer technologies, and may function as markers of severe progression. Inhibition of SR could reduce adenoviral uptake and improve gene therapy and vaccination, as well as reduce pathogenesis. In this review, we will examine the crucial role of SR play in cell entry of different types of human virus, which will allow us to further understand their role in protection and pathogenesis and its potential as antiviral molecules. The recent discovery of SR-B1 as co-factor of SARS-Cov-2 (severe acute respiratory syndrome coronavirus 2) entry is also discussed. Further fundamental research is essential to understand molecular interactions in the dynamic virus-host cell interplay through SR for rational design of therapeutic strategies.

Usutu virus escapes langerin-induced restriction to productively infect human Langerhans cells, unlike West Nile virus

Usutu virus (USUV) and West Nile virus (WNV) are phylogenetically close emerging arboviruses and constitute a global public health threat. Since USUV and WNV are transmitted by mosquitoes, the first immune cells they encounter are skin-resident dendritic cells, the most peripheral outpost of immune defense. This unique network is composed of Langerhans cells (LCs) and dermal DCs, which reside in the epidermis and the dermis, respectively. Using human skin explants, we show that while both viruses can replicate in keratinocytes, they can also infect resident DCs with distinct tropism: WNV preferentially infects DCs in the dermis, whereas USUV has a greater propensity to infect LCs. Using both purified human epidermal LCs (eLCs) and monocyte derived LCs (MoLCs), we confirm that LCs sustain a faster and more efficient replication of USUV than WNV and that this correlates with a more intense innate immune response to USUV compared with WNV. Next, we show that ectopic expression of the LC-specific C-type lectin receptor (CLR), langerin, in HEK293T cells allows WNV and USUV to bind and enter, but supports the subsequent replication of USUV only. Conversely, blocking or silencing langerin in MoLCs or eLCs made them resistant to USUV infection, thus demonstrating that USUV uses langerin to enter and replicate in LCs. Altogether, our results demonstrate that LCs constitute privileged target cells for USUV in human skin, because langerin favours its entry and replication. Intriguingly, this suggests that USUV efficiently escapes the antiviral functions of langerin, which normally safeguards LCs from most viral infections.

Dengue virus infection modifies mosquito blood-feeding behavior to increase transmission to the host

Because dengue viruses are spread by mosquitoes during biting, transmission capacity depends on mosquito-biting behavior. For this reason, it is critical to understand how infection in mosquitoes influences biting. To answer this question, we deployed a multidisciplinary approach including high-resolution, multivariate biting behavior monitoring on mice, in vivo transmission assay, and mathematical modeling. We demonstrated that infected mosquitoes are more attracted to mice and bite more often to get the same amount of blood as uninfected mosquitoes. While the effect of increased attraction to host on transmission capacity is trivial, we showed that increased number of bites results in successive transmission. Eventually, we calculated that the infection-induced behavior changes tripled transmission capacity of mosquitoes.

Mosquito blood-feeding behavior is a key determinant of the epidemiology of dengue viruses (DENV), the most-prevalent mosquito-borne viruses. However, despite its importance, how DENV infection influences mosquito blood-feeding and, consequently, transmission remains unclear. Here, we developed a high-resolution, video-based assay to observe the blood-feeding behavior of Aedes aegypti mosquitoes on mice. We then applied multivariate analysis on the high-throughput, unbiased data generated from the assay to ordinate behavioral parameters into complex behaviors. We showed that DENV infection increases mosquito attraction to the host and hinders its biting efficiency, the latter resulting in the infected mosquitoes biting more to reach similar blood repletion as uninfected mosquitoes. To examine how increased biting influences DENV transmission to the host, we established an in vivo transmission model with immuno-competent mice and demonstrated that successive short probes result in multiple transmissions. Finally, to determine how DENV-induced alterations of host-seeking and biting behaviors influence dengue epidemiology, we integrated the behavioral data within a mathematical model. We calculated that the number of infected hosts per infected mosquito, as determined by the reproduction rate, tripled when mosquito behavior was influenced by DENV infection. Taken together, this multidisciplinary study details how DENV infection modulates mosquito blood-feeding behavior to increase vector capacity, proportionally aggravating DENV epidemiology. By elucidating the contribution of mosquito behavioral alterations on DENV transmission to the host, these results will inform epidemiological modeling to tailor improved interventions against dengue.

