Pak2-KO HPCs display increased sensitivity to GM-CSF signaling. (A) MDSCs were generated from Pak2-KO or WT BM cells in the presence of GM-CSF, IL-6, and G-CSF. (B) Colony formations of sorted CD45.2⁺ BM cells cultured in methylcellulose with GM-CSF. The numbers of colonies (colony-forming units in response to GM-CSF [CFU-GM]) per femur are shown. (C) GM-CSF receptor α (Csf2ra) and β (Csf2rb) chain gene expression was measured in progenies collected from the GM-CSF colony assay shown in Figure 3B. Pak2-KO or WT BM c-kit⁺ cells were cultured with GM-CSF for 7 days. (D-E) Cells were stained for CD11b and Gr1 (D) and cocultured with T cells to test their suppressive function (E). The suppressive function on CD8⁺ T-cell proliferation is shown. Representative data from at least 3 experiments with 3 to 6 mice per genotype are shown. **P < .01; ***P < .001.