Skin innate immune response to flaviviral infection

Skin is a complex organ and the largest interface of the human body exposed to numerous stress and pathogens. Skin is composed of different cell types that together perform essential functions such as pathogen sensing, barrier maintenance and immunity, at once providing the first line of defense against microbial infections and ensuring skin homeostasis. Being inoculated directly through the epidermis and the dermis during a vector blood meal, emerging Dengue, Zika andWest Nile mosquito-borne viruses lead to the initiation of the innate immune response in resident skin cells and to the activation of dendritic cells, which migrate to the draining lymph node to elicit an adaptive response. This literature review aims to describe the inflammatory response and the innate immune signalization pathways involved in human skin cells during Dengue, Zika and West Nile virus infections.     

Immunization with DNA through the skin

The skin has evolved as a barrier to prevent external agents, including pathogens, from entering the body. It has a complex and efficient immune surveillance system, which includes Langerhans cells and dendritic cells. By targeting the body's natural defense system, skin-DNA immunization attempts to produce an efficient immune response. Nucleic acid vaccines provide DNA for protein expression in a variety of cells, including keratinocytes, Langerhans cells, and dendritic cells, which are located in the two main areas of the skin, the epidermis (the most superficial layer) and the dermis. After maturation, Langerhans cells and dermal dendritic cells can migrate to local lymph nodes where presentation of antigens to T cells can occur and thus start a variety of immunologic responses. Dermal immunization methods described in this article target the epidermis, the dermis, or both and include: (a) stripping; (b) chemical modification; (c) trans-epidermal immunization (transcutaneous immunization or non-invasive vaccination of the skin); (d) gene gun technology; (e) electroporation; (f) intradermal injections; and (g) microseeding. These techniques all require the removal of hair, the circumvention or modification of the stratum corneum layer of the epidermis, and the addition of DNA or amplification of DNA signal. As the biology of the skin and the mechanisms of DNA vaccination are elucidated, these skin immunization techniques will be optimized. With refinement, skin-DNA immunization will achieve the goal of producing a reliable and efficacious immune response to a variety of pathogens.     

Langerhans cells: mediators of immunity and tolerance

Langerhans cells provide the epidermis with a surveillance network that samples the external environment influencing the decision between immunity and tolerance. Langerhans cells are immature dendritic cells acquiring antigens from foreign invaders as well as damaged native tissue for display to the immune response. The current paradigm suggests that the state of maturity of Langerhans cells, defined by the display of molecules that provoke immune responses (histocompatibility, co-stimulators, adhesion and homing receptors), determines whether emigration of the Langerhans cell to lymph nodes signals immunity or tolerance. Other factors such as type of immunogen ingested, environmental danger signals and the level of cell death may also play a role in tipping the balance towards immunity or immunosuppression. As modulators of the immune response, Langerhans cells play a role in cutaneous autoimmunity in lupus and in cancers that have an affinity for the epidermis such as cutaneous T cell lymphoma.     

Nitric oxide-mediated depletion of Langerhans cells from the epidermis may be involved in UVA radiation-induced immunosuppression.

Ultraviolet A (UVA) irradiation of the dorsal skin of mice reduced the contact hypersensitivity (CHS) response and the density of epidermal Langerhans cells (LC). The roles of nitric oxide (NO) and reactive oxygen species (ROS) in these biological effects of UVA were investigated. Topical application of N(G)-monomethyl-L-arginine acetate, an inhibitor of NO production, 2,2'-dipyridyl, an iron chelater, or 4-hydroxy-tempo, a superoxide dismutase mimicking agent, inhibited UVA-induced suppression of the CHS response. N(G)-monomethyl-L-arginine acetate but not the ROS inhibitors prevented UVA from reducing LC numbers in the epidermis. This suggests that NO but not ROS produced in response to UVA mediates a depletion of LC from the epidermis, probably by signaling these cells to migrate from the skin. This could be responsible for UVA-induced immunosuppression. UVA-induced ROS can also cause immunosuppression, but by a different mechanism. Agents that inhibit or modulate NO or ROS production may be useful for preventing damage caused by the UVA component of sunlight to the skin immune system.     

