Ultraviolet radiation‐induced tolerance (1999)

Ultraviolet (UV) radiation is one of the most important environmental factors. Besides its well-known advantages and its indispensable effects on human life, UV light, particularly the middle wavelength range (290–320 nm [UVB]), can harm human health by inducing skin cancer, premature skin aging, inflammation, and cell death ( 1-4). Over the last 25 years, it has become apparent that exposure to UVB radiation may also suppress immune reactions. Review of the numerous studies on the immunosuppressive properties of UV radiation led to two quite surprising observations: that UV exposure can have systemic immunologic consequences that UV radiation can induce antigen-specific tolerance. This paper briefly reviews the phenomenon of UV-induced tolerance. Photoimmunology was born in the mid-1970s in the recognition of an association between UV radiation and immune suppression during photocarcinogenesis. Kripke and coworkers induced skin tumors in mice by chronic UVB exposure. These tumors appeared to be highly antigenic since they were rejected when transplanted onto syngeneic healthy recipients ( 5-7). When the recipients were treated with immunosuppressive drugs, the transplanted tumors were not rejected, indicating that this type of rejection was immunologic in nature. Rejection also failed to occur when recipients were exposed to rather low doses of UVB ( 8-10). This was the first indication that UVB radiation can exert immunosuppressive properties. Since the tumors grew even when transplanted into sites not directly exposed to UV radiation, it was concluded that UV radiation exerts also a systemic immunosuppressive effect ( 9, 10). Suppression of tumor rejection could be transferred by injecting T cells from UV-irradiated mice into normal recipients, which subsequently became unable to reject UV-induced tumors ( 11, 12). In contrast, rejection of non-UV-induced neoplasms (e.g., chemically induced tumors) was not affected ( 13-16). These findings suggested that UV exposure induced T cells which inhibit the immune response specifically to UV-induced tumors (T suppressor cells). The phenotype of these T suppressor cells is generally Thy-1+, Lyt-1+ (CD5), Lyt-2− (CD8), L3T4+ (CD4), and I-A−, but varies among the different tumor systems and wavelengths used for irradiation ( 11, 12, 17-19). Although the initial photoimmunologic studies investigated the effect of UV radiation on the immune response to UV-induced tumors, most of the subsequent studies used the model of allergic contact hypersensitivity (CHS). CHS is a special form of the delayed-type hypersensitivity response which is induced by epicutaneous application of low-molecular-weight reactive chemical compounds (e.g., dinitrofluorobenzene or oxazolone). Since these molecules bind to a host protein presumably in the skin to form the antigen, they are called haptens. Application of haptens to skin which has been exposed to rather low doses of UVB radiation (around 1 kJ/m2) failed to induce CHS, whereas application of the hapten at an unirradiated site produced a normal CHS response ( 20). The induction of this immunologic alteration by UV radiation correlated with a reduction in the number of Langerhans cells at the site of exposure and with changes in their structure ( 20). The UV-mediated alterations of Langerhans cells, which are the crucial cells in the epidermis for sensitization, were confirmed shortly thereafter by Aberer et al. ( 21). The ability of low-dose UVB to impair the induction of CHS appears to be genetically determined, since inhibition of CHS was observed only in some particular strains (e.g., C3H/HeN, C57BL/6), called UV susceptible, while other strains (C3H/HeJ, Balb/c), called UV resistant, responded with normal sensitization despite UV exposure ( 22). The phenotypic traits of UVB-resistant and -susceptible mice are polygenically inherited. The relevant loci at which polymorphic alleles reside are LPS and TNF-α ( 23). Accordingly, tumor necrosis factor (TNF)-α seems to be relevant to the inhibition of CHS by low-dose UV exposure, since UV-susceptible mice can be sensitized upon receiving neutralizing anti-TNF-α antibodies when the hapten is applied on UVB-exposed skin ( 24). In addition, the CHS response was significantly impaired when the hapten was painted on murine skin shortly after the site had received an injection of a subinflammatory dose of recombinant TNF-α ( 24). On the other hand, whether mice are UV-resistant or-susceptible may also depend on the time lag between UV exposure and sensitization ( 25) and on the concentration of the hapten ( 26). However, UV exposure not only inhibits the induction of CHS, but even induces hapten-specific tolerance, since the affected animals cannot be resensitized with the same hapten at a later time ( 20). This long-term suppression is hapten specific, since the same mice can be perfectly sensitized to another unrelated hapten, indicating