Ultraviolet radiation‐induced tolerance
Authors: T. Schwarz
Journal: Allergy (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 genetic disruption of the C3 gene were used to test whether C3 activation is essential for UV-induced immunosuppression. Indeed, C3-deficient mice did not develop tolerance after hapten application through a skin area which received a single low-dose UVB exposure ( 51). Inhibition of C3 activation partially blocked UV-induced infiltration of CD11bbright macrophages but did not prevent UV-induced depletion of Ia+ CD11blow Langerhans cells. Taken together, these findings indicate that the ability of UV-exposed skin to induce tolerance critically depends on inflammatory Ia+ CD11bbright monocytic/macrophagic cells, which infiltrate UV-exposed skin, 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 area which was not exposed to UV radiation ( 27, 37) was the first indication that this radiation can suppress the immune system in a systemic fashion. How the events occurring at the irradiated site led to an abnormal response to an antigen applied at a distant, non-UV-exposed skin area remained unclear for quite a long time, since Langerhans cells, which were found to be critically involved in local immunosuppression ( 20), were unaffected in non-UV-exposed skin ( 45). UV radiation has turned out to be an effective inducer of cytokine release. Since keratinocytes were identified as a potent source of cytokines ( 52), and by virtue of their anatomic location are the natural target for UV radiation, Schwarz et al. postulated that UV-exposed keratinocytes could be the source of an immunosuppressive soluble mediator which enters the circulation and thereby mediates systemic immunosuppressive effects ( 53). Accordingly, it was shown that intravenous injection of supernatants obtained from UV-exposed murine keratinocytes into naive mice 5 days before hapten application results in suppression of the induction of CHS ( 53). Meanwhile, the concept of UV-exposed keratinocytes as a source of immunosuppressive factors was supported by various other studies. For example, injection of supernatants from UV-exposed murine keratinocytes suppressed the induction of delayed-type hypersensitivity to alloantigen and trinitrophenyl-modified self-antigens in syngeneic and allogeneic mice ( 54). There is also evidence of in vivo release of suppressor factors. Injection of plasma obtained from UV-exposed mice into normal animals suppressed the ability of the recipient mice to generate CHS reactions ( 55). Moreover, a UV-inducible serum factor that suppresses allergic contact dermatitis has also been described in guinea pigs ( 56). With regard to immunosuppression, two cytokines appear to be of particular interest: TNF-α and IL-10. Injection of anti-TNF-α antibodies was found to block the induction of UV-induced immune suppression, suggesting that TNF-α is a critical mediator in this type of compromise of the immune system ( 57). This is also supported by the finding that injection of TNF-α mimics the effects of UVB radiation ( 46). However, as already mentioned above (see section V), neutralization of TNF-α does not prevent the induction of tolerance ( 47, 48), confirming that different pathways are responsible for the impairment of CHS on the one hand, and for the induction of tolerance on the other hand. IL-10 was found to be able to interfere with the antigen-presenting capacity of Langerhans cells. In an in vitro system, it was observed that incubation of Langerhans cells with IL-10 abrogates the ability of these cells to present antigen to Th1 clones and even tolerizes them ( 58). In addition to macrophages, B cells, and T cells, keratinocytes can function as a source of IL-10 ( 59, 60). IL-10 production in the skin is enhanced upon application of contact allergens, whereas tolerogens or irritants have no effect ( 61). In addition, injection of IL-10 in the skin area of hapten application prevents the induction of CHS and induces hapten-specific tolerance ( 62). Furthermore, it was demonstrated that UV radiation upregulates IL-10 in both murine and human keratinocytes ( 60). Due to its ability to downregulate inflammatory and immune reactions, keratinocyte-derived IL-10 may play an important role in UV-induced immunosuppression ( 60). Accordingly, injection of an anti-IL-10 antibody into UV-irradiated mice prevented systemic UV-induced suppression of the induction of delayed-type hypersensitivity ( 60). Moreover, it was observed that spleen cells from UV-exposed animals did not present antigen to Th1 cells, while presentation to Th2 cells was even enhanced ( 63). Both of these effects could be prevented by injection of an anti-IL-10 antibody, suggesting a crucial role of IL-10 in UV-induced systemic immunosuppression. On the other hand, IL-10 was also found to influence not only the induction, but also the elicitation phase of both contact and delayed-type hypersensitivity ( 64). As a whole, these data suggest that UV radiation with the help of IL-10 tolerizes Th1 cells and activates Th2 