Retarded Immunological Recovery in Sublethally X-Irradiated Mice by Additional Thymic Exposure. Reversal with Injected Marrow Cells ∗

The pronounced inhibition of immune responses following sublethal X-irradiation and the requirement of an intact thymus for the subsequent recovery of immune competence are well documented [see reviews by Taliaferro (1), and Miller (2)]. Recent reports have indicated that transplanted cells of the thymus interact synergistically with cells present in the bone marrow to elicit immuno-competence in X-irradiated animals (3–5). In vitro observations showed that thymus-marrow cell combinations can reactivate immunological competence of irradiated spleen tissue (6). In the irradiated animal, therefore, recovery of the immune capacity might be dependent not only on recovery of thymic function but also on the recovery of the immunologic stem cells of the marrow. In the present study, the effect of these 2 parameters was investigated by re-exposing the thymus of sublethally whole-body X-irradiated mice to high doses of radiation in order to further delay thymic recovery, and by augmenting the pool of marrow cells in other similarly treated animals. The immunological capability of the mice was then measured at various time intervals. The results show: (i) the residual reticular-epithelial cells of the thymus in sublethally irradiated mice are functionally radiosensitive; (ii) that both the recovery of this thymic function and the supply of marrow-derived immunogenic precursor cells are important determinants of the recovery of immunocompetence in sublethally irradiated mice.

Methods. Adult (3–4 month old) (C57LXA/He) F₁ hybrid mice bred at this Laboratory, were exposed to 500 rad whole-body X-radiation in perforated Lucite tubes placed on a rotating table. The radiation factors were as follows: 250 kVp, 15 mA, 0.5 mm Cu and 1 mm Al filters, 28 rad/min dose rate, and 40 cm TSD. Within 1 hr some of these mice were anesthetized (sodium pentobarbital) and taped into 1/8 in.