Seeding efficiency of primitive human hematopoietic cells in nonobese diabetic/severe combined immune deficiency mice: implications for stem cell frequency assessment

Nonobese diabetic/severe combined immune deficiency (NOD/SCID) mouse repopulating cells (SRC) have been proposed to represent a more primitive human stem cell subset than the cobblestone area-forming cell (CAFC) week (wk) 6 or the long-term culture-initiating cell (LTC-IC) wk 5 on the basis of their difference in frequency, phenotype, transfectibility, and multilineage outgrowth potential in immunodeficient recipients. We have assessed the percentage of various progenitor cell populations (colony-forming cell [CFC] and CAFC subsets) contained in unsorted NOD/SCID BM nucleated cells (nc), human umbilical cord blood (UCB) nc, bone marrow (BM) nc, peripheral blood stem cells (PBSC), and CD34(+) selected UCB nc, seeding in the BM and spleen of NOD/SCID mice within 24 hours after transplantation. The seeding efficiency of NOD/SCID BM CAFC wk 5 was median (range) in the spleen 2.9% (0.7% to 4.0%) and in the total BM 8.7% (2.0% to 9.2%). For human unsorted UCB nc, BM nc, PBSC, and CD34(+) UCB cells, the seeding efficiency for CAFC wk 6 in the BM of NOD/SCID mice was 4.4% (3.5% to 6.3%), 0.8% (0.3% to 1.7%), 5.3% (1. 4% to 13.6%), and 4.4% (3.5% to 6.3%), respectively. Using flow cytometry, the percentage CD34(+) UCB cells retrieved from the BM of sublethally or supralethally irradiated NOD/SCID mice was 2.3 (1.4 to 2.8) and 2.5 (1.6 to 2.7), respectively. Because we did not observe any significant differences in the seeding efficiencies of the various stem cell subsets, it may be assumed that the SRC seeding efficiency in NOD/SCID mice is similarly low. Our data indicate that the seeding efficiency of a graft can be of great influence when assessing stem cell frequencies in in vivo repopulation assays.