Langerhans Cell Homeostasis and Activation Is Altered in Hyperplastic Human Papillomavirus Type 16 E7 Expressing Epidermis

It has previously been shown that expression of human papillomavirus type 16 (HPV) E7 in epidermis causes hyperplasia and chronic inflammation, characteristics of pre-malignant lesions. Importantly, E7-expressing epidermis is strongly immune suppressed and is not rejected when transplanted onto immune competent mice. Professional antigen presenting cells are considered essential for initiation of the adaptive immune response that results in graft rejection. Langerhans cells (LC) are the only antigen presenting cells located in normal epidermis and altered phenotype and function of these cells may contribute to the immune suppressive microenvironment. Here, we show that LC are atypically activated as a direct result of E7 expression in the epidermis, and independent of the presence of lymphocytes. The number of LC was significantly increased and the LC are functionally impaired, both in migration and in antigen uptake. However when the LC were extracted from K14E7 skin and matured in vitro they were functionally competent to present and cross-present antigen, and to activate T cells. The ability of the LC to present and cross-present antigen following maturation supports retention of full functional capacity when removed from the hyperplastic skin microenvironment. As such, opportunities are afforded for the development of therapies to restore normal LC function in hyperplastic skin.     

Dynamic nature and function of epidermal Langerhans cells in vivo and in vitro: a review, with emphasis on human Langerhans cells.

Summary Epidermal Langerhans cells (LC) are Birbeck granule-containing bone-marrow-derived cells, which are located mainly in the suprabasal layer of the epidermis. They can be readily identified by their strong expression of CDIa and MHC class II molecules. In addition to these classical properties, an extensive phenotypic profile of normal human LC, summarized in this review, is now available. The powerful capacity of LC to activate T lymphocytes is clearly documented and, to date, LC are recognized as the prominent antigen-presenting cells of the skin immune system. They are generally believed to pick up antigens encountered in the epidermis and to migrate subsequently from the epidermis to the skin-draining lymph nodes. Upon arrival in the paracortex of lymph nodes, the antigen-laden LC transform into interdigitating cells and they present antigen to naive T lymphocytes in a MHC class II-restricted fashion; this results in the generation of antigen-specific immune responses. It has also been demonstrated that transformation of LC into interdigitating cells occurs when LC are culturedin vitro. Bothin vivo andin vitro studies have indicated that properties of LC, such as phenotype, morphology and the stimulatory potential to activate T lymphocytes, are dependent on the local microenvironment in which the LC reside. The essential role of LC in the induction of contact allergic skin reactions and skin transplant rejection is well established.     

The skin as an immune organ.

As a protective interface between internal organs and the environment, the skin encounters a host of toxins, pathogenic organisms, and physical stresses. To combat these attacks on the cutaneous microenvironment, the skin functions as more than a physical barrier: it is an active immune organ. Immune responses in the skin involve an armamentarium of immune-competent cells and soluble biologic response modifiers including cytokines. Traversed by a network of lymphatic and blood vessels, the dermis contains most of the lymphocytes in the skin, other migrant leukocytes, mast cells, and tissue macrophages. Although the epidermis has no direct access to the blood or lymphatic circulation, it is equipped with immune-competent cells: Langerhans cells, the macrophage-like antigen-presenting cells of the epidermis; keratinocytes, epithelial cells with immune properties; dendritic epidermal T lymphocytes, resident cells that may serve as a primitive T-cell immune surveillance system; epidermotropic lymphocytes, migrants from vessels in the dermis; and melanocytes, epidermal pigment cells with immune properties. Although the components of the epidermis and dermis work in concert to execute immune responses in the skin, for purposes of this review, we focus on the cells and cytokines of the epidermal immunologic unit, the frontline of immune protection against environmental toxins and microbes.     

Immunohistochemical localization of Toll-like receptor 2 in skin Langerhans’ cells of striped dolphin (Stenella coeruleoalba)

The skin is the primary interface between the body and the environment, and has a central role in host defence. In the epidermis, Langerhans’ cells form an interconnecting network of dendritic cells, that play a central role within inflammatory and immune responses of terrestrial and aquatic mammals, but few studies aimed at their characterization have been carried out in cetaceans, so far. Toll-like receptors are crucial players in the innate immune response to microbial invaders. These receptors are expressed on immune cells, such as monocytes, macrophages, dendritic cells, and granulocytes.     

Fine structure of lymphoid cells in the nasal skin of primates.

Electron microscopy of the nasal skin of primates shows that lymphoid cells are normally present in this region. No degeneration or mitosis of these cells or their transformation into other cell types was seen. These intraepidermal lymphocytes contained cytoplasmic fibrils. There was no evidence that they were phagocytic in the epidermis. The present study suggests that these lymphoid cells migrate into the lamina propria but not into the epidermis.     

