FLT3 signaling augments macrophage production from human pluripotent stem cells.

Recent advances in cell engineering technologies enable immune cells to be utilized for adoptive cell transfer (ACT) immunotherapy against cancers. Macrophages have the potential to directly and indirectly exterminate cancers and are therefore an attractive option for therapies. To develop new ACT therapies using macrophages, a great number of macrophages are required. Human induced pluripotent stem cells (iPSCs) are expected to be a source of macrophages; therefore, a system to efficiently produce macrophages from human iPSCs is needed. Here, we demonstrated that human iPSCs were robustly differentiated into macrophages by enforced FMS-like tyrosine kinase-3 (FLT3) signaling via the introduction of exogenous FLT3 into iPSCs and the addition of its ligand FLT3L to the macrophage induction culture. These iPSC-derived macrophages were identical to those obtained by standard differentiation induction methods. Thus, our novel system enables the preparation of scalable macrophages from human iPSCs. We believe that this system will be useful to develop a novel ACT therapy using macrophages.

An Interferon-γ/FLT3 Axis Positively Regulates Hematopoietic Progenitor Cell Expansion from Human Pluripotent Stem Cells.

Since it became possible to differentiate human pluripotent stem cells (hPSCs) into hematopoietic cells in vitro, great efforts have been made to obtain highly potent hematopoietic stem/progenitor cells (HSPCs) from hPSCs. Immunophenotypical HSPCs can be obtained from hPSCs, but their repopulating potential in vivo is low. Here, we developed a novel hematopoietic differentiation method for human-induced pluripotent stem cells (hiPSCs) to determine why the existing hPSC differentiation systems are inadequate. hiPSC-derived CD45+CD34+ cells in our system were mostly CD38- immunophenotypical HSPCs. The vast majority of human CD45+CD34+ cells in umbilical cord blood, fetal liver, and bone marrow are CD38+ hematopoietic progenitor cells (HPCs); therefore, the poor production of CD38+ HPCs was indicative of a systematic problem. hiPSC-derived CD45+CD34+ cells did not express FLT3, a receptor tyrosine kinase. Exogenous FLT3 activity significantly enhanced the production of CD38+ HPCs from hiPSCs. Thus, poor production of CD38+ HPCs was due to a lack of FLT3 expression. Interferon-γ upregulated expression of FLT3 and increased the number of CD38+ HPCs among hiPSC-derived CD45+CD34+ cells. These results suggest that the poor production of CD38+ HPCs with hPSC differentiation systems is due to a lack of FLT3 expression, and that the addition of interferon-γ can solve this problem.

Modification of Lhx2 activity for ex vivo amplification of human iPSC-derived hematopoietic stem/progenitor cells.

Hematopoietic stem cells (HSCs) obtained from patient-derived human induced pluripotent stem cells (iPSCs) are a promising tool for curing various hematological disorders. We previously demonstrated that enforced expression of the LIM-homeobox transcription factor Lhx2, which is essential for mouse embryonic hematopoiesis, leads to generation of engraftable and expandable hematopoietic stem cells (HSCs) from mouse iPSCs. However, it remained unknown whether Lhx2 can induce HSCs from human iPSCs. Here, we investigated the effect of Lhx2 overexpression on hematopoietic differentiation of human iPSCs. Unexpectedly, Lhx2 severely inhibited proliferation of human iPSC-derived hematopoietic cells. Thus, Lhx2 exhibited differential effects on mouse and human hematopoietic cells. Further studies implied that the inhibitory effect of Lhx2 on human iPSC-derived hematopoietic cells was due to insufficient transcriptional activation ability. Therefore, we modified Lhx2 to strengthen its activity as a transcriptional activator. This modified Lhx2 could induce ex vivo amplification of human iPSC-derived hematopoietic stem/progenitor cells (HSPCs). We believe that these findings will facilitate the development of a method to efficiently produce HSCs from human iPSCs.