Activated natural killer cell-mediated immunity is required for the inhibition of tumor metastasis by dendritic cell vaccination

Immunization with dendritic cells (DCs) pulsed with tumor antigen can activate tumor-specific cytotoxic T lymphocytes (CTL), which is responsible for tumor protection and regression. In this study, we examined whether DCs pulsed with necrotic tumor lysates can efficiently prevent malignant melanoma tumor cell metastasis to the lung. DCs derived from mouse bone marrow were found to produce remarkably elevated levels of IL-12 after being pulsed with the tumor lysates. Moreover, immunization with these DCs induced CTL activation and protected mice from metastasis development by intravenously inoculated tumor cells. In addition, these DCs activated NK cells in vitro in a contact-dependent manner, and induced NK activities in vivo. Furthermore, NK cell depletion before DC vaccination significantly reduced the tumor-specific CTL activity, IFN-gamma production, and IFN-gamma- inducible gene expression, and eventually interfered with the antitumor effect of tumor-pulsed DCs. Finally, similar findings with respect to NK cell dependency were obtained in the C57BL/ 6J-bg/bg mice, which have severe deficiency in cytolytic activity of NK cells. These data suggest that the antitumor effect elicited by DC vaccination, at least in a B16 melanoma model, requires the participation of both cytolytic NK and CD8(+) T cells. The findings of this study would provide important data for the effective design of DC vaccines for cancer immunotherapy.     

Cytolytic Response to HIV in Patients with HIV Disease Treated with Extracorporeal Photochemotherapy: Preliminary Study

Extracorporeal photochemotherapy (photopheresis), an immunomodulatory therapy that targets circulating T helper lymphocytes, has been applied to the management of human immunodeficiency virus (HIV) disease. Any therapy that exerts its actions on CD4⁺ T cells has the potential of exacerbating HIV infection. Therefore, it was necessary to observe immune function during treatment. Because cytotoxic T lymphocytes (CTL) and natural killer cells are thought to play an important role in the response against HIV infection, we examined the effect of photopheresis on HIV cytolytic activity. The study group consisted of seven patients with late-stage HIV disease who had not received any previous treatment for HIV infection. Patients were treated exclusively with photopheresis on two consecutive days each month for 14–32 months (average, 25 months). Peripheral lymphocytes, collected at various points during treatment, were used as effectors in a ^Wr release assay. Epstein-Barr virus (EBV)-transformed autologous B cell lines transfected with recombinant vaccinia vectors that expressed the HIV env (gp120, gp41) and gag (p24) proteins were used as target cells. All seven patients demonstrated relatively constant levels of cytolysis (>10% above controls) during treatment in the context of stable CD4⁺ T cell counts and a stable clinical status. These results suggest that extracorporeal photochemotherapy did not impair the cytolytic response to HIV infection and may have enhanced it in some patients.     

Antitumor immunity induced by tumor cells engineered to express a membrane-bound form of IL-2

Transduction of cytokine gene into tumor cells is a promising method of tumor therapy, but the value is limited by accompanying side effects. To focus antitumor immune response to tumor antigen-specific CTL, we developed an antitumor vaccine by transfecting modified IL-2 gene in a membrane-bound form (mbIL-2) into B16F10 melanoma cells. The mbIL-2 clone showed reduced tumorigenicity and metastatic ability, and inhibited metastasis and prolonged the survival of mice against B16F10 cells. The inhibition of B16F10 metastasis by mbIL-2 was accompanied by the increment of CD8(+) T cells. The metastasis of mbIL-2 clone was significantly increased in the CD8(+) T cell-depleted mice, but not in CD4(+) T cell depleted mice. Spleen cells immunized with the mbIL-2 clone showed higher CTL activity towards B16F10 cells than those immunized with control cells. The size of CD8(+) T cell population in the lung of mice injected with the mbIL-2 clone was markedly greater than that of mice injected with B16F10 cells, but there was no detectible change in CD4(+) and CD8(+) T cell populations of lymph nodes and spleen. These results suggest that when the mbIL-2 clone is introduced into the blood stream, it migrates mainly to lung and activates CD8(+) T cells in situ, possibly by direct priming. Such a tumor vaccine may ameliorate the toxic side effects encountered with conventional cytokine gene therapy.     