that the initial low-dose UV exposure immunosuppressed the animals in a specific rather than general way. The use of higher UVB doses (around 20 kJ/m2) can also affect immune reactions initiated at distant, non-UV-exposed sites (“systemic immunosuppression”). Accordingly, CHS cannot be induced in mice which are exposed to high doses of UVB even if the hapten is applied at unirradiated sites ( 27). Furthermore, as observed with local immunosuppression, hapten-specific tolerance develops, since the mice cannot be resensitized to the very same hapten at a later time point. There were two investigations of whether UVB induces hapten-specific tolerance also in man. In one study, only 10% of the subejcts failed to develop specific CHS when subsequently reimmunized with the same hapten ( 28). Tolerance demonstrated in these human subjects was hapten specific, since they revealed pronounced CHS responses upon subsequent immunization with another, unrelated hapten ( 28). Cooper et al. reported that a higher proportion of subjects developed tolerance when the hapten was applied onto skin areas exposed to erythemogenic UV doses ( 29). Despite these differences, which may be due to different protocols, both studies show that a human subset who fail to develop CHS when the hapten is first applied to UVB-exposed skin also develop tolerance. The study by Cooper et al. ( 29) also demonstrated that the application of erythemogenic UVB doses depleted Langerhans cells (CD1a+, HLA-DR+) and induced the infiltration of CD1a− HLA-DR+ macrophages, thereby confirming previous observations. In earlier studies, Cooper et al. showed in vivo that UV-exposed human epidermis contains a population of CD1a− HLA-DR+ CD36+ infiltrating macrophages with antigen-presenting capabilities ( 30). These macrophages were observed only after UV exposure conditions which induce tolerance of contact allergens, but not after regimens unassociated with tolerance induction. Accordingly, these infiltrating macrophages were found to activate autoreactive T cells ( 31), specifically CD4+ suppressor-inducer cells ( 32), which in turn induce the maturation of suppressor T cells ( 33). These CD11b+ macrophages infiltrating the epidermis after in vivo UV exposure were found to produce interleukin (IL)-10 potently and to represent the major secretory source of epidermal IL-10 protein in the human system ( 34). This observation might be important, since IL-10 can act in an immunosuppressive way and appears to be involved in UV-induced immunosuppression (see section VI). In addition, UV-induced epidermal macrophages were found to induce in vitro CD4+ T cells, which are characterized by deficient expression of the IL-2 receptor alpha chain ( 35). Inhibition of the IL-2 receptor alpha chain apparently depends on transforming growth factor-beta. In the murine system, the same group recently demonstrated convincingly that these UV-induced macrophages might indeed play an important role in tolerance induction in vivo (see section V). As observed in the tumor model (see section I), UV-mediated suppression can be transferred by injecting T cells obtained from the lymph nodes and spleens of UV-exposed and hapten-treated animals into naive recipients. Thus, transferred suppression is hapten specific, since the recipients do not respond to the specific hapten, whereas a normal CHS response can be obtained to non-cross-reacting haptens ( 36, 37). Taken together, these findings suggest that tolerance is mediated via induction of hapten-specific T suppressor cells. Although transfer of tolerance could be observed in both the high- ( 37) and the low-dose models ( 36), different types of T cells appear to be responsible for transfer of suppression. In the systemic form of UV-induced suppression (high-dose model), transfer of tolerance is mediated by the induction of antigen-specific, CD3+, CD4+, and CD8− suppressor cells ( 38). In the local form of UV-induced immunosuppression (low-dose model), Elmets et al. ( 36) showed that treatment of cells from UV-irradiated animals with antibodies against Lyt-1 (CD5) completely abrogated their ability to transfer suppression, while treatment of cells with antibodies against Lyt-2 (CD8) inhibited suppression partially ( 36). Accordingly, Schwarz et al. recently reported that transfer of suppression was lost in the low-dose model when T cells were depleted of CD8+ cells ( 39). When UV-induced T suppressor cells are transferred into sensitized but not naive mice, the CHS response in the recipients is not affected ( 40). This indicates that T suppressor cells can affect the induction of CHS but cannot interfere with the elicitation of CHS. Although the adoptive transfer experiments in both the low- and high-dose UV models were quite convincing, implying that sensitization in combination with UV light induces T suppressor cells, attempts to purify and clone these cells were unsuccessful for many years. Hence, the term “T suppressor cells” was