CD207+ Langerhans cells constitute a minor population of skin-derived antigen-presenting cells in the draining lymph node following exposure to Schistosoma mansoni

Infectious cercariae of Schistosoma mansoni gain entry to the mammalian host through the skin where they induce a transient inflammatory influx of mononuclear cells. Some of these cells have antigen-presenting cell function (MHCII+) and have been reported to migrate to the skin-draining lymph nodes (sdLN) where they have the potential to prime CD4+ cells of the acquired immune response. Here, in mice exposed to vaccinating radiation-attenuated schistosome larvae, which induce high levels of protective immunity to challenge infection, we describe the parasite-induced migration of Langerhans cells (LCs) from the epidermal site of immunisation to the sdLN using a specific monoclonal antibody that recognises langerin (CD207). CD207+ cells with dendritic morphology were abundant in the epidermis at all times and their migration into the dermis was detected soon after vaccination. All CD207+ LCs were MHCII+ but not all MHCII+ cells in the skin were CD207+. LCs migrated from the dermis in enhanced numbers after vaccination, as detected in dermal exudate populations recovered after in vitro culture of skin biopsies. Elevated numbers of CD207+ LCs were also detected in the sdLN from 24h to 4 days after vaccination. However, compared with other dermal-derived antigen-presenting cells that were CD207-MHCII+ or CD207-CD11c+, the relative numbers of CD207+ cells in the dermal exudate population and in the sdLN were very small. Furthermore, the migration of CD207+ cells after exposure to 'protective' radiation-attenuated, compared with 'non-protective' normal cercariae, was similar in terms of numbers and kinetics. Together, these studies suggest that CD207+ LCs are only a minor component of the antigen-presenting cell population that migrates from the epidermis and they are unlikely to be important in the priming of protective CD4+ cells in the sdLN.     

Langerhans cells require MyD88-dependent signals for Candida albicans response but not for contact hypersensitivity or migration

Langerhans cells (LC) are a subset of skin-resident dendritic cells (DC) that reside in the epidermis as immature DC, where they acquire Ag. A key step in the life cycle of LC is their activation into mature DC in response to various stimuli, including epicutaneous sensitization with hapten and skin infection with Candida albicans. Mature LC migrate to the skin-draining LN, where they present Ag to CD4 T cells and modulate the adaptive immune response. LC migration is thought to require the direct action of IL-1β and IL-18 on LC. In addition, TLR ligands are present in C. albicans, and hapten sensitization produces endogenous TLR ligands. Both could contribute to LC activation. We generated Langerin-Cre MyD88(fl) mice in which LC are insensitive to IL-1 family members and most TLR ligands. LC migration in the steady state, after hapten sensitization and postinfection with C. albicans, was unaffected. Contact hypersensitivity in Langerin-Cre MyD88(fl) mice was similarly unaffected. Interestingly, in response to C. albicans infection, these mice displayed reduced proliferation of Ag-specific CD4 T cells and defective Th17 subset differentiation. Surface expression of costimulatory molecules was intact on LC, but expression of IL-1β, IL-6, and IL-23 was reduced. Thus, sensitivity to MyD88-dependent signals is not required for LC migration, but is required for the full activation and function of LC in the setting of fungal infection.     

Dietary selenium levels determine epidermal langerhans cell numbers in mice

Selenium (Se) is a dietary trace element that is essential for effective immunity and protection from oxidative damage induced by ultraviolet radiation (UVR). Langerhans cells (LC) represent the major antigen-presenting cells resident in the epidermis; a proportion migrate from the skin to the draining lymph nodes in response to UVR. Because it is known that Se deficiency impairs immune function, we determined what effect this has on LC numbers. CH3/HeN mice were weaned at 3 wk and placed on diets containing <0.005 ppm of Se (Se deficient) or 0.1 ppm of Se (Se adequate, control mice). After 5 wk on the diet, the epidermal LC numbers in the Se-adequate group were 966±51 cells/mm² and LC counts in the epidermis of the Se-deficient mice were 49% lower (p<0.05). Glutathione peroxidase-I (GPx) activity was measured in the epidermis, lymph nodes, and liver. In the epidermis, the activity of GPx in the Se-deficient mice was only 39% (p<0.01) of that seen in epidermis from Se-adequate mice (1.732 U/mg protein). The mice were then irradiated with one dose of 1440 J/m² of broadband UVB or mock irradiated. After 24 h, the decrease in LC number after UVB was greater in the Se-adequate mice, (40% decrease) compared to the Se-deficient group (10%). Thus, Se deficiency reduces epidermal LC numbers, an effect that might compromise cutaneous immunity.     

