IL-23 Antagonizes UVR-Induced Immunosuppression through Two Mechanisms: Reduction of UVR-Induced DNA Damage and Inhibition of UVR-Induced Regulatory T Cells

Systematic review on gene-sun exposure interactions in skin cancer

BACKGROUND

The risk of skin cancer is determined by environmental factors like ultraviolet radiation (UVR), personal habits like time spent outdoors and genetic factors. This review aimed to survey existing studies in gene-environment (GxE) interaction on skin cancer risk, and report on GxE effect estimates.

METHODS

We searched Embase, Medline (Ovid) and Web of Science (Core Collection) and included only primary research that reported on GxE on the risk of the three most common types of skin cancer: basal cell carcinoma (BCC), squamous cell carcinoma (SCC) and melanoma. Quality assessment followed the Newcastle-Ottawa Scale. Meta-analysis was not possible because no two studies examined the same interaction. This review was registered on PROSPERO (CRD42021238064).

RESULTS

In total 260 records were identified after exclusion of duplicates. Fifteen studies were included in the final synthesis-12 used candidate gene approach. We found some evidence of GxE interactions with sun exposure, notably, with MC1R, CAT and NOS1 genes in melanoma, HAL and IL23A in BCC and HAL and XRCC1 in SCC.

CONCLUSION

Sun exposure seems to interact with genes involved in pigmentation, oxidative stress and immunosuppression, indicating that excessive UV exposure might exhaust oxidative defence and repair systems differentially, dependent on genetic make-up. Further research is warranted to better understand skin cancer epidemiology and develop sun exposure recommendations. A genome-wide approach is recommended as it might uncover unknown disease pathways dependent on UV radiation.

Mouse Models of Allergic Contact Dermatitis: Practical Aspects

Allergic contact dermatitis is a frequently observed dermatosis, especially in industrialized countries. Regarded as a classical type IV immune reaction (delayed type), the process can be separated into two pathogenetic parts: the induction phase where sensitization takes place and the elicitation phase in which inflammation is induced upon re-exposure to the same antigen. A murine model was established decades ago, which reliably reproduces both phases. Epicutaneously applied low-molecular-weight sensitizers bind to proteins (haptens) and become full antigens, which results in sensitization. Subsequent administration of the same hapten onto ear skin causes a swelling response. This reaction is antigen specific because it cannot be induced in nonsensitized mice or in sensitized mice with a different hapten. This model was used to study the mechanisms involved in allergic contact dermatitis and also was intensively utilized to study immunologic mechanisms, including antigen presentation and development of T effector or regulatory T cells. The model's major merit is its antigen specificity. It is highly reproducible, reliable, and simple to perform. In this paper, the methods of this technique are described to help researchers successfully establish this widely used model in laboratories. Describing the complex pathomechanisms underlying the model is beyond the scope of this article.

Nucleic Acid Sensing Pathways in DNA Repair Targeted Cancer Therapy

The repair of DNA damage is a complex process, which helps to maintain genome fidelity, and the ability of cancer cells to repair therapeutically DNA damage induced by clinical treatments will affect the therapeutic efficacy. In the past decade, great success has been achieved by targeting the DNA repair network in tumors. Recent studies suggest that DNA damage impacts cellular innate and adaptive immune responses through nucleic acid-sensing pathways, which play essential roles in the efficacy of DNA repair targeted therapy. In this review, we summarize the current understanding of the molecular mechanism of innate immune response triggered by DNA damage through nucleic acid-sensing pathways, including DNA sensing via the cyclic GMP-AMP synthase (cGAS), Toll-like receptor 9 (TLR9), absent in melanoma 2 (AIM2), DNA-dependent protein kinase (DNA-PK), and Mre11-Rad50-Nbs1 complex (MRN) complex, and RNA sensing via the TLR3/7/8 and retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs). Furthermore, we will focus on the recent developments in the impacts of nucleic acid-sensing pathways on the DNA damage response (DDR). Elucidating the DDR-immune response interplay will be critical to harness immunomodulatory effects to improve the efficacy of antitumor immunity therapeutic strategies and build future therapeutic approaches.

