Early stages in human and mouse T-cell development.

One important question in lymphopoiesis is where stem cells commit to T-, B- and natural killer (NK)-cell lineages. Recent findings in human and mouse systems suggest that the thymus is seeded by a yet uncommitted progenitor cell. The earliest murine thymic progenitor cells have the capacity to develop into B, T and NK cells when introduced into the appropriate microenvironment. The mechanisms underlying T-cell commitment are unknown, but cytokines might be involved. The gamma-chain of the interleukin (IL)-2 receptor seems to play a role in development of T and NK cells, but the current data argue against a critical role for IL-2 in T- and NK-cell development. This suggests that the IL-2 receptor gamma-chain is part of a receptor for another cytokine, important for T- and NK-cell development. IL-7 might be involved in regulating T-cell receptor rearrangements and in proliferation of cells within the thymus.