Human cytomegalovirus inhibits differentiation of monocytes into dendritic cells with the consequence of depressed immunological functions

Human cytomegalovirus (HCMV) infections in immunocompromised patients are associated with impaired immunological functions. Blood monocytes, which can differentiate into dendritic cells upon cytokine stimulation, play a central role in adequate immune reactivity and are believed to carry latent HCMV. We demonstrate here that HCMV infection of monocytes results in a block in the cytokine-induced differentiation of monocytes into functionally active CD1a-positive dendritic cells, which exhibited severely depressed immunological functions in vitro. The HCMV-infected cells exhibited a significantly reduced ability to endocytose fluorescein isothiocyanate-labeled dextran particles as well as a more than 90% reduced ability to migrate in response to the chemoattractant factors RANTES, MIP-1alpha, and MIP-3beta. Interestingly, HCMV-infected cells expressed high levels of the costimulatory molecule CD86, in contrast to the low levels of expression that was observed on uninfected monocytes and uninfected immature dendritic cells. Furthermore, HCMV-infected CD1a-negative cells were unable to induce a T-cell response. Thus, these observations suggest that HCMV infection of monocytes in vitro blocks cytokine-induced dendritic cell differentiation, and since dendritic cells play a central role in initiating immune responses, these findings suggest a powerful tactic to avoid immune recognition and to blunt the immune response at early phases of infection.     

Phagocyte sabotage: disruption of macrophage signalling by bacterial pathogens

Macrophages function at the front line of immune defences against incoming pathogens. But the ability of macrophages to internalize bacteria, migrate, recruit other immune cells to the site of infection and influence the nature of the immune response can provide unintended benefits for bacterial pathogens that are able to subvert or co-opt these normally effective defences. This review highlights recent advances in our understanding of the many interference strategies that are used by bacterial pathogens to undermine macrophage signalling.     

Evidence for involvement of lymphocyte function-associated antigen 1 in T cell migration to epidermis.

Although it is well known that in various T cell-mediated skin diseases T cells migrate preferentially to epidermis, no direct evidence has been presented in which molecules on T cells are important in directing T cell traffic to epidermis. We have previously established CD4+ autoreactive cloned T cells with a special tropism for epidermis in vitro as well as in vivo. Antibody inhibition studies demonstrated that only anti-lymphocyte function associated Ag 1 (anti-LFA-1) mAb completely inhibited the in vitro migration of the T cells toward the epidermis, whereas mAb against other T cell surface molecules had little or no effect. Monovalent F(ab) fragment of the anti-LFA-1 mAb, although less efficient, also inhibited the T cell migration. The apparent dependency of the inhibition on the anti-alpha-chain mAb suggested a major role for the alpha-chain of LFA-1 in T cell migration to epidermis. The relevance of an LFA-1-dependent mechanism to the epidermotropic migration of T cells was further strengthened by the findings that the T cell migration to epidermis was inhibited by divalent cation depletion, cytochalasin B, and low temperature. These findings indicate that the LFA-1 molecule, which is thought to be primarily involved in cell-to-cell adhesions, also plays an important role in directing T cell migration to epidermis.     

Depletion of Langerhans cells in human papillomavirus type 16-infected skin is associated with E6-mediated down regulation of E-cadherin

Human papillomavirus type 16 (HPV16) is an oncogenic virus that causes persistent infections in cervical epithelium. The chronic nature of HPV16 infections suggests that this virus actively evades the host immune response. Intraepithelial Langerhans cells (LC) are antigen-presenting cells that are critical in T-cell priming in response to viral infections of the skin. Here we show that HPV16 infection is directly associated with a reduction in the numbers of LC in infected epidermis. Adhesion between keratinocytes (KC) and LC, mediated by E-cadherin, is important in the retention of LC in the skin. Cell surface E-cadherin is reduced on HPV16-infected basal KC, and this is directly associated with the reduction in numbers of LC in infected epidermis. Expression of a single viral early protein, HPV16 E6, in KC reduces levels of cell surface E-cadherin thereby interfering with E-cadherin-mediated adhesion. Through this pathway, E6 expression in HPV16-infected KC may limit presentation of viral antigens by LC to the immune system, thus preventing the initiation of a cell-mediated immune response and promoting survival of the virus.     

