Immunosuppressive therapy in bone marrow aplasia: the stroma functions normally to support hematopoiesis

In aplastic anemia (AA) patients responsive to antilymphocyte globulin (ALG) therapy, abnormalities in both stroma and progenitor cell (PC) pool have been described. The relevance of each pathophysiologic defect was characterized in 16 individuals, and data were compared to results from seven normal volunteers. Bone marrow mononuclear cells were split into two fractions. Stromal layers (SL) were prepared from the first, and a CD34+ enriched population was obtained by immunomagnetic selection from the second. In cross-culture experiments, 1 x 10(4) of the latter from patients or controls were seeded on preformed SL, and adhesive PC were scored for the formation of blast colonies (CFU-Bl) on day 5 of culture. Nonadherent progenitors were recovered and quantitated in a standard clonogenic assay (CFU-GM). There were significantly fewer CD34+ cells in the AA group (median 0.65%, SD 0.39%, vs. 1.62%, SD 1.4%; p = 0.002). No morphological or cytologic differences between normal and aplastic SL were detected. Both equally supported the growth of CFU-Bl from normal progenitors (mean 117, SD 20.4, and 103.1, SD 30.4), while this value was reduced for the aplastic PC (mean 41.06, SD 42.9; p = 0.0002, exact two-tailed test). Similarly, the AA nonadherent PC had a decreased CFU-GM growth (mean 142.6, SD 104.8, vs. mean 361.7; SD 91.3), with a lower total clonogenic output (p = 0.0009). We conclude that aplastic stroma appropriately supports the growth of normal progenitors, whereas the depressed clonogenicity of the corresponsing population derived from AA is unrelated to their attachment to SL but intrinsic to the CD34+ cells, whether adherent or not.