In this article, we report that the human fetal thymus contains CD34(bright) cells (+ lineage negative (lin-) thymocytes contained a low number of primitive myeloid progenitor cells, thus suggesting that the different hematopoietic lineages present in the thymus may be derived from primitive hematopoietic progenitor cells seeding the thymus. To investigate whether the thymus is permissive for the development of non-T cells, human fetal organ culture (FTOC) assays were performed by microinjecting sorted CD34+ lin- fetal liver cells into fragments of HLA- mismatched fetal thymus. Sequential phenotypic analysis of the FTOC-derived progeny of CD34+ lin- cells indicated that the differentiation into T cells was preceded by a wave of myeloid differentiation into CD14+CD11b+CD4(dull) cells. Donor-derived B cells (CD19+CD20+) were also generated, which produced immunoglobulins (IgG and IgM) when cultured under appropriate conditions, as well as functional CD56+CD3- NK cells, which efficiently killed K562 target cells in cytotoxicity assays. These results demonstrate that the microinjection of fetal liver hematopoietic progenitors into fetal thymic organ fragments results in multilineage differentiation in vitro.
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