4HPR-X radiation mechanisms of interactions in non-small cell lung cancer (NSCLC) cells in vitro

Lung cancer is the leading cause of cancer death. However, poor survival using conventional therapies fuel the search for more rational interventions. The objective of this study was to design and implement a 4HPR-radiation interaction model in NSCLC, employing a traditional clinical modality (radiation), a relatively new, therapeutically unexplored agent (4HPR) and rationally combining them based on molecular mechanistic findings pertaining to their interactions. To test the hypothesis that 4HPR sensitizes cells to radiation-induced cell death via G2+M accumulation, we designed a working model consisting of H522 adenocarcinoma cells (p53, K-ras mutated) derived from an NSCLC patient; 4HPR at concentrations up to 10 μM; and X radiation up to 6 Gy generated by a patient-dedicated Phillips RT-250 X ray unit at 250 KV, 15 mA, 1.85 Gy/min. We found that 4HPR produced time- and dose-dependent morphological changes, growth inhibition, and DNA damage-inducing enhancement of reactive oxygen species. A transient G2+M accumulation of cells maximal at 24 h of continuous 4HPR exposure was used for irradiation time scheduling. Our data demonstrated enhanced cell death (both apoptotic and necrotic) in irradiated cells pre-treated with 4HPR versus those with either stressor alone. 4HPR's effect of increased NSCLC cells' radioresponse was confirmed by clonogenic assay. To explore these practical findings from a molecular mechanistic perspective, we further investigated and showed that levels of cyclin B1 and p34cdc2 kinase—both components of the mitosis promoting factor (MPF) regulating the G2/M transition—did not change following 4HPR treatment. Likewise, cdc25C phosphatase was not altered. However, enhanced p34cdc2 phosphorylation on its Thr14Tyr15 residues—indicative of its inactivation and increased expression of MPF negative regulators chk1 and wee1 kinases—were supportive of explaining 4HPR-treated cells' accumulation. Hence, p34cdc2 phosphorylation, chk1, and wee1 warrant further evaluation as potential molecular targets for 4HPR-X radiation combination. In summary, we (1) demonstrated that 4HPR not only induces cell death by itself, but also increases NSCLC cells' subsequent radioresponse, indicative of potential clinical applicability, and (2) for the first time, shed light on deciphering 4HPR-X radiation molecular mechanisms of interaction, including the finding of 4HPR's role as a p34cdc2 inactivator via Thr14Tyr15 phosphorylation. ^

Investigation of mechanisms involved in ultraviolet B radiation-induced, T cell -mediated immune suppression

Cutaneous exposure to ultraviolet-B radiation (UVR) results in the suppression of cell-mediated immune responses such as contact hypersensitivity (CHS) and delayed-type hypersensitivity (DTH). This modulation of immune responses is mediated by local or systemic mechanisms, both of which are associated with the generation of antigen-specific suppressor T lymphocytes (Ts). UV-induced Ts have been shown to be CD3+CD4+CD8 − T cells that control multiple immunological pathways. However, the precise mechanisms involved in the generation and function of these immunoregulatory cells remain unclear. We investigated the cellular basis for the generation of UV-induced Ts lymphocytes in both local and systemic models of immune suppression, and further examined the pleiotrophic function of these immunoregulatory cells. ^ We used Thy1.1 and Thy1.2 congenic mice in a draining lymph node (DLN) cell transfer model to analyze the role played by epidermal Langerhans cells in the generation of Ts cells. We demonstrate that T cells tightly adhered to antigen-presenting cells (APC) from UV-irradiated skin are the direct progenitors of UV-induced Ts lymphocytes. Our studies also reveal that UV-induced DNA-damage in the form of cyclobutyl pyrimidine dimers (CPD) in the epidermal APC is crucial for the altered maturation of these adherent T cells into Ts. ^ We used TCR transgenic mice in an adoptive transfer model and physically tracked the antigen-specific clones during immune responses in unirradiated versus UV-irradiated mice. We demonstrate that UV-induced Ts and effector TDTH cells share the same epitope specificity, indicating that both cell populations arise from the same clonal progenitors. UVR also causes profound changes in the localization and proliferation of antigen-specific T cells during an immune response. Antigen-specific T cells are not detectable in the DLNs of UV-irradiated mice after 3 days post-immunization, but are found in abundance in the spleen. In contrast, these clones continue to be found in the DLNs and spleens of normal animals several days post-immunization. Our studies also reveal that a Th2 cytokine environment is essential for the generation of Ts in UV-irradiated mice. ^ The third part of our study examined the pleiotrophic nature of UV-induced Ts. We used a model for the induction of both cellular and humoral responses to human gamma-globulin (HGG) to demonstrate that UV-induced Ts lymphocytes can suppress DTH as well as antibody responses. (Abstract shortened by UMI.) ^

Ultraviolet radiation intensity predicts the relative distribution of dermatomyositis and anti-Mi-2 autoantibodies in women.

