The influence of neonatal thymectomy on the development of radiation myelopathy in rats.

To investigate the possible contribution of cellular immunity in the development of radiation injury of the central nervous system, Wag/Rij rats were thymectomized at birth and irradiated to the cervical spinal cord at the age of 3 months. At the time of paralysis or at the end of the follow-up period (when rats were 1-year-old) the animals were sacrificed and the mediastinum was examined histologically. In 95% of the neonatally thymectomized animals no thymus was left. These rats showed a firm impairment of the cellular immunity, as they had a 40% reduction of the T-lymphocytes in the spleen, and a 70% reduction of the mixed lymphocyte reaction, compared to age-matched controls. Both single dose and two-fraction irradiation experiments were performed. No modification of the latency time to develop paralysis was observed comparing thymectomized and age-matched controls. The incidence of foreleg paralysis after cervical spine irradiation (single dose or two-fraction) was identically distributed in the follow-up period for both neonatally thymectomized and control Wag/Rij rats. The ED50 value derived in the single dose experiments was 20.3 Gy for the control animals, and 20.9 Gy for thymectomized rats, and in the two fraction experiments 29 Gy for controls and 29.6 Gy for thymectomized rats. None of these differences are significant. It appears that neonatal thymectomy, in spite of its firm suppression of the cellular immunity, has no major influence on the development of radiation myelopathy in rats.