Advertisement

Topical Imiquimod Treatment Prevents UV-Light Induced Loss of Contact Hypersensitivity and Immune Tolerance

      Imiquimod (1-(2-methylpropyl)-1H-imidazo[4,5-c]quinolin-4-amine) is a TLR7 agonist that induces cytokine production in TLR7 bearing antigen-presenting cells (APCs), including IL-12, a cytokine that has been demonstrated to be a critical effector molecule for contact hypersensitivity (CHS). To test our hypothesis that topical applications of imiquimod may protect the skin immune system against the deleterious effects of UV light exposures, we treated animals with this agent, or its vehicle or nothing before UV exposures. Although topical imiquimod exposures before UV light did not prevent the depletion of epidermal Langerhans cells, it did prevent the loss of CHS. IL-12 was important in the protective role of imiquimod in preventing UV-induced loss of CHS, as systemic treatment of mice with an anti-IL-12 p70 monoclonal antibody blocked the protective effects of imiquimod. Additionally, only imiquimod-treated mice were resistant to hapten-specific tolerance induction after UV irradiation at the site of the initial sensitization with the hapten 2,4 dinitro-1-fluorobenzene. To model for the effects of TLR7 activation on the UV effect on antigen-APCs, XS52 cell line was used to study this interaction in an in vitro model system. This cell line expressed mRNA for TLR7, downregulated IκB, phosphorylated c-Jun N-terminal kinase, and secreted cytokines after exposure to imiquimod or lipopolysaccharide. Activation of the TLR7 signaling pathway on XS52 before UV-light exposures enhanced IL-12p70 secretion by this cell line. Similarly, activation of TLR7 on XS52 before UV-light exposure also prevented the UV-induced loss of IFN-γ triggering in T cells during an allogeneic mixed lymphocyte reaction. Imiquimod-treated, UV-irradiated XS52 triggered a more vigorous IFN-γ production than did either imiquimod-treated XS52 or UV-irradiated XS52, again suggesting a synergy between the two treatments. Lastly, enriched lymph node CD11c+ APCs from mice treated with UV irradiation, imiquimod alone or the combination of UV irradiation and imiquimod indicated the same in vivo synergy between imiquimod irradiation and UV irradiation in enhancing IL-12p70 production. These data suggest that topical imiquimod applications may play a role in preventing UV-induced impairment of the skin immune system, which is thought to be one of the critical events that allow the development of UV-induced skin cancers.
      APC
      antigen-presenting cell
      CHS
      contact hypersensitivity
      DNFB
      2,4 dinitro-1-fluorobenzene
      DNP
      dinitrophenyl
      Imiquimod
      1-(2-methylpropyl)-1H-imidazo[4,5-c]quinolin-4-amine
      JNK
      c-Jun N-terminal kinase
      LC
      Langerhans cell
      LPS
      lipopolysaccharide
      MLR
      mixed lymphocyte reaction

      Introduction

      UV light has a variety of acute and chronic deleterious effects on mammalian skin, which are thought to be important in the development of skin cancer. One of the acute effects of UV-light exposure is its adverse effects on contact hypersensitivity (CHS), which is dependent on interactions between epidermal Langerhans cells (LCs) and skin-associated lymphoid tissue. UV light is known to physically deplete LCs from the epidermis (
      • Toews G.B.
      • Bergstresser P.R.
      • Streilein J.W.
      Epidermal Langerhans cell density determines whether contact hypersensitivity or unresponsiveness follows skin painting with DNFB.
      ;
      • Aberer W.
      • Schuler G.
      • Stingl G.
      • Honigsmann H.
      • Wolff K.
      Ultraviolet light depletes surface markers of Langerhans cells.
      ), which is thought to be an important mechanism of UV suppression of CHS. Those LCs that survive UV exposure are impaired in the ability to mature, and hence to present haptens to Th1-lymphocytes. The result of this impaired maturation and ability to activate Th1-lymphocytes is clonal anergy in T-lymphocytes that would normally develop into hapten-specific Th- and Tc-lymphocytes that would normally be effector cells for CHS (
      • Cruz Jr, P.D.
      • Nixon-Fulton J.L.
      • Tigelaar R.E.
      • Bergstresser P.R.
      Disparate effects of in vitro low-dose irradiation on intravenous immunization with purified epidermal cell subpopulations for the induction of contact hypersensitivity.
      ,
      • Cruz Jr, P.D.
      • Tigelaar R.E.
      • Bergstresser P.R.
      Langerhans cells that migrate to skin after IV infusion regulate the induction of contact hypersensitivity.
      ;
      • Simon J.C.
      • Tigelaar R.E.
      • Bergstresser P.R.
      • Edelbaum D.
      • Cruz Jr, P.D.
      UVB radiation converts LC from immunogenic to tolerogenic antigen presenting cells. Induction of specific clonal anergy in CD4+ T helper 1 cells.
      ). In addition to the loss of responsiveness to hapten sensitization through UV-irradiated skin, specific immune tolerance develops when UV-irradiated hosts are re-challenged through non-irradiated skin (
      • Elmets C.A.
      • Bergstresser P.R.
      • Tigelaar R.E.
      • Wood P.J.
      • Streilein J.W.
      Analysis of the mechanism of unresponsiveness produced by haptens painted on skin exposed to low dose ultraviolet-radiation.
      ). This non-responsiveness is adoptively transferable, and is a result of the development of CD4+CD25+ T-regulatory cells, which downregulate effector T-cell responses either by cell–cell contact or the elaboration of immunosuppressive cytokines (
      • Schwarz A.
      • Maeda A.
      • Wild M.K.
      • Kernebeck K.
      • Gross N.
      • Aragane Y.
      • et al.
      UV-induced regulatory T-cells do not only inhibit the induction but can also suppress the effector phase of contact hypersensitivity.
      ). Most of these data are the result of studies in animal model systems. However, these findings are relevant to UV immunosuppression in humans. It has been demonstrated that humans also develop hapten-specific tolerance when sensitized with experimental haptens through UV-irradiated skin (
      • Cooper K.D.
      • Oberhelman L.
      • Hamilton T.A.
      • Baadsgaard O.
      • Terhune M.
      • LeVee G.
      • et al.
      UV exposure reduces immunization rates and promotes tolerance to epicutaneous antigens in humans: relationship to dose, CD1aDR+ epidermal macrophage induction, and Langerhans cell depletion.
      ).
      Imiquimod (1-(2-methylpropyl)-1H-imidazo[4,5-c]quinolin-4-amine) is an immune response modifier that is a TLR7 agonist (
      • Gorden K.B.
      • Gorski K.S.
      • Gibson S.J.
      • Kedl R.M.
      • Kieper W.C.
      • Qiu X.
      • et al.
      Synthetic TLR agonists reveal functional differences between human TLR7 and TLR8.
      ). Activation of TLR7 results in activation of the critical transcription factor, NF-κB, and cytokine production by TLR7 bearing antigen-presenting cell (APC). Most notably, the genes encoding the cytokines IL-12, IFN-α and tumor necrosis factor-α, among others, are rapidly induced after APC are exposed to imiquimod. These cytokines play a critical role in the inflammatory process, and also shape adaptive immune responses, favoring Th1-lymphocyte development when imiquimod-activated APC present antigens to T-lymphocytes in vitro or in vivo. Because of these properties, imiquimod has anti-viral and antitumor activities in a variety of animal model systems. These preclinical properties have translated well in the clinical use of imiquimod, which is utilized to treat external genital warts, actinic keratosis, and basal cell carcinoma (
      • Villa A.M.
      • Berman B.
      Immunomodulators for skin cancer.
      ;
      • Vender R.B.
      • Goldberg O.
      Innovative uses of imiquimod.
      ).
      Among the families of APCs that are targeted by imiquimod are dendritic cells, including epidermal LCs. In vivo, with topical application to mouse skin, imiquimod induces migration of LCs from the epidermis to the local lymph node. As well, imiquimod enhances CHS responses (
      • Suzuki H.
      • Wang B.
      • Shivji G.M.
      • Toto P.
      • Amerio P.
      • Tomai M.
      • et al.
      Imiquimod, a topical immune response modifier, induces migration of Langerhans cells.
      ). It has been demonstrated that imiquimod and its hydroxylated derivative, R848, also induce maturation of human epidermal LCs, enhanced IL-12 production, and significantly increase IFN-γ production by CD4+ T-lymphocytes during antigen presentation by imiquimod-treated LCs compared to control LCs (
      • Burns Jr, R.P.
      • Ferbel B.
      • Tomai M.
      • Miller R.
      • Gaspari A.A.
      The imidazoquinolines, imiquimod and R-848, induce functional, but not phenotypic, maturation of human epidermal Langerhans' cells.
      ). It is thought that these effects of imiquimod on epidermal LCs are an important mechanism of action by which imiquimod induces Th1-dominant cellular immune responses in situ during the treatment of external genital warts and basal cell carcinoma (
      • Kaisho T.
      • Akira S.
      Regulation of dendritic cell function through Toll-like receptors.
      ).
      Previously, it has been reported that IL-12 protects mice from UV-induced loss of CHS responses (
      • Schmitt D.A.
      • Owen-Schaub L.
      • Ullrich S.E.
      Effect of IL-12 on immune suppression and suppressor cell induction by ultraviolet radiation.
      ;
      • Schwarz A.
      • Grabbe S.
      • Aragane Y.
      • Sandkuhl K.
      • Riemann H.
      • Luger T.A.
      • et al.
      Interleukin-12 prevents UVB-induced local immunosuppression and overcomes UVB-induced tolerance.
      ;
      • Schwarz A.
      • Grabbe S.
      • Mahnke K.
      • Riemann H.
      • Luger T.A.
      • Wysocka M.
      • et al.
      Interleukin-12 breaks UV light induced immunosuppression by affecting CD8+ rather than CD4+ T-cells.
      ). Because of the adjuvant effects that imiquimod induces after topical application to rodent or human skin, and because of its direct effects on epidermal LCs, including the production of IL-12, we hypothesized that imiquimod, when applied topically to mouse skin, may exhibit protective effects against the acute effects of UV. Herein, we report that topical applications of imiquimod to mouse skin before UVB exposures prevent the loss of CHS, hapten-specific tolerance, and loss of the hapten-specific antibody response. In vitro studies of XS52, a LC-like cell line indicated that imiquimod initiates UV-resistant maturation, and that imiquimod and UVB synergize to enhance IL-12 production by XS52, and migratory LCs in vivo, preserving CHS. These data suggest that topical imiquimod treatment may play a role in preventing UV-induced immunosuppression. Thus, it is possible that this agent may play a role in preventing skin cancer development as well as treating fully developed skin cancers.

