miR-155 knockdown in myeloid cells accelerates tumor growth. (A) Experimental scheme: The BM of 5.5-week-old PyMT mice was reconstituted with wild-type FVB HSPCs that were transduced with the miR-155/KD or ScrT Ctrl vectors. Mean vector copy numbers per cell ± SEM are indicated. Chimeras are referred to as PyMT Kd mice and PyMT Ctrl mice. (B) Mammary tumor growth was measured by MRI (mean volume ± SEM of the 3 anterior mammary gland pairs) at 9 and 12 weeks of age in PyMT Ctrl (n = 9) and PyMT Kd (n = 10) mice. (C) Tumor burden at the experimental end point (13 weeks) was determined by weighing the mass of surgically resected tumor specimens from each mouse. Shown is the mean relative tumor weight in PyMT Ctrl mice (n = 33) and PyMT Kd mice (n = 27). Within each of 4 independent experiments, tumor mass was normalized to the median tumor weight of the respective PyMT Ctrl group. (D) White blood cell (WBC) count was measured in wild-type FVB mice (n = 12), PyMT Ctrl (n = 33), and PyMT Kd mice (n = 39) at 9 weeks of age. (E) Complete blood cell counts and hemoglobin levels were measured in wild-type FVB mice (n = 12), PyMT Ctrl (n = 30), and PyMT Kd mice (n = 25) at 13 weeks of age. Results are from 4 independent experiments. (F) Differential WBC counts were obtained by multiplying the absolute WBC shown in (E) by the fraction of CD11b⁺side scatter (SSC)^hi cells (granulocytes, left panel), CD11b⁺SSC^low cells (monocytes, middle panel), and CD19⁺ cells (B lymphocytes, right panel) as measured by flow cytometry. (G) The graph shows tumor growth in FVB mice transplanted with miR-155/KD or Ctrl vector–transduced HSPCs and orthotopically challenged with late-stage PyMT mammary tumor cells 6 weeks after transplantation. Shown is the mean tumor weight ± SEM; n = 6 mice per group. HGB, hemoglobin; PLT, platelets; Wt, wild type.