Selection of normal human hematopoietic stem cells for bone marrow transplantation using immunomagnetic microspheres...

Complete yet nontoxic removal of tumor cells from autologous marrow grafts has proved difficult. New methods for separating normal stem cells from tumor cells are needed. The CD34+ cells in bone marrow, 1-2% of the low-density leukocytes, include precursors of all lymphohematopoietic lineages and probably also the primitive cells responsible for engraftment. A nontoxic, inexpensive, reproducible, and clinically applicable method for positive selection of CD34+ cells was developed. Paramagnetic microspheres coated with goat anti-mouse IgG1 are used to partition the cells; brief incubation with chymopapain is used to release them from the beads. Chymopapain exposure does not injury colony-forming cells or delay engraftment in rodents. Clinical volumes of bone marrow can be processed rapidly. In pilot experiments, the resulting grafts have a purity of 85-99% CD34+ cells and 40% median recovery of the assayable colony-forming cells. These studies form the background for a Phase I trial of autologous BMT using CD34+ stem cells.