ENTPD1 (CD39) Expression Inhibits UVR-Induced DNA Damage Repair through Purinergic Signaling and Is Associated with Metastasis in Human Cutaneous Squamous Cell Carcinoma

UVR and immunosuppression are major risk factors for cutaneous squamous cell carcinoma (cSCC). Regulatory T cells promote cSCC carcinogenesis, and in other solid tumors, infiltrating regulatory T cells and CD8⁺ T cells express ectonucleoside triphosphate diphosphohydrolase 1 (ENTPD1) (also known as CD39), an ectoenzyme that catalyzes the rate-limiting step in converting extracellular adenosine triphosphate (ATP) to extracellular adenosine (ADO). We previously showed that extracellular purine nucleotides influence DNA damage repair. In this study, we investigate whether DNA damage repair is modulated through purinergic signaling in cSCC. We found increased ENTPD1 expression on T cells within cSCCs when compared with the expression on T cells from blood or nonlesional skin, and accordingly, concentrations of derivative extracellular adenosine diphosphate (ADP), adenosine monophosphate (AMP), and ADO are increased in tumors compared with those in normal skin. Importantly, ENTPD1 expression is significantly higher in human cSCCs that metastasize than in those that are nonmetastatic. We also identify in a mouse model that ENTPD1 expression is induced by UVR in an IL-27-dependent manner. Finally, increased extracellular ADO is shown to downregulate the expression of NAP1L2, a nucleosome assembly protein we show to be important for DNA damage repair secondary to UVR. Together, these data suggest a role for ENTPD1 expression on skin-resident T cells to regulate DNA damage repair through purinergic signaling to promote skin carcinogenesis and metastasis.

A deep dive into UV-based phototherapy: Mechanisms of action and emerging molecular targets in inflammation and cancer

UV-based phototherapy (including psoralen plus UVA (PUVA), UVB and UVA1) has a long, successful history in the management of numerous cutaneous disorders. Photoresponsive diseases are etiologically diverse, but most involve disturbances in local (and occasionally systemic) inflammatory cells and/or abnormalities in keratinocytes that trigger inflammation. UV-based phototherapy works by regulating the inflammatory component and inducing apoptosis of pathogenic cells. This results in a fascinating and complex network of simultaneous events-immediate transcriptional changes in keratinocytes, immune cells, and pigment cells; the emergence of apoptotic bodies; and the trafficking of antigen-presenting cells in skin-that quickly transform the microenvironment of UV-exposed skin. Molecular elements in this system of UV recognition and response include chromophores, metabolic byproducts, innate immune receptors, neurotransmitters and mediators such as chemokines and cytokines, antimicrobial peptides, and platelet activating factor (PAF) and PAF-like molecules that simultaneously shape the immunomodulatory effects of UV and their interplay with the microbiota of the skin and beyond. Phototherapy's key effects-proapoptotic, immunomodulatory, antipruritic, antifibrotic, propigmentary, and pro-prebiotic-promote clinical improvement in various skin diseases such as psoriasis, atopic dermatitis (AD), graft-versus-host disease (GvHD), vitiligo, scleroderma, and cutaneous T-cell lymphoma (CTCL) as well as prevention of polymorphic light eruption (PLE). As understanding of phototherapy improves, new therapies (UV- and non-UV-based) are being developed that will modify regulatory T-cells (Treg), interact with (resident) memory T-cells and /or utilize agonists and antagonists as well as antibodies targeting soluble molecules such as cytokines and chemokines, transcription factors, and a variety of membrane-associated receptors.

The use of tissue-engineered skin to demonstrate the negative effect of CXCL5 on epidermal ultraviolet radiation-induced cyclobutane pyrimidine dimer repair efficiency

BACKGROUND

Ultraviolet radiation (UVR) is responsible for keratinocyte cancers through the induction of mutagenic cyclobutane pyrimidine dimers (CPDs). Many factors influence CPD repair in epidermal keratinocytes, and a better understanding of those factors might lead to prevention strategies against skin cancer.