OBJECTIVE: Because studies suggest that ultraviolet (UV) radiation modulates the myositis phenotype and Mi-2 autoantigen expression, we conducted a retrospective investigation to determine whether UV radiation may influence the relative prevalence of dermatomyositis and anti-Mi-2 autoantibodies in the US. METHODS: We assessed the relationship between surface UV radiation intensity in the state of residence at the time of onset with the relative prevalence of dermatomyositis and myositis autoantibodies in 380 patients with myositis from referral centers in the US. Myositis autoantibodies were detected by validated immunoprecipitation assays. Surface UV radiation intensity was estimated from UV Index data collected by the US National Weather Service. RESULTS: UV radiation intensity was associated with the relative proportion of patients with dermatomyositis (odds ratio [OR] 2.3, 95% confidence interval [95% CI] 0.9-5.8) and with the proportion of patients expressing anti-Mi-2 autoantibodies (OR 6.0, 95% CI 1.1-34.1). Modeling of these data showed that these associations were confined to women (OR 3.8, 95% CI 1.3-11.0 and OR 17.3, 95% CI 1.8-162.4, respectively) and suggests that sex influences the effects of UV radiation on autoimmune disorders. Significant associations were not observed in men, nor were UV radiation levels related to the presence of antisynthetase or anti-signal recognition particle autoantibodies. CONCLUSION: This first study of the distribution of myositis phenotypes and UV radiation exposure in the US showed that UV radiation may modulate the clinical and immunologic expression of autoimmune disease in women. Further investigation of the mechanisms by which these effects are produced may provide insights into pathogenesis and suggest therapeutic or preventative strategies.

Mechanisms of perceptual learning of depth discrimination in random dot stereograms.

Perceptual learning is a training induced improvement in performance. Mechanisms underlying the perceptual learning of depth discrimination in dynamic random dot stereograms were examined by assessing stereothresholds as a function of decorrelation. The inflection point of the decorrelation function was defined as the level of decorrelation corresponding to 1.4 times the threshold when decorrelation is 0%. In general, stereothresholds increased with increasing decorrelation. Following training, stereothresholds and standard errors of measurement decreased systematically for all tested decorrelation values. Post training decorrelation functions were reduced by a multiplicative constant (approximately 5), exhibiting changes in stereothresholds without changes in the inflection points. Disparity energy model simulations indicate that a post-training reduction in neuronal noise can sufficiently account for the perceptual learning effects. In two subjects, learning effects were retained over a period of six months, which may have application for training stereo deficient subjects.

Mechanisms of Adenovirus-Mediated Autophagy

A patient diagnosed with a glioma, generally, has an average of 14 months year to live after implementation of conventional therapies such as surgery, chemotherapy, and radiation. Glioblastomas are highly lethal because of their aggressive nature and resistance to conventional therapies and apoptosis. Thus other avenues of cell death urgently need to be explored. Autophagy, which is also known as programmed cell death type II, has recently been identified as an alternative mechanism to kill apoptosis- resistant cancer cells. Traditionally, researchers have studied how cells undergo autophagy during viral infection as an immune response mechanism, but recently researchers have discovered how viruses have evolved to manipulate autophagy for their benefit. Extensive studies of viral-induced autophagy provide a rationale to investigate other viruses, such as the adenovirus, which may be developed as part of a therapy against cancers resistant to apoptosis. Despite the present and relatively poor understanding of the mechanisms behind adenoviral-induced autophagy, adenovirus is a promising candidate, because of its ability to efficiently eradicate tumors. A better understanding of how the adenovirus induces autophagy will allow for the development of viruses with increased oncolytic potency. We hypothesized that adenovirus induces autophagy in order to aid in lysis. We found that replication, not infection, was required for adenovirus-mediated autophagy. Loss of function analysis of early genes revealed that, of the early genes tested, no single gene was sufficient to induce autophagy alone. Examination of cellular pathways for their role in autophagy during adenovirus infection revealed a function for the eIF2α pathway and more specifically the GCN2 kinase. Cells lacking GCN2 are more resistant to adenovirus-mediated autophagy in vitro; in vivo we also found these cells fail to undergo autophagy, but display more cell death. We believe that autophagy is a protective mechanism the cell employs during adenoviral infection, and in the in vivo environment, cells cannot recover from virus infection and are more susceptible to death. Congruently, infected cells deficient for autophagy through deletion of ATG5 are not able undergo productive cell lysis, providing evidence that the destruction of the cytoplasm and cell membrane through autophagy is crucial to the viral life cycle. This project is the first to describe a gene, other than a named autophagy gene, to be required for adenovirus- mediated autophagy. It is also the first to examine autophagic cell death as a means to aid in viral-induced cell lysis.