virtually banned and the entire concept of UV-induced tolerance brought into question. On the other hand, the transfer experiments failed to detect whether T suppressor cells originate in the donor, as usually claimed, or in the recipient. The latter possibility could be due to the transfer of cells which induce the development of cells with negative regulatory function in the recipient. Although it was recently shown that transferable suppression was mediated by a small number of donor-derived CD4+ T cells which copurify with donor hapten-bearing, antigen-presenting cells ( 41), this did not clarify whether the donor T cells were T suppressor precursors or whether they induced T suppressor cells in the recipients. By using Thy-1.1+ and Thy-1.2+ congenic mice, Shreedar et al. very recently showed that the T suppressor cells in the recipients (Thy-1.2) were derived from T cells in the draining lymph nodes of the UV-exposed donors (Thy-1.1) ( 42). In addition, these authors werethe first to clone T cells from UVB-exposed mice which were sensitized with fluorescein isothiocyanate (FITC). Cells cloned from UV-exposed mice were CD4+, CD8−, TCR-α/β+, MHC restricted, and specific for FITC. They produced IL-10, but not IL-4 or interferon-γ, whereas cells from unirradiated animals produced high amounts of interferon-γ and little IL-4 and IL-10. T cells from UV-exposed mice blocked antigen-presenting cell functions and IL-12 production. Even more importantly, injection of 5×104 cloned T cells into untreated recipients suppressed the induction of CHS against FITC. Since impairment of CHS and induction of tolerance are the consequence of the same event, i.e., low- or high-dose UV exposure followed by hapten application, it was thought for years that the same mechanisms are involved. Irradiation of skin with either low- or high-dose UVB results in depletion of epidermal Langerhans cells ( 21), which are crucial for the induction of CHS ( 43). Under normal conditions, Langerhans cells take up the hapten and migrate with it to the regional lymph nodes, where the Langerhans cells, which have developed during their journey into potent immunostimulatory dendritic cells, present the antigen to T cells ( 44). Thus, impairment of the induction of CHS upon application of the hapten onto UVB-exposed skin can be easily explained by the depletion of Langerhans cells. However, this does not apply to systemic UV-mediated immunosuppression, since even after multiple UVB exposures, Langerhans cells are not affected in skin areas which are not directly exposed to UV radiation ( 45). In this case, secretion of soluble mediators by keratinocytes appears to be important (see section VI). Since application of haptens onto UVB-exposed mice results in both the impairment of CHS and the induction of hapten-specific tolerance, it was long postulated that the same mechanisms are involved and that induction of tolerance is the consequence of the inhibition of CHS induction. However, there is growing evidence that the molecular basis of UVB-induced tolerance is different from the mechanism responsible for UVB-impaired induction of CHS. Although TNF-α induces alterations in epidermal Langerhans cells like those of UVB ( 46) and although injection of anti-TNF-α antibodies inhibits UV-mediated impairment of CHS, development of tolerance cannot be prevented by injection of neutralizing TNF-α antibodies ( 47, 48). Hammerberg et al. observed that application of a hapten immediately after a single low-dose UVB exposure resulted in inhibition of the induction of CHS but failed to induce tolerance ( 49). A state of tolerance could be achieved only if a delay of 72 h was allowed between the UV exposure and the initial sensitization. By 72 h after in vivo UV exposure, Langerhans cells were depleted, and Ia+ CD11bbright macrophages had appeared in the epidermis. The appearance of this macrophage cell type was initially observed by the same group in the human system (see section III). Intracutaneous injection of haptenated epidermal cell suspensions obtained from in vivo UV-exposed skin induced hapten-specific tolerance. Induction of tolerance was lost when CD11bbright cells were removed from the epidermal cell suspensions before injection. In addition, induction of UV-mediated tolerance was blocked when mice were treated with an antibody blocking CD11b ( 50). CD11b can serve as a receptor for the fragment of the complement component 3, iC3b. Mice with a of the were used to whether is for UV-induced mice did not develop tolerance after hapten application a skin which received a single low-dose UVB exposure ( Inhibition of partially blocked UV-induced infiltration of CD11bbright macrophages but did not UV-induced depletion of Ia+ Langerhans cells. Taken together, these findings that the ability of UV-exposed skin to induce tolerance depends on Ia+ CD11bbright cells, which UV-exposed and not on the depletion of Langerhans cells. The observation that