Hematopoietic Cells from Ube1L-Deficient Mice Exhibit Impaired Short-Term Reconstitution Capacity Under the Stress of Bone Marrow Transplantation

Following bone marrow transplantation, donor stem cells are recruited from their quiescent status to promote the rapid reconstitution of a depleted hematopoiesis system in recipients. This dynamic process is tightly regulated by a complex of internal and external signals. The proper proliferation and differentiation of hematopoietic stem cells (HSCs) are crucial for reconstitution of hematopoiesis in recipients with bone marrow ablation. Therefore, thoroughly understanding the complex regulation of HSCs during transplantation will be very helpful for future therapeutic applications. Type I interferons (IFNs) are induced in various types of cells by different stimuli. Protein modification by the ubiquitin like modifier ISG15 (ISGylation) is strongly induced by Type I IFNs. To understand whether Type I IFN signaling and protein ISGylation are important for the rapid reconstitution of the hematopoietic system upon depletion, we examined the expression levels of IFNs and protein ISGylation during transplantation. Our results showed a clear increase in Type I IFNs in the bone marrow cells of recipient mice at 4, 6, 8, and 10 days after transplantation, and obviously higher levels of protein ISGylation in different tissues at 2, 4, 6, and 8 days after transplantation. To further investigate the role of protein ISGylation in hematopoiesis, we examined the phenotypes of the Ube1L knockout mouse. Ube1L is a critical enzyme for protein ISG15 modification. Ube1L deficient cells do not have protein ISGylation upon IFN stimulation. Steady-state hematopoiesis was unperturbed in Ube1L −/− mice. In addition, Ube1L deficiency did not affect the colony-forming ability of progenitors in vitro. The in vivo colony-forming unit spleen (CFU-S) assay showed that the frequency of CFU-S12 was normal, however, the size of the colonies derived from Ube1L deficient cells were clearly reduced compared to wild-type cells. In transplant experiments, the homing capacity of Ube1L-deficient donor cells was not affected, however, at 3 weeks posttransplant, noncompetitive transplantation experiment revealed a 50% reduction in repopulation potential of Ube1L −/− bone marrow cells compared with Ube1L +/+ cells (p = 0.0015). A competitive transplantation experiment magnified and extended the Ube1L −/− phenotype showing that the Ube1L −/− cells had a significantly lower repopulating ability after 3 weeks (p = 0.0141), but less pronounced differences at 6 and 12 weeks. The lineage commitment of engrafted Ube1L-deficent cells was not affected. Further study showed that after transplantation in the multipotent progenitor (MPP) population, the absence of Ube1L resulted in fewer Ube1L −/− donor cells than Ube1L +/+ wild-type donor cells when compared to the competitors (p = 0.0367). These results indicate that in conditions that call for rapid proliferation, such as bone marrow transplantation, protein ISGylation appears to be critically involved in promoting and enhancing the proliferative response of short-term repopulation. This study will contribute to a thorough understanding of stem cell characteristics under transplantation, and may provide novel strategies to predict the potential risk factors related to ISGylation defects and enhance the success rate of HSC transplantation.