cells. The Th2 shift in systemic immunosuppression is further supported by the observation that immune suppression is blocked in mice treated with an anti-IL-4 antiserum ( 65). However, UV light does not directly induce IL-4 release. This might be mediated indirectly via UV-induced release of prostaglandin E2 by keratinocytes. Accordingly, cylcooxygenase-2 inhibitors blocked IL-4 production. This suggests that UV exposure activates a cytokine cascade (prostaglandin E2→ IL-4 → IL-10) that finally results in systemic immunosuppression ( 65). Hence, there is extensive evidence that in vivo exposure to UV radiation induces a shift toward a Th2 immune response in vivo, and this explains why mostly Th1-mediated cellular immune reactions are impaired by UV radiation. However, IL-10 appears to be involved not only in mediating tolerance induced by high-dose UV, but also in tolerance induced by low-dose UV. Niizeki & Streilein observed that tolerance develops when haptens are applied onto skin areas in which IL-10 was injected intraperitoneal injection of an anti-IL-10 antibody prevented tolerance induced by low-dose UV ( 66). In contrast, tolerance induced by intracutaneous injection of cis-urocanic acid (UCA) was only partially inhibited by anti-IL-10 antibodies ( 66). cis-UCA is the photoisomer of trans-UCA, a natural component of the stratum corneum ( 67). Since removal of the stratum corneum by tape stripping prevented the particular immunosuppressive effects of UVB radiation ( 68), it was supposed that cis-UCA is involved in UV-induced immunosuppression. Indeed, cis-UCA has immunosuppressive properties, since hapten application onto skin in which cis-UCA was injected results in the induction of hapten-specific tolerance which can even be adoptively transferred ( 48). Although these findings clearly show the immunosuppressive properties of cis-UCA, the relative contribution of cis-UCA to UV-induced tolerance is not yet completely clarified. As already mentioned, anti-IL-10 antibodies block the induction of UV-induced tolerance completely, while cis-UCA induced tolerance only partially ( 66). Moreover, complete inhibition of UV-induced tolerance by application of antibodies against cis-UCA ( 69) has not yet been demonstrated. The cytokine IL-12 plays an important role in the development of Th1 cells ( 70), which are crucial effector cells in contact and delayed-type hypersensitivity reactions ( 71). The critical functional role of IL-12 during cutaneous sensitization was proven by the finding that intraperitoneal injection of an anti-IL-12 antibody into naive mice before epicutaneous hapten application resulted in the failure to induce sensitization in these animals ( 72). Similar findings were obtained by Müller et al. ( 73). In addition, in vivo blocking of IL-12 before sensitization appears to induce tolerance; when mice which were initially anti-IL-12 treated and sensitized were resensitized with the same hapten after a resting period of 14 days, sensitization again was not inducible, suggesting that these animals had become tolerant to this hapten ( 72). Tolerance was hapten specific, since the animals could be successfully sensitized to another unrelated hapten. Since IL-12 and IL-10 counteract in some sense ( 74, 75), it was investigated whether IL-12 could overcome the systemic immunosuppression induced by UV radiation. Administration of IL-12 blocked systemic suppression of contact and delayed-type hypersensitivity in animals exposed to a single high-doseUV exposure ( 76). IL-12 was supposed to prevent the generation of UV-induced T suppressor cells, since adoptive transfer of spleen cells from UV-exposed animals treated with IL-12 had no effect on the CHS response of the recipient mice, while transfer of cells from UV-irradiated, but not IL-12-treated, mice inhibited sensitization ( 76). IL-12 is able to overcome UV-induced local immunosuppression as well. Schwarz et al. ( 77) reported that a single intraperitoneal injection of IL-12 given after the last of four low-dose UV exposures before hapten application to the irradiated skin area completely restored the CHS response. Moreover, it was observed that IL-12 enables a full immune response when injected into animals that were initially sensitized through UV-exposed skin, before resensitization. This clearly demonstrates that IL-12 not only prevents UV-induced immunosuppression upon early injection, but also destroys UV-induced tolerance when injected into animals that were initially tolerized by application of haptens through UV-exposed skin ( 77). After transfer of lymphoid cells from UV-irradiated and IL-12-treated donors, no immunosuppression was detectable in the recipient mice, suggesting that injection of IL-12 abrogates the induction of suppressor cells. The observation that the reconstitutive effect of IL-12 in the transfer experiments was not lost when CD4+ T cells were depleted implies that IL-12 does not restore the immune response by inducing Th1 cells; instead, this observation suggests that IL-12 acts on CD8+ cells, which seem