Mechanisms of Adenovirus-Mediated Autophagy

A patient diagnosed with a glioma, generally, has an average of 14 months year to live after implementation of conventional therapies such as surgery, chemotherapy, and radiation. Glioblastomas are highly lethal because of their aggressive nature and resistance to conventional therapies and apoptosis. Thus other avenues of cell death urgently need to be explored. Autophagy, which is also known as programmed cell death type II, has recently been identified as an alternative mechanism to kill apoptosis- resistant cancer cells. Traditionally, researchers have studied how cells undergo autophagy during viral infection as an immune response mechanism, but recently researchers have discovered how viruses have evolved to manipulate autophagy for their benefit. Extensive studies of viral-induced autophagy provide a rationale to investigate other viruses, such as the adenovirus, which may be developed as part of a therapy against cancers resistant to apoptosis. Despite the present and relatively poor understanding of the mechanisms behind adenoviral-induced autophagy, adenovirus is a promising candidate, because of its ability to efficiently eradicate tumors. A better understanding of how the adenovirus induces autophagy will allow for the development of viruses with increased oncolytic potency. We hypothesized that adenovirus induces autophagy in order to aid in lysis. We found that replication, not infection, was required for adenovirus-mediated autophagy. Loss of function analysis of early genes revealed that, of the early genes tested, no single gene was sufficient to induce autophagy alone. Examination of cellular pathways for their role in autophagy during adenovirus infection revealed a function for the eIF2α pathway and more specifically the GCN2 kinase. Cells lacking GCN2 are more resistant to adenovirus-mediated autophagy in vitro; in vivo we also found these cells fail to undergo autophagy, but display more cell death. We believe that autophagy is a protective mechanism the cell employs during adenoviral infection, and in the in vivo environment, cells cannot recover from virus infection and are more susceptible to death. Congruently, infected cells deficient for autophagy through deletion of ATG5 are not able undergo productive cell lysis, providing evidence that the destruction of the cytoplasm and cell membrane through autophagy is crucial to the viral life cycle. This project is the first to describe a gene, other than a named autophagy gene, to be required for adenovirus- mediated autophagy. It is also the first to examine autophagic cell death as a means to aid in viral-induced cell lysis.

Mechanisms of UV induced systemic immunosuppression: An essential role for interleukin-10

The recognition of the skin as an immunocompetent organ has focused attention on the complex interaction between ultraviolet radiation and the immune system. How UV-radiation, which hardly penetrates past the epidermis, induces systemic immune suppression is not entirely clear. We propose that suppressive cytokines, released by UV-irradiated keratinocytes, play a role in the induction of immune suppression. Injecting supernatants from UV-exposed murine keratinocytes into mice impairs their ability to mount a delayed-type hypersensitivity response against allogeneic histocompatibility antigens. We tested the hypothesis that the down regulation of the immune response by UV is precipitated by the release of IL-10 after keratinocytes are UV-irradiated. After UV exposure IL-10 mRNA was upregulated. Western analysis indicated immunoreactive IL-10 was secreted by UV-exposed keratinocytes. The addition of supernatants from UV-irradiated keratinocytes to Th1 clones diminished their IFN production, whereas the addition of supernatants from normal keratinocytes had no suppressive effect on IFN production. Furthermore, treating supernatants from UV-irradiated keratinocytes with anti-IL-10 antibodies blocked the induction of immune suppression. To determine if IL-10 was responsible for the immunosuppression seen after total-body UV irradiation, UV-exposed mice were treated with anti-IL-10 antibodies. Treating UV-irradiated mice with anti-IL-10 reversed the induction of immune suppression. These findings suggest that keratinocyte-derived IL-10 was mediating UV-induced suppression in vivo. We also tested the hypothesis that UV-induced suppressor cells are Th2 cells. Mice were injected with spleen cells from either normal or UV-exposed donor mice immunized with alloantigen. At the time of spleen cell infusion, the recipient mice were then resensitized. Spleen cells from UV-exposed mice suppressed DTH. Mice treated identically and injected with anti-IL-10 antibodies were able to generate a DTH response. Taken together these data suggest that the suppressor cells that are induced by UV radiation are Th2 cells which mediate their suppressive effect by release of IL-10. ^

