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MECHANISMS OF APOIPTOSIS INDUC TION AND SUPPRESS ION IN RAT THYMOCYTES BY UV-IRRADIATION. I.Lonskaya*, V.Afanasyev#, V.Pechatnikov#, V.Dolgachev#, N.Nicolaeva# *Institute of Cytology, St.-Petersburg, Russia # Institute of Cell Biophysics, Pushchino (Moskow Region), Russia Last time great interest has been focused on investigation of the effects of UV-irradiation in cells of immune system. Data from recent investigations says about contradictirity of UV-action: in many articles immunostimulative and immunosuppressive effects of UV has been described [1, 2]. A lot of investigations is dedicated to a proof of carcinogenic effect of UV [3, 4, 5], and sucsessful experience of using of UV-irradiation in cancer treatment has been described simultaneously. Physiological cell death such as that occuring during differentiation in the immune system, normal cell turnover, and regulation of tissue homeostasis is often recognizable by tipical morfological and biochemical characteristics which define a mode of cell death known as apoptosis [6]. Apoptosis is declared as an essential element of the regulation of normal development and functional activity of immune system cells. Recently it has been speculated that apoptosis might serve protective role to prevent tumorogenicity. Apoptosis has been reported an effitient mechanism to eliminate cells that have sustained mutations resulting in uncheduled division and cancer risks [7]. Due to this fact we decided to investigate the effects of UV -irradiation on apoptosis realization. Induction of apoptosis in some lymphoid lines by UV has been described early [8, 9, 10], but the molecular mechanisms of this effect are not understanded completely yet. We begun present work to investigate molecular mechanisms of UV-induced apoptosis. We have chosen rat thymocytes as a more good investigated experimental model in apoptosis investigation. Thymocytes suspensions were prepared from thymuses of mal rats (200g). Dead cells in thymocytes population were registered as a cells with DNA content <2C by DNA-flow cytometry after dyeing by Hoecst 33258. Type of cell death (apoptosis or necrosis was determinated from registration both cells with degradated chromatin and cells with permeabeled membrane ( as described [11]). Membrane permeability was monitored by flow cytometry after dyeing by supravital dyes (propidium iodide).