Hormetic Response to Low-Dose Radiation: Focus on the Immune System and Its Clinical Implications

The interrelationship between ionizing radiation and the immune system is complex, multifactorial, and dependent on radiation dose/quality and immune cell type. High-dose radiation usually results in immune suppression. On the contrary, low-dose radiation (LDR) modulates a variety of immune responses that have exhibited the properties of immune hormesis. Although the underlying molecular mechanism is not fully understood yet, LDR has been used clinically for the treatment of autoimmune diseases and malignant tumors. These advancements in preclinical and clinical studies suggest that LDR-mediated immune modulation is a well-orchestrated phenomenon with clinical potential. We summarize recent developments in the understanding of LDR-mediated immune modulation, with an emphasis on its potential clinical applications.

Effects of low-dose-gamma rays on the immune system of different animal models of disease

We reviewed the beneficial or harmful effects of low-dose ionizing radiation on several diseases based on a search of the literature. The attenuation of autoimmune manifestations in animal disease models irradiated with low-dose γ-rays was previously reported by several research groups, whereas the exacerbation of allergic manifestations was described by others. Based on a detailed examination of the literature, we divided animal disease models into two groups: one group consisting of collagen-induced arthritis (CIA), experimental encephalomyelitis (EAE), and systemic lupus erythematosus, the pathologies of which were attenuated by low-dose irradiation, and another group consisting of atopic dermatitis, asthma, and Hashimoto's thyroiditis, the pathologies of which were exacerbated by low-dose irradiation. The same biological indicators, such as cytokine levels and T-cell subpopulations, were examined in these studies. Low-dose irradiation reduced inter-feron (IFN)-gamma (γ) and interleukin (IL)-6 levels and increased IL-5 levels and the percentage of CD4(+)CD25(+)Foxp3(+)Treg cells in almost all immunological disease cases examined. Variations in these biological indicators were attributed to the attenuation or exacerbation of the disease's manifestation. We concluded that autoimmune diseases caused by autoantibodies were attenuated by low-dose irradiation, whereas diseases caused by antibodies against external antigens, such as atopic dermatitis, were exacerbated.

Immunomodulatory effects of ultraviolet B irradiation on atopic dermatitis in NC/Nga mice

BACKGROUND

Atopic dermatitis (AD) is a common pruritic inflammatory skin disease, which occurs primarily in childhood. Recently, narrow-band ultraviolet B (UVB) phototherapy has been used to treat AD, but the mechanism involved is unknown. In this study, we investigated whether UVB irradiation influences AD in the NC/Nga mouse.

METHODS

The mice were separated into three groups: control, AD-control (immunized with mite antigens), and AD+UVB-irradiated (immunized with mite antigens and UVB irradiation) groups. The mice in the irradiation group were exposed to 1 kJ/m(2)/day twice a week from 6 to 12 weeks of age. Animals in the control and AD-control groups were shaved, but not irradiated.

RESULTS

In the AD+UVB-irradiated group, the atopy score, ear thickness, and total immunoglobulin E (IgE) were increased in comparison with the AD-control group. On day 40, the levels of interleukin (IL)-4, IL-5, and IL-10 in the spleen lymphocytes were significantly increased compared with the AD-control group, resulting in a marked decrease of the interferon (IFN)-gamma/IL-4 ratio compared with the AD-control group. In addition, the levels of IL-6, tumor necrosis factor (TNF)-alpha, and NO(x) production by peritoneal macrophages were significantly elevated.

CONCLUSION

These results indicate that UVB irradiation promotes the development of AD-like skin lesions in NC/Nga mice.