CHANGES IN THYMIC EXPORT OF L-SELECTIN(-DELTA AND ALPHA-BETA T-CELLS DURING FETAL AND POSTNATAL-DEVELOPMENT() GAMMA)

We have used the technique of in situ intrathymic injection of fluores cein isothiocyanate to examine L-selectin expression on gamma delta an d alpha beta T cells immediately after emigrating from the thymus of f etal and postnatal animals. We found that the percentage of L-selectin (+) thymocytes exported per day decreased by half after birth and that the export of T cells from the thymus does not rely on expression of the peripheral lymph node homing receptor, L-selectin. Analysis of L-s electin on emigrant and mature T cell subsets revealed a remarkable he terogeneity of expression, both in terms of the numbers of cells expre ssing this molecule as well as the level of expression. gamma delta T cells, reportedly not having a propensity for homing to lymph nodes, n ot only contained the highest proportion of L-selectin(+) cells, but a lso expressed far more of this molecule than either CD4(+)CD8(-) or CD 4(-)CD8(+) alpha beta T cells. Furthermore, those emigrant T cells exp ressing L-selectin are somewhat immature in their expression of this m olecule. Subsequent maturation resulted in up-regulation of L-selectin on mature peripheral blood T cells, maturation that was clearly indep endent of extrinsic antigen. This antigen-independent post-thymic matu ration appeared to occur as part of the normal progression from immatu re thymocyte to mature peripheral T cell in both fetal and postnatal a nimals.