Interleukin 6 receptor expression by human cord blood- or peripheral blood-derived primitive haematopoietic progenitors implies acquisition of different functional properties

The significance of interleukin 6 receptor (IL-6R) expression by cord blood (CB)- and peripheral blood (PB)derived primitive haematopoietic progenitor s was investigated, IL-6R was preferentially expressed by PB-derived myeloi d progenitors. Most PB-derived erythroid bursts (BFU-E) and mixed colony-fa rming cells (CFU-Mix) did not express this receptor. However, CB-derived pr imitive progenitor cells possessed multipotentiality, irrespective of IL-6R expression. Interestingly the long-term culture-initiating cell (LTC-IC) p opulation was enriched in PB-derived CD34(+) IL-6R(+) cells, but the extend ed LTC-IC (ELTC-IC) population, which represents a less mature class of hae matopoietic progenitors, seemed to be equally distributed in the IL-6R(+) a nd IL-6R(-) cell populations. In contrast, the number of LTC-ICs and ELTC-I Cs was similar in CB-derived CD34(+) IL-6R(+) or IL-6R(-) cells. It is note worthy that the number of LTC-ICs and ELTC-ICs in CB-derived CD34(+) cells was markedly higher than that in PB-derived CD34(+) cells regardless of IL- 6R expression. Telomerase activity was consistently lower in PB-derived CD3 4(+) IL-6R(-) cells than in CD34(+) IL-6R(+) cells. In contrast, telomerase activity was similar in CB-derived CD34(+) IL-6R(+) or IL-6R(-) cells. The pattern of telomerase induction upon cytokine stimulation differed between CB- and PB-derived CD34(+) IL-6R(+) or IL-6R(-) cells. However, overall te lomerase activity per dish was well correlated with the proliferative poten tial of both cell populations, suggesting that induction of telomerase play s an important role in the escape from replicative senescence of primitive haematopoietic progenitors. Collectively, these results suggest that CB-der ived primitive progenitors are less mature than PB-derived progenitors and that the expression of IL-6R by primitive haematopoietic progenitors may ha ve different implications for PB- and CB-derived CD34(+) cells.