LYMPHOID AND MYELOID DIFFERENTIATION OF FETAL LIVER CD34-) CELLS IN HUMAN THYMIC ORGAN-CULTURE( LINEAGE()

In this article, we report that the human fetal thymus contains CD34(b right) cells (<0.01% of total thymocytes) with a phenotype that resemb les that of multipotent hematopoietic progenitors in the fetal bone ma rrow. CD34(bright) thymocytes were CD33(-/dull) and were negative for CD38, CD2, and CD5 as well as for the lineage markers CD3, CD4, and CD 8 (T cells), CD19 and CD20 (B cells), CD56 (NK cells), glycophorin (er ythrocytes), and CD14 (monocytes). In addition, total CD34(+) lineage negative (lin(-)) thymocytes contained a low number of primitive myelo id progenitor cells, thus suggesting that the different hematopoietic lineages present in the thymus may be derived from primitive hematopoi etic progenitor cells seeding the thymus. To investigate whether the t hymus is permissive for the development of non-T cells, human fetal or gan culture (FTOC) assays were performed by microinjecting sorted CD34 (+)lin(-) fetal liver cells into fragments of HLA-mismatched fetal thy mus. Sequential phenotypic analysis of the FTOC-derived progeny of CD3 4(+)lin(-) cells indicated that the differentiation into T cells was p receded by a wave of myeloid differentiation into CD14(+) CD11b(+) CD4 (dull) cells. Donor-derived B cells (CD19(+)CD20(+)) were also generat ed, which produced immunoglobulins (IgG and IgM) when cultured under a ppropriate conditions, as well as functional CD56(+)CD3(-) NK cells, w hich efficiently killed K562 target cells in cytotoxicity assays. Thes e results demonstrate that the microinjection of fetal liver hematopoi etic progenitors into fetal thymic organ fragments results in multilin eage differentiation in vitro.