An Abscopal Effect of X-Ray upon Mouse Spleen Weight

The loss of weight of the spleen after whole-body irradiation has been shown to be a sensitive indicator of radiation effect in the mouse (1) and has been used as a means of comparing different ionizing radiations (2). This weight loss may be due to direct effects on the radiosensitive lymphoid cells of the spleen. It has also been suggested that adrenal responses may be a cause of spleen weight loss in irradiated animals (3-5). The present experiments were designed to distinguish between splenic weight changes resulting from the action of X-rays on the spleen itself and those arising ill response to radiation effects elsewhere in the body. Mole (6) has proposed the use of the term to designate effects produced in one tissue volume by irradiation of another, thus avoiding the term indirect, which has another wellestablished meaning in radiobiology at the molecular level. The following, then, is an investigation of the extent to which the weight loss of the spleen after whole-body irradiation is an abscopal effect. The basic plan was to irradiate the surgically mobilized spleens of a pair of littermate mice simultaneously, and under identical conditions, except that one member of the pair also received the X-ray dose over the entire body and the other member's body was shielded. Spleen weights were determined on the fifth postirradiation day. Any consistent difference between the shielded and the exposed members of the pairs must then be due to irradiation of tissue other than the spleen, since the spleen X-ray dose was the same for both members of the pair.