A Review on the Current Knowledge on ZIKV Infection and the Interest of Organoids and Nanotechnology on Development of Effective Therapies against Zika Infection

Zika virus (ZIKV) acquired a special relevance due to the pandemic that occurred in the Americas in 2015, when an important number of fetal microcephaly cases occurred. Since then, numerous studies have tried to elucidate the pathogenic mechanisms and the potential therapeutic approaches to combat the virus. Cellular and animal models have proved to be a basic resource for this research, with the more recent addition of organoids as a more realistic and physiological 3D culture for the study of ZIKV. Nanotechnology can also offer a promising therapeutic tool, as the nanoparticles developed by this field can penetrate cells and deliver a wide array of drugs in a very specific and controlled way inside the cells. These two state-of-the-art scientific tools clearly provide a very relevant resource for the study of ZIKV, and will help researchers find an effective treatment or vaccine against the virus.

Skin Viral Infections: Host Antiviral Innate Immunity and Viral Immune Evasion

The skin is an active immune organ that functions as the first and largest site of defense to the outside environment. Serving as the primary interface between host and pathogen, the skin’s early immune responses to viral invaders often determine the course and severity of infection. We review the current literature pertaining to the mechanisms of cutaneous viral invasion for classical skin-tropic, oncogenic, and vector-borne skin viruses. We discuss the skin’s evolved mechanisms for innate immune viral defense against these invading pathogens, as well as unique strategies utilized by the viruses to escape immune detection. We additionally explore the roles that demographic and environmental factors, such as age, biological sex, and the cutaneous microbiome, play in altering the host immune response to viral threats.

Mobilization and Activation of the Innate Immune Response to Dengue Virus

Dengue virus is an important human pathogen, infecting an estimated 400 million individuals per year and causing symptomatic disease in a subset of approximately 100 million. Much of the effort to date describing the host response to dengue has focused on the adaptive immune response, in part because of the well-established roles of antibody-dependent enhancement and T cell original sin as drivers of severe dengue upon heterotypic secondary infection. However, the innate immune system is a crucial factor in the host response to dengue, as it both governs the fate and vigor of the adaptive immune response, and mediates the acute inflammatory response in tissues. In this review, we discuss the innate inflammatory response to dengue infection, focusing on the role of evolutionarily conserved innate immune cells, their effector functions, and clinical course.

Crosstalk Between Dermal Fibroblasts and Dendritic Cells During Dengue Virus Infection

Dengue virus infection (DENV-2) is transmitted by infected mosquitoes via the skin, where many dermal and epidermal cells are potentially susceptible to infection. Most of the cells in an area of infection will establish an antiviral microenvironment to control viral replication. Although cumulative studies report permissive DENV-2 infection in dendritic cells, keratinocytes, and fibroblasts, among other cells also infected, little information is available regarding cell-to-cell crosstalk and the effect of this on the outcome of the infection. Therefore, our study focused on understanding the contribution of fibroblast and dendritic cell crosstalk to the control or promotion of dengue. Our results suggest that dendritic cells promote an antiviral state over fibroblasts by enhancing the production of type I interferon, but not proinflammatory cytokines. Infected and non-infected fibroblasts promoted partial dendritic cell maturation, and the fibroblast-matured cells were less permissive to infection and showed enhanced type I interferon production. We also observed that the soluble mediators produced by non-infected or Poly (I:C) transfected fibroblasts induced allogenic T cell proliferation, but mediators produced by DENV-2 infected fibroblasts inhibited this phenomenon. Additionally, the effects of fibroblast soluble mediators on CD14+ monocytes were analyzed to assess whether they affected the differentiation of monocyte derived dendritic cells (moDC). Our data showed that mediators produced by infected fibroblasts induced variable levels of monocyte differentiation into dendritic cells, even in the presence of recombinant GM-CSF and IL-4. Cells with dendritic cell-like morphology appeared in the culture; however, flow cytometry analysis showed that the mediators did not fully downregulate CD14 nor did they upregulate CD1a. Our data revealed that fibroblast-dendritic cell crosstalk promoted an antiviral response mediated manly by type I interferons over fibroblasts. Furthermore, the maturation of dendritic cells and T cell proliferation were promoted, which was inhibited by DENV-2-induced mediators. Together, our results suggest that activation of the adaptive immune response is influenced by the crosstalk of skin resident cells and the intensity of innate immune responses established in the microenvironment of the infected skin.

