Further biological characterization of small molecules UM171 and SR1: In vitro effects on three hematopoietic cell populations from human cord blood

Small molecules UM171 and SR1 have already been taken into clinically-oriented protocols for the ex vivo expansion of hematopoietic stem (HSCs) and progenitor (HPCs) cells. In order to gain further insight into their biology, in the present study we have assessed their effects, both individually and in combination, on the in vitro long-term proliferation and expansion of HSCs and HPCs contained within three different cord blood-derived cell populations: MNCs (CD34⁺ cells = 0.8 %), LIN⁻ cells (CD34⁺ cells = 41 %), and CD34⁺ cells (CD34⁺ cells >98 %). Our results show that when added to cultures in the absence of recombinant stimulatory cytokines, neither molecule had any effect. In contrast, when added in the presence of hematopoietic cytokines, UM171 and SR1 had significant stimulatory effects on cell proliferation and expansion in cultures of LIN⁻ and CD34⁺ cells. No significant effects were observed in cultures of MNCs. The effects of both molecules were more pronounced in cultures with the highest proportion of CD34⁺ cells, and the greatest effects were observed when both molecules were added in combination. In the absence of small molecules, cell numbers reached a peak by days 25–30, and then declined; whereas in the presence of UM171 or/and SR1 cell numbers were sustained up to day 45 of culture. Our results indicate that besides CD34⁺ cells, LIN⁻ cells could also be used as input cells in clinically-oriented expansion protocols, and that using both molecules simultaneously would be a better approach than using only one of them.