Changes in thymic export of L‐selectin+ γδ and αβ T cells during fetal and postnatal development

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Abstract
We have used the technique of in situ intrathymic injection of fluorescein isothiocyanate to examine L‐selectin expression on γδ and αβ T cells immediately after emigrating from the thymus of fetal 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‐selectin on emigrant and mature T cell subsets revealed a remarkable heterogeneity of expression, both in terms of the numbers of cells expressing this molecule as well as the level of expression. γδ T cells, reportedly not having a propensity for homing to lymph nodes, not only contained the highest proportion of L‐selectin+ cells, but also expressed far more of this molecule than either CD4+CD8− or CD4−CD8+ αβ T cells. Furthermore, those emigrant T cells expressing L‐selectin are somewhat immature in their expression of this molecule. Subsequent maturation resulted in up‐regulation of L‐selectin on mature peripheral blood T cells, maturation that was clearly independent of extrinsic antigen. This antigen‐independent post‐thymic maturation appeared to occur as part of the normal progression from immature thymocyte to mature peripheral T cell in both fetal and postnatal animals.