Repetitive bone marrow transplantation in nonmyeloablated recipients

Transplantation of 200 million male BALB/c marrow cells into normal nonmyeloablated female BALB/c hosts results in relatively high levels of engraftment, whether the cells are infused repetitively over time or in a single infusion. These high engraftment rates suggested that repetitive injections of high levels of male BALB/c cells might be able to totally replace host marrow. Accordingly, we transplanted 40x10(6) male BALB/c bone marrow cells into female BALB/c recipients over a 7-week period for a total of 20 injections (800x10[6] marrow cells). Engraftment in this experimental group was compared to that seen when female recipients received 2x10(6) male marrow cells or phosphate-buffered saline (PBS) over the same injection schedule. Engraftment was quantitated at 5 and 7 weeks after the final infusion by detection of male-specific sequences in female host marrow, spleen, and thymus by Southern blot analysis using a Y-specific cDNA probe. Male DNA levels were quantitated with a Molecular Dynamics phosphorimager. Engraftment of male cells into female marrow at 5 and 7 weeks posttransplantation ranged from 19 to 88%, whereas that in spleen and thymus ranged between 30 and 100% and 28 and 50%, respectively. The mean percent engraftments for marrow, spleen, and thymus were 41, 69, and 39%, respectively. Mean percent engraftments for 2x10(6) cell infusions at 5 and 7 weeks for marrow, spleen, and thymus were 4, 6, and 4%, respectively. Marrow and spleen cellularity and total high proliferative potential colony-forming cell numbers were determined in PBS- and cell-injected mice. No significant differences between these groups were observed. For marrow engraftment, 20 injections of 40x10(6) cells was not more effective than five, but donor DNA in thymus and spleen was increased with 20 injections. Primitive progenitor cell levels and marrow cellularity do not increase in mice injected with large numbers of marrow cells, suggesting that host marrow cells are replaced rather than augmented by infused donor cells.