Hematopoiesis by iPSC-derived hematopoietic stem cells of aplastic anemia that escape cytotoxic T-cell attack

Hematopoietic stem cells (HSCs) that lack HLA-class I alleles as a result of copy-number neutral loss of heterozygosity of the short arm of chromosome 6 (6pLOH) or HLA allelic mutations often constitute hematopoiesis in patients with acquired aplastic anemia (AA), but the precise mechanisms underlying clonal hematopoiesis induced by these HLA-lacking (HLA^-) HSCs remain unknown. To address this issue, we generated induced pluripotent stem cells (iPSCs) from an AA patient who possessed HLA-B4002-lacking (B4002^-) leukocytes. Three different iPSC clones (wild-type [WT], 6pLOH⁺, and B∗40:02-mutant) were established from the patient's monocytes. Three-week cultures of the iPSCs in the presence of various growth factors produced hematopoietic cells that make up 50% to 70%of the CD34⁺ cells of each phenotype. When 10⁶ iPSC-derived CD34⁺ (iCD34⁺) cells with the 3 different genotypes were injected into the femoral bone of C57BL/6.Rag2 mice, 2.1% to 7.3% human multilineage CD45⁺ cells of each HLA phenotype were detected in the bone marrow, spleen, and peripheral blood of themice at 9 to 12weeks after the injection, with no significant difference in the human: mouse chimerism ratio among the 3 groups. Stimulation of the patient's CD8⁺ T cellswith the WTiCD34⁺ cells generated a cytotoxic T lymphocyte (CTL) line capable of killingWT iCD34⁺ cells but not B4002^- iCD34⁺ cells. These data suggest that B4002^- iCD34⁺ cells show a repopulating ability similar to that of WT iCD34⁺ cells when autologous T cells are absent and CTL precursors capable of selectively killing WT HSCs are present in the patient's peripheral blood.