(A and B) In vivo paracrine effect of dko and SHIP^–/– cells. Equal numbers of CD45.1⁺ (Ly5.1) BM cells and CD45.2⁺ (Ly5.2) BM cells derived from either WT, dko, or SHIP^–/– mice were cotransferred into lethally irradiated Ly5.1 recipients. Eight weeks later, CD45.1 or CD45.2 cells in peripheral blood were gated and Gr-1⁺CD11b⁺ myeloid cells were enumerated. Percentages of Gr-1⁺CD11b⁺ cells within each CD45 cohort are indicated. (C) RNAs from WT, dko, or SHIP^–/– KSL cells transduced with the indicated bicistronic vectors were submitted to real-time PCR analysis. *P < 0.05 versus WT/vector, ^#P < 0.05 versus dko/vector values, by Student’s t test. (D) Intracellular staining of IL-3 and GM-CSF in WT and dko KSL cells. (E) Lin^– cells (Ly5.1) were cocultured for a week with either WT, dko, or SHIP^–/– Lin^– cells (Ly5.2⁺) in the presence of control IgG, anti–IL-3, anti–GM-CSF, or anti–IL-3 plus anti–GM-CSF. CD11b⁺ cells were enumerated by flow cytometry. (F) A proposed model. Both Lyn and Hck are required to fully activate SHIP that in turn inhibits the production of several cytokines. In the absence of Lyn and Hck, secreted cytokines, including IL-3 and GM-CSF, can activate the Jak2/Stat5 pathway, eventually disrupting the homeostasis of HSCs.