Hematopoietic Activity of Human Short Term Repopulating Cells in Mobilized Peripheral Blood Cell Transplants Is Restricted to the First 5Months after Transplantation.

Short-term repopulating cells (STRC) are largely enriched in human mobilized peripheral blood (mPB) as compared to bone marrow (BM) and cord blood (CB) transplants. By comparative analysis of human hematopoietic transplants in NOD. Cg-Prkdc^scid B2m^tm1Unc/J(B2m^−/−) mice we have previously identified and characterized two distinct classes of STRC that dominate the early hematopoietic reconstitution after transplantation: myeloid-restricted STRC-M and STRC-ML that generate lymphoid as well as myeloid progeny. However, due to the limited lifespan of B2m^−/− mice, the kinetics of the hematopoietic contribution of STRC remained unclear. Here we compared the long-term kinetics of the hematopoietic reconstitution of human transplants from mPB (high STRC content) to cord blood transplants (low STRC content) after xenotranplantation into long-living NOD. Cg-Prkdc^scid Il2rg^tm1Wjl/SzJ (IL2RG^−/−) mice to assess the relative hematopoietic contribution of STRC over time. For this, CD34⁺ CB and mPB cells were injected intravenously into irradiated B2m^−/− and IL2RG^−/− mice and the engraftment and differentiation of human cells in the peripheral blood and bone marrow of the mice was monitored in serial blood samples and bone marrow aspirates. Multilineage engraftment of CD34⁺ CB and mPB cells was reliably achieved in the BM of both mouse strains. Lineage distribution and engraftment levels of the human transplants in both mouse strains were similar with the exception of a 4-fold larger proportion of erythroid cells at three weeks after transplantation(p<0.05) and a statistically non-significant trend towards higher engraftment levels after mPB transplantation in IL2RG^−/− as compared to B2m^−/− mice, indicating at least equally efficient engraftment of human STRC in both strains. Serial analysis of BM aspirates of transplanted IL2RG^−/− mice for up to 44 weeks after transplantation revealed largely different engraftment kinetics of mPB as compared to CB transplants. mPB transplants generated the maximum number of progeny early at 3 weeks and then remained constant for up to 8 weeks before the engraftment levels gradually declined to very low levels at 28 weeks after transplantation, indicating that STRC-ML contributed quantitatively to hematopoiesis only for 5 months after transplantation. In contrast, maximum engraftment of CB transplants was detected at 8 weeks post-transplant and then remained stable for the whole observation period, demonstrating a larger contribution of long-term repopulating cells to the overall engraftment of CB than mPB transplants. Comparison of the peripherilization of CB and mPB transplants revealed further differences. Whereas the percentage of human cells in the peripheral blood of the mice correlated to the BM after CB transplantation (r=0.62), mPB transplants did not reliably peripheralize. In addition, the full differentiation potential of the human transplants was only detectable in the murine BM. A very low proportion of myeloid cells were detectable in the peripheral blood of the mice after CB as well as mPB transplantation. In summary, we provide direct evidence that the hematopoietic activity of human STRC is restricted to the first 5 months after transplantation and that the ratio of STRC to long-term repopulating cells dramatically changes during ontogeny. Our results emphasize that strategies able to selectively amplify STRC are needed to overcome extended cytopenia after clinical CB transplantation.