Radiobiological properties of the human hematopoietic microenvironment: contrasting sensitivities of proliferative capacity and hematopoietic function to in vitro irradiation

We describe the effects of in vitro irradiation on the proliferative capacity and hematopoietic supportive function of human marrow stromal cells. To assess the effects on the proliferative capacity of stromal progenitors and differentiated fibroblasts, marrow cell suspensions and trypsin-dispersed marrow fibroblasts were treated with a single dose of gamma radiation at 100 rad/min. Fibroblastic progenitors (CFU-F) showed an exponential decrease in colony formation with increasing doses of irradiation, with a Do slightly higher than that of granulomonopoietic progenitors (CFU-GM); Do values for CFU-F and CFU-GM were 130 and 115, respectively. However, although the CFU-F survival curve exhibited a shoulder (n = 1.3), the CFU-GM curve did not (n = 1.0), indicating that only fibroblastic progenitors have the potential to repair irradiation- induced damage. Passaged marrow fibroblast colony-forming cells also showed a shouldered exponential survival curve with a Do of 110 and n value of 1.4. Marrow stromal progenitors giving rise to adherent layers in long-term marrow cultures also demonstrated a highly radiosensitive proliferative capacity. Stromal layers derived from irradiated marrow suspensions failed to establish adherent layers after relatively low doses of irradiation (over 240 rad) in a dose-response manner. To assess any functional damage in stromal progenitors surviving irradiation, stromal layers derived from marrow suspensions irradiated up to 240 rad were cocultured with freshly isolated autologous hematopoietic cells and assayed for their capacity to support prolonged CFU-GM production. Confluent stromal layers derived from irradiated marrow suspensions sustained CFU-GM production as well as controls. To study the effects of irradiation on the hematopoietic supportive capacity of established marrow-derived stromal layers, 4 to 6-week-old adherent layers were irradiated as described and cocultured with autologous marrow cells enriched for colony-forming cells. Stromal layers irradiated up to 1,320 rad sustained prolonged CFU-GM production, indicating that the hematopoietic supportive function remained intact at this dose of irradiation. In conclusion, we demonstrated that the proliferative capacity of human marrow stromal progenitors, as well as that of their differentiated descendants, is quite sensitive to in vitro radiation, while the hematopoietic supportive function of differentiated stromal cells is relatively resistant to the effects of radiation.