The fetal thymus stores immature hemopoietic cells capable of differentiating into non-T lineage cells constituting the thymus stromal element

Bcl11a is essential for normal lymphoid development

Bcl11a (also called Evi9) functions as a myeloid or B cell proto-oncogene in mice and humans, respectively. Here we show that Bcl11a is essential for postnatal development and normal lymphopoiesis. Bcl11a mutant embryos lack B cells and have alterations in several types of T cells. Phenotypic and expression studies show that Bcl11a functions upstream of the transcription factors Ebf1 and Pax5 in the B cell pathway. Transplantation studies show that these defects in Bcl11a mutant mice are intrinsic to fetal liver precursor cells. Mice transplanted with Bcl11a-deficient cells died from T cell leukemia derived from the host. Thus, Bcl11a may also function as a non-autonomous T cell tumor suppressor gene.

Generation of macrophages from early T progenitors in vitro

Early T progenitors in the thymus have been reported to have the capacity to develop into B cells, thymic dendritic cells, and NK cells. Here we describe conditions that induce early T progenitors to develop into macrophages. Initially, we observed that early T progenitors could be induced to develop into macrophages by cytokines produced from a thymic stromal cell line, TFGD, and later we found that the cytokine mixture of M-CSF plus IL-6 plus IL-7 also induced macrophage differentiation from pro-T cells. M-CSF by itself was unable to induce macrophage differentiation from early T progenitors. To correlate this observation with the developmental potential of early T progenitors, mouse embryonic thymocytes were sorted into four populations, pro-T1 to pro-T4, based on the expression of CD44 and CD25, and then cultured with TFGD culture supernatant. We found that pro-T1 and pro-T2 cells, but not pro-T3 and pro-T4 cells, generate macrophages. Limiting dilution analysis of the differentiation capability of sorted pro-T2 cells also confirmed that pro-T2 cells could generate macrophages. These results suggest that T cells and thymic macrophages could originate from a common intrathymic precursor.

The linkage between T-cell and dendritic cell development in the mouse thymus

Thymic dendritic cells (DC) mediate negative selection at a relatively late stage of the T-cell developmental pathway. We present evidence that the development of thymic DC and of T-lineage cells is linked via a common precursor at an early stage of thymocyte development. T-lineage precursor populations from the adult mouse thymus, prior to T-cell receptor gene rearrangement, display a capacity to produce DC as well as T cells in the thymus, and are very efficient precursors of DC in culture. These lymphoid/DC precursors have little capacity to form myeloid cells, indicating that thymic DC are a lymphoid-related rather than myeloid-related lineage. In contrast to myeloid-related DC, granulocyte-macrophage colony-stimulating factor is not required for the development of these lymphoid-related DC in vivo or in vitro. DC can develop in mutant mice lacking mature T cells, provided the common precursors are present. However, in mutant mice lacking functional Ikaros transcription factors, there are deficiencies in lymphoid precursor cells, in mature lymphoid cells and in DC.

Flt3 Ligand Plus IL-7 Supports the Expansion of Murine Thymic B Cell Progenitors That Can Mature Intrathymically

Flt3 ligand (flt3L) has potent effects on hemopoietic progenitors, dendritic cells, and B lymphopoiesis. We have investigated the effects of flt3L on intrathymic precursors. The addition of flt3L + IL-7 to lobe submersion cultures of murine fetal thymic lobes resulted in the expansion of an immature population of Thy-1low, CD44high, HSAhigh cells. This population contained cells with precursor activity, as determined by their capacity to repopulate deoxyguanosine-treated fetal thymic lobes. Upon reentry to the thymic lobe, flt3L + IL-7-cultured Thy-1low, CD44high, HSAhigh cells underwent expansion and differentiation into B cells. Two weeks after fetal thymic organ culture following thymic lobe reconstitution, intrathymic cells were Thy-1−, B220+, and a subset was sIgM+. The intrathymic B cells shared features of adult thymic B cells, including CD5 expression and proliferative responses to IL-4 + IL-5 + CD40 ligand, but not to LPS or soluble anti-IgM. Ig production was noted upon stimulation with IL-4 + IL-5 + LPS and IL-4 + IL-5 + CD40 ligand. In conclusion, we have demonstrated that flt3L + IL-7 supports the expansion of a subset of progenitors present in the fetal thymus. The cultured progenitors can repopulate a fetal thymic lobe and develop into mature functional B cells, demonstrating that the fetal thymus is able to support B cell as well as T cell development.

