Proliferation of thymic stem cells with and without receptors for interleukin 2. Implications for intrathymic antigen recognition

We have tested the dividing cells in the mouse thymus for expression of interleukin 2 (IL-2) receptors (IL-2-R) using the rat monoclonal antibody 7D4. A discrete subpopulation of the lymphoblasts clearly expressed IL-2-R at levels comparable to those on mitogen-activated peripheral T cells. This subpopulation, however, represented a small minority of the proliferating cells. IL-2-R-bearing cells were depleted from the PNA⁺ (peanut agglutinin) lymphoblast population, which contains the direct precursors of most of the cells in the thymus. The majority of receptor-bearing cells were found in the PNA^- lymphoblast population, where they constituted only ~12% of the cells. Thus, virtually all the PNA⁺ and most of the PNA^- blast cells were in cycle without detectable IL-2-R expression. This indicates that they were not dividing in response to IL-2, and implies that they were not dividing in response to antigen, but rather to novel thymus-specific mitogenic stimuli. On the other hand, the proliferating cells that do express IL-2-R were enriched 4-5-fold in the rapidly growing neonatal thymus, suggesting that they may also play a key role in T cell development.