Environmental Cues Can Promote Symmetrical Division of Functional Human Hematopoietic Stem Cells.

Widespread clinical use of ex-vivo expanded human umbilical cord blood (CB) grafts has been limited by lack of proper understanding of factors regulating self-renewal type of symmetric cell divisions. The expansion of the number of functional hematopoietic stem cells (HSC) ex-vivo requires the creation of an environment which favors symmetrical division. In our current studies, addition of late acting cytokines, (GM-CSF, IL-6, Epo) with early acting cytokines (thrombopoietin, SCF, Flt-3 ligand) resulted in loss of expansion of stem/progenitor cells. These data indicate that modification of HSC fate is not fully independent of external humoral influences. We have previously demonstrated that following treatment of CD34+ cells with 5-aza-2-deoxycytidine (5azaD) and trichostatin A (TSA) there is a 10- fold increase in the number of SCID mouse repopulating cells (SRC). This increase of SRC, however, occurred concomitantly with an increase in absolute number of CD34⁺CD90⁺ cells as well as primitive progenitors which gives rise to colony forming unit Mix lineage (CFU-Mix). We hypothesized that if the primary CD34+ cells generates CFU-Mix/CFU-GM in a ratio of ‘X’, then to observe a higher rate of symmetric cell division we would expect to see the ratio increased (>X) in the 5azaD/TSA treated cells in comparison to cells cultured in the absence of 5azaD/TSA (< X). Interestingly, analyses of our data suggest that when 5azaD/TSA treated CD34+ cells are cultured for 5 days and assayed for colonies we observed a significant increase in the ratio of CFU-Mix/CFU-GM in contrast to cells cultured in cytokines alone, 0.373 ± 0.06 and 0.066 ± 0.032 respectively. The ratio of CFU-Mix/CFU-GM of CB CD34+ cells (day 0) was 0.262 ± 0.045. These findings indicate that 5azaD/TSA treatment promotes the ratio of CFU-Mix/CFU-GM possibly by enhancing symmetric division of CFU-Mix while in the absence of 5azaD/TSA treatment the culture condition likely induces differentiation. In addition, we have also investigated the ratio of progenitor cells/differentiated cells by assessing the ratio of human CD34+ cells/CD33+ cells in the bone marrow of immunodeficient mice following transplantation (8 weeks) of equal numbers of CD34+ cells. The ratio of CD34+ cells/CD33+ cells following transplantation of 5azaD/TSA treated cells was 0.52 ± 0.14 (n = 11) while in the absence of 5azaD/TSA the ratio dropped to 0.31± 0.16 (n = 4). The ratio following transplantation of primary CD34+ (day 0) cells was 0.62 ± 0.14 (n = 6). These data suggest that 5azaD/TSA treated cells maintain the balance of generation of CD34+ cells/CD33+ cells at a comparable rate to that of primary CD34+ cells, while the CD34+ cells generated in the absence of 5azaD/TSA promotes generation of more differentiated cells. Alternatively, it is also possible that 5azaD/TSA treatment of CD34+ cells in the culture results in inhibition of myeloid differentiation at the cost of proliferation. However, the latter possibility is unlikely, since treatment of CB cells with 5azaD/TSA results in an increase in the absolute number of progenitors including SRC possessing both myeloid and lymphoid differentiation potential. Taken together, these data support our hypothesis that chromatin modifying agents in the culture is capable of promoting self-renewal type of symmetric cell division possessing in vivo multilineage marrow repopulating potential.