mice which are exposed to higher doses of UV radiation cannot be sensitized even when the antigen is applied on a skin which was not exposed to UV radiation ( 37) was the first indication that this radiation can suppress the immune system in a systemic the at the site led to an response to an antigen applied at a distant, non-UV-exposed skin for quite a long since Langerhans cells, which were found to be involved in local immunosuppression ( were in non-UV-exposed skin ( 45). UV radiation has to be an of Since keratinocytes were as a potent source of ( and by of their are the for UV radiation, Schwarz et al. postulated that UV-exposed keratinocytes could be the source of an immunosuppressive soluble which the and thereby systemic immunosuppressive effects ( Accordingly, it was shown that injection of obtained from UV-exposed murine keratinocytes into naive mice before hapten application results in suppression of the induction of CHS ( the concept of UV-exposed keratinocytes as a source of immunosuppressive was by other injection of from UV-exposed murine keratinocytes suppressed the induction of delayed-type hypersensitivity to and in syngeneic and mice ( There is also evidence of in vivo of suppressor factors. of obtained from UV-exposed mice into normal animals suppressed the ability of the mice to CHS reactions ( a factor that allergic contact has also been in ( to immunosuppression, two appear to be of particular TNF-α and IL-10. of anti-TNF-α antibodies was found to the induction of UV-induced immune suppression, that TNF-α is a in this type of of the immune system ( This is also by the that injection of TNF-α the effects of UVB radiation ( However, as (see section of TNF-α does not the induction of tolerance ( 47, confirming that different are responsible for the impairment of CHS on the one hand, and for the induction of tolerance on the other IL-10 was found to be to interfere with the antigen-presenting of Langerhans cells. In an in vitro system, it was observed that of Langerhans cells with IL-10 the ability of these cells to present antigen to and even ( In to macrophages, cells, and T cells, keratinocytes can function as a source of IL-10 ( IL-10 in the skin is upon application of contact allergens, whereas or have effect ( In addition, injection of IL-10 in the skin of hapten application the induction of CHS and induces hapten-specific tolerance ( Furthermore, it was demonstrated that UV radiation IL-10 in both murine and human keratinocytes ( to its ability to and immune IL-10 may play an important role in UV-induced immunosuppression ( Accordingly, injection of an antibody into UV-irradiated mice prevented systemic UV-induced suppression of the induction of delayed-type hypersensitivity ( it was observed that cells from UV-exposed animals did not present antigen to cells, while to cells was even ( of these effects could be prevented by injection of an a crucial role of IL-10 in UV-induced systemic On the other hand, IL-10 was also found to not only the but also the elicitation of both contact and delayed-type hypersensitivity ( As a these suggest that UV radiation with the of IL-10 cells and cells. The in systemic immunosuppression is by the observation that immune suppression is blocked in mice treated with an ( However, UV light does not directly induce IL-4 This might be mediated via UV-induced of by Accordingly, blocked IL-4 production. This that UV exposure a IL-4 that results in systemic immunosuppression ( Hence, there is evidence that in vivo exposure to UV radiation induces a a immune response in and this immune reactions are impaired by UV However, IL-10 appears to be involved not only in tolerance induced by high-dose but also in tolerance induced by low-dose observed that tolerance when haptens are applied onto skin areas in which IL-10 was injection of an antibody prevented tolerance induced by low-dose UV ( In contrast, tolerance induced by injection of was only partially inhibited by antibodies ( is the of a component of the ( Since of the by prevented the particular immunosuppressive effects of UVB radiation ( it was that is involved in UV-induced has immunosuppressive since hapten application onto skin in which was results in the induction of hapten-specific tolerance which can even be transferred ( 48). Although these findings show the immunosuppressive properties of the of to UV-induced tolerance is not completely As antibodies the induction of UV-induced tolerance while induced tolerance only partially ( inhibition of UV-induced tolerance by application of antibodies against ( has not been The IL-12 an important role in the development of cells ( which are crucial cells in contact and delayed-type hypersensitivity reactions ( The role of IL-12 during sensitization was by the that injection of an antibody into naive mice before epicutaneous hapten application resulted in the to induce sensitization in these animals ( findings were obtained by et al. ( In addition, in