      Results

      To test our hypothesis that local application of the TLR7 agonist, imiquimod can prevent the induction of UV-light suppression of CHS, mice were first treated with imiquimod, vehicle, or no treatments before UV irradiation, then sensitized with 0.5% 2,4 dinitro-1-fluorobenzene (DNFB) using standard methods (
      • Gaspari A.A.
      • Katz S.I.
      ;
      • Glass M.J.
      • Bergstresser P.R.
      • Tigelaar R.E.
      • Streilein J.W.
      UVB radiation and DNFB skin painting induce suppressor cells universally in mice.
      ). Consistent with previous reports, mice UV-irradiated before DNFB sensitization exhibited impaired ear swelling after challenge with DNFB when compared to non-irradiated control mice (Figure 1a). Similarly, vehicle treated, UV-irradiated mice also exhibited impaired CHS. Mice that were pretreated with imiquimod 5% cream before UV irradiation and subsequent DNFB sensitization exhibited ear swelling responses similar to that of non-irradiated, control mice (Figure 1a). Histologic sections of the ear pinna of mice taken 24 hours after hapten challenge confirmed that imiquimod-treated UV-irradiated, DNFB-sensitized mice exhibited the same robust histologic changes as DNFB-sensitized mice, which was comprised of a dense mononuclear infiltrate, vasodilation, dermal edema, and spongiosis of the epidermis. In contrast, vehicle-treated UV-irradiated, DNFB-sensitized mice, or UV-irradiated, DNFB-sensitized mice exhibited histologic changes that were similar to naïve mice after DNFB challenge on the ear pinna (Figure 1b). Topical imiquimod, when applied 24 hours before sensitization, has been demonstrated to enhance CHS (
      • Suzuki H.
      • Wang B.
      • Shivji G.M.
      • Toto P.
      • Amerio P.
      • Tomai M.
      • et al.
      Imiquimod, a topical immune response modifier, induces migration of Langerhans cells.
      ). We asked the question if our regimen of two consecutive doses of imiquimod, followed by a 4-day untreated interval before sensitization, would have similar effects. As depicted (Figure 1c), this treatment slightly enhanced CHS, but this enhancement was not statistically significant compared to untreated or vehicle-treated mice.
      Figure thumbnail gr1
      Figure 1Topical imiquimod, when applied to mouse skin before UV irradiation, prevents loss of CHS. (a) Groups of mice (N=5/group) were treated with topical imiquimod cream, its vehicle on abdominal skin for 2 days or nothing before four consecutive doses of UV light (see ). They were subsequently sensitized at the UV irradiation site, and challenged on the ear pinna 4 days later; 24 hours later, ear swelling was measured (*P<0.05 compared to non-irradiated, DNFB-sensitized mice). Data represent mean±SD of ear swelling responses from a single representative experiment that was repeated three times. (b) Histologic sections were taken from representative mice at 24 hours after hapten challenge, stained with hematoxylin and eosin, and photomicrographs were taken (original magnification × 20 for all sections). (c) Two doses of topical imiquimod, followed by a 4-day untreated interval before sensitization, does not significantly augment CHS. (d) Administration of anti-IL-12 monoclonal antibody blocks the protective effects of imiquimod on UV immunosuppression (*P<0.05 compared to the non-sensitized control mice or the imiquimod-treated UV-irradiated mice).
      We tested the role of IL-12 in preventing the impairment of CHS induced by UV-light exposure. Groups of experimental mice were treated with an anti-IL-12 monoclonal antibody or a control monoclonal IgG during imiquimod treatments before UV-light exposure, and subsequent DNFB sensitization. Anti-IL-12, but not control IgG blocked the ability of imiquimod to prevent the loss of DNFB sensitization induced by UV-light exposure (Figure 1d). Because topical imiquimod prevented the loss of responsiveness to hapten sensitization after UV exposures, we studied epidermal LCs density using indirect immunofluorescence by staining epidermal sheets with a monoclonal antibody specific for class II major histocompatibility antigens, a well-characterized LCs marker. LCs were counted using digital image analysis. Epidermal LC densities were studied after no treatments, two treatments with topical vehicle or imiquimod in the absence or presence of UV-light exposures. LC densities were studied at the time that hapten sensitization would occur. In the absence of UV light, neither vehicle nor imiquimod topical treatments affected LCs density (Figure 2a and b). In the presence of UV light, neither vehicle nor imiquimod protected LCs from being depleted. Thus, imiquimod does not protect LCs depletion after exposure to UV light under these conditions.
      Figure thumbnail gr2
      Figure 2Topical imiquimod treatments does not protect epidermal LC from UV-induced depletion. (a) Mice (N=5/group) were treated as described in , but were not sensitized with DNFB. On day 0 (time of sensitization), the abdominal skin was taken, and stained with anti-class II MHC monoclonal antibodies and a fluoresceinated secondary antibody. (b) Digital image analysis of epidermal LC frequencies (LC numbers/high powered field at original magnification × 400) in the epidermis of mice (*P<0.05 compared to normal control mice). Data represent mean±SD of LC counts, from a single representative experiment, that was repeated three times.
      Untreated and vehicle treated, UV-irradiated mice were tolerant to DNFB sensitization (Figure 3a). In contrast, non-UV irradiated mice and imiquimod treated, UV-irradiated mice were responsive to a second DNFB sensitization. These data indicate topical imiquimod treatments before UV irradiation prevent the induction of immunological tolerance to hapten sensitization.
      Figure thumbnail gr3
      Figure 3Topical imiquimod treatments prevents UV-light-induced tolerance to hapten sensitization. (a) Mice (N=5/group) were treated as in . Instead of elicitation on the ear, the groups of mice were re-sensitized with 20 ml of DNFB 0.5% on shaved, non-irradiated back skin. Four days later, the animals were challenged on the pinna of the ear with 0.2% DNFB, and ear swelling was measured 1 day later (*P<0.05 compared to the UV-irradiated, or vehicle-treated, UV-irradiated mice). Baseline (before the experiment began), and 3 weeks after the first sensitization, blood was collected from the groups of mice by tail vein bleeding. Hapten-specific (b) IgG2a and (c) IgG1 were assayed using a specific ELISA (see Materials and Methods) (*P<0.05 compared to naïve sera, or sera from UV-irradiated, or vehicle-treated, UV-irradiated mice). Data in ELISA represent the mean±SEM for the OD of sera from each of the five animals in each group. The data are from a single representative experiment that was repeated three times.
      Because anti-hapten antibody response after topical sensitization is dependent upon T-lymphocyte-derived help in the form of cytokines to drive isotype switching and the maturation of the humoral immune response (
      • Thomson J.A.
      • Troutt A.B.
      • Kelso A.
      Contact sensitization to oxazolone: involvement of both interferon-gamma and IL-4 in oxazolone-specific Ig and T-cell responses.
      ), we reasoned that assay of this aspect of the immune response is a robust, downstream measure of epidermal antigen presentation by skin-derived LCs. Therefore, 21 days after the initial hapten sensitization, sera were obtained from all groups of mice to study for the presence of dinitrophenyl (DNP)-specific IgG1 and IgG2a using a specific ELISA. Whereas naïve mice and UV-irradiated, hapten-sensitized (tolerant) and vehicle-treated, UV-irradiated (tolerant) mice did not produce detectable DNP-specific IgG2a in their sera, non-irradiated, DNFB-sensitized and imiquimod-treated, UV-irradiated, hapten-sensitized mice produced high levels of DNP-specific IgG2a (Figure 3b). The dilution curves for these two latter groups were not significantly different, indicating the imiquimod treatment maintained full immunologic responsiveness for antibody production, even in the presence of UV light. In the absence of UVB exposures, sera from mice treated with imiquimod before sensitization with DNFB were not significantly greater than mice sensitized in the absence of imiquimod (data not shown).
      Similarly, for the IgG1-DNP-specific antibody response, UV-irradiated, hapten-sensitized mice produced a low level of antibody that was similar to that of vehicle-treated, UV-irradiated, hapten-sensitized mice. In contrast, non-irradiated, hapten-sensitized mice produced significantly greater levels of hapten-specific antibody, which was identical to that of imiquimod-treated, UV-irradiated, hapten-sensitized mice (Figure 3c). These data are consistent with the findings for the IgG2a hapten-specific response (Figure 3b).
