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
(HLA2) HSCs remain unknown. To address this issue, we generated induced pluripotent
stem cells (iPSCs) from an AA patient who possessed HLA-B4002–lacking (B40022)
leukocytes. Three different iPSC clones (wild-type [WT], 6pLOH1, 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 CD341 cells
of each phenotype. When 106 iPSC-derived CD341 (iCD341) cells with the 3 different genotypes
were injected into the femoral bone of C57BL/6.Rag2 mice, 2.1% to 7.3% human multilineage
CD451 cells of each HLA phenotype were detected in the bone marrow, spleen, and peripheral
blood of the mice at 9 to 12 weeks after the injection, with no significant difference in the human:
mouse chimerism ratio among the 3 groups. Stimulation of the patient’s CD81 T cells with the
WT iCD341 cells generated a cytotoxic T lymphocyte (CTL) line capable of killing WT iCD341 cells
but not B40022 iCD341 cells. These data suggest that B40022 iCD341 cells show a repopulating
ability similar to that of WT iCD341 cells when autologous T cells are absent and CTL precursors
capable of selectively killing WT HSCs are present in the patient’s peripheral blood.