Role of the Immune System in Mediating the Antitumor Effect of Benzophenothiazine Photodynamic Therapy

The role of the host immune system in contributing to tumor regression following benzophenothiazine photodynamic therapy (PDT) was examined. Photodynamic therapy with 2-iodo-5-ethylamino-9-diethylaminobenzo[a]-phenothiazinium chloride (2I-EtNBS) eradicated EMT-6 mammary fibrosarcomas in 75-100% of treated mice. In contrast, PDT failed to inhibit tumor growth in T-cell-deficient nude mice. Furthermore, T-cell depletion studies with anti-CD8 antibody revealed that the CD8+ T-cell population was critical for an effective PDT response (tumor volume 14 days post-PDT: 262 mm3 vs 59 mm3 in controls; P < 0.01). Because anti-CD4 antibody inhibited tumor growth in the absence of PDT, the role of CD4+ T cells remains unclear. Depletion of natural killer (NK) cells in vivo with anti-asialo-GM1 antibody significantly reduced a suboptimal PDT effect relative to vehicle controls (tumor volume 13 days post-PDT: 513 mm3 vs 85 mm3, respectively; P < 0.001). However, splenic NK cells obtained from PDT-treated tumor-bearing mice were not cytotoxic in vitro against EMT-6 cells, suggesting that NK cells contribute to the PDT effect in vivo by an indirect mechanism. In addition, when mice with complete tumor regression following PDT were rechallenged 28 days later with 5 x 10(5) EMT-6 cells, tumor growth was significantly inhibited as compared to controls (tumor volume 40 days postrechallenge: 137 mm3 vs 833 mm3 in controls; P < 0.03; percent animals without tumor in five experiments: 67% vs 8% in controls). Collectively, these results demonstrate that CD8+ T cells are required to prevent tumor regrowth after 2I-EtNBS-PDT, NK cells contribute to this response and such PDT can elicit protective antitumor immunity.     

Selective addition of CXCR3+CCR4-CD4+ Th1 cells enhances generation of cytotoxic T cells by dendritic cells in vitro

Increasing importance is being given to the stimulation of Th1 response in cancer immunotherapy because its presence can shift the direction of adaptive immune responses toward protective immunity. Based on chemokine receptor expression, CXCR3⁺CCR4^-CD4⁺ T cells as Th1-type cells were investigated its capacity in monocyte-derived dendritic cell (DC) maturation and polarization, and induction of antigen specific cytotoxic T lymphocytes (CTL) in vitro. The levels of IL-4, IL-5 and IL-10 were decreased to the basal level compared with high production of IFN-γ, TNF-α, and IL-2 in CXCR3⁺CCR4^-CD4⁺ T cells stimulated with anti-CD3 and anti-CD28 antibodies. Co-incubation of activated CD4⁺ or CXCR3⁺CCR4^-CD4⁺ T cells with DC (CD4⁺/DC or CXCR3⁺CD4⁺/DC, respectively) particularly up-regulated IL-12 and CD80 expression compared with DC matured with TNF-α and LPS (mDC). Although there was no significant difference between the effects of the CXCR3⁺CCR4^-CD4⁺ and CD4⁺ T cells on DC phenotype expression, CXCR3⁺CD4⁺/DC in CTL culture were able to expand number of CD8⁺ T cells and increased frequencies of IFN-γ secreting cells and overall cytolytic activity against tumor antigen WT-1. These results demonstrated that the selective addition of CXCR3⁺CCR4^-CD4⁺ T cells to CTL cultures could enhance the induction of CTLs by DC in vitro, and implicated on a novel strategy for adoptive T cell therapy.     

