Depletion of recipient CD169⁺Mφs reduces engraftment of donor HSCs posttransplantation. (A) Schematic of the transplantation model using lethally irradiated CD169-DTR or C57BL/6 primary (1°) recipients and congenic UBC-GFP LSK cells as the 1° donor graft. BM from 1° recipients was collected at 5 weeks posttransplant, mixed with C57BL/6 competitor BM cells, and transplanted into secondary (2°) lethally irradiated C57BL/6 recipients. (B-F) As no significant differences were observed in any of the outcome measures between saline-treated CD169-DTR and DT-treated C57BL/6 1° recipient mice, these groups were pooled as the control (Cont). (B) Number of recipient resident Mφs in the BM of 1° recipients at 5 weeks posttransplant. (C) Number of 1° donor HPCs (blue bars), MPPs (yellow bars), and HSCs (red bars) in the BM of 1° recipients at 5 weeks posttransplant. (D) Number of recipient resident Mφs in spleen of 1° recipients at 5 weeks posttransplant. (E) Number of 1° donor HPCs (blue bars), MPPs (yellow bars), and HSCs (red bars) in the spleen of 1° recipients at 5 weeks posttransplant. (F-G) Number of leukocytes (total white blood cells) in the BM (F) and spleen (G) of 1° recipients at 5 weeks posttransplant. (H and I) Number of donor Mφ in BM (H) or spleen (I) of 1° recipients at 5 weeks posttransplant. Data are presented as mean ± SD (n = 8-20 1° recipients/group). (J) Blood GFP⁺ chimerism of 2° recipients (n = 10-25 mice/group) Statistical analysis was performed using unpaired Student t test, with colored asterisks matching relevant cell populations where appropriate (*P < .05 and **P < .01). Data were pooled from 3 independent biological replicate experiments, and 300 000 events were collected by flow cytometer to analyze myeloid population; 1 × 10⁶ events were collected to analyze LSK populations, and 100 000 events were collected to analyze tail bleeds. ***P = .0002. WBC, white blood cell.