Down-regulated CD14⁺ LC-engagement property of PLTs is associated with IVIg-DC–mediated amelioration of ITP. Experimental outline (A) for the PLT count (B) and PLT-CD14⁺ LC engagement (C-D) analyses of recipient mice after adoptive transfer of IVIg-primed splenic cells (LC, CD11c⁺ DC, and CD11c⁻) and ITP induction (MWReg30; 0.1 mg/kg). PLTs and LCs were labeled with lineage specific fluorescent Ig (PLT: CD41, LC: CD14) before the engagement. Recipient PLTs and LCs were incubated with respective LC and PLT counterparts from control mice (untreated naive, UN mice; wild type, WT) for 15 minutes and then the percentage of CD14⁺CD41⁺ engaged cells were measured by flow cytometry (C-D). *P < .05, ***P < .001 versus respective vehicle groups; #P < .05 versus CD11c⁻ groups. Naive groups n = 10, vehicle and IVIg groups n = 6 (5 and 3 experiments with 2 replicates, respectively). (E-G) As the experiment outline indicated (A), PLT counts (E) and PLT-CD14⁺ LC engagements (F-G) in the recipient mice were analyzed after the cell-transfer of IVIg-primed splenic CD11c⁺ DCs (IVIg-DCs; WT, Fcgr2b^−/− and Fcgr3^−/− donors to WT recipients). (C,F) PLTs from the recipients were engaged with LCs from control mice (UN mice, WT). (D,G) LCs from the recipients were engaged with PLTs from control mice (UN mice, WT). Engagement levels of control PLTs plus control LCs (both from control UN mice, WT) were normalized to 100% (C-D, F-G). *P < .05 represents significant amelioration, versus respective vehicle groups. n = 6 (3 experiments with 2 replicates). D: donor (green labels); R: recipient (red labels); Control: untreated naive mice (blue labels). Data are mean ± SD. Naive: without cell transfer. Untreated: without ITP induction and vehicle/IVIg treatments.