Association between interferon lambda 3 rs12979860 polymorphism and clinical outcome in dengue virus-infected children

Single nucleotide polymorphisms (SNPs) in immune-related genes have been shown to play a role in driving the development of the severe phenotypes of dengue virus (DENV) infection. We assessed the association between IFNL3 gene SNP (rs12979860) and dengue clinical outcomes in children. Patients with dengue-related symptoms (aged 1-15 years) admitted at a public hospital in Northeast Brazil were invited to participate. The association between rs12979860 polymorphism and dengue classification and clinical signs and symptoms were analysed. A total of 206 DENV-infected children were included: 53.4% of the infections were classified as severe dengue. The T allele carriers had higher risk of developing severe dengue when compared to CC genotype carriers (OR: 1.81; 95% CI: 0.98-3.32 p = .054). The T allele carriers also showed longer fever episodes when compared to patients with the CC genotype (OR: 1.90; 95%CI: 1.07-3.38; p = .027). On the other hand, the ones carrying the CT/TT genotype had 70% lower chance of developing thrombocytopenia when compared to those with the CC genotype (OR: 0.30; 95%CI: 0.08-0.88; p = .042). Our findings demonstrated that the T allele carriers of the IFNL3 gene had higher risk of developing severe dengue, suggesting a link between IFN-λ expression and DENV immunopathogenesis.

Antiviral and Immunomodulatory Properties of Antimicrobial Peptides Produced by Human Keratinocytes

Keratinocytes, the main cells of the epidermis, are the first site of replication as well as the first line of defense against many viruses such as arboviruses, enteroviruses, herpes viruses, human papillomaviruses, or vaccinia virus. During viral replication, these cells can sense virus associated molecular patterns leading to the initiation of an innate immune response composed of pro-inflammatory cytokines, chemokines, and antimicrobial peptides. Human keratinocytes produce and secrete at least nine antimicrobial peptides: human cathelicidin LL-37, types 1–4 human β-defensins, S100 peptides such as psoriasin (S100A7), calprotectin (S100A8/9) and koebnerisin (S100A15), and RNase 7. These peptides can exert direct antiviral effects on the viral particle or its replication cycle, and indirect antiviral activity, by modulating the host immune response. The purpose of this review is to summarize current knowledge of antiviral and immunomodulatory properties of human keratinocyte antimicrobial peptides.

IL-27 signaling activates skin cells to induce innate antiviral proteins and protects against Zika virus infection

In the skin, antiviral proteins and other immune molecules serve as the first line of innate antiviral defense. Here, we identify and characterize the induction of cutaneous innate antiviral proteins in response to IL-27 and its functional role during cutaneous defense against Zika virus infection. Transcriptional and phenotypic profiling of epidermal keratinocytes treated with IL-27 demonstrated activation of antiviral proteins OAS1, OAS2, OASL, and MX1 in the skin of both mice and humans. IL-27-mediated antiviral protein induction was found to occur in a STAT1- and IRF3-dependent but STAT2-independent manner. Moreover, using IL27ra mice, we demonstrate a significant role for IL-27 in inhibiting Zika virus morbidity and mortality following cutaneous, but not intravenous, inoculation. Together, our results demonstrate a critical and previously unrecognized role for IL-27 in cutaneous innate antiviral immunity against Zika virus.

La Crosse Virus Infection of Human Keratinocytes Leads to Interferon-Dependent Apoptosis of Bystander Non-Infected Cells In Vitro

Resident cells in the skin serve as the first innate line of defense against insect-borne pathogens, but the role of these cell types in promoting or limiting arbovirus replication is not completely understood. Here, we have examined the outcome of infection of cultured human keratinocyte cells with La Crosse virus (LACV), using a spontaneously transformed cell line, HaCaT. In single cycle infections, keratinocyte HaCaT cells supported rapid and high level LACV replication, resulting in high virus yields and extensive caspase-dependent cell death. By contrast, multi-cycle LACV replication in HaCaT cells was restricted by an antiviral response elicited by the production of both IFN-β and IFN-λ. During low multiplicity LACV infections, HaCaT cell death was seen in non-infected bystander cells. Media from LACV-infected cells induced caspase-dependent killing of naïve non-infected HaCaT cells, and this bystander cell death was relieved by IFN-β neutralizing antibodies or by an inhibitor of JAK-STAT signaling. Naïve HaCaT cells showed dose-dependent killing by treatment with exogenous IFN-β but not IFN-λ. Our data suggest a model whereby keratinocytes produce IFNs which limit virus spread through both antiviral signaling and by induction of bystander cell death of potential new target cells for infection.

