Frequent HLA-DR loss on hematopoietic stem progenitor cells in patients with cyclosporine-dependent aplastic anemia carrying HLA-DR15

To determine whether antigen presentation by HLA-DR on hematopoietic stem progenitor cells (HSPCs) is involved in the development of acquired aplastic anemia (AA), we studied the HLA-DR expression on CD45^dimCD34⁺CD38⁺ cells in the peripheral blood of 61 AA patients including 23 patients possessing HLA-class I allele-lacking (HLA-class I[−]) leukocytes. HLA-DR-lacking (DR[−]) cells accounted for 13.0–57.1% of the total HSPCs in seven (11.5%) patients with HLA-DR15 who did not possess HLA-class I(–) leukocytes. The incubation of sorted DR(–) HSPCs in the presence of IFN-γ for 72 h resulted in the full restoration of the DR expression. A comparison of the transcriptome profile between DR(–) and DR(+) HSPCs revealed the lower expression of immune response-related genes including co-stimulatory molecules (e.g., CD48, CD74, and CD86) in DR(–) cells, which was not evident in HLA-class I(–) HSPCs. DR(–) cells were exclusively detected in GPI(+) HSPCs in four patients whose HSPCs could be analyzed separately for GPI(+) and GPI(–) HSPCs. These findings suggest that CD4⁺ T cells specific to antigens presented by HLA-DR15 on HSPCs may contribute to the development of AA as well as the immune escape of GPI(–) HSPCs in a distinct way from CD8⁺ T cells recognizing HLA-class I-restricted antigens.