      In order to define the mechanisms of the preservation of hapten sensitization and the prevention of tolerance induction by topical applications of imiquimod before UV exposure and subsequent hapten sensitization, an in vitro model system was used to study the interactions of TLR7 stimulation and UV on epidermal LCs. XS52 is an LC-like cell line that has been adapted to tissue culture, is growth factor dependent, and can be maintained in an immature state, but has the ability to mature after exposure to a variety of stimuli (
      • Kitajima T.
      • Ariizumi K.
      • Mohamadazadeh M.
      • Edelbaum D.
      • Bergstresser P.R.
      • Takashima A.
      T cell-dependent secretion of IL-1 beta by a dendritic cell line (XS52) derived from murine epidermis.
      ). To determine whether XS52 can be utilized as a model for TLR7 effects, it was necessary to demonstrate that this cell line expressed transcripts for TLR7. As demonstrated in Figure 4a, two sets of nested primers to amplify the 5′ or the 3′ ends of the cDNA encoding either TLR7 or TLR8 (labeled as TLR7 #1 or TLR8 #1, or TLR7 #2 or TLR8 #2, respectively) were utilized to identify TLR7 or TLR8 transcripts, respectively. XS52 (labeled XS52+RT) expressed transcripts for TLR7 and TLR8. As a control, RNA in the absence of reverse transcription was subjected to PCR amplification (labeled XS-RT), and specific PCR amplification products were not identified (Figure 4a). To confirm that XS52 responded to TLR7 activation by initiating the characterized biochemical MyD88 signaling pathways (
      • Zhang G.
      • Ghosh S.
      Molecular mechanisms of NF-kappaB activation induced by bacterial lipopolysaccharide through Toll-like receptors.
      ), steady-state cytoplasmic IκB was studied (using Western blotting). Within 15 minutes after exposure to 5 μg/ml of imiquimod or 50 ng/ml of lipopolysaccharide (LPS), IκB rapidly decreased from cytoplasmic lysates studied by Western blotting when treated with imiquimod or LPS (Figure 4b). c-Jun N-terminal kinase (JNK) is known to be rapidly phosphorylated after TLR stimulation by the MyD88-dependent pathways (
      • Ozato K.
      • Tsujimura H.
      • Tamura T.
      Toll-like receptor signaling and regulation of cytokine gene expression in the immune system.
      ). As expected, XS52 rapidly phosphorylated JNK after exposure to imiquimod or LPS (Figure 4c). Finally, XS52 also secreted both IL-12p70 and tumor necrosis factor-α 24 hours after exposure to imiquimod 5 μg/ml or the TLR4 agonist, LPS, 50 ng/ml, which is a late event that occurs after activation of TLR7 or TLR4 (Figure 4d).
      Figure thumbnail gr4
      Figure 4XS52, an LC-like mouse cell line, expresses TLR7 and TLR8 transcripts, activates biochemical signaling pathways, and secretes cytokine after exposure to imiquimod in vitro. (a) XS52 cells express transcripts for TLR7 and TLR8 (+ indicates reverse transcription; - indicates absence of reverse transcription) (b) Nuclear-free lysates from XS52 stimulated with medium alone, or imiquimod or LPS downregulate IκB as demonstrated by Western blotting. (c) XS52 stimulated with LPS (+L) or imiquimod (+I) rapidly phosphorylate JNK as demonstrated by Western blotting. (d) Imiquimod or LPS rapidly induce tumor necrosis factor-α and IL-12p70 secretion by XS52 over a 24-hour period. Data represent mean±SD for data depicted in the ELISA (*P<0.05 compared to control (unstimulated) XS52; **P<0.001 compared to control (unstimulated) XS52; analysis of variance).
      Having demonstrated that XS52 expresses TLR7 transcripts, activates the appropriate biochemical signaling pathways after exposure to imiquimod, leading to cytokine secretion, the interactions of TLR7 and UV-light exposures were then examined. XS52 was exposed to 5 μg/ml imiquimod, 5 or 25 mJ/cm2 of UVB or 5 μg/ml imiquimod for 4 hours, followed by 5 or 25 mJ/cm2 of UVB. XS52 produce low levels of IL-12p70 (specific ELISA) in the absence of imiquimod (Figure 5a). Similarly, UV exposures did not result in any increase in IL-12p70 above that of control XS52. As expected, 5 μg/ml of imiquimod increased IL-12p70 secretion. When XS52 was stimulated with imiquimod for 4 hours, followed by UV exposure, low doses of UVB did not interfere with XS52 IL-12p70 secretion. At the higher dose of UVB, there was a significant augmentation of IL-12p70 secretion compared to XS52 stimulated with imiquimod alone.
      Figure thumbnail gr5
      Figure 5Imiquimod treatment of XS52 synergizes with UV light in the enhancement of IL-12p70 secretion and triggering of IFN-γ secretion by T-cells. (a) XS52 was incubated with medium alone or with imiquimod for 4 hours before UV irradiation. Twenty hours later, supernatants were collected and assayed for IL-12p70 content (*P<0.05 compared to medium alone or UV-irradiated XS52). (b) Imiquimod-treated, UV-irradiated XS52 and control XS52 (H-2d) were co-cultured with T-lymphocytes from C57BL/6 mice (H-2b); after 72 hours, supernatants were collected, and IFN-γ content was assayed using a specific ELISA (*P<0.05 compared to non-irradiated XS52; **P<0.001 compared to XS52 treated with imiquimod alone).
      A functional assay was used to study the interaction of TLR7 activation with UVB exposures. In this experiment, XS52 was exposed to 5 or 10 μg/ml of imiquimod, for 24 hours, or exposed to these doses of imiquimod for 4 hours, washed thoroughly to remove this agent, and then irradiated with 10 mJ/cm2 of UVB. Then, the treated XS52 were studied for their ability to trigger IFN-γ secretion in an allogeneic mixed lymphocyte reaction (H-2b CD4+ T-lymphocytes) (
      • Steinman R.M.
      • Inaba K.
      Stimulation of the primary mixed leukocyte reaction.
      -30). T-lymphocytes cultured in medium alone produced low levels of IFN-γ, whereas XS52 induced robust IFN-γ secretion into culture supernatants (Figure 5b). XS52 irradiated with UVB (10 mJ/cm2) triggered an impaired IFN-γ secretion by allogeneic T-lymphocytes. Neither 5 nor 10 μg/ml of imiquimod-treated XS52 augmented IFN-γ secretion by allogeneic T-lymphocytes. When XS52 was treated with 5 or 10 μg/ml of imiquimod, followed by 10 mJ/cm2 of UVB, there was an augmented IFN-γ response, which was significantly greater than that triggered by UVB-treated XS52 or imiquimod-treated XS52.
      To determine whether the synergy between TLR7 activation and UVB exposure in augmenting cytokine secretion by XS52 was relevant to the in vivo model system, another in vivo experiment that focused on cellular events in local lymph node tissues was then completed. Groups of animals were treated on abdominal skin with vehicle alone, imiquimod alone, UVB alone, or imiquimod and UVB, and local lymph nodes in the inguinal area were dissected, followed by enrichment of CD11c+ dendritic APCs. These enriched APCs were then cultured in medium alone, or medium containing two concentrations of LPS (10 or 50 μg/ml). IL-12p70 secretion was studied using a specific ELISA (Figure 6). APCs cultured in medium alone did not produce detectable IL-12p70 (data not shown). Whereas APCs from vehicle-treated animals produced low, but detectable IL-12p70 in response to LPS, there was significant augmentation of the IL-12p70 response in APCs from imiquimod-treated mice. APCs from vehicle/UV-irradiated animals exhibited an impaired IL-12p70 response, which was less than the vehicle alone treated animals, or the imiquimod-treated animals. There was a marked augmentation of the IL-12p70 secretion by the APCss from the imiquimod/UV-treated animals. These data indicate that the synergy between TLR7 activation and UV exposures in augmenting IL-12p70 secretion by XS52 was indeed relevant to the mechanism of imiquimod in the in vivo model system in which there TLR7 activation in vivo renders mice resistant to the deleterious effects of UVB.
      Figure thumbnail gr6
      Figure 6Topical applications of imiquimod followed by UVB exposures significantly enhances IL-12p70 secretion by local lymph node CD11c+ dendritic APC populations. Mice (N=5/group) were treated in vivo with vehicle, vehicle and UV, imiquimod or imiquimod and UV. Local lymph nodes were dissected pooled together, and CD11c+ dendritic cell were enriched, and then were stimulated with LPS for 24 hours, supernatants were collected and assayed for IL-12p70 content (*P<0.05 significantly different when compared to vehicle alone; **P<0.01 compared to imiquimod alone). Data are derived from a single, representative experiment that was repeated three times.