The effects of ultraviolet B radiation on immune responses to {\it Borrelia burgdorferi}, the causative agent of Lyme disease

Ultraviolet B (UVB) radiation, in addition to being carcinogenic, is also immunosuppressive. Immunologically, UVB induces suppression locally, at the site of irradiation, or systemically, by inducing the production of a variety of immunosuppressive cytokines. Systemic effects include suppression of delayed-type hypersensitivity (DTH) responses to a variety of antigens (e.g. haptens, proteins, bacterial antigens, or alloantigens). One of the principal mediators of UV-induced immune suppression is the T helper-2 (Th2) cytokine interleukin-10 (IL-10); this suggests that UV irradiation induces suppression by shifting the immune response from a Th1 (cellular) to a Th2 (humoral) response. These "opposing" T helper responses are usually mutually exclusive, and polarized Th1 or Th2 responses may lead to either protection from infection or increased susceptibility to disease, depending on the infectious agent and the route of infection.^ This study examines the effects of UVB irradiation on cellular and humoral responses to Borrelia burgdorferi (Bb), the causative agent of Lyme disease (LD) in both immunization and infectious disease models; in addition, it examines the role of T cells in protection from and pathology of Bb infection. Particular emphasis is placed on the Bb-specific antibody responses following irradiation since UVB effects on humoral immunity are not fully understood. Mice were irradiated with a single dose of UV and then immunized (in complete Freund's adjuvant) or infected with Bb (intradermally at the base of the tail) in order to examine both DTH and antibody responses in both systems. UVB suppressed the Th1-associated antibodies IgG2a and IgG2b in both systems, as well as the DTH response to Bb in a dose dependent manner. Injection of anti-IL-10 antibody into UV-irradiated mice within 24 h after UV exposure restored the DTH response, as well as the Th1 antibody (IgG2a and IgG2b) response. In addition, injecting recombinant IL-10 mimicked some of the effects of UV radiation.^ Bb-specific Th1 T cell lines (BAT2.1-2.3) were generated to examine the role of T cells in Lyme borreliosis. All lines were CD4$\sp+,$ $\alpha\beta\sp+$ and proliferated specifically in response to Bb. The BAT2 cell lines not only conferred a DTH response to naive C3H recipients, but reduced the number of organisms recovered from the blood and tissues of mice infected with Bb. Furthermore, BAT2 cell lines protected mice from Bb-induced periarthritis. ^

Tumors in the contralateral breast following radiation therapy for breast cancer

The proportional distribution of independent malignant tumors in the contralateral breast following treatment for breast cancer was investigated to assess the influence of scattered radiation as a cause of these tumors. In a population of 172 patients the proportion of contralateral tumors in each quadrant and the center (the nipple-areolar complex) was compared with the expected, or natural, distribution found in the general population, in the absence of radiation. The observed/expected ratio for contralateral tumors was 1.43 for the upper-inner quadrant; 0.97, lower-inner quadrant; 1.51, center; 0.76, upper-outer quadrant; and 0.64, lower-outer quadrant. In each quadrant, except the lower-inner, the observed/expected ratio differed from 1.00 with statistical significance at the 5% level (one-tail). The same analysis, stratified by age and menopausal status, showed a similar shift of tumors, with more than expected in the inner quadrants and center and less than expected in the outer quadrants, although the results did not show statistical significance at the 5% level for all strata. For each patient the mean absorbed radiation dose for each quadrant and center of the breast was estimated, based on measurements in a tissue-equivalent phantom. Among patients the doses ranged from 0.5 to 8 Gy; within individuals, doses to the inner quadrants typically were a factor of three times higher than doses to the outer quadrants. The results suggest that radiation may be a risk factor for contralateral breast tumors and warrants further investigation. ^

