T-CELL APOPTOSIS DETECTED IN-SITU DURING POSITIVE AND NEGATIVE SELECTION IN THE THYMUS

BECAUSE of positive and negative selection to molecules of the major h istocompatibility complex (MHC)(1), only a small proportion of the mas sive numbers of T cells generated is the thymus are selected for exper t(2,3). Immature thymocytes have a rapid turnover(2), and it has long been assumed that most thymocytes die in situ(4,5), presumably from ap optosis(6). This has yet to be proved, however, and conventional stain ing techniques have shown only minimal evidence of cell death in the n ormal thymus(7,8). Using a method for detecting cells with DNA strand breaks, we now present direct evidence for apoptosis in the normal thy mus. In sections of thymus from adult mice, apoptotic cells are scatte red throughout the cortex and are engulfed locally by F4/80(+) macroph ages. Apoptosis in the thymic cortex is not reduced in MHC-deficient m ice, which suggests that T-cell death is primarily a reflection of lac k of positive selection rather than negative selection. Direct evidenc e for apoptosis due to negative selection was obtained by crossing a V beta 5 transgenic line(9) to I-E(+) and I-E(-) mice: I-E(+) mice are known to eliminate V beta 5(+) T cells in the thymus whereas I-E(-) mi ce do not(10). In marked contrast to I-E(-) mice, the medulla of I-E() V beta 5 transgenic mice contains dense aggregates of apoptotic cell s; these cells are engulfed by a distinct population of F4/80(-) MAC-3 (+) macrophages. Negative selection of V beta 5(+) cells is thus restr icted to the medulla.