      Discussion

      Application of topical imiquimod to mouse skin prevented the loss of CHS induced by UV, as measured by ear swelling responses, and analysis of histologic specimens of hapten challenge sites in mice (Figure 1a and b). The role of IL-12 in the mechanism of action of imiquimod on UV immunosuppression was confirmed by the administration of a specific anti-IL-12 monoclonal antibody, which blocked the protective effects of topical imiquimod (Figure 1c). These data are consistent with the observation that the administration of recombinant IL-12 prevents UV suppression of CHS, and overcomes UV-induced tolerance (
      • Schwarz A.
      • Grabbe S.
      • Aragane Y.
      • Sandkuhl K.
      • Riemann H.
      • Luger T.A.
      • et al.
      Interleukin-12 prevents UVB-induced local immunosuppression and overcomes UVB-induced tolerance.
      ). However,
      • Schwarz A.
      • Grabbe S.
      • Aragane Y.
      • Sandkuhl K.
      • Riemann H.
      • Luger T.A.
      • et al.
      Interleukin-12 prevents UVB-induced local immunosuppression and overcomes UVB-induced tolerance.
      utilized a recombinant cytokine to protect mice from UV-immunosuppression by systemic (intraperitoneal) injections. The current study utilized a TLR7 agonist, which was administered topically, and is a known IL-12 inducer. Both studies demonstrate the critical role of IL-12 in preserving CHS in the setting of UV-light irradiation. Additionally, topical applications of imiquimod before UV exposure also prevented the induction immune tolerance, further demonstrating the similarities between the two methods of modulating UV-light damage to the skin immune system.
      The frequency of epidermal LCs is an important factor in determining whether sensitization or tolerance occurs after hapten application (
      • Toews G.B.
      • Bergstresser P.R.
      • Streilein J.W.
      Epidermal Langerhans cell density determines whether contact hypersensitivity or unresponsiveness follows skin painting with DNFB.
      ;
      • Aberer W.
      • Schuler G.
      • Stingl G.
      • Honigsmann H.
      • Wolff K.
      Ultraviolet light depletes surface markers of Langerhans cells.
      ). UV light is thought to impair CHS and induce immune tolerance because it depletes epidermal LCs (
      • Cruz Jr, P.D.
      • Nixon-Fulton J.L.
      • Tigelaar R.E.
      • Bergstresser P.R.
      Disparate effects of in vitro low-dose irradiation on intravenous immunization with purified epidermal cell subpopulations for the induction of contact hypersensitivity.
      ,
      • Cruz Jr, P.D.
      • Tigelaar R.E.
      • Bergstresser P.R.
      Langerhans cells that migrate to skin after IV infusion regulate the induction of contact hypersensitivity.
      ;
      • Simon J.C.
      • Tigelaar R.E.
      • Bergstresser P.R.
      • Edelbaum D.
      • Cruz Jr, P.D.
      UVB radiation converts LC from immunogenic to tolerogenic antigen presenting cells. Induction of specific clonal anergy in CD4+ T helper 1 cells.
      ). This depletion is thought to be the result of UV-light-induced migration (
      • Kolgen W.
      • Both H.
      • van Weelden H.
      • Guikers K.L.
      • Bruijnzeel-Koomen C.A.
      • Knol E.F.
      • et al.
      Epidermal LC depletion after artificial UVB irradiation of human skin in vivo: apoptosis versus migration.
      ), whereby the UV-impaired LCs in local lymph nodes result in the formation of T-regulatory cells that suppress CHS, and are adoptively transferable to naïve mice (
      • Schwarz A.
      • Maeda A.
      • Wild M.K.
      • Kernebeck K.
      • Gross N.
      • Aragane Y.
      • et al.
      UV-induced regulatory T-cells do not only inhibit the induction but can also suppress the effector phase of contact hypersensitivity.
      ). Studies of the protective effects of IL-12 on UV immunosuppression demonstrated that injections of this cytokine immediately before UV exposures prevented LCs depletion from the epidermis (
      • Schwarz A.
      • Maeda A.
      • Kernebeck K.
      • van Steeg H.
      • Beissert S.
      • Schwarz T.
      Prevention of UV radiation-induced immunosuppression by IL-12 is dependent on DNA repair.
      ). Topical imiquimod therapy has been demonstrated to induce migration of epidermal LCs from the skin to local lymph nodes within 24 hours of application (
      • Suzuki H.
      • Wang B.
      • Shivji G.M.
      • Toto P.
      • Amerio P.
      • Tomai M.
      • et al.
      Imiquimod, a topical immune response modifier, induces migration of Langerhans cells.
      ). Thus, the combination of topical imiquimod followed by UV irradiation presented the potential for exaggerated migration (ie, depletion) from the epidermis before sensitization with hapten. It is noteworthy that the regimen of topical imiquimod exposures in the current studies did not affect LCs, which is in contrast to previous studies of LCs migration that used a different treatment regimen (
      • Suzuki H.
      • Wang B.
      • Shivji G.M.
      • Toto P.
      • Amerio P.
      • Tomai M.
      • et al.
      Imiquimod, a topical immune response modifier, induces migration of Langerhans cells.
      ).
      Because topical imiquimod prevented loss of CHS (Figure 1) and tolerance induction (Figure 3a), epidermal LC densities were studied. In contrast to the data derived from the studies of recombinant IL-12, topical applications of imiquimod did not prevent UV-induced depletion of epidermal LCs (Figure 2). It is noteworthy that the studies with the IL-12 involved exposure to UV light first, then administration of IL-12 injection immediately before DNFB sensitization. Our protocol involved two daily doses of imiquimod, then four daily doses of UVB at 70 mJ/cm2. In our study, DNFB hapten was not applied to mouse skin before immunochemistry to study LC densities. Thus, differences in experimental methods may have resulted in differing outcomes regarding LC densities in response to UV depletion.
      The rationale for studying hapten-specific antibody responses are related to observations that epicutaneously hapten-sensitized mice produce antibodies specific for the allergens, which is the result of T-cell-derived help in the form of cytokines that occur in germinal centers as a result of antigen presentation and polarized cytokine production. Study of hapten-specific IgG2a production (an antibody response that is driven by Th1-derived cytokines such as IFN-γ) (
      • Jurado A.
      • Carballido J.
      • Griffel H.
      • Hochkeppel H.K.
      • Wetzel G.D.
      The immunomodulatory effects of interferon-gamma on mature B-lymphocyte responses.
      ) indicated that UV irradiation rendered DNFB-sensitized mice totally unresponsive to DNP, with antibody production similar to naïve mice, or mice treated with vehicle before DNFB sensitization. In contrast, imiquimod-treated, UV-irradiated mice responded as vigorously as did non-irradiated, DNFB-sensitized mice, indicating that topical imiquimod preserved Th1-lymphocyte help to drive cytokine production. Study of hapten-specific IgG1 production (an antibody response that is driven by Th2-derived cytokine such as IL-4) (
      • Coffman R.L.
      • Savelkoul H.F.
      • Lebman D.A.
      Cytokine regulation of immunoglobulin isotype switching and expression.
      ) indicated that UV irradiation rendered these mice DNP hyporesponsive, with an antibody responsive significantly greater than that of naïve mice. Imiquimod treatment before UV irradiation also reversed the hyporesponsiveness, rendering this group's antibody response the same as non-irradiated, DNFB-sensitized mice. Collectively, these data suggest that UV irradiation before hapten sensitization impairs antigen presentation, and subsequent T-lymphocyte help for hapten-specific antibody production, and that treatment of mouse skin with topical imiquimod prevents this impairment.
      XS52 is derived from the epidermis from neonatal mouse skin, is growth factor-dependent, and is maintained in an immature state, but responds to maturational stimuli (
