Risk assessment for the harmful effects of UVB radiation on the immunological resistance to infectious diseases

Risk assessment comprises four steps: hazard identification, dose-response assessment, exposure assessment, and risk characterization. According this scheme, we have analysed the effects of UVB radiation on basal immune functions in rats and man, and the immunological resistance to infectious diseases in rats. Non-threshold mathematical methods were used in order to estimate the risk for the human population after increased exposure to UVB radiation. These data demonstrate that UVB radiation, at doses relevant to outdoors exposure, may affect the immunological resistance to infectious diseases in human individuals. This study may also provide a basis for a strategy to assess the risk of adverse effects of exposure to immunotoxic agents.

A rat cytomegalovirus infection model as a tool for immunotoxicity testing

A rat cytomegalovirus infection model for use in immunotoxicity testing has been developed. In resistance against viruses, natural killer cells and cytotoxic T-cells play an important role. Therefore, this model complements other rat host resistance models for immunotoxicity testing, i.e. existing bacterial and parasitic infection models in which cytotoxic T-cells and natural killer cells play a minor role. Host resistance against cytomegalovirus infections in the rat was determined by titrating infectious virus levels in organs after cytomegalovirus infection in an in vitro infectivity test denoted as the Plaque Forming Unit (PFU) Test. In this test, homogenates of different organs were investigated for infectious virus titers on rat embryonic cell monolayers. We demonstrated that in the salivary gland, the major target organ for rat cytomegalovirus, virus was detectable from 8 days onward after intraperitoneal infection. To show that this model is suitable for the detection of immunotoxicity four different methods for immunosuppression were investigated: 1. gamma-irradiation, 2. congenitally athymic rats, 3. chemically induced immunosuppression, 4. ultraviolet-B (UVB) irradiation. Rat cytomegalovirus titers in the salivary glands of irradiated (500 rad 1 day prior to infection) or congenitally athymic rats were significantly increased as compared to non-irradiated rats and euthymic control rats respectively. In TOX-Wistar rats, given 20 or 80 mg bis(tri-n-butyltin)oxide (TBTO) per kg food beginning 6 weeks before cytomegalovirus infection, a regimen known to have immunotoxic effects, cytomegalovirus titers in the salivary glands were significantly increased as compared to non-TBTO-treated cytomegalovirus infected rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of in vitro exposure to ultraviolet radiation on the functional activity of lymphocytes, with emphasis on susceptibility of different species

In this study lymphocytes from blood and/or spleen of different species (rat, mouse, human) were exposed to different doses of ultraviolet radiation (UVR). The functional activity of these lymphocytes was determined using assays for mitogen proliferation and the mixed lymphocyte response (MLR). These experiments demonstrated that in vitro exposure to UVR causes a dose-dependent decrease of the MLR activity of the irradiated lymphocytes. Viability of lymphocytes and mitogen proliferation responses were also decreased by UVR exposure but less severe in comparison to the MLR. Lymphocytes of rats seem to be more sensitive to UVR as compared to lymphocytes of mice and humans.

Effects of UV-B on the resistance against infectious diseases

It is known that ultraviolet-B light (UV-B) affects human health. In addition to deleterious effects on the skin and the eyes, such as erythema, photoageing, keratitis and cataract, UV-B is also able to impair the resistance against skin-associated tumours and infections. Our data implicate that UV-B can impair the resistance against certain non-skin-associated infections in rats, such as Listeria monocytogenes, Trichinella spiralis and Ratcytomegalovirus (RCMV). Rats, infected with T. spiralis, had an increased amount of T. spiralis larvae in their carcasses after UV-B exposure in comparison to control animals, indicating that the resistance to this parasite was decreased by UV-B. Exposure to UV-B caused an increase of RCMV load in the salivary gland 26 days after infection with this virus, indicating that especially the resistance against the second generation of viruses was impaired. In L. monocytogenes-infected rats, UV-B exposure caused an increased number of bacteria in the spleen, coupled to a decreased specific response of T lymphocytes to the bacteria. We conclude that UV-B radiation may affect the resistance against several non-skin-associated infectious diseases, which is probably caused by a defect in the specific lymphocyte response to the antigen.

UV-B exposure impairs resistance to infection by Trichinella spiralis

To assess the possibility that increases in UV-B exposure on the earth's surface could lead to impaired resistance to several infectious diseases, we studied the effect of UV-B exposure on resistance against Trichinella spiralis. Wistar rats, orally infected with T. spiralis larvae, were exposed to suberythemal doses of UV-B radiation daily for 5 days at different time periods before or after infection. A significant increase in the number of Trichinella larvae was found in the carcasses of rats irradiated with UV-B between 6 and 10 days after infection. These data indicate that exposure to UV-B radiation suppresses the resistance to a parasitic infection. We suggested that UV-B radiation especially suppresses cellular immune responses against these worms because specific IgM, IgG, and IgE titers were not significantly altered by UV-B exposure. These data indicate that UV-B irradiation plays a role in the course of infection with T. spiralis, which suggests that increases of UV-B exposure might also lead to problems with other infectious diseases and might affect vaccination because of the interaction of UV-B irradiation with memory T-cells.

UV-B and the immune system. A review with special emphasis on T cell-mediated immunity

The immunosuppressive activity of ultraviolet light-B (UV-B) has become a major topic of interest, especially now that there are indications of an increased exposure to UV-B on the earth's surface, caused by a decreased thickness of the ozone layer. This review indicates that the thymus-dependent immune system is a prime target for damage by UV-B. Especially the systemic effects of UV-B on T cell mediated immunity are described and analyzed with respect to the mode of action. In summary, this review demonstrated that UV-B can alter T cell mediated immune responses by different pathways in which cytokines (e.g. TNF-alpha) and other soluble mediators (e.g. cis-urocanic acid) may play a role. Effects of UV-B on the location and morphology of different cells in the skin affect functionality of the immune system. Thus, UV-B may suppress local immunity against skin tumours and skin-associated infections as well as systemic immunity against non skin-associated infectious diseases and tumours.

Adjuvant activity of all-trans-retinoic acid in C57Bl/6 mice

The effects of all-trans-retinoic acid were investigated on the immune responses in C57Bl/6 mice after daily oral administration for one week. In selected experiments the immunosuppressive chemicals, cyclophosphamide and cyclosporin A were used in conjunction with retinoic acid. Retinoic acid stimulated the production of antibodies against sheep red blood cells and DNP-Ficoll; however, retinoic acid did not reverse the depression caused by immunosuppressive chemicals. In non-immunized animals retinoic acid stimulated the production of IL-1 but not of IL-2. The mitogenic responses of splenocytes against concanavalin A, phytohemagglutinin and pokeweed mitogen were depressed after the retinoic acid treatment; those against lipopolysaccharide were not influenced. Treatment with retinoic acid did not alter the mixed leukocyte responses but increased the activity of NK cells. Results indicate that retinoic acid may act as an adjuvant via activating macrophages, however, retinoic acid cannot reverse the immunosuppression induced by potent chemicals.