The earliest progenitor cells in the thymus have the capacity of developing into multiple hematopoietic lineages. Upon further development within the thymus, these progenitor cells progressively lose their multipotentiality (1). There is considerable evidence that the human (2) and the mouse thymuses (3, 4) contain bipotential T/NK progenitors. However, given the paucity of mature NK cells in the thymus, the thymic environment strongly favors development of these bipotential T/NK progenitor cells into T cells. The molecular mechanisms controlling T/NK lineage specification in the thymus are not yet elucidated, but it seems fair to assume that this developmental choice is under transcriptional control.
Several transcription factors that orchestrate lymphoid development have now been identified in the mouse. Ikaros is a key factor that affects development of all lymphoid (T, NK, and B) cells (5). Several other factors appear to be critical for development of specific lymphoid lineages like GATA-3 (6) and TCF-1 (7) for T cells and Pax5 (8), EBF (9), Sox4 (10), and the products of the E2A gene (11, 12) for B cells. The E2A proteins E12 and E47 belong to the family of basic helix loop helix (bHLH) transcription factors, which are involved in differentiation of many cell types (13). E12 and E47 probably regulate B cell development by controlling expression of immunoglobulin, RAG, and a number of B cell–specific genes including mb-1,
5, CD19, and Pax5 (11, 12, 14). Transcriptional activity of bHLH factors is controlled by the inhibitor of DNA binding (Id) proteins. This family of HLH factors comprises four members, Id1, 2, 3, and 4, which are highly homologous in their HLH domains and have distinct tissue distributions (15, 16). Id factors can heterodimerize with bHLH factors, but lack a basic DNA binding domain, and therefore they block transcriptional activity by bHLH factors. Constitutive overexpression of Id1 under control of a B cell–specific mb-1 promotor leads to inhibition of B cell development (17), similar to the one observed in E2A-deficient animals (11, 12).
Recent evidence has suggested a role for bHLH factors in development of T cells as well. E2A-deficient mice were initially reported to have no gross abnormalities in T cell development, but the size of the thymus is smaller than that of wild-type animals (12). Mice deficient for another bHLH factor, HEB, displayed a partial block in T cell development at an early stage of development (18). Whether HEB and E2A deficiencies affect NK cell development is not known.
In this paper we have investigated the role of bHLH factors in human T and NK cell development. We have made use of retrovirus-mediated gene transfer to enforce expression of one of the Id proteins, Id3, in CD34+ progenitors using a bicistronic vector harboring Id3 and the marker gene green fluorescent protein (GFP). Id3 can interact with a wide range of bHLH factors and blocks transcriptional activity of E12, E47, and HEB (19). We found that enforced expression of Id3 strongly inhibits development of CD34+ progenitors to CD3+ T cells in an in vitro fetal thymic organ culture (FTOC), but promotes development of NK cells.
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Materials and Methods
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Construction of the Vectors and Transduction of Target Cells.
Previously we have successfully used bicistronic vectors with a gene of interest linked to a downstream internal ribosomal entry site (IRES) and a marker gene that allow independent translation of the products of both genes in the transduced target cells (20). The IRES-GFP sequence was ligated into the LZRS vector (21), and a polylinker was placed downstream of the gag and upstream of the IRES sequences. The Id3 coding sequence was cloned from the pCDNAId3 (gift of Dr. C. Murre, University of California at San Diego, San Diego, CA) plasmid by PCR using oligonucleotide primers with appropriate linkers. The product was ligated between the XhoI and SnaBI site of the polylinker from our plasmid LZRS-linker-IRES-GFP, to obtain the retroviral vector LZRS-Id3-IRES-GFP. (A modified version of GFP [enhanced GFP] was used in this study and was obtained from Clontech, Palo Alto, CA.) Helper-free recombinant retrovirus was produced after transfection into a 293T-based amphotropic retroviral packaging cell line, Phoenix (21). Purified CD34+ cells were cultured for 24 h in the presence of 10 ng/ml human IL-7 and 10 ng/ml stem cell factor (SCF) (both from R&D, Abingdon, UK). The cells were then transduced by overnight incubation with virus supernatant in the presence of 10 µg/ml Dotap (Boehringer Mannheim GmbH, Mannheim, Germany).
Isolation of CD34+ Cells from Fetal Liver and Postnatal Thymus.
Fetal liver was obtained from elective therapeutic abortions. Gestational age was determined by crown–rump length and ranged from 14 to 17 wk. The use of this tissue was approved by the medical ethical committee of the Netherlands Cancer Institute and was contingent upon informed consent. Human fetal liver cells were isolated by gentle disruption of the tissue by mechanical means, followed by density gradient centrifugation over Ficoll-Hypaque (Lymphoprep; Nycomed Pharma, Oslo, Norway). The CD34+ cells were isolated from these samples by immunomagnetic cell sorting, using a CD34 separation kit (varioMACS; Miltenyi Biotec, Bergisch Gladbach, Germany) and were further purified by sorting with a FACStar plus® (Becton Dickinson, San José, CA). Postnatal thymus samples were from children undergoing open-heart surgery. CD34+ cells, enriched by immunomagnetic cell sorting, were labeled with anti-CD34 (HPCA-2; Becton Dickinson) and anti-CD1a (Coulter, Hialeah, FL) antibodies and the CD34+CD1a– cells were purified by FACS® sorting. The purity of the populations used in this study was >99%.
