iPSC-derived modeling of HLA-lacking hematopoiesis reveals clonal diversity in eltrombopag response in acquired...

Acquired aplastic anemia (AA) is an immune-mediated bone marrow failure in which cytotoxic T lymphocytes (CTLs) target hematopoietic stem cells (HSCs). Approximately 30% of AA patients develop immune escape clones lacking specific HLA class I alleles (HLA[-]) through loss of heterozygosity in chromosome 6p (6pLOH) or somatic loss-of-function mutations. Eltrombopag (EPAG), a thrombopoietin receptor agonist (TPO-RA), demonstrates clinical efficacy in AA in combination with immunosuppressive therapy; however, its impact on HLA(-) HSCs and hematopoietic progenitor cells (HPCs) remains poorly understood. In this study, we evaluated the hematopoietic effects of EPAG using umbilical cord blood-derived HPCs and a humanized hematopoiesis model in immunodeficient (BRGS) mice. Furthermore, we established induced pluripotent stem cell (iPSC)-derived hematopoietic models encompassing five wild-type (WT) clones and seven HLA-lacking clones, differentiated them into HPCs, and assessed their responses to EPAG. EPAG selectively conferred a proliferative advantage to specific hematopoietic fractions in HLA(-) HPCs, distinct from that observed in WT HPCs. Molecular analyses revealed clone-dependent differences in CD110 expression and downstream effectors, including phosphorylated STAT5, FOXM1, and E2F1, indicating differential activation of TPO receptor-mediated signaling pathways among clones. These findings highlight the functional diversity of HLA(-) hematopoiesis and suggest that the hematopoietic response to EPAG is governed by clone-intrinsic signaling programs. Furthermore, our results provide new insights into how eltrombopag modulates clonal competition and hematopoietic recovery in immune-escape hematopoiesis, with potential implications for optimizing therapeutic strategies and predicting clinical response in patients with acquired AA.