Enrichment and characterization of thymus-repopulating cells in stroma-dependent cultures of rat bone marrow.

The bone marrow precursor cells seeding the thymus have been difficult to investigate using fresh bone marrow and in vivo thymus reconstitution assays. We have therefore designed a short-term bone marrow culture system allowing the study of thymus-repopulating cells in the marrow microenvironment. Low-density rat bone marrow cells were grown on pre-established mouse bone marrow stromal cell layers. Cocultured cells were maintained either under steroid-free conditions (Whitlock/Witte-type culture) or in the presence of 10(-7) M hydrocortisone (Dexter-type culture). After 3 days in vitro, the unanchored cell fractions were tested for their ability to colonize and repopulate fetal mouse thymic lobes in vitro. Both fresh low-density cells and Whitlock/Witte-type cultures, but not Dexter-type cultures, gave rise intrathymically to significant numbers of rat donor-type Thy-1.1high CD2+ CD5low CD43+ cells accounting for 50% to 90% of the organ-cultured cells at day 14. Repopulation of fetal mouse thymic lobes by rat Thy-1.1high cells could be used as a readout assay for initiation of thymopoiesis from bone marrow precursor cells, since 90% of the cells were CD3-/low and TCR alpha beta-/low and 15% of the cells co-expressed CD4 and CD8. Dose-response analysis showed that thymus repopulating cells were at least maintained, if not amplified during the 3-day culture period, leading to at least a 10-fold enrichment as compared to unfractionated bone marrow. Unlike fresh low-density cells before culture, short-term Whitlock/Witte-type cultures were depleted in myeloid-restricted precursor cells. In culture, the thymus-repopulating activity was predominantly associated with a 10% lymphoid cell subset which did not express the B-lineage-associated antigens revealed by HIS24 (the rat B220 equivalent) and HIS50 mAbs. We propose that unanchored thymus-repopulating cells enriched in Whitlock/Witte-type cultures may represent lymphoid-restricted, T-cell precursors of the bone marrow capable of emigrating and colonizing the thymus.