Fetal Thymic Organ Cultures.
In vitro development of human T and NK cells from CD34+ fetal liver cells or thymocytes was studied using the hybrid human/mouse FTOC using fetal thymuses from embryos of RAG-1–deficient mice as described (22). TCR-
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+ and NK cells were expanded in Yssels medium (23) from the FTOC cultures using a feeder cell mixture as described previously (24).
NK Cell Assays.
10,000 CD34+ sorted cells were cultured in Yssels medium in the presence of 10 ng/ml SCF, 10 ng/ml IL-7, and 100 U/ml IL-2 (Eurocetus, Amsterdam, Netherlands). After 1 wk, cells were harvested, counted, and analyzed by FACS®, using anti-CD3 and anti-CD56 (provided by Dr. J.H. Phillips, DNAX, Palo Alto, CA). The cytotoxic activity of NK cells was determined with a standard 51Cr–release assay using cells of NK-sensitive (K562) and insensitive (the EBV-transformed B cell line EBV225) cell lines as target cells.
Analysis of D-Jβ Rearrangements.
D-Jβ rearrangements were determined by PCR as described previously (25). Primers and probes used for TCR-β rearrangement were: 5-TGGTGGTCTCTCCCAGGCTCT-3' (Dβ1.1), 5'-CCAGCTGTCCAGCCTTGACTT-3' (Jβ1.3-1.4) and 5'-CAAAGCTGTAACATTGTGGGGAC-3' (Dβpan probe). To control for the amount of DNA in the PCR, genomic amplification of the RAG-2 gene was performed. Primers and probe used were: 5'-TGTGAATTGCACAGTCTTGCCAGG-3' (RAG-2 sense), 5'-GGGTTTGTTGAGCTCAGTTGAATAG-3' (RAG-2 antisense), and 5'-CAAGATATGGTTTGGAAGCAACATGGGAAA-3' (RAG-2 probe).
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Results and Discussion
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Enforced Expression of Id3 Inhibits Development of CD1a–CD34+ Thymocytes, but Promotes NK Development in an FTOC.
To study the role of bHLH factors in T cell development, we overexpressed the dominant negative HLH protein Id3 in T cell progenitors and monitored the fate of the transduced cells. Id3 has a very broad tissue distribution and CD34+ fetal liver and thymic progenitor cells express Id3 messenger RNA as determined by reverse transcription PCR (results not shown). Purified CD34+ fetal liver cells were cultured with a combination of SCF and IL-7 for 24 h. The cells were then transduced by overnight coculture with supernatants of packaging cells producing recombinant viruses harboring either Id3-IRES-GFP or IRES-GFP, and cultured in an FTOC for 4 wk (22). The flow cytometric analysis shown in Fig. 1 demonstrates that the GFP marker was transferred to 23% of the progeny of the IRES-GFP and 13% of the Id3-IRES-GFP–transduced progenitor cells. The patterns of CD3, CD4, and CD5 stainings of the GFP+ cells from the IRES-GFP–transduced cells are identical to that of the nontransduced GFP– cells (Fig. 1). The great majority of the GFP+ cells express CD1a, and very few CD56+ cells were present in these samples. By contrast, cells harvested from the FTOC with Id3-IRES-GFP–transduced fetal liver cells did not express CD1a, and 32% of these cells were positive for the NK cell marker CD56. Almost no CD3+ cells were observed (Fig. 1). Not only the proportions of CD56+ cells were different, but also the absolute numbers since >30-fold more GFP+CD56+ cells were found in the FTOC, populated with Id3-transduced CD34+ cells. These data indicate that CD34+ fetal liver cells, overexpressing Id3, develop preferentially into CD3–CD56+ cells in the FTOC. It was important to ensure that these GFP+CD3–CD56+ cells represent functional NK cells. To obtain enough cells for testing their cytolytic activity, we cultured the cells harvested from the FTOC with a feeder mixture consisting of irradiated PBMCs, the EBV cell line JY, PHA and IL-2. Under these conditions we can expand NK cells and TCR-
/
, but not TCR-
/β cells from an FTOC (24); the failure to obtain TCR-
/β cells is because the mouse thymic environment is unable to induce functional maturation in these cells (24). Fig. 2 demonstrates that the GFP+ cells expanded from the control FTOC contained GPF+ TCR
/
+ cells and a few CD56+CD3– cells. By contrast, GFP+ cells expanded from the FTOC with the Id3+ progenitor cells were CD56+ and did not express CD3, underscoring the strong inhibitory effect of Id3 on generation of TCR-
/
+ cells. The GFP+CD3–CD56+ cells were sorted from the Id3-IRES-GFP cultures and were shown to be highly cytotoxic for the NK sensitive target cell K562, but not for an NK-resistant cell line (Fig. 2 b). These results confirm that the CD56+CD3–GFP+ cells observed in these FTOC are functional NK cells.
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