Differences in the Kinetics of Hematopoietic and Stem Cell (SC) Engraftment Following Murine Newborn/Cord Blood (NB) and Bone Marrow (BM) Transplantation.

Abstract

Cord blood is being used at an increasing frequency as a source of stem and progenitor cells in human hematopoietic transplantation. However, very little is known about the kinetics of engraftment of cord blood relative to bone marrow derived stem cells. We have used a murine model of newborn/cord blood transplantation to address this question. Our studies demonstrated that murine NB can provide long-term engraftment in primary, secondary, and tertiary transplant recipients. Although NB engrafts more slowly than BM, at one year, engraftment was similar in both types of recipients. We compared the rate of recovery of blood counts, hematopoietic progenitor counts and putative stem cell [SC] counts (i.e. Sca-1+, c-kit+, Lin- cells) in recipients of NB and BM transplantation. Interestingly, the SC ratio in mononuclear cells from donor NB/donor BM was 3.25 ± 0.8 (range = 2.45 – 4.75, n=15). Similarly, donor NB mononuclear cells contained approximately 30% of the number of megakaryocytic progenitors, 12% of the myeloid progenitors and 5% of the erythroid progenitors of donor BM mononuclear cells. The repopulation kinetics in recipients of donor BM and NB transplantation were analyzed at regular intervals, up to 8 months after transplantation. During the first two weeks, NB recipients had lower hemoglobin, WBC and platelet counts than BM recipients. However, by 1 month, the hemoglobin and WBC counts were at similar levels in NB and BM recipients. In contrast, NB transplantation recipients required 2 to 3 months to achieve platelet counts similar to those in BM recipients. These results are reminiscent of the well-known delayed platelet recovery following human cord blood transplantation. Progenitor cell counts in the bone marrow of recipients paralleled the hematological recovery described above. At 2 weeks post-transplantation, progenitor counts of all lineages in NB recipients were 25 to 35% of those in BM recipients. By 1 month, erythroid and myeloid progenitor numbers were similar in NB and BM recipients. In contrast, the appearance of megakaryocytic progenitors was delayed following NB transplantation and did not reach the same level as BM recipients until about 2 months after transplantation. During recovery, the number of SC in the bone marrow of both types of recipients increased gradually over time. At one month after transplantation, the number of SC in BM recipients was significantly greater than that in NB recipients, with a ratio of 4.2 ± 0.2. This SC ratio decreased gradually during the next several months. At 2, 4, and 8 months following transplantation, the ratios of SC in BM recipients/NB recipients were 3.5 ± 0.4, 2.6 ± 0.5, and 2.2 ± 0.3, respectively (n=5). This gradual decrease in the ratio of BM/NB SC suggests that NB SC increase more rapidly than BM SC. In conclusion, these data demonstrate that the kinetics of hematopoietic and stem cell recovery following NB and BM transplantation are significantly different. Although hematopoietic recovery after NB transplantation is slow at first, final engraftment is similar following NB and BM transplantation. Furthermore, the number of SC in NB recipients increases at a faster rate than the number of SC in BM recipients. These differences in SC recovery may be a reflection of differences either in the homing capacity or in the functional maturity of NB relative to BM SC. Further investigation is required to distinguish between these two possibilities.