Thymic nurse cells: differentiation of thymocytes within complexes.

Thymic nurse cell complexes (TNC-c) were isolated from thymuses of BDF1 mice at pre-determined intervals during the 12-week latency period that precedes the development of leukemias. T-cell leukemias were induced by a single i.v. injection of 50 mg/kg of methylnitrosourea (MNU). In order to clarify processes taking place in TNC-c, the complexes of mice after MNU injection were compared with TNC-c of age-matched control mice, with respect to their number per thymus, the distribution of TNC-c according to their size (the number of intra-TNC thymocytes reflects the type of TNC-c), the number of intra-TNC thymocytes that undergo DNA synthesis, and the phenotype of thymocytes inside TNC-c. During the latency period of leukemogenesis, the effects of MNU were shown to involve, in addition to changes in number of TNC-c, a decrease in the number of thymocytes incorporating labeled thymidine, viz., the number of dividing cells, thus affecting the size distribution of TNC-c types. Intra-TNC thymocytes of control mice were heterogeneous in their phenotype and represented cells at varying stages of their maturation cycle. MNU administration was followed by selective differentiation of thymocytes within TNC-c to Lyt 1-thymocytes in some and to Lyt 2-thymocytes in others. Lyt 1 and Lyt 2 being specific antigens expressed by thymocytes.