Ms4a3 depletion preserves stemness of BCR-ABL1⁺ HSPCs, but does not affect normal hematopoiesis in mice. (A-B) BM Lin⁻ cells from Scl-tTA⁺;TRE-BCR-ABL1⁺ mice were transduced with shMs4a3 and cultured ex vivo. (A) Ms4a3 expression was quantified by qRT-PCR in cells cultured with dox (BCR-ABL1 off). (B) Colony formation was assessed with or without BCR-ABL1 induction. (C-E) Ms4a3^−/−;Scl-tTA⁺;TRE-BCR-ABL1⁺ and Ms4a3^{+/+ or +/−};Scl-tTA⁺;TRE-BCR-ABL1⁺ mice were monitored for 16 weeks after BCR-ABL1 induction (dox withdrawal). Blood cell counts (C), spleen weight (D), and BM stem cell and progenitor counts (E) were quantified by veterinary hematology analyzer and flow cytometry. (F) Blood counts of recipient mice after lethal irradiation and allograft of BM Lin⁻ cells from leukemic donors in panel E. Counts are normalized to week 4, the initial successful engraftment time point. (G) Granulocyte counts and percentage of leukemic mice undergoing 7 days of IM challenge (400 mg/kg) and 7 days of IM withdrawal and recovery. IM were administered by oral gavage. (H-I) PB counts in Ms4a3^+/+ and Ms4a3^−/− mice, of adult (n = 13 and 10) (H) and old (n = 8 and 14) (I) individuals. The cell count units are: 10¹²/L for red blood cell (RBC); 10⁹/L for white blood cell (WBC), lymphocyte (LYM), monocyte (MONO), granulocyte (GRAN), and platelet (PLT). (J) Flow cytometry analysis of BM stem and progenitor populations in Ms4a3^+/+ and Ms4a3^−/− mice.