Our inability to purify hematopoietic stem cells (HSCs) precludes direct study of many aspects of their behavior in the clinical hematopoietic stem cell transplantation (HSCT) setting. We indirectly assessed stem/progenitor cell behavior in the first year after HSCT by examining changes in neutrophil telomere length, X-inactivation ratios, and cycling of marrow progenitors in 25 fully engrafted allogeneic HSCT recipients. Donors were sampled once and recipients at engraftment and 2 to 6 months and 12 months after HSCT. Telomere length was measured by an in-gel hybridization technique, X-inactivation ratios were measured by the human androgen receptor assay, and cell cycle status was determined by flow cytometric analysis of pyronin Y-and Hoechst 33342-stained CD34 ؉ CD90 ؉ and CD34 ؉ CD90 ؊ marrow cells. Compared with their donors, recipients' telomeres were shortened at engraftment (؊424 base pairs [bp]; P < .0001), 6 months (؊495 bp; P ‫؍‬ .0001) after HSCT, and 12 months after HSCT (؊565 bp; P < .0001). There was no consistent pattern of change in telomere length from 1 to 12 months after HSCT; marked, seemingly random, fluctuations were common. In 11 of 11 informative recipients, donor X-inactivation ratios were faithfully reproduced and maintained. The proportion of CD34 ؉ CD90 ؉ progenitors in S/G 2 /M was 4.3% in donors, 15.7% at 2 to 6 months (P < .0001) after HSCT, and 11.5% at 12 months after HSCT (P < .0001, versus donors; P ‫؍‬ .04, versus 2-6 months). Cycling of CD34 ؉ CD90 ؊ progenitors was largely unchanged. We infer that (1) HSCT-induced accelerated telomere loss is temporary and unlikely to promote graft failure or clonal hematopoietic disorders and (2)

Summary. Recent studies have demonstrated excessive telomeric shortening in peripheral blood leucocytes of bone marrow transplant (BMT) recipients. This finding has raised concerns about accelerated haemopoietic ageing that might predispose to clonal disorders and late graft failure. We studied the peripheral blood neutrophils and T cells of 14 fully engrafted long-term survivors of BMT.We found that in both neutrophils and T cells there was significant telomere shortening in the recipient (0·6 and 0·5 kb, respectively; P < 0·001 and < 0·04, respectively). We found no relationship between degree of shortening and the nucleated cell dose given at the time of transplant. We also demonstrated significantly longer telomeres in T cells than neutrophils from the same individual (mean 11·6 kb and 10·6 kb, respectively; P¼0·0001).We propose mechanisms to account for these observations. The replicative stress that causes this telomere shortening does not necessarily occur at the level of the most primitive haemopoietic stem cell.