Telomere Length of Hematopoietic Cells in Long-Term Post-Autograft Survivors: No Evidence of Accelerated Replicative Cell Senescence Despite the Persistent Reduction of Both Committed and Immature Progenitor Cell Compartments.

Telomere length is considered a valuable replicative capacity predictor of human hematopoietic stem cells. Indeed, a progressive telomere shortening affects hematopoietic cells upon in vitro expansion. However, less is known on the dynamics of telomere shortening in vivo following a non-physiological replicative stress. Aim of this study was to investigate markers for cellular senescence of hematopoietic cells exposed to replicative stress induced by bone marrow reconstitution following stem cell autograft. Thus, both telomere length and in vitro functional characteristics of bone marrow (BM) and peripheral blood (PB) were evaluated at long-term in subjects who had received intensive chemotherapy and autograft. Thirty-two adults with a previous diagnosis of lymphoma were examined, at a median time of 73 months (range 42–125) since autograft. They all had received a high-dose sequential chemotherapy treatment followed by peripheral blood progenitor cell (PBPC) autograft. There were 20 male and 12 female (median age at autograft: 40 yrs., range 21–60). A Southern blot procedure using a chemiluminescence-based assay was employed to determine telomere length on samples from grafted PBPC as well as on BM and PB samples obtained at long-term during follow-up. These latter samples were also studied for their in vitro growth characteristics, assessed by short and long-term culture assays. In all cases, autograft had been performed with large quantities of hematopoietic stem cells (median autografted CD34+ve cells/kg: 9.8 x 10⁶, range 2–24), allowing a rapid and stable hematologic reconstitution. Telomere length was found slightly shorter in BM mononuclear cells from samples taken at follow-up compared to samples from grafted material (median telomere length: 6,895 bp vs 7,073 bp, respectively; p=ns). No marked differences were observed in telomere evaluation between BM and PB cells. No significant differences were observed as well when PB telomere length of follow-up samples was compared with telomere length of PB from age-related normal subjects. BM and PB samples were then assessed for their in vitro growth characteristics. Committed and stromal progenitors were grown from all samples in good though variable quantities. However, as compared to normal controls, a statistically significant reduction of marrow-derived hematopoietic progenitors (CFU-GM - BFU-E - CFU-Mix) as well as stromal progenitors (CFU-F) was observed. Additionally, the more immature LTC-IC progenitor cell compartment was dramatically reduced, both in BM and PB samples. The results indicate that: i. the proliferative stress induced by intensive chemotherapy and post-graft hematopoietic reconstitution does not imply marked telomere loss in BM and PB cells at long-term, provided that large quantities of PBPC are used for autograft; ii. stem cells present in the graft or surviving after high-dose therapy are capable of reconstituting a sufficiently adequate hematopoiesis although the committed progenitor cell compartment and even more the immature LTC-IC progenitors are persistently reduced even at up to 10 years since autograft.