Platelet-associated TGF-β1 stimulated the function of MDSCs in vitro. (A) Representative flow-cytometric plots of MDSCs (CD11b⁺ CD33⁺ HLA-DR^low cells) in peripheral blood mononuclear cells (PBMCs) from healthy controls and patients with ITP before and after TPO-RA treatment. (B) TGF-β1 level in control and low-platelet and high-platelet cocultured groups. (C) MDSC-mediated suppression of CD4⁺ T_effs proliferation was measured according to the division index. (D-E) Relative mean fluorescence intensity (MFI) of Arg-1, PD-L1, PD-L2, and iNOS expressed by MDSCs cocultured with low-platelet, high-platelet, high-platelet plus TGF-βi, or none. Relative MFI is the fold of average MFI in controls. (D) MDSC from patients with ITP. (E) MDSC from healthy controls.