2B9-Fc Ameliorates Immune Thrombocytopenia by Suppressing STAT1-Mediated M1 Macrophage Polarization and Expanding Regulatory T Cells

Immune thrombocytopenia (ITP) is an autoimmune bleeding disorder in which enhanced platelet destruction and impaired platelet production coexist, and a substantial proportion of patients remain refractory to available therapies. Here, we evaluated 2B9-Fc, a thrombopoietin (TPO) mimetic peptide engineered from a cyclic peptide with high affinity for both human and mouse c-MPL, in passive (anti-CD41 monoclonal antibody R300) and active (guinea pig anti-mouse platelet serum, GP-APS) immunization male C57BL/6J and humanized c-MPL receptor transgenic mouse models of ITP. 2B9-Fc (500 μg/kg, subcutaneously) significantly elevated peripheral platelet counts in both models (p < 0.01), without affecting WBC, RBC, or hemoglobin, and alleviated skin purpura and bleeding scores in the active model. In addition, 2B9-Fc treatment was associated with a reduced splenic M1/M2 macrophage ratio, increased regulatory T cells (Tregs), downregulated IL-6 and iNOS while upregulating TGF-β1 and Foxp3, and suppressed STAT1 phosphorylation. In humanized c-MPL receptor transgenic (MT) mice, 2B9-Fc promoted CD41+ megakaryocyte generation in vitro and sustained platelet recovery in vivo through day 14. These results indicate that 2B9-Fc ameliorates ITP and is associated with promotion of megakaryocyte maturation and restoration of immune tolerance, likely involving inhibition of STAT1-mediated M1 polarization and Treg expansion, supporting its potential for clinical translation.