Suppression of marrow stromal cells and microenvironmental damage following sequential radiation and cyclophosphamide

Persistent defects in marrow stroma may contribute to hemopoietic insufficiency in patients treated with combined modality therapy for malignancy. To assess the bone marrow failure following combined therapy, mice received intraperitoneal administration of four weekly doses of cyclophosphamide, 160 mg/kg (CY) one week after 1500 rad leg irradiation (LI). This treatment inhibited repopulation of endogenous nucleated cells to less than 60% of normal at one, two, four and six months post-irradiation. Leg irradiation alone suppressed the repopulation to about 75% of normal and cyclophosphamide alone suppressed to 80% of normal. Normal body weight and recovery of peripheral leukocytes and hematocrit to normal levels demonstrated the localized nature of the lesion. Differential marrow counts revealed that 1500 rad LI + CY suppressed erythroblasts, myeloid, and lymphoid cells more than either modality alone. To directly assess the damage of sequential 1500 rad LI + CY on the microenvironment, marrow stromal cells were flushed from the femoral marrow and cultured as adherent cell colonies. They were suppressed to less than 30% of normal for three months following combined modality treatment. Other progenitors, granulocyte-macrophage and spleen colony forming units (CFU-C and CFU-S respectively), were suppressed but exhibited different patterns of partial recovery. We conclude that multiple courses of cyclophosphamide starting one week after 1500 rad LI produced persistent damage to the microenvironment reflected by decreased marrow stromal cells and failure of hemopoietic cells and their progenitors to completely repopulate femoral marrow.