As more older patients become candidates for transplantation in the treatment of hematologic malignancies and nonhematologic diseases, the effects of aging on the homing of stem/progenitor cells are clinically relevant yet remain largely unexplored.
Hematopoiesis is maintained throughout life by self-renewing stem cells with a high potential for proliferation and multilineage differentiation.
Accumulating evidence indicates that as animals age, the number and the functional properties of hematopoietic stem cells (HSCs) become altered.
However, these effects of aging on stem cells and their bone marrow (BM) microenvironment are not clearly defined.
For example, marrow from old C57BL/6 mice contains more HSCs (measured by cobblestone area formation, primitive phenotype, and competitive repopulating ability) than BM from young mice.
In contrast, the stem-cell pool from DBA/2 and all other mouse strains studied contracts during aging.
There is compelling evidence that strain-specific variation in this and other stem/progenitor-cell parameters is regulated by cell-intrinsic mechanisms and is affected by several quantitative trait loci (QTL).
In serial transplantation experiments, marrow cells from old animals was less able to engraft later passage recipients than young BM cells.
Moreover, old HSCs exhibit a differentiation pattern skewed toward the myeloid lineage at the expense of lymphopoiesis.
Further evidence of age-related changes in stem cells include the finding that a higher proportion of Thy-1loSca-1+Lin-Mac-1-CD4-c-kit+ cells from old mice are in S/G2/M phases of the cell cycle6 and the findings of Henckaerts et al, who showed that the proliferative response of Lin-Sca-1+ckit + marrow cells to the early-acting cytokines kit ligand (KL), FMS-like tyrosine kinase 3 ligand (Flt3L), and thrombopoietin (TPO) decreases dramatically with age.
In addition to such quantitative and functional changes of HSCs with aging, the density or activity of several cell-surface antigens and membrane transporters that facilitate their identification and isolation fluctuates during ontogeny and throughout adulthood.
Therefore, functional measurements of HSC properties may actually reflect the effects of aging on this important population.
The competitive repopulation assay is the most rigorous test for defining HSCs by their capacity for long-term reconstitution of the lympho-hematopoietic system.
When combined with a limiting dilution design, this assay enables measurement of HSC numbers in vivo.
To test the hypothesis that aging has negative effects on stem-cell homing and engraftment, young or old C57BL/6 bone marrow (BM) cells were injected, using a limiting-dilution, competitive transplantation method, into old or young Ly5 congenic mice.
Numbers of hematopoietic stem cells (HSCs) and progenitor cells (HPCs) recovered from BM or spleen were measured and compared with the numbers initially transplanted.
Although the frequency of marrow competitive repopulation units (CRUs) increased approximately 2-fold from 2 months to 2 years of age, the BM homing efficiency of old CRUs was approximately 3-fold lower than that of young CRUs.
Surprisingly, the overall size of individual stem-cell clones generated in recipients receiving a single CRU was not affected by donor age.
However, the increased ages of HSC donors and HSC transplant recipients caused marked skewing of the pattern of engraftment toward the myeloid lineage, indicating that HSC-intrinsic and HSC-extrinsic (microenvironmental) age-related changes favor myelopoiesis.
This correlated with changes after transplantation in the rate of recovery of circulating leukocytes, erythrocytes, and platelets.
Recovery of the latter was especially blunted in aged recipients.
Collectively, these findings may have implications for clinical HSC transplantation in which older persons increasingly serve as donors for elderly patients.
In all experimental and clinical stem-cell transplantations, the critical first step leading to successful engraftment is homing of stem cells to the BM. Szilvassy et al. previously showed that CRUs from fetal liver (FL) and young adult BM had roughly the same seeding efficiency to the marrow of lethally irradiated young recipients when measured 24 hours after intravenous transplantation.
However, Morrison et al. found that old stem cells have only approximately one fourth the competitive repopulating activity of young stem cells.