CD43 expression in a B cell lymphoma, WEHI 231, reduces susceptibility to G1 arrest and extends survival in culture upon serum depletion

CD43 is a major surface sialoprotein on hemopoietic cells, whose extracellular domain is heavily O-glycosylated. The functional role of CD43 in the hemopoietic system is not fully understood; however, it has been suggested that CD43 may have a role in cell-cell repulsion and in modifying T cell proliferation and activation. CD43 is expressed in immature B cells in the bone marrow, but not by peripheral B cells, except for B-1 B cells and plasma cells. To analyze the biological effect of CD43 in B-lineage cells, we transfected mouse CD43 cDNA into a CD43- B cell lymphoma, WEHI 231, and the growth and survival in culture were compared to those of a parental cell line, human CD8 transfectants, and CD43- revertants established from CD43+ clones. We observed that CD43 expression supported cell growth in culture upon serum reduction, whereas growth of CD43- cell lines was barely detected under this condition. CD43- cell lines accumulated in G1 phase of the cell cycle, and the numbers of viable cells were greatly reduced during culture upon serum depletion, whereas expression of CD43 reduced the susceptibility to G1 arrest and temporarily retarded the apoptotic process, which, in turn, resulted in an increase and maintenance of the number of viable cells in culture. The results suggest that CD43 may have some role in the survival and expansion of B-lineage cells. The biological effect of CD43 was initiated without stimulation by cross-linking and was significantly impaired by replacement of the extracellular domain by the human CD8 extracellular domain. The basis of these regulatory processes is discussed.

Early T lymphocyte progenitors

The earliest steps along the pathway leading to T cells in mice and humans are reviewed. These are the steps between the multipotent hemopoietic stem cell (HSC) and the fully committed precursors undergoing T cell receptor (TCR) gene rearrangement. At this level significant differences between adult and fetal lymphopoiesis have been demonstrated. The extent of lymphoid commitment of precursors within bone marrow is still unresolved, although HSCs clearly undergo developmental changes before migration to the thymus. Both multipotent and T-restricted precursors have now been isolated from fetal blood, suggesting both may seed the thymus. Within the thymus, several minute but discrete populations of T precursors precede the stage of TCR gene rearrangement. They include precursors that are not exclusively T-lineage committed, although they are distinct from HSCs. These precursors have a potential to form NK cells, B cells, dendritic cells, and sometimes other myeloid cells. Some factors that control early lymphoid development are discussed, including IL-7 and the Ikaros transcription factors. These will eventually help to clarify the process of T-lineage commitment.

Haemopoietic progenitor cell differentiation: flow cytometric assessment in bone marrow and thymus

We have recently shown that expression of any of the lineage-associated molecules CD2, CD7, CD10, CD19 or CD33 does not ensure lineage-commitment of CD34+ progenitor cells. Further, normal progenitor cells and leukaemic blast cells have been shown to coexpress molecules associated with more than one haemopoietic lineage. Five-dimensional flow cytometric analysis of normal bone marrow cells was exploited to investigate the hypothesis of a developmental stage in haemopoiesis comprising CD34+ cells coexpressing CD2, CD5, CD7, CD10, CD19 and CD33 or any combination of these molecules. We report on a subpopulation of CD34+ bone marrow cells constituting < 5% of the CD34+ cells and characterized by extensive coexpression of several molecules associated with the B lymphoid, T lymphoid and myeloid lineages. There is every probability that some cells display the CD34+ CD2+ CD5+ CD7+ CD10+ CD19+ CD33+ phenotype. Studies on postnatal thymocytes suggest that this may be the phenotype or one of a few phenotypes of a candidate thymus-seeding progenitor cell population. Finally, our findings that CD34+ as well as CD34+ CD5+ thymocytes can be driven into non-T-lymphoid differentiation by cytokines, support the notion that the thymus is seeded by uncommitted progenitors.

Extrathymic intestinal T-cell development: virtual reality?

Extrathymic T-cell development is a topic of considerable interest and debate, with important implications for the mechanisms of T-cell maturation and repertoire selection. Recent evidence has suggested that intraepithelial T lymphocytes (IELs) of the small intestine can mature and undergo selection in the absence of a thymus. However, IEL precursors are present in the thymus and IEL development is known to be influenced by the thymus. Here, Leo Lefrançois and Lynn Puddington discuss these data and suggest that the differentiation pathway of IEL precursors is dependent on whether or not a thymus is present.