Small Molecule Combination Enhances CD34⁺/CD38^- Cell Proliferation Via Inhibition of Differentiation

Hematopoietic stem cells (HSCs) have become increasingly attractive for the therapy of various hematological system disorders. The aim of this study is to identify approaches that promote the expansion of HSCs. We present here the identification of a combination of small molecules and cytokines that is effective in retaining high stemness of hematopoietic stem/progenitor cells while promoting cell proliferation by inhibiting differentiation.

Firstly, five small-molecule candidates were screened for their individual effects on ex vivo expansion of human peripheral blood CD34⁺ cells in the presence of selected cytokines. The best compounds at their optimal concentrations were further analyzed in combination, to achieve maximum capacity for stimulating the CD34⁺CD38^- cell expansion ex vivo. The extent of cell expansion and the immunophenotype of expanded cells were assessed through flow cytometry. Additional cell and molecular assays were performed to confirm that the expanded CD34^± cells are functionally normal in vitro. Subsequently, the expanded cells were transplanted into sublethally irradiated NOD/SCID mice for the assessment ofhuman cell viability and engraftment potential in vivo. Furthermore, the expression of several genes in the cell proliferation and differentiation pathways was analyzed through qPCR during the process of CD34^±cell expansion.

Following multiple rounds of screening, an optimal formula (named as "SVC cocktail") was obtained, which consisted of four cytokines (stem cell factor, flt-3 ligand, thrombopoietin and interleukin-6) and three small molecules (Stem Regenin 1, valproic acid and CAY10433). CD34⁺ cells cultured with SVC cocktail had a purity of 76.2%±7.5% and reached expansion folds of 27.9±4.3 for CD34⁺/CD38^- HSCs on day 7. In contrast, CD34⁺ cells cultured with the cytokines alone displayed a purity of 27.4%±6.3% and expansion folds of 15.5±2.2 for CD34⁺/CD38^- cells. The groups with small molecules only (plus DMSO, the vehicle), or with basal medium only, showed no surviving cells on day 4. Furthermore, cell cycle analysis indicated that the SVC cocktail-induced CD34⁺/CD38^- cells stayed in a more quiescent state (G₀/G₁: 75.2%±3.6%; S: 9.2%±2.4%). On the other hand, the cells cultured without the three small molecules had active DNA synthesis (G₀/G₁: 56.0%±2.0%; S: 31.8%±3.2%), implicating a trend of enhanced cell differentiation in the cytokine alone group. RT-qPCR analysis further demonstrated that the expression of HSC stemness markers CD90, CD133, CD117, ALDH1, Bmi1, HoxB4, GATA-2, Runx1, and CXCR4 were elevated in the SVC cocktail-induced CD34⁺ cells, but dramatically reduced or barely detectable in the cytokine alone group. In addition, CFU assays for the SVC cocktail group vs the cytokine alone group demonstrated BFU-E of 54.0±4.6 vs 11.7±1.5, CFU-GM of 71.0±2.7 vs 8.3±2.5, CFU-GEMM of 40.7±3.8 vs 5.0±2.0 and CFU-Mk of 6.7±1.5 vs 0.7±0.6, respectively. For the in vivo engraftment in mouse bone marrow, human CD45 rate in the SVC cocktail group was much higher than in the cytokine alone group (21.1%±2.7% vs 0.5%±0.1%); similar group differences were also found in the CD34⁺ and CD34⁺CD38^- rate (7.7%±1.4% vs 1.6%±1.2% and 6.8%±2.2% vs 1.6%±0.1% respectively), all at 8 weeks post transplantation. Moreover, qPCR analysis of Notch and Wnt signaling pathways for cultured cells on day 7 showed that the expression of Notch target genes (related to high activation of HSC property) was enhanced in the SVC cocktail group compare to the cytokine group (HES5: 9.2±2.3 vs 3.6±1.4 in arbitrary units; HEY1: 6.3±1.9 vs 2.6±1.2; HES1: 3.2±1.3 vs 1.3±0.4; Notch1: 1.4±0.3 vs 1.2±0.3), whereas the expression of Wnt target genes (related to activation of HSC differentiation) was greater in the cytokine alone group than in the SVC cocktail group (CCND1: 10.1±4.3 vs 1.2±0.8; LEF1: 4.3±0.6 vs 2.9±0.2; PPAR D: 3.4±0.3 vs 1.5±0.1; FZD2: 1.8±0.2 vs 1.0±0.1).

Taken together, our results show that the new SVC cocktail is able to retain the characteristics of HSCs remarkably well, by enhancing their expansion while inhibiting their differentiation. Mechanistically, it appears that the three small molecules can effectively inhibit the cytokines' pro-differentiation effects on CD34⁺CD38^- cells without affecting the cytokines' ability to stimulate cell proliferation.