Increased engraftment efficiency of CFP⁺ HSC exposed for 12 hours to D2 osteoblasts compared with HSC exposed to untreated osteoblasts. (A) Experimental strategy. A total of 100 000 Lin⁻ BM cells (HPC) from untreated mice were exposed for 12 hours in vitro to 2000 OPN⁺ osteoblasts isolated from D2 mobilized mice (which had received 1 dose of Cy + 2 daily doses of G; black) and tested for HSC function by measuring reconstitution potential. (B) Lethally irradiated recipient mice were transplanted with cells from 12-hour HPC only cultures (), or HPC:untreated () or HPC:D2 (▬) cocultures. The average engraftment was calculated by determining the percentage donor-derived myeloid (top; Mac-1⁺, Gr-1⁺), B (middle, B220⁺), and T (bottom, CD3⁺, CD4⁺, CD8⁺) cells at the indicated time points after transplant. Donor-derived cells were identified based on CFP transgene expression and were corrected for the percentage of CFP⁺ myeloid or lymphoid populations (80%-90%) in unmodified CFP⁺ transgenic hosts (*P < .05; ‡P = .0598). Error bars represent SD.