The thymus and negative selection.

Self-tolerance induction is largely a reflection of negative selection (deletion) of autoreactive T cells in the thymus. Evidence is presented that negative selection occurs at a relatively late stage of thymocyte differentiation and affects a population of semimature HSA(hi) CD4+8- cells found in the medulla. Negative selection involves a number of cell-surface molecules on T cells, including Fas, CD28, CD5, and CD43. These molecules appear to act in consort, thereby ensuring that negative selection is highly efficient.