Immunological mechanism of low-dose priming radiation resistance in walker-256 tumor model mice

Abstract

The aim of the present study was to investigate whether low-dose priming radiation induces antitumor immunity that can be augmented by the modulation of natural killer (NK) cell and cytokine activity using a mouse tumor model. Walker-256 cells were injected into the right flank of male BALB/c mice. At 7 days after inoculation, mice were divided into three groups, including group 1,2,3. In group 1 the mice were without radiation, in group 2 the mice were received 2 Gy radiation only, and in group 3 the mice were radiated with a priming dose of 75 mGy followed by 2 Gy radiation after 24 h. On day 21 following the radiation, the tumors were removed and the tumor index (tumor weight as a percentage of body weight) was calculated. At 1, 7, 14 and 21 days following the 2 Gy radiation, mouse splenocytes were isolated to analyze the NK activity and measure the production of the cytokines interleukin-1β, interferon-γ and tumor necrosis factor-α by ELISA. Apoptosis was also measured by flow cytometry. The results demonstrated that priming radiation significantly delayed the tumor growth and prolonged the median survival time to 38 days compared with the 31-day survival in the 2 Gy radiation group. The percentage of apoptotic cells was significantly higher in the mice that received 75 mGy + 2 Gy radiation compared with that in the mice that received 2 Gy alone; by contrast, mice that were not irradiated exhibited a relatively low level of apoptosis. The primed mice had a higher level of NK activity as compared with the mice exposed to 2 Gy radiation only or mice that were not irradiated. Furthermore, cytokine expression remained at a higher level in mice receiving priming dose of radiation compared that in the mice receiving only 2 Gy radiation. In conclusion, the results indicated that low-dose priming X-ray radiation may enhance the NK activity and the levels of cytokines, and that the immune response serves an important role in anticancer therapy, including radiotherapy.

Keywords: antitumor; apoptosis; cytokines; low-dose radiation; natural killer activity; radiation resistance.

Figure 1.

Effect of radiation on the growth of tumors in mice. The effect is presented in term of the tumor index (mg/g) at day 21 after tumor induction. Group 1 comprises of mice with no exposure, group 2 comprises mice exposed to 2 Gy radiation, and group 3 comprises mice exposed to priming 75 mGy + 2 Gy radiation. Data are presented as the mean ± standard deviation (8 mice were in each group). ^#P<0.05 vs. group 2.

Figure 2.

Effect of radiation on apoptosis in terms of the percentage of cell apoptotic cells. Group 1 represents tumor model mice with no exposure to radiation, group 2 represents mice exposed to 2 Gy radiation and the group 3 represents mice exposed to 75 priming mGy + 2 Gy radiation. Data are presented as the mean ± standard deviation (5 mice were in each group at each time point). *P<0.05 vs. group 1; ^#P<0.05 vs. group 2.

Figure 3.

NK cell activity in the tumor model mice treated with radiation. Group 1 represents tumor model mice with no exposure, group 2 represents mice exposed to 2 Gy and group 3 represents mice exposed to priming 75 mGy + 2 Gy. Data are presented as the mean ± standard deviation (5 mice were in each group at each time point). ^#P<0.05 vs. group 2. E:T, effector cells:target cells; NK, natural killer.

Figure 4.

Cytokine expression levels in the peripheral blood of tumor model mice exposed to radiation. Alterations in the (A) IL-1β, (B) IFN-γ and (C) TNF-α expression levels are shown. Group 1 represents tumor model mice with no exposure, group 2 represents mice exposed to 2 Gy and group 3 represents mice were exposed to priming 75 mGy + 2 Gy. Data are presented as the mean ± standard deviation (5 mice were in each group at each time point). *P<0.05 vs. group 1; ^#P<0.05 vs. group 2. IL, interleukin; IFN, interferon; TNF, tumor necrosis factor.