Reciprocal immune enhancement of dengue and Zika virus infection in human skin

Dengue virus (DENV) and Zika virus (ZIKV) are closely related mosquito-borne flaviviruses that co-circulate in tropical regions and constitute major threats to global human health. Whether preexisting immunity to one virus affects disease caused by the other during primary or secondary infections is unknown but is critical in preparing for future outbreaks and predicting vaccine safety. Using a human skin explant model, we show that DENV-3 immune sera increased recruitment and infection of Langerhans cells, macrophages, and dermal dendritic cells following inoculation with DENV-2 or ZIKV. Similarly, ZIKV immune sera enhanced infection with DENV-2. Immune sera increased migration of infected Langerhans cells to the dermis and emigration of infected cells out of skin. Heterotypic immune sera increased viral RNA in the dermis almost 10-fold and reduced the amount of virus required to infect a majority of myeloid cells by 100- to 1000-fold. Enhancement was associated with cross-reactive IgG and induction of IL-10 expression and was mediated by both CD32 and CD64 Fcγ receptors. These findings reveal that preexisting heterotypic immunity greatly enhances DENV and ZIKV infection, replication, and spread in human skin. This relevant tissue model will be valuable in assessing the efficacy and risk of dengue and Zika vaccines in humans.

Insight into the Tropism of Dengue Virus in Humans

In tropical and subtropical zones, arboviruses are among the major threats to human life, affecting a large number of populations with serious diseases. Worldwide, over three hundred million people are infected with dengue virus (DENV) every year as per the World Health Organization (WHO). DENV-mediated disease severity ranges from a mild fever to hemorrhagic fever and shock syndrome. Patients suffering from severe infection might experience multi-organ failure, cardiomyopathy and even encephalopathy, further complicating the disease pathogenesis. In life-threatening cases, DENV has been reported to affect almost all organs of the human body. In this review, we discuss the organ tropism of DENV in humans in depth as detected in various autopsy studies. Keeping in mind the fact that there is currently no DENV-specific antiviral, it is of utmost importance to achieve a vivid picture of the susceptible cells in humans which might help in designing antivirals against DENV, especially targeting those tissues in which infection might lead to life-threatening conditions.

Targeting of host cell receptor tyrosine kinases by intracellular pathogens

Intracellular pathogens use complex and tightly regulated processes to enter host cells. Upon initial interactions with signaling proteins at the surface of target cells, intracellular microbes activate and co-opt specific host signaling pathways that mediate cell surface-cytosol communications to facilitate pathogen internalization. Here, we discuss the roles of host receptor tyrosine kinases (RTKs) in the establishment of productive infections by major intracellular pathogens. We evaluate the gaps in the current understanding of this process and propose a comprehensive approach for assessing the role of host cell signaling in the biology of intracellular microorganisms and viruses. We also discuss RTK-targeting strategies for the treatment of various infections.

Hijacking the Host Immune Cells by Dengue Virus: Molecular Interplay of Receptors and Dengue Virus Envelope

Dengue virus (DENV) is one of the lethal pathogens in the hot climatic regions of the world and has been extensively studied to decipher its mechanism of pathogenesis and the missing links of its life cycle. With respect to the entry of DENV, multiple receptors have been recognized in different cells of the human body. However, scientists still argue whether these identified receptors are the exclusive entry mediators for the virus. Adding to the complexity, DENV has been reported to be infecting multiple organ types in its human host. Also, more than one receptor in a particular cell has been discerned to take part in mediating the ingress of DENV. In this review, we aim to discuss the different cells of the human immune system that support DENV infection and their corresponding receptors that DENV deploy to gain access to the cells.

Organization of the Skin Immune System and Compartmentalized Immune Responses in Infectious Diseases

The skin is an organ harboring several types of immune cells that participate in innate and adaptive immune responses. The immune system of the skin comprises both skin cells and professional immune cells that together constitute what is designated skin-associated lymphoid tissue (SALT). In this review, I extensively discuss the organization of SALT and the mechanisms involved in its responses to infectious diseases of the skin and mucosa. The nature of these SALT responses, and the cellular mediators involved, often determines the clinical course of such infections. I list and describe the components of innate immunity, such as the roles of the keratinocyte barrier and of inflammatory and natural killer cells. I also examine the mechanisms involved in adaptive immune responses, with emphasis on new cytokine profiles, and the role of cell death phenomena in host-pathogen interactions and control of the immune responses to infectious agents. Finally, I highlight the importance of studying SALT in order to better understand host-pathogen relationships involving the skin and detail future directions in the immunological investigation of this organ, especially in light of recent findings regarding the skin immune system.

