PURIFIED MURINE LONG-TERM IN-VIVO HEMATOPOIETIC REPOPULATING CELLS ARE NOT PROTHYMOCYTES

Analysis of kinetics of thymic repopulation by Rh123(low), Lin(-), Ly6 A/E(+), c-kit(+) (RH123(low)) cells, highly enriched for long-term in vivo hematopoietic repopulating cells, reveals that this population is deficient in thymic repopulation at week 3 after intravenous transpla ntation when compared to normal bone marrow cells. This suggests that the marrow prothymocytes have been depleted from this population, and analysis of thymic repopulation at week 3 can therefore be used to dif ferentiate prothymocytes and their precursors. Using this short-term a ssay, the Rh123(high) Lin(-), Ly6A/E(+), c-kit(+) (Rh123(high)) popula tion has been found to be relatively more efficient at early thymic re population, suggesting that this population contains the prothymocytes . In addition, the differentiation potential. and reconstitution behav ior of the Rh123(high) population observed after intravenous and intra thymic transfer strongly indicates that this population is at the tran sitional stage between the marrow primitive pluripotential and thymic more mature lymphoid restricted stem cells. We propose that the thymic repopulating ability of the Rh123(low) population is through generati on of the more mature Rh123(high) progeny, presumably in the marrow.