INTRATHYMIC DELTA-SELECTION EVENTS IN GAMMA-DELTA CELL-DEVELOPMENT

The major pathway of gamma delta cell development is shown to be regul ated by in-frame rearrangements at the T cell receptor (TCR) delta loc us. Such ''delta selection'' occurs at or around the same point in thy mocyte development as selection for in-frame rearrangements at the TCR beta locus. However, there are at least two major differences with be ta selection: first, delta selection commonly involves selection on th e cognate TCR chain, gamma, suggesting that there is no ''preT gamma'' chain of major biological significance; second, most gamma delta-sele cted thymocytes differentiate rather than proliferate. Nonetheless, so me delta selection events seemingly facilitate thymocyte expansion, si milar to alpha beta T cell development. In these cases, TCR gamma sele ction is less obvious. Furthermore, the capacity of individual gamma c hains to facilitate gamma delta selection is shown to vary with develo pmental age. The results further clarify early T cell development at t he beta selection/delta selection stage and place clear constraints on models of cell fate determination.