OBJECTIVES

To evaluate the impact of dermal components on epidermal CPD repair efficiency and to investigate potential factors responsible for the dermal-epidermal crosstalk modulating UVR-induced DNA damage repair in keratinocytes.

METHODS

A model of self-assembled tissue-engineered skin containing human primary keratinocytes and fibroblasts was used in this study.

RESULTS

We showed that CPD repair in keratinocytes is positively influenced by the presence of a dermis. We investigated the secretome and found that the cytokine CXCL5 is virtually absent from the culture medium of reconstructed skin, compared with media from fibroblasts and keratinocytes alone. By modulating CXCL5 levels in culture media of keratinocytes, we have shown that CXCL5 is an inhibitor of CPD repair.

CONCLUSIONS

This work outlines the impact of the secreted dermal components on epidermal UVR-induced DNA damage repair and sheds light on a novel role of CXCL5 in CPD repair.

Human and Murine Evidence for Mechanisms Driving Autoimmune Photosensitivity

Ultraviolet (UV) light is an important environmental trigger for systemic lupus erythematosus (SLE) patients, yet the mechanisms by which UV light impacts disease are not fully known. This review covers evidence in both human and murine systems for the impacts of UV light on DNA damage, apoptosis, autoantigen exposure, cytokine production, inflammatory cell recruitment, and systemic flare induction. In addition, the role of the circadian clock is discussed. Evidence is compared in healthy individuals and SLE patients as well as in wild-type and lupus-prone mice. Further research is needed into the effects of UV light on cutaneous and systemic immune responses to understand how to prevent UV-light mediated lupus flares.

Interleukin (IL)-12 and IL-23 and Their Conflicting Roles in Cancer

The balance of proinflammatory cytokines interleukin (IL)-12 and IL-23 plays a key role in shaping the development of antitumor or protumor immunity. In this review, we discuss the role IL-12 and IL-23 plays in tumor biology from preclinical and clinical data. In particular, we discuss the mechanism by which IL-23 promotes tumor growth and metastases and how the IL-12/IL-23 axis of inflammation can be targeted for cancer therapy.

Hydrogen Water Drinking Exerts Antifatigue Effects in Chronic Forced Swimming Mice via Antioxidative and Anti-Inflammatory Activities

Purpose

This study was performed to evaluate antifatigue effect of hydrogen water (HW) drinking in chronic forced exercise mice model.

Materials and Methods

Twelve-week-old C57BL6 female mice were divided into nonstressed normal control (NC) group and stressed group: (purified water/PW-treated group and HW-treated group). Stressed groups were supplied with PW and HW, respectively, ad libitum and forced to swim for the stress induction every day for 4 consecutive weeks. Gross antifatigue effects of HW were assessed by swimming endurance capacity (once weekly for 4 wk), metabolic activities, and immune-redox activities. Metabolic activities such as blood glucose, lactate, glycogen, blood urea nitrogen (BUN), and lactate dehydrogenase (LDH) as well as immune-redox activities such as reactive oxygen species (ROS), nitric oxide (NO), glutathione peroxidase (GPx), catalase, and the related cytokines were evaluated to elucidate underlying mechanism. Blood glucose and lactate were measured at 0 wk (before swimming) and 4 wk (after swimming).

Results

HW group showed a higher swimming endurance capacity (p < 0.001) than NC and PW groups. Positive metabolic effects in HW group were revealed by the significant reduction of blood glucose, lactate, and BUN in serum after 4 wk (p < 0.01, resp.), as well as the significant increase of liver glycogen (p < 0.001) and serum LDH (p < 0.05) than PW group. In parallel, redox balance was represented by lower NO in serum (p < 0.01) and increased level of GPx in both serum and liver (p < 0.05) than PW group. In line, the decreased levels of serum TNF-α (p < 0.01), IL-6, IL-17, and liver IL-1β (p < 0.05) in HW group revealed positive cytokine profile compared to PW and NC group.

