Changes in thymic export of γδ and αβ T cells during fetal and postnatal development

Abstract

The thymus plays an essential role in the generation and selection of T cells and exports approximately 0.5-1% of thymocytes per day in young animals and considerably fewer in older animals. To date there have been no studies directly examining fetal thymic export in any species. Using the technique of intrathymic injection of fluorescein isothiocyanate, followed by an assay for green fluorescent cells in the periphery and for the expression of cell surface antigens on these cells, we have compared directly the export of T cells from the fetal and postnatal ovine thymus. While the thymus exports both alpha beta and gamma delta T cells, our results demonstrate that the proportion of thymic gamma delta T cells that are exported per day is much higher than that of thymic alpha beta T cells. Moreover, the export rate of gamma delta T cells increased from approximately 1 in every 60 gamma delta thymocytes per day emigrating from the fetal thymus to 1 in every 20 from the postnatal thymus. In addition, we identify a population of CD5+CD4-CD8-gamma delta-. T cells emigrating from the fetal thymus but greatly reduced among thymic emigrants after birth. These findings have several implications regarding the mechanisms and control of selection of both gamma delta and alpha beta T cells.

l‐selectin expression on thymic emigrants defines two distinct tissue‐migration pathways

“…Table 1 shows the average daily thymic export rate was 0·9% thymocytes/day in young lambs, which is similar to that previously reported in sheep, 6 , 22 mice, 7 rats, 8 chickens 9 and pigs. 11 The majority of thymic emigrants were found in the spleen (72%), while blood contained 10%, and LN 18% of thymic emigrants recovered 24 hr after in situ labelling with FITC.…”

Cell death and thymic export during fetal life

“…This population constituted 20% of the cells exported from the fetal thymus each day (4.52 Â 10 7 cells). They were described initially by Witherden [17] and are not a population of CD5 + B cells (unpublished observations). SPCD8 thymocytes comprise <1% of fetal thymocytes, but account for almost half of the thymic emigrants exported per day.…”

Rapid turnover of the recirculating lymphocyte pool in vivo

“…This would suggest that 1X10 11 /29.8 or 3 36X10 9 lymphocytes divide each day, presumably within the peripheral lymphoid organs. It has been estimated that the thymic input from a 3-month-old lamb is 4.0X10 8 lymphocytes per day (36). Export rates from the thymus in more mature animals are likely to be somewhat lower.…”