Modification of immune response by low dose ionizing radiation: role of apoptosis
Introduction
The immune system has long been known to be highly sensitive to moderate to high doses of ionizing radiation. Immunosuppression is one of the most important causes of death in acute radiation syndrome. However, the dose effect relationship of immune functions has been found to be quite different in the low dose range, especially for doses below 50 cGy and stimulation of certain immunological functions has also been reported [1], [2], [3]. Equally interestingly, epidemiological studies show decreased cancer mortality in high natural background radiation areas compared to the control areas [4] and in nuclear power workers [5] and increased life span in chronic low dose radiation (LDR) exposed experimental animals [6] and A-bomb survivors [7].
Whole-body LDR given in acute (7.5 cGy; [3], [8]) or chronic exposures (4 cGy/day for 5–10 days; [9], [10]) has been found to enhance response of spleen cells to the polyclonal mitogen Con A as well as phytohemagglutinin (PHA) in C57BL/6 mice. The proliferation induced by the B cell mitogen lipopolysaccharide (LPS) was, however, not affected [11]. In rats too, 2.5 cGy X-ray dose has been shown to augment T cell responses to Con A and PHA [12]. In C57BL/6 mice the augmentative effect of fractionated LDR was observed to be mainly at the T cell level though the contribution of accessory cells in the irradiated animals could not be ruled out [11]. Liu et al. [8] have shown enhancement in several types of immunological activities such as MLR, ADCC, secretion of lymphokines, etc. in 7.5 cGy exposed C57BL/6 mice. Even in the A-bomb survivors significant increase in NK cell activity was observed in population exposed to relatively lower doses (<0.01–1 Gy) compared to the population exposed to higher doses [13]. It was, therefore, interesting to examine whether such an augmentation of immune responses would be observed in different strains of mice and for different types of responses following exposure to LDR.
Selective changes in the expression of proteins are reported to accompany LDR exposure. Increased levels of stress proteins were observed in LDR exposed, mitogen stimulated mouse lymphocytes [11], [14]. In PHA-stimulated as well as resting human T lymphocytes, acute LDR strongly enhanced the expression of CD2 antigen, due to transcriptional activation of CD2 gene [15]. Most importantly, an acute dose of 25 cGy was reported to induce significant accumulation of p53 in mouse adrenal glands and pancreas but not in spleen, testis or kidney [16]. It was therefore, possible that the process of apoptosis (activation induced cell death) could be affected in low dose irradiated animals. There could be differences in the interactions among the various signal transduction pathways which function in specific tissues in a co-ordinated manner [16]. Differences in the susceptibility to radiation-induced apoptosis are known among mouse strains of different genetic backgrounds [17].
The present studies were, therefore, undertaken to evaluate T cell responses to different antigens in two different strains of mice following fractionated LDR and the contribution of apoptosis and p53 to the LDR induced immunomodulation.
Section snippets
Mice
Four-week-old female mice of C57BL/6 and BALB/c strains bred and maintained in the animal breeding facility of Bhabha Atomic Research Centre were used in this study.
Irradiation schedule
The protocol developed by James and Makinodon [9] was followed. Mice were kept in perspex covered boxes and exposed to whole body γ irradiation from a 60Co source. The dose was given in fractions of 4 cGy/day for 5 days/week (Monday–Friday). The total dose administered was 20 cGy/mouse. Sham-irradiated controls were treated in the
Con A-induced proliferation in lymphocytes of C57BL/6 and BALB/c mice after LDR exposure
Table 1 shows the [3H]thymidine incorporation in the pooled spleen cells of sham-irradiated and 20 cGy irradiated mice. In each of these experiments there was no significant change in the spontaneous [3H]thymidine incorporation (in absence of con A) in the spleen cells from sham-irradiated and 20 cGy-irradiated groups of C57BL/6 or BALB/c mice. However, when stimulated with Con A for 72 h there was a significant enhancement (30–40%; P<0.05) in proliferation in the spleen cells of LDR exposed
Discussion
Effects of LDR on biological systems have attracted special attention in recent years especially in the light of several observations describing beneficial or hormetic effects of LDR [23]. This concept is supported by low cancer incidence and mortality in nuclear industry workers which could not be attributed to the healthy worker effect [24]. Furthermore, the data on different immunological parameters studied in the A-bomb survivors also suggested that various immunological functions may be
Acknowledgements
We wish to thank Dr Sumedha Sahani and Dr Maria Borges, Speciality-Ranbaxy Laboratories, Andheri (E), Mumbai for their help with the flow cytometric studies.
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