Conclusion

This study shows antifatigue effects of HW drinking in chronic forced swimming mice via metabolic coordination and immune-redox balance. In that context, drinking HW could be applied to the alternative and safety fluid remedy for chronic fatigue control.

In vivo quantification of quantum dot systemic transport in C57BL/6 hairless mice following skin application post-ultraviolet radiation

BACKGROUND

Previous work has demonstrated size, surface charge and skin barrier dependent penetration of nanoparticles into the viable layers of mouse skin. The goal of this work was to characterize the tissue distribution and mechanism of transport of nanoparticles beyond skin, with and without Ultraviolet Radiation (UVR) induced skin barrier disruption. Atomic absorption spectroscopy (AAS), flow cytometry and confocal microscopy were used to examine the effect of UVR dose (180 and 360 mJ/cm² UVB) on the skin penetration and systemic distribution of quantum dot (QD) nanoparticles topically applied at different time-points post UVR using a hairless C57BL/6 mouse model.

RESULTS

Results indicate that QDs can penetrate mouse skin, regardless of UVR exposure, as evidenced by the increased cadmium in the local lymph nodes of all QD treated mice. The average % recovery for all treatment groups was 69.68% with ~66.84% of the applied dose recovered from the skin (both epicutaneous and intracutaneous). An average of 0.024% of the applied dose was recovered from the lymph nodes across various treatment groups. When QDs are applied 4 days post UV irradiation, at the peak of the skin barrier defect and LC migration to the local lymph node, there is an increased cellular presence of QD in the lymph node; however, AAS analysis of local lymph nodes display no difference in cadmium levels due to UVR treatment.

CONCLUSIONS

Our data suggests that Langerhans cells (LCs) can engulf QDs in skin, but transport to the lymph node may occur by both cellular (dendritic and macrophage) and non-cellular mechanisms. It is interesting that these specific nanoparticles were retained in skin similarly regardless of UVR barrier disruption, but the observed skin immune cell interaction with nanoparticles suggest a potential for immunomodulation, which we are currently examining in a murine model of skin allergy.

IL-23 Inhibits Melanoma Development by Augmenting DNA Repair and Modulating T Cell Subpopulations

In animal models, IL-12 and IL-23 participate in the development of malignant neoplasms of keratinocytes. However, the role of these cytokines in pigmented lesion development and their progression to melanoma has received little attention. IL-12p35, IL-23p19, and IL-12/IL-23p40 knockout mice on a C3H/HeN background, subjected to a melanomagenesis protocol, demonstrated profound differences in susceptibility to nevus initiation, transformation, tumorigenicity, and metastatic potential. IL-23 was found to be essential for melanocyte homeostasis, whereas IL-12 supported nevus development. A direct action of IL-23 on primary melanocytes, shown to be IL-23R⁺, demonstrated that DNA repair of damaged melanocytes requires IL-23. Furthermore, IL-23 modulated the cutaneous microenvironment by limiting regulatory T cells and IFN-γ and inhibiting IL-10 production. Neutralizing Ab to IFN-γ, but not IL-17, inhibited nevus development (p < 0.01).

Sunlight Effects on Immune System: Is There Something Else in addition to UV-Induced Immunosuppression?

Sunlight, composed of different types of radiation, including ultraviolet wavelengths, is an essential source of light and warmth for life on earth but has strong negative effects on human health, such as promoting the malignant transformation of skin cells and suppressing the ability of the human immune system to efficiently detect and attack malignant cells. UV-induced immunosuppression has been extensively studied since it was first described by Dr. Kripke and Dr. Fisher in the late 1970s. However, skin exposure to sunlight has not only this and other unfavorable effects, for example, mutagenesis and carcinogenesis, but also a positive one: the induction of Vitamin D synthesis, which performs several roles within the immune system in addition to favoring bone homeostasis. The impact of low levels of UV exposure on the immune system has not been fully reported yet, but it bears interesting differences with the suppressive effect of high levels of UV radiation, as shown by some recent studies. The aim of this article is to put some ideas in perspective and pose some questions within the field of photoimmunology based on established and new information, which may lead to new experimental approaches and, eventually, to a better understanding of the effects of sunlight on the human immune system.

