Pak2-deficient T cells induce MDSC generation in vitro. Purified splenic CD4⁺ T cells from mice reconstituted with Pak2-KO or WT BM were stimulated with CD3/CD28 beads for 3 days before supernatant was collected. (A-C) Amounts of GM-CSF (A), TNF-α (B), and IFN-γ (C) in the supernatant were determined by enzyme-linked immunosorbent assay. Naive C57BL/6 mice BM cells were cultured in the presence of supernatant from Pak2-KO or WT splenic CD4⁺ T cells for 5 days, collected, and then cocultured with purified splenic T cells from naive C57BL/6 mice to measure their suppressive function on T-cell proliferation. (D-E) The percentage of the CD11b^highGr1^high population in panel D and the percentage suppression of T-cell proliferation by BM cells cultured in CD4⁺ T-cell supernatant (E) are shown. Representative data from 2 or 3 experiments with 3 to 6 mice per genotype are shown. *P < .05; **P < .01; ***P < .001; ***P < .0001.