DEVELOPMENT OF AN in vitro SYSTEM FOR THE ANALYSIS OF ULTRAVIOLET RADIATION-INDUCED SUPPRESSION OF NATURAL KILLER CELL ACTIVITY

Previous studies have shown that natural killer (NK) cell activity was suppressed in volunteer subjects exposed to ultraviolet radiation (UVR) from solarium lamps. The present studies were carried out to determine that spectrum of UVR responsible for suppression of NK activity and to develop in vitro methods to analyze the effectivenes of sunscreen agents in prevention of UVR-mediated suppression of NK activity and other aspects of immune function. UVR from a xenon are lamp source was used to irradiate peripheral blood lymphocytes (PBL) in wells of tissue culture flasks, and transmission interference filters were used to eliminate UVR of particular wavelengths. The results indicated that UVR from this source inhibited NK activity of PBL in a dose-dependent manner with a 50% inhibitory dose of 5.5 mJ/cm² when unfiltered and 29.6 mJ/cm² when diluted through cellulose acetate, which gave a UV spectrum similar to that in solar radiation. Equivalent suppression of NK activity was mediated by UV-A (UVR > 315 nm) at dose levels of 4.2 J/cm², which was approximately 140 times greater than the amount of UV-B (UVR > 315 nm) needed to suppress NK activity. Similar dose-response curves were seen for inhibition of mitogenic responses to phytohemagglutinin except that the latter appeared less sensitive than NK to inhibition by UV-A. These studies suggest that whe the greater proportion of UV-A in solar radiation adn its greater penetration into skin is taken into account, UV-A may have equivalent or greater direct immunosuppressive effects than UV-B. The mechanisms of their immunosuppressive effects may, however, differ. The in vitro system described here would appear to provide a simple test system for further analysis of UVR-indued imunosuppression.     

Construction of Recombinant Fowlpox Virus （rFpv） Expressing HIV-1 Gag-pol Protein

IntroductionWHO estimated that currently there are more than40 million individuals living with HIV and there are16000 new individuals infected daily worldwide. HIVstimulates strong immune CD8 cytotoxic T lympho-cytes(CTL) response in the infected people…     

Therapeutic Perspectives of CD26 Inhibitors in Imune-Mediated Diseases

The enzymatic activity of CD26/DPP4 (dipeptidyl peptidase 4/DPP4) is highlighted in multiple studies to play a vital role in glucose metabolism by cleaving and inactivating the incretins glucagon-like peptide-1 (GLP) and gastric inhibitory protein (GIP). A large number of studies demonstrate that CD26 also plays an integral role in the immune system, particularly in T cell activation. CD26 is extensively expressed in immune cells, such as T cells, B cells, NK cells, dendritic cells, and macrophages. The enzymatic activity of CD26 cleaves and regulates numerous chomokines and cytokines. CD26 inhibitors have been widely used for the treatment of diabetes mellitus, while it is still under investigation as a therapy for immune-mediated diseases. In addition, CD26’s involvement in cancer immunology was also described. The review aims to summarize the therapeutic effects of CD26 inhibitors on immune-mediated diseases, as well as the mechanisms that underpin them.     

Antioxidant protection in cultured corneal cells and whole corneas submitted to UV-B exposure

Several corneal pathologies are characterized by the presence of reactive oxygen species (ROS); therefore, we evaluated the protection afforded by pirenoxine and melatonin to corneal cell culture and whole rabbit cornea from ultraviolet exposure and other oxidant systems. Rabbit cornea cell (SIRC) plates and whole corneas were exposed to UV-B (80 or 800 mJ/cm2) or incubated with fMLP-stimulated autologous macrophages, in the presence or absence of pirenoxine or melatonin (10(-5) M). The protective activity of compounds was assessed by measuring superoxide anion formation, inhibition of oxidation and mitochondrial viability. Moreover the ex vivo protective effect of pirenoxine and melatonin was verified in the whole cornea submitted to UV-B exposure in vitro. Our experimental data demonstrate that pirenoxine and melatonin were able to inhibit the superoxide formation and oxidative effect in cell culture and whole rabbit corneas submitted to UV-B exposure or to incubation with fMLP-stimulated autologous macrophages. Mitochondrial viability was restored in epithelial cells of rabbit cornea but not in SIRCs. Moreover, both compounds are also able to increase ex vivo epithelial corneal cell defences against the in vitro UV-B induced lipid peroxidation.     