      • Kitajima T.
      • Ariizumi K.
      • Mohamadazadeh M.
      • Edelbaum D.
      • Bergstresser P.R.
      • Takashima A.
      T cell-dependent secretion of IL-1 beta by a dendritic cell line (XS52) derived from murine epidermis.
      ). We demonstrate that XS52 cells express mRNA encoding TLR7 and TLR8. After exposure to imiquimod, XS52 cells degrade IκB (an event associated with NF-κB transport to the nucleus), phosphorylate JNK, and secrete IL-12p70 and tumor necrosis factor-α (Figure 4a–e). These represent the molecular events associated with TLR signaling by the MyD88-dependent pathway (
      • Zhang G.
      • Ghosh S.
      Molecular mechanisms of NF-kappaB activation induced by bacterial lipopolysaccharide through Toll-like receptors.
      ). Thus, XS52 cell line is representative of an imiquimod responsive cell type, such as epidermal LCs (
      • Burns Jr, R.P.
      • Ferbel B.
      • Tomai M.
      • Miller R.
      • Gaspari A.A.
      The imidazoquinolines, imiquimod and R-848, induce functional, but not phenotypic, maturation of human epidermal Langerhans' cells.
      ;
      • Suzuki H.
      • Wang B.
      • Shivji G.M.
      • Toto P.
      • Amerio P.
      • Tomai M.
      • et al.
      Imiquimod, a topical immune response modifier, induces migration of Langerhans cells.
      ). Direct studies of the effects of TLR7 activation before UV irradiation demonstrate that even brief pulse incubations of XS52 with imiquimod (4 hours before UV-irradiation) render this cell type able to secrete IL-12p70. Because the combination of imiquimod before UV irradiation results in a greater production of IL-12p70 than either alone, this suggests that TLR7 and UV irradiation are synergistic in enhancing the production of this critical cytokine (Figure 5a).
      Because Th1-lymphocytes play a critical role in CHS (
      • Hauser C.
      Cultured epidermal LC activate effector T-cells for contact hypersensitivity.
      ) and UV irradiation has been demonstrated to result in impaired antigen presentation specifically to Th1-lymphocytes (
      • Simon J.C.
      • Tigelaar R.E.
      • Bergstresser P.R.
      • Edelbaum D.
      • Cruz Jr, P.D.
      UVB radiation converts LC from immunogenic to tolerogenic antigen presenting cells. Induction of specific clonal anergy in CD4+ T helper 1 cells.
      ), the ability of imiquimod to preserve Th1-cytokines in the setting of UV irradiation was studied using an allogeneic mixed lymphocyte reaction (MLR) as a measure of antigen presentation (Figure 5b). Whereas UV-irradiated XS52 triggered impaired IFN-γ production by CD4+ T-lymphocytes, imiquimod treatment of XS52 before UV irradiation significantly enhanced IFN-γ production by CD4+ T-lymphocytes. This IFN-γ production was greater than that stimulated by UV-irradiated XS52 or imiquimod-treated XS52. XS52, cultured in medium alone or in the presence of imiquimod, did not produce IFN-γ (data not shown). These data again suggest that imiquimod and UV irradiation synergize to enhance IFN-γ production, which is a known effect of IL-12 (
      • Langrish C.L.
      • McKenzie B.S.
      • Wilson N.J.
      • de Waal Malefyt R.
      • Kastelein R.A.
      • Cua D.J.
      IL-12 and IL-23: master regulators of innate and adaptive immunity.
      ). The lack inability of imiquimod by itself to act on XS52 to enhance IFN-γ triggering in T-lymphocytes (compared to control XS52) is in contrast to previous studies of human epidermal LCs in which imiquimod enhances the ability of the cells to trigger IFN-γ in T-lymphocytes (
      • Burns Jr, R.P.
      • Ferbel B.
      • Tomai M.
      • Miller R.
      • Gaspari A.A.
      The imidazoquinolines, imiquimod and R-848, induce functional, but not phenotypic, maturation of human epidermal Langerhans' cells.
      ). It is possible that higher doses of imiquimod may be necessary in this cell line.
      Our ex vivo studies of IL-12p70 production by lymph node APCs demonstrated that such CD11c+ APC from UV-irradiated animals were indeed deficient in their ability to produce IL-12p70 after stimulation with LPS when compared to non-irradiated mice, or imiquimod-treated mice (Figure 5b). Consistent with our in vitro studies of the interactions of TLR7 activation and UV irradiation on XS52, there was a synergy between imiquimod treatment and UV irradiation in the enhancement of IL-12p70 secretion when compared to lymph node APCs from imiquimod alone treated mice or UV-irradiated mice. Thus, these data consistently indicate that TLR7 activation before exposure to UV irradiation results in an enhanced secretion of IL-12p70, which is likely to be responsible for conferring resistance to UV immunosuppression, and subsequent loss of CHS. One possible explanation for this synergy between TLR7 and UV irradiation in triggering enhanced IL-12 secretion is both these stimuli activate NF-κB to move into the nucleus and activate gene transcription (
      • Abeyama K.
      • Eng W.
      • Jester J.V.
      • Vink A.A.
      • Edelbaum D.
      • Cockerell C.J.
      • et al.
      A role of NF-κB-dependent gene transactivation in sunburn.
      ;
      • Zhang G.
      • Ghosh S.
      Molecular mechanisms of NF-kappaB activation induced by bacterial lipopolysaccharide through Toll-like receptors.
      ). This possibility warrants further study.
      Given that imiquimod augments LC function (
      • Burns Jr, R.P.
      • Ferbel B.
      • Tomai M.
      • Miller R.
      • Gaspari A.A.
      The imidazoquinolines, imiquimod and R-848, induce functional, but not phenotypic, maturation of human epidermal Langerhans' cells.
      ) and CHS (
      • Suzuki H.
      • Wang B.
      • Shivji G.M.
      • Toto P.
      • Amerio P.
      • Tomai M.
      • et al.
      Imiquimod, a topical immune response modifier, induces migration of Langerhans cells.
      ), it is possible that all of the observations related to its effects on UV on the skin immune system can be explained by the ability of imiquimod to enhance LC function (ie, adjuvant activities), and hence CHS, rather than a true prevention of UV immunosuppression.
      Recombinant IL-12 renders mice resistant to UV immunosuppression because it triggers DNA repair of UV-induced cyclobutane pyrimidine dimers (
      • Schwarz A.
      • Maeda A.
      • Kernebeck K.
      • van Steeg H.
      • Beissert S.
      • Schwarz T.
      Prevention of UV radiation-induced immunosuppression by IL-12 is dependent on DNA repair.
      ), which are known to be a critical lesion in migratory APCs, resulting in immune defects after UV exposure (
      • Kripke M.L.
      • Cox P.A.
      • Alas L.G.
      • Yarosh D.B.
      Pyrimidine dimers in DNA initiate systemic immunosuppression in UV-irradiated mice.
      ). This raises the possibility that imiquimod, by stimulating IL-12 p70 gene expression, may stimulate DNA repair in APCs and other epidermal cells. This would present a novel function for TLR signaling, mediated directly by TLR signaling itself, or indirectly by induced cytokines such as IL-12.
      These results suggest that there may be a new paradigm for the prevention of photoimmunosuppression using topical agents. Previous studies have suggested that topical sunscreens (
      • Whitmore S.E.
      • Morison W.L.
      Prevention of UVB-induced immunosuppression in humans by a high SPF sunscreen.
      ), and possibly topical DNA repair enzymes (
      • Yarosh D.B.
      • Klein J.
      DNA repair enzymes in prevention of photocarcinogenesis.
      ), can dampen photoimmunosuppression. Topical imiquimod and its derivatives may present an opportunity to prevent photoimmunosuppression by manipulating dendritic cell responses to UV light by stimulating TLR signaling. This possibility warrants further study, and suggests that imiquimod may play a role in the prevention of skin cancer development in the setting of chronic UV-light exposures.