Immune responses to dengue virus in the skin

Dengue virus (DENV) causes infection in humans and current estimates place 40% of the world population at risk for contracting disease. There are four DENV serotypes that induce a febrile illness, which can develop into a severe and life-threatening disease in some cases, characterized primarily by vascular dysregulation. As a mosquito-borne infection, the skin is the initial site of DENV inoculation and also where primary host immune responses are initiated. This review discusses the early immune response to DENV in the skin by both infection target cells such as dendritic cells and by immune sentinels such as mast cells. We provide an overview of the mechanisms of immune sensing and functional immune responses that have been shown to aid clearance of DENV in vivo. Finally, we discuss factors that can influence the immune response to DENV in the skin, such as mosquito saliva, which is co-injected with virus during natural route infection, and pre-existing immunity to other DENV serotypes or to related flaviviruses.

Innate Immune Response of Primary Human Keratinocytes to West Nile Virus Infection and Its Modulation by Mosquito Saliva

West Nile Virus (WNV) is a flavivirus involved in many human infections worldwide. This arthropod-borne virus is directly co-inoculated with mosquito saliva through the epidermis and the dermis during blood meal. WNV starts replicating in the skin before migrating to the draining lymph node, leading to widespread viremia and in some cases to neurological symptoms. Skin is a complex organ composed of different cell types that together perform essential functions such as pathogen sensing, barrier maintenance and immunity. Keratinocytes, which represent 90% of the cells of the epidermis, are the organism's first line of defense, initiating innate immune response by recognizing pathogens through their pattern recognition receptors. Although WNV was previously known to replicate in human primary keratinocytes, the induced inflammatory response remains unknown. The aim of this study was first to characterize the inflammatory response of human primary keratinocytes to WNV infection and then, to assess the potential role of co-inoculated mosquito saliva on the keratinocyte immune response and viral replication. A type I and III interferon inflammatory response associated with an increase of IRF7 but not IRF3 mRNA expression, and dependent on infectious dose, was observed during keratinocyte infection with WNV. Expression of several interferon-stimulated gene mRNA was also increased at 24 h post-infection (p.i.); they included CXCL10 and interferon-induced proteins with tetratricopeptide repeats (IFIT)-2 sustained up until 48 h p.i. Moreover, WNV infection of keratinocyte resulted in a significant increase of pro-inflammatory cytokines (TNFα, IL-6) and various chemokines (CXCL1, CXCL2, CXCL8 and CCL20) expression. The addition of Aedes aegypti or Culex quinquefasciatus mosquito saliva, two vectors of WNV infection, to infected keratinocytes led to a decrease of inflammatory response at 24 h p.i. However, only Ae. Aegypti saliva adjunction induced modulation of viral replication. In conclusion, this work describes for the first time the inflammatory response of human primary keratinocytes to WNV infection and its modulation in presence of vector mosquito saliva. The effects of mosquito saliva assessed in this work could be involved in the early steps of WNV replication in skin promoting viral spread through the body.

Insights into ZIKV-Mediated Innate Immune Responses in Human Dermal Fibroblasts and Epidermal Keratinocytes

Zika virus (ZIKV) has emerged as a global pathogen causing significant public health concern. ZIKV infections in humans principally occur via mosquito bites. Thus, host skin cells are permissive to ZIKV infection and are the first line of defense against the virus. Here, we examined the role and mechanisms of antiviral skin immunity against ZIKV infection. ZIKV infection (African lineage MR766) in human dermal fibroblasts, human epidermal keratinocytes, and HaCaT keratinocytes resulted in distinct expression changes in RIG-I-like receptors, such as RIG-I and MDA5. Inhibition of RIG-I using small interfering RNA resulted in increased viral gene expression and reduced induction of IFNs and IFN-stimulated genes. Furthermore, ZIKV NS1 directly interacted with RIG-I or MDA5 and down-regulated RIG-I-like receptor-mediated antiviral signaling pathways. Asian lineage ZIKV (PRVABC59) infection also showed a distinct pattern of antiviral immunity in human skin cells, compared with other ZIKV strains. Additionally, ZIKV infections in human neural progenitor cells induced the robust activation of RIG-I-like receptor-mediated signaling, followed by highly enhanced IFN-stimulated gene expression. Our findings provide important insights into ZIKV tropism and subsequent antiviral signaling pathways that regulate ZIKV replication in human dermal fibroblasts and human epidermal keratinocytes.