The symbiosis of phototherapy and photoimmunology

The health benefits of natural sunlight have been noted since the rise of civilization, even without the knowledge of its mechanisms of action. Currently, phototherapy remains an effective and widely used treatment for a variety of skin diseases. Ultraviolet radiation, from either the sun or artificial light sources, has a profound immunomodulatory effect that is responsible for its beneficial clinical outcomes. Ultraviolet radiation mostly induces the innate while suppressing the adaptive immune system, leading to both local and systemic effects. It is antigen specific, acts on both effector and regulatory T cells, alters antigen-presenting cell function, and induces the secretion of cytokines and soluble mediators. This review provides an overview of the immunologic mechanisms by which ultraviolet radiation is responsible for the therapeutic effects of phototherapy.

Ultraviolet radiation signaling through TLR4/MyD88 constrains DNA repair and plays a role in cutaneous immunosuppression

UV radiation (UVR) induces DNA damage, leading to the accumulation of mutations in epidermal keratinocytes and immunosuppression, which contribute to the development of nonmelanoma skin cancer. We reported previously that the TLR4-MyD88 signaling axis is necessary for UV-induced apoptosis. In the dinitrofluorobenzene contact hypersensitivity model, UV-irradiated MyD88-deficient (MyD88(-/-)) C57BL/6 mice had intact ear swelling, exaggerated inflammation, and higher levels of dinitrofluorobenzene-specific IgG2a compared with wild-type (WT) mice. Even with normal UV-induced, dendritic cell migration, DNA damage in the local lymph nodes was less pronounced in MyD88(-/-) mice compared with WT mice. Cultured, UV-irradiated WT APCs showed cleavage (inactivation) of the DNA damage-recognition molecule PARP, whereas PARP persisted in MyD88(-/-) and TLR4(-/-) APCs. Epidermal DNA from in vivo UV-irradiated MyD88(-/-) mice had an increased resolution rate of cyclobutane pyrimidine dimers. Both in vitro treatment of MyD88(-/-) APCs with and intradermal in vivo injections of PARP inhibitor, PJ-34, caused WT-level cyclobutane pyrimidine dimer repair. Lymphoblasts deficient in DNA repair (derived from a xeroderma pigmentosum group A patient) failed to augment DNA repair after MyD88 knockdown after UVR, in contrast to lymphoblasts from a healthy control. These data suggest that interference with the TLR4/MyD88 pathway may be a useful tool in promoting DNA repair and maintaining immune responses following UVR-induced damage.

Interleukin-17 mediated inflammatory responses are required for ultraviolet radiation-induced immune suppression

Ultraviolet radiation (UVR) induces immunosuppression and is a major factor for development of skin cancer. Numerous efforts have been made to determine mechanisms for UVR-induced immunosuppression and to develop strategies for prevention and treatment of UVR-induced cancers. In the current study, we use IL-17 receptor (IL-17R) deficient mice to examine whether IL-17 mediated responses have a role in UVB (290-320)-induced immunosuppression of contact hypersensitivity responses. Results demonstrate that IL-17 mediated responses are required for UVB-induced immunosuppression of contact hypersensitivity responses. The systemic immune suppression and development of regulatory T cells are inhibited in UVB-treated IL-17R deficient mice compared to wild-type animals. The deficiency in IL-17R inhibits the infiltration and development of a tolerogenic myeloid cell population in UVB-treated skin, which expresses CD11b and Gr-1 and produces reactive oxygen species. We speculate that the development of the tolerogenic myeloid cells is dependent on IL-17-induced chemokines and inflammatory mediators in UVB-treated skin. The inhibition of the tolerogenic myeloid cells may be attributed to the suppression of regulatory T cells in UVR-treated IL-17R(-/-) mice. The findings may be exploited to new strategies for prevention and treatment of UVR-induced skin diseases and cancers.