      Materials and Methods

      Experimental mice

      C57BL/6 female mice, 8 weeks old were purchased from Jackson Laboratories (Bar Harbor, ME). These animals were chosen because they have been demonstrated to exhibit a phenotype of susceptibility to UV-induced suppression of CHS (
      • Toews G.B.
      • Bergstresser P.R.
      • Streilein J.W.
      Epidermal Langerhans cell density determines whether contact hypersensitivity or unresponsiveness follows skin painting with DNFB.
      ;
      • Elmets C.A.
      • Bergstresser P.R.
      • Tigelaar R.E.
      • Wood P.J.
      • Streilein J.W.
      Analysis of the mechanism of unresponsiveness produced by haptens painted on skin exposed to low dose ultraviolet-radiation.
      ). Animals were kept in standard housing conditions, fed mouse chow ad libatum, and exposed to a 12 hours light/dark cycle. All experimental procedures were reviewed and approved by the University of Maryland Institutional Animal Care and Use Committee.

      Chemicals

      Imiquimod (R-837) 5% cream and its vehicle cream were used for topical applications, and were provided by 3M Pharmaceuticals (St Paul, MN). For the in vitro studies, imiquimod powder was used (also provided by 3M Pharmaceuticals). It was prepared as a stock solution in sterile water at 1 mg/ml, and diluted to the appropriate working concentration in complete medium

      Immunofluorescence staining

      Abdominal skin samples were scraped of subcutaneous fat, incubated with ammonium thiocyanate, washed with phosphate-buffered saline, and fixed in acetone. The epidermal sheets were stained overnight for LCs with an anti I-A/I-E monoclonal antibody (M5.114, hybridoma from American Type Culture Collection), then incubated with a fluorescein conjugated secondary anti-rat antibody (IgG) and examined using an epifluorescence microscope (Nikon E600) equipped with a digital spot camera. A monoclonal rat IgG of irrelevant specificity was used as an isotype control for staining.
      Digital image analysis was used to count epidermal LCs density on abdominal skin after topical imiquimod or vehicle or no treatments before UV exposure. Image Pro Plus Version 4.5 for Windows (Media Cybernetics, Silver Spring, MD) software program was used to count stained epidermal LCs.

      Hapten sensitization

      For hapten sensitization, the experimental allergen DNFB (Eastman Chemicals, Kingsport, TN) was utilized. To sensitize mice, 0.5% DNFB in 4:1 acetone/olive oil in a 20 μl volume was pipetted onto shaved abdominal skin on days 0 and +1; to elicit CHS, the allergic mice (or naïve mice as a negative control) were challenged with 0.2% DNFB in acetone/olive oil in a 20 μl volume (half of the total volume to each side of the ear). Ear swelling was measured on the right ear 24 hours after challenge using an engineer's thickness gauge caliper (Mitutoyo, Cole-Parmer Instrument company, Tylertown, MS). Change in ear thickness was calculated by subtracting the baseline ear thickness to that measured 24 hours after hapten challenge (
      • Gaspari A.A.
      • Katz S.I.
      ). Additionally, skin biopsy specimens were taken from the pinna of the ear in selected animals at 24 hours after hapten challenge to study the histologic changes. The specimens were paraffin-embedded, sectioned onto glass slides, stained with hematoxylin–eosin, and examined and photographed using a E600 Nikon microscope equipped with a Spot Digital camera (Diagnostic Instruments, Inc., Sterling Heights, MI) in the brightfield mode.
      To block the biologic effects of IL-12 during topical imiquimod treatments before UV irradiation, rat anti-mouse IL-12 p70 (C15.6, Biosource, Camarillo, CA) or rat IgG1 (isotype control) (BD Pharmingen, San Diego, CA), anti-IL-12 or the isotype control antibody was administered by intraperitoneal injections (150 μg/injection) daily during the 2 days of imiquimod treatments.
      For all of these in vivo experiments, there were five mice in each experimental group. The depicted data represent the mean±SD for each experimental group. Every in vivo experiment was repeated three times to assure reproducibility.

      UV light source

      Groups of mice were irradiated with a panel of 48” Q-Sun light bank (Q-Panel Laboratory products, Cleveland, OH) (equipped with a UVC WG320 filter) at a distance of 12 inches from the light source to their shaved abdominal skin. The spectral emission profile of this light source closely mimics that of natural sunlight, emitting predominantly UVA. A UVB radiometer (National Biologic Corporation, Twinsburg, OH) was used to determine UVB output, and calculate the time necessary to deliver the desired doses of UVB. Mice received four daily doses of 70 mJ/cm2 of UVB, a schedule of UV radiation that has been demonstrated to inhibit the afferent phase of CHS (
      • Glass M.J.
      • Bergstresser P.R.
      • Tigelaar R.E.
      • Streilein J.W.
      UVB radiation and DNFB skin painting induce suppressor cells universally in mice.
      ).
      The protocol for studying the prevention of UV-induced loss of CHS is summarized in Figure 7. Briefly, groups of mice (N=5/group) were treated with topical 5% imiquimod cream (0.1 ml dispensed from a tuberculin syringe) or a similar volume of vehicle or nothing on shaved abdominal skin for 2 consecutive days before the four consecutive daily doses of UV radiation. Animals were fitted with soft collars to prevent the ingestion of the topical applications as a result of their grooming activities. On the day of the last dose of the UV radiation and the following day, all experimental groups were sensitized with 0.5% DNFB (on Day 0 and +1). The first sensitizing dose of DNFB was applied 4 hours after the final dose of UV radiation.
      Figure thumbnail gr7
      Figure 7Protocol for studying the prevention of UV suppression of CHS with topical applications of imiquimod or its vehicle. Groups (N=5/group) of mice were treated with imiquimod 5% cream, its vehicle alone, or nothing for two consecutive days before four daily doses of UV light before sensitization with 0.5% DNFB. They were subsequently challenged on the ear pinna with 0.2% DNFB, and then ear swelling was measured. This critical experiment was repeated three times, and found to be reproducible.
      To study the prevention of UV-induced tolerance to hapten sensitization, this protocol was used with minor modifications. All animals received the topical treatments, UV exposures and initial sensitization through UV-irradiated abdominal skin as outlined in Figure 7. Instead of eliciting on the ear pinna on day +4, animals received a second sensitization with 0.5% DNFB on shaved, non-UV-irradiated dorsal skin of the back (days +7 and +8). Four days later (day +11), the mice were challenged on the ear pinna with 0.2% DNFB, and the following day (day +12), ear swelling was measured.

      ELISA to detect hapten-specific antibody responses

      ELISA plates were coated for 8 hours with DNP-conjugated ovalbumin at a concentration of 10 μg/ml. Blocking solution (1% bovine serum albumin and 0.05% Tween) was then added for 2 hours. The wells were then washed with phosphate-buffered saline 0.05% Tween. The test sera were added for 8 hours at 4°C. The wells were washed, and then biotinylated goat anti-mouse IgG1 (BD Pharmingen) (1:8,000 dilution) or biotinylated goat anti-mouse IgG2a (BD Pharmingen) (1:6,000) was added for 2 hours, followed by streptavidin-conjugated horse radish peroxidase for 45 minutes. Color substrate was added, color was allowed to develop, and then stopped by adding 2 N H2SO4. The plates were read at 450 nm in a Benchmark Biorad ELISA plate reader.

      ELISA to detect secreted cytokines

      Commercially available ELISA (BD OptEIA) were used to detect secreted cytokines into cell-free culture supernatants. For IL-12p70 and tumor necrosis factor-α, supernatants were collected over a 24-hours period. The limit of detection for these ELISAs was 10 ng/ml for the cytokines of interest. For IFN-γ, culture supernatants were collected after 72 hours of the mixed lymphocyte reaction.

      Mixed lymphocyte reaction

      For the MLR, XS52 (H-2d) (a generous gift from Dr Akira Takashima, UTSW) were utilized as the stimulator APC, at 20,000 cells/well of a round-bottomed 96-well plate. As responder T-cells, the spleen was removed from a C57BL/6 mouse (H-2b), passed through a nylon wool column to deplete accessory cells, counted, and then added to a 96-well microtiter plate at 200,000 T cells/well in RPMI 1640 with 10% fetal calf serum, glutamine, and antibiotics. The cells were incubated in a 5% CO2 atmosphere at 37°C for 72 hours before collecting supernatants, which were frozen at −20°C until cytokine ELISA were performed.
      To treat XS52 before the MLR, this cell line was cultured in complete medium without growth factors for 24 hours before adding imiquimod at 5–10 μg/ml for 4 hours. The culture medium containing imiquimod was removed, and the cells were rinsed with phosphate-buffered saline before UV irradiation with the Q-panel light source (see above). A UVB radiometer was used to measure UVB doses. These doses were 5–25 mJ/cm2, and have been demonstrated to induce NF-κB signaling in the XS cell line in the absence of cell death (
      • Abeyama K.
      • Eng W.
      • Jester J.V.
      • Vink A.A.
      • Edelbaum D.
      • Cockerell C.J.
      • et al.
      A role of NF-κB-dependent gene transactivation in sunburn.
      ). After the UV radiation, the XS52 were cultured in complete medium without growth factors, then harvested for MLR 20 hours after UVB exposure. Thus, with both the imiquimod and UVB treatments, a total of 24 hours elapsed before the XS52 were added to the MLR. This experiment was repeated three times to assure reproducibility.