Effect of ultraviolet-B radiation on intact cells of the cyanobacterium Spirulina platensis: characterization of the alterations in the thylakoid membranes

Intact trichomes of Spirulina platensis are exposed to ultraviolet- B (UV-B) radiation (270-320 nm; 1.9 mW m(-2)) for 9 h. This UV-B exposure results in alterations in the pigment-protein complexes and in the fluorescence emission profile of the chlorophyll-protein complexes of the thylakoids as compared with thylakoids isolated from control dark-adapted Spirulina cells. The UV-B exposure causes a significant decrease in photosystem II activity, but no loss in photosystem I activity. Although there is no change in the photosystem I activity in thylakoids from UV-B-exposed cells, the chlorophyll a emission at room temperature and at 77 K indicates alterations associated with photosystem I. Additionally, the results clearly demonstrate that the photosystem II core antennae of chlorophyll proteins CP47 and CP43 are affected by UV-B exposure, as revealed by Western blot analysis. Furthermore, a prominent 94 kDa protein band appears in the sodium dodecylsulfate polyacrylamide gel electrophoresis (SDS-PAGE) profile of UV-B-exposed cell thylakoids, which is absent from the control thylakoids. This 94 kDa protein appears not to be newly induced by UV-B exposure, but could possibly have originated from the UV-B-induced cross-linking of the thylakoid proteins. The exposure of isolated Spirulina thylakoids to the same intensity of UV-B radiation for 1-3 h induces losses in the CP47 and CP43 levels, but does not induce the appearance of the 94 kDa protein band in SDS-PAGE. These results clearly demonstrate that prolonged exposure of Spirulina cells to moderate levels of UV-B affects the chlorophyll a-protein complexes and alters the fluorescence emission spectral profile of the pigment-protein complexes of the thylakoid membranes. Thus, it is clear that chlorophyll a antennae of Spirulina platensis are significantly altered by UV-B radiation.     

EFFECT OF VITAMIN E ON CYTOTOXICITY, DNA SINGLE STRAND BREAKS, CHROMOSOMAL ABERRATIONS, AND MUTATION IN CHINESE HAMSTER V-79 CELLS EXPOSED TO ULTRAVIOLET-B LIGHT

The effect of pretreatment with vitamin E on cytotoxicity, DNA single strand breaks, and chromosomal aberrations as well as on mutation induced by ultraviolet-B light (UV-B) was investigated in Chinese hamster V-79 cells. Cellular pretreatment with non-toxic levels of 25 microM alpha-tocopherol succinate (vitamin E) for 24 h prior to exposure resulted in a 10-fold increase in cellular levels of alpha-tocopherol. Using a colony-forming assay, this pretreatment decreased the cytotoxicity of UV-B light. However, alkaline elution assays demonstrated that pretreatment with vitamin E did not affect the number of DNA single strand breaks caused by UV-B light. In addition, UV-B exposure produced a dose-dependent induction of chromosomal aberrations and mutations at the HGPRT locus, and neither of these actions of UV-B was influenced by pretreatment with the vitamin. These results suggest that vitamin E protects cells from UV-B-induced cytotoxicity, possibly through its ability to scavenge free radicals. The results also suggest that the extent of genotoxicity induced by UV-B light may not correlate directly with the cytotoxic action of this wavelength region in sunlight.     

