The understanding of human thymic function and evaluation of its contribution to T cell homeostasis are matters of great importance. Here we report the development of a novel assay to quantitate the frequency and diversity of recent thymic emigrants (RTEs) in the peripheral blood of humans. Such cells were defined by the presence of T cell receptor (TCR) rearrangement deletion circles (DCs), episomal byproducts of TCR-β V(D)J rearrangement. DCs were detected in T cells in the thymus, cord blood, and adult peripheral blood. In the peripheral blood of adults aged 22 to 76 years, their frequency was highest in the CD4+CD45RA+ CD62L+ subpopulation of naive T cells. TCR DCs were also observed in other subpopulations of peripheral blood T cells, including those with the CD4+CD45RO−CD62L+ and CD4+CD45RO+CD62L+ phenotypes. RTEs were observed to have more than one Vβ rearrangement, suggesting that replenishment of the repertoire in the adult is at least oligoclonal. These results demonstrate that the normal adult thymus continues to contribute, even in older individuals, a diverse set of new T cells to the peripheral circulation.

It is still uncertain whether multilineage hematopoietic progenitor cells are affected by human immunodeficiency virus-1 (HIV-1) infection in vivo. The SCID-hu Thy/Liv model is permissive of long-term multilineage human hematopoiesis, including T lymphopoiesis. This model was used to investigate the effects of HIV-1 infection on early hematopoietic progenitor function. We found that both lineage-restricted and multilineage hematopoietic progenitors were depleted from grafts infected with either a molecular clone or a primary isolate of HIV-1. Depletion of hematopoietic progenitors (including CD34+ cells, colony-forming units in methylcellulose, and long-term culture-initiating cells) occurred several days before the onset of thymocyte depletion, indicating that the subsequent rapid decline in thymocyte numbers was due at least in part to loss of thymocyte progenitors. HIV-1 proviral genomes were not detected at high frequency in hematopoietic cells earlier than the intrathymic T-progenitor cell stage, despite the depletion of such cells in infected grafts. Proviral genomes were also not detected in colonies derived from progenitor cells from infected grafts. These data demonstrate that HIV-1 infection interrupts both lineage-restricted and multilineage hematopoiesis in vivo and suggest that depletion of early hematopoietic progenitor cells occurs in the absence of direct viral infection.

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