      Enrichment of dendritic APCs from local lymph nodes

      Regional lymph nodes were dissected from the inguinal area of mice treated on abdominal skin with vehicle alone, vehicle and UV irradiation, imiquimod alone, or imiquimod treatment before the UV exposures (N=5 mice/group) as summarized in Figure 7. The lymph nodes from each experimental group were pooled together, then mechanically disrupted into a single-cell suspension, and pipetted onto a density gradient as previously described (
      • Gorman S.
      • Tan J.W.-Y.
      • Thomas J.A.
      • Townley S.L.
      • Stumbles P.A.
      • Finlay-Jones J.J.
      • et al.
      Primary defect in UVB-induced systemic immunomodulation does not relate to immature or functionally impaired APCs in regional lymph nodes.
      ). The cells at the interface were collected, washed, counted, and then incubated with anti-CD11c (BD Biosciences, San Diego, CA) that had been coupled onto the surface of Tosyl activated magnetic beads (Dynal Corporation, Oslo, Norway), and then placed over a magnet. Non-adherent cells were removed. The adherent cells were released from the magnet, washed, counted, and used as lymph node dendritic cells. The resulting cells were >90% M5/114 (class II MHC) positive, and exhibited a dendritic morphology. These dendritic cells were cultured in medium alone (50,000 cells/well) or with imiquimod or LPS (a generous gift from Dr Stephanie Vogel) in a 200 μl volume for 24 hours, and supernatants were collected and studied for IL-12p70 content using a commercially available ELISA.

      Western blotting

      To study IκB cytoplasmic content in XS52, Western blotting was used. Before and 15 minutes after stimulation with imiquimod 5 μg/ml or LPS, 1 × 106 cells were lysed with lysis buffer (RIPA buffer, Sigma, St Louis, MO) and then scraped from the culture vessel, placed on ice and then on a rocking platform for 10 minutes. The lysate was then centrifuged at 15,000 × g for 3 minutes. The cytosolic fraction (supernatant) was collected. Protein concentration was determined (Bradford method). The lysate was then run on a 10% Bis-Tris NuPage gel under reducing conditions with MOPS SDS running buffer. The separated proteins were transferred to a nitrocellulose membrane, stained with a rabbit polyclonal antiserum specific for mouse IκB (Santa Cruz #SC-371, Santa Cruz, CA). Staining was detected using Western Breeze Chemiluminescent Immunodetection kit (Invitrogen, Carlsbad, CA); a Chemdoc (Biorad, Hercules, CA) digital imaging system was used to record the resulting images.
      To detect phosphorylation of JNK-1, 2 in XS52 or those cells stimulated with imiquimod or LPS, 1 × 106 cells were lysed with lysis buffer (RIPA buffer with a phosphatase inhibitor cocktail II, Sigma, St Louis, MO) scraped from the culture vessel. Cellular debris was pelleted and removed, and protein concentration determined (Bradford method). The lysate was then run on a 10% Bis-Tris NuPage gel under reducing conditions with MOPS SDS running buffer. The separated proteins were transferred to a nitrocellulose membrane, and stained with a rabbit polyclonal antibody specific for JNK1, 2. Staining was detected using Western Breeze Chemiluminescent Detection kit (Invitrogen); a Chemdoc (Biorad) digital imaging system was used to record the resulting images.

      RNA extraction and cDNA synthesis (RNeasy Mini Protocol, Qiagen, Valencia, CA)

      Cultured cells (from one well of six-well plate) were lysed by adding 350 μl of β-mercaptoethanol in buffer RLT (1:100) directly into the culture vessels. The cell lysate was placed into a microcentrifuge tube and vortexed to ensure good mixing. The sample was then homogenized by placing it into a QIAshredder spin column and centrifuging for 2 minutes at maximum speed. 350 μl of 70% ethanol in DEPC H2O was added to the homogenized lysate and mixed well. The 700 μl sample was placed in an RNeasy mini column and centrifuged for 15 seconds at ≥10,000 r.p.m., and the flow-through discarded. Seven hundred microliters of Buffer RW1 was added to the RNeasy column, and centrifuged again for 15 seconds at ≥10,000 r.p.m. After transferring the RNeasy column into a new 2 ml collection tube, 500 μl of Buffer RPE was added to the column and centrifuged for 15 seconds at ≥10,000 to wash the column. Another 500 μl of Buffer RPE was added and column centrifuged for 2 minutes at ≥10,000 r.p.m. to dry the silica-gel membrane. The column was centrifuged at full speed for 1 minute in order to eliminate any chance of possible Buffer RPE carryover. The RNA was eluted with 30 μl of RNAse-free water.
      RNA samples (2 μg) were placed in microcentrifuge tubes and RNAse-free water was added to bring the samples to 8 μl (or 20 μl). The RNA solution was heated to 65°C for 10 minutes, then chilled at 4°C for 5 minutes. Five microliters (or 11 μl) of the bulk first-strand cDNA reaction mix was then added to the heat-denatured RNA solution, followed by 1 μl of DTT solution and 1 μl of pd(N)6 (First-Strand cDNA Synthesis Kit; Amersham, Piscatway, NJ). The mixture was pipetted up and down several times to ensure a good mix, and incubated at 37°C for 1 hour. After the incubation, the completed first-strand reaction product was heated to 90°C for 5 minutes to denature the RNA-cDNA duplex and to inactivate the reverse transcriptase. It was then cooled at 4°C for 5 minutes. In all, 0.5–1.0 μl of this final cDNA product was used for PCR amplification.

      Reverse transcription-PCR

      Detection of TLR7 and TLR8 mRNA was performed using reverse transcription-PCR according to previously published methods (
      • Gaspari A.A.
      • Burns Jr, R.P.
      • Kondo S.
      • Nasir A.
      • Kurup A.
      • Mlodynia D.
      • et al.
      Characterization of the altered cutaneous reactivity in transgenic mice whose keratinocytes overexpress B7-1.
      ). The annealing temperature was 55°C, followed by 35 cycles of amplification using a PCR Sprint (Hybaid, Inc., Waltham, MA). Primers were designed according to published sequences (AY035889.1 for murine TLR7 and AY035890.1 for murine TLR8), and spanned one intron.
      TLR7 primers (first set, 5′ end of gene; amplification product: 167 bp):
      • 5′-CCTGTTCTACTGGGGTCCAA-3′
      • 3′-GCCTCAAGGCTCAGAAGATG-5′
      TLR7 primers (second set, 3′ end of gene; amplification product: 161 bp):
      • 5′-GGAGCTCTGTCCTTGAGTGG-3′
      • 3′-CAAGGCATGTCCTAGGTGGT-3′
      TLR8 primers (first set, 5′ end of gene; amplification product: 195 bp):
      • 5′-GGCACAACTCCCTTGTGATT-3′
      • 3′-CATTTGGGTGCTGTTGTTTG-5′
      TLR8 primers (second set, 3′ end of gene; amplification product: 163 bp):
      • 5′-CTTCCAAACTTGGCAACCAT-3′
      • 3′-GAAGACGATTTCGCCAAGAG-5′
      After amplification, the samples were loaded on a 4% agarose gel, stained, and photographed. Omission of reverse transcriptase controlled for DNA contamination.

      Statistical analyses

      Graph Pad Instat and Prism software programs (Graph Pad, San Diego, CA) were used to compare quantitative data from experimental groups for statistical significance. For comparisons between multiple groups, analysis of variance was used; P<0.05 were considered to be statistically significant.

      Conflict of Interest

      This study was sponsored by an unrestricted research grant from 3M Pharmaceuticals to Dr Gaspari. Dr Tomai and Dr Miller are employed by 3M Pharamaceuticals.