AN INVESTIGATION INTO THE MOLECULAR MECHANISMS OF ANDROGEN ACTION IN THE SERTOLI CELL

Steroid hormones regulate target cell function via quantitative and qualitative changes in RNA and protein synthesis. In the testis, androgens are known to play an important role in the regulation of spermatogenesis. The Sertoli cell (SC), whose function is thought to be supportive to the developing germ cell, has been implicated as an androgen target cell. Although cytoplasmic androgen receptors and chromatin acceptor sites for androgen-receptor complexes have been found in SC, effects on RNA synthesis have not previously been demonstrated. In this study, SC RNA synthetic activity was characterized and the effect of testosterone on SC nuclear transcriptional activity in vitro assessed. SC exhibited two fold increases in RNA and ribonucleotide pool concentrations during sexual maturation. These changes appeared to correlate with a previously observed increase in protein concentration per cell over an age span of 15-60 days. Following incubation with ('3)H-uridine, SC from older animals incorporated more label into RNA than SC from younger animals. Since the relative concentration of cytidine nucleotides was higher in SC from older rats, the age-related increase in tritium incorporation may reflect an associated increase in incorporation of ('3)H-CMP into RNA. Alternatively, the enhanced labeling may be the result of either a change in the base composition of the RNA resulting in a higher proportion of CMP and UMP in the RNA, or compartmentalization of the nucleotide pools. The physiologic consequences of these maturational alterations of nucleotide pools remains to be elucidated. RNA polymerase activities were characterized in intact nuclei obtained from cultured rat SC. (alpha)-Amanitin resistant RNA polymerase I+III activity was identical when measured in low or high ionic strength (0.05 M or 0.25 M ammonium sulfate (AS)) in the presence of MnCl(,2) or MgCl(,2), with a divalent cation optimum of 1.6 mM. RNA polymerase II was most active in 0.25 M AS and 1.6 mM MnCl(,2). The apparent Km of RNA polymerase II for UTP was 0.016 mM in 0.05 M AS and 0.037 mM in 0.25 M AS. The apparent Km values for total polymerase activity was 0.008 mM and 0.036 mM at low and high ionic strenghts, respectively. These data indicate that Sertoli cell RNA polymerase activities have catalytic properties characteristic of eukaryotic polymerase activities in general. In the presence of 21 (mu)M testosterone, RNA polymerase II activity increased two fold at 15 minutes, then declined but was still elevated over control values six hours after androgen addition. Polymerase I+III activity was not greatly affected by testosterone. The stimulation of polymerase II measured at 15 minutes was dose-dependent, with a maximum at 0.53 nM and no further stimulation up to 10('-5) M (ED(,50) = 0.25 nM testosterone), and was androgen specific. The results of preliminary RNA isolation and characterization experiments suggested that the synthesis of several species of RNA was enhanced by testosterone administration. These findings have great potential importance since they represent the first demonstration of a direct effect of androgens on the transcriptional process in the Sertoli cell. Furthermore, the results of these studies constitute further evidence that the Sertoli cell is a target for androgen action in the testis. ^

STUDIES OF THE MOLECULAR MECHANISMS OF CHROMOSOME ABERRATION FORMATION (DNA REPAIR, CROSSLINKS, MITOMYCIN C)

The objective of this research has been to study the molecular basis for chromosome aberration formation. Predicated on a variety of data, Mitomycin C (MMC)-induced DNA damage has been postulated to cause the formation of chromatid breaks (and gaps) by preventing the replication of regions of the genome prior to mitosis. The basic protocol for these experiments involved treating synchronized Hela cells in G(,1)-phase with a 1 (mu)g/ml dose of MMC for one hour. After removing the drug, cells were then allowed to progress to mitosis and were harvested for analysis by selective detachment. Utilizing the alkaline elution assay for DNA damage, evidence was obtained to support the conclusion that Hela cells can progress through S-phase into mitosis with intact DNA-DNA interstrand crosslinks. A higher level of crosslinking was observed in those cells remaining in interphase compared to those able to reach mitosis at the time of analysis. Dual radioisotope labeling experiments revealed that, at this dose, these crosslinks were associated to the same extent with both parental and newly replicated DNA. This finding was shown not to be the result of a two-step crosslink formation mechanism in which crosslink levels increase with time after drug treatment. It was also shown not to be an artefact of the double-labeling protocol. Using neutral CsCl density gradient ultracentrifugation of mitotic cells containing BrdU-labeled newly replicated DNA, control cells exhibited one major peak at a heavy/light density. However, MMC-treated cells had this same major peak at the heavy/light density, in addition to another minor peak at a density characteristic for light/light DNA. This was interpreted as indicating either: (1) that some parental DNA had not been replicated in the MMC treated sample or; (2) that a recombination repair mechanism was operational. To distinguish between these two possibilities, flow cytometric DNA fluorescence (i.e., DNA content) measurements of MMC-treated and control cells were made. These studies revealed that the mitotic cells that had been treated with MMC while in G(,1)-phase displayed a 10-20% lower DNA content than untreated control cells when measured under conditions that neutralize chromosome condensation effects (i.e., hypotonic treatment). These measurements were made under conditions in which the binding of the drug, MMC, was shown not to interfere with the stoichiometry of the ethidium bromide-mithramycin stain. At the chromosome level, differential staining techniques were used in an attempt to visualize unreplicated regions of the genome, but staining indicative of large unreplicated regions was not observed. These results are best explained by a recombinogenic mechanism. A model consistent with these results has been proposed.^

