Genetic disruption of Pak2 in HSPCs induces MDSC expansion in the spleen. CD45.1⁺ naive BoyJ mice were noncompetitively transplanted with CD45.2⁺Pak2^flox/floxMx1Cre⁺ (Pak2-KO) or CD45.2⁺Pak2^flox/floxMx1Cre⁻ (WT) BM cells and treated with polyIC. (A) Gr1^highLy6G⁺ cells that were isolated from spleens suppressed T-cell proliferation. (B) The numbers of phenotypic granulocytic (CD11b⁺Ly6G⁺Ly6C^low) and monocytic (CD11b⁺Ly6G^low/−Ly6C^hi) MDSCs are shown per spleen. (C-D) WT (C) and Pak2-KO (D) splenic CD45.2⁺Gr1^highLy6G⁺ cells were stained with Giemsa. Inserts show morphology of cells at a high magnification. All scale bars represent 20 mm. Representative data from at least 3 experiments with 3 to 9 mice per genotype are shown.