Lymphoid progenitors exhibit accelerated thymimic and marrow engraftment relative to stem cells

We have recently described a mouse bone marrow population defined by the phenotype Thy1.1Neg, Sca1Pos, LinNeg (ThyNeg) which contains lymphoid progenitor cells. To evaluate the engraftment kinetics of these progenitors in comparison to Thy1.1Low, Sca1Pos, LinNeg hematopoietic stem/progenitor cells (HSPC), each population was transplanted intravenously into sublethally irradiated (6.5 Gy) Ly5 congenic mice at 103 cells per mouse. Over a course of 14 to 28 days post-transplant, both thymic lobes and one femur from each mouse were collected and assayed for the presence of donor cells by immunofluorescent staining and flow cytometry. Engraftment of CD19Pos B cells was observed in bone marrow of ThyNeg recipients by day 14, three days earlier than recipients of HSPC. Myeloid engraftment (Gr-1Pos) was also observed in HSPC transplant recipients by day 17, but was not observed in ThyNeg recipients. Thymic reconstitution was observed at significant levels (>106 cells per lobe) in 25% of ThyNeg recipients by day 14. In contrast, donor-derived thymocytes were not observed in recipients of HSPC until day 21, when 1 of 20 thymic lobes was positive for significant engraftment compared to 9 of 24 lobes assayed from ThyNeg recipients. HSPC engraftment proceeded rapidly after day 21, and by day 28 all of the lobes assayed were positive for >106 donor-derived thymocytes. ThyNeg recipients achieved this degree of engraftment in only 33% of lobes assayed. We conclude from these observations that the ThyNeg population contains lymphoid progenitors capable of more rapid thymic engraftment relative to HSPCs. This suggests that specific commitment steps have predisposed these cells to thymic homing and engraftment. The difference in engraftment kinetics will allow us to distinguish the activity of lymphoid progenitors from HSPCs. This assay will facilitate enrichment and characterization of bone marrow-derived T cell progenitors.