Stem cells in the hair follicle bulge contribute to wound repair but not to homeostasis of the epidermis

The discovery of long-lived epithelial stem cells in the bulge region of the hair follicle led to the hypothesis that epidermal renewal and epidermal repair after wounding both depend on these cells. To determine whether bulge cells are necessary for epidermal renewal, here we have ablated these cells by targeting them with a suicide gene encoding herpes simplex virus thymidine kinase (HSV-TK) using a Keratin 1-15 (Krt1-15) promoter. We show that ablation leads to complete loss of hair follicles but survival of the epidermis. Through fate-mapping experiments, we find that stem cells in the hair follicle bulge do not normally contribute cells to the epidermis which is organized into epidermal proliferative units, as previously predicted. After epidermal injury, however, cells from the bulge are recruited into the epidermis and migrate in a linear manner toward the center of the wound, ultimately forming a marked radial pattern. Notably, although the bulge-derived cells acquire an epidermal phenotype, most are eliminated from the epidermis over several weeks, indicating that bulge stem cells respond rapidly to epidermal wounding by generating short-lived 'transient amplifying' cells responsible for acute wound repair. Our findings have implications for both gene therapy and developing treatments for wounds because it will be necessary to consider epidermal and hair follicle stem cells as distinct populations.     

CCR7 coordinates the primary immune response by establishing functional microenvironments in secondary lymphoid organs.

The proper function of immune surveillance requires well-coordinated mechanisms in order to guide the patrolling immune cells through peripheral tissues and into secondary lymphoid organs. Analyzing gene-targeted mice, we identified the chemokine receptor CCR7 as an important organizer of the primary immune response. CCR7-deficient mice show severely delayed kinetics regarding the antibody response and lack contact sensitivity and delayed type hypersensitivity reactions. Due to the impaired migration of lymphocytes, these animals reveal profound morphological alterations in all secondary lymphoid organs. Upon activation, mature skin dendritic cells fail to migrate into the draining lymph nodes. Thus, in order to bring together lymphocytes and dendritic cells to form the characteristic microarchitecture of secondary lymphoid organs, CCR7 is required to rapidly initiate an adoptive immune response.     

A combination of MIP-3alpha and TGF-beta1 is required for the attraction of human Langerhans precursor cells through a dermal-epidermal barrier.

Signals regulating the traffic of Langerhans cell precursors from blood to the epidermis are not yet fully understood. The observations that TGF-beta1 is of unique importance in Langerhans cells (LC) ontogeny and that macrophage inflammatory protein-3alpha (MIP-3alpha) is able to attract LC within the epidermis, prompted us to study the effect of MIP-3alpha and TGF-beta1 on the migration of LC precursors. The migratory capacity of immature dendritic cells (DC) was assessed using a reconstituted basement membrane assay (Matrigel), mimicking the prerequisite passage through the dermal-epidermal basement membrane on the way into the epidermis. DC differentiated from cord blood CD34 cells in the presence of GM-CSF plus TNF-alpha were subjected to migration using modified Boyden chambers. Day-6 DC progenitors migrated in a dose-dependent fashion in response to MIP-3alpha, and CD1alpha+ LC precursors responded preferentially to the chemokine. Immature DC did not respond strongly to TGF-beta1 alone in migration assays, but up to 68% of the cells migrated in response to MIP-3alpha plus TGF-beta1. Among them, at least 50% expressed CD1a and E-cadherin and can be considered LC precursors. The allostimulatory function of these cells was significantly more potent than that which migrated in response to MIP-3alpha alone. Our results show that a significant proportion of immature DC is able to migrate through a dermal-epidermal basement membrane equivalent. In the presence of TGF-beta1, the DC which respond to MIP-3alpha have the phenotype and the functional capacity of epidermal LC. Our findings underline the role of MIP-3alpha and TGF-beta1 in attraction and localization of immature LC within the epidermis under normal conditions.     

Characteristics of the reaction of healthy human blood serum with the epidermis

Indirect immunofluorescence has shown that the blood serum of normal subjects reacts with cytoplasm antigens of epidermis differentiated cells in 100% of the cases. The level of antibodies and the immunomorphological picture of the reaction are marked by high constancy and intense fluorescence of the cytoplasm of epidermis differentiated cells, while the reaction with basal layer cell antigens is observed comparatively seldom and little pronounced. The authors discuss possible participation of antibodies contained by the normal blood serum and of their complexes with tissue antigens in the regulation of vital activity of the cells and immune response to the host antigens.