Thymic control in expression of natural killer activity in AKR and C57BL/6 mice

The natural killer (NK) activity of 2-month old AKR mice was markedly depressed compared with that of C57BL/6 mice of the same age. It decreased with age in C57BL/6 mice, whereas it increased and reached a maximum at 5 months in AKR mice. When the animals developed leukaemia, NK activity completely disappeared. A considerable increase in NK activity was observed in AKR and C57BL/6 mice which had been thymectomized at 30 days of age. This activity remained at a high level for 3 months and slowly decreased afterwards. The NK activity of C57BL/6 mice fell after sublethal irradiation (1.75 Gy X 4) and did not recover within 12 weeks, except in mice restored with bone marrow (BM). When mice were thymectomized prior to radiation exposure, NK activity remained at a normal level and BM restoration resulted in a weak but significant enhancement. NK activity, which was already low in AKR mice, was not significantly modified by sublethal irradiation, whether the animals were previously thymectomized or not. In this strain, BM restoration increased the NK activity after sublethal irradiation, and a higher level was reached in thymectomized animals than in intact ones. Lethally irradiated mice restored with bone marrow displayed a high level of NK activity, which was comparable in both strains and was augmented by thymectomy. Taken together, these results indicate that spleen NK activity is, at least in part, controlled by the thymus. The increased NK activity in thymectomized animals might result from the decline or inactivation of a thymus-dependent suppressive activity. The low NK activity observed in AKR mice seems to be related to a thymus-mediated suppressive effect rather than to a defect of BM in NK precursors.