Hematopoietic stem cells (HSCS) have the ability to self-renew and give rise to more HSCs or differentiate to produce all blood lineages. Due to their role in sustaining the hematopoietic system throughout life, it was thought that HSCs might be protected from the effects of aging that have an impact on differentiated cells. However, we and others found that aged HSCs were approximately three fold less active than young HSCs in competitive repopulating assays (Chambers et al. 2007; Plos Biology). Paradoxically, there was an expansion in the HSC pool and the population was more homogenous for stem cell marker expression (Sca-1 and c-Kit). Besides the diminished regenerative capacity of the HSC, there was also a skewing of lineage potential from lymphopoiesis toward myelopoiesis with age (Sudo et al. 2000; J Exp Med).
While the effects of aging on HSC have been characterized, limited focus has been spent on what role the surrounding bone marrow environment (stem cell niche) plays in pathways dysregulated with age in the HSC. In order to understand age-related changes in the microenvironment, we performed a cytokine protein array in young versus old mice. From this study, we found an increase in two pro-inflammatory cytokines Mig and Rantes in aged mice, indicating the presence of an inflammatory microenvironment in aged mice. We also found that Rantes expression increases with age in the bone and lineage positive cells of the bone marrow. With enforced expression of Rantes, we saw a significant change in the differentiated progeny of HSCs 16 weeks after transplantation. There were significantly fewer (p<0.05) T cells and more myeloid cells in the Rantes over-expressing group. We also analyzed bone marrow for HSC and progenitor populations after 18 weeks of transplantation and saw significantly more LT-HSCs in the Rantes over-expressing group. We also tested effects of Mig and Rantes on LT-HSCs by ex-vivo treatment followed by functional transplantion assay. We saw the same lineage skewing phenotype after 16 weeks of transplantation (more myeloid cells and significantly less lymphoid cells with Rantes treatment), consistent with the over-expression study. When we transplanted old or young bone marrow cells into either old or young mice and compared either old-into-old with old-into-young, or young-into-young with young-into-old transplants, we observed a significant decrease in T cells and an increase in myleoid cells 16-weeks post-transplant when bone marrow cells were transplanted into old mice. The heterochronic transplantation results demonstrated that an old environment favors myeloid differentiation against T cell differentation. Since enforced expression of Rantes had the same lineage skewing phenotype with those transplants, one possible explanation would be that enforced expression of Rantes or ex-vivo treatment mimicks the old environment and leads to myeloid skewing phenotype. We are now testing Rantes knock-out stem cells for their ability to reconstitute blood. We are expecting to see a better engraftment and more T cells in the peripheral blood. Collectively, Rantes is a key inflammatory cytokine that contributes to myeloid skewing and functional deficits in HSCs of aged mice.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.