Toll-like receptor-4 deficiency enhances repair of UVR-induced cutaneous DNA damage by nucleotide excision repair mechanism

UVB-induced DNA damage plays a critical role in development of photoimmunosuppression. The purpose of this study was to determine whether repair of UVB-induced DNA damage is regulated by Toll-like receptor-4 (TLR4). When TLR4 gene knockout (TLR4^-/-) and TLR4 competent (TLR4^+/+) mice were subjected to 90 mJ/cm² UVB radiation locally, DNA damage in the form of CPD, were repaired more efficiently in the skin and bone marrow dendritic cells (BMDC) of TLR4^-/- mice in comparison to TLR4^+/+ mice. Expression of DNA repair gene XPA (Xeroderma pigmentosum complementation group A) was significantly lower in skin and BMDC of TLR4^+/+ mice than TLR4^-/- mice after UVB exposure. When cytokine levels were compared in these strains after UVB exposure, BMDC from UV-irradiated TLR4^-/- mice produced significantly more interleukin (IL)-12 and IL-23 cytokines (p<0.05) than BMDC from TLR4^+/+ mice. Addition of anti-IL-12 and anti-IL-23 antibodies to BMDC of TLR4^-/- mice (before UVB exposure) inhibited repair of CPD, with a concomitant decrease in XPA expression. Addition of TLR4 agonist to TLR4^+/+ BMDC cultures decreased XPA expression and inhibited CPD repair. Thus, strategies to inhibit TLR4 may allow for immunopreventive and immunotherapeutic approaches for managing UVB-induced cutaneous DNA damage and skin cancer.

A balance of interleukin-12 and -23 in cancer

Interleukin (IL)-12 and IL-23 share the IL-12p40 molecule. IL-12 promotes T helper (Th)1 immunity and IL-23 promotes Th17 immunity, and it has recently become apparent that the balance between IL-12 and IL-23 is important in carcinogenesis. A series of studies demonstrated that, where tumor initiation, growth, and metastasis are concerned, IL-12 may act independently of interferon (IFN)-γ, and IL-23 independently of IL-17A. This review explores the activity of IL-23 in carcinogenesis. In the context of the tumor-inhibitory effects of IL-12, and tumor-promoting effects of IL-23, we discuss the use of anti-IL-12p/23 monoclonal antibodies (mAbs) in autoimmune inflammatory disorders and the alternative specific neutralization of IL-23.

TLR signaling and DNA repair: are they associated?

Toll-like receptor (TLR) signaling is a well-characterized, innate immune cellular defense mechanism used to detect and respond to pathogen-associated molecular patterns (PAMPs). TLR signaling is highly conserved and has evolved to have both extracellular and endosomal receptors that recognize PAMPs from a wide range of microbial pathogens. Recent literature has emerged to show that activation of TLRs not only leads to the upregulation of cellular defense mechanisms, but also results in upregulation of DNA repair genes and increased functional DNA repair. Endosomal TLR agonists result in increased survival and repair after both ionizing and UV radiation, suggesting that the repair pathways for single- and double-strand breaks are affected. This review brings together these and other experimental findings to examine how DNA repair pathways may be linked to TLR signaling. Also discussed are the varied outcomes and related physiological implications that increased DNA repair after injury might have.

Inflammasome activation of IL-1 family mediators in response to cutaneous photodamage

Although keratinocytes are relatively resistant to ultraviolet radiation (UVR) induced damage, repeated UVR exposure result in accumulated DNA mutations that can lead to epidermal malignancies. Keratinocytes play a central role in elaborating innate responses that lead to inflammation and influence the generation of adaptive immune responses in skin. Apart from the minor cellular constituents of the epidermis, specifically Langerhans cells and melanocytes, keratinocytes are the major source of cytokines. UVR exposure stimulates keratinocytes to secrete abundant pro-inflammatory IL-1-family proteins, IL-1α, IL-1β, IL-18, and IL-33. Normal skin contains only low levels of inactive precursor forms of IL-1β and IL-18, which require caspase 1-mediated proteolysis for their maturation and secretion. However, caspase-1 activation is not constitutive, but dependents on the UV-induced formation of an active inflammasome complex. IL-1 family cytokines can induce a secondary cascade of mediators and cytokines from keratinocytes and other cells resulting in wide range of innate processes including infiltration of inflammatory leukocytes, induction of immunosuppression, DNA repair or apoptosis. Thus, the ability of keratinocytes to produce a wide repertoire of proinflammatory cytokines can influence the immune response locally as well as systematically, and alter the host response to photodamaged cells. We will highlight differential roles played by each IL-1 family molecule generated by UV-damaged keratinocytes, and reveal their complementary influences in modulating acute inflammatory and immunological events that follow cutaneous UV exposure.