Molecular regulatory mechanisms and embryonic tissue specification in sea urchin development

To understand how a eukaryote achieves differential transcription of genes in precise spatial patterns, the molecular details of tissue specific expression of the Strongylocentrotus purpuratus Spec2a gene were investigated by functional studies of the cis-regulatory components in the upstream enhancer. Regional activation of Spec2a in the aboral ectoderm is conferred by a combination of activators and repressors. The positive regulators include previously identified SpOtx and a trans-regulatory factor binding at the CCAAT site in the Spec2a enhancer. The nuclear protein binding to the CCAAT box was determined to be the heterotrimeric CCAAT binding factor (SpCBF). SpCBF also mediates general activation in the ectoderm. The negative regulators consist of an oral ectoderm repressor (OER), an endoderm repressor (ENR), and an S. Purpuratus goosecoid homologue (SpGsc). OER functions to prevent expression in the oral ectoderm, while ENR is required to repress endoderm expression. SpGsc antagonizes the SpOtx function by competing for binding at SpOtx target genes in oral ectoderm, where it functions as an active repressor. Thus, SpOtx and SpGsc perform collectively to establish and maintain the oral-aboral axis. Finally, purification of ENR and OER proteins from sea urchin blastula stage nuclear extracts was performed using site-specific DNA-affmity chromatography. ^

Mechanisms involved in suppression of the elicitation of immune responses by ultraviolet light

The ultraviolet radiation (UVR) present in sunlight is the primary cause of nonmelanoma skin cancer and has been implicated in the development of cutaneous malignant melanoma. Ultraviolet radiation also suppresses the immune response. In the majority of studies investigating the mechanisms regulating UV-induced immune suppression, UV is used to suppress the induction of immune responses. Equally important, is the ability of UVR to suppress established immune responses, such as the recall reaction in humans, which protects against microbial infections. We established a murine model to help elucidate the immunological mechanisms governing UV-induced suppression of the elicitation of immune responses. 80 kJ/m2 of UVR nine days after sensitization consistently suppressed the elicitation of delayed type hypersensitivity reaction to C. albicans . We found ultraviolet A (320±400 nm) radiation was as effective as solar-simulated ultraviolet A + B (290±400 nm) in suppressing the elicitation of an established immune response. The mechanisms involved in UV-induced suppression of the induction & elicitation of the immune response are similar. For example, mice irradiated with UV after immunization generated antigen-specific T suppressor cells. Injection of monoclonal antibodies to IL-10 or recombinant IL-12 immediately after exposure to UVR blocked immune suppression. Liposomes containing bacteriophage T4N5 to the skin of mice also prevented immune suppression, demonstrating an essential role for ultraviolet-induced DNA damage in the suppression of established immune reactions. ^ In addition to damaging DNA, UV initiates immune suppression through the isomerization of urocanic acid in the epidermis. Here we provide evidence that cis-UCA induces systemic immunosuppression via the serotonin (5-hydroxyyryptamine; 5-HT) receptor. Biochemical and immunological analysis indicate that cis-UCA binds to, and activates, the serotonin receptor. Moreover, serotonin specific antibodies block UV- and/or cis-UCA-induced immune suppression. Our findings identify cis-UCA as novel serotonin receptor ligand and indicate that serotonin receptor engagement can activate immune suppression. Cumulatively, our data suggest that similar immune regulatory mechanisms are activated regardless of whether we expose mice to solar-simulated UV (UVA + UVB) radiation or UVA only, and that ultraviolet radiation activates similar immunologic pathways to suppress the induction or the elicitation of the immune response. ^