CD34-expressing human thymocyte precursors proliferate in response to interleukin-7 but have lost myeloid differentiation potential

Abstract

CD34 is a marker for pluripotent stem cells also present on lineage- committed hematopoietic progenitors from bone marrow and a subpopulation of immature thymocytes. To characterize these early immature thymocytes, we have studied 24 pediatric thymus samples for CD34/7 expression. Three subpopulations could be defined from these T- cell receptor (TcR-) immature thymocytes: CD34+7++ (12.0 +/- 5.8), CD34- 7++ (12.6 +/- 8.6), and CD34-7+ (71.5 +/- 17.0%). CD7++ represents upregulation of this antigen and is expressed by cells of a blast-like morphology. Three-color flow cytometric analysis of these three subsets suggests the following ordered differentiation sequence: CD34+7++1-4-8- 45RA+-->CD34+7++1+ 4+8-45RA+/- -->CD34-7++1+4+8-+45RO+-->CD34- 7+1++4+8+45RO+. Early immature thymocyte cell division is essential in the thymus to generate a large number of precursors before the initiation of the selection process. We observed that both CD2 as well CD28 activation pathways were inefficient to serve as costimulant with phorbol ester 12-O-tetradecanoyl phorbol 13-acetate or interleukin-2 (IL-2) to induce the proliferation of the three CD34/7 subsets isolated by cell sorting. However, whereas IL-1, IL-2, IL-3, IL-4, granulocyte colony-stimulating factor, and granulocyte-macrophage colony- stimulating factor were ineffective, IL-7 was a potent cytokine, alone or in synergy with stem cell factor (SCF) to induce immature thymocyte proliferation. The proliferation induced by IL-7 or IL-7 + SCF is restricted to the CD34+ cells and, after 4 or 8 days of culture with IL- 7, some CD34+7++ acquire the expression of CD4 and/or CD8, but remain CD3/TcR-. We also tested the myeloid differentiation capacity of these CD34 immature thymocytes. Using two different approaches, myeloid colony formation in methylcellulose and limiting dilution analysis in the presence of myeloid growth factors, we were unable to detect myeloid differentiation capacity from CD34+ early thymocytes, whereas CD34+7+ from bone marrow contained about 10% of the clonogenic cells present in the CD34+7- fraction. Together, these data support the concept that thymic CD34+7++ represents the earliest thymic subset of fully committed T-lineage cells, capable of proliferating specifically to IL-7.

The Majority of Lymphocytes in the Bone Marrow, Thymus and Extrathymic T Cells in the Liver Are Generated In Situ from Their Own Preexisting Precursors

“…Many investigators concluded that c-kit+ stem cells migrate very slowly to the thymus (4). This consideration comes from the fact that c-kit+ cells are present in the thymus but are not able to reconstitute other hematopoietic cells when c-kit+ cells are injected into lethally irradiated mice (5,13,17,20). In a recent study, we confirmed this result.…”

“…Although the latter interpretation appears to be unnatural, there is one reason why such interpretation is possibly valid. c-kit Lin cells which are isolated from the thymus do not seem to be able to reconstitute lymphocytes in the thymus nor in other organs (5,13,17,20). We further investigated this subject by using a parabiotic method.…”

Neoplastic thymic epithelial cells of human thymoma support T cell development from CD4−CD8− cells to CD4+CD8+ cellsin vitro

“…In the human, CD34 þ CD4 ¹ CD8 ¹ cells express IL-7R a-chain and common g-chain in the normal thymus [22], and their cultures in the presence of IL-7 show excellent viability of the lymphocytes. Although some studies have previously demonstrated that CD4 ¹ CD8 ¹ cells cultured with IL-7 alone differentiated to TCRab ¹ CD4 þ CD8 þ cells [23][24][25], we could not find significant induction of CD4 þ CD8 þ cells in cultures of CD4 ¹ CD8 ¹ cells with IL-7 alone. However, in these cultures we noted the appearance of a minor proportion of CD4 single-positive cells which are intermediates between the CD4 ¹ CD8 ¹ cells and CD4 þ CD8 þ cells.…”

CD135 (Flk2/Flt3) Expression by Human Thymocytes Delineates a Possible Role of FLT3‐Ligand in T‐Cell Precursor Proliferation and Differentiation

“…CD7 high , CD3 À CD4 À CD8 À triple negative thymocytes were used as a source of autologous Tcell precursors, because this triple negative population express high levels of CD135. CD7 high thymocytes were shown to contain one of the most immature population in the human thymus [15,16,19], although this population is heterogeneous with respect to CD34 expression. In fact, 70 6 10% of CD7 high CD3 À CD4 À CD8 À thymocytes expressed the CD34 antigen.…”

“…The proliferation of CD7 high T-cell precursor in TSCC stimulated by FL, IL-7 or SCF alone were compared to unstimulated control cultures. The IL-7 and SCF were used as positive controls because other showed authors [15] that these two cytokines induced the proliferation of intrathymic T-cell precursors. The T-cell precursor proliferation was assessed by counting the number of cells harvested from the culture at each half depletion of the medium.…”