      REFERENCES

        • Aberer W.
        • Schuler G.
        • Stingl G.
        • Honigsmann H.
        • Wolff K.
        Ultraviolet light depletes surface markers of Langerhans cells.
        J Invest Dermatol. 1981; 76: 202-210
        • Abeyama K.
        • Eng W.
        • Jester J.V.
        • Vink A.A.
        • Edelbaum D.
        • Cockerell C.J.
        • et al.
        A role of NF-κB-dependent gene transactivation in sunburn.
        J Clin Invest. 2000; 105: 1751-1759
        • Burns Jr, R.P.
        • Ferbel B.
        • Tomai M.
        • Miller R.
        • Gaspari A.A.
        The imidazoquinolines, imiquimod and R-848, induce functional, but not phenotypic, maturation of human epidermal Langerhans' cells.
        Clin Immunol. 2000; 94: 13-23
        • Cruz Jr, P.D.
        • Nixon-Fulton J.L.
        • Tigelaar R.E.
        • Bergstresser P.R.
        Disparate effects of in vitro low-dose irradiation on intravenous immunization with purified epidermal cell subpopulations for the induction of contact hypersensitivity.
        J Invest Dermatol. 1989; 92: 160-165
        • Coffman R.L.
        • Savelkoul H.F.
        • Lebman D.A.
        Cytokine regulation of immunoglobulin isotype switching and expression.
        Semin Immunol. 1989; 1: 55-63
        • Cooper K.D.
        • Oberhelman L.
        • Hamilton T.A.
        • Baadsgaard O.
        • Terhune M.
        • LeVee G.
        • et al.
        UV exposure reduces immunization rates and promotes tolerance to epicutaneous antigens in humans: relationship to dose, CD1aDR+ epidermal macrophage induction, and Langerhans cell depletion.
        Proc Natl Acad Sci USA. 1992; 89: 8497-8501
        • Cruz Jr, P.D.
        • Tigelaar R.E.
        • Bergstresser P.R.
        Langerhans cells that migrate to skin after IV infusion regulate the induction of contact hypersensitivity.
        J Immunol. 1990; 144: 2486-2492
        • Elmets C.A.
        • Bergstresser P.R.
        • Tigelaar R.E.
        • Wood P.J.
        • Streilein J.W.
        Analysis of the mechanism of unresponsiveness produced by haptens painted on skin exposed to low dose ultraviolet-radiation.
        J Exp Med. 1983; 158: 781-794
        • Gaspari A.A.
        • Burns Jr, R.P.
        • Kondo S.
        • Nasir A.
        • Kurup A.
        • Mlodynia D.
        • et al.
        Characterization of the altered cutaneous reactivity in transgenic mice whose keratinocytes overexpress B7-1.
        Clin Immunol Immunopathol. 1998; 86: 259-270
        • Gaspari A.A.
        • Katz S.I.
        Coligan J. Kruisbeek A.M. Margulies D.H. Shevach E.M. Strober W. Contact hypersensitivity in current protocols in immunology. John Wiley & Sons, New York1990: 1-5
        • Glass M.J.
        • Bergstresser P.R.
        • Tigelaar R.E.
        • Streilein J.W.
        UVB radiation and DNFB skin painting induce suppressor cells universally in mice.
        J Invest Dermatol. 1990; 94: 273-278
        • Gorden K.B.
        • Gorski K.S.
        • Gibson S.J.
        • Kedl R.M.
        • Kieper W.C.
        • Qiu X.
        • et al.
        Synthetic TLR agonists reveal functional differences between human TLR7 and TLR8.
        J Immunol. 2005; 174: 1259-1268
        • Gorman S.
        • Tan J.W.-Y.
        • Thomas J.A.
        • Townley S.L.
        • Stumbles P.A.
        • Finlay-Jones J.J.
        • et al.
        Primary defect in UVB-induced systemic immunomodulation does not relate to immature or functionally impaired APCs in regional lymph nodes.
        J Immunol. 2005; 174: 6677-6685
        • Hauser C.
        Cultured epidermal LC activate effector T-cells for contact hypersensitivity.
        J Invest Dermatol. 1990; 95: 436-440
        • Jurado A.
        • Carballido J.
        • Griffel H.
        • Hochkeppel H.K.
        • Wetzel G.D.
        The immunomodulatory effects of interferon-gamma on mature B-lymphocyte responses.
        Experientia. 1989; 45: 521-526
        • Kaisho T.
        • Akira S.
        Regulation of dendritic cell function through Toll-like receptors.
        Curr Mol Med. 2003; 3: 373-385
        • Kitajima T.
        • Ariizumi K.
        • Mohamadazadeh M.
        • Edelbaum D.
        • Bergstresser P.R.
        • Takashima A.
        T cell-dependent secretion of IL-1 beta by a dendritic cell line (XS52) derived from murine epidermis.
        J Immunol. 1995; 155: 3794-3800
        • Kolgen W.
        • Both H.
        • van Weelden H.
        • Guikers K.L.
        • Bruijnzeel-Koomen C.A.
        • Knol E.F.
        • et al.
        Epidermal LC depletion after artificial UVB irradiation of human skin in vivo: apoptosis versus migration.
        J Invest Dermatol. 2002; 118: 812-817
        • Kripke M.L.
        • Cox P.A.
        • Alas L.G.
        • Yarosh D.B.
        Pyrimidine dimers in DNA initiate systemic immunosuppression in UV-irradiated mice.
        Proc Natl Acad Sci USA. 1992; 89: 7516-7520
        • Langrish C.L.
        • McKenzie B.S.
        • Wilson N.J.
        • de Waal Malefyt R.
        • Kastelein R.A.
        • Cua D.J.
        IL-12 and IL-23: master regulators of innate and adaptive immunity.
        Immunol Rev. 2004; 202: 96-105
        • Ozato K.
        • Tsujimura H.
        • Tamura T.
        Toll-like receptor signaling and regulation of cytokine gene expression in the immune system.
        Biotechniques. 2002; 66-68 (72 passim): 70
        • Schmitt D.A.
        • Owen-Schaub L.
        • Ullrich S.E.
        Effect of IL-12 on immune suppression and suppressor cell induction by ultraviolet radiation.
        J Immunol. 1995; 154: 5114-5120
        • Simon J.C.
        • Tigelaar R.E.
        • Bergstresser P.R.
        • Edelbaum D.
        • Cruz Jr, P.D.
        UVB radiation converts LC from immunogenic to tolerogenic antigen presenting cells. Induction of specific clonal anergy in CD4+ T helper 1 cells.
        J Immunol. 1991; 146: 485-491
        • Schwarz A.
        • Grabbe S.
        • Aragane Y.
        • Sandkuhl K.
        • Riemann H.
        • Luger T.A.
        • et al.
        Interleukin-12 prevents UVB-induced local immunosuppression and overcomes UVB-induced tolerance.
        J Invest Dermatol. 1996; 106: 1187-1191
        • Schwarz A.
        • Grabbe S.
        • Mahnke K.
        • Riemann H.
        • Luger T.A.
        • Wysocka M.
        • et al.
        Interleukin-12 breaks UV light induced immunosuppression by affecting CD8+ rather than CD4+ T-cells.
        J Invest Dermatol. 1998; 110: 272-276
        • Schwarz A.
        • Maeda A.
        • Kernebeck K.
        • van Steeg H.
        • Beissert S.
        • Schwarz T.
        Prevention of UV radiation-induced immunosuppression by IL-12 is dependent on DNA repair.
        J Exp Med. 2005; 201: 173-179
        • Schwarz A.
        • Maeda A.
        • Wild M.K.
        • Kernebeck K.
        • Gross N.
        • Aragane Y.
        • et al.
        UV-induced regulatory T-cells do not only inhibit the induction but can also suppress the effector phase of contact hypersensitivity.
        J Immunol. 2004; 172: 1036-1043
        • Steinman R.M.
        • Inaba K.
        Stimulation of the primary mixed leukocyte reaction.
        Crit Rev Immunol. 1985; 5: 331-348
        • Suzuki H.
        • Wang B.
        • Shivji G.M.
        • Toto P.
        • Amerio P.
        • Tomai M.
        • et al.
        Imiquimod, a topical immune response modifier, induces migration of Langerhans cells.
        J Invest Dermatol. 2000; 114: 135-141
        • Thomson J.A.
        • Troutt A.B.
        • Kelso A.
        Contact sensitization to oxazolone: involvement of both interferon-gamma and IL-4 in oxazolone-specific Ig and T-cell responses.
        Immunology. 1993; 78: 185-192
        • Toews G.B.
        • Bergstresser P.R.
        • Streilein J.W.
        Epidermal Langerhans cell density determines whether contact hypersensitivity or unresponsiveness follows skin painting with DNFB.
        J Immunol. 1980; 124: 445-453
        • Vender R.B.
        • Goldberg O.
        Innovative uses of imiquimod.
        J Drugs in Dermatol. 2005; 4: 58-63
        • Villa A.M.
        • Berman B.
        Immunomodulators for skin cancer.
        J Drugs Dermatol. 2004; 3: 533-539
        • Whitmore S.E.
        • Morison W.L.
        Prevention of UVB-induced immunosuppression in humans by a high SPF sunscreen.
        Arch Dermatol. 1995; 131: 1128-1133
        • Yarosh D.B.
        • Klein J.
        DNA repair enzymes in prevention of photocarcinogenesis.
        Photochem Photobiol. 1996; 63: 445-447
        • Zhang G.
        • Ghosh S.
        Molecular mechanisms of NF-kappaB activation induced by bacterial lipopolysaccharide through Toll-like receptors.
        J Endotoxin Res. 2000; 6: 453-457