Immunomodulatory effect of 1, 25 dihydroxy vitamin D3 on the expression of RNA sensing pattern recognition receptor genes and cytokine response in dengue virus infected U937-DC-SIGN cells and THP-1 macrophages

Dengue virus (DENV) infections are straining public health systems worldwide. Vitamin D, a secosteroid hormone, is currently being investigated for its immunomodulatory effects in DENV infections. The objectives of the present study was to study the effect of 1, 25 dihydroxy vitamin D3 (1,25(OH)₂D₃) on the expression of genes coding for RNA sensing pattern recognition receptors, downstream signaling components including oligoadenylate synthetases (OAS) and interferon stimulated gene 15 (ISG15) and T helper (Th)1, Th2 and Th17 cytokine response in DENV infected U937-DC-SIGN cells and THP-1 macrophages. U937-DC-SIGN RNA was investigated for the expression of TLR3, DDX58, IFIH1, OAS1, OAS2, OAS3, CAMP and ISG15 genes using gene expression assays. Interleukin (IL)-12p70, IL-10, IL-4 and IL-17A levels were assessed in the THP-1 macrophage culture supernatants. The results revealed that 1,25(OH)₂D₃ increased the expression of DDX58, OAS1, OAS2 and OAS3 at 0.1 μM while higher concentration had diminishing effect. 1,25(OH)₂D₃ enhanced the expression of ISG15 and CAMP genes. 1,25(OH)₂D₃ suppressed the levels of IL-4 and IL-17A. Lower concentration of 1,25(OH)₂D₃ suppressed IL-12p70 and IL-10 levels while a higher concentration enhanced the levels. The results suggest that 1,25(OH)₂D₃ may have concentration dependent immunomodulatory effects. Higher dose of 1,25(OH)₂D₃ might have an immunoregulatory role in ameliorating inflammation during dengue infections. Further studies are needed to evaluate the efficacy of different doses of 1,25(OH)₂D₃ in preventing severe dengue.

Human keratinocyte cultures (HaCaT) can be infected by DENV, triggering innate immune responses that include IFNλ and LL37

The skin is the first anatomical region that dengue virus (DENV) encounters during the natural infection. Although the role of some skin resident cells like dendritic cells and fibroblasts has been demonstrated to be crucial to elucidate the role of resident cells and molecules participating during the early events of the innate immune response, the participation of keratinocytes during DENV infection has not been fully elucidated. In this paper we aimed to evaluate the use of the HaCaT cell line as a model to study the immune responses of skin keratinocytes to DENV infection. We demonstrated productive DENV-2 infection of HaCaT cells and their capability to establish an antiviral response through production of type I and type III interferons (IFN-β and IFN-λ). The production of these cytokines by HaCaT cells correlated with upregulation of IFN-inducible transmembrane protein-3 (IFITM3) and viperin in bystander, uninfected cells. We also observed an increase in secretion of IL-6 and IL-8. Skin keratinocytes are known to secrete antimicrobial peptides (AMPs) during viral infections. In our model, DENV-2 infected HaCaT cells upregulate the production of cytoplasmic LL-37. We evaluated the dual role of LL-37, HBD2, and HBD3 antiviral activity and immunoregulation during DENV-2 infection of HaCaT cells and found that LL-37 significantly reduced DENV-2 replication. This indicates that the HaCaT cell line can be used as a model for studying the innate response of keratinocytes to DENV infection. Our results also suggest that skin keratinocytes play an important role in the skin microenvironment after DENV infection by secreting molecules like type I and type III IFNs, pro-inflammatory molecules, and LL-37, which may contribute to the protection against arboviral infections.

Experimental in vitro and in vivo systems for studying the innate immune response during dengue virus infections

Dengue is the most prevalent arboviral disease in humans and leads to significant morbidity and socioeconomic burden in tropical and subtropical areas. Dengue is caused by infection with any of the four closely related serotypes of dengue virus (DENV1-4) and usually manifests as a mild febrile illness, but may develop into fatal dengue hemorrhagic fever and shock syndrome. There are no specific antiviral therapies against dengue because understanding of DENV biology is limited. A tetravalent chimeric dengue vaccine, Dengvaxia, has finally been licensed for use, but its efficacy was significantly lower against DENV-2 infections and in dengue-naïve individuals. The identification of mechanisms underlying the interactions between DENV and immune responses will help to determine efficient therapeutic and preventive options. It has been well established how the innate immune system responds to DENV infection and how DENV overcomes innate antiviral defenses, however further progress in this field remains hampered by the absence of appropriate experimental dengue models. Herein, we review the available in vitro and in vivo approaches to study the innate immune responses to DENV.