to mediate 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 described more than 15 years ago ( 36), the mechanisms by which T suppressor cells mediate tolerance still remains to be determined. There is increasing evidence that apoptosis plays an important role in immune reactions ( 78). Fas, also called CD95 or APO-1, is a surface molecule ( 79) which induces apoptotic cell death after interaction with its natural ligand FasL, also called CD95L ( 80). Consequently, cells or organs with high expression of FasL can kill attacking T cells which bear Fas, thereby escaping the immune response and obtaining an immune privileged status ( 81, 82). Tolerance can be considered a form of immune privilege, and Schwarz et al. were the first to investigate whether the Fas/FasL system is important for UV-induced tolerance ( 83). To do so, they utilized lpr mice, which lack Fas, and gld mice, which lack FasL ( 84, 85). Exposure of both strains to low doses of UV radiation inhibited the induction of CHS after hapten application to UV-irradiated skin, indicating that both strains are UV susceptible. However, in contrast to UV-exposed wild-type mice, UV-exposed lpr and gld mice did not develop tolerance. This clearly suggested that the Fas/FasL system is crucial for the development of tolerance ( 83). Transferred suppression was observed when cells from UV-exposed and hapten-treated lpr donors were injected into naive wild-type mice, but also when cells from UV-exposed and hapten-treated gld mice were transferred into naive wild-type recipients. In contrast, both naive lpr and naive gld recipients of T suppressor cells from UV-exposed and hapten-treated wild-type mice could be normally sensitized. Thus, these findings suggested that the transfer of tolerance does not require Fas or FasL expression on the suppressor cells but does require both molecules on cells in the recipient ( 83). These findings were compatible with the hypothesis that T suppressor cells do not kill their targets via expression of FasL themselves, but rather drive into apoptosis cells in the recipient that are essential during sensitization, and that this process may be mediated via the Fas/FasL system. When bone-marrow-derived dendritic cells used as antigen-presenting cells were coincubated with T cells from UV-tolerized mice in the presence of the specific hapten, enhanced apoptosis of dendritic cells was observed. This implied that T suppressor cells may mediate their inhibitory property by inducing apoptosis of antigen-presenting cells. Addition of IL-12 to cocultures of T suppressor cells and dendritic cells significantly reduced the number of dead dendritic cells ( 83). This is a very interesting observation since, as mentioned above (see section VII), IL-12 is the first and, so far, the only cytokine known to be able to destroy established UV-mediated tolerance ( 77). By rescuing antigen-presenting cells from apoptotic cell death induced by T suppressor cells, IL-12 might be able to enable resensitization and thereby destroy established tolerance. The critical role of the Fas/Fas-ligand system in UV-induced immunosuppression was recently confirmed by another study. Using the high-dose UV model, Hill et al. showed that host-derived, but not recipient-derived, FasL expression is essential for the generation and/or function of UV-induced suppressor cells ( 86). These findings differ from those of Schwarz et al. ( 83), a fact which may reflect differences in studying local (low-dose) or systemic (high-dose) UV-induced immunosuppression. Nevertheless, Fas/FasL interactions appear to be essential for both types of suppression. UV-induced tolerance is one of the most fascinating and puzzling phenomena in photoimmunology, and thus it is one of the most rapidly growing research areas in photobiology. Although the induction of tolerance by UV radiation was first described more than 25 years ago, we are still far from understanding all the mechanisms involved. This may be due to the fact that different models of UV-induced tolerance exist, such as high- and low-UV dose tolerance, and systemic and local tolerance. Accordingly, in recent decades, we have learned that the UV dose applied, the concentration of the hapten, and probably also the type of the hapten can cause differences ( 87). Thus, one has to be careful when comparing studies using different models of UV-induced tolerance. Nevertheless, in recent years, numerous studies have made major contributions to this field. These include the recognition that impairment of CHS and induction of tolerance are mediated by different mechanisms, the first successful attempts to isolate UV-induced T suppressor cells, and the identification of the link between apoptosis and tolerance, to mention just a few. These findings will certainly have tremendous influence on research on other tolerance models such as oral tolerance ( 88) or high- ( 89) and low-dose tolerance ( 90). T.S. is supported by grants from the German Research Foundation (DFG, Schw 625/1-3) and the European Community (ENV4-CT97-0556).