Studies on restoration of sensitivity of thymectomized rats to viral leukemia.

Thymectomy at 3-4 days of age prevented the induction of leukemia by virus in WF rats; grafts of neonatal thymus cor rected this deficiency. Multiple thymus grafts were much more effective than single ones. Similarly, thymectomy in rats caused a lasting reduction of lymphocyte levels and the capacity to produce hemagglutinins; these deficiencies were also corrected by thymus grafts. In restoring susceptibility to leukemia and the lymphocyte levels, 5 thymuses were more effective than a single thymus, and 10 thymuses were more effective than 5 thymuses. In restoring immunologie capacity, 5 and 10 thymuses were more effective than a single thymus. Transplantation assays of the induced lymphomas for genetic and histocompatibility factors suggested that the induced lymphoma cells were of 3 types: (a) those which possessed the transplantation pattern of the host; (6) those which had the transplantation pattern of the donor cells; and (c) those which were immunologically altered. This alteration is indicated by lack of transplantability in adult rats with transplantability in young rats and/or by regression of tumors in adult hosts. Electron micrographs revealed the presence of replicating virus in all 3 lymphomas arising in thymus grafts that were examined. The mechanism by which the thymus and the virus bring about neoplastic transformation of normal lymphoid cells remains conjectural.