Cell proliferation and thymocyte subset reconstitution in sublethally irradiated mice: compared kinetics of endogenous and intrathymically transferred progenitors.

Abstract

After sublethal (6 Gy) whole-body irradiation, the C57BL/Ba (Thy-1.1) murine thymus regenerated in two waves, on days 3-10 and 25-32, separated by a severe relapse. The second phase of depletion-reconstitution reproduced the first one, in a less synchronous manner. The depletion affected all cell subsets, but CD4+ CD8-cells decreased later than immature cells. Cell proliferation, measured by BrdUrd incorporation, started on day 3 after irradiation and concerned CD4-CD8-, CD4-CD8+, and CD4+ CD8+ cells, sequentially. CD4+ CD8-cells never represented a significant percentage of cycling cells. When irradiation was immediately followed by an intrathymic inJection of 105 C57BL/Ka (Thy-1.2) bone marrow cells, the relapse in thymus reconstitution was no longer observed. Detected with anti-Thy-1.2 antibodies, donor cells started cycling on day 14 and showed only one wave of proliferation. In these chimeras, recipient thymocytes behave exactly like thymocytes ofsolely irradiated mice. Intrathymically transferred CD4-CD8-thymocytes (105) showed the same proliferation kinetics as endogenous cells, with a peak in number on day 10 but completely disappeared from the thymus on days 14-21. These data reflect maturational differences between intrathymic and bone marrow precursor cells and suggest different radiosensitivities not linked to proliferative status. The resting state of the thymus immigrants was shown by the absence of Thy-i acquisition by bone marrow cells continuously labeled for 10 days with BrdUrd in vivo before intrathymic transfer. When such labeled bone marrow cells were injected in the thymus, only the minor BrdUrd-subset gave rise to Thy-1+ cells.All thymocyte subsets represent intermediary stages of the T-cell differentiation pathway, which can be followed in particular by CD4 and CD8 surface expression (1,2). In the fetus (3-5) as well as in the adult (6), all thymocytes develop from exogenously produced precursors that colonize the organ. In the adult steady-state thymus, the study of the properties of recently immigrated precursors and of the early events leading to differentiation is rendered difficult by the asynchronous coexistence of successive steps and by the fact that these precursors represent a very minor subset.Cell proliferation is an essential event in T-cell differentiation (7,8), and it is clear that cell activation is the first in a cascade of events driving thymocyte precursors (CD4-CD8-) to cortical thymocytes (CD4+ CD8+) and to mature medullary cells (single positive, essentially CD4+ CD8-) (9-12). Treatment of mice by specific cell-proliferation inhibitors leads to transient cessation of cell generation and of differentation. Thymus regeneration then takes place by a synchronous development of progenitors. By combining DNA synthetic labeling and cell-surface phenotype analysis, we have been able to elucidate the early events leading to sequential differentiation of thymocyte subsets (11).Irradiation has been used widely to deplete the thymus; although its effects are not restric...

Hematopoietic reconstitution after lethal irradiation and bone marrow transplantation: effects of different hematopoietic cytokines on the recovery of thymus, spleen and blood cells

“…HPC usually reside in the bone marrow for 2 or 3 weeks before migrating to the thymus, where they remain quiescent for another 10-14 days before cycling. 23 Thus, treatment of irradiated mice with hematopoietic cytokines significantly shortens the time required for the repopulation of the thymus by the injected bone marrow cells. Indeed, previous findings 24 suggest that early acting hematopoietic growth factors are able to reduce the time required by HPC to go through the G 1 phase of the cell cycle.…”

Long-Term Retention of Mature NK1.1+ NKT Cells in the Thymus

“…In such instances, host-derived T cell precursors and progenitors are largely ablated, but some cells persist to produce a limited chimerism between host-and donor-derived, hemopoietically derived cells (26,27). In the endogenous thymus, this creates a circumstance similar to that in the transplanted thymus, where surviving endogenous thymocytes are exported or otherwise lost as donor-derived progenitors enter and repopulate the thymus microenvironment.…”

Flow Cytometric Analysis of Thymocyte Subpopulations in Mice after Whole-Body X-Irradiation.

“…From 96 to 168 hr after irradiation, the number of CD4 + 8 + cells increased from 1.06 × 10 4 to 64.75 × 10 4 , while the number of CD4 -8 -cells remained unchanged. The increased number of CD4 + 8 + cells may be differentiated from CD4 -8 -subset [3,14].…”