vivo blocking of IL-12 before sensitization appears to induce when mice which were initially treated and sensitized were resensitized with the same hapten after a of sensitization was not that these animals had become to this hapten ( Tolerance was hapten specific, since the animals could be sensitized to another unrelated Since IL-12 and IL-10 in some ( it was investigated whether IL-12 could the systemic immunosuppression induced by UV of IL-12 blocked systemic suppression of contact and delayed-type hypersensitivity in animals exposed to a single exposure ( IL-12 was to the of UV-induced T suppressor cells, since adoptive transfer of cells from UV-exposed animals treated with IL-12 had effect on the CHS response of the mice, while transfer of cells from but not mice inhibited sensitization ( IL-12 is to UV-induced local immunosuppression as Schwarz et al. ( reported that a single injection of IL-12 after the last of low-dose UV before hapten application to the skin completely the CHS it was observed that IL-12 a immune response when into animals that were initially sensitized UV-exposed before This that IL-12 not only UV-induced immunosuppression upon but also UV-induced tolerance when into animals that were initially by application of haptens UV-exposed skin ( transfer of cells from UV-irradiated and immunosuppression was in the mice, that injection of IL-12 the induction of suppressor cells. The observation that the effect of IL-12 in the transfer experiments was not lost when CD4+ T cells were depleted that IL-12 does not the immune response by inducing this observation that IL-12 on CD8+ cells, which to tolerance in the low-dose UV model ( 39). Although the transfer of UV-induced tolerance by injecting T cells from UV-exposed mice into naive recipients was more than years ( 36), the mechanisms by which T suppressor cells tolerance to be There is evidence that an important role in immune reactions ( also called or is a ( which induces cell death after with its also called ( cells or with high expression of can T cells which thereby the immune response and an immune ( Tolerance can be a form of immune and Schwarz et al. were the first to whether the system is important for UV-induced tolerance ( do they mice, which and mice, which ( of both strains to low doses of UV radiation inhibited the induction of CHS after hapten application to UV-irradiated indicating that both strains are UV However, in to UV-exposed mice, UV-exposed and mice did not develop tolerance. This suggested that the system is crucial for the development of tolerance ( suppression was observed when cells from UV-exposed and hapten-treated donors were into naive mice, but also when cells from UV-exposed and hapten-treated mice were transferred into naive recipients. In contrast, both naive and naive recipients of T suppressor cells from UV-exposed and hapten-treated mice could be Thus, these findings suggested that the transfer of tolerance does not or expression on the suppressor cells but does both molecules on cells in the ( These findings were with the that T suppressor cells do not their via expression of but rather into cells in the that are during sensitization, and that this may be mediated via the When dendritic cells used as antigen-presenting cells were with T cells from mice in the of the specific hapten, of dendritic cells was This that T suppressor cells may their by inducing of antigen-presenting cells. of IL-12 to of T suppressor cells and dendritic cells significantly the number of dendritic cells ( This is a very observation as (see section IL-12 is the first the only to be to UV-mediated tolerance ( By antigen-presenting cells from cell death induced by T suppressor cells, IL-12 might be to and thereby tolerance. The role of the system in UV-induced immunosuppression was recently confirmed by another the high-dose UV et al. showed that but not expression is for the function of UV-induced suppressor cells ( These findings from those of Schwarz et al. ( a which may in local or systemic UV-induced appear to be for both types of suppression. UV-induced tolerance is one of the most and in and it is one of the most growing areas in Although the induction of tolerance by UV radiation was first more than 25 years are from the mechanisms involved. This may be due to the that different models of UV-induced tolerance as high- and dose tolerance, and systemic and local tolerance. Accordingly, in have that the UV dose the concentration of the hapten, and also the type of the hapten can ( Thus, one has to be when studies using different models of UV-induced tolerance. in years, numerous studies have major to this These the recognition that impairment of CHS and induction of tolerance are mediated by different the first attempts to UV-induced T suppressor cells, and the of the between and tolerance, to a These findings have on on other tolerance models as tolerance ( or high- ( and low-dose tolerance ( is by from the and the