Age-related intrinsic defects in B-lineage progenitors diminish their leukemogenic potential. (A) Pro/pre-B cells (Lin⁻ CD19⁺CD45R⁺AA4.1⁺) purified from the BM of young and old mice were transduced with BCR-ABL^P210, and the same number of young and old cells was transplanted into Rag1^−/− recipients. Eight weeks later, 25% of the recipients of BCR-ABL–transduced young pro/pre-B cells (n=8) developed B-lymphoid leukemia, whereas recipients of old pro/pre-B cells (n=8) did not develop any characteristics of disease. Young (B) and old (C) BM cells (3 × 10⁵) transduced with EGFP or BCR-ABL^P210 were used to establish hematopoietic cultures in B-lineage–permissive conditions.⁴⁵  Cultures were examined 3 weeks later. Young BM^BCR-ABL cells expanded 100-fold, whereas only a 6-fold expansion was observed with old cells compared with controls. (D) Phenotypic and morphologic analysis of cultures described for B and C. Cultures derived from young BM^BCR-ABL had increased cellularity and a higher frequency of GFP⁺ B-lineage cells compared with those that were initiated from old BM^BCR-ABL, which produced cells primarily with a myeloid morphology. Images of live, cultured cells were viewed with a Nikon Diaphot TMD inverted phase microscope (Nikon, Tokyo, Japan) using a 20×/0.40 NA phase-contrast objective. Micrographs were taken using an Olympus DP11 camera (Olympus), and photographs were prepared using Adobe Photoshop image-acquisition software (Adobe Systems). (E) BCR-ABL^P210–expressing (GFP⁺) young and old pro/pre-B cells (3.8 × 10⁵) were seeded on stromal layers in B-lineage–permissive conditions.⁴⁵  After 5 days, the number of GFP⁺ cells increased 10-fold in the cultures seeded with young pro/pre-B cells, whereas the old pro/pre-B cells did not show any significant expansion. One of 2 representative experiments is shown.