A bone marrow-derived stroma cell line, ST2, can support the differentiation of fetal thymocytes from the CD4(-) CD8(-) double negative to the CD4(+)CD8(+) double positive differentiation stage in vitro

T-cell precursors differentiate into mature T cells predominantly in the th ymus. However, it has also been reported that T-cell precursors mature in e xtrathymic organs such as the liver, bone marrow, or intestines. In order t o investigate the nature of the extrathymic microenvironment that supports T-cell maturation, we examined the effect of a bone marrow-derived stroma c ell line, ST2, on T-cell precursors by using a reaggregate thymic organ cul ture (RTOC) system. We found that ST2 cells supported the differentiation o f fetal thymocytes at day 14.5 of gestation from a CD4(-) CD8(-) double neg ative (DN) to a CD4(+) CD8(+) double positive (DP) differentiation stage in a manner similar to that observed in thymus. Anti-interleukin-7 receptor ( IL-7R) and anti-c-kit antibodies blocked the growth of thymocytes in RTOC w ith ST2 cells, but did not inhibit the generation of DP thymocytes. These d ata indicate that a bone marrow-derived stroma cell, ST2, which supports B- cell differentiation, is also able to support T-cell development and may co nstitute one of the microenvironmental components for extrathymic T;cell de velopment.