HEALTH EFFECTS OF LOW DOSE IONIZING RADIATION: NON-LINEAR DOSE-RESPONSE RELATIONSHIP IN THE IMMUNE SYSTEM

摘要

The health effects of low dose radiation (LDR) have been the concern of thernacademic spheres, regulation bodies, governments and the public. Among theserneffects the most important is carcinogenesis. Currently the linear no-thresholdrn(LNT) hypothesis is adopted for cancer risk assessment, which is based onrnobservations from the carcinogenic effect of high doses of ionizing radiation (IR).rnHowever, data from inhabitants in areas of high natural radioactivity and workersrnin nuclear industry have not shown increase in cancer risk after exposure to LDR.rnThe LNT hypothesis emphasizes the stochastic nature of DNA damage caused byrnIR. But the defense and adaptive reactions in the body are often activated by LDRrnand suppressed by high dose radiation (HDR). Among these reactionsrnantioxidants, DNA repair, apoptosis of damaged cells, adaptive response inrnmutagenesis and immune surveillance are important ones. The present paper willrnconcentrate on the dose-response relationship of the immune system after IR. Therndose-response relationship of the changes in different cell types of the immunernsystem after whole-body irradiation is analyzed on the basis of systemic datarnfrom the author's laboratory in combination with recent reports in the literature.rnFor T lymphocytes J- or inverted J-shaped curves are usually demonstrated afterrnirradiation while for macrophages curves of sustained stimulation with irregularrnpatterns are often observed. The intercellular reactions between the antigenrnpresenting cell (APC) and T lymphocyte (TLC) in the immunologic synapse andrnsecretion of cytokines by the two cell types after different doses of radiationrntogether with the signal transduction thus initiated are illustrated to explain thernmechanism of the phenomenon of low dose stimulation and high dosernsuppression of immunity. Experimental and clinical data are cited to show thatrnLDR retards tumor growth, reduces metastasis, increases the efficacy ofrnconventional radiotherapy and chemotherapy as well as alleviates the suppressionrnof immunity due to tumor burden. The incidence of thymic lymphoma after highrndose radiation is lowered by pre-exposure to LDR and its mechanism is supposedrnto be related to low dose stimulation of immunity. Recent reports on lowering ofrnstandardized cancer mortality rate and total death rate of cohorts occupationallyrnexposed to low dose radiation from the US, UK and Canada are cited.