Imiquimod-induced TLR7 signaling enhances repair of DNA damage induced by ultraviolet light in bone marrow-derived cells

Imiquimod is a TLR7/8 agonist that has anticancer therapeutic efficacy in the treatment of precancerous skin lesions and certain nonmelanoma skin cancers. To test our hypothesis that imiquimod enhances DNA repair as a mechanism for its anticancer activity, the nucleotide excision repair genes were studied in bone marrow-derived cells. Imiquimod enhanced the expression of xeroderma pigmentosum (XP) A and other DNA repair genes (quantitative real-time PCR analysis) and resulted in an increased nuclear localization of the DNA repair enzyme XPA. This was dependent on MyD88, as bone marrow-derived cells from MyD88(-/-) mice did not increase XPA gene expression and did not enhance the survival of MyD88(-/-)-derived bone marrow-derived cells after UV B exposure as was observed in bone marrow-derived cells from MyD88(+/+) mice. Imiquimod also enhanced DNA repair of UV light (UVL)-irradiated gene expression constructs and accelerated the resolution of cyclobutane pyrimidine dimers after UVL exposures in P388 and XS52. Lastly, topical treatment of mouse skin with 5% imiquimod cream prior to UVL irradiation resulted in a decrease in the number of cyclobutane pyridimine dimer-positive APC that were found in local lymph nodes 24 h after UVL irradiation in both wild-type and IL-12 gene-targeted mice. In total, these data support the idea that TLR7 agonists such as imiquimod enhance DNA repair in bone marrow-derived cells. This property is likely to be an important mechanism for its anticancer effects because it protects cutaneous APC from the deleterious effects of UVL.

Molecular mechanisms of ultraviolet radiation-induced immunosuppression

Solar ultraviolet radiation (UVR) is well known for its immunosuppressive properties. UVR can suppress immune reactions both in a local and a systemic fashion. One of the major molecular mediators of photoimmunosuppression is UVR-induced DNA damage. In contrast to immunosuppressive drugs, UVR does not act in a general but antigen-specific fashion. This is due to the induction of regulatory T cells. Epidermal Langerhans cells harboring UVR-induced DNA damage appear to be essentially involved in the induction of these cells. Cytokines including interleukin (IL)-12, -18 and -23 exert the capacity to reduce UVR-induced DNA damage via induction of DNA repair. Accordingly, these cytokines prevent UVR-mediated immunosuppression. In contrast to IL-18, IL-12 and IL-23 can also inhibit the suppressive activity of regulatory T cells by a mechanism which still needs to be determined. Clarification of the molecular mechanisms underlying UVR-induced immunosuppression will help to develop new immunosuppressive therapeutic strategies by utilizing UVR-induced regulatory T cells for the treatment of immune-mediated diseases. In addition, these insights will contribute to a better understanding of photocarcinogenesis since suppression of the immune system by UVR essentially contributes to the induction of skin cancer.

The other end of the rainbow: infrared and skin

Although infrared radiation (IRR) is ubiquitous in the terrestrial milieu, its effects on human skin have until now been largely ignored. Recent studies suggest an important role for infrared A (IRA) radiation (760-1440 nm) in dermal inflammation, photoaging, and photocarcinogenesis. In this issue, Calles et al. identify and analyze the IRA-induced transcriptome in human dermal fibroblasts. Their work paves the way for new research directions in IRA photobiology and raises important clinical questions regarding photoprotection and IRR-based dermatotherapy.