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The long-term maintenance of hematopoietic stem cells (HSCs) relies on the regulation of endoplasmic reticulum (ER) stress at a low level, but the underlying mechanism remains poorly understood. Here, we demonstrate that suppression of ER stress improves the functions of HSCs and protects HSCs against ionizing radiation (IR)-induced injury. We identify epithelial membrane protein 1 (EMP1) as a key regulator that mitigates ER stress in HSCs. Emp1 deficiency leads to the accumulation of protein aggregates and elevated ER stress, ultimately resulting in impaired HSC maintenance and self-renewal. Mechanistically, EMP1 is located within the ER and interacts with ceramide synthase 2 (CERS2) to limit the production of a class of sphingolipids, dihydroceramides (dhCers). DhCers accumulate in Emp1-deficient HSCs and induce protein aggregation. Furthermore, Emp1 deficiency renders HSCs more susceptible to IR, while overexpression of Emp1 or inhibition of CERS2 protects HSCs against IR-induced injury. These findings highlight the critical role played by the EMP1-CERS2-dhCers axis in constraining ER stress and preserving HSC potential. A new study shows EMP1 protects hematopoietic stem cells by suppressing sphingolipid metabolism and ER stress. EMP1 interacts with CERS2 to limit dihydroceramide production, which causes protein aggregation when elevated.

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 CD45dimCD34+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-$\gamma$ 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.

Hematopoietic clones with leukemogenic mutations arise in healthy people as they age, but progression to acute myeloid leukemia (AML) is rare. Recent evidence suggests that the microenvironment may play an important role in modulating human AML population dynamics. To investigate this concept further, we examined the combined and separate effects of an oncogene (c-MYC) and exposure to IL3, GM-CSF and SCF on the experimental genesis of a human AML in xenografted immunodeficient mice. Initial experiments showed that normal human CD34+ blood cells transduced with a lentiviral MYC vector and then transplanted into immunodeficient mice produced a hierarchically organized, rapidly fatal and serially transplantable blast population, phenotypically and transcriptionally similar to human AML cells, but only in mice producing IL3, GM-CSF and SCF transgenically, or in regular mice in which the cells were exposed to IL3 or GM-CSF delivered using a co-transduction strategy. In their absence, the MYC+ human cells produced a normal repertoire of lymphoid and myeloid progeny in transplanted mice for many months but, upon transfer to secondary mice producing the human cytokines, the MYC+ cells rapidly generated AML. Indistinguishable diseases were also obtained efficiently from both primitive (CD34+CD38-) and late (GMPs) cells. These findings underscore the critical role that these cytokines can play in activating a malignant state in normally differentiating human hematopoietic cells in which MYC expression has been deregulated. They also introduce a robust experimental model of human leukemogenesis to further elucidate key mechanisms involved and test strategies to suppress them.