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
) 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
) 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, λ5, CD19, and Pax5 (11
). 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
). 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
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.