AUGMENTATION OF ULTRAVIOLET ERYTHEMA BY INDOMETHACIN IN ACTINIC PRURIGO: EVIDENCE OF MECHANISM OF PHOTOSENSITIVITY

Abstract— The effect of topical indomethacin on the intensity of erythema induced by ultraviolet radiation was measured by reflectance spectrophotometry in six patients with actinic prurigo. The intensity of UV-C erythema was decreased by indomethacin in five patients. The intensity of UV-B erythema was increased by indomethacin in five patients, and UV-A erythema was increased by indomethacin in all patients. The increased inflammatory response induced by UV-B and UV-A with indomethacin application was related to erythemal sensitivity at these wavelengths. Topical indomethacin caused no change in the intensity of UV-A erythema in a group of non-photosensitive subjects.
That inhibition of cyclo-oxygenase augments the inflammatory response to ultraviolet radiation suggests that lipoxygenase metabolites of arachidonic acid may be involved in the mechanism of photosensitivity in actinic prurigo.     

Photochemically generated elemental selenium forms conjugates with serum proteins that are preferentially cytotoxic to leukemia and selected solid tumor cells

The objective of this study was to determine if and how photoproducts contribute to the antitumor effect of merocyanine-mediated PDT. A panel of barbituric, thiobarbituric and selenobarbituric acid analogues of Merocyanine 540 was photobleached, and the resulting photoproducts were characterized by absorption, fluorescence emission, mass, energy dispersive X-ray, and X-ray photoelectron spectroscopy and tested for cytotoxic activity against tumor cell lines and freshly explanted bone marrow cells. While all dyes were readily photobleached, only photoproducts of selone dyes showed cytotoxic activity. One-hour incubations with micromolar concentrations of selone-derived photoproducts were sufficient to reduce leukemia/lymphoma cells ≥10 000 fold, whereas preserving virtually all normal CD34-positive bone marrow cells. Of six multidrug-resistant tumor cell lines tested, five were as sensitive or more sensitive to photoproducts than the corresponding wild-type lines. Physicochemical characterizations of the cytotoxic activity indicated that it consisted of conjugates of subnano particles of elemental selenium and (lipo)proteins. The discovery of cytotoxic Se-protein conjugates provides a rare example of photoproducts contributing substantially to the antitumor effect of PDT and challenges the long-held view that Se in oxidation state zero is biologically inert. Agents modeled after our Se-protein conjugates may prove useful for the treatment of leukemia.     

Comparative photobiology of growth responses to two UV-B wavebands and UV-C in dim-red-light- and white-light-grown cucumber (Cucumis sativus) seedlings: physiological evidence for photoreactivation

We examined the influence of short-term exposures of different UV wavebands on the elongation and phototropic curvature of hypocotyls of cucumbers (Cucumis sativus L.) grown in white light (WL) and dim red light (DRL). We evaluated (1) whether different wavebands within the ultraviolet B (UV-B) region elicit different responses; (2) the hypocotyl elongation response elicited by ultraviolet C (UV-C); (3) whether irradiation with blue light-enriched white light (B/WL) given simultaneous with UV-B treatments reversed the effect of UV in a manner indicative of photoreactivation; and (4) whether responses in WL-grown plants were similar to those grown in DRL. Responses to brief (1-100 min) irradiations with three different UV wavebands all induced inhibition of elongation measured after 24 h. When WL-grown seedlings were irradiated with light containing proportionally greater short wavelength UV-B (37% of UV-B between 280 and 300 nm), inhibition of hypocotyl elongation was induced at a threshold of 0.5 kJ m(-2), whereas exposure to UV-B including only wavelengths longer than 290 nm (and only 8% of UV-B between 290 and 300 nm) induced inhibition of hypocotyl elongation at a threshold of 1.6 kJ m(-2). The UV-C treatment induced reduction in elongation at a threshold of <0.01 kJ m(-2) for DRL-grown plants and <0.03 kJ m(-2) for WL-grown plants. B/WL caused 50% reversal of the short-wavelength UV-B-induced inhibition of elongation in DRL-grown seedlings but did not reverse the effect of long-wavelength UV-B. B/WL caused 30% reversal of the UV-C-induced inhibition of elongation in WL-grown seedlings but did not affect the response to short-wavelength UV-B. Short-wavelength UV-B also induced positive phototropic curvature in both types of seedlings, and this was reversed 60% or completely in DRL-grown and WL-grown seedlings, respectively. The similarity of responses between the etiolated (DRL-grown) and de-etiolated (WL-grown) seedlings indicates that the short-wavelength specific response may be relevant to natural light environments, and the apparent photoreactivation implicates DNA damage as the sensory mechanism for the response.     

Antitumor Effects of Newcastle Disease Virus Hemagglutinin-neuraminidase Used as a Molecular Adjuvant

Gene therapy is a potentially powerful tool used in cancer therapy. The strength of immune responses induced by some strategies is usually low, therefore, the development of agents capable of enhancing these responses is highlighted. The authors investigated the potential of an approach based on the hemagglutinin-neuraminidase(HN) of Newcastle disease virus(NDV) as a potential immune adjuvant. It was found that recombinant adenovirus(Ad) infected SGC7901 cells expressing HN exhibited both hemagglutinin(HA) and neuraminidase(NA) activities. It was demonstrated that administration of HN induced higher levels of the effector cytokines TNF-α, IFN-α and IFN-γ and increased natural killer(NK) cell activity. Based on the therapeutic tumor model, the results show that the administration of HN with Apoptin led to improved survival and tumor suppression. In conclusion, this study indicates that HN stimulates innate immune responses to make the activity of NK cells increased, which highlights the potential adjuvant activity of HN in cancer gene therapy.     

UV-B effects on Antarctic Chlorella sp cells.

Growth of Antarctic Chlorella sp cells was measured in cultures irradiated with 30 kJ m(-2) UV-B (280-320 nm). The specific growth rate immediately after the lag phase was 0.36+/-0.06 and 0.26+/-0.03 day(-1) for unirradiated cultures and cultures irradiated with UV-B, respectively, UV-B irradiation significantly decreased ascorbate content by 54.5%, and increased the ascorbyl radical content/ascorbate content ratio by 2.25-fold in algae cultures in log phase. UV-B exposure significantly decreased by 95, 62 and 71% the content of alpha-tocopherol, beta-carotene and total thiols, respectively, in cells in log phase of development. The cellular content of alpha-tocopherol, beta-carotene and total thiols was reduced by 78, 43 and 44%, respectively in stationary phase, as compared to the antioxidant content in the cells during log phase of development. UV-B exposure reduced the content of alpha-tocopherol and total thiols in stationary phase of development by 64 and 91%, respectively, as compared to unirradiated cells. The content of beta-carotene in stationary phase was not affected by UV-B exposure. The results presented here suggest that increased UV-B radiation was responsible for the development of oxidative stress conditions, assessed as the ascorbyl radical content/ascorbate content ratio, in Antarctic Chlorella sp cells. Moreover, a significant decrease in the content of both lipid and water soluble antioxidants might contribute to establish oxidative stress in the cells.     

A screening platform for compounds with potential immuno-regulatory activities using human cord blood mononuclear cells

A systematic and combinatorial approach was adopted using human umbilical cord blood mononuclear cells (hUCB-MNCs) to screen for potential immuno-regulatory compounds. The hUCB-MNCs contain several types of immunogenic cells, which are a suitable material to mimic the in vivo immuno-response after drug treatment. hUCB-MNCs were treated with various natural products such as quercetin, astaxanthin, caffeic acid, bilobalide, eugenol, rutin and gamma-dodecalactone (gamma-DDL). Phenotypic expression analysis revealed that the subpopulation of CD3(+) T cells, CD56(+) NK cells and CD1a(+) dendritic cells apparently increased after being treated with gamma-DDL for 6 days. The expression of CD56 reached a maximum at 72 h with a dose-dependent relationship. The NK cells activation marker (CD69) also elevated following gamma-DDL treatment. These results demonstrated that the gamma-DDL has immuno-regulatory effects to enhance cord blood NK cells population and bioactivities. Such a high-throughput methodology using hUCB-MNCs may be an effective platform for systematically screening potential immuno-regulatory compounds.     

DIFFERENTIAL EFFECT OF ULTRAVIOLET-B RADIATION ON CERTAIN METABOLIC PROCESSES IN A CHROMATICALLY ADAPTING Nostoc

The impact of UV-B radiation on growth, pigmentation and certain physiological processes has been studied in a N2-fixing chromatically adapting cyanobacterium, Nostoc spongiaeforme. A brownish form (phycoerythrin rich) was found to be more tolerant to UV-B than the blue-green (phycocyanin rich) form of N. spongiaeforme. Continuous exposure to UV-B (5.5 W m-2) for 90 min caused complete killing of the blue-green strain whereas the brown strain showed complete loss of survival after 180 min. Pigment content was more strongly inhibited in the blue-green strain than in the brown. Nitrogenase activity was completely abolished in both strains within 35 min of UV-B treatment. Restoration of nitrogenase occurred upon transfer to fluorescent or incandescent light after a lag of 5-6 h, suggesting fresh synthesis of nitrogenase. Unlike the above processes, in vivo nitrate reductase activity was stimulated by UV-B treatment, the degree of enhancement being significantly higher in the blue-green strain. Like the effect of UV-B on nitrogenase, 14CO2 uptake was also completely abolished by UV-B treatment in both strains. Our findings suggest that UV-B may produce a deleterious effect on several metabolic activities of cyanobacteria, especially in cells lacking phycoerythrin. Strains containing phycoerythrin appear to be more tolerant to UV-B, probably because of their inherent property of adapting to a variety of light qualities.     

The influence of enhanced UV-B radiation on Batrachium trichophyllum and Potamogeton alpinus -- aquatic macrophytes with amphibious character

The responses of two amphibious species, Batrachium trichophyllum and Potamogeton alpinus to different UV-B environments were studied. Plant material from natural environments, as well as from outdoor treatments was examined. In long-term outdoor experiments plants were grown under three different levels of UV-B radiation: reduced and ambient UV-B levels, and a UV-B level simulating 17% ozone depletion. The following parameters were monitored: contents of total methanol soluble UV-absorbing compounds and chlorophyll a, terminal electron transport system (ETS) activity and optimal and effective quantum yield of photosystem II. No effect of the different UV-B levels on the measured parameters was observed. The amount of UV-B absorbing compounds seems to be saturated, since no differences were observed between treatments and no increase was found in peak season, when natural UV-B levels were the highest. Physiological measurements revealed no harmful effects; neither on potential and actual photochemical efficiency, nor on terminal ETS activity. The contents of UV-B absorbing compounds were examined also in plant material sampled in low and high altitude environments during the growth season. Both species exhibited no seasonal dynamics of production of UV-absorbing compounds. The contents were variable and showed no significant differences between high and low altitude populations.     

AMELIORATION OF UV-B DAMAGE UNDER HIGH IRRADIANCE. I: ROLE OF PHOTOSYNTHESIS

Sensitivity to ultraviolet-B radiation (UV-B,280–315 nm) is generally reduced when background irradiance is high. We tested the involvement of photosynthesis in the amelioration of UV-B damage by treating plants at high PAR (photosynthetically-active radiation, 400–700 nm; 1000 μmol m-² s-¹) with supplemental UV-B at double ambient levels of biologically-effective radiation (18 kJ m-²d-¹) and either “ambient” (450 μmol mol-¹) or short term elevated (750 μmol mol-¹) CO₂ levels. Responses to UV-B were assessed by photosynthetic gas exchange, leaf expansion and production of UV-absorbing compounds (presumptive flavonoids) in cultivars of cucumber (Cucumis sativus L.) previously demonstrated to be relatively sensitive (cv. Poinsett) and insensitive (cv. Ashley) to UV-B. Except for marginal leaf interveinal chlorosis observed in Poinsett, both cultivars responded similarly. UV-B had little direct effect on leaf photosynthesis, but it did cause reductions in leaf area and corresponding increases in leaf dry matter per area. Increased CO, stimulated plant growth, counteracting the effect of UV-B on leaf growth and indicating an important role for photosynthesis. In contrast, the accumulation of UV-absorbing flavonoid compounds was enhanced by UV-B exposure but was not affected by COz enrichment.