Increased Recruitment of Hematopoietic Progenitor Cells Underlies the Ex Vivo Expansion Potential of FLT3 Ligand

Influence of Cytokines on the Growth Kinetics and Immunophenotype of Daughter Cells Resulting From the First Division of Single CD34+Thy-1+lin− Cells

There is a need to determine whether culture conditions may exist for ex vivo expansion of hematopoeitic stem cells (HSC), which favor solely proliferative self-renewal of HSC as opposed to proliferation with differentiation. Using single cells, we studied the effects of individual and combinations of cytokines in serum-free medium on the kinetics of the first cell doubling and the resulting phenotype of each of individual daughter cell. CD34+Thy-1+lin−cells were plated 1 cell per well in Terasaki plates in serum-free medium containing cytokines. Each well containing a single cell was monitored daily over 7 days for maintenance, division, or death. When division occurred in an individual well, the phenotype of the daughter cells was determined by staining with anti-CD34 fluorescein isothiocyanate (FITC)- and phycoerythrin (PE)-conjugated lineage specific antibodies. The cumulative percent of wells with an undivided single cell, wells in which the cell had divided, and wells in which the cell had died were scored. The number of doublets with conserved phenotype (CD34+lin−) was compared to those wells with one or more differentiated daughter cells (CD34+lin+). Over 7 days, cells cultured in single factors showed that between 13% (interleukin-6 [IL-6]) and 29% (thrombopoietin [TPO]) of the cells were undivided, between 13% (IL-1) and 35% (TPO) of the cells doubled, and between 35% (TPO) and greater than 60% (IL-11, IL-1, or hepatocyte growth factor [HGF]) died. When combinations of cytokines were used over 7 days, between 5% (FLT-3 ligand [FLT-3L], stem cell factor [SCF], IL-3, IL-6, granulocyte colony-stimulating factor [G-CSF], β nerve growth factor [βNGF]) and 22% (FLT-3L + HGF) of the cells remained undivided, between 15% (HGF, IL-1, IL-11, G-CSF) and 68% (SCF + TPO) of the cells had doubled and between 27% (FLT-3L + TPO) and 70% (HGF, IL-1, IL-11, G-CSF) died. The combination of FLT-3L + TPO induced the highest total percent (64.6%) of cells with conserved phenotype (percent conserved doublets + percent with 1 cell conserved), followed by SCF + TPO, (50%) and the combination of FLT-3L, SCF, IL-3, IL-6, G-CSF, βNGF (53%). These combinations also produced the highest yield of cells with conserved phenotype after one division (FLT-3L + TPO − 81 cells/100 initial cells, SCF + TPO − 68 cells/100 initial cells) (P = .01). Observation of the time of the initial cell division and phenotype of the daughter cells allowed us to identify candidate combinations of cytokines that promote maintenance of lin− cells (TPO), or recruit the primitive cells to divide and undergo phenotypic self-renewal (FLT-3L + TPO, SCF + TPO).

Flt3 Ligand Promotes the Generation of a Distinct CD34+Human Natural Killer Cell Progenitor That Responds to Interleukin-15

Interleukin-15 (IL-15) is produced by human bone marrow (BM) stromal cells and can induce CD34+ hematopoietic progenitor cells (HPCs) to differentiate into CD56+CD3−natural killer (NK) cells in the absence of stromal cells. IL-15 mediates its effects by signaling through the β and γcchains of the IL-2/15 receptor (R). The c-kit ligand (KL), also produced by stromal cells, enhances the expansion of NK cells from CD34+ HPCs in the presence of IL-15, but alone has no ability to differentiate NK cells. Mice deficient in KL do not appear to have a quantitative deficiency in NK cells, suggesting that other stromal cell factors may contribute to NK cell expansion. Flt3 ligand (FL) is also produced by BM stromal cells and has homology with KL. Furthermore, mice with a targeted disruption of the FL gene have reduced numbers of NK cells. We evaluated here the effects of FL on human NK cell development and expansion from CD34+ HPCs. Like KL, FL significantly enhanced the expansion of NK cells from CD34+ HPCs in the presence of IL-15, compared with IL-15 alone. However, FL alone had no effect on NK cell differentiation. We therefore explored the mechanism by which FL promotes IL-15–mediated NK cell development. FL was found to induce IL-2/15Rβ (CD122) expression on CD34bright HPCs. The CD34brightCD122+ cell coexpressed CD38, but lacked expression of CD7, CD56, NK cell receptors (NKRs), or cytotoxic activity in the absence of IL-15. Using limiting dilution analysis in the presence of IL-15 alone, we demonstrated that the FL-induced CD34brightCD122+ HPCs had an NK cell precursor frequency 20- to 60-fold higher than the CD34dim/negCD122− HPCs and 65- to 235-fold higher than fresh CD34+ HPCs. KL had similar effects as FL, but induced a significantly lower percentage of CD34brightCD122+ cells (P ≤ .01). Both FL and KL also increased IL-15R transcript in CD34+ HPCs. Culture of CD34+ HPCs in FL or KL, followed by culture in IL-15 alone, induced expression of both C-type lectin and Ig-superfamily NKRs on CD56+ cells. These data collectively support a role for FL in early human NK cell development. FL or KL generate a unique CD34brightCD122+CD38+ human NK cell intermediate from CD34+ HPCs that lacks NK features yet is IL-15–responsive. IL-15 is then required for the induction of CD56 and NKRs, LGL morphology, cytotoxic activity, and the ability to produce abundant cytokines and chemokines.

Ex Vivo Expansion of Genetically Marked Rhesus Peripheral Blood Progenitor Cells Results in Diminished Long-Term Repopulating Ability

The possibility of primitive hematopoietic cell ex vivo expansion is of interest for both gene therapy and transplantation applications. The engraftment of autologous rhesus peripheral blood (PB) progenitors expanded 10 to 14 days were tracked in vivo using genetic marking. Stem cell factor (SCF)/granulocyte colony-stimulating factor (G-CSF)–mobilized and CD34-enriched PB cells were divided into two equal aliquots and transduced with one of two retroviral vectors carrying the neomycin-resistance gene (neo) for 4 days in the presence of interleukin-3 (IL-3), IL-6, and SCF in the first 5 animals, IL-3/IL-6/SCF/Flt-3 ligand (FLT) in 2 subsequent animals, or IL-3/IL-6/SCF/FLT plus an autologous stromal monolayer (STR) in the final 2. At the end of transduction period, one aliquot (nonexpanded) from each animal was frozen, whereas the other was expanded under the same conditions but without vector for a total of 14 days before freezing. After total body irradiation, both the nonexpanded and expanded transduced cells were reinfused. Despite 5- to 13-fold higher cell and colony-forming unit (CFU) doses from the expanded fraction of marked cells, there was greater short- and long-term marking from the nonexpanded cells in all animals. In animals receiving cells transduced and expanded in the presence of IL-3/IL-6/SCF/FLT, engraftment by the marked expanded cells was further diminished. This discrepancy was even more pronounced in the animals who received cells transduced and expanded in the presence of FLT and autologous stroma, with no marking detectable from the expanded cells. Despite lack of evidence for expansion of engrafting cells, we found that the addition of FLT and especially STR during the initial brief transduction period increased engraftment with marked cells into a clinically relevant range. Levels of marked progeny cells originating from the nonexpanded aliqouts were significantly higher than that seen in previous 4 animals receiving cells transduced in the presence of IL-3/IL-6/SCF, with levels of 10% to 20% confirmed by Southern blotting from the nonexpanded IL-3/IL-6/SCF/FLT/STR graft compared with 0.01% in the original IL-3/IL-6/SCF cohort. These results suggest that, although expansion of PB progenitors is feasible ex vivo, their contribution towards both short- and long-term engraftment is markedly impaired. However, a brief transduction in the presence of specific cytokines and stromal support allows engraftment with an encouraging number of retrovirally modified cells.

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