The thymus and negative selection

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