Revival: Biological Effects of Low Level Exposures to Chemical and Radiation (1992)

106 BIOLOGICAL EFFECTS OF LOW LEVEL EXPOSURES

Figure 5.8. Effect of in vitro irradiation (15 rads) of donor spleen cells upon tumor size in a

Winn assay. Mice were inoculated subcutaneously with 104 Sal cells; after 2

days the mice were killed, and their spleens irradiated or sham-irradiated and

then used in a Winn assay with an equal number of Sal cells. Results

represent mean tumor area ± S.E. Adapted from Anderson et al.23

DISCUSSION

Under appropriate experimental conditions, radiation exposure can be

associated with augmentation of the immune response. This phenomenon

can be demonstrated both in vivo and in vitro and appears to be due to the

loss of a T cell that under normal circumstances exerts a suppressive influ­

ence. According to this hypothesis, radiation-induced injury to this cell type

causes a loss of suppression and thereby results in augmentation.

As a part of normal differentiation, T cells undergo spontaneous cell

death within the thymus. This process, known as apoptosis, is thought to be

responsible for the elimination of cells with the potential of reacting against

the host and thereby eliciting an autoimmune reaction. The morphological

similarities between apoptosis and radiation-induced interphase cell death

are striking and suggest the possibility of one or several common

denominators.

Apoptosis is an energy-dependent phenomenon characterized by the con­

densation of nuclear chromatin and the fragmentation of DNA at internu-

cleosomal linker sites. As a consequence, electrophoresis of DNA obtained

from cells undergoing apoptosis yields discrete bands with multiples of 180

to 200 base pairs.25 In normal thymus cells, this phenomenon